Team Integra Forums banner
1 - 20 of 29 Posts

· Registered
Joined
·
21,139 Posts
Discussion Starter · #1 ·
Dynamic Compression Ratio Calculator (click for the hyperlink)

relationship between cam spec and CR combination calculated!!

Use this to determine your cranking pressures using stock CR with stock cams and then add aftermarket cams and vary the CR to find the best cranking pressure.

No more excuses. You shouldn't be overcamming your engine package when you upgrade your cams now.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #2 ·
amazingly when I plugged in my numbers for the Toda Spec B intake cam ABDC degree valve closing degree spec (50 degrees), 12:1 static CR , and bore/stroke for the B18C1, my dynamic CR was only 10.4:1...assuming that the induction system and exhaust scavenging generates zero boost.

Using the stock GSR static CR and Toda B cams, the Dynamic CR was only 8.7:1!!!!

no wonder long duration cams lose power at stock static CR..the actual compression is worst than an B18B with stock cams!

In actuality, the Hytech header is said to generate as much as 2 psi boost...acoustic supercharging..which would raise my dynamic CR to 11.9:1 at 600 ft altitude here in Toronto.

However, this is a good teaching tool....using a systems approach.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #5 ·
NathanJ on June/19/02 said:
how exactly do you figure the ABDC degree from the specs?
usually if you have the (zero lash) cam duration and the intake valve degree opening angle BTDC, you can calculate ABDC intake valve closing by

ABDC = Intake Cam Duration - Intake Valve Opening Crank Angle BTDC - 180.

Usually the cam supplier will give you the spec sheet for intake & exhaust valve opening degrees.

For the stock B18C cams: Intake Valve ABDC degrees are:

ITR: 45

GSR: 40

Someone with a Helms manual for the B18B, please list the intake valve ABDC degrees closing value.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #6 ·
SurferX on June/20/02 said:
Is the real number to be concerned with the dynamic compression number? Say if I am planning to upgrade my pistons before cams, would that put me into more dangerous levels of compression because of a milder cam setup?
There are practical limits like whether you have sufficient octane gas to support very high compressions, fuel delivery, and of course ECU tuning on a wideband.

for the most part, you want a dynamic CR above stock dynamic CR for an all motor Honda B18.

Dynamic CR's

GSR at 600 ft 9.1:1

ITR @ 600 ft: 9.46:1

ITR @ 600 ft with 11.3:1 static CR (JDM ITR pistons and Mugen head gasket) - 10.1:1

ITR @ 600 ft with stock static CR and Toda B cams - 9.2:1

ITR @ 600 ft with 11.3:1 static CR with Toda B cams - 9.8:1

ITR @ 600 ft with 12:1 to 12.3:1 static CR and Toda B cams - 10.4:1 to 10.7:1 DR

Here's the insights from this exercise:

No wonder you don't gain as much power as you expect in an ITR with Toda B's. The DR is below stock DR!

Notice that there is no co-incidence as to why Toda only sells 12-12.5:1 CR pistons. They had their long duration cams and engine package in mind. The DR is well above 10:1 even with the late ABDC intake closing spec.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #7 ·
The shorter duration cams need less of a static CR increase to compensate for the cylinder pressure or cranking pressure loss with a longer duration intake cam spec. It's less crucial to get more than 11:1 static CR.

But if you go for a Jun type 3 or Toda B cam, they are begging for 11.6:1 static CR minimum to get above 10:1 DR and prefer 12:1 if you can tune for it.

So, if you are shopping for cams and they equal or exceed 50 degrees ABDC intake valve closing, then the minimum static CR pistons you want to order are 11.6:1 using a stock 3 layer head gasket (no cam retarding issues from thinner gaskets).
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #23 ·
Acuranerd on Jul/01/02 said:
See, this is what I love. I know a "certain" hatchback that has a 21.53:1 Dynamic CR........oN PUMP GAS!!!

