Engine Package I: CamTiming-Rod Ratio Relatnships - Team Integra Forums - Team Integra
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Engine Package I: CamTiming-Rod Ratio Relatnships

Posted 02-08-2002 at 12:00 AM by MichaelDelaney

This article is quoted and modified from the excellent series of tech articles at http://victorylibrary.com/tech/tech.htm . It will go into cam gear tuning, valve lash setting, and the influence of rod ratio on your engine package in terms of camshaft selection, port sizing for headporting, and connecting rod-crankshaft choice. CAM TIMING BASICS A. CAM GEAR TUNING Cam timing can affect where your peak torque will be located along the RPM range. Cam timing (and peak torque location) can be moved by adjusting or rotating the cam gears. When you advance the cam gear it opens and closes the valve earlier. Retarding or delaying the cam gear opens and closes t valve later. We will first begin with a simple single overhead cam (SOHC) system with a fixed cam overlap to show the relationship between advancing or retarding the intake valve opening and where the power is shifted to along the rpm range. Once you grasp the concepts with an SOHC system, we will then move to a more complex double overhead cam (DOHC) system, like in the Integra, to show the effects on separately adjusting the intake & exhaust valve timing events and changing the cam overlap. SINGLE OVERHEAD CAM (SOHC) TUNING In a SOHC engine, like in a Honda Civic Dx's D16Z6 SOHC VTEC engine, the intake cam lobe and the exhaust cam lobe are both located on the same single camshaft. Advancing or retarding the cam timing (using a single cam gear) rotates both the intake and exhaust valve opening/closing events together simultaneously , in the same direction, by the same amount. The event most affected by these changes in an SOHC engine is the INTAKE VALVE CLOSING POINT After Bottom Dead Center (ABDC). [ Aside for beginners : BDC refers to point in time when the piston reaches the very bottom of its travel. After BDC refers to time when the piston begins to travel upwards again relative to this bottom most BDC reference point.] What Happens When We Advance A SOHC Cam Gear?: - Advancing the Cam : closes the intake valve earlier, increasing cylinder pressure and power in the low to mid-range rpms, and reducing peak power. - Retarding (or Delaying) the Cam : closes the intake valve later, which reduces low to mid-range power (and may reduce pinging), but potentially improves peak power. Summary Effects of Advancing/Retarding SOHC Cam Timing |-------------------------------------------------------------------------------- | Advancing ................................. | ..... Retarding |--------------------------------------------------------------------------------| |Begins Intake Event Sooner........ | Delays Intake Event,Closes Intake |Open Intake Valve Sooner........ ...| Keeps Intake Valve Open Later |Builds More Low-End Torque ...... | Builds More High-End Horsepower |Decrease Piston-Intake Valve Cl. | Increase Piston-Intake Valve Cl. |Increase Piston-Exhaust Valve Cl.| Decrease Piston-Exhaust Valve Cl. |--------------------------------------------------------------------------------| Cl. = Clearance DOUBLE OVERHEAD CAMS (DOHC) TUNING WHAT IS LSA VERSUS CAM OVERLAP?: In SOHC engines, the lobe separation angle (LSA) and Overlap are fixed and cannot be changed. Both the intake and exhaust valves are either both advanced or both delayed by the same amount in the same direction of adjustment. With a DOHC engine, like the Integras, we can advance or retard the individual intake and exhaust cam lobes independently or separately. The intake cam can be adjusted in the opposite direction to the exhaust cam if the tuner wishes. The tuner can also adjust both cams in the same direction like in the SOHC. There is more freedom to change the overlap in a DOHC engine since the LSA and Overlap are not fixed. Therefore, we have a changeable LSA and Overlap. What is LSA and Overlap? We will now discuss and define these in detail below. The LSA can be changed and tuned by 2 adjustable cam gears instead of 1 cam gear. We could not do this in a SOHC engine and were stuck with one cam Overlap and LSA. Figure 1a. View of Intake Camshaft Lobe and Exhaust Camshaft Lobe from their ends: Notice the Intake and Exhaust Cam Lobes. This diagram shows LSA , each cam lobe's Centerlines (i.e. the red