Checking exhaust temperatures?

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Jun 20, 2004 | 08:34 PM

DansYellow66

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From: Central Arkansas

Checking exhaust temperatures?

Took my car for a drive today and checked the coolant temp with an IR temperature gage when I returned. I was glad to see a good coordination between my gage and shooting the top of the thermostat housing - about 190 to 195 idling in the driveway. The VC1810 vacuum advance unit really holds the temperature of my big-block down. My question is that I took a few hurried shots on the top of the exhaust manifold on the pass side before I had to leave for a car show. The temperatures varied quite a bit from exhaust port to exhaust port - from about 350 F to 425F at idle. The rear pass exhaust runner was the hottest. What kind of temperature spread should I expect at hot-idle with reasonably good jetting and fuel distribution? My motor is a pieced together L-88 with solids, high compression, an unknown Cam Dynamics camshaft, open chamber aluminum heads, L-88 intake without a center divider. I have a restored 800 cfm (427/425) Holley that I stagger jetted per the Chevrolet Power Book for the L-88 intake. I want to spend some more time checking this out next week - probably at idle and again with some revs. Is there anything to be concluded from doing this as far as adjusting jetting goes - or is finding out I have some lean cylinders at low RPMs about the only thing this will accomplish?

Jun 20, 2004 | 10:44 PM

magicv8

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From: Going too fast over the hill. Iowa

Re: Checking exhaust temperatures? (DansYellow66)

Got me. My SB exhaust manifold runs 500-600 on the IR guage.

Jun 20, 2004 | 10:50 PM

62fuelie

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From: Pleasant Grove AL

Re: Checking exhaust temperatures? (DansYellow66)

Both my small blocks seem to show IR gun reading in the 5-600 Deg F. range as well. There is some variation from one spot on the exhaust to another, but I don't attribute it to the mixture. My fuelie has the same variations as the 67 L-79, and at least in theory, the fuelie should have very little cylinder to cylinder A/F variation.

Jun 20, 2004 | 11:57 PM

nitram

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From: Sulphur LA

Re: Checking exhaust temperatures? (DansYellow66)

#5 will normally be the hottest on a SBC because it fires just before #7 and they are physically located right next to each other. I've always found #5's exhaust gas temperture to be about 70-100 deg. hotter than the others.

Jun 21, 2004 | 12:05 AM

SWCDuke

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Re: Checking exhaust temperatures? (DansYellow66)

Surface temp of the manifold will vary with manifold thickness and local geometry, air flow, and whether reflective or absorbtive surfaces are nearby. Try to shoot them at the same relative location on each branch like right between the bolts, equidistant from the head surface on each branch.

If you're total idle timing is correct - 20-26 degrees for medium performance and 26-32 for SHP or equivalent they should be in the range of 500F at idle.

SB ram horns might measure a little hotter at the center two ports than the front and rear ports.

Duke

Jun 21, 2004 | 09:46 AM

Matt Gruber

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From: central FL, near the beach.

Re: Checking exhaust temperatures? (DansYellow66)

should be hot enough to bake fish in 10-15 minutes.

Jun 21, 2004 | 07:29 PM

DansYellow66

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From: Central Arkansas

Re: Checking exhaust temperatures? (Matt Gruber)

Huh, sounds like mine is running pretty cool. I had driven it about 15 minutes. I use a dial back timing light and shoot for 36 deg total initial and centrifugal timing which works out to about 12 initial if I remember correctly. I forget what the CV1810 vacuum advance unit bumps it up to but after putting as big a bushing as I could on it, I think I was still getting 12 or 14 deg additional timing on top of initial. Like I said, after shooting the thermostat housing I quickly shot about 3 cylinders in about 10 seconds and had to quit and leave. I may not have been getting a good reading or hitting the head instead of the exhaust manifold itself. These open plenum motors are not models of good fuel distribution from what I understand and checking it on the idle circuit probably doesn't tell me anything about jetting under throttle. I thought there might be a shot at finding out some useful tuning information from checking manifold temperatures but it probably falls more in the category of useless info.