A friend of mine has been plying with the big L6's for years (like the 292) and has had much success. He has a fairly mild 292 with a ported head and a sheetmetal intake that he made with a 750 holley and a blow tru turbo setup using a 60mm turbo. He uses a powerglide and 4.11 gears in a mini tubbed 67' camaro and goes 10.80's with ease at low boost on pump gas and at 3000 lbs.

He has another lil' sick project.....a chevette with a 250 L6 with the same carb, intake, turbo setup but runs race gas and high boost (28 lbs) in the lil' tubbed a tube chassied chevette at 2300 lbs goes 9.60's

The modern Chevy in-line engine families all have a bore of 3.875" and differ in stroke from 2.25" for the 230 to 4.125" for the 292 with the 250 in the middle with a stroke of 3.53" (the 194 Thrift Six was odd man out with a small 3.56" bore and a 3.25" stroke, same as the Iron Duke’s 4 cylinder engine which had a 3.875" bore like the rest of the in-lines).
I say modern to differentiate them from the older "Stove Bolt" sixes designed in the 1930’s. The 3.785" bore was also common to the venerable 283 which has a 3.000" stroke so contrary to what some think pistons do not swap.

The 270 cid and 292cid in-line sixes use a different tall truck block to hold their longer strokes (the 270 is another small bore orphan with a 3.781" bore but it is hard to find and most bore out the tall truck block to the 292 displacement). The biggest limitation on the engine is never the bottom end as it doesn’t contribute to making power, but can cost power due to frictional losses. Here the in-line means that there are thee additional main bearings to rob a little friction. The plus comes from the fact that a straight engine has single throw for each cylinder; which means the power can be smoother, with each cylinder being 60° out of phase instead of the more common 90° found on V-8’s (and 4 cylinder engines).

The 60° phase angle and additional support of added main caps means there is not as much harmonic vibration or flex in the crank shaft so if the heads and cam will support it will rev higher than a V-8. Unfortunately the heads on most inline engines are dismal. They are designed for economy and low RPM operation. They commonly have cast in place log manifolds for the intake and occasionally the exhaust as well. The ports and valves are small with no thought given to high RPM gas flow. If ever there was a head that the aftermarket needed to cast it would be for a six cylinder in-line engine. Yet no one does because there is no perceived demand for it.

If modern raised port head technologies were applied to the in-line six you would see power approaching that of what a V-8 could produce even though they are 25% smaller in displacement. Because the phase angle is more favorable they produce more torque per cube than the modern V-8 which is why they are favored as truck engines and torque applied over time is horse power. It would be easier to add cylinders to the inline configuration because you just make everything longer inside with out additional worries about added stress. So why were Buick and Olds the only ones to mass produce in-line eight cylinder engines? Because they were the only ones to like the longer hood lines I guess, but they also were making more power than the comparable displacement Cadillac V-8 of the time.

Heads are the limitation, but they share the same bore spacing and head bolt pattern of the 283 V-8 so who is to say we can not take some "junk" aluminum SBC heads and add two cylinders to the middle of the head with a TIG welder. Could be done, then you would need to fabricate two thee cylinder intakes and two thee cylinder headers to have an eye opening ride. Crane and Comp both grind cams to pep up the existing in-line sixes (total number of grinds are limited but a custom cast core grind is not out of the question).

Years ago Clifford sold parts to owners of six cylinder Chevy II owners with a slogan of Six=Eight and with a light car you could keep up, but it didn’t have the machismo that a V-8 did. For the same reason the big GMC pick-up truck V-6’s never caught on even though they displaced over 432 cubic inches with a 4.875 inch bore and a 3.86 inch stroke (they started out at 305 cid but they grew bigger).


Big Dave

a buddy of mine has a 43 stude pickup with a twin turbo slant 6 from a dart its his daily driver so no numbers from the track but that little truck goes
he made the intake and exhaust manifolds and has fuel injectors from a jeep 4.2 I'll see if I can get some pics its pretty cool looking
Jim

I had a 70 Nova with a 250 with a stock TRW rebuild and a T04 turbo, 4- speed and 336 gears. It got 24+ mpg and ran 7.90's in the 1/8 mile.
That was one of the most fun cars I ever had- the V-8 guys hated me.