[MichaelDelaney]

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.

 

[MichaelDelaney]

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.

 

[NathanJ]

how exactly do you figure the ABDC degree from the specs?

 

[SurferX]

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?

 

[MichaelDelaney]

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.

 

[MichaelDelaney]

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.

 

[MichaelDelaney]

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).

 

[MichaelDelaney]

you sell yourself short if you go for 11.4:1 with Toda B's or C's or Jun 3's.

 

[SurferX]

Interesting specs, I don't have the money to drop into a complete internals package right away so that's why I was wondering. I think there is no point in getting big cams first thing and wasting time/money to tune them on stock CR when they will have to be re-tuned once the new pistons go in. I was thinking pistons would be the obvious first choice if I had to do things one at a time but was concerned with leaving the stock cams if dynamic compression would be over the top. Looks like I will have to think about things a little differently with more information and that's always a good thing.

on a side note do you have a link to a good place that explains how the exhaust system can create an acoustic supercharging effect? I'd really love to learn about that.

 

[SurferX]

By the way Mike nowhere in my Helms does it say the intake valve duration values. Where did you find that info?

 

[MichaelDelaney]

By the way Mike nowhere in my Helms does it say the intake valve duration values. Where did you find that info?

you can try Frank Lin's website which has a section on b series specs...

http://www.g-speed.com/pbh/

 

[IntegraBad18c]

amazingly when I plugged in my numbers for the Toda Spec B intake cam ABDC degree valve closing degree, 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.

I'm curious, you have longer rods, no? If so, did you throw that into the calculation? Also, boost from the scavenging effect is important, so is there any way we could throw that into the equation? Any way we could measure this?

 

[IntegraBad18c]

Oh, what a kick in the pants. It fails to amaze me as to how many bike guys school car people on a daily basis...

 

[MichaelDelaney]

longer rods would require the pin location to be changed or added deck height...neither of which appeal to me...

yes the bike guys are the source of my schooling...

 

[NathanJ]

you sell yourself short if you go for 11.4:1 with Toda B's or C's or Jun 3's.

So how does that work with the skunk2 stage 2 (which sound alot like toda b's) and a 84mm overbore? should I still go with the 11.5 compression or would I need to go higher?

edit:
Nevermind I pluged in the specs that I found for the skunk2's and got this;

Your engine summary is as follows: Bore 3.307 inches, stroke 3.43 inches, rod c-c length 5.429 inches, with a static compression ratio of 11.5 :1. Your camshaft specifications call for an inlet valve closing of 49 degrees ABDC (after bottom dead center).

Your chamber volume is 45.98 cc's. With this camshaft your dynamic, or effective stroke is 3.00 inches. Your dynamic compression ratio is 10.06 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 213.28 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 0 PSI is 10.06 :1.

Knowledge is power.

 

[MichaelDelaney]

I'm glad you worked it out yourself without any hand holding...cheers

 

[RStoR]

good stuff tuan.

 

[NathanJ]

also o/t, but I found it on the calculator page.

http://www.rbracing-rsr.com/secrets.htmSecrets[/url] ]


1. Study Sir Harry Ricardo's work, The High Speed Internal Combustion Engine (multiple editions) and you will find it's unlikely you will do anything that's new, and if you don't read it you will probably do everything wrong.

2. Study Charles Fayette Taylor's two volume The Internal Combustion Engine in Theory and Practice (M.I.T. Press) and realize for a second time you don't know what you are doing.

3. Read anything that Smokey Yunick wrote...Then try to figure out what he did that he didn't write about. Smokey died May 2001 from complications related to Lukemia...he left us a three volume set of books covering his life, racing and inventions.

4. When Honda spends $500,000,000.00 on a racing engine try to pay attention to what they do.

Don't ask them, they won't tell you anyway.

5. Most experts aren't.

6. Research is the difference between speculation and investment.

7. Just because someone goes fast doesn't mean you should do whatever you think they did.

This path assumes they knew what they were doing, which isn't necessarily the case. Take this path and you will be guaranteed second place.

8. Test Test Test.

9. A new engine never makes too much power.

10. The salt bears will win everytime

I don't know what the hell a salt bear is, or why they always win, but I hate them anyway

 

[MichaelDelaney]

I can tell you that site is one of the best ones I've surfed and found.

The fact is motorcycle racers can teach us import enthusiasts a lot....

 

[Acuranerd]

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.

Even so, it's fun to play with.