I want to do a 396, but I was wondering how hard or likely it is to bore and stroke a lt1 to, say, 427? Also what bore and stoke for the 396, I don't have a formula to figure it out.

 

I read about some place awhile back stroking out lt1's to 427.
I think a old issue of GM high performance 18 to 24 months ago possibly.

 

GM says you should not bore it over more than 30. The more you bore, weaker the block gets.

 

421 is the biggest I have seen.

 

I think that just boring .030 over only brings you to a 355. Its when you stroke the crankshaft that you gain CI's. Remember that when calculating displacement that its the bore and stroke that the number comes from. Increase the stroke, increase displacement.

 

I raced and a 398.....got destroyed!

 

you have to clearence the block even if you only go 30 over with a 383. on even the 396 walls get thin motor runs hotter. i am going for the 409 but i have a couple replacment blocks just incase. but for a street motor just go with a 383 or lower i am prob taking my car off the road perm. due to tickets i'll just run it on sat n sundays so its ok for me to go bigger and even use alum rods (not sure if i am gonna ) but you can build one hell of a 383 to like 650 hp

 

I think the "standard" 396 is a 30 over bore, and a 3.875 inch stroke.


I say "standard" because that is what the 396 lt1's usually are, unless you want to spend $$$$ to get a custom crank with the stroke of your choice made and run a different bore size.

30 over and 3.875 is the standard off the shelf 396 lt1. But of course you can spend more $$ and play around with the bore and stroke if you want.

 

I think there is a diminishing return on LT1 motors over 383/396. Since you really can't overbore more than .040", even if you do stuff in a huge stroke, the bore size is too small to allow proper valve sizing for a large cubic inch motor. Plus, a 400+ LTI motor has such a big stroke, in order to run the RPMs, you'd need very pricey high end rods/pistons in order to stand up to the pistons speeds needed to make HP up top.

 

Mark Montalvo, who used to post here fairly regularly has built two different versions of the 421ci LT1:

4.03 X 4.125 stroke

4.09 X 4.000 stroke

The 90-thou overbore took extensive block measurements, and recentering the bores in the block to put the strength on the thrust side of each cylinder. The engine made over 800HP on nitrous.

 

Thanks for the replys, I guess I'll go 396 (Not doing it till I get out of college), Didn't think you couls safely bore and stroke the LT1 to 427, But Damn, I love to say its got a 427 ,That just sounds cool to say. A guy that I worked for had a 97 Z28 with a 427 BBC, But I don't won't to do that much work to get something like that in a Z28!

 

The lt1 block can be bored to .130, but I wouldn't bore more than .060.