Compression vs Boost

[Bud]

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.

Indeed

 

[05stroker]

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.

What is your timing set at?

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.

I will do a dynamic comparison Sat. instead of the combined I posted tonight.

 

[Bud]

About 18-19, more power to be had there, but stock coyote blocks suck :/

 

[rcm90]

I will do a dynamic comparison Sat. instead of the combined I posted tonight.

Out of curiosity what are your cam specs?

 

[05stroker]

About 18-19, more power to be had there, but stock coyote blocks suck :/

On my 9.01:1 setup I always found 23* was the sweet spot on C16 and E85 and ran 18* on 93 oct. On my 3v.

Out of curiosity what are your cam specs?

They are Comp 127550 cams. Here is a pic of the cam card another member posted in my build thread. And you already know about my heads.

 

[05stroker]

Dynamic Compression Ratio Calculator
(Use seat to seat specs for Intake spec for best results) Number of Cylinders : Bore in Inches : Stroke in Inches : Rod Length in Inches : Static Compression Ratio : 1) Inlet Valve Closes ABDC : º Boost Pressure in PSI : Target Altitude : (Feet)

Static compression ratio of 12.0:1.
Effective stroke is 3.50 inches.
Your dynamic compression ratio is 11.17:1 .
Your dynamic cranking pressure is 243.45 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 21 PSI is 27.13 :1.

 

[05stroker]

Dynamic Compression Ratio Calculator
(Use seat to seat specs for Intake spec for best results) Number of Cylinders : Bore in Inches : Stroke in Inches : Rod Length in Inches : Static Compression Ratio : 1) Inlet Valve Closes ABDC : º Boost Pressure in PSI : Target Altitude : (Feet)

Static compression ratio of 12.0:1.
Effective stroke is 3.50 inches.
Your dynamic compression ratio is 11.17:1 .
Your dynamic cranking pressure is 243.45 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 21 PSI is 27.13 :1.

The numbers entered for this were.

Cylinders 8
Bore 3.582
Stroke 3.75
Rod length 3.85
C/R 12.0:1
ABDC 40*
21 PSI
500 FT

 

[gray1622]

I am not sure about this but I would think that this test is point less. In my mind the higher compression motor would have higher mass flow. To me what would be interesting is a test where you run two turbo charged motors that are the same except for compression .Then in test run more boost on low compression motor so that the maf count is the same in both motors.. Then do a low HP test and High Hp test.


This is good read http://www.xcceleration.com/cr-boost%20101.htm

Here what is striking to me in the link. "So now we can target a certain ECR, say 12.0:1. We see that at 8.5:1 CR we can run 14.7psi of boost. But at 7.5:1 we can run 23psi of boost (and still maintain the 12.0:1 ECR). We only gave up 1 point of compression (3% max power) and yet we gained 28% more oxygen (28% more power potential)." Now I think there more consider than just total oxygen such as different amount of timing your run due to CC temp. and higher compression will burn more efficient. I would say that 28% more power is BS figure. Also at the end of the day you are limited by size of turbo you can put on car and the octane of the fuel. Plus a low compression and high boost motor is going be a dog down low. I would say for a street car on e85 high compression is the way to go since you dont have to worry about detonation and your limited by 3 in down pipe unless you want 4 in down pipe sticking out your front bumper.

 

[19COBRA93]

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.

My 4.0L Whipple will do it without a doubt. I've already made 26lbs and still have 3 big jumps in pulley sizes to go. This blower just moves air. I had to put a stock throttle body on it to get the boost down to pump gas levels as it is (3lb difference between the SCJ monoblade and stock). And the blower is flowing this kind of air at 5000ft. At sea level it would be even more.

I agree, melting pistons wasn't exactly the compressions fault, it was my fault by trying to make just decent power on pump at that compression level. It was on the ragged edge, and I pushed it too far. Lower compression motors tend to be much more forgiving. This is why Ford chose low compression as well for both the Cobra and GT500.

With that said, I'm not saying one is better than the other. As I said before, it depends on too many things for one answer to be more right than the other. I'm just offering my opinion on which way I went, and how it's working for me.

800rwh on 91 octane. Safe as could be. Good gas, bad gas, hot air, cold air, street driven, drag raced...It will do it all day long. On higher compression and less boost, on this motor, I don't think would be as forgiving. That's just my opinion though.

 

[Dubstep Shep]

My 4.0L Whipple will do it without a doubt. I've already made 26lbs and still have 3 big jumps in pulley sizes to go. This blower just moves air. I had to put a stock throttle body on it to get the boost down to pump gas levels as it is (3lb difference between the SCJ monoblade and stock). And the blower is flowing this kind of air at 5000ft. At sea level it would be even more.



I agree, melting pistons wasn't exactly the compressions fault, it was my fault by trying to make just decent power on pump at that compression level. It was on the ragged edge, and I pushed it too far. Lower compression motors tend to be much more forgiving. This is why Ford chose low compression as well for both the Cobra and GT500.



With that said, I'm not saying one is better than the other. As I said before, it depends on too many things for one answer to be more right than the other. I'm just offering my opinion on which way I went, and how it's working for me.