line on the lobe and is point of maximum valve lift), and Overlap in an Integra, where the crankshaft spins counterclockwise when the engine is viewed from the driver's side. DEFINITION OF LSA Lobe Separation Angle (LSA) is the number of degrees between the intake and exhaust cams' lobe center lines. The Lobe Centerline (red line on the diagram above) is an imaginary line that passes through the camsh rotation axis and represents the point of maximum valve lift. To calculate LSA, you need to determine the INTAKE CAM Centerline (C/L in) and EXHAUST CAM Centerline (C/L ex). LSA = [ C/L in + C/L ex ] / 2 C/L in = [INTAKE CAM DURATION @ 1mm lift /2 ] - INTAKE LOBE OPENING SPEC BTDC C/L ex = [EXHAUST CAM DURATION @ 1mm lift /2] - EXHAUST LOBE CLOSING SPEC ATDC Adjustable cam gears can change the lobe Centerline by increasing or decreasing the point at which valve events take place during engines cycles, without actually changing the cam duration spec. WHAT IS THE RELATIONSHIP BETWEEN LSA AND OVERLAP? Notice that in a DOHC engine, Cam Overlap is inversely affected by LSA . The size of the LSA affects the size of the cam overlap proportionately but in the opposite direction to one another. Increase LSA and you decrease Overlap. Reduce LSA and you increase Overlap. More Cam Overlap gives you more Upper RPM power gains. Wilder DOHC cams have more overlap in their specs. In other words, LSA can be changed in DOHC engines by independently advancing or retarding intake and exhaust cams separately by using adjustable, aftermarket cam gears.: - Increasing the LSA by separately retarding the intake cam and advancing the exhaust cam will decrease overlap, this will give you more midrange rpm power, with less peak hp gains. - Reducing LSA by separately advancing the intake cam and retarding the exhaust cam, this will give more overlap and therefore, give more peak power, with less midrange gains. Notice it's a tradeoff or compromise. You get gains at one end of the rpm range for less gains or even a loss of power at the other opposite end of the rpm range. It's never the same amount of gain throughout the entire rpm range when cam timing tuning is involved, unless you have variable cam timing like i-VTEC. RANGES FOR LSA Above 114 Deg. = Extremely Wide (MIDRANGE ORIENTED) 114-112 Deg. = Wide 112-110 Deg. = Moderately Wide 110-108 Deg. = Moderate 108-106 Deg. = Moderately Tight 106-104 Deg. = Tight Below 104 Deg. = Extremely Tight (PEAKY) The tighter the LSA: the higher up the powerband shifts along the rpm range and the narrower the powerband becomes. Overlap is increased. Widen the LSA: the lower the powerband shifts along the rpm range and the wider the powerband becomes. Overlap is decreased. We would like to narrow the LSA in the Integras for N/A applications (more overlap) and widen the LSA for boosted applications (less overlap). Figure 2. Effects of Changing Lobe Separation On Power Gain Location. Compare the cam with 116 degrees lobe separation to the one having a smaller 106-degree lobe separation (with dots) . Red lines are hp curves. Green lines are torque curves. Look particularly at the the Green torque lines. The wider lobe separation angle (no dots) produces more midrange torque but with a loss of peak torque. Narrower lobe separation angle (with dots) is better for a drag racing engine than a high performance street engine, due to an increase in valve overlap which creates more scavenging and a stronger upper rpm powerband. DOHC ENGINE CAM GEAR ADJUSTMENT SUMMARY Summary of the Effects of Adjusting Lobe Separation Angle ------------ | Tighten ..........................................| Widen |-------------------------------------------------------------------------| | Moves Torque to Higher RPM....... | Moves Torque to Lower RPM | Increases Maximum Torque........ | Reduces Maximum Torque | Narrow Powerband....................... | Broadens Power Band | Builds Higher Cylinder Pressure.. | Reduce Maximum Cylinder Pressure | Increase Chance of Engine Knock | Decrease Chance of Engine Knock | Increase Cranking Compression.. | Decrease Cranking Compression | Increase Efective Compression... | Decrease Efective Compression | Idle Vacuum is Reduced............... | Idle Vacuum is Increased | Idle Quality Suffers...................... | Idle Quality Improves | Open Valve-Overlap Increases... .| Open Valve-Overlap Decreases | Closed Valve-Overlap Increases. .