Compression vs Boost

[Unreal]

With big v8s these days most people are running out of compressor/blower/turbo. Sure if you have twin 80mm that can support 30psi it would make more but if the turbos/blower/etc tap out at 16-17psi then I'll take the compression.

My car is running 18psi and that is all the blower has. Only thing would be a bigger blower which means all sorts of custom work/etc. Or I could bump compression up a point.

Just did a TT Camaro the other day, 416ci 11:1 compression, 10psi made 920rwhp. Few weeks before that did a 427 TT 9:1 and took 16psi to make similar power, didn't spool as nicely and wasn't nearly as fun to drive.

 

[skwerl]

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?

Sorry, I don't have $20,000 to buy 30 lbs of boost. Your proposition is nonsensical. I didn't realize you were going off on another one of your nonsensical "what if" threads with no restraint of practicality.

 

[19COBRA93]

I chose low compression (8.4:1) and lots of boost. I also still wanted to run higher boost on 91. I have a big blower capable of 30lbs, but I'm limited on octane, as I just run 91 on the street, and 100 at the track. If I had the octane though (like E85), I'd go high compression, also with lots of boost. As it is though, I need my compression to be 91 octane friendly. My motor isn't a Coyote, so it needs a lot of boost to make the big numbers. I can make 800rwh on 91 octane without a worry. On this motor, there's no way I could do that on high compression/lower boost and 91.

On my 10.8:1 3v with a Procharger, I couldn't run shit for boost on 91 octane. Which also resulted in shit for power, and later 3 melted pistons. Not doing that again. It seem like the Coyote is the only motor able to get away with it, and still make big power.

So there is no right answer. It depends on the engine, the fuel, and what you're using it for.

 

[Bud]

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?

In the real world, what blower is going to push 30psi on our motors? How much power will it take to spin that blower, how much heat will that blower make (intercool all you want, there are limits), how much will all that extra load and possibly heat impact the longevity of the motor?

Theoretical is nice, but what people are doing in the real world seems to be better data to go off of.

 

[BruceH]

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?

10.7 at 15psi gives a boost compression ratio of 21.62. 9.5 at 30psi gives a boost compression ratio of 28.89.

Think about how much more power it will take to turn a supercharger to 30psi. Also think about o ringing the block and being restricted to race gas or e85. It's not a valid comparison imo.

9.5 at 19psi is a closer comparison. It's making the boost compression 21.78. Turning 4 psi more will rob power from the motor and make iats higher which will affect the detonation point.

9.5 isn't too bad imo. It really gets ridiculous at 8.5 or so. A few years ago that was the thought but with higher compression factory cars surviving forced induction it's a no brainer to go with higher compression imo.

FYI I have a 2012 Focus that runs 12:1 compression with 87 octane. Technology is great imo. 30 years ago a 12:1 motor wouldn't of been possible on 87.

 

[lito]

I prefer more this technical approach:

http://www.modularfords.com/threads/51059-Increasing-CR-vs-boost-pressure

 

[BruceH]

I prefer more this technical approach:

http://www.modularfords.com/threads/51059-Increasing-CR-vs-boost-pressure

That's an interesting read. However, if I understand his method correctly he used an 8.5 compression motor and "scaled" the results up by 6% for the higher compression trace on the graph. Nothing more than a guess.

To me it looks like he pushed everything toward the result he wanted.

 

[weather man]

I prefer more this technical approach:

http://www.modularfords.com/threads/51059-Increasing-CR-vs-boost-pressure

Imaginary reps to black2003cobra...good shit.

 

[skwerl]

I just remembered that I'm a registered member over on Modular Fords. Unfortunately I don't own a sn-95 or an 03-04 Cobra so there's really nothing of interest for me there.

 

[BruceH]

Reading into the thread some more and found a real result from Justin. He had a motor with 10:1 making 440rwhp with 7psi and 14 degrees of spark advance. When a Cobra block with 8.5 compression was used with the same setup and tune he got 380rwhp. Increasing the spark by 6 degrees resulted in 420rwhp.

