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Discussion Starter · #1 ·
This is a scenario I am using as a question based on Page 2 of Engine Package I: Cams, Rod Ratios, Port Volumes

Lets say we have two motors, Motor A and Motor B. Both of their engine packages are the same(same cam lift, duration, exhaust system, intake system, etc.) except they have different rod ratios. The intake valve closes when the piston hits BDC. They are both, needless to say, naturally aspirated.

Motor A: 150mm rod and 90mm stroke= rod ratio of 1.66:1
Motor B: 150mm rod and 100mm stroke= rod ratio of 1.50:1

Both motors have different rod ratios, but they are both low rod ratios.

Motor A can completely fill the cylinder (maximum volumetric efficiency?) at WOT and when the mean flow velocity has been achieved (240fps-260fps).

Now,can Motor B completely fill the cylinder at WOT and when mean flow velocity has been achieved?

To my best of knowledge I don't think it can unless you change the lift/duration of the cams and/or port volumes (changing the engine package) or changing out to a longer rod to accomodate the longer stroke (changing the rod ratio).

Here are some reasons I think why a cylinder in Motor B won't fill as well as Motor A:

1.) Even though Motor B has a lower rod ratio and faster speeds away from TDC, there is a longer distance to the floor of the combustion chamber from the intake port b/c of the longer stroke. I think this will have an affect on dynamic CR b/c the cylinder stops filling at BDC and the piston has "ran away" from the air/fuel mix, leaving unfilled dead space (BTW, would that be negative pressure?).
2.) The engine package doesn't fit this particular low rod ratio. Even though speeds away from BDC are slower, allowing longer cam duration during this period, I jury-rigged the whole scheme by stopping cylinder filling at BDC.

Did I leave out any important factors? If any of this seems out of line, plz point out any misconceptions....I was at a bar when I wrote this.

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Just some factual details to straighten out first before going to your answer:

First, the intake valve does NOT instantly close at BDC. Even the gsr cams have 40 degrees after BDC before they close. The wilder cams take even longer after BDC to close. Why? because we don't have F1 pneumatic or electromagnetic valve actuaters. We use valvesprings which don't instantaneously open and close a valve (like a square wave pattern...on/off switch). The valve isn't slammed onto the valve seat too hard to cause valve bounce. In wilder cams, higher intake lift dictates the need for longer intake duration. Now, this ABDC valve closure period is where we lose vacuum/cylinder pressure/(dynamic) compression even if the rings are good (dynamic compression is affected by late intake valve closure). The later the intake valve closure is an attempt to capture some more intake mixture using the "ram effect" (see the Ramming the Rat Article from Hot Rod 1999 that I quoted in one of the IM design threads here). However, the mix is being compressed in these 40 or more degrees and the path of least resistance is back up the intake port. We won't get into the balance between the exhaust cam duration relative to the intake cam's duration and LSA today.

Secondly, anything above 1.65 rod ratio is considered good and still "high". 1.66 is NOT low. "Ideal" is 1.75-1.77. You get into powerbands that are located to high and too narrow to be useful once you exceed 2.05:1 and would require a 7 speed, close ratio gearbox to keep you there in that powerband.

Now, onto cylinder filling.

cylinder filling is a function of port volume/flow capacity, efficiency (loss from friction or pumping losses), IM design, intake valve size and the number of intake valves (4-5 valve head vs 2 valve head), cam profile, rpm, bore size relative to rod ratio, and displacement.

Let's deal with some of these factors in sequence.

In terms of port size/flow capacity, displacement, efficiency, rpm, and displacement....In a 1.8L (110 cubic inches) with 8000 rpm redline, you have:

110 cu. in. x 0.5 = 55 cu in. per revolution

[55 cu in/rev]/ 1,728 = 0.032 cu. ft/ rev.

0.032 cu. ft/rev x 8000 rev/ min. = 254 cu. ft./ min. (cfm) flow needed to fill the cylinder.

This is the port flow capacity you need to fill the cylinder at 100% efficiency (no viscous or pumping losses).

Now, port volume as it relates to filling only and rod ratio:

The relationship between rod ratio and port size goes un-noticed by a lot of people. Low rod ratio engines like larger "lazy" ports, moreso than high rod ratio engines. This combination of low rod ratio piston geometry and big ports biases the powerband gains towards the upper rpm range.