| Closed Valve-Overlap Decreases | Decreases Piston-to-Valve Cl. ......| Increases Piston-to-Valve Cl. | |-------------------------------------------------------------------------| Cl. = Clearance B. VALVE LASH TUNING (Valve Adjustment) Slight adjustments can be made to the cam overlap & total duration by making minor changes in valve lash or how long the rocker arm sits on the cam lobe. The domestic engine builders vary the lash settings beyond the stock valve range in a systematic way called "valve lash loops" to determine indirectly if the engine would prefer a bigger or smaller cam duration without actually changing the cams. This will help them decide if they want to go out and purchase a bigger cam with the current piston dome (static CR) they have. Going beyond stock valve lash setting range is done for testing purposes to see if your engine package likes even more or less cam overlap than that obtained from the cam and cam gear tuning. It is an additional level of fine tuning available or "tuning trick" for enthusiasts.Valve lash setting however must be done on a bone cold engine preferably or at the very least an engine which has been shut off for at least 3-4 hr. Most experts suggest +.004 in. & -.008 in. of valve lash as the maximum valve lash limits to experiment beyond the stock lash specs. Go in 0.002 in. increments. Increasing Valve Lash (Smaller Valve Clearance or "Tighter" Lash) - will increase cylinder pressure at low to midrange RPM's, close the intake valve sooner, and reduce cam overlap & total cam duration. The effect is slightly similar to ADVANCING the cam gear . Too much lash increases wear on all components, lowers maximum RPM, and increases the possibility of breakage. Reducing Valve Lash (Larger Valve Clearance or "Looser" Lash) - will do the opposite: reduced cylinder pressure at low to midrange RPM's, later intake valve closure, and increased cam overlap & cam total duration. The effect is slightly similar to retarding the cam. NOTE: Too little (or too loose of a ) valve lash may result in a valve (especially an exhaust valve) not seating completely when the engine temperature rises (such as a full-throttle 1/4 mile pass). This is how exhaust valves get burned commonly. Part of the hot exhaust gases travel up the valve stem towards the valve guides instead of out the exhaust port and burn the valve. Dynoing an engine after changing the valve lash each time in order to determine if the engine performs better objectively would be impractical for the average enthusiast. I suggest doing timed acceleration runs after each new valve lash setting instead. CONNECTING ROD RATIO BASICS AND IT'S EFFECTS INTRODUCTION There are 3 other important factors which affect how camshafts make power other than the cam's specs and cam gear or valve lash tuning methods: 1. The * Rod Ratio= connecting rod length divided by stroke (also known as the "internal piston geometry" of the motor ) 2. The Intake Valve Closing Point ( in crankshaft degrees after bottom dead center [ABDC] ) 3. The Static Compression Ratio As you tune the valve timing (camshaft overlap or LSA) with the cam gears and set the valve lash, remember that valve opening and closing affects the pressure inside the cylinder. Cylinder pressure is what turns the crankshaft and makes power at the wheels. Static CR, the intake valve ABDC closing point, and rod ratio can also influence the cylinder pressure and therefore, whether you will make big power gains. In this next section of this article, I'll focus in on one of these 3 factors, the rod ratio. In other separate articles in this series, I'll discuss the definition of static & dynamic CR and the cam spec's effect on how much static CR you need. LINKS TO OTHER RELATED TI.NET ARTICLES - Compression Ratio (Static & Dynamic) Explained -> http://www.team-integra.net/sections...?ArticleID=233 - Static CR & Intake Cam Duration Relationship -> http://www.team-integra.net/sections...?ArticleID=472 But for now, let's learn and discuss the concept and consequence of rod ratio: WHAT IS THE ROD RATIO IN INTEGRAS? i/ LS or SE (B18A/B18B) connecting rod length = 137.0 mm stroke (determined by crankshaft) = 89.0 mm rod ratio = 137/89 = 1.54 ii/ GSR or ITR (B18C1/B18C5) connecting rod length = 137.9 mm stroke (determined by crankshaft) = 87.2 mm rod ratio = 137.9/87.2 = 1.58 DEFINITION OF ROD RATIO The geometric relationship between the rod and rod journal