79 Caddy Overheating Issues
I am still new to this board and appreciate the honest answers and helpful attitude all of you have. This is not necessarily a hot rod question, but is about a racing application. I have a pretty good background, especially with SBC stuff, but am running a 79 caddy with a 425 in an entry level dirt track class. Because most rounders don't mess with caddy stuff, I thought that maybe you guys might have some ideas. Here are the details:
-425 stock compression
-shimmed SB valve springs (reduce float above 4000)
-Big 4 core radiator
-Q-jet with choke removed, larger primary main jets/matching rods, slight mods on secondary rods
-Notched plenum wall below secondaries, cast iron intake
-Timing at 10 @ 1000, 28 @ 3500+ (total advance)
-Stock, open exhaust
-High octane pump fuel with 20% racing fuel mixed (recommended by the other stock class racers)
-I removed the thermostat "button" and put the base in to act as a flow restrictor
-RPM: 5500-6000 peak at end of straights
The problem that I have is the temp. If I sit in the staging area too long, as with a long yellow in the race prior, it starts to get hot pretty quick. (250 - 260) If I can get on the track right away, it is a little happier, but will still get hot before the end of the race. I think that I'm getting enough fuel, and it seems like it has decent accel out of the corners until it gets hot, then the valves start to float a bit easier. I am running with water wetter and staright water, also suggested by the boys at the track.
The temp gauge is sure distracting when the needle gets to the up side. Best finish has been 2nd in the feature, and I'd sure like to figure this out so that I can worry about going faster instead of runing cooler.
Any thoughts or help would be greatly appreciated.
one suggestion i can make is to do a plug chop, run it at high RPM, then quickly let off the accelerator and shut it down, then pull a few plugs and see what they look like, if they are white it`s too lean, which leads me to believe it could be with a open exhaust, even so you richened the mixture to compensate for this, it could still be too lean if your Q-jet came off a "smog mobile" which were already too lean from the factory, also check the fuel pressure, if the fuel pump can`t keep up with the demand, it could be running right on the edge of the bowl being dry also causing a lean condition as Q-jets have a small bowl, but have massive secondary`s and when you stomp it, it`s dumping some fuel in there and it don`t take long to empty the bowl. it`s also possible the trick you did to the thermostat isn`t slowing the water flow down enough, I`d just stick a 160 degree job in it with a small hole drilled in the outer edge so it`ll help to burp some of the air out of the system. stock fan meaning clutch fan? if so check it also to make sure it`s still good and that the thermal unit on the front of it isn`t missing in action. you do have the air dam still under the car? if not this can cause it to overheat also.
In the Cad Company catalog, they mention a little bit about the carbs running dry on a dyno run. Granted this were on 400hp 500ci. But it may be happening on the 425, just takes a little longer of a run for the fuel system to fall short. Take a look a the following site (if you are not already familiar with it), it may offer some help.
Thanks for the help.
I have read the plugs a few times after race and they looked slight lean/hot. Fan clutch is good, shroud/box are intact. I will try the thermo change, as well as fuel pump check and bowl supply. One thought, because it doesn't like to sit and idle, is I may have compound problems. Your diagnosis of fuel starvation makes sense to me, but I also get hot at idle. One change at a time, re-test, try again. At least the testing phase is a lot of fun.
I really appreciate your help. This gives me a few new ideas to work through.
A stock Caddy thermostat has a bell shape deal on the bottom of it. That bell covers the bypass that Caddy installed to warm the motor quickly. You can tap and plug this hole so that a standard Chevy type thermostat can be used, or none at all if you choose. You will need a 1" plug and tap the hole under the thermostat housing with a standard pipe tap. Drill a small, 1/8" or so, hole in the plug so that it won't air lock.
What type of mods are you allowed in this class? A dual plane intake off a 472/500 flows better than a 425s single plane. A ported 472/500 intake will come close to a stock Edelbrock as far as power and flow. A 1" carb spacer will do wonders as well. Your heads have 2" intake, and 1.625 exhaust valves stock. You can use Pontiac valves in your Cad heads for a cheap upgrade. A 2.11" valve will flow much more than a 2". You should shoot for 32* total in the timing department. Fuel pump flow is also a real issue with the Cad. They can't hardly keep up with a stock motor, let alone under racing conditions. You can use a high flow Olds pump by changing the arm out for the Caddy piece. A 472 or 500 will also bolt in place of the 425. Being a Cadillac, nobody would know anyway......Dan
One other thing. Have you had the car across the scales yet? They aren't as heavy as everybody thinks........
Cadillac's also have a problem with the intake manifold gasket in the front below the intake. My caddy was overheating for a while it kept eating up thermostats and it was the gasket was doing it, I replaced it and never had the problem again
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