1967 Corvette L71 Running Hot

[Joe T.]

Re: 1967 Corvette L71 Running Hot

Just my opinion and a little of experience. If your motor is pure stock, didn't have a rebuild, low miles, not using a higher lift cam, no over bore and so on. The factory settings will work very well. As i said further up in this thread, I had to go to a B28 can and light springs along with the timing because my motor isn't PURE stock. So, my point is, what works best for your motor and heat range. The settings and parts you use will be different. I hope your problem is resolved. Let us know your outcome.

[Charles F.]

[Charles F.]

[Larry M.]

Re: 1967 Corvette L71 Running Hot

I should mention that I missed the part Duke said about hitting between 36-40 with VAC not attached. I had the VAC attached with the new setup reading of 24 at 900 RPM idle then upwards of 50 at 4000+ RPM’s. Is that normal or am I getting to much advance at high RPM’s?

Charles:

I will say this for the third and (my) final time. Measure your centrifugal advance curve at 750, 1000, 2000,3000, and 4000. RPM. Vacuum can disconnected and line plugged. Only then can anyone advise you.

At 4000 RPM and no significant engine load (like testing it in the driveway) the vacuum advance is still likely active and adding timing advance to the engine. So with the vacuum advance hooked up, yes , 50 degrees is possible. But again, this is at essentially no engine load. Detonation should not be a problem under this condition.

WOT is just that...... WIDE OPEN THROTTLE. Your carbs (all 3) are wide open and the engine is producing its maximum horsepower........like a 1/4 mile drag race with your foot clean to the floor on the gas pedal. In this case, engine vacuum is very, very low and the vacuum advance contribution goes to ZERO. You cannot measure engine advance at WOT with a timing light, except perhaps on a chassis dyno. It can only be assumed from your initial and centrifugal advance curve and settings, and by testing and verifying that the vacuum advance actually does go to zero advance at 1-2 inches HG vacuum.

I.....or Duke.......or anyone......cannot help you piecemeal wise. You need to take the time and do the complete homework assignment. Then and only then can someone advise further. As Joe T has already mentioned, all engines are a bit different depending on what work has been done to them. So we need to know what your car distributor curve actually is?? Factory stock?? Modified?? By how much??

Also, what vacuum can did you install?? Provide it's ID number so we can see what you have.

Larry

EDIT: If you have not already done so, get and read Duke's and Lar's Technical Papers and presentation on this subject. That may clear up some of the questions and reasons for doing certain things. I have personally done this and it has helped me a lot.......even though I have been wrenching on these cars for over 50 years.

[Charles F.]

[Joe T.]

Re: 1967 Corvette L71 Running Hot

Charles: Sounds like you are getting some really good advice from some really experienced individuals like Duke and Larry. You are headed in the right direction. Glad your temperature is coming down.

Joe T.

[Duke W.]

Re: 1967 Corvette L71 Running Hot

So I set the initial timing at 10 and used the B26 vacuum can. I now have around 24 degrees BTDC at 900RPM. The car at idle parked for a while hits about 210 but now cruises between 180-200 and cools down whereas before it could not recover. At WOT and at about 4000 RPM’s I am surpassing 40 degrees, maybe upwards of 46-50. Does that mean I need heavier springs to bring the limit down to 40? The car smells richer and seems to have less get up and go as opposed to the ported config but the hot motor issue was killing me, so if that is a trade-off and I can live with it. Any advice on how to bring the full degrees of 46-50 down at WOT?

As has been repeatedly stated, WOT advance has to be checked with the VAC signal line, disconnected and plugged and set a few hundred RPM ABOVE the point that the centrifugal is all in that you established by testing. The maximum OE 30 deg. centrifugal is all in at 3800, so total WOT advance must be set at something over 4000 and then blip the throttle and check that advance does not increase, if the centrifugal is still OE, but you must test first as stated above to verify, or, if modified set WOT advance a few hundred revs above the engine speed that centrifugal is all in.

Say you set it at 40, which would be about 10 initial. Then with the B26 VAC connected "total idle advance" with full time vacuum advance should be about 8 + 16 = 24, and "total cruise timing" which is above the point of maximum centrifugal at light load should be about 8 + 30 + 16 = 54. If there is any detonation reduce total WOT advance in two degree increments until the detonation goes away.

You still haven't told us for sure at what RPM centrifugal is all in. That's important to reveal, so we can make a judgment if you are following the proper procedure.

Duke

[Dan D.]

Re: 1967 Corvette L71 Running Hot

Charles, you can duplicate WOT in the garage by removing and plugging the vacuum canister hose, and then reviving the engine until the centrifugal is all the way in.

You can do this using your dial back timing light, and you can also map your advance curve this way.

This is how the advance system operates:

Cranking - Initial only

Idle: - Initial + vacuum

Cruise - Initial + vacuum + plus centrifical

WOT - Initial + centrifical

-Dan-

[Larry M.]

Re: 1967 Corvette L71 Running Hot

Charles, you can duplicate WOT in the garage by removing and plugging the vacuum canister hose, and then reviving the engine until the centrifugal is all the way in.