is one not generally understood by many mechanics but plays a key role in the motor's breathing characteristics and overall power characteristics. This rod ratio is calculated by dividing the connecting rod length by the stroke length. The rod's length is measured from the center of the piston-pin opening to the center of the big-end bore. Rod ratio greatly affects the way an engine performs and how long it lasts. RANGE OF VALUES FOR ROD RATIO How big or small can a rod ratio get? There is a small range of rod ratios for most conventional piston engines: The rod is between roughly 1.4 and 2.2 times the stroke length. It's not possible for the rod to be the same length as the stroke. Rods much longer than twice the stroke make the motor very tall and these engines cannot fit inside the tight engine bays of modern aerodynamic cars with low hoodlines. They are impractical except for racing engines. WHAT IS THE BEST ROD RATIO? A rod ratio value of 1.75 is considered "ideal" by many respected engine builders, if the breathing (i.e. cylinder head port sizes and IM size) is also optimized for the design. Notice that the Integras have a low rod ratio. CONSEQUENCE OF ROD RATIO The ROD RATIO dictates the ANGLE at which the piston travels up and down the cylinder. Lower rod ratios produce a larger or steeper angle between the rod and the crankshaft. The rod angle must not be too steep such that it encourages excessive friction at the cylinder wall and piston skirt as the piston travels: - A lower rod ratio (eg. B18B) , resulting in a steeper angle at which the piston travels, can be made by installing a shorter rod or by increasing the stroke (crankshaft). - A higher rod ratio (eg. in a B16A) , resulting in a less steep angle for the piston travel, can be made by using a longer rod or a shorter stroke (crankshaft). a low rod ratio produces a steeper rod to crankshaft angle and MORE PISTON SIDELOADING FORCES against the cylinder wall, as it travels up and down the cylinder. The consequence of a low rod ratio and resultant larger rod angle is MORE WEAR & ENGINE VIBRATION as the engine revs higher and higher: you are eventually forced to use a lower redline in order to prevent engine damage. Secondly, when the rod ratio value becomes smaller, it has other effects: mechanically, on breathing ability, and on how you set your spark timing. I. MECHANICAL EFFECTS OF A LOW ROD RATIO Motors with low rod ratios (like our 2nd & 3rd gen. integras ) typically exhibit the following characteristics (compared to high rod ratio motors): - shorter pistons, measured from the pin center to the bottom of the skirt. - higher level of vibration - greater wear on piston skirts and cylinder walls - slightly higher operating temperature & oil temperature due to friction - physically shorter engine (more oil pan, header, and air cleaner clearance), allowing for a lower hood line. - lower block weight. II. LOW ROD RATIO AND CHANGES IN SPARK IGNITION TIMING - earlier timing (MORE IGNITION TIMING ADVANCE) is required with a low rod ratio, since the combustion chamber volume is larger (i.e. the piston is farther from TDC) at the same point of crankshaft rotation compared to a high rod ratio motor. - the motor may also be less knock-sensitive with a low rod ratio, as the chamber volume increases more rapidly ATDC, lowering cylinder combustion pressure (this is useful for nitrous & supercharged motors). III. LOW ROD RATIO EFFECTS ON CYLINDER FILLING AND VOLUMETRIC EFFICIENCY (HOW WELL A MOTOR BREATHES) - with a lower rod ratio, intake vacuum rises sooner ATDC and higher flow speeds can be reached earlier in the rpm range: this allows you to have larger cylinder head intake ports & intake manifold (IM) plenum volumes without the loss of throttle response. This fact is important for headporting and IM selection. - a too small or badly shaped port will "run out of breath" sooner in the upper rpms with a low rod ratio motor. - piston speed or motion away from BDC is slower. The result of this is a trapping a higher percentage of cylinder volume. This, in turn, makes the motor less sensitive to late intake valve closing (i.e. later ABDC intake valve closing with longer duration "wilder" cams like Skunk2 Stage 2, Jun 3, or Toda B/C). IV. CHANGING ROD RATIO BY CHANGING CONNECTING ROD LENGTH A. Effects of Long Rods (High Rod Ratio) PROS: Provides longer piston dwell time at & near TDC, which maintains