Lower compression effects
 

All,

I am getting ready to build the bottom end of the engine with rods, pistons, etc.

I am going between keeping stock compression vs going to 10:1

Aiming for 350+ whp on pump and 400+ on e85

What are the other effects that I will notice or will there be if I lower compression?

Turbo spool time increase?
Low end torque loss?

Running a custom setup with a GT2871r turbo

Torque and HP drop across the board, much more resistance to detonation, worse spool time on the turbo, and probably worse fuel mileage

There is zero reason to lower compression when you could limit boost, or do boost by gear or progressive boost programming with a boost controller. The only reason to lower compression is because you don’t have e85 and want to make more power than you could on pump.

I'm no expert on this, but with a turbo that size and access to e85, I would leave stock compression and run lower boost. I also feel like you're gonna max out that 2871r running e85 with a built engine.

I should rephrase: if you expect to get to 350whp on pump then you will need to use e85 or lower compression. High compression is good for fuel economy and NA power and even for getting low end power for a turbo to spool faster, but boost raises the effective compression ratio, so that is a problem. Boost on low compression gives the power of having high compression, while having low compression at low rpms where knock risk is higher. The other solution is the Atkinson Cycle in modern engines that reduces the effective ratio while maintaining the expansion ratio, or there are these new variable compression motors like what GM and Nissan have introduced. With such a motor, it could be possible to run a static compression ratio that was variable from 12:1 to 20:1 without a turbo, but on to you.

Evo has a static compression ratio of 9:1 with 19psi of peak boost making the effective compression ratio is 20:1. The BRZ has a static compression ratio of 12.5:1 with 9psi of boost on a basic kit making the effective compression ratio is 20:1 and both cars will dyno around 250whp +/-, but which is better? Our cars will have more power, and it will come on sooner, but the Evo can add more boost without dramatically raising the effective compression ratio or without raising much of the effective compression ratio down low, which is why it is easier to run on pump gas.

So, how to get around this? Bleed off boost with a variable boost controller, so you institute a progressive boost pattern by rpm and boost by gear, so that you can run a lower effective boost ratio at low rpms and at higher loads (lower gears) when knock is at a higher risk. If you do this, you will be able to have a static compression of 12.5:1 as a minimum, so spool time is decent, gas milage might be better and pickup will be better for putting around town. Then you will have the benefits of higher compression when you add E85.

Or you can lower static compression.

With your situation, there's no reason to lower compression. If you were making some monster drag engine, then maybe. Just get a good tuner.

I could chime in because I am currently running that turbo with my setup.

Built, 10:1 compression
flex fuel, 1000cc injectors, radium rails, ebcs, walbro 450 w/ fuel return

My car is getting tuned today so I could post numbers later, but so far from its base numbers with still lots more to go on pump 93 it made over 345/315. Looking forward on the e85 numbers.

Edited: Car isnt ready yet :(

I’m curious what you mean by this.

I’m lost on this as well because everyone raves about how great e85 is in higher compression engines because of it being “knock resistant.”


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Same; pretty much the opposite is true in my opinion.

From what I have experienced with E85, it does have an optimum operating temperature. This is common knowledge for starting the car, that cold starts aren't easy on E85, and I feel that the car runs better once it is warm, but not just a little like how every car runs better when warm.

I doubt E85 has an ideal compression ratio for any motor (in a parabolic distribution), including our motor. I'm sure the benefits or maximums are logarithmic, tapering off at some effective compression ratio that is far from 10:1. Considering the stock compression is 12.5:1 and works great with E85, his statement doesn't seem accurate at all.

Can anyone share any experience relative to fuel consumption of a 10:1 motor versus 12.5 ? I am trying to understand if lowering the comp ratio will automatically result in worst fuel economy because of an inherent less efficient engine, or if careful tuning of the turbo on a built 10:1 engine can actually allow to maintain equivalent fuel economy (at equal performance) to a stock 12.5:1 by having boost coming on sooner and reach the same overall effectiveness (once spooled).

At 10:1, my torque dip prior to boost is greater than stock, I notice less HP, and with a high flow fuel pump, I get more consumption.

A friends twin with stock compression, with a bigger blower, high flow fuel pump, makes more useable HP prior to boost, and it gets better fuel consumption, until boost. He says its comparable to stock, as long as he stays out of boost.
I expect his car to grenade on the track as it has stock rods + pistons.
:burnrubber:

@Marcoscrdo, would you have a dyno run on 93 octane you can share ? Would you be able to comment on fuel economy with the built 10:1 motor compared to before ? How did you find the low end response and spool up after the tuning ?