Higher Compression motors

[JeremysGN]

Has anyone had any problems runnig higher compression in these motors ~ 9 to 1. I'm taking my stuff to the machine shop this week and I'm gonna try something a little different. Also is the exact standard bore & stroke on these 3.80 & 3.4 or where can you find these specs. THANKS Jeremy

[Blown&Injected]

lots of everything here:
http://www.gnttype.org/

You know you got me thinking about my compression test numbers - time for a new thread. Really wish I had measured the heads and done the work to check CR while it was apart.

[Ormand]

You can run higher compression if you want- but it will limit the amount of boost you can run. It will make the motor more responsive, and make more power "off boost", but it will limit the boost. For max power, lower compression, and more boost is the route, but that makes the engine less responsive, and slightly less efficient. So, you pick what matches your application.

[chevyII]

There two trains of thought 8:1 and 40lbs or 9.0 and 30lbs. Both will be about the same power. I think that turbo engines NEEDING low compression is a myth. Ive seen some run 10:1 on this board and are some of the fastest cars. My engine is 9:1 and I myself wouldnt run anyless. But if pump gas is involved go lower.

[Ormand]

Don't think anyone said that turbo engines NEED low compression. It's just a fact that higher compression allows less boost. And since power is a function of boost, then when you run compression too high, you have limited power. For any combustion chamber/cam/exhaust/etc, there will be a limit- fuel has its limits, too, so you can just keep pumping up the boost, and adding octane. Power is actually a function of the mass flow of air/fuel, but in practical terms, you only have a few choices to increase that flow, and boost is one. Others are rpm and cubic inches. If you want to turn 8,000 rpm, then you can use less boost, more compression. Mm, how much would that cost????

[John Wilde]

One reason to run more compression with the V-6 is to help the exahaust velocity. The V-6 with low compression takes a lot of work to get to launch with a big single 88MM and bigger and larger twins like 70MM and bigger with an automatic transmission. If you have a clutch it does not matter, Duttweiler ran low compression in his Olds and has gone 6.8 @ 204 and it could launch on a pro tree. His clutch and BB Turbos helped the car explode off of the line.
I have see several single turbo buicks that sit at the line so long they could never race heads up. They are basically limited to test and tune numbers.

[wiked87gn]

I have a turbo T with compression around 8 to 1 to 8.5 to 1 something like that, ported stock elbow with ported stock heads and stock atr size downpipe, let me tell you this thing has no lagg whatsoever, i mean you hit the pedal from a dead stop and the tires get roasted in a hurry, nevermind from a roll you hit the gas at 60mph and the boost hits 17psi in less than 1/2 second no lagg at all, i guess the higher compression helps spool turbos no matter what but then again it is true about running pump gas i cant get more than 10psi on pump gas at without knocking but at 17psi feels like 22psi

[Jerryl]

Here is a thread to read.


Raise Compression or Boost?

[bruce]

Many years ago, Bruce Crower, did an article for one of the hotrod magazines, about what CR made the most HP. They used a VW engine, and did nothing but change pistons to drop the CR, and then cranked up the boostwith appropriate jetting), to once and for all answer, what CR made the most HP. He got down to 4.5:1, and was still making more HP. The problem at that stage, was just starting the motor....

Figuring out what CR suits you best is as much a personal item, as anything else. There are lots of people that have their own style of tuning, and can just get some combs to work out best for them....

Personally, I still think there's still alot on the table, code wise, for making a lower compression motor, more responsive, then what's currently *out there*. For awhile the aftermarket ecms continually talked about being easy to program, and while that's a good thing, it takes a fair amount of complexity to really optimise a tune. It's evident in the latest aftermarket offerings they're beginning to see the light about what's really needed. I'd wager, that in the next generation, of aftermarkets, they continue that trend.

Not to mention the converter makes a ton of difference, in how an engine initially spools up the turbo.

My 4.1L wound up at just under 9:1, instead of 7.5:1, and while it was almost like having a big block under the hood with a TA-62, I would perfered, the 7.5, just since there was no need for that much low end grunt.
Again, that's personal preference.

[JeremysGN]

I'm gonna try ~ 9 to 1 on my motor & see what happens. It seems like nearly everyone has made nearly every combination out there work. It seems to me that there is a critical state in which there is not enough fuel to feed the fire ie det. I'm curios how feeding more air through the motor is naturally better if its not as efficient. The way I see it is there are three factors, two of which are completely tuneable (fuel & air) & the third is compression. On the VW motors I wonder what there stats were comparing how much fuel increase there was to boost increase to decrease in compression. Period if more air or more compression you need more fuel, RIGHT?? THANKS GUYS

[Ormand]

I'm curios how feeding more air through the motor is naturally better if its not as efficient

Well, if you start with an engine, with no blower, how much can you improve efficiency by increasing compression? Maybe 10%, depending on the initial compression ratio. Higher compression gives higher thermal efficiency, but once you get to about 8 to 1, or 9 to 1, the gains fall off, real fast. For example, doubling the compression ratio, from 10:1 to 20:1 improves thermal efficiency from 0.60 to 0.695, and you obviously can't go THAT high! On the other hand, by going from zero boost to 15 pounds, you can almost double the mass flow. By going the 30 pounds, it's almost tripled. (IF you have a perfect intercooler) So, there are small gains to be made by increasing the compression, but huge gainst to be made by feeding more air/fuel. 10% vs 100%, or 200%.???