Optimum compression ratio
 

I'm adding lt4 fueling and a cam kit to my Maggie equipped SS this winter and I'm on the fence regarding swapping out the stock pistons. I'm working with Ted Jannetty on my build but thought I would post my question here rather than calling Ted direct to help out and get input from other members.
When swapping pistons on a FI motor, is it best to drop the compression a point or so? There appear to be pistons/piston rod combos available that will retain the stock compression ratio or drop it a point or so.
I expect that lower compression is advantageous at extremely high levels of boost. I'll probably be somewhere around 10 - 12 psi.

What fuel are you willing to run?

This question is more pertinent than your desired boost level.

As much e85 as I can get away with. I'd love to be able to run e85 exclusively but recognize that I will probably need to stay around 70%

If running E85 don't touch the compression... pump gas only, drop it a point.

I am not a tuner but generally, compression is your friend.
If your car is likely to never see 87 octane (or lower) with a good tune you can safely run higher than stock compression. Being DI, pre-ignition is even less likely.

All this said, consult the tuner you intend to use. Ultimately, it's between you two if you are not pleased with the end result.

My opinion is, if you are only going to have LT4 fuel parts with a cam lobe, running E85, then you will not be able to run enough boost to justify lowering the compression ratio (due to limited fuel supply). Lowering the compression ratio actually makes the engine less efficient, and you'd need to then up the boost just to get back to where you are right now. So unless you are doing a setup to run quite a bit of boost, I think that lowering the CR would be a step backwards. Therefore, with the parts and fuel that you're listing, I think that the drop-in pistons from Vengeance Racing (stock CR) would be a better choice than the lowered-CR pistons.

What he said^^

If you have plenty of compressor and fuel and plan on turning it up then lowering the compression makes sense.

Intuitively I agree. In most cases, compression is definitely a good thing, especially on DI motors. I just want to get inputs from others with more experience. I've done numerous NA builds over the past 40 years but am still a newbie regarding forced induction. I wonder why the lt4 has lower compression given it's modest amount of boost?

The reason why I said what I did was for the simple fact that on stock compression the octane wall is around 700whp with 93 octane. So if you don't plan on supplementing with meth or c16, then dropping compression would be required if you plan on going over 700whp with pump fuel.

That being said I would run E85 at all costs and leave the compression alone unless you plan on building a 1k whp monster, then it might be a good idea to go ahead and lower the compression a point .

I'm rebuilding my motor right now and I'll be running 10.5 compression. Should have less issues with running higher boost and hopefully less detonation with the lower compression ratio. I run VP Ms 109 and 2-1000 cc nozzles of meth/water. We pushed it pretty hard at 16.5 lbs of boost with the higher compression on the stock Lt1 and turning it a little higher then stock rpm.

The drop in piston is still the stock CR. With your 95mm pulley you are probably at about 7 lbs of boost, so your on pretty low boost right now. Stock compression will work best if you plan on running about 10-11 lbs. I plan on running about 17-18 lbs with the new motor.

FYI - I did add the forged pistons with the stock compression ratios. I can run full pump e85 which is between e70-80 (apparently the gas stations in Illinois have additional environmental additives that cause the percent e to be reduced).

I have two e gas stations right by my house. The first station's pump has e80 and the other is closer to e70. Once I up to 10psi, I expect that we will be kind of topped out around e70 so I will generally fill-up at the station with e70. At the same time, when I go other places, I will use HP tuner to check my mixture... That should allow me to skip mixing. If I am further away from home and use 93, then I will simply add from the station with higher e until numbers are where I want them.

My intent is to keep the car setup so I don't really have to mix fuels which means staying with about 10psi.

I have started a couple Jannetty Super Street Brawler threads, check those out and see what you think. I worked with Magnuson and Laynlo at first and then went with Ted for the second stage of my setup...couldn't be happier. I am going to have them overnight the pulley to up boost tomorrow

My guess on this one is, because it has a warranty, GM is being extra cautious.