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Understanding Compression Ratio(s) And Forced Induction

6K views 21 replies 10 participants last post by  LSG 
#1 ·
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Hello,

I am building a 408 stroker with EFI and timing retard. I plan to run 93 octane fuel and ~8psi of boost.

I am trying to figure out what compression ratio I should run. I see others with a similar setup running 9-10:1 static compression and running 8-15psi of boost. However, after doing tons of reading, are you not supposed to go over 12:1 expected compression ratio?

Please see this chart and the note below:


What am I missing?

Thanks
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G
#2 ·
With real CR above 12:1, you are going to have detonation and preignition issues without running race gas. That can be effected by the A/F ratio some, and water injection and other band-aides . . but the end result can be a problem. Is this a supercharger or turbocharger?
 
#3 · (Edited)
I am running 9:1 static CR on a supercharged 351. I max out at about 10 psi boost. I looked at the same tables you are looking at when I put my engine together. I have not experienced any detonation problems. I always run 93 octane gas. My supercharger has a water/air intercooler in the intake manifold, I start pulling ignition timing as soon as I get into boost and I richen up the mixture as soon as I get into boost. I think if you have an effective timing retard strategy and make sure you don't go lean, you should be fine at 9:1 and 8 psi.

Baxter
 
#7 ·
I wasn't planning on doing an intercooler because of the pain on plumbing it. Could I get away with running 9:1 CR supercharged with ~8psi of boost? I am running a tunable EFI that can manage the ignition system or I can use and MSD box to do it. EFI has a 2.5bar MAP sensor and is designed for blow through.

All is crazy to me. Some say "yes you can run high compression no problem" while others say "don't do it per the #s from that math."

Got any pics of your water/air intercooler setup?
 
#4 ·
There's also the school of thought to run a high CR with low boost. It's suppose to retain low end torque, good throttle response with good power. With the lower boost you have less mechanical heat induced into the inlet track. With this method the engine is already starting off making good power and SC builds upon it. The 3.5 Ecoboost has 11.1 CR with 16 pounds of boost and uses 87 octane. But truthfully that's a loaded reply as it's a whole different SC system. With the DI there is no fuel during compression stroke to spontaneously ignite and during injection of the fuel going from a high pressure to low pressure comparatively speaking, vaporized the fuel better as well as cooling the compressed air. It's a very detonation resistant design. At the end of the day it's the net improvement in power no matter what route you take.
 
#5 ·
It also depends upon if you're intercooled as you can run more boost with a cooler intake temperature.

On my 03 Cobra with a stock compression ratio of 8.5 to 1 and intercooled I ran 17psi on 91 pump gas. Now that I'm on e85 I run between 21-23 psi regularly and the car does it with no issue.
 
#13 ·
Don't know this information will help the OP or not, but with 6.0 lbs boost from a vintage Paxton I had zero detonation issues. And was using the stock HiPo compression Pistons that gave me a measured 10.0:1 CR.

I did however reduce the total ignition timing from 40 degrees BTDC to 36 degrees BTDC.

Of course this was on a 289 wih vintage HiPo heads. A 408 might act differtly

Z
 
#20 ·
Back to the OP, here is the simplified formula for determining what the true boosted Compression Ratio is:

Original Compression ratio (OCR) + (OCR x Boost / 14.7)

So if the OCR is 8.0 and the boost is 6 psi

8.0 + (8.0 x (6/14.7))

or

8.0 + (8.0 x .41)

or

8.0 + 3.2

=

11.3

Which is the same value as the table listed.

This is based on "sea level" pressures 14.7 Pounds per Square Inch. It is also assumed a "normal" day, not in the middle of a low or high barometric pressure day.

So, if you build your motor in Denver, in the winter, in the middle of a snowstorm (low pressure + thin air due to elevation), and take your car, as is, to the beach in Florida on a gorgeous summer day, and go max boost, bring along an engine diaper to pick up all the parts.

This is why boosts on planes varies based on altitude. The higher the plane, the less air (base pressure) there is to work with, so more boost just brings you back to a lower elevation performance.
 
#21 ·
I had a measured CR on my '66 GT350 with a HiPo 289 engine of 10.0:1 and had a maximum boost of 6 psi. I don't recall what thickness of head gasket the machine shop used to get that CR.

Using the posted formula, my adjusted CR on my GT350 with vintage Paxton was a hair over 14.0:1 @ maximum boost. Didn't have any detonation issues, but I did blow a head gasket into the water jacket early on with my first Paxton. After that it was smooth sailing.


Z
 
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