350 tbi to 406 tbi

So I had a old 400. It was bored to a 406 with a very mild towing cam kit from comp cams, I do not remember the exact specs on it. Built it for a old truck that got totaled. Long story short the motor is now in my fathers 95 tahoe. The 350 blew due to water pump failure and he put the 406 in there. I beleive the only thing he did was swap the intake manifold and heads. He says it runs but he doesn't drive anymore. Anyone have any experience getting the original tbi to run a 400? I have read that we may to up the injectors and put in an adjustable fuel pump. My goal is to get it running well enough to tow a trailer to from Virginia to the Houston area and then take the truck over as mine. Eventually build it up and restore it with my son. Would be pretty cool to restore grandads truck with a motor that he and I built together, with my son.

I built a healthy 406 for a truck I had years ago. Extreme energy 268 cam, headers, stock intake/TB, stock tune. Ran like a mother f’er. That motor is the reason I bought another 400 to build.

b454rat said:

I built a healthy 406 for a truck I had years ago. Extreme energy 268 cam, headers, stock intake/TB, stock tune. Ran like a mother f’er. That motor is the reason I bought another 400 to build.

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Did you have to get any tuning done?

Nope. But I think I got lucky. I put bout 10k or so on it, and only real issue I had was a sensor went bad, never did figure out what one. But it wouldn't start cold without giving it a lil gas. Once started it was fine. But check engine light never came on. I sold it, guy who bought it hammered the living **** out of it, and blew motor. Talked to guy who did work on it, and kept asking if it was a stick, cause I guess it had a lot of lateral movement, like a clutch pushing the crank forwards. Tranny was tired, but had nothing to do with the motor. It did have 225K on it lol. Might build the 406 I have now for my Yukon with Vortecs and a Whipple, but still debating on what way to go.

Cool. I think I just need to get my brother to drive it around for a bit and ser how it does.

b454rat said:

I built a healthy 406 for a truck I had years ago. Extreme energy 268 cam, headers, stock intake/TB, stock tune. Ran like a mother f’er. That motor is the reason I bought another 400 to build.

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I helped spec and tune one for a 95 G30 cutaway van based RV to replace the 350.

Used a Comp HE 268 cam, stock 350 1-ton 810 casting swirl ports with a spring upgrade and steam holes drilled. Stock TBI intake bored to 46mm and a 46mm throttle body. Adjustable regulator, 96 Vortec fuel pump, and stock 350 injectors. Stock van 2.25" exit log manifolds. Stock van 2.25" to 3" Y-pipe, a 3" high flow cat and a magnaflow long case 3" muffler.

Was driveable on the stock 350 tuning at 18 psi fuel pressure and timing bumped a little but ran alot better after some chip tuning. Old man used it to tow a 3,500 lbs boat all over the US. I guessed it to make about 450 ft/lbs @ 2,800 and 300 hp @ 4,500 based off the fuel I was feeding it. Roughly 50% power gain compared to the stock 350.

So 350 heads fit just by drilling a few holes? I was under the impression you were stuck with the smog heads.

Can run any small block head on a 400. Dont need to drill steam holes, but think that’s for racing or over 4K rpms

b454rat said:

Can run any small block head on a 400. Dont need to drill steam holes, but think that’s for racing or over 4K rpms

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High RPM racing stuff does not need to be drilled per say. However low rpm street and heavy duty use they need to be drilled. I think the steam holes are a double edged sword myself. The blocks tend to crack from the steam holes but they run much cooler with them.

Supercharged111 said:

So 350 heads fit just by drilling a few holes? I was under the impression you were stuck with the smog heads.

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That is correct. Vortec heads fit too. The Holley L31 timing cover also fits a 400 block. With a factory 400 block you are stuck with a flat tappet unless you buy the expensive conversion roller stuff. Also the 400s have larger mains. If I were building a roller cam 400 for a modern truck like a L31 application I would start with a Dart SHP 400 bore/350 main/1-piece rear seal that uses factory roller cam stuff. Block is much thicker and has stronger/better caps. Bare block sells for $1750 and Darts block prep kit is about $100. At that point all they require is a finish hone to match the piston and ring selection. I would not hesitate to say 1/2 the factory 400 cores are non-rebuildable junk that are floating around. They were prone to core shift and some left the factory with very thin cylinder walls at stock bore. 400s are also prone to deck cracking. The dart 400 block has thicker decks.