Compression vs Boost

06gtimpact · Mar 20, 2014

I know a lot of us here have been curious about using high compression and boost together, I thought this was pretty interesting given the controls put in place to see how it all works.

http://www.musclemustangfastfords.com/tech/mmfs_070022_ford_compression_ratio/viewall.html

Dubstep Shep · Mar 20, 2014

The only thing I don't like about that article is that it doesn't address overall compression as it relates to fuel octane and detonation.

Yes, a higher compression motor will make more power than a lower compression motor all other things being constant.

That doesn't address that a 8.5:1 motor can run a heck of a lot more boost than say a 10:1 motor could before detonation.

Just my $0.02

Unreal · Mar 20, 2014

With modern piston design/ecus/head design/etc I think 10-10.4 is perfect for boost, of course that depends on gas. I'm running 9.7:1 on my blower setup but wish I went higher. Lots of friends running 10.4-11:1 with great results. More power, more low end, spools turbos faster (if turbo) etc.

07TGGT · Mar 20, 2014

Higher compression equates to more pressure, thus more heat, which leads to detonation. I even know that and I don't know jack shit bout anything.

Dubstep Shep · Mar 20, 2014

Unreal said:
With modern piston design/ecus/head design/etc I think 10-10.4 is perfect for boost, of course that depends on gas. I'm running 9.7:1 on my blower setup but wish I went higher. Lots of friends running 10.4-11:1 with great results. More power, more low end, spools turbos faster (if turbo) etc.

You're correct that a higher compression ratio is better for spoiling turbos.

But that effect is unnecessary with a PD blower, is it not?

Not to mention that you can ALWAYS be in boost with a PD, so it really doesn't matter how laggy the motor is.

Bud · Mar 20, 2014

07TGGT said:
Higher compression equates to more pressure, thus more heat, which leads to detonation. I even know that and I don't know jack shit bout anything.

Lower compression appears to need to push the power adder harder to make the same power, which could mean higher IATs, which could lead to det problems. If you aren't limited to power adder size then it isn't really an issue unless you have belt slip issues. If you are limited to a particular sized power adder you could jump out of the efficiency range of it, and compressing air more creates more heat. Parasitic losses increase chasing a certain power level by spinning a blower faster as well, so more stress on the engine there.

High compression, turbo, and e85 ftw

Unreal · Mar 20, 2014

If you have the octane (meth, e85, etc) then go higher compression. Like Bud said, you don't have to spin the blower/turbos as hard. Plus the motor is more efficient off boost. When my heads come off I'm milling them for 10.2 or so. No issues making 1050rwhp on 91 octane+meth.

Dubstep Shep · Mar 20, 2014

Bud said:
Lower compression appears to need to push the power adder harder to make the same power, which could mean higher IATs, which could lead to det problems. If you aren't limited to power adder size then it isn't really an issue unless you have belt slip issues. If you are limited to a particular sized power adder you could jump out of the efficiency range of it, and compressing air more creates more heat. Parasitic losses increase chasing a certain power level by spinning a blower faster as well, so more stress on the engine there.

High compression, turbo, and e85 ftw

Yes, you'll have to push the power adder harder to make the same power with a lower compression engine.

However, unlike with engine compression, you can cool power adder compression with an intercooler.

Additionally, the added compression from a power adder actually means you add more fuel.

The engine compression is more how well that air fuel mixture burns.

For instance, you could drop your NA engine from a 9.8:1 compression ratio to a 8.5:1, and run 19lb of boost and still maintain your overall compression ratio. 14.7lb of boost is equal to atmospheric pressure, so 14.7lb boost is essentially double your normal air intake.

Sky Render · Mar 20, 2014

Threads like this make me pissed off that it's hard to find E85 around here.

BruceH · Mar 20, 2014

I've run boost with 11:1 and 92 octane gas. I've also run it with E85. More compression will always give more power with less boost. IMO that is always desireable.

WTF are you guys doing discussing a magazine article for? If you want real experiences search the forum. Plenty of us have put the low compression with boost myth to bed years ago.

rcm90 · Mar 20, 2014

Bud said:
High compression, turbo, and e85 ftw

/End Thread.. J/k

Bruce how much boost were you running with the 11:1c/r and 92oct?

Dubstep Shep · Mar 20, 2014

BruceH said:
I've run boost with 11:1 and 92 octane gas. I've also run it with E85. More compression will always give more power with less boost. IMO that is always desireable.

WTF are you guys doing discussing a magazine article for? If you want real experiences search the forum. Plenty of us have put the low compression with boost myth to bed years ago.

So why do the pros run like a 5:1 compression ratio and boost out the ass?

rcm90 · Mar 20, 2014

I believe it has something to do with having enough volume for the fuel after its compressed in the combustion chamber.

Bud · Mar 20, 2014

Dubstep Shep said:
So why do the pros run like a 5:1 compression ratio and boost out the ass?

A lot of people that are limited on blower or turbo size are upping the compression to get every last bit of power. Like the article in the OP states the more power a motor makes N/A the better it seems to be as a base for a power adder. Mihovetz is running 52psi through an 11:1 motor. It's all about tune and fuel: http://www.accufabracing.com/john-mihovetz-mustang/2-uncategorised/34-john-mihovetz-mustang-2

My car has had 21psi through it repeatedly at 11:1. E85 is unicorn piss though

Dubstep Shep · Mar 20, 2014

rcm90 said:
I believe it has something to do with having enough volume for the fuel after its compressed in the combustion chamber.

True. But I bet it has something to do with just being able to run more boost.

You're always better off making compression that's pre fuel and can be cooled.

Unreal · Mar 20, 2014

What pros? Anyone running a fast gas car I know is running as much compression as they can. 11:1 and 30+psi on good gas.

What a nitro car does isn't relevant to a street car.

rcm90 · Mar 20, 2014

Dubstep Shep said:
True. But I bet it has something to do with just being able to run more boost.

You're always better off making compression that's pre fuel and can be cooled.

For a nitro motor it's all about the fuel.

BruceH · Mar 20, 2014

Dubstep Shep said:
So why do the pros run like a 5:1 compression ratio and boost out the ass?

I'm not a pro. I'm also not burning nitromethane. I would have no idea why they do what they do.

I do know that having cylinder pressures higher is what makes more power. That can be accomplished with forced induction, higher compression, or both.

I'm just very surprised that this would be a topic of discussion on this forum. High compression and boost has been covered many times. It's not a new thought at all. I would expect magazine articles to be talked about on svtp et al where the posters only know what they read and what others with no experience tell them but this forum is far different from those.

I ran 17psi with a Whipple and 92. Compression was 9.5:1. The 11:1 motor saw 12psi with 92 iirc. It could of only been 10. The same motor with different heads had a compression of 10.76:1 and saw a high of 13-14psi with 92 and 19psi with e85.

IMO modern engine management systems with knock sensors and multiple safeguards help us to achieve the high compression and boost.

I also think E10 is allowing more compression and boost with pump gas. Ethanol cools and adds power in a way that isn't measured with octane or btu. That's another discussion though.

skwerl · Mar 20, 2014

I'm at 10.7:1 running 15 lbs of boost on 93 octane, drive it almost every day.

Dubstep Shep · Mar 20, 2014

I'm not saying it isn't possible to run high compression with boost on pump gas. You're just limited.

Take that 10.7:1 at 15 lbs boost and drop it to 9.5:1 and run 30 lbs boost. Which one do you think will make more power?

Which is gonna add more power? An increase of one to the compression ratio or nearly 15lbs of boost?

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