The problem is that there are other factors that contribute to the dynamic compression ratio of an engine under boost, that arent accounted for in this calculator. i.e. head flow, turbo flow, and cam overlap.
head flow and turbo flow contribute to boost which you can plug into the calculator.

how does cam overlap affect dynamic CR? the factor that affects dynamic CR as I understand it is: as the piston is at the start of squeezing the air/fuel mix at BDC on the compression stroke and the intake valve has started to close, the intake valve closing point after BDC dictates dynamic CR, once the piston is on it's way up and squeezing the mix. This is where you lose cylinder pressure and dynamic cranking pressure up the intake port . Cam overlap affects After TDC filling and scavenging not dynamic CR. which involves the 180 degrees before BDC. Can you explain that one please?
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #25 ·
I see. Are you sure that the boost entry they used in that calculator was manifold pressure and not actual boost? I can ask the website to confirm that. thanks for the clarification.

Intake valve closure ABDC is also an estimate that reflects cam overlap. I thought the calculator entry of intake ABDC closing angle would take cam overlap into account.

here's another one ...just for clarity, understanding, and not to be pedantic (I'm not trying to insult your intelligence here. So bare with me):

on a 4 stroke, we agree that cam overlap occurs starting at almost the end of the exhaust stroke and finishes around the beginning of the compression stroke. Is dynamic compression not measured during the compression stroke and exhaust stroke, as the intake valve is just closing or just opening respectively? So intake ABDC closing angle indicates how far into cam overlap the intake valve will go. once the piston has almost reached TDC on the compression or exhaust stroke, do we continue to measure dynamic compression? Does ABDC angle entered into the calculator not reflect cam overlap?

Just clearing that one up too....

For novices: here is a flash video of a 4 stroke engine at work. It is not accurate since it does not show cam overlap when in reality, there is a time when both the intake and exhaust valves are simultaneously partially open...not the discreet separate opening and closing for each valve that you see on the flash player.

http://www.howstuffworks.com/engine.htm

compare that to a 2 stroke engine...

http://www.howstuffworks.com/two-stroke2.htm
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #27 ·
but compression during the exhaust stroke has no meaning!! it's the compression stroke and the positions of the piston at TDC vs BDC while the mix is being squeezed before and during the combustion process (started by the spark) that we are interested in.

dynamic CR relates to how cylinder pressure or dynamic cranking pressure affects the combustion process (compression stroke NOT exhaust stroke). CR affects thermodynamic efficiency.

your reasoning is incorrect and misapplied unfortunately.

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=233
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #29 ·
Acuranerd on Jul/08/02 said:
DCR is your way to put limit's on boost pressure. It really doesn't matter. That calculator is based on the Static CR and manifold pressure. Just stick to your N/A Applications and I'll stick to turbochargers. I think we'll get along better.
I put no limits on boost pressure. The engine will.

you're right, it doesn't matter.

the calculator uses intake cam opening time, displacement, static CR, and boost to calculate dynamic CR (DCR). The point of this thread is that cam spec can affect burn rate via affecting DCR. The longer the intake cam duration spec, the more DCR you lose: That's the point of the calculator...not boost...which seems to be your fixation.

If boost (in the cylinder not the manifold) did not affect dynamic CR (in the cylinder) , then it would not have been incorporated in the calculator. I'm not sure if you understood the purpose of the calculator. It's to relate cam spec to dynamic cranking pressure and how dynamic cranking pressure affects power (through burn rate or thermodynamic efficiency). This is true whether it is FI or N/A.

The lesson from the calculator is that you must factor in the cam's effect on dynamic CR or you will lose power. It's not just about getting the longest possible duration cam spec to make power N/A or the shortest possible duration cam spec to make power FI. Again, it re-emphasises the idea that a single part by itself (like a cam) is not going to tell you whether that part will make power on it's own because it affects the function of other parts (cylinder pressure in this case). Parts don't work in isolation. They "talk" to each other. You don't just buy a wild cam because of the nasty spec for N/A or whether the overlap is shortened for FI to limit boost loss into the turbo exhaust manifold.

Also, it's not a boost calculator.

Dynamic CR is a standard definition. Not something you or I can make up as we go along. Everyone defines it the same way.When it comes to turbos or N/A , dynamic CR still means the same. You don't use dynamic CR during the exhaust stroke. So I don't know where you got that impression. Any type of CR affects air fuel mix burn rate whether it was force fed or passively fed...