If you look at a stock B16a head it has smaller port sizes. When you combine that head made for a high revving high rod ratio motor and put it on a low rod ratio block like a B18B (1.54 r/s ratio), the midrange torque is phenomenal. The LS/VTEC's combination of high piston speeds early after TDC from a low rod ratio and smaller ports for higher flow speeds will fill the cylinder at low rpms better.

We already know that in a 2 stage IM, the resonance frequency increases when the secondary butterfly valves open and this creates a second sonic wave that provides higher pressure arriving at the intake valve as it opens. This 2 stage IM idea assists in cylinder filling but at the cost of added flow drag. Initially the 2nd stage of a 2 stage IM improves upper rpm power but at higher and higher rpms, the added drag from the additional runners begin to cancel out their advantage on filling (poorer flow quality).

We haven't said a word about flow quality yet, up to this point. Filling the cylinder as much as possible (quantity) versus obtaining a stratified charge in the cylinder (quality) are separate but equally important issues. It's not only mixture quantity but also mixture quality for a more complete fast burn that gains power.

Flow quality as the mix enters at the valve seat can be affected by the initial energy of the flow from the time the intake valve opens to about 12-14 degrees afterwards and can be affected by the valvespring rate and intake cam lobe's shape/grind (ramp angle) which dictates the speed at which the intake valve opens. The critical initial speed is what helps completely fill the cylinder. You not only want to fill the cylinder with the volume of mix needed for the level of power wanted but you also want to stratify charge inside the cylinder. We already mentioned valve seat angles and people use different strategies to help fill the cylinder in an article already (enhance low lift flow or don't enhance low lift flow for antireversion).

You do not want any swirling of the mixture to happen in the port above the valve seat. The mixture should approach the short turn radius at speed, slow down to make the bend, enter past the valve seat, and then swirl fill the cylinder. It is critical that the mix is then pushed over by the rising piston, on the compression stroke, over to the exhaust side of the cylinder to regain speed and induce stratification. Stratification in a compact combustion chamber ensures complete burning of the mix...making power.

You'll notice the dome shapes on the pistons for N/A monsters all have deeper valve pockets on the exhaust side compared to the intake side and that there is a slope deep into the piston dome from the intake side down to the exhaust side. The piston is the "floor" of the chamber and this "floor" is rising up, as it compresses the mix. This squeezing action and the shape of the dome compresses AND pushes the mix towards the exhaust side.

The last thing to consider is bore/rod ratio combinations. Honda chose a small bore with the higher rod ratio motors. A shorter stroke with a smaller bore will make torque in the midrange. The D series motors with 1.6L can smoke a B16a 1.6L in the midrange (all other things being equal).


With the basics out of the way, back to your question.

If we have the same small port size, 1.8L displacement, 8,100 rpm redline, IM, cams, valve seat angles, intake valve size & number, bore:

The higher rod ratio engine, the faster the piston speed further away from TDC or "later down the intake stroke" in filling. The lower the rod ratio, the faster the piston speed earlier on the intake stroke or closer to TDC.

A high rod ratio engine has more piston speed earlier going away from BDC. In contrast, a low rod ratio has lower piston speed away from BDC, relatively speaking.

You are correct in thinking that piston speeds can dictate filling. Since rod ratio determines piston speed and where the max piston speed occurs, it is oNE factor among many that will dictate filling.

Here's a formula for exact piston position (inches of stroke) for crank position in degrees:

SE = (S ÷ 2) + R + [(S ÷ 2) × cosA] - SQRT [(R2) - ((S ÷ 2) × sinA)2]

where "SE" is the effective stroke, "S" is the nominal (full) stroke, "R" is the rod length, "A" is the crank- shaft angle in degrees ABDC from 0 to 90°, and "SQRT" (or "SQR") is the square root function. to convert from degrees to radians:

Radians = Degrees × .017453, or Degrees × Pi ÷ 180.

A simpler equation but only for maximum piston speed:

Z = N2 × S (1 + (1 ÷ 2n)) ÷ 2189

where "Z" is piston acceleration in feet per second squared, "N" is engine RPM, "S" is the stroke length in inches, "n" is the rod-to-stroke ratio, and 2189 is a constant. A safe limit for "Z" is about 100,000
ft/s2, although this will cause ring flutter with 1/16" compression rings.

