68 - L79 Overheat Problem

[Mark H.]

Re: 68 - L79 Overheat Problem

Can the wrong vacuum can, fully deployed at idle and producing too much advance, also cause overheating?

[Duke W.]

Re: 68 - L79 Overheat Problem

You've got it backwards. You WANT the vacuum advance pulled to the limit at idle, so the vacuum required for max advance should be about 2" less than idle vacuum. The L-79 can (GM "236" or NAPA/Echlin VC1810) is speced at 0@4", 16@8". Since the L-79 pulls about 14" at idle, full vacuum advance is locked in at idle.

This can is also used on 30-30 cam engines, which only pull 10" at idle, and I recommend it be retrofitted to '63 SHP engines.

Total idle timing for SHP/FI engines should be in the range of 26-32 degrees.
This minimizes EGT and heat rejection to the cooling system, which means the engine is operating on minimum fuel flow.

Duke

[Michael H.]

Re: Here We Go Again....

Here we go again. Duke, I strongly disagree with your recommendations of using a 236 on any 1963 engine. I know that you're going to respond to this by saying you have a 236 on the 63 that you bought new but here are the facts.

As you well know, the 236 is designed to operate off a totally different system. I agree that the 236 would supply full vac adv at idle to help minimize chamber temp's etc but the negative side of all this is the problem encountered at part throttle. While the correct 201 adv unit for 63 was designed with a stronger spring, the weaker spring of the 236 would have the timing at near peak at part throttle, when the vacuum signal would be the highest. In a direct manifold vacuum source application, vacuum is only at it's highest with the throttle closed but with "above the throttle plate" vacuum source, the high readings come in at a time that the timing should be dropping, not advancing. These are two totally different vacuum advance units and are to be used with two totally different vacuum sources. I think you would be ahead by keeping the correct 201 and switching to direct manifold vacuum. You would then have the full vac adv at idle but at critical part throttle, it would relax much earlier than the 236 would. Running about 55 MPH up a slight incline at part throttle, the vacuum would probably not drop below the 8" range of the 236 so timing would be at it's maximum. With the correct 201, the can would relax and drop the timing. I think all of this is even more important with todays fuel. You certainly don't need more timing at the critical RPM/load areas than was originally designed in when regular gas was 98 octane.

Let me know what your thoughts are on this. I may be missing something here.

Michael

[Duke W.]

Re: Here We Go Again....

With the original vacuum can on my '63 L-76 idle stability was a constant problem because the Duntov cam didn't pull enough idle vacuum (about 12")to keep the plunger at full advance. The normal idle lope would cause the advance to back off, which reduced idle speed, which lost vacuum causing more speed loss until the engine was barely running. With FI it may be a mute point if it has ported vacuum advance. Why Cheverolet did this I have no idea, but beginning in '64 ALL SHP/FI engine were set up with the 236 can and full time vacuum advance. This did not change because they finally got it right.

I also set up my '63 L-76 with the '64-'65 mechanical lifter cam centrifugal advance, which is all in by 2350. Thus, cruising the total advance was on the order of 54 degrees. Car got 22 MPG (3.08 axle) on the road and didn't detonate on leaded premium. I might have to slow down the centrifugal a bit on today's fuel unless I want to mix it with race or avgas.

I recommend the 236 can and quick advance as a starting point for all SHP/FI engines and then slow the mechanical or back off on initial timing from there if the engine detonates. This applies to all mechanical lifter cams and the L-79 cam. Though L-79s use the 236 can, they have a slower centrifugal curve than the 30-30 cam engines. Because the L-79 cam closes the inlet valve a little earlier it has higher dynamic compression. The Duntov cam closes the inlet valve at about the same point as L-79. The LT-1 inlet closing is later, and later still with the 30-30.

There is also a 12" can, which is used on SHP big blocks, and this can be a tuning option on a SHP SB, particularly L-79 since it pulls about 14" at idle. The Duntov and LT-1 cam pull about 12", 30-30 cam, 10".

