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Old 05-17-2010, 03:00 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by 34jeff
My street rod has a nearly stock (except mild cam) 305 Chevrolet carbureted engine with 350 turbo transmission in a 1932 Plymouth PB Coupe that is running hot at around 70 mph (approx. 2500 RPM). Rear axle is mid 70's Maverick with 2:73 gearing. We use electric fan (ON all the time) typical 50/50 antifreeze to water coolant, and am running a full, stock, hood. Radiator is Griffin alum. for street rod. Transmission has after market oil cooler. Was driving a few days ago in 70 - 75 degree weather when temp gauge reached about 230 - 240 degrees. Pulled off freeway and when I slowed down to 45 mph gauge returned to 180 - 190 degrees. Went back on freeway and kept speed around 60 mph and didn't seem to have temp problem all the way home. I have changed thermostats multiple times (current one is Stant 45356), flushed system, changed water pumps, and I don't know what is going on. Can anyone help figure this overheating problem out.
Thanks in advance.
34Jeff
The radiator needs to be sized for the engine, unfortunatly most old car shapes just don't provide enough space for enough radiator. A 305 is going to push into and aluminum radiator with two tube core of 1-1/4 tubes with a face area of about 430 square inches.

Most rods push the radiator right up to the front fairline of the radiator housing. You will note that modern cars set the radiator back a bit and box the inlet in on the sides. This is done to force a high pressure area to form over the face area of the radiator inside the radiator frame. When the radiator is in the classic old car position, the high pressure area forms ahead of the car and spills considerable air around the radiator frame and hood rather than thru the core. It was suggest that you put an air dam under the forward position of the engine to create a low presser area under the engine, I agree, this would help. I'd add to that the thought of bringing the radiator as far back in the nose cowl as possible and box it to the grill tightly to build a ram air box leading into the core.

The engine needs vacuum advance. It's eliminated for race cars because they don't cruise on partial throttle, therefore, don't need it. Street engines need it, especially where the engine has a lot of power against the resistance and weight it has to move. In this situation, the throttle is mostly closed and with the 2.73 gears the revs pretty low. Combine that with the 305's rather lazy combustion chamber, there just isn't enough advance. In this situation the actual charge density is quite low which results in long burn time. The burn doesn't go to completion and is still flaming when the exhaust valve opens which overheats the valve, and the exhaust port, which gets to be a cooing system problem. So at low RPMs and a nearly closed throttle the manifold has a lot of vacuum and the engine needs a lot of advance. So using a vacuum controlled advance is just the perfect solution for this situation. At low RPMs with a mostly closed throttle the vacuum advance pulls the timing up to get an early start on the slow burning mixture. As the throttle is opened and the RPMs come up, the mixture density increases and so does it's burn speed. But there is a hole in the burn speed increase up to about 3000 RPM. Here the vacuum is going down and the vacuum advance with it, but the RPMs are coming up so the centrifugal takes over and hits it's max around 3000, where nature takes over and continues to burn faster in almost perfect proportion to RPM till you get up around the horsepower peak where charge density starts to drop off as the cylinder just can't get a full breath in the time it has for the intake stroke.

So for a street driven engine, especially one that doesn't need much throttle opening and turns at modest cruise RPMs. A vacuum advance is a requirement to get the burn over with before the exhaust valve opens. You can soften the centrifugal springs to overcome this, but this usually over-advances the engine in the 2000-3000 RPM range and makes it really sensitive to fuel octane where more is better, but in many cases you just can't get that much octane, so the engine tends to be pingy.

Point ignition is way obsolete, again at low RPMs it has big problems with arcing due to the time of opportunity. Add to that the coil build up time is slow as the voltage these things operate at is just to low to shock the coil into getting on with business. I ditch it for an HEI, that overcomes many of the inductive ignitions problems of weak and slow spark. Or you can keep the points and take the power load off them and just use them to switch a CDI box like those from MSD. Pertronix, or Mallory. These things really boost coil output putting an end to miss and late fires typical with points and low density mixtures. this should do a lot to putting the heat of combustion into pushing the piston instead of cooking the coolant.


Bogie
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