You can do this using your dial back timing light, and you can also map your advance curve this way.

This is how the advance system operates:

Cranking - Initial only

Idle: - Initial + vacuum

Cruise - Initial + vacuum + plus centrifical

WOT - Initial + centrifical

-Dan-

Agree with above.

Also agree that you can "DUPLICATE" WOT in your garage, BUT to do it correctly in my opinion, you must also verify exactly where in "inches HG" the vacuum can will completely drop out. This is easily done, but it must be done to make certain everything is working as it should. I test the can at both ends of the spec to make certain the can is performing as it should. A Mighty-Vac is a good tool to use for this.

To me WOT is just that......Wide Open Throttle.........it can be duplicated at home, but you need to do all the steps.

FWIW.

Larry

[Dan D.]

Re: 1967 Corvette L71 Running Hot

I need to clarify one thing about my previous post defining advance. Idle is initial + vacuum as long as the vacuum is full manifold vacuum. If it is port vacuum ,then idle is just initial.

-Dan-

[Duke W.]

Re: 1967 Corvette L71 Running Hot

Correct. If ported vacuum advance, total idle advance should be initial only, but may included a few degrees centrifugal if centrifugal starts below idle speed.


Total idle advance = initial plus vacuum advance (if any) plus, maybe, a few degrees centrifugal if it starts below idle speed, like on 30-30 cam engines.

Total WOT advance = initial plus full centrifugal, no vacuum advance.

Max cruise advance = initial advance + full centrifugal advance + full vacuum advance. This may never be achieved on some OE engines because full centrifugal may not be in until up to 5100 RPM.

For best idle quality and lowest idle fuel consumption, total idle advance should be in the range of low 20s to low 30s, and the biggest variable is valve overlap - low overlap cams in the low to mid range and high overlap cams in the mid upper range.

Ported vacuum advance is an emission control strategy. By having less than optimum total idle advance EGT increases, which helps oxidized HC and CO, but it also transfers a lot more heat to the cooling system as the exhaust traverses the exhaust port. This is why ported vacuum advance engines tend to run hotter in low speed stop and go traffic.

All of this is explained in great detail in my tuning seminar.

Duke

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Ok. Got a good timing light. At idle with no vacuum and plugged port, 10 advanced initial. At 1000RPM 10, 2000 RPM 24, 3000RPM 30, 4000RPM 36, it appears to top out at 36 even with throttle blip. What is my next move seeing how most people try to keep to 40? Lower initial to 4 to give me 18 idle with manifold vac? (Idle with VAC attached and 10 initial totals 24 )

[Charles F.]

Re: 1967 Corvette L71 Running Hot

Nevermind, I am breathing too many gas fumes. 10 initial plus centrifugal at 4000 is giving me 36 total with vacuum detached. I guess I am within the 40 total? Should I add 4 more to initial to get to 40 total 14 initial plus 36 centrifugal?

[Larry M.]

Re: 1967 Corvette L71 Running Hot

Congratulations..............

Most BB like around 36-38 degrees total mechanical advance (no vacuum). I would simply stop where you are for now. Drive the car a bit and see how it behaves and if you hear any detonation when accelerating on the road. You need to establish exactly how the new vacuum can is working with your car.

For BB cars, 40 BTDC total mechanical advance is pushing the limit.......even with high octane gas. If you always run special leaded racing gas, 40 degrees may be fine. But on 93 octane Shell or Exxon etc 40 degrees may be too much.

The difference in noticeable horsepower will be small.

Your initial advance of 10 degrees may have a little bit of centrifugal advance built into it, as the factory weights for your car start to bring in the centrifugal advance in around 900 RPM........and your lowest reported RPM was 1000 RPM.

Now take a mighty-vac and make sure the vacuum can does not begin to add any advance until the vacuum is about 5-7 inches HG. This actually represents the point where the vacuum advance timing will drop to zero when you are racing or reving your engine for maximum power (WOT) IT MUST DROP OUT TO ZERO around the 5-7 in HG for you to keep from having periodic detonation when cruising and playing with your car.

If after a few weeks or a month and you feel all is fine, then you can bump the total mechanical timing to 38 degrees. If after another month you feel more timing is needed........and you feel lucky..........then go to 40 degrees total mechanical advance. But this level or above, you risk detonation problems with your engine...........so don't be greedy.

My small block SHP car calls for 40 degrees and I am running 36 degrees max with a 50/50 mix of 93 octane no-ethanol gas and 110 octane leaded racing gas in the tank. No sense in taking chances at my age. I might be on road trip and need to fill-up at a gas station with 93 octane max gas, so I do not want to be on the rough edge regarding gas octane number and total timing. Take this "for what its worth". At may age, and with my car running factory side pipes, I can't hear any detonation that may be occurring.

Larry

EDIT: The factory timing for your car when new was 35 BTDC maximum mechanical advance (initial + centrifugal).......all in at 3800 RPM. So you are essentially at this point now..........but you have an improved vacuum advance can and strategy. So try things out and see how it goes.

[Charles F.]