Exhaust Manifold Temperatures

[Stuart F.]

I, as a matter of routine, give my engine a once over with an I.R. gun before I put the car back in the garage. Usually, I've just returned from a full temp run which is easy to do here in Florida during the summer. I was wondering if anyone else did that and, just for the heck of it, what kind of temps do you see on your exhaust manifolds - specially near the down spouts. I have always shown about 100 degrees more on the passenger side than the driver side, even though I have changed a few things like wiring open the heat riser butterfly for the summer. My water temps, both at the thermostat housing and slightly upstream from the housing on the hose are w/i one degree of the thermostat - and the engine runs perfectly normal. I even did a vlave adjustment since taking note of this, but that made no difference.

Stu Fox

[Stuart F.]

Re: Exhaust Manifold Temperatures

I know, there ya go with that acronym kinda stuff with the IR meaning InfraRed - sorry. I didn't know what BTW meant until my wife (from Thailand) told me a few weeks ago that it meant "Better Than Whisky" (think she was talking about Mekong). By the way, did I tell you the one about ------

Stu Fox

[Stuart F.]

Re: Exhaust Manifold Temperatures

Looks like I'll have to take this up by E-Mail with a few of the fellows who are more into this sort of thing. I kind of like to think it is in keeping with Engine Diagnostics, i.e. maybe I'm looking for trouble where none exits.

Stu Fox

[Gary S.]

Re: Exhaust Manifold Temperatures

Stuart,
I just came back from a nice little cruise in the 72 and thought I would put my IR gun on the manifolds, out of curiosity and to help you with a second set of numbers. All temps were at idle, blipping the throttle once or twice to see what happened with the temps.

Left side, cylinders 2, 4, 6, 8 all read within five degrees of each other at around 400 F.

Right side, cylinders 1, 3, 5, 7 are a little skewed so I will do another test tomorrow. 1 & 3 read in 420+ range while 3 & 5 read in the 500 range.

Odd? Any explanation for 3 & 5?

Hope this helps.

[Stuart F.]

Re: Exhaust Manifold Temperatures

Gary;

Thanks so much for doing a test like this. It tells me a lot. First, mine are running hotter than yours, perhaps due to the higher compression and the Duntov Cam in my 340 hp. I'm seeing in the 450 to 500 degree range on the left bank, but on the right bank #4 & #6 are running near 600 degrees w/# 2 & #8 near the upper end of the left bank range. I'm thinking the downspout area on the right bank has the choke hot air tube in it and it is probably glowing cherry red constantly. I have my heat riser butterfly wired open, otherwise I would point the finger at it. It may still be a contributing factor, but I won't know until I replace it with a F.I. spacer (waiting for cooler weather to do the work). I have taken readings at the exhaust cross over passages in the intake manifold, but I didn't write them down as they weren't as significant as the exhaust manifolds. I know my gun is right as it reads the thermostat housing area exactly at the thermostat temp (160 degrees).

Interesting. Thanks again for your taking the time to check yours and post it. I really appreciate it.

Stu Fox

Just noticed your cylinder numbering. I think you have them mixed; left should be odd and right even, right?

[Duke W.]

Re: Exhaust Manifold Temperatures

All things equal a higher compression ratio yields lower EGT. Peak combustion temperature is about the same in the range of automotive compression ratios, but the higher the CR the higher the expansion ratio, and as the gas expands it cools.

Another variable is exhaust valve openig timing. The earlier it opens the higher the EGT.

If total idle timing is in the correct range (non-emission controlled engine), the manifolds should read about 500 F, but it will vary with engine configuration.

I should also be about the same side to side measured at the same point in the manifold. The center of the manifolds may be a bit more than the ends.

I think you would be better off with a 180 degree thermostat. You are overcooling the engine with a 160, which will increase fuel consumption and won't let the oil get hot enough in relative short trip driving to burn off blowby condensate.

Duke

[Kenneth T.]

Re: Exhaust Manifold Temperatures

Gotta love this organization! Doing IR tests prior to bed down?
"BTW", I have an IR gun too, go figure.

Ken

[Stuart F.]

