Compression Ratio Questions...

[smh0101]

I was considering going with higher compression for more low-end power... Like 9.5:1

BUT I'm not sure if that is going to be too high for boost around 16-20lbs... Without Knocking. Would it knock?

What do you guys think?
It seems a lot of people lower their compression.

[555BCTurbo]

9.5:1 with that much boost would probably not be so piston friendly...unless you run Aclohol

[greg donovan]

9.5:1 with that much boost would probably not be so piston friendly...unless you run Aclohol

alcohol or water injection would be the way to go on a high compression/high boost motor i would think.

[smh0101]

Ah... Gotcha...

Didnt think about that...

Not sure if I'm wanting to use alcohol/water injection... Just because it complicates the system...

[PhyrraM]

Personally, I think that with a 2.35 you'll be happy with the low end even with an 8.0 compression ratio. Also, once its running at 15 PSI, your gonna want 20. Then 25, then 30. You know it's true. Hard to change the compression ratio after the engine is built, but easy to up the boost.

Also, keep in mind that so far your talking about using the stock phase 1 EJ25 rods. From the reading I've done these are suspect as to how much they can handle. If I was going to run these rods, and I might as I've got a few 22t blocks around, I think a higher compression ratio with 12-15 psi boost would make a killer combo for street use. Of course, I'm old enough to know I won't get greedy (in a street car).

[BXSS]

I agree with the above.

[93forestpearl]

I vote against higher compression in a decent to agressively boosted motor. All the higher compression does for you is give you a little more pep off boost. On the other hand, it limits what you can do on the high end. You don't want to have to run a ton of fuel or retard the timing a bunch to fight knock. You'll end up with more power and a happier motor in the long run if you stay with boost friendly compression.

There is a reason factory turbo motors use lower compression. As far as I know, the highest factory compression on a gasoline turbo motor is 9.5:1, and thats on Mazda's DISI 2.3 in the MS3 and MS6. They can get away with it due to the cooling and stratified charge effects of direct injection.

I'll quote Simple Digital Sytems on this one, " There are sound scientific reasons why there are no factory 10 to 1 CR turbocharged engines which produce specific outputs of 175 hp/L. In fact, there is NO production, piston, automotive engine which I am aware of which can achieve a specific output of this level on 92 octane pump fuel anywhere. Despite this fact, many people try to do this with expensive results. High compression ratios and high boost simply don't mix on pump fuel. If you try this, you will either be unhappy with the results or blow up the engine."

[evolutionmovement]

You won't much notice the performance difference off boost with the lower compression, I would guess. I ran a turbo as N/A for a while and there's little between them, if anything, below 3k. Torque didn't seem affected - it just ran out of breath sooner and revved a touch slower. (8.0 vs. 9.5)

[asc_up]

So I'm picking up that you shouldn't go high CR. Lol.


But I think that you'll be happy with the 2.35L. My friend has an STi and he still has a lot of low end torque with the 2.5L off of boost.

[smh0101]

Cool. This is why I posted about this...

So I'll stick with 8.5:1.

Its definitely easier to add boost... but your stuck with CR.

And as for wanting 15lbs... 20lbs... 25lbs... 30lbs....

No Comment

[BXSS]

I think the new 2.0t Audis have 10.5:1 compression, but they are direct injected.

But like everyone else said your stuck with your CR once the engine is built.

[93forestpearl]

Seriously, 8.5:1 is totally fine. My buddy is rocking a 22T with 2.5 DOHC heads and 22T headgaskets which puts him at what, 7.5:1? Still not bad. A nice turbo (BB Garret with or w/out twin scroll) makes up for it. Lots of boost from a good sized turbo will make you forget all about it, believe me.