Best 275 4Cam Distributor Curve

This is a confusing subject. 4 Cams were built with both S85A 15 degree and S85E 9 degree distributors. Check the plate on the side of your's to see what you have. Also, the flywheels are different, one marked AF10 for the S85A and one marked AF24 for the S85E. I have seen cars with the flywheels and distributors mixed up and they run very poorly. Both setups work, but overall, the S85A combination gives the smoothest performance. We used to get complaints about what we called "4 Cam leap", where the car would bog off the line and then suddenly take off as the rpm came up. Switching to S85A distributors and initial timing settings usually cured this problem.

 

you need to determine which distributor is in the car first.

 

Standard S85A advance curve (as shown in the 275 manual) is fine.

 

Dyke, I am curious as to how closely you'd match the left/right distributor curves. Thanks in advance.

john

 

John: The factory was usually happy if the curves were within a couple of distributor degrees of each other. My friend Dave North has really gotten into these distributors and gets them almost EXACTLY together all the way up the rev range. I have gotten lazy, and let him set them up for me now.

 

Get the engine or car on the dyno and find the optimum timing/advance to suit the best (and most consistent) fuel you can now run on. Then get your distributors modified (if necessary) to produce the right advance curve and reinstall them at what you've determined to be the correct static timing.
Obviously check everything else is right first and get the jetting spot on too whilst doing the dyno test.
Why on earth not do this on a multi million dollar car?

 

Thanks Dyke. Your friend can probably do it quickly by now, but it does take a long time to get them perfectly matched.

john

 

John: No, it takes everybody a long time. No shortcuts to get good results. He has made sleeves and various shims to alter the advance curve in very small increments.

 

Dyke, can his mods take the full advance beyond 15 degrees? By how much?

john

 

John: Yes, you can go beyond 15 degrees, but there is no reason to do that. On the dyno, the engines always make the most power at 38-41 degrees total advance (inside plug motors like 41, outside plug like 38 degrees). If anything, you might want less centrifugal advance and more static advance for better low end response. However, with more static advance comes more load on the starter on hot starts. I think the factory got it right the first time.

 

I have found that even a variation of 0.25 N/mm on the spring rating can throw the curve right off! (on the weaker longer spring).

 

Raising spring tension can be done by dropping shims inside the sleeves, but I'd be interested in knowing how Dyke's friend lowers tension.

john