a longer state of compression by keeping the chamber volume small. This has obvious benefits: better combustion, higher cylinder pressure after the first few degrees of rotation past TDC, and higher temperatures within the combustion chamber. This type of rod will produce very good mid to upper RPM torque. The longer rod will reduce friction within the engine, due to the reduced angle which will place less stress at the thrust surface of the piston during combustion. These rods work well with numerically high gear ratios and lighter vehicles. For the same total deck height, a longer rod will use a shorter (and therefore lighter) piston, and generally have a safer maximum RPM. CONS: They do not promote good cylinder filling (volumetric efficiency) at low to moderate engine rpms due to reduced air flow velocity. After the first few degrees beyond TDC, piston speed will increase in proportion to crank rotation, but will be biased by the connecting rod length. The piston will descend at a reduced rate and gain its maximum speed at a later point in the crankshaft's rotation. Longer rods have greater interference with the cylinder bottom & water jacket area, pan rails, pan, and camshaft - some combinations of stroke length & rod choice are not practical. To take advantage of the energy that occurs within the movement of a column of air, it is important to select manifold and port dimensions that will promote high velocity within both the intake and exhaust passages. Long runners and reduced inside diameter air passages work well with long rods. Camshaft selection must be carefully considered. Long duration cams will reduce the cylinder pressure dramatically during the closing period of the intake cycle. B.Effects of Short Rods (Low Rod Ratio) Recall that the B18A/B18B rod ratio is 1.54 and B18C rod ratio is 1.58. Both have Low Rod Ratios. PROS: Provides very good intake and exhaust velocities at low to moderate engine rpms causing the engine to produce good low end torque, mostly due to the higher vacuum at the beginning of the intake cycle. High intake velocities also create a more homogenous (uniform) air/fuel mixture within the combustion chamber. This will produce greater power output due to this effect. The increase in piston speed away from TDC on the power stroke causes the chamber volume to increase more rapidly than in a long-rod motor - this delays the point of maximum cylinder pressure for best effect with supercharger or turbo boost and/or nitrous oxide. Cam timing (especially intake valve closing) can be more radical than in a long-rod motor. CONS: Causes an increase in piston speed away from TDC which, at very high RPM, will out-run the flame front causing a decrease in total cylinder pressure (Brake Mean Effective Pressure) at the end of the combustion cycle. Due to the reduced dwell time of the piston at TDC, the piston will descend at a faster rate with a reduction in cylinder pressure and temperature as compared to a long-rod motor. This will reduce total combustion. We discuss another way to change the rod ratio by changing the stroke via swapping crankshafts from other Bseries engines in the Engine Package II (Swapping Parts For Power) Article later on. V. ROD RATIO EFFECTS ON HEADPORTING (PORT SIZE) The intake system must be matched in proportion to the motor's displacement so you can achieve the largest power at the right place along the rpm range. If you choose a camshaft or IM that is too big or too small....or you remove too much material off the head's intake port,...compared to your redline, the largest power gain may occur at the wrong rpm. The rod ratio value can be used to compensate for less-than-perfect match between the intake system's parts (intake, TB, intake manifold, and cylinder head intake port size and shape) to the motor's size (i.e. displacement) & rpm range (cam redline). The reverse is also possible: After the bottom end or block has been upgraded (eg. bigger displacement from boring out or stroking the engine), there are still choices for the top end or intake system(intake, TB, intake manifold, head intake port size). Again, the rod ratio value can be used as a correction factor to better "match" the intake parts to the block. So here we are talking about compatibility between parts. Just as some girls or guys (depending on your gender and orientation) are too "high maintenance" for you to date, some connecting