Maximum compression ratio at sea level

[BobDcuda]

What is the highest compression ratio that can safely be run WOT at sea level?  We're buttoning up the new engine for my salt racer, and the compression ratio pencils out to 13.72:1.  Running at Bonneville's altitude is no problem for that, but what is a safe limit for testing & tuning at sea level (Seattle)?  The engine is a Mopar slant six.

[rouse]

Depends on how good your head gaskets are. 13.5 should be fine with the right tuneup.

But I found a limit on just ho much nitrous you can get by with at sea level.

Rouse

[BobDcuda]

This would be NA and on race gas only, and just for initial tuning before trailering up and the trek to Wendover. 

[Sumner]

Your cam is going to play a big part as it can really effect the dynamic compression ratio, which is what the engine is going to see when running.

http://cochise.uia.net/pkelley2/DynamicCR.html

Here is a calculator.

http://www.wallaceracing.com/dynamic-cr.php

Sumner

[TheBaron]

It is possible to simulate Bonneville altitude when actually at sea level..

OK, here we go,,, every 1000 feet above sea level reduces atmospheric pressure one inch of Hg (mercury)...

I'd use 6000 feet for Bonneville because of the average day temps in August...

So, install a manifold vacuum gauge and run the motor up to 6" of Hg vacuum and no more.

Now with carburetors, this part throttle running will not open the power enrichment circuit and the A/F ratio will be wrong from full throttle running at Bonneville.

You can jet, at sea level, for the A/F ratio you plan to run at Bonneville and your Dyno numbers should be valid for Bonneville...

Good Luck on the salt!

Smitty

[wobblywalrus]

There are three compression ratios.  One is static and one is dynamic.  It is possible to use a long duration cam to lower the dynamic ratio to a reasonable level to run a relatively high static comp ratio at sea level.  This was done on BSA's and Triumphs in the 1970's.

The other ratio depends on how well the intake and exhaust system work.  These can use wave action to cram more or less mixture into the combustion chambers than either the static or dynamic ratios calculate.  Also, combustion chamber shape has an influence.  So, its hard to give an answer.

What we do is to use the same gas for dyno testing that is used for racing.  The mixture is adjusted on a dyno near Portland, Oregon to give max HP.  The timing is set to give max torque on the dyno.  A knock sensor is used during the tuning to detect any problems.  The engine is telling me what it wants.

Sometimes, after both timing and mixture are optimized, the knock light continues to blink.  This indicates the drive chain is loose or more octane is needed. 

All said, it is possible to run a pretty high static compression ratio and get good HP with high octane gasoline, a long cam, and an intake/exhaust system combo that does not work well at lower rpm.   

   

[BobDcuda]

Thanks all for your comments.  All good information.  Using the linked calculator (thank you Sumner), it says my dynamic compression ratio is 11.95:1.  This is quite a bit higher than what the other linked article recommended for "race engines, using high octane race gas" at around 9:1 DCR.  As for your point Wobbly, my intake is a dual carb/plenum, long runner design, and the combustion chambers have been welded to produce a sorta kidney shape chamber.  Which if I understand you correctly says my DCR is likely something more than the calculated 11.95:1.

[John Burk]

I have questions about this . The cam that has the lowest "dynamic compression ratio" is intended to aid cylinder filling at high rpm which would raise peak cylinder pressure .

[panic]

DCR decays in importance as engine speed increases, and is completely gone before you reach your peak torque.
Unless you need to use WOT at low speed, don't worry about it.
WRT max CR: there are about 5 more factors involved, engines differ significantly as to knock sensitivity.