Increasing the C/R?

[jcc911]

Couple of questions.
1st what does increasing the C/R of an engine by .5 or 1 relate to in increase in HP?

2nd Could I put high comp/ratio pistons in my 3.0 in order to get more HP?

3rd At what c/r does one have to consider much higher octane levels in order to safely run higher C/Rs? 10.5:1 or higher?

Note my 3.0L heads are already ported for dual plug, but is currently not being used. I was thinking about putting some high c/r pistons in using the stock cylinders for more HP.
Is this doable or waste of time?

Thanks. J.

[emcon5]

Originally Posted by jcc911

Couple of questions.
1st what does increasing the C/R of an engine by .5 or 1 relate to in increase in HP?

It depends on where you start. A 1 point increase will do more for you if your starting compression is 9:1 than 10:1.

Some info here:
http://www.popularhotrodding.com/tech/0311_phr_compression_ratio_tech/index.html

Your SC should be 8.5:1, so a 1 point bump to 9.5 should get you a ~3% increase according to the Vizard article I just linked.

Frankly, a larger advantage of increasing compression is to try and offset some of the dynamic compression loss of running a hotter cam.

For example, I ran the numbers a while back for my race car, a stock 1.8 VW GTI:

Stock static compression ratio: 8.5:1
Dynamic compression ratio with the stock cam 7.42:1
Dynamic compression ratio with a Elgin 308º cam 6.18:1

To get the dynamic compression back to the stock level with that cam, you would need to start at ~10.5:1.

2nd Could I put high comp/ratio pistons in my 3.0 in order to get more HP?

Sure.

3rd At what c/r does one have to consider much higher octane levels in order to safely run higher C/Rs? 10.5:1 or higher?

Steve Wiener says for air cooled 911 is 9.8:1 is marginal for pump gas one one plug.

Note my 3.0L heads are already ported for dual plug, but is currently not being used. I was thinking about putting some high c/r pistons in using the stock cylinders for more HP. Is this doable or waste of time?

It is doable.

[emcon5]

Damn thing posted before I was done with it.

Note my 3.0L heads are already ported for dual plug, but is currently not being used. I was thinking about putting some high c/r pistons in using the stock cylinders for more HP. Is this doable or waste of time?

It is doable. Are you sticking with CIS? Do you have to pass any type of emissions tests?

Another option would be to get new 98mm cylinders to go along with new higher compression pistons to make a short stroke 3.2 liter engine. You can also get the cam reground while you have the engine apart and go to a hotter profile, 964 is a fair improvement if you have to pass smog, and will work fine with CIS.

Here is a pretty extreme example of how far you can go with CIS:
http://forums.pelicanparts.com/showthread.php?t=166093

Tom

[Eagledriver]

Those "dynamic" compression numbers are very misleading. The actual effective compression ratio depends on the volumetric efficiency of the motor including the intake and exhaust system. This varies wildly with things like port size, RPM, intake tuning, exhaust tuning, and cam shaft timing. An F1 race engine at 15000 RPM has a very high VE. I suspect that if you measured the "dynamic" compression ratio based on the valve timing at that RPM you'd get a very low number. The tendency of an engine to knock is greatest at the max torque RPM, this is the same place that the engine has it's greatest VE. There is very little you can tell about the allowable compression ratio just by the timing of the valves. Static compression ratio gives you much better data because it assumes a VE of 1.0 which most high performance engines can do.

-Andy

[emcon5]

I used the dynamic CR calculator here:

http://www.empirenet.com/pkelley2/DynamicCR.html

Static compression ratio uses the full swept area of the piston, BDC to TDC. Dynamic CR in the context I am referring to uses the swept area of the piston after the intake valve has closed.

I used true numbers for the Elgin race cam, I had to make some educated guesses about the stock GTI cam since the numbers are not published by VW.

I also said nothing about that figure being safe, just that in that example engine, to get the dynamic CR back to the stock level with a 308º cam, I needed to increase the static compression about 2 full points.

[Eagledriver]

In the web page you reference the author states that "compression cannot begin until the intake valve is closed". This is incorrect and the source of the problem with using DCR. The compression begins when the pressure in the cylinder goes above ambient outside pressure. This may occur much earlier than the intake valve closing. The reason we use cams that don't close at BDC is because the cylinder is still filling due to the velocity of the air flowing in. This air flowing in starts the compression process. If an engine is running at a VE of 1 then it is getting a full volume of the cylinder worth of air. In that case the pressure at TDC will be the same as the static compression ratio would predict. The web page that the author references has a passing mention of cylinder pressure (this is the actual useful number in deciding what compression ratio you should use). You are completly correct in how DCR is calculated but my point is that it is a meaningless number.

-Andy