I have some friends who did this with a pair of Vortec 305's - their hybrid toyota corolla wagon/ chevy 350/powerglide/ started doing 12.0 with 125hp shot with a best of 11.8 - - by the way the cranking pressure on that 350 was over 230lbs

puffs of smoke this past weekend ended all the fun just off the line - result the top ring lands of 4 pistons - Trw stock forged we checked the numbers and they are .60 over too! - just cracked up all around the edges

no damage to the block - nothing on the head to suggest impact!
it happened right on the line and so there are no holes to suggest melt down detonation - no plugs melted (mind you it happened just on the line) etc. to suggest nitro fault

I have heard from one other web chat mate that this has happened to him and to another friend - all trying the 305/350 combo??

Is there any answers you can help us with?

Definately sounds like detonation to me...I have never seen one hole a piston, but all of the detonated pistons I have seen are broken around the ring lands.


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375hp 78 Chevy truck
77 Chevy Nova
95 Chevy Lumina 3.4L
and building a 78 Nova

I ran close to the same combo, 245 on cranking pressure with flat tops 54cc heads.cheap gas ended it all, less than 60 feet i heard it right before it blew. It broke the cylinder wall in 3 places and broke the ring land out of 1 piston. I repaired it all new pistons thicker head gasket and 210 on cranking pressure it ran better and was not so finicky

Could the ring end gap be too close? I know that with NOS the top compression ring will see more heat and expand more. If the ring ends butt togeter they WILL break the land - take a close look at the rings.

When I installed my KB pistons in my 383 they included detailed directions on correct gap for the top ring. The formula depended on whether you were going to be supercharged, use NOS, alcohol, N/A, etc.

I seriously doubt the problem is a direct result of the 305 heads. Back when Summit racing was building Chevy crate engines they used ported 305 heads extensively on their some of their 355 cid motors.

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68 Camaro, 383 small block with TH350 trans. 12.4's and never trailered.

And I bet that Summit opened up the chamber and also used the right pistons to drop the CR. down to a street gasoline range if they didn't have knock problems w/ their combo. IMHO!!! pdq67

OK I pulled my 1995 Summit catalog out (amazing that I actually save that stuff huh) and looked it up. They used 305 heads on their 300HP, 325HP and 350HP 355 engines.

The 300HP engine uses rebuilt 58cc 305 heads BUT with a D-shaped dish piston.

The 350 HP engine uses ported 66cc 305 heads and TRW flat tops. Sounds like some combustion chamber work to me too.

They also used a 58cc Corvette head on a 325 HP motor. These heads were also 58cc and used a dished piston.

If my math is correct, a flat top with valve reliefs on a 355 engine and 58cc heads would be 11.1:1. Too high for pump gas, and a little dicey with NOS, but certainly doable on race gas.