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.
Sounds like at higher speeds it is unable to get hot air out of the engine compartment fast enough. You could try a chin spoiler under the bottom of the radiator/core support like modern cars have(usually rubber), this creates a low pressure area behind the spoiler helping to draw the hot air that has come through the radiator out the bottom of the compartment.
Are you running a vacuum advance on the distributor?? Vacuum advance creates cooler running at cruise speeds.
Thanks for the advice. My 305 is using an Accel points distributor that does NOT have vacuum advance. Also, the front of the coupe is pretty low to ground so I'm not sure there would be room for a spoiler but I will check. I am also thinking about adding heat control wrapping to headers to try and keep heat out of engine compartment. I would appreciate hearing from any one with experience using exhaust wrap, (it's like tape you wrap around header pipes). Does it really reduce underhood temps?
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.
Here's the steps:
1) Be sure to have a wire inside of the lower radiator hose so it won't collapse at RPM. Wrap a coat hanger around a broom handle and make one if you need to.
2) Check the temp gauge calibration
3) Be sure your radiator cap will actually hold 16 psi.
4) Drive it with the sides or all the hood off and see if the problem continues.
5) Be sure that your water pump is at least overdriven, shoot for 120%. SBC have horrible hot spots in the head passages so you must shove a bunch of water through them to stop the steam pockets. You cannot shove water through a SBC head too fast. Velocity/water pressure through the heads is extremely important. High flow thermostat strongly recommended.
6) Does the fan shroud restrict ram air through the radiator? Drive it without the shroud if needs be.
7) Definitely install a properly calibrated vacuum advance. It will probably increase your gas mileage 2 mpg or more, and improve part throttle response, as well as make the engine run cooler, as stated above.
8) Any decent system with 50/50 antifreeze should cool without any "magic from a can" coolant additive.
9) Stop the electric fan from running above 35 mph. You should not need any fan above 35. The wind will make it windmill anyway.
10) High flow thermostat strongly recommended.
geez they wiped out my post total.
Wow! you guys have really impressed me with your knowledge. I will be getting an HEI distributor with vacuum advance shortly and try that. If no improvement I'll remove hood sides and try that. The electric fan I am currently running does not have a shroud and I have about 1/2 inch clearance from the front of the water pump to fan. I know if I had built the car originally things would be a lot different but I'm trying to make do with what I have.
Thanks to everyone for your ideas and comments. I will let you know how it works out.
If it is ok at 60 and not at 70 you most likely have an air flow through the radiator issue. If you park the car and run it at 2500 rpm does it heat up with the fan on. Air flow or an under sized radiator can exhibit the issues you have described. Timing as well but it would not be the first place I would look. You can eliminate it being timing by following F-Birds advise and it wont cost you a dime.
Small changes to the grill area and radiator position can make a huge difference to the airflow.
Also as mentioned the fan does little at highway speed.
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