>Yes, for two cars running the same boost but one at 9.5 to 1
>and the other at 12.5 to 1, the 12.5 to 1 compression car is
>going to be faster.
>
>Although, as you described, running higher compression gives
>you a "modest" HP increase. (1 point is 4%) Now modest is
>a reletave term. 4% would be a huge increase for some forms
>of racing (which is why, as you pointed out, that wins
>races) but it could be considered modest when you compare
>the HP difference between a turboed vs. non turboed car,
>especially a "street" car.

But then you not comparing apples and apples. The 4% is exclusive of other mods. If you took our base 100 hp engine, and added headers, air intake, intake manifold, cams etc, and got it to 150 hp, then put the high compression pistons in, then you still get 4% or 6 hp.

>
>My point being, since there is a "total" cylinder pressure
>you can run (which comes from a combination of your
>compression ratio and your boost pressure), if you are
>maxing out your engine internals to this "total cylinder
>pressure," wouldn't it be more advantageous to run a lower
>compression ratio and more boost?

That was the thinking a few years ago. That idea is a hold over from a bygone era. With today's ignitions and fuel management systems you can run a lot closer to the edge. The lower is better idea came in an era of carbs and points, we have much better systems now. Volvo sells a factory 10.5 to 1 turbo engine now.

> That way the total power
>would be the same, since you would be running the same total
>cylinder pressure.

That is not what Boyles Law states. With the higher compression you are starting an explosion in a smaller space. So the intial hit is harder since it is not spread out over a a larger area. The flame travel propogates better.

> You are just doing it in a different
>way, with lower compression and more boost, not more
>compression and less boost.

It is different. But the higher compression works better in practice. In Pro Star Funny Bike classes, which are running turbos, they are running high compression some 15 to 1. Once the first guy went to high compression everybody was left scratching there heads, as they could not keep up. When they went to high compression they caught back up. So in reallity the high compression guys, are beating the lower compression guys.


>
>That seems to be the route some 4G63 owners continue to
>take. I would think it is "safer" and more economical to
>run a lower compression ratio.

It is safer, but it is not faster. If your racing high compression will win, on the street we can probably find a compromise. 10.5 with 8 psi, or 12.5 with 5 psi. The higher compression will yield better fuel economy as well.

> It would be safer because
>you do not risk getting knock as much when you are not at
>the track. You can be safe running lower octane gas and
>save money in the process (lower octance costs less). When
>it's time to race at the track, you just turn that boost up
>to the max level the internals can take and run that race
>gas. The power loss from the lower compression, and the
>lower fuel economy, isn't too significant on the street
>since that drop in power is, again, "modest" with a drop in
>compression ratio.

It all comes down to who is the most modest, the guy willing to let it hang out there for the world to see wins. We learn new stuff everyday, if you don't build on what you know you stay in the same place. If you keep running low compression, you may not learn the next secret.
>
>If you are running 12.5 to 1, even if you turn your boost
>controller to 0 you STILL need high octane. You are going
>to be spending more on higher octane gas and at 12.5 to 1
>you are putting your engine more at risk for knock.

Pull timing out, we have been runing 12.5 pistons on the street for 2 years in a 420A, with a timing retard close to 30K miles now on pump gas, and then we crank the timing and fuel to it at the track.

>
>Just want your opinion on this. I always like a good
>technical discussion

It comes down when you playing the game who get to the other end first, and that is the only rule, then you have to bring your biggest gun out first. Playing it safe is not an opition.

Gary

Visit the Howell Automotive Open House August 20th, 2005. The largest gathering of Chrysler 2.0 engines on the planet.

US Small Business Administration Export Business of the Year for 2003

Visit Howell Automotive at www.howellautomotive.com