rod lengths are not compatible with certain intake system designs and are more "comfortable" with certain types of intake system designs. How can a change or swap in connecting rods with different lengths do this? Which rod lengths go with which port sizes? Which rod lengths go with largest power gain location along the rpm range? : A. High Rod Ratios 1.65 - 2.1 (ideal = 1.75) are more compatible with : - Small intake port volume vs. motor size (displacement) - High rpm powerband (higher redline rpm & peak power more important) B. Low Rod Ratios 1.45 - 1.64 (eg. Integras 1.54-1.58) are more compatible with : - Large intake port volume vs. motor size - Midrange rpm powerband (for pick-up,launch, midrange rpm, hauling or towing) IV. ROD RATIO AFFECTS ON CAMSHAFT SELECTION AND PURCHASE Low rod ratio motors (like the integras) have slower piston movement or piston speed upwards and away from bottom dead center (BDC) crankshaft rotation position on the compression stroke, along with a shorter piston dwell time at top dead center (TDC) (i.e. how long the piston pauses at the very top of it's travel, as it flip flops from the compression stroke to the power stroke or from the exhaust stroke to the intake stroke). The result is low rod ratio motors will capture more air:fuel mixture at the same point of intake valve closure on the intake stroke compared to a high rod ratio motor. This makes them more "tolerant" of late intake valve closing, meaning these type of engines with this low rod ratio won't lose too much hp, if you choose a cam that is too big for the engine. Therefore, low rod ratio motors like longer duration cams or at least can handle them better compared to high rod ratio motors like a Civic's b16a. V. CAMSHAFT SELECTION EFFECTS ON CYLINDER PRESSURE AND STATIC COMPRESSION RATIO: IS A BIGGER WILDER CAM ALWAYS BETTER? Can a cam's specs alone (i.e. lift & duration) determine if you make big power? No it can't. Why can't you just go out and buy the biggest wildest cam available on the market? Because the intake cam's valve closing point also determines how much cylinder pressure can leak (yes, you read right...leak) up the intake port during the compression stroke. If you lose a lot cylinder pressure during the compression stroke, you will lose power. Longer duration cams generally need more static compression or a larger piston dome to compensate for this pressure loss . Remember the Cam's jobs (also known as valve events) are to determine the : Intake Valve's Opening Point, Intake Valve's Closing Point, Exhaust Valve's Opening Point, Exhaust Valve's Closing Point, the size of the Overlap (Lobe Separation Angle) and how much Valve Lift. The only cam function which affects cylinder pressure directly is the Intake Closing Point - where the intake valve has just closed after the start of the compression stroke. Intake valve closure after bottom dead center (ABDC) always causes some of the intake mixture to be compressed backwards out of the cylinder by the rising piston at very low RPMs. All high-performance cams use later and later intake valve closure points as a method of increasing peak power. Late intake valve closure causes some mixture to escape (reversion) even at moderate RPMs, reducing cylinder pressure . The point in the engine's RPM range where this reversion stops and full-stroke capture occurs is at the peak torque. This RPM at which there is peak torque depends on many factors you can control including: cam timing, port efficiency, intake manifold runner area, cylinder head intake port volume, resonance, harmonics, exhaust system design, etc. If you have a long-duration cam, you can regain some of this lost cylinder pressure by raising the static compression ratio (by using a bigger piston dome ,and/or thinner head gasket, and/or milling the head) but the 2 effects do not always "cancel each other out". You can't get something for nothing in return. Even though a higher compression ratio will give you back a higher cylinder pressure reading, the power may still be lower, at least at low to moderate engine speeds (rpms). The problem lies in the fact that a smaller volume of air-fuel mixture from choosing a bigger piston dome is being compressed into a higher compression ratio. Even though the cylinder pressure gauge reading taken during cranking or idling is higher, the total of cylinder pressure multiplied by the actual air:fuel mixture