Ask Away! with Jeff Smith: Cooling Down a Big Block Chevy Occasionally Running Over 220 Degrees F

Can you help me with a problem? I have a big block Chevy in my 1966 Chevelle and I need a little advice on cooling. The engine runs okay at 200-210 degrees F. which won’t kill me, but it will creep above 220 degrees at times and that scares me a bit.

I’m currently running a Griffin radiator with two electric fans.

J.P.

This reader and I traded emails and among the information I got from him was that his initial timing was set at roughly four degrees Before Top Dead Center (4° BTDC). This pointed me toward the solution that really has little to do with his cooling system but has everything to do with basic engine operation. The engine photo he sent shows the big block with an MSD ignition box, a big single plane intake manifold and a large Holley four barrel carburetor so without digging too deep I’m pretty sure that means this engine is equipped with a performance camshaft and perhaps a little higher compression than stock.  

All of these additions to the engine will want a little more timing at idle to improve the idle quality. This is important because street engines spend an overly large amount of time at idle. So optimizing the idle mixture, initial, and off-idle timing is critical to make this engine run cooler.

I get push-back from this next statement all the time—but it is nevertheless true.

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An engine idling with a too-rich mixture and insufficient ignition lead will run hotter than an engine running a too-lean air-fuel ratio.

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That sounds counter-intuitive but it’s true.

As an example of this point, a number of years ago I was doing a story on one of Kenny Duttweiler’s turbocharged small block engines. I asked him if we could do a glowing red header photo for my story and he agreed. He added a richer mixture and deliberately retarded the ignition timing. He pulled the handle under load and in moments the headers and exhaust housing on the turbo were glowing bright red. This occurred because the late timing created a situation where combustion continued to occur even past the exhaust valve and down into the header pipes and turbo housing. As soon as he put the timing back where it belonged and leaned the mixture to the proper ratio—the pipes never came close to glowing red.

First thing I’d recommend is to check your initial timing and advance curve. A good place to start is with 12 to 15 degrees initial and make sure the mechanical advance is all in by 3,000 rpm with roughly 36 degrees total. It also appears from the photo of your engine that the vacuum advance canister is not connected. Try hooking a vacuum line to ported manifold vacuum. This will definitely help the operating temperature although not at idle since the vacuum canister will only advance timing after the throttle is opened.

I’ve built several big block Chevys with performance cams and put nearly all of them at 15 degrees initial. An initial timing of less than 10 degrees is insufficient and could be the reason the engine runs warm. If you are concerned, set the initial at 10 degrees and do not run it at WOT but check to see if the engine runs cooler at part throttle. I think this will certainly help with the cooling—as will the addition of the vacuum advance.

Think about it: Vacuum advance only works at part throttle and that is when the engine needs the additional timing. Don’t be concerned if the timing reads 45 degrees (or more) at 3,000 rpm with the vacuum advance hooked up. This only occurs with very high manifold vacuum with very little cylinder filling due to a mostly closed throttle. If you add the initial and the vacuum advance and the engine rattles at a certain point, then reduce the amount of vacuum advance by using an adjustable vacuum canister, ACCEL (among others) sells one that works well.

You can also try slowing the rate of the mechanical advance slightly as this might also help. By adding stiffer advance springs, you will still have the same total advance—it will just come in at a higher rpm.

If after these changes the engine still runs warm, then the next suggestion would be to check the pulley ratio on the water pump because it appears your engine is running an aftermarket accessory drive. If the crank pulley is the same diameter as the water pump pulley, this is a 1:1 relationship. You might consider using a larger drive pulley on the crank to create an overdrive ratio to spin the water pump faster.

It’s an old urban myth that driving the water pump too fast will tend to overheat the engine. I’ve never found that to be true. But if the water is moving too slowly, it can affect cooling since the coolant is not moving as fast as it should.

Finally, are the electric fans covering the entire face of the radiator core? If not, then you are sacrificing cooling efficiency. The fans should be combined with a fan shroud that covers the entire radiator core surface. That will pull air in across the entire core, not just the area underneath the fans.

Just the addition of a shroud will make a huge difference. It’s possible that you may have to include all these recommendations—but if so, I can almost guarantee a cooler running engine.