Re: Exhaust Manifold Temperatures

Gary, Duke and Ken;

Thanks loads for your input. I'll sleep better tonight confident that I'm not seeing the early signs of a major problem. I have been monitoring it closer lately to see how my distributor rehab w/ change to a B28 VAC is going to work out. I backed off my initial timing to start with by 2 degrees to ensure I won't get any PI, and I believe I am seeing a small amount of EGT increase (I had a B20 VAC before). Withthe B28, I feel a little more response at off idle to midrange, but the pull up to red line is about the same - maybe a little flat at the upper end which may be accounted for by the 2 degrees less total. Still, I do get the Florida stumble when my coolant temp rises above 160, and that is my main deterent for going with a 180 thermostat. I still intend to replace my wired open heat riser with the FI spacer when I feel a little more comfortable doing the crawl under again. And, I know it is frowned on, but I may block my exhaust cross over passages with tin plates as in the old days - at least during the hot summer. I will also be trying an electric choke. I know I don't need much of a choke here, but I want to eliminate the hot air bleed through the choke housing (as a test).

Thanks again guys.

Stu Fox

[Gary S.]

Re: Exhaust Manifold Temperatures

Duke,
your comments that the centers should be hotter than the ends makes sense and seems to be what my right side is showing.

Stuart,
I didn't do you justice because I failed to mention that this is on a LT-1. Does that make any difference to you? Clearly though, 100 degree difference is a lot, but then I know so little about this level of exhaust temperatures, valve opening and cams.

Gary

[Stuart F.]

Re: Exhaust Manifold Temperatures

Gary;

I believe an LT-1 would certainly make a difference, but as Duke has mentioned, that valve timing is important specially on the exhaust. I'm sure if we keep this interesting to him, we stand to learn a lot.

Stu

[Stuart F.]

Re: Exhaust Manifold Temperatures

Gary;

I know you don't hear it much today anymore, but;

GO JETBLUE!!

Stu

[Stewart A.]

Re: Exhaust Manifold Temperatures

Stu a quick note from the other Stew spelt from the Stewart Clan but that's another story.
I fired up the 60 today for the first time ever and felt better than my first heroin shot thats another story.
I ran the cam in at 2500 rpm for half an hour like comp cams specify,
but I did notice that the heat riser tube was glowing like a formula 1 motor. Whats the go with that is it normal. I have punched out the butterfly valve to give it less restriction ( I don't like butterflies) and welded a metal clip on the inside of the shaft so when bolted up it looks factory and the little spring weight still can move but not work the butterfly inside because it's been removed with the shaft. I don't like it being that hot. I do see alot of second hand driver side manifold for sale compared to passenger side ones. I wonder If it's because of the heat difference damages that side. Interesting !!! Are they designed to get that hot ?
Stewy

[Stewart A.]

Re: Exhaust Manifold Temperatures

Hey Duke have you ever thought about working for NASA. You have a great skill in mechanical engineering. I think you could easily out do those guys who designed the space shuttle.

[Duke W.]

Re: Exhaust Manifold Temperatures

I did work for the Air Force on a communications satellite back in the seventies, but I'm now retired.

Stuart - The Duntov cam has a lot of overlap and likes total idle timing of near 30 degrees. This can be accomplished with the B28 VAC (16 deg. at idle) and 10-16 initial. Assuming your centrifugal is still the OE 24 degrees this will yield 34-40 degrees total WOT timing.

The OE centrifugal curve is lazy - peaks at 4600, so you might be able to run close to the upper limit without detonation. If you have quickened the centrifugal, maybe not.

The actual EGT is higher than the manifold measured temperature, but the manifolds rapidly loose heat thorugh radiant and convective heat transfer, and their surface temperature is a function of both EGT and the amount of exhaust gas flowing through them. This is why the center will typically measure a little higher than the ends.

At full load, high revs the manifolds will glow orange and will run in the range of 1100-1300 deg. F. - nearly the same as the actually exhaust gas in the exhaust ports.

I found blocking the heat riser to be unsatisfactory even in S. Cal. The engine would stumble on initial take off from a dead stop without excessive clutch slippage until the engine had been running for at least 20 minutes. This characteristic was probably exascerbated by the CR trans/3.08 axle. IIRC you have a CR/3.36.

The base of the manifold needs some heat to evaporate the fuel sufficiently to create a combustable mixture at low revs. One suggestion I have put forth before is to just block one side of the heat riser. This creates a dead end passage that will allow some heat to migrate up to the manifold base, but much less than if the passage is completely open. This might be a happy medium between good low speed driveability and percolation.

On your car you can block either side. On configurations with the choke thermostatic coil mounted above the inlet manifold heat riser passage, block the side opposite.


Duke

[Clem Z.]

Re: Exhaust Manifold Temperatures

i have seen the exhaust manifolds on a 454 motor home get so hot they were transparent. you could see the shape of the rectangular exhaust port showed as a dark image. i was running the motor home without the "dog house" engine cover on. this extra heat is why they had trouble with the exhaust manifolds cracking till they went to ones that were 2 piece with a stainless steel expansion bellows in the middle