Lower compression effects

[86TOYO2k17]

Originally Posted by falcon_wizard

For what it’s worth and more from a holistic perspective... the final tuning work was recently completed on my engine. Since I was going to use 91/94 octane (no e85 in my location) The builder/tuner had decided to proceed with 10:1 to maximize long term reliability and detonation protection. I was initially concerned about how this could affect off boost performance, however Final tuning dissipated all my doubts, the car performs amazingly, with power delivery being just as gradual as before, with essentially no lag at all. The car is simply mind boggingly fun to drive, and they we able to bring TQ about 400rpms earlier than before in the low end, as well as achieving a higher tq plateau (up to +60lbft @crank) across the entire mid/top end as well, resulting in lots of extra “area under the curve”. The garrett gt2871 was able to maintain steam to 7500rpm. Note that I was not seeking maximum top end power, but just more usable tq across the entire range for daily driving with the odd hpde. My low boost 91 map now has higher tq across the entire rpm range than my previous stock engine high boost 94 map, with the rest of the setup and unchanged. So for those not going towards higher octane fuel, my feedback on those considering lowering to 10:1 cr would be that very satisfactory results (for my goals) was achieved.

how many PSI you running and what was your TQ/WHP?
Got a dyno sheet?

[falcon_wizard]

Max psi was 17. Max hp 398 crank, 307 whp on a Maha dyno. Max Crank TQ was about 314 pdft, about 258wtq. It was tuned for overall daily drivability/reliability and overall performance across the entire rpm range (not just top end or numbers bragging purposes) Chart is on 94 octane on high boost. With stock engine (9psi) chart shape Looked similar except tq curve was about 400rpm later in the low end and plateaued at 260/340 crank tq/hp vice 314/398.

[86TOYO2k17]

Originally Posted by falcon_wizard

Max psi was 17. Max hp 398 crank, 307 whp on a Maha dyno. Max Crank TQ was about 314 pdft, about 258wtq. It was tuned for overall daily drivability/reliability and overall performance across the entire rpm range (not just top end or numbers bragging purposes) Chart is on 94 octane on high boost. With stock engine (9psi) chart shape Looked similar except tq curve was about 400rpm later in the low end and plateaued at 260/340 crank tq/hp vice 314/398.

Interesting, thanks. Seems like your effective compression is 21.5:1 about the same as stock compression with 10.6psi which with a turbo on 93/94 most people hit 300+whp with. Basically the same.

I guess the question is which one is running “safer” or has a higher probability of lasting longer before blowing up? I’m sure your making a bit less whp under 3500rpm compared to stock compression so maybe your stressing it less daily driving and thus would last longer. But would it be more or less or equally stressed when both are max boost at those PSI?

[falcon_wizard]

Originally Posted by 86TOYO2k17

Interesting, thanks. Seems like your effective compression is 21.5:1 about the same as stock compression with 10.6psi which with a turbo on 93/94 most people hit 300+whp with. Basically the same.

I guess the question is which one is running “safer” or has a higher probability of lasting longer before blowing up? I’m sure your making a bit less whp under 3500rpm compared to stock compression so maybe your stressing it less daily driving and thus would last longer. But would it be more or less or equally stressed when both are max boost at those PSI?

The expectation indeed was that my target goals could be met either way with 12.5 or lower cr. My understanding is that detonation is a higher risk at lower rpms and thus would indeed be most relevant is that lower range. Initially we were planning on sticking with 12.5, but moved towards 10:1 to maximize long term reliability which was a high priority factor for me. The question then was how much of a trade off was that to going to be from a performance standpoint. There is probably a small difference in the lower rpm, but in the end I did not find it objectionable. I noticed that turbo’ed porsches, AMGs and BMWs in the 350-450 hp range that I saw all had CRs between 9.3 and 10.3 or around there, so I figured that was a good benchmark when looking for OEM level reliability. The GT4 and GT3 atmo engines from porsche were the ones I saw with 12+ CR that I saw.

[falcon_wizard]

The closest dyno on a stock motor to mine seems to be the one from R/T Tuning, running at 14psi on 93 octane. https://www.ft86club.com/forums/show...t=42290&page=3

The Dynojet was at 320whp, with a very similar torque curve to mine, where max TQ is reached around 4000rpm is roughly maintained to redline with a small downslope from 6000 onward. My understanding is that maha dynos read below many other ones, so that sounds about right. Pitman’s Dyno on a Maha dyno at max 10.5 psi reaches about 240whp 340 crank.

My initial calculations and models seemed to require at least 13psi. So could it be that the conventional effective compression calculations do not paint the whole picture ? 14psi on a stock motor seems to be by most account I’ve seen pushing the boundaries of reliability.