You're the first person I know who says that you can run 20 lb boost on a stock bottom end. The issue isn't the boost. The issue is the heat and ability to withstand detonation should it occur. BTW the Nuformz blockguard only protects the top of the sleeve. The top of the block is not where the sleeve fails with detonation. If you can tune 20 psi properly (WHICH IS THE MAIN POINT) and have the right combustion chamber shape, then those reduce the detonation risk. I guess the question is how long you will be running.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #31 ·
I don't think we are talking about the same thing.

A race motor can be setup to take a certain level of boost. But for how many runs? I certainly would not recommend 20 psi on a daily driver to any friend I know. It's one thing to run 20 psi boost for a race weekend motor or even an occasional street racer. It's another thing to advise it for someone who uses the car as their daily grinder/commuter as well. Larry runs 18 psi boost on his ITR that occasionally can generate up to 21 psi boost in the upper rpms. I don't think he would advocate blasting it down Texas Motor Speedway for 100 laps at full boost.

I don't think I've ever said anywhere on any of my articles that an LS can't rev to 8000 rpm. But let's see you take her up to 9000 rpm on a regular basis. Rod ratios don't make any difference? I don't see too many 1.54 rod ratio LS owners revving to 9k daily driven, day in , day out. Even a mild increase from 1.54 in the LS to 1.58 in a GSR/ITR makes all the difference in the world. They can rev to 9k in street form quite safely as long as the valvesprings were also upgraded.

I can tell you that you have to be careful about making cavalier blanket statements across the board like the way you do. Tell all the Toda cam owners out there that DCR makes no difference in their stock static CR motor.You'll find a lot of disappointed owners out there wondering why their $1000 cams can't beat a Bseries motor with just ITR cams.

Quoting peak dyno numbers shouldn't impress anyone here. Anyone worth their salt in racing knows that a motor's peak hp number alone won't tell you who's the fastest car out there. Making 768 whp and spinning your wheels and hopping at the line is literally just blowing smoke. How sad would it be if a lower hp car beat that 768 peak whp at the 1/4 mile because he/she had a better hook-up and transients? But I'm sure you already know that if you ran a 9.4-9.97 pass.

What I'm a little curious about is why you haven't answered the simple questions I posed to you in the page before in this thread? Your response has been more about being defensive than about sharing info.

It's one thing to say that I don't no a thing about boost and quote impressive numbers from your shop. It's another thing to pass along useful info people can use reliably. If you don't explain how and why, then that means you don't understand why you do certain manoevers or modifications. If you can't explain it to someone else, it means you don't really understand what is happening when you do what you do. I for one refused a long time ago to blindly do what someone tells me to, without telling me why and how.....and that in a nutshell, is what this forum is about. Not a place for someone to beat their fists against their chest and brag about how great they are.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #32 ·
Acuranerd on Jul/08/02 said:
DCR is your way to put limit's on boost pressure. It really doesn't matter. That calculator is based on the Static CR and manifold pressure. Just stick to your N/A Applications and I'll stick to turbochargers. I think we'll get along better.

It's easy to use math to calculat things, but until you actually try it, the numbers mean nothing. That's why we can run 20 lbs. of boost on a stock LS motor and not blow anything up.
I applaud your efforts. I just want to see some reliability numbers to go along with the big peak hp numbers and big boost numbers. Saying "go ahead and try it because we did" is doing people a disservice without showing how you prevented a meltdown.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #35 ·
Acuranerd on Jul/09/02 said:
It's kind of a shop secret. If you found a way to do something nobody else knew how to do, I don't think you would run around telling everybody. As for reliability numbers, the car has been running 20 lbs. of boost, everyday, for the last 8 months, and not a single problem. The Hondata also hasn't been readjusted with the changing of the weather, and here in Houston, the weather goes from about 50 in the Winter, to almost 100 in the Summer, so I think were doing pretty well.
I would like to help everybody out by telling you guys what exactly has been done to the engines, but I've agreed not to. I'm just trying to get people to think on their own and not to listen to what most people will tell you, especially the magazines.

I know virtually nothing about setting up a car for a NA motor. So I couldn't tell you anything about the Toda cams, other than I wouldn't run them in a boosted motor.

Oh, BTW, the hatch I've told you about will be in an upcomming issue of SCC, you should check it out. I don't know if you get that up their, though.

