True Compression L-71

[Gary Noser]

I am wondering about the true compression of my engine. The heads are #3919840, It has comp. cam (cca-11-218-4) (Int./Exh.) 236/236; lift- (Int/Exh) 0.561”/0.561”. The pistons are TRW-L2268F60.(60 over) Bore= 4.310" stroke= 3.76"

I know the pistons claim to be 11.25:1 compression, but there are a lot of factors besides just the pistons, correct? (Head cc etc.) Isn’t this cam tamer than the original?

I really just want to know if there is any hope of getting this car to run on pump gas (93, non-oxygenated) without run-on (I do not hear any knocks while running).

Thanks for the Help,

Gary

[William C.]

Re: True Compression L-71

If you are having problems with run-on, I would ask if you vacuum advance system is functional and runninf from manifold vacuum. Excessive throttle opening at idle due to lack of advance was usually the culprit with BB run-on in the '60s

[Dave Stafford]

Re: True Compression L-71

Gary, if I remember right, you do not have vacuum advance, so if your distributor is set up for only mechanical advance, set your timing about 10 degrees and see what you get.

Dave

[Gary Noser]

Re: True Compression L-71

Dave,

You are right I do not have vacuum advance. It is a complete MSD ignition. Is it possible to change the springs in the distributor to lighter ones. It did come with a bag of springs. I do not have that clear of an understanding of all this. If I set my timming to 10 won't that affect the engines ability run on pump gas? It was at one time set at 12 deg and still ran-on.

Thanks,
Gary

[William C.]

Re: True Compression L-71

Street engines are set up to idle at about 30 degrees of timing, initial plus the vacuum advance. On a bigblock this allows a greater degree of throttle closure at idle than you can obtain without vacuum advance. On street applications using ported vacuum, we (GM) used a throttle solenoid that extended when the engine ran and retracted with the engine off to allow the throttle to close more completely. This was specifically added to the bigblock stickshift engines to deal with run-on. Set the engine back up the way it was designed and you will eliminate the problem, ie- install the proper vacuum advance system and readjust the idle speed as low as possible using good initial timing and full vac. advance.

[Clem Z.]

a quick fix for run on

leave the trans in gear and let out the clutch as you shut off the ignition.

[Michael H.]

Hey Duke, can you "splain" better?

Gary,

As long as all we have to work with is todays fuel, I'm not sure if you will ever eliminate the run-on problem. These engines didn't have that problem nearly as much when they were new and there was good fuel available but with today's 93 octane, you sometimes have to shut down the engine by leaving it in gear and letting the clutch out just enough to slightly load the engine while turning off the key.

As far as your timing is concerned, you're going to have to get some idea of what you have for a total advance. In your previous post, I recommended a max of 4 to 6 degrees initial which would have given you about 36 total but at that time, I asumed that you had the correct original Delco distributor for the car. The MSD that you have would probably be a completely different story as far as total "in distributor" advance is concerned so you will have to start with the total and work backwards. You're going to need a timing tape on the balancer or at least a dial back timing light to find the total at high RPM. Because the MSD doesn't have a vacuum advance unit to pick up the timing at idle, you will have to start at a higher than normal initial timing setting, possibly around 10 or 12 degrees. However, it is critically important that the total not exceed the 36-38 degree range on the street with trash fuel. If the initial timing is set near this figure, I wouldn't use real soft springs on the advance as the timing would probably be to high for low RPM part throttle operation.

Hopefully, Duke will see this and explain it more clearly for you. He's a lot better at getting it in print than I am.

Michael

[Terry M.]

Run-on was a problem back in the day

Not to drag this off topic, but 1971 BB AC Corvettes (and that drive train in some other carlines) had a system to engage the AC clutch on engine shut-down to place a load on the engine to prevent run-on. Thankfully a one year only feature.
But it was a problem when they were new, and the engineers were trying to fix it back then.

[Michael H.]

Re: Run-on was a problem back in the day

Absolutely correct. And all the big car engines had that same problem all through the 60's and early 70's. Buick, Olds, Cadillac.... all of em, and that was with only 10-1 CR. I remember my neighbor with his big Electra coming home from work every day and listening to his car run-on for at least 5 minutes after he was already in the house. One day, a tow truck hauled it away because a piston fizzled.