[86TOYO2k17]

Originally Posted by falcon_wizard

The closest dyno on a stock motor to mine seems to be the one from R/T Tuning, running at 14psi on 93 octane. https://www.ft86club.com/forums/show...t=42290&page=3

The Dynojet was at 320whp, with a very similar torque curve to mine, where max TQ is reached around 4000rpm is roughly maintained to redline with a small downslope from 6000 onward. My understanding is that maha dynos read below many other ones, so that sounds about right. Pitman’s Dyno on a Maha dyno at max 10.5 psi reaches about 240whp 340 crank.

My initial calculations and models seemed to require at least 13psi. So could it be that the conventional effective compression calculations do not paint the whole picture ? 14psi on a stock motor seems to be by most account I’ve seen pushing the boundaries of reliability.

I’ve seen dozens at 10-11psi 93octane making 300+whp. Too lazy to pull them alll up, but one quick search http://www.ft86club.com/forums/showthread.php?p=2649457 has several.
Idk about “exact” same torque curve as yours. You are on different compression and boost so different driving characteristics, but most are hitting hard at about 4500rpm at which point both setups have the same effective compression ratio.

Also a lot of dynos “read low” So it is what it is.

Still cool to have something different, I bet when that 17lbs kicks in it puts you into the back of the seat pretty hard with such a drastic fast torque ramp up like that.

[falcon_wizard]

Originally Posted by 86TOYO2k17

I’ve seen dozens at 10-11psi 93octane making 300+whp. Too lazy to pull them alll up, but one quick search http://www.ft86club.com/forums/showthread.php?p=2649457 has several.
Idk about “exact” same torque curve as yours. You are on different compression and boost so different driving characteristics, but most are hitting hard at about 4500rpm at which point both setups have the same effective compression ratio.

Also a lot of dynos “read low” So it is what it is.

Still cool to have something different, I bet when that 17lbs kicks in it puts you into the back of the seat pretty hard with such a drastic fast torque ramp up like that.

I have to say my first few «*attempts*» at going wot on that new setup really surprised me... It kept wanting to pull and pull, until redline. When I though I was at wot, I realized I wasn’t and it was ready for more when I though it would run out of out steam. Although on paper 55tq extra doesn't not look like that much, the fact it’s over a full 3500 rpm range adds up to a lot of extra acceleration by the time you get to redline. On a car this light, it’s more than most should need for daily driving imho. Numbers mean only so much, and the way power delivery is done is hard to express in numbers, but I have to say I find it very impressive

Cheers

[Irace86.2.0]

Lower compression will be safer than higher compression at the same effective ratio. E85 is the great equalizer, but this isn’t the case at the extreme’s.

[falcon_wizard]

Originally Posted by 86TOYO2k17

I’ve seen dozens at 10-11psi 93octane making 300+whp. Too lazy to pull them alll up, but one quick search http://www.ft86club.com/forums/showthread.php?p=2649457 has several

You are correct that several reach 300whp, which basically is about 210wtq at 7500rpm. I have not seen many that maintain about 240-250wtq from 4000 to 6600 rpm on 93 octane however, although many reach that with Ethanol. The one from R/T tuning was the one I saw in the thread that basically have an identical tq curve. King tut also has a very similar tq curve at 12.5psi with 93 and same turbo as my setup.

Cheers

[Sonic86]

Originally Posted by falcon_wizard

For what it’s worth and more from a holistic perspective... the final tuning work was recently completed on my engine. Since I was going to use 91/94 octane (no e85 in my location) The builder/tuner had decided to proceed with 10:1 to maximize long term reliability and detonation protection. I was initially concerned about how this could affect off boost performance, however Final tuning dissipated all my doubts, the car performs amazingly, with power delivery being just as gradual as before, with essentially no lag at all. The car is simply mind boggingly fun to drive, and they we able to bring TQ about 400rpms earlier than before in the low end, as well as achieving a higher tq plateau (up to +60lbft @crank) across the entire mid/top end as well, resulting in lots of extra “area under the curve”. The garrett gt2871 was able to maintain steam to 7500rpm. Note that I was not seeking maximum top end power, but just more usable tq across the entire range for daily driving with the odd hpde. My low boost 91 map now has higher tq across the entire rpm range than my previous stock engine high boost 94 map, with the rest of the setup and unchanged. So for those not going towards higher octane fuel, my feedback on those considering lowering to 10:1 cr would be that very satisfactory results (for my goals) was achieved.

That sounds truth.
That’s because with lower CR you can set ignition and variable cam timing much more aggressively.
That helps a lot with throttle response and low-mid torque gain.
In case of running pump 91 the CR 10:1 is perfect in my opinion.
Just look at factory boosted subarus/hondas. Their engineers aren’t stupid. They know that running boosted with CR 12.5:1 you can forget about propper ignition timings that causes a lot of problems such as:
- lack of boost in low rpm range
- poor throttle response
- retarded ignition timings causes exhaust valves to overheat and overall higher EGTs