Performance Q&A Questions - Performance Q&A

Kevin McClelland Jan 1, 2009 0 Comment(s)

Stovebold Six
Q

I'm running a 250 six-cylinder and a four-speed with 3.08:1 gears in my '70 Camaro. I just bought the car, so I haven't driven it yet. My plan was to pull it and install a big-block, but with the price of gas I was thinking of just fixing the six so I can drive it as a commuter rather than buying a freakin' Kia or something. So, before I go and spend some money on fixing the six (it leaks oil and has a flat cam), I wanted to know what kind of mileage it would get rather than putting a mild small-block in it. I have most of the parts to small-block it, but if it will get better mileage with a six I want to keep the six. What do you suggest?
Glen Yotter
Via e-mail

A
Back in the day, we built many 230, 250, and 292 six-cylinder Chevys. They were great, reliable engines, but they weren't the best in mileage. We even built a 0.060-over 292 that was our "302" in a '63 Chevy truck. That thing was a kick to drive with a Muncie four-speed, 3.73 gears, and long 41/4-inch stroke. It didn't like anything north of 5,000 rpm, but it would haul the mail to that point.

The main problem with this Chevy engine design was the cylinder head. It used a Siamese, or common, inlet port for two cylinders. With this you didn't have much tuning going on in the intake tract. On our engine, we even went to the extreme of modifying the Offenhauser single four-barrel manifold with a divider wall that traveled down the center inlet port to feed cylinders No. 3 and No. 4. This was done in an attempt to divert some fuel into the center cylinders at higher engine speeds. At slow speed the center cylinders ran rich and the outboard cylinders ran lean. Dividing the center ports to about 3/4 inch from the floor of the port was a decent Band-Aid. We came up with this solution by watching the header tubes glow at different engine speeds.

Back to the sharing of inlet ports. Without any tuning, the engine doesn't get any benefit from a properly sized inlet port and manifold runner. When building torque for mileage you need everything you can get. If we were to guess, you should be able to knock down 15-18 mpg with a fresh 250 six. With a conservatively built small-block, this is also easily attainable and a whole lot more fun to drive.

Back to the "freakin' Kia." If gas keeps going the way it is, we may all be looking to smaller cars and keeping our toys to entertain us only on the weekends. Let's hope it doesn't come to this, but with GM closing four truck plants, they may have gotten the message. America, at least, needs a choice, and The General is stepping up to the plate. They will have small performance/high-efficiency cars for us all soon.

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