Compresion ratio ?

[JOHNBOY]

I have been running at just under 15:1. I dont care for it. I lost a good bit of bottomend. My next engine is going to be much higher compression. No way I would go under 16:1 after dealling with this engine. Spending the money on big studs and o rings would be worth every penny IMHO.

 

[SmokeShow]

I like his thinking, especially if it's gonna be dedicated race vehicle pretty much. ^^ :thumb:

 

[Gasuout]

I guess its just the true real use purpose ..... full race or both daily driving and race .

Its just a balance and you give on one end or the other .

Then the total boost you plan on running of coarse .

 

[McRat]

I have been running at just under 15:1. I dont care for it. I lost a good bit of bottomend. My next engine is going to be much higher compression. No way I would go under 16:1 after dealling with this engine. Spending the money on big studs and o rings would be worth every penny IMHO.

Normally until you hit limitations, the higher compression the more power. The charge expands faster and burns fuel more completely.

The beauty behind compression is it's "free" horsepower all across the curve, from low RPM to high RPM. But finding out you have too much compression is normally done by building a bonfire using hundred dollar bills...

 

[Gasuout]

But finding out you have too much compression is normally done by building a bonfire using hundred dollar bills...

:thumb: ..... :rofl: cha ching .... cha ching ....

 

[JOHNBOY]

Normally until you hit limitations, the higher compression the more power. The charge expands faster and burns fuel more completely.

The beauty behind compression is it's "free" horsepower all across the curve, from low RPM to high RPM. But finding out you have too much compression is normally done by building a bonfire using hundred dollar bills...

I like fires!

With the cam design I run you blead some CP to keep the flow going at the rpms I run. So it makes the effects of low compression seem worse in the low end.

The issue with high compression is CP. I make great power with the less compression but my powerband is to narrow. I need a wider powerband to be competitve. Another 200hp sure would not hurt either. I am worried about driving the bottomend out of it but I think with careful tuning I can keep it alive. Bogging the engine down below 3000rpm is what will kill it. That is where peak torque and cylinder pressure is. This a pretty big reversal of thought for me. I was all for lower compression and more boost. But in the big single turbo world I am in that is just not cutting it.

Burns hundreds in bonfire is nothing new. The is diesel performance after all!

 

[duramaximizer]

Everyone I know that has run normal compression went lower and is now running higher than stock FWIW. I sure a heck won't go with less than stock if anything I would go up.:thumb:

 

[IdahoRob]

Everyone I know that has run normal compression went lower and is now running higher than stock FWIW. I sure a heck won't go with less than stock if anything I would go up.:thumb:

At 32 lbs of boost, I wouldn't either.

 

[JOHNBOY]

At 32 lbs of boost, I wouldn't either.

Even at 60 I would not go lower. Just tune around big bottomend torque. I can't put into words how much I dislike 14.9 and a big cam. I tried all season to tune some bottomend back in to it but no real luck. To be competitive in single turbo land you need compression IMHO. It is just getting it to hold up. New build (JBCE 2.0) will be half filled block, 14MM head studs, big bore 16.5 pistons, better heads and cam, and more turbo.

 

[TrentNell]

Everyone I know that has run normal compression went lower and is now running higher than stock FWIW. I sure a heck won't go with less than stock if anything I would go up.:thumb:

The problem is if I were to chose wrong and not go low enough, and underestimated the strain that 80 psi boost will cause , i more than likely would not have the money to clean up the mess . This isnt a daily driver this is a strip only vehicle with a tiny bit of street use . By the time i'm done i wont even be able to get it registered .

 

[S Phinney]

The main purpose of lowering the CP is to help pistons last a bit longer. That is the weakest link in our builds I believe.

 

[Brayden]

I hear 15.5 to 1 is the magic # for all around best results .

I hear 19.5:1 is nice too...

 

[JOHNBOY]

The main purpose of lowering the CP is to help pistons last a bit longer. That is the weakest link in our builds I believe.

If your using a Stock piston yes. My present compression ratio was determined by two main things.
1 I need .100 deep valve releifs because of my big cam
2 I wanted to delip the pistons and radius the transition.

With the new Forgings you have lots of options that you did not with a stock casting.

FWIW my cut stock pistons held up great to the beatings I gave them. Many passes between 1600 and 2000 degress at over 800rwhp.

 

[Brayden]

Even at 60 I would not go lower. Just tune around big bottomend torque. I can't put into words how much I dislike 14.9 and a big cam. I tried all season to tune some bottomend back in to it but no real luck. To be competitive in single turbo land you need compression IMHO. It is just getting it to hold up. New build (JBCE 2.0) will be half filled block, 14MM head studs, big bore 16.5 pistons, better heads and cam, and more turbo.

So you've seen the motor on the front page of our site huh?

455 cubes....


I agree 100 percent with the statements above.
Decompressing makes no sense to me with the tuning ability we have.. If you have a fixed timing 12 Valve then yeah you need to lose some compression to keep from killing the bottom end. We have timing control and fuel control everywhere through the RPM range so you may as well control it with the electronics instead of limiting yourself mechanically..

Just my opinion.

We've got a Cummins running over 19:1 and the torque peaked over 1700lb/ft.

http://www.youtube.com/watch?v=e_VQ6qxH5gg

845hp/1707tq ..pulling in Direct with 3.73's and 33" STS's. 2.6 charger and underfueled.

 

[Brayden]

The problem is if I were to chose wrong and not go low enough, and underestimated the strain that 80 psi boost will cause , i more than likely would not have the money to clean up the mess . This isnt a daily driver this is a strip only vehicle with a tiny bit of street use . By the time i'm done i wont even be able to get it registered .

How much boost are you making now?

Say you lose 3 points of compression... You've got to make nearly 45 more psi of boost to make that up.. Assuming I'm thinking correctly .

How long did the motor last?

How long do you want to go before a rebuild?

If a high compression motor lasts me a season and puts 200hp on the next closest truck I'd be OK with putting new bearings, rings etc into it for that.

 

[racinmike77]

Say you lose 3 points of compression... You've got to make nearly 45 more psi of boost to make that up.. Assuming I'm thinking correctly .

You add pretty well :thumb: I totally agree with you and johnboy

 

[whitetrash21]

You add pretty well :thumb: I totally agree with you and johnboy

I'm guessing 14.7:1 atmoshperic to cylinder pressure?? That how you get those numbers??

 

[racinmike77]

I'm guessing 14.7:1 atmoshperic to cylinder pressure?? That how you get those numbers??

Yea, its not exact though. 60psix16=960 and 74x13= 960 difference of 14psi. As boost goes up variation in compression ratio has a smaller effect on total pressure.

Any way you cut it compression is cheap power.

 

[Gasuout]

I hear 19.5:1 is nice too...

Nice ..... compression is free HP .

 

[Stingpuller]

Nice reading

You can BET! my next motor build will be higher after running it lower I don't like it. I guess it's just like a Nitrous gas motor you never give up anything. Jeff