I can post the dyno for the hatch if you like, but I don't have any webspace to host it, so I would need somebody to host it for me.
how convenient that everything is a secret...there are some basics you can discuss without selling the farm...it would lend some credibility to some of your comments...

oh well, I tried guys...I did ask again.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #38 ·
Quetzolcotl

you'll want to call or email crower / crane and get the intake valve closing spec in degrees ABDC for those cams since it is needed for the dynamic cr calculator. the specs you have listed here are useless for the calculator.

if you want me to explain it again, then you may want to check out the article.
 

· Registered
Joined
·
21,139 Posts
Discussion Starter · #39 ·
Acuranerd on Jul/06/02 said:
Here is my reasoning. In short, the reason why overlap affects the DCR is because while you are on the compression stroke, the exhaust valve is still open, therefore you are losing some of your compression out of the exhaust. That is why FI cars use short overlap cams.

As you can all see this was a discussion I had with Acuranerd in the summer about Dynamic CR (DCR) .

Just to make sure everyone is on the same page, we were clarifying whether the exhaust side AT CAM OVERLAP affects DCR. THIS WAS A RED HERRING. I'll tell you why later on in this post. But to make sure you understand what he was trying to get at:

from the "STATIC CR - INTAKE CAM DURATIoN RELATIoNSHIP" article (please read the fine print below. the 3 underlined sections are entitled "Immediately Prior to Overlap, Valve Overlap Period Begins, Valve Overlap Period Ends" ...a little hard to read that if you don't squint). P represents the pressure in each section indicated.




at the start of cam overlap at very low valve lifts,
P3 (chamber) > P2 (exhaust manifold [EM] ) > P1 (intake port) . "Reversion" back up the intake port can occur here.

then, P1 (intake) rises to a high enough level to overcome P3 (chamber) and fills the cylinder.

if the exhaust valve is not closed at this point where P1 overcomes P3, then we get what is called "Overscavenging" and part (not all) of the intake charge goes into the header or turbo exhaust manifold, since P1 > P2 (exhaust manifold) as well.

Acuranerd was saying that DCR is also affected by the exhaust side as well since you can get the loss of cylinder pressure out the EM at overlap from overscavenging.



THE PROBLEM

The problem was in the next series of posts he goes on to relate DCR to the exhaust stroke, not the compression stroke.


PURPOSE OF THIS THREAD

The DCR Calculator that I linked to in this thread was to help you work out which cam you should get with your current static CR, since you don't want to get a cam that is too big (Overcamming the engine).

For those of you looking to make the big move to adding very aggressive aftermarket cams, the DCR Calculator tells you how much change in static CR you need to make, in order to MAINTAIN and NOT LOSE cylinder pressure below the point that you see in the stock setup.

The DCR Calculator relates to bleeding off of the cylinder pressure or cranking pressure DURING THE COMPRESSIoN STROKE. Why is this an important distinction?

The compression stroke is what builds up the baseline cranking pressure. The spark ignition of that baseline cranking pressure adds pressure on top of that to give you the huge turning force on the crank. This force that turns the crank is what generates the power (on the power stroke).

If you lose DCR (i.e. up the intake port) during this critical moment (i.e. compression and power strokes), it affects power output (i.e. less cranking pressure).


THE RED HERRING


Acuranerd diverted the attention to this critical moment and started talking about DCR and cam overlap (i.e. exhaust stroke followed by the intake stroke).

It's a red herring to divert your attention and making you look away from the key time that DCR makes a difference.

It's like focusing on the penny you just dropped on the sidewalk when a tornado is coming right at you.

Compression Ratios relate to COMPRESSIoN and the COMPRESSIoN STROKE. Whether they are static or dynamic CR. CR's relate to COMBUSTIoN or BURN. They affect the size of the burn or explosion and whether the burn is complete or not. A complete burn means there is no unburnt air-fuel mix leftover after the combustion process is finished. If you have unburnt air-fuel mix leftover, you haven't made use of any improved cylinder filling (volumetric efficiency) and you increase your emissions.


Compression Ratios don't relate to exhaust gas removal. This is what he was diverting your attention to erroneously. The hook to get your attention was the outrageous numbers he stated at the beginning (20 psi boost on a stock block and 24:1 DCR). The hook is like that penny.

This thread was about why you should use the DCR Calculator and how you should use or apply the DCR Calculator results.

Focus, focus, focus.

Don't get diverted or lose your attention from the tornado coming at you.
 
1 - 20 of 29 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top