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So I've mapped out the temp's for each cylinder once the motor was well heated up. Its a SBC 406 w/AFR 195 Heads & Hooker Super Comp Headers. One thing that popped out to me was the #5 position having an approx. 70-100 deg increase in temp compared to others. I doubled & triple checked this. I did blast/primer/paint these headers but I dont think a coating buildup would cause such a difference.
Also, #4 & #6 positions have dimples in the header tubes (around the steering knuckle). I'm wondering if the reduced cross-sectional area is causing a back pressure and perhaps not allowing these two to get as hot as most of the others? The #2 position is also a little low and doesn't have a dimple.
For a header pipe to be that much hotter indicates that cyl is either firing late, which I doubt, or a Exh Valve is not seating / sealing 100%.
I think I would pull the Valve Cover on that side and with the #5 Intake valve open, check the lash on the Exh valve. That valve head or even the valve seat may have a flaw in it allowing hot gases to escape constantly.
Last edited by HeadsU.P.; Dec 29, 2020 at 07:07 PM.
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
Do not assume each cylinder at idle speed is getting the exact same volume of fuel mixture. My header tube temps are higher on 3, 4, 5 and 6 than the other (outer) cylinders. I've been told that would not be the case running around at cruising speed.
HI ALL I CHECK MY 70 SMALL BLOCK WITH HEADERS ABOUT 5 MONTHS AGO AND THE SAME WAS GOING ON AND WAS GOING TO POST THIS BUT SAW THIS AND GLAD THAT THERE WAS NO PROBLEM BECAUSE THE MOTOR WAS JUST REBUILT.THANKS RESDOGGIE I FEEL BETTER
My BBC was around 750 at idle and around 1250 on the dyno run. High and low cylinders were always around 100 up or down.
So I am not sure you have anything to worry about. If you have carb, fuel distribution is not that precise.
You have to have a load on the motor to actually get good measurements. We ran motors to about 75% on the dyno. We were looking at 1100 to 1300 degrees
So there was no load on the motor. Simply starting it up and letting it all warm up.
@ HeadsU.P. - Heads are older but still new (maybe a few miles on them since bought 8 yrs ago). I do have 1.6 ratio roller rockers in them. I will definitely check this...
@ Resdoggie - I charted out the compression of each cylinder a while back. Oddly enough, #5 had the most but not by much:
1 = 170 psi
2 = 168 psi
3 = 165 psi
4 = 168 psi
5 = 175 psi
6 = 165 psi
7 = 170 psi
8 = 168 psi
@ leigh1322 - Carb is Holley 770 Street Avenger & intake manifold is an Edelbrock RPM Airgap - dual plane. I cant remember the port routing but could the left/right port diff cause passenger side of motor to run cooler?
@ gkull - if you added engine load (and time) won't the headers would get to a state of thermal equilibrium. The hotter pipe would cool down to the others.
From: Las Vegas - Just stop perpetuating myths please.
What type of intake manifold do you have? Are you having any current problems with the engine? Higher exhaust temperatures would indicate a leaner mixture. But on most V8 motors with a single carb manifold the inner cylinders run richer than the outer cylinders.
IR gun readings depend on the emissivity of the material and that can be affected by issues you can't see. The best way to measure exhaust gas temperature is to use thermal couples within the tube - at the same location with respect to the exhaust port - but thats a chore to do. An even more modern method is to use an O2 sensor in each tube and that's even more of an effort.
What I'm saying is the IR gun readings are not convincing and a more accurate method is much more of a research project effort.
I was always under the impression that as long as you are within 100 degrees you are good to go. And here's a Dyno shot of my brand new engine under full load with eight exhaust pipe thermocouples.. Lowest 1175 and highest about 1350. This is a perfectly normal spread. All close. #2 & #6 slightly low. A 10% variation. This is kind of like compression readings, you want them all close. They will never be the same. Now if one is 20-30% low, that's different, and you have a problem. Your compression readings are even less than 10% spread, more like 6%.
No you want in a perfect engine all 8 cylinders exhaust thermal couples to be the same temperature. When all cylinders are making the same amount of power. In Individual runner intake manifolds it's easier to accomplish. Very easy with adjustable 8 port sequential fuel injection. You just change the pulse width and even out the temperature of all cylinders
Racing single plane engines they bench flow the intakes and heads and modify the intake manifold until you get equal flow. This gets close, but that's why they came up with cams with altered valve events and lift for the outer cylinders
Remember- at idle - you really don't have much airflow passing through the carb or the intake manifold. I am not at all surprised that one cylinder is somewhat hotter - I'm actually impressed with how uniform the rest are. Intake manifold design - like most things - is a matter of compromises - with flow, space, geometry considerations, manufacturing considerations and cost considerations all in contention with one another.
And as far as the header dimples- it would be nice if they weren't necessary- but they are - and while there is NO question that they will impact flow - I highly doubt that if you ran the engine on a dyno - and swapped out that set of headers with the dimples - and another set of headers without the dimples - that you would ever be able to see the difference...
These temp readings were just approximate. I was OK with the 535F-590F but the 650F stood out by itself and that's what caught my attention.
Another thing I read is that there could be a small intake leak for that cylinder. Is there a good way to check for that or just spray some water area?
If you have a vacuum leak. Inducing flammables in the area of a leak will change the idle speed. Hot start ether, carb cleaner, propane, highly volatile solvents....... will make an instant change if it is sucked into a vacuum leak
And its a 50-50 chance that a Intake gasket is leaking on the top side for the propane to find anyway.
Sometimes the gasket leaks on the bottom side, from the lifter valley. May, or may not smoke from oil vapors.
Another test is to run the engine in a dark garage with the hood up. Take note of that cyl #5. If its cherry red in the dark, the Exh Valve is leaking.
Header Exhaust Temp Differences by Cylinder?
