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Forced Induction Turbo, Supercharger, Methanol, Nitrous


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Old 04-27-2019, 10:58 AM   #15
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Originally Posted by falcon_wizard View Post
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).
It will be inherently less efficient. Raising the effective compression ratio by adding boost is not as efficient as raising compression. While everyone always talks about the parasitic effects of a supercharger, there are still losses with a turbo. Recall too that turbos add boost under load and are subject to turbo lag and boost threshold. Because the turbo requires energy to create boost, inherently having less energy means more rpms are needed to get to that threshold.

Remember too that manufactures will painfully innovate to get three more mpgs out of a car because going from thirty to thirty-three mpgs is a ten percent improvement, which gets them closer to government mandates, but is that really a big deal for a car enthusiast? Usually not.
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Old 04-27-2019, 11:24 AM   #16
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Originally Posted by RFB View Post
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.
Thanks RFB, would you be able to quantify the off boost fuel consumption difference before and after the engine build, or what your current off boost consumption is ?
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Old 04-28-2019, 02:57 PM   #17
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Originally Posted by falcon_wizard View Post
Thanks RFB, would you be able to quantify the off boost fuel consumption difference before and after the engine build, or what your current off boost consumption is ?
That info would not be useful as I have a temporary Jackson supercharger, insufficient boost for 10 to 1, (awaiting enough coin for a 500 hp turbo and much more boost), not an ideal situation. Currently very poor consumption little usable hp at low rpm, necessitating boost for normal driving.

My buddy with a 350 whp turbo, stock internals, reports normal driving at stock hi compression out of boost, like a normal torque dip in an oem car.
But he will blow up his motor on the track, the rods will bend.
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Old 04-28-2019, 03:18 PM   #18
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Yea a centrifugal sc on lower compression motor would suck because all the boost is up top.
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Old 04-30-2019, 10:09 AM   #19
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@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 ?
Not yet, I did not get my car back yet lol, going to be awhile before I can do testing in between 93 and e.

Sorry if any my information was confusing.
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Old 04-30-2019, 12:03 PM   #20
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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.
So it looks like most people are saying lowering compression isn't recommended. what about increasing comp to 13.5:1 ?
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Old 04-30-2019, 10:17 PM   #21
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So it looks like most people are saying lowering compression isn't recommended. what about increasing comp to 13.5:1 ?
I don't know if the FA20 uses any types of trickery to alter its compression/expansion ratio. My belief is that they are always a 1/1 and that there is just alterations in the cams and injector profiles to compensate for engine knock risks at lower rpms in varying conditions. I have no idea if Delicious Tuning, who tuned my BRZ, makes changes to modify compression/expansion ratio.

With that said, an increase to 13.5:1 would increase the risk of knock, especially at lower rpms. This risk of knock would raise with higher AITs or other factors, and the fear would be that the motor would reach some limit where it couldn't adjust timing or alter cam profiles enough to compensate on those hot days or whatever. This is where a tuner could institute an Atkinson cycle to the motor, altering the compression/expansion ratio, but I don't know if this is done; I'm not a tuner.

If higher compression was used then I would limit the car to E85 for safety.

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Old 05-01-2019, 01:13 AM   #22
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Stay with 12.5:1 because
- smaller turbo for same power
- smaller turbo spools faster
- you get a high compression NA engine at the low rpm range rather than a low compression NA engine before the turbo spools
- the direct injectors don't work so great at different compression ratios
- e85 and DI does an amazing job of preventing detonation which is the only reason to lower static compression ratio

I honestly can't think of a single good reason to lower to 10:1
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Old 05-01-2019, 10:05 AM   #23
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Stay with 12.5:1 because
- smaller turbo for same power
- smaller turbo spools faster
- you get a high compression NA engine at the low rpm range rather than a low compression NA engine before the turbo spools
- the direct injectors don't work so great at different compression ratios
- e85 and DI does an amazing job of preventing detonation which is the only reason to lower static compression ratio

I honestly can't think of a single good reason to lower to 10:1
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Old 05-01-2019, 04:07 PM   #24
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Before I spun a rod bearing, I was running an Edelbrock supercharger (original pulley 11psi) on California 91. Delicious tuning was always battling my IAM's dropping down anywhere from .9-.75. IAM's were the worst when autocrossing in hot weather.

After my rebuild, I am now running 10:1 pistons with a smaller pulley (18psi). IAM's are rock solid on 1. Power is significantly higher despite the closely comparable effective compression ratios. I never have to worry about Cal 91 gas anymore. However, Delicious has stated I won't be able to take advantage of e85. In any case, I am content with the power for now.

Would I personally choose 10:1 pistons again? No. I would choose maybe 11.5:1 to give me a buffer on shitty california gas, while still being able to take advantage of e85. This is only my take on my roots supercharged setup. Every boosted setup is different.
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Old 05-03-2019, 10:28 AM   #25
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Good info. Pulled the trigger on JE 12.5:1 pistons.
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Old 05-03-2019, 04:35 PM   #26
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Originally Posted by Hags86 View Post
Stay with 12.5:1 because
- smaller turbo for same power
- smaller turbo spools faster
- you get a high compression NA engine at the low rpm range rather than a low compression NA engine before the turbo spools
- the direct injectors don't work so great at different compression ratios
- e85 and DI does an amazing job of preventing detonation which is the only reason to lower static compression ratio

I honestly can't think of a single good reason to lower to 10:1

To add to that... higher boost pressure = more heat in the intake charge too.
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Old 05-03-2019, 07:11 PM   #27
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To add to that... higher boost pressure = more heat in the intake charge too.
I don’t think this is a factor. It is more of a product of the trade off of lower compression and higher boost motors versus higher compression and lower boost. Higher IATs are bad, but higher compression creates higher temps, so 300hp is 300hp, no matter where the temps originate. There might be an argument for better manifold air velocity with colder IATs, but I would have to see the data. The bigger differences are at the low end—not the high end.
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Old 05-04-2019, 01:30 AM   #28
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Before I spun a rod bearing, I was running an Edelbrock supercharger (original pulley 11psi) on California 91. Delicious tuning was always battling my IAM's dropping down anywhere from .9-.75. IAM's were the worst when autocrossing in hot weather.

After my rebuild, I am now running 10:1 pistons with a smaller pulley (18psi). IAM's are rock solid on 1. Power is significantly higher despite the closely comparable effective compression ratios. I never have to worry about Cal 91 gas anymore. However, Delicious has stated I won't be able to take advantage of e85. In any case, I am content with the power for now.
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