The formulas used in that thread would of predicted 411rwhp for 11:1 vs the actual 440rwhp for 10:1. IMO that's a significant difference and more than likely it's a dynamic, non linear increase that can't be scaled without the use of logarithms in the mathematical forumula.

 

[05stroker]

I am doing a high C/R high boost build now. I will be running 12.0:1 CR with 21psi on e85. I used the Wallace Racing calculators and the new setup will be about the same combined CR as running 33 psi on the old 9.01:1 pistons.

21 psi
9.01:1 CR
500 ft
21.78 CCR


21 psi
12.0:1 CR
500 ft
29.04 CCR


33 psi
9.01:1 CR
500 ft
29.14 CCR

 

[05stroker]

I am doing a high C/R high boost build now. I will be running 12.0:1 CR with 21psi on e85. I used the Wallace Racing calculators and the new setup will be about the same combined CR as running 33 psi on the old 9.01:1 pistons.

21 psi
9.01:1 CR
500 ft
21.78 CCR


21 psi
12.0:1 CR
500 ft
29.04 CCR


33 psi
9.01:1 CR
500 ft
29.14 CCR

I should add that I run a triple nozzle AEM meth system with a 30/70 mix also. And that may change to a 50/50 mix for track days.

 

[Unreal]

I chose low compression (8.4:1) and lots of boost. I also still wanted to run higher boost on 91. I have a big blower capable of 30lbs, but I'm limited on octane, as I just run 91 on the street, and 100 at the track. If I had the octane though (like E85), I'd go high compression, also with lots of boost. As it is though, I need my compression to be 91 octane friendly. My motor isn't a Coyote, so it needs a lot of boost to make the big numbers. I can make 800rwh on 91 octane without a worry. On this motor, there's no way I could do that on high compression/lower boost and 91.

On my 10.8:1 3v with a Procharger, I couldn't run shit for boost on 91 octane. Which also resulted in shit for power, and later 3 melted pistons. Not doing that again. It seem like the Coyote is the only motor able to get away with it, and still make big power.

So there is no right answer. It depends on the engine, the fuel, and what you're using it for.

What blower will make 30psi on a 4.6 or 5.0? I'm running a Ysi and even over spinning it can't make over 20psi on my 7L. Even a F2/etc won't make 30psi on a good high flowing big motor. I guess you could put tiny heads and a small cam just to say you are running 30psi.

Most people with large v8s with decent heads/cam run out of compressor. SBF, SBC, LS motors, etc all like higher compression. Melting some pistons isn't the compressions fault, that is just a bad tune or setup.

 

[05stroker]

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

Do you run Methanol as well or just E85?

 

[BruceH]

Billy,

Ethanol/gasoline mixtures aren't supposed to detonate once the ethanol content is over 50%. I'll bet you won't need meth with e85.

 

[Unreal]

Either way the motor will only make what the octane supports. You can do that with compression and boost or more boost. If you have the octane to make 800rwhp you will max at there no matter what the compression/boost is since in the end it is cylinder pressure that matters. High compression (10+) will drive a lot better off boost, have more low end, a broader power band, better gas mileage, and many other positives so in the end if it is 11:1 and 10psi to make 800rwhp or 9:1 and 18psi to make 800rwhp I will take the high compression any day of the week.

 

[05stroker]

Billy,

Ethanol/gasoline mixtures aren't supposed to detonate once the ethanol content is over 50%. I'll bet you won't need meth with e85.

That is a BIG if. I would love to loose the meth, but 21 psi 12.0:1 CR and 150+* intake temps in Texas heat scares me.

 

[weather man]

 

[Bud]

Do you run Methanol as well or just E85?

Just e85. I won't be going over 21psi until I sleeve the block, but still don't plan on running meth. Guess timing would come into play too for overall cylinder pressure, I'm not crazy on timing on my setup.

 

[rcm90]

Cam profile plays a big part in dynamic compression ratio which is what causes an engine to detonate.

Some engines struggle to run 10.5:1 on 93 where some people run 13.1 on 93 with zero issues and the only big difference between are cam profiles.