volume captured may still be lower (compared to the original stock or milder cam and lower moderate compression ratio). The gas present in the combustion chamber at top dead center (TDC)of the crankshaft rotation is considered to be non-combustible inert exhaust gas remaining from the previous cycle, and is therefore, not included in the fresh mixture volume for making power. It just takes up space inside the cylinder without combusting again. It cannot make power. At higher engine rpms, cam overlap does cause this residual exhaust gas to be blended in with the fresh intake mixture of air & fuel and made to be partially combustible, but this is not true at low to moderate engine rpms. So, a cam upgrade that is too big (called "overcamming the engine")can't be "cured" completely by raising the compression ratio, but it's still a good idea to reduce the bad effects of this problem. These same conditions generally affects low engine speed (rpm) response & flexibility, knock-resistance, etc. for spark advance settings, axle ratio choices, etc. We will discuss this issue in more detail in a separate Dynamic Compression Ratio Article in the TI.net Performance Section. Another useful thing to consider when upgrading to new cams is to anticipate the effect of higher altitudes on cylinder pressure. People have to estimate how much adjustment to the static compression ratio is needed to compensate for the lower air density (oxygen content) at higher altitude like in Colorado. You may need more displacement and not want to give up the combustion chamber size by choosing a big cam and needing a huge piston dome to compensate for the big cam. You do not want to lower combustion chamber volume too much from a higher static CR piston, as you go higher in elevation. VI. Engine Package Summary rod ratio is calculated by taking the connecting rod length and dividing it by the crankshaft's stroke. A good rod ratio is considered to be 1.65-1.77. Some people say the ideal rod ratio is 1.75-1.77:1. Guess what? the Civic b16a 's rod ratio is 1.75:1 and the CTR b16b is 1.77:1. Co-incidence? I don't think so. Honda has lots of race engineering experience with high revs in it's Superbike and Formula 1 engines...like 18,000+ rpm. They trickle this knowledge and tech down to us plebs or common folk in a street car...cool huh? Unfortunately, the Integras got a low rod ratio: B18B 1.54:1 and B18C 1.59:1...not ideal. So what is the importance of rod ratio anyway? Rod ratio describes internal piston geometry (3 things): 1. piston speed away from TDC and BDC 2. piston dwell time at TDC 3. the amount piston sideloading force against the cylinder wall as the piston travels up and down the swept volume. What does this rod ratio have to do with anything? Engine package: How does one part (ie. the connecting rod) affect the choice of another part (cam, piston, cylinder head port size)? A. REV LIMIT & ROD RATIO The rod ratio can limit how high you can rev, since a low rod ratio places more side loads on the wall as you rev higher and higher. And you thought the valvespring stiffness was the only thing that limited how high the redline can go.... B. CYLINDER HEAD PORT SIZE & ROD RATIO The rod ratio in a naturally aspirated engine affects how well the cylinder is filled and dictates cylinder head port size. The faster the piston pulls away from TDC on the intake stroke means you can get more suck (mixture capture) to fill the cylinder. How fast the piston transitions or flip-flops from squeezing the exhaust gas out at TDC during the exhaust stroke to dropping down and begin filling on the intake stroke (i.e. TDC piston dwell time), affects your cam overlap scavenging effect and cylinder filling: Low rod ratio engines have short piston dwell times at TDC and fast piston speeds away from TDC (or faster piston speed dropping down on the intake stroke, compared to a long rod ratio engine). So a low rod ratio motor generates high air flow velocities for filling through the intake port at low to mid rpm's. These low rod ratio engines prefer bigger cylinder head intake port sizes, compared to a long rod ratio motor like the Civic Si's b16a. C. CAM SPECS - STATIC COMPRESSION & ROD RATIO A low rod ratio, ALL MOTOR, engine prefers camshaft specs with less lobe separation angle, more duration, and more cam overlap, since it has short piston dwell time at TDC and needs help scavenging fresh air & fuel. You