January 2012 Chevy High Performance Q&A

Kevin McClelland Nov 23, 2011 0 Comment(s)
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We love letters, especially technical questions. Submit your tech questions to Kevin McClelland at chevyhi@sorc.com. Regular shout-outs and good tidings are also always welcome.

Traveling Salesman

1201chp 01 O  January 2012 Chevy High Performance Q A Editor 2/2

Recently, as I was getting ready to make a run, a traveling salesman came by our trailer. He’d been there a few minutes, looking over my roadster. He came over to the door and asked, “Have you ever thought of running exhaust gas temperature sensors?” Well, I was the wrong person for him to ask this question. I immediately responded that EGTs were a waste of time on a race car! Knowing that wasn’t a very fair response to his question, I felt compelled to explain myself. I don’t know if that helped or hurt, but at least now he understands where I’m coming from.

EGT is a great tool for the engine dyno and when you’re doing intake manifold development. Once you’ve worked out the distribution in your intake tract, nothing is going to change. Using an EGT sensor to set the jetting of your carburetor isn’t very accurate, sometimes not even close.

Many factors affect an engine’s EGT. The fuel mixture is one of them, but more important is the combustion space. This is the piston crown and combustion chamber. The more efficient the combustion space is at extracting heat from the fuel (work), the lower the EGT will be at the header tube. I’ve seen EGTs as low as 1,150 degrees F at peak power, to 1,500 degrees F with unleaded street fuel on properly fueled engines. EGT sensor instructions recommend tuning your carburetor until you have an EGT in the 1,300- to 1,350-degree range! Once a good friend brought his small-block–powered race car over to the shop and ran it on the chassis dyno. He was a very good racer and hadn’t been able to hit his tail with both hands in about a year. He wanted me to check everything out and see if I saw something. On the first pull we recorded O2 readings in the 15:1 air/fuel ratio at the tailpipe! Earlier he and his partner had installed an EGT sensor in one of his header tubes and had been trying to achieve that elusive 1,350-degree temperature. I richened up his carb six jet sizes to get the AFR back to a safe 12.8-13.0:1. The EGT immediately dropped into the low 1,200-degree range. The thing picked up 50 hp at the rear wheels and was very happy.

Next race out, he was back to his winning ways and dominated that year for the track championship in Super Pro. Sometimes I think that I’m my own worst enemy! I don’t know if the salesman approached any other trailers this past weekend and got the type of response that I gave him. No disrespect intended, but EGTs have a place, and they are not to help you set the fueling of your race car. That’s what O2s are for.




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