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TR6 TR6 Centrifugal Advance

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vettedog72

Jedi Knight
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Is the TR6 centrifugal advance curve linear? I need spec’s such as when it should start the advance (in RPM’s) and if the curve is linear with increased RPM’s and what RPM should be full advance. Also, has this spec changed between the high compression TR6 and the later low compression motors?

Using a timing light, I set my advance with the motor running at about 800 RPM's (with the vacuumed blocked off) to 8* BTC. I now understand the timing is way off because the 8* setting should be a static setting and not with the engine running. I want to think I have a lot of potential for increased power when I get the timing corrected. I want to insure the mechanical advance is working optimally from low RPM's to the full advance spec.
 
There is a guy in the upper midwest who has a good rep for rebuilding dists. I have to dig forhis info but I bet if you get in touch with him he has all that info.I would like to have a copy of that graph.
 
Looking at my 30 year old Chilton's manual, they show static is 10 btdc and dynamic 4 atdc. The static and and running at idle with the vacuum hoses plugged is really the same thing. Back in the 60's timing lights weren't common so you rotated the engine to 10 btdc number 1 and connected a 12 volt test light to the points wire in the distributor. You loosened the pinch bolt on the distributor and rotated the distributor until the test light went on. That was the instant the points opened and fired the coil. Tighten the pinch bolt and you were done with static. If you wanted to know the advance curve you took the distributor out and put it in a distributor machine that let you spin it up and check the advance curves, vacuum and centrifugal.

The problem with using the test light is it didn't show all the play in the timing gears or chains and the gear drive to the distributor itself. With the timing light on an operating engine you can adjust all that out.

If you don't have an advance checking timing light you should probably get one. Then you could disconnect the vacuum hoses, rev the engine and plot the centrifugal curve at 500 rpm intervals. You should see about 10 to 15 degrees total centrifugal advance.

Then reinstall the hoses and do the same 500 rpm checks, probably about 35 to 40 degrees with everything hooked up.

You should have a dual diaphragm distributor on that engine. The reason they started using those was when the EPA regs hit in 68 the engines produced lots of emissions at idle speed. They found out if the butterfly in the carb was opened a bit, more air was flowing and mixed the fuel better and the emissions went down. To get the idle speed back down from 1300 rpm they installed the retard diaphragm. It was connected to the manifold vacuum which is strongest at idle. It pulled the advance plate around to get the 4 atdc spec I quoted.

If you don't have an advance checking light you can shade tree by measuring how far apart the tdc mark and the 10 btcd are. Say it's half an inch. 20 would be an inch from tdc, 30 an inch and half and so on. Put some nice white marks on the pulley and away you go.

If you weren't concerned with idle or starting the engine with the starter, you could pull all the advance stuff out and set the static timing to about 30 btdc and from 2000 to redline it would run fine. The centrifugal is there to allow the starter motor to crank the engine and have a sane idle speed. Go crank in 30 btdc and see if you can start it. No, don't, the engine will try to turn backwards and might break the starter. I had that happen with a customer's El Camino. He had a 396 in it and had left the distributor clamp loose and we noticed right away that something was wrong. He tried to start it, it back fired and broke the starter right off the engine. Starter bolts pulled out of the block and took a corner of the block with it. We ended up in Small Claims Court, he said we caused it. We won.

The vacuum advance is there to sense the load on the engine. When you're cruising at 2 or 3000 rpm the ported vacuum is strongest and advances the timing. At those engine speeds the mixture from the carb is pretty thin and it takes longer to burn. Starting the burn earlier compensates for that.

When you nail it and go to full throttle, the port vacuum falls off and the return spring in the vacuum advance pushes the advance plate back to the no advance position. You need to retard the timing at full throttle because the rich mixture burns quicker. You also produce more heat and heat means it's more likely to detonate or ping. Now all of this is done with sensors on the engine and the ECU decides what to do.

Hope this helps, Ken
 
Hello VD,

no the advance is not linear but in two steps as the mechanical advance uses two different strength springs.
There is quite a rapid advance to about 1,200 or so rpm then a slower up to maximum advance at about 4,000.
The steps do vary, quite a bit in some cases, but that is the general idea. The vacuum advance is added to that if the throttle is not wide open.

Alec
 
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