Why did Honda choose a lower rod ratio? Shorter rod allows for a lower hoodline, since the engine fits nice and neat inside a shallower engine bay. They don't use a crazy high redline either. They don't like having recalls for pistons going through cylinder walls.

The low rod ratio motor will fill better with the small ports and a big cam at lower to mid rpms. The low rod ratio motor will not fill well when you shorten the time between one rev to the next. It runs out of steam at high rpms BECAUSE OF THE LIMITATIoN OF THE PORT SIZE AND VALVE SEAT (especially if the valve seat angle was set for enhanced low lift flow).

The high rod ratio motor will fill better with small ports and a big cam at high rpms. It has no inertial or haul in the low to mid rpms to generate the energy for filling the cylinder at low rpms. At high rpms, the higher piston speed at the furthest point away from TDC will help fill the cylinder when the time for the intake stroke is briefer (say from 6500-8000 rpm).

This is why the powerband locations are where they are.

So your view of filling is very static and a snapshot of one space in time. If you look at filling at low, mid, and high rpm, each rod ratio fills more efficiently at different places.


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if you now change the scenario to larger port size, the low rod ratio motor will shift the powerband location to a higher rpm. The high rod ratio motor will have a narrower powerband at the higher rpm. It won't shift downward...just become narrower with less driveability.

The big picture and take home here is again package, powerband location, and filling quantity vs. quality & complete burn. All are important.

You fill but don't burn all of huge power gain.

Many things affect filling and they interact with one another to give you different filling characteristics at different parts of the powerband.

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misael on Aug/03/02 said:
WoW! Everyday I learn more
that's the point of the exercise....

a smarter membership means the discussion becomes a higher level of discussion. It's not just about whether you buy a part or tune a certain way. It's about why you do it and how you do it with intelligence.

we are supposed to be one of the more informative boards for beginners and intermediate enthusiasts....hopefully we will get to a level of attracting high level enthusiasts.

The kicker is that the higher level enthusiasts don't do it the "conventional" way and so, it will throw some beginners off. However, the high level methods are usually evolutionary or derivative from the basics...we then can get into how they arrived at the nonconventional way in our discussions....

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from article by Bill Clemmons said:
Short Rod is slower at BDC range and faster at TDC range.

Long Rod is faster at BDC range and slower at TDC range.


A. Intake Stroke -- will draw harder on cyl head from 90-o ATDC to BDC.

B. Compression Stroke -- Piston travels from BDC to 90-o BTDC faster than short rod. Goes slower from 90-o BTDC to TDC--may change ign timing requirement versus short rod as piston spends more time at top. However; if flame travel were too fast, detonation could occur. Is it possible the long rod could have more cyl pressure at ie. 30-o ATDC but less crankpin force at 70-o ATDC. Does a long rod produce more efficient combustion at high RPM--measure CO, CO2? Find out!!

C. Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20 to 75 ATDC less force is exerted on the crank pin than a shorter rod. However, the piston will be higher in the bore for any given crank angle from 90-o BTDC to 90-o ATDC and thus cylinder pressure could be higher. Long rod will spend less time from 90-o ATDC to BDC--allows less time for exhaust to escape on power stroke and will force more exhaust out from BDC to 90-o BTDC. Could have more pumping loss! Could be if exhaust port is poor, a long rod will help peak power.

D. Exhaust Stroke -- see above.

II. Short Rod

A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.

B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.

C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC.Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.

D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.


A. Rod Length Changes -- Appears a length change of 2-1/2% is necessary to perceive a change was made. For R & D purposes it appears a 5% change should be made. Perhaps any change should be 2 to 3%--ie. Ignition timing, header tube area, pipe length, cam shaft valve event area, cyl head flow change, etc.

B. Short Rod in Power Stroke -- Piston is higher in the bore when Rod-Crank angle is at 90-o even though at any given crank angle the piston is further down. Thus, at any given "time" on the power stroke between a rod to crank pin angle of 10o and ie. 90-o, the short rod will generate a greater force on the crank pin which will be in the 70-o to 75-o ATDC range for most engines we are concerned with.

