Compression Ratio/Cam/Gas Choices - Questions

What has been your experience with cam/compression ratio/and type of gas? I am looking for recomendations.

We are building a performing daily driver 200 with OZ head, Holley 4V (350 cfm), duraspark/MSD6al, T5, and headers. We will be putting in a cam (probably in the 270 range). With this cam and running pump gas - hopefully regular - what is the max compression ratio we should shoot for. To provide an out, if the compression turns out to be too great, we are planning the build toward 87 octane gas and a 0.045 head gasket so we will be able to lower the compression with a 0.055 gasket and if need be go to higher octane. We are running at sea level in hot and humid north Florida.

To kick off this discussion, with the stock cam Falcon Performance Handbook's rule of thumb recommends the compression ratio be limited to the octane of the gas. This is in keeping with the original design of the car: 9.2:1 compression ratio back in the day when regular gas was 92 octane.

Clifford reports that with a performance cam (not specified) they run 10.0:1 on pump gas - because of the cam.

So what has worked?

Thanks, Dave

I'm running a cliffy 272h cam with 10:1 compression (block decked .040/2.3 hsc pistons/head milled.060/steel shim gasket) with a Holley water injection unit.

It runs fine (no pinging) up to around 90 degrees....anything higher than that temperature in the summer, requires premium fuel ( I live in Indiana though) so it might not work as well for you in Florida where I'm sure it gets hotter!

I like the water injection! You can find some good fairly inexpensive setups on ebay every now and then (you might want to check out the topic about water injection in the hardcore forum) Usually I have to fill it with water or a water/alcohol mix every other fuel fillup, so its not that troublesome to me.

You might want to try to keep the compression ratio closer to 9.5:1 so you dont have as much trouble with detonation, but another thing to keep in mind is I'm not sure how a 270/272/280 cam will work with the lower compression ratio....you might lose some of the bottom end with that cam and lower compression??

Later,

Doug

Howdy back DaveLS:

The best advice is to run only as much compression as you want to have to manage. The higher it goes, the more you'll have to do to manage it. Doug can certainly tell you about that.

That said, and properly done, run as much CR as you can.

You may want to reread the piece on page 19 of the latest edition of the FPH, "High Compression VS Pre-ignition", for tips on avoiding "Knock".

The difference in cylinder pressure between a 260 and a 270 cam will be minimal. The difference between these cams and a stock cam will be very noticeable. For a daily driver, I'd go with the 260/264 duration cam and give up a little less low end.

The OZ head may help with equalizing fuel distribution. I do not know what other advantages it may offer in controlling pre-ignition.

If you have done all you can to safely run higher than stock CR (polished chambers, quench...), and if you have a standard trans with lower rear gearing, (no lugging), And if you are willing to buy a tank of 92 octane gas, now and then, in the heat of the summer, and/or retard your initial advance setting a degree or two, and if you have an effective cooling system and a cold air intake for induction, you should be able to safely run CR as high as 9.5:1.

If any of the above descriptors are not in place reduce that figure by a 1/10th for each missing factor. A good, safe goal CR, in your case, might be 9.2:1.

That's my guess-ti-mate. What do you think.

Adios, David

David and Doug,

Thanks for the info.

In fact I re-read page 19 and re-visited Doug's web site before posting.

The new wrinkle to me was the effect of cam on max CR, and the need for increased CR with longer cams.

Your answers were exactly what I needed.

We have a 4cyl T5 and a 3.20 rear end, so our overall first gear is over 12.

The Plan: 9.5 compression with 260/264. Water injection, thicker head gasket, and higher octane will be the fall back.

Thanks Dave