800rwh on 91 octane. Safe as could be. Good gas, bad gas, hot air, cold air, street driven, drag raced...It will do it all day long. On higher compression and less boost, on this motor, I don't think would be as forgiving. That's just my opinion though.

Kinda exactly what I was getting at. The bigger Whipples and KBs make that kind of boost no problem.

 

[05yellowgt]

Then again the 2013-2014 5.8 GT500 upped the compression to 9.5 to get the power they wanted. So Ford realized they could get better power, and likely better emissions due to a more efficient burn by upping the compression.

You can make big power with low compression and high boost and with high compression and lower boost. The big variables are quality of fuel you have available to you, the off boost driveability, the efficiency of the power adder at the lower and higher boost numbers (AIT, drive losses for blowers/backpressure for turbos)

 

[05yellowgt]

Kinda exactly what I was getting at. The bigger Whipples and KBs make that kind of boost no problem.

They can push that kind of flow (boost is just a measurement of restriction remember) but how much additional horsepower is needed to drive the blowers at higher speeds to give you that amount of boost? Combine that with the increase of AIT and you may come ahead, may break even, or may come out behind the same combo with higher boost and lower compression.

Not that it matters that much, but all other things being equal, a higher compression combo with larger pulley will get better mileage than a lower compression motor with smaller pulley

 

[Dubstep Shep]

Then again the 2013-2014 5.8 GT500 upped the compression to 9.5 to get the power they wanted. So Ford realized they could get better power, and likely better emissions due to a more efficient burn by upping the compression.

You can make big power with low compression and high boost and with high compression and lower boost. The big variables are quality of fuel you have available to you, the off boost driveability, the efficiency of the power adder at the lower and higher boost numbers (AIT, drive losses for blowers/backpressure for turbos)

Well said. There are many variables at play and no single answer. There are many different configurations that'll work

Engine compression increases will certainly lower emissions and increase efficiency.

 

[19COBRA93]

Then again the 2013-2014 5.8 GT500 upped the compression to 9.5 to get the power they wanted. So Ford realized they could get better power, and likely better emissions due to a more efficient burn by upping the compression.

You can make big power with low compression and high boost and with high compression and lower boost. The big variables are quality of fuel you have available to you, the off boost driveability, the efficiency of the power adder at the lower and higher boost numbers (AIT, drive losses for blowers/backpressure for turbos)

Let's keep the facts straight here. The 13/14 GT500's are 9.0:1, AND they added piston cooling jets. Those two things are kind of important.

 

[05yellowgt]

Sorry my memory fails me now than I'm out of my 20's!

 

[19COBRA93]

Not that it matters that much, but all other things being equal, a higher compression combo with larger pulley will get better mileage than a lower compression motor with smaller pulley

I'll agree with that all day long.

 

[Dubstep Shep]

They can push that kind of flow (boost is just a measurement of restriction remember) but how much additional horsepower is needed to drive the blowers at higher speeds to give you that amount of boost? Combine that with the increase of AIT and you may come ahead, may break even, or may come out behind the same combo with higher boost and lower compression.



Not that it matters that much, but all other things being equal, a higher compression combo with larger pulley will get better mileage than a lower compression motor with smaller pulley

Yep. Each combo had it's merits and balancing YOUR setup is the most important thing to good performance. There's no single right answer to any of this. Everything has a trade off.

And I completely agree about the mileage vs compression.

 

[Dubstep Shep]

I'll agree with that all day long.

Ditto

 

[05yellowgt]

Turbo's and Centri's probably benefit the most as far as street/strip setups go by having higher compression. Centri's don't make much boost at part throttle and low to mid RPM WOT and turbo's can be extremely laggy in the same scenarios with low compression. Upping the compression helps make up for the low rpm power outpout of Centris and help turbos spool more quickly.

Roots/hybrids/twin screws are a bit of a different animal, but I think they still benefit from higher compression, especially when you can run a larger blower and turn it more slowly. You can control the AIT's much more easily and still make great power throughout the powerband (gmitch's build is a prime example)


When you get into true race cars and the exotic fuels they can run all bets are off as far as the best way to make power, though a lot of how their combos are built are dictated by the rule book rather than what would make the best power and potentially make the motor live longer.

 

[Dubstep Shep]

Turbo's and Centri's probably benefit the most as far as street/strip setups go by having higher compression. Centri's don't make much boost at part throttle and low to mid RPM WOT and turbo's can be extremely laggy in the same scenarios with low compression. Upping the compression helps make up for the low rpm power outpout of Centris and help turbos spool more quickly.

Roots/hybrids/twin screws are a bit of a different animal, but I think they still benefit from higher compression, especially when you can run a larger blower and turn it more slowly. You can control the AIT's much more easily and still make great power throughout the powerband (gmitch's build is a prime example)


When you get into true race cars and the exotic fuels they can run all bets are off as far as the best way to make power, though a lot of how their combos are built are dictated by the rule book rather than what would make the best power and potentially make the motor live longer.

Another build to consider is CrownAviation. He was making crazy power with a 2013 TVS supercharger on a 8.5:1 CR motor. I wanna say like 750hp or something near that.