can get even more overlap by adjusting the cam gears and adjusting the valve lash. Remember to add more static compression ratio, if you upgrade to a significantly longer duration cam, in N/A integras, since you will lose cylinder pressure up the intake port due to a more radical (later) intake valve closing ABDC. A loss in cylinder pressure reduces the gains in hp. You get less power than you expected for an expensive ($800-1000) hot cam upgrade, if you don't also compensate with a little more compression. D. BOOST & ROD RATIO The importance of rod ratio and engine package all has to do with revving ability, proper intake port sizing, proper cam overlap, and cylinder filling IN ALL MOTOR SETUPS which depend on passive filling using lower pressure in the cylinder compared to the atmospheric pressure (14.7 psi). In turbos, you push in the air to fill the cylinder and so rod ratio plays a very MINOR role in cylinder filling. The importance of rod ratio in a boosted engine relates to how efficiently the inert burnt exhaust gases are removed from the cylinder after combustion. The piston speed away from BDC to push the exhaust gas out is important in a boosted engine and we know piston speed away from BDC is slower, in low rod ratio motor. Remember exhaust gases aren't burnt twice and cannot make power and so if they aren't removed, they just take up space in the cylinder...preventing room for fresh air/fuel from coming in to do their job. Burnt exhaust gases are like unwanted tenants not paying rent: you want to evict them from the cylinder. Short piston dwell time at TDC is less important in a boosted engine because you don't want big cam overlap. More cam overlap here would cause some of the boost to shoot over into the turbo manifold instead of going into the combustion chamber (assuming the boost pressure is higher than the exhaust manifold pressure). You don't need big overlap cams to help filling. In fact, boosted engines prefer big lift but short duration and short overlap. So a high rod ratio actually helps a boosted engine, since the piston speed away from BDC is higher than in a low rod ratio engine (to help evacuate the extra burnt exhaust gases). The importance of piston dwell time at TDC and piston speed away from TDC on cylinder filling and cam selection effects on cylinder filling and pressure are less important, in a boosted application . REMEMBER: in a boosted engine or a N/A engine, don't rev the sh*t out of a low rod ratio engine (all motor or boosted)...the shorter rod ratio still causes the piston to sideload the cylinder wall harder causing more risk of a piston going through a wall, the higher the rpms go. ---------------------- E. ENGINE PACKAGE: COMPARING HONDAS & THE EFFECT OF ROD RATIO AND POWER the LS bottom end has a lower rod ratio than the GSR or ITR bottom end. as the rod ratio on a naturally aspirated (i.e. No Boost) engine becomes lower, low-mid rpm hauling power increases. you see the peak torque shift to a lower and lower rpm, as the rod ratio drops. this is probably why Honda chose a shorter rod ratio with a higher displacement on the integras compared to the civics. As you decrease displacement to 1.6L on a civic , peak torque becomes smaller and you have to rely upon higher rpms to generate horsepower. So the civic gets a longer rod ratio because of where it's powerband will be located (in the higher rpms). The 1.8L , with more displacement, makes more torque and has a lower rpm "hauling" or "pull" powerband. It gets a lower rod ratio. This summary ties together the concept of "engine package" or "engine combination". You cannot buy one part without thinking about how it will affect the other parts of the engine and performance. It also hints at the concept of how a combination of parts can affect powerband location. A low rod ratio motor (eg. b18) likes a certain intake manifold plenum size, intake manifold runner length/diameter, intake port shape and size in headporting, cam spec, header length/diameter, exhaust diameter. A higher rod ratio motor (eg. b16a) likes a different set of parts with different design characteristics. With these different packages, the powerband (which is about 2000-3000 rpm in width if you are a good tuner) can be moved up or down the rpm range to your liking. Where it is placed dictates your tranny gear ratios. A narrower rpm band (
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