high compression on an inline 6

I just built a 292 (bored to 301) Chev inline 6 cylinder. Had severe overheating problems that I couldn't cure. New aluminum radiator, waterpump change( both were new, different style impellers), thermostat change from stock to high flow, changed from shrouded 6 blade stock type 15 in. fan to twin electrics, (850 cfm each, coming on at 180 degrees) also shrouded, nothing made a difference. The pistons were supposed to bump the compression from a stock 8 to1 to 8.5 to 1. After pulling the motor I ran a compression check, was getting 185 to 190 psi. I pulled the head and CC'd it, stock 64cc. Crank is stock. The cam was a little hot, designed for 2000 to 5500 rpm. Ordered a new low rev cam from Comp. cams. Just put it in and ran a compression check. Still getting 190psi. I feel that the high compression is the cause of the overheating. any thoughts on this? thanks, RON

 

I would have you check a few things.
1. What is the timing curve for idle, cruise, WOT?
2. Have you drilled a small burp hole in the thermostat? 1/8"
3. Where is the temp sensor located? I like to put them into the thermostat housing rather than the head.

Tom

 

Higher compression should make it run cooler, at least that's what thermodynamics tells us (because of the more complete expansion/cooling of the exhaust gas)

A higher RPM cam will also lower the effective compression ratio at lower rpm, as it bleeds off cylinder pressure.

Instead of chasing mystery problems with the engine, how about letting us see some good pictures of the engine in the vehicle? specifically the cooling system, such as how big the radiator is, etc.

What ignition timing are you running, initial and total?

 

A half a point on the compression is not going to cause overheating.

Timing will have much more effect.

Was it boiling or just showing high on the gauge?

 

what temp was it getting too? maybe you have an air pocket. i use an air lift to fill cooling systems, pull a vacum and let the coolant in. no air pockets. your engine combo is nowhere radical enough to cause overheating. maybe a restriction in the cooling system. once had a buddy chase an overheat in a tow truck. found a rag in the lower hose placed there by previous mechanic that did some engine work and stuffed the rag in the hose to stop the dripping.

 

In addition...where is the temp sensor location?

 

I had the same problem with a built 235. I discovered two things.

1. Air bubbles in the radiator was causing some issues. I resolved it by drilling a small hole in the ring of the thermostat.

after fixing the air bubble problem my temp guage showed a decrease in the temp but was still way too high. it would boil over.

2. I corrected the timing. I noticed I would have to adjust the timing a little after running it for a while. I discovered the hold down bolt was stripped out a bit and not allowing it to bite down hard to hold down the dizzy. I also discovered that with all the engine mods; pistons, boring, cam, milling, headers, etc...the factory setting for timing didn't work for my motor. Once I figured out where the new timing should be...it ran PERFECT.

I was in the Pismo Beach Car Show Cruise in the middle of the summer. It was bumper to bumper and very slow to stopped during the cruise. I cruised for about 1 1/2 without going over 180 degrees.

take your time and dont overlook the obivous.

good luck
jason

 

Good to point out. I used a thermal tempature gauge to get a true reading of the entire block and head. I discovered that the area where the sensor was located was the hottest part of the entire motor. the head was almost 10 to 15 degrees cooler.

 

Back in the day Combines used 292 GM sixes if they were bored 060" they would often overheat. Another engine that suffers from this is the 401 AMC V8. Not incurable but certainly annoying. Does it overheat idling in gear or at high speed or all the time. You will find the clues to fixing it in those 3 questions.
Don