C. Stroke -- Trend of OEM engine mfgs to go to longer stroke and/or less over square (bore numerically higher than stroke) may be a function of L/R. Being that at slower engine speeds the effect of a short rod on Intake causes few problems. Compression/Power Stroke should produce different emissions than a long rod. Short rod Exhaust Stroke may create more reversion--EGR on a street engine.

D. More exhaust lobe or a earlier exhaust opening may defeat a longer rod. I am saying that a shorter rod allows a earlier exhaust opening. A better exhaust port allows a earlier exhaust opening.

E. Definition of poor exhaust port. Becomes turbulent at lower velocity than a better port. Flow curve will flatten out at a lower lift than a good port. A good exhaust port will tolerate more exhaust lobe and the engine will like it. Presuming the engine has adequate throttle area (so as not to cause more than 1" Hg depression below inlet throttle at peak power); then the better the exhaust port is, the greater the differential between optimum intake lobe duration and exhaust lobe duration will be--ie. exh 10-o or more longer than intake Carbon buildup will be minimal if cyl is dry.


Short Rod -- Min Rod/Stroke Ratio -- 1.60Max Rod/Stroke Ratio -- 1.80

Long Rod -- Min Rod/Stroke Ratio -- 1.81Max Rod/Stroke Ratio -- 2.00

Any ratio's exceeding these boundaries are at this moment labeled "design screw-ups" and not worth considering until valid data supports it.

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Discussion Starter · #9 ·
I just got time to read through this about three hours ago! Can you believe it? I haven't gotten to any of the math on the rod angles and trig, yet, but it is coming!!

A. Long Rod Ratios 1.75 - 2.1 more compatible with:

- Small intake port volume vs. motor size (displacement)

- High engine speed or rpm powerband (higher redline rpm & peak power more important)

B. Short Rod Ratios 1.45 - 1.75 more compatible with:

- Large intake port volume vs. motor size
- Moderate engine speed or rpm powerband (pick-up,launch, midrange rpm, hauling or towing)

I'm, sorry, but I read that and I figured that 1.76:1 and up rod ratios were high and that anything below is considered to be a low rod ratio. I understand now that 1.75-1.77:1 is an ideal rod ratio and that 1.65:1 and above is a high rod ratio.

I've got a question about this:

We haven't said a word about flow quality yet, up to this point. Filling the cylinder as much as possible (quantity) versus obtaining a stratified charge in the cylinder (quality) are separate but equally important issues. It's not only mixture quantity but also mixture quality for a more complete fast burn that gains power.

Is this a tradeoff or compromise? Could too much fuel destroy the flow quality? I understand that when increasing displacement and when increasing airflow by a significant amount that added fuel will be necessary.

Other than that nit picky crap, I'm as sound as a pound!

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Older combustion chambers like the Chrysler hemi had high surface area to volume ratios which make them inefficient combustionwise since they lose too much heat from the added surface area. The burn was incomplete.

Modern fuel efficient combustion chambers like the pentroof chambers of Hondas are more COMPACT and therefore more efficient. And yet they make the same amount of power as the "bigger and more is better" OLD way of thinking. The OPEC oil embargo of the 70's drove the world to learn how to make the same amount of torque with less fuel. The result is the modern compact chamber designs and lean fast burn theory.

Use the web browser search engine and enter the words "lean burn theory combustion engine" and you should get a ton of info on this.

So there is no tradeoff...we've learned that as long as you stuff in enough air (and you don't have to stuff a lot) the key is to stratify and burn more efficiently/completely.

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Discussion Starter · #11 ·
Yes, I've been thinking about doing a review on older hotrod engine packages vs. modern engine packages. Why? To assist some of us in the argument of old vs new. I know a lot of old guys that tell me BS like, "You should sell that japcrap" or "Put a hemi in it". The Hemi is a strong motor, but certainly not perfect. I don't like it when I don't get any credibility. Credibility around here comes with age and how big your motor is.

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the hemi is from the bigger and more is better era when we had lotsa fossil fuels and were just starting to add car pollution to the environment.

those days and that way of thinking are long gone. apply them now and you will be parked at the side of the road having run out of gas and not making any greater power advantage and spewing more hydrocarbons and CO in the air.

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Discussion Starter · #13 ·
Tell that to this 300lb guy I know....I'm at 140lb's or so...remember, I live in Louisiana, home of drive-thru daquiries. Some people here are pretty thick headed. "Bigger is better" still reigns down here.
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