Compression ratio: What is the practical maximum?

[DDS]

I have set up my heads to run as high as 9.1:1 CR with a .040" (8.6 with .060") deck height, and a FAT442 cam.
I've read that 8.5:1 is a conservative max for a stock T4, yet other engines (even without knock sensors) run much higher than that on pump gas without problems.

What is the practical max. a street T4 can run on pump gas?
Does this become a function of the duration of the cam? (true versus static CR?)

I've read that deck heights are generally set at .040-.060" What is the maximum deck height one ought to run? Other than a lower CR, what problems are caused by excessive deck height?


TIA, Dave.

[Dave at Pelican Parts]

Compression and cam choice very definitely affect each other. With a wild lumpy cam, you can afford (and need!) a higher static compression ratio than with a mild cam. You can run higher compression when you use higher-octane fuel, so the people out on the east coast where they can get real 93 and 94 octane can run higher compression than we in California with our crappy 91.

Automotive engineering has come a long way since the mid-60s when the Type IV engine was designed. Flow theory, combustion theory, fuel mixture and spark control, materials, and on and on and on. Not to mention the whole water-cooling bit... The wonder isn't that we can't run the same compression as modern cars, but that we can get as close as we do!!

8.5:1 should be just fine with a stock cam on 91 octane fuel. 9:1 should be doable with 93 or 94 octane, and if you go to a lumpy cam grind you can get to 9.5:1 with the good fuel. Higher is possible, but depends on a whole lot of things.

--DD

[DDS]

Thanks for the solid info (as usual) Dave. I'm getting excited about this engine now!

[MASSIVE TYPE IV]

The cam makes a huge difference...it affects dynamic CR, not static..

8.5to 9:1 is very safe with a better cam and head work...8>3:1 is safe with mostly stock parts..

[DDS]

Thanks Jake, that's reassuring. It looks like I have some 'headroom' I elect to stroke this beast eventually too.

Based on your experience, do you have any hp estimates/ranges for a carbed (42DCNF) , street cammed (FAT442 .465/.280 - .240@.050" similar to Webcam 494), Ported big valve (44mmX38mm) 9:1 CR 2056cc like this?

Many TIA.

[Jim T]

I researched that when I rebuilt my stock 2.0, I went with 8.5 to 1.

Those heads crack up with even the stock compression ratio, Euro is 8.0, and I did not want to go too much higher, esp. given the gas that is available.