Having recently tuned a new high compression build, I have to say I've acquired a newfound appreciation for the effects of SCR. Aaron EnglishRacing mentioned in the past that raising compression does not improve spool, and that might be the case since he has the data to show it, but the response from this new engine running an FP white is better than a stroked motor. The powerband is wider than I ever thought possible on a 38mm restrictor, with power not dying off well into the 7000rpm range, unlike the previous 9:1 stroker that choked at 5500rpm. The engine sounds and behaves like a naturally aspirated engine, and that was the purpose of the build.

The input to this thread from David, Ted B and English is much appreciated, but I am still undecided on what compression I would use for a stroked street car running a stock frame turbo running pump gas and meth injection. There is a trend by OEM's to go for small turbo + high compression in the last few years (e.g. VW's TFSI and 1.8T), resulting in some impressively wide powerbands. IMO this is an avenue worth exploring for a fun street 2.3 evo running circa 400whp. For big power with a big turbo I have to agree with Ted B that boost is the power maker, and your fuel of choice dictates your choice of SCR.

I use an Exedy Twin Disc HD clutch with a Sheptrans Stage 2 Transmissiona and Transfer Case.

I really am thinking that the 9.5:1 is the way for me to go.

I love my 10.1 but it's weird, when I do a compression test it only reads 180 across the board. I have Jun 272's but didn't think they would cause it to read that low.

I think I'm going to take my chances and try 11.5 to 12.1 on my next build or rebuild. The worst that could happen is I will have to pull out timing. On E100 I should be okay though, guess we'll see.

Whats wrong with runnning a quartermaster or tilton in the mountain?S

That's perfectly normal for the JUNs and/or any cam set with long valve timing.


Neither give enough inertia to pull the skin off a grape from idle, which becomes a real PITA when trying to move the car from a stop when pointed uphill. Been there, done that, never again. Just FYI.

Very true, but once your car is moving up the hill, the response is quite nice ....

Not as nice as what I have now. If one doesn't like the idea of being stalled at an uphill stop with traffic going around him, than he should avoid these tiny clutches like the plague. Again, just FYI.

Back on topic please ...

Okay but is that compression lost due to the valves hanging open, or is it still 10.1 that reads like 8.5:1 only at slow engine rotation?

So this should allow me to get away with slightly higher compression pistons would it not.... I'm new to this, bare with me. Also can you make your answer very detailed...

Thanks..

The engine only cranks at 450 RPM. large duration cams hold valve open long enough to see a significant drop in air velocity at low RPMs. Thus less gets sucked in a low RPM. 450 is definitely low RPM. Put a small cam in car and watch cranking compression jump to 210 or so.

At high RPMs that long duration now allows more to enter chamber. This making more power at high RPMs. I have explained this to you at least three times. Does it help to see in in writing?

The reason builders say high compression and large duration cams work hand in hand is because large duration cams give up large amounts of low end torque. you can get back some of that lost torque with the higher compression. second reason is increases in compression increase likely hood of detonation. detonation is most likley to occur at low piston speeds. so an decrease of air velocity (large duration) becomes welcome

Jerry what's wrong with you, no need to get your panties in a bunch. I value what you say above most but you forget that some people value many opinions, not just one and run with it. I like to hear what all the evom wise men have to say as well as share this knowledge with others interested in this thread.


See now thats all you had to say.... Thanks for another great contribution to this thread

So is there a formula for matching static compression to cam duration? Or is it always just a guessing game?

That isn't something that can be worked out with a calculator. There are just too many variables. Good engine modeling software is needed to make accurate predictions.

I was so hoping for a different answer...... lol

Trial and error eh

Pot meet kettle.................you sound like what you accuse me of in that post. Just so you know.

FWIW, compression is the last item I would choose when building a motor. Its effect just isn't THAT great and other items can easily dominate over it. Look at the fuel and the turbo first. Everything else should be built around them to reach your goal.

The relation of 4%/point of compression is probably excessive. From a strictly theoretical sense, based on otto cycle efficiency, going from 8.8:1 to 10:1 is worth about 2% efficiency.
12:1 ~= 4% over 8.8:1
14:1 ~= 6% over 8.8:1

If you want it to make power, you better make sure that you don’t have to drop absolute boost levels lower than the percentage gained by compression. Just as a rough estimate, say 70 psia (roughly 55 psi of boost) is the limit on 8.8:1 on your setup. You better be able to run ~68.5 psia (53.5 psi gauge) on 10:1, 67 psia (52 psi gauge) on 12:1, and ~66 psia (51 gauge) on 14:1 if you don’t want to loose power. As you can see, boost pressure is a lot more effective at making power.

Now if you look at the flip side, let’s say the turbo can only produce 35 psi and you can run e98 on it. Now you pick the compression based on the turbo and fuel. You can probably get away with 14:1 (not to be taken literal) compression on this setup over 8.8:1 since it doesn’t limit the engines boost capabilities.

First hand experience though, 91 octane and 10:1 will limit you to about 18-20 psi. 30 psi in the middle of the summer on e70 is pretty tricky too. I've seen e70 at 40+ psi on 9:1 though.