These engine need LOTS of TOTAL idle timing (26-32 degrees) because the overlap causes a lot of exhaust gas dilution, and this slows flame propagation speed. This is compensated by lots of advance - relatively high initial and full vacuum advance at idle.

Duke

[Michael H.]

Re: I agree

I thought you were saying use the 64-65 236 with all else remaining the same for 63. I know exactly what you mean about the diminishing idle speed because of timing, or diminishing timing because of idle speed. (what came first, the timing or the idle speed) Amazingly, an engine can run at 800 RPM for a long time with no variation and suddenly drop off to 550 because of that very problem.

I've also used the 12 deg big block vac adv that you suggested on a several small block cars and that works well too.

By the way, in the early 70's, I drove a 63 FI conv with 4.11 from Chicago to Florida and was getting roughly 28 MPG on the road. This was in the beginning of the gas crunch and the speed limit was 55 so that may have had a lot to do with it but I was still quite impressed.
The car was a Daytona blue conv., #20031 and if anyone has that car today, I MIGHT know where your original block is.

[Mark H.]

Re: 68 - L79 Overheat Problem

I'm wondering if there is such a thing as too much vacuum advance, (can causes the engine to advance further than specs call for at idle) and if in an over-advanced state overheating occurs. Practically it would be the same situation as a proper can that is poorly timed with too much base timing advance. Is overheating a symptom of that situation?

[Richard D.]

Re: 68 - L79 Overheat Problem

Jim:

This is the response I got from the Mallory help desk.

Solution:
With this distributor you need to set the base timing at idle to 12 deg's BTDC. This should give you atotal of 36 degrees after 3200 rpm. Unless the mechanical advance is broken of not fully advancing, tghis should be fine for your motor.

I have followed up with them to ask what in the distr. compensates for the lack of the 16 degrees of advance that a stock vacuum advance gives at idle.

This may help your friend out, and I will post the further info I get from them when it comes in.

Rich

[Jim T.]

Re: 68 - L79 Overheat Problem

Richard I told my co-worker when we were discussing his heating problem at idle that he would benefit by installing the original type tach drive distributor with vacuum advance and also improve economy. Removing the valve that closes off the flow to your heater core is going to warm up the interior of the car for summer driving, I'm sure someone installed it to help keep the heat out. Hot coolant circulates through the heater core all the time otherwise.

[Michael H.]

Re: 68 - L79 Overheat Problem

Mark,

That's a good question. I've never actually tried it to see at idle speed only but I suppose it could be responsible for some rise in temp but if we're just talking about idle conditions, I don't think it would be a major factor. I don't think a vacuum advance unit, even an aftermarket, would ever be off more than a few degrees and that's probably not enough to cause a problem. Off idle and any load condition would be a totally different story.

Michael

[Duke W.]

Re: 68 - L79 Overheat Problem

I've never run across a situation where an engine overheated because of too much advance, but overheating due to too LITTLE advance is common. If the ignition map - initial, centrifugal, and vacuum advance - is properly tailored to the engine, overheating will not be an issue if the cooling system is in reasonably good condition.

Selecting a vacuum advance first requires that you know the idle speed vacuum with 20-30 degrees of advance. You can check for this by just advancing the distributor to about 25 with the vacuum can disconnected and then take a idle vacuum reading.

Select a vacuum can that provides full advance at 2" or more LOWER than idle vacuum. Thus, with the 30-30 cam and 10" of idle vacuum you need an 8" can. Even though L-79s idle at 14" GM used the 8" can and they run fine.

Select an initial timing to yield 36-38 degrees, sum of initial plus full centrifugal.

Last, get the centrifugal in as quick as possible. The limiting factor will usually be low speed detonation. Because of the low dynamic compression ratio generated by the 30-30 cam, GM got it all in at 2350, and they are usually conservative. It also worked well with the Duntov cam, so it should also work well with the LT-1 cam. Not sure about the L-79 cam, so you L-79 guys will just have to try quicker centrifugal curves if you want to find the ideal setup.

Duke