I picked up a pair of 243 heads to mill and swap on my cammed LS1. I wanted to check my PTV with them before milling.

I installed them with a gm mls gasket and set lash to zero with an adjustable pushrod and solid lifter. I used checking springs and set my dial indicator on the spring retainer. I measured PTV from 20* BTDC to 20* ATDC in 2* increments.

My closest PTV was .086 intake at 8* ATDC and .086 exhaust at 8* BTDC.

My cam specs are:
227/235 .614”/.621” 113LSA +2 advance (with 1.7 ratio rockers)
Comp LSL/LSL lobes
13017R/13019R HR113LSA +2 advance (111 installed intake centerline)

I did degree the cam and its only .5 degrees off.

Also my pistons are .006-7 out of the hole.

Do these measurements seem normal? Every ptv thread I've read the intake was closer. Its also my understanding that PTV with checking springs will be less than with the dual springs im going to run with. Is that correct?

 

Yep....it can be the same.....move the cam one degree in either direction and it will be different....just kinda a coincidence of all your valve events, margin of the valve thickness, valvejob depths, etc. etc.

And you could mill .025 or so off them and still be pretty good....you can basically add .015-.020 or so to the number you get with checking springs to simulate real world values when you have "real" spring pressure and all the typical deflection of parts in the valvetrain (delaying the opening of the valve and increasing P to V). Clay it yourself with a regular spring installed to check. Thick/larger diameter pushrods will get you closer to the checking spring value however....keep that in mind.

Your in good shape....mill the heads some to bump your CR

I would run a thinner .040 gasket.....you get better quench and a larger bump in your actual compression ratio without milling the heads as much which effect your piston to valve pretty close to a 1 to 1 ratio. With a thinner gasket you can mill the heads less and have your compression better optimized.

Details details.....its all about the right combination guys

-Tony

 

Thank you very very much. That makes me feel better and makes sense. I double checked today and doubled up on the springs I was using for checking springs and got 0.040 on the intake and exhaust. So they are definitely the same ptv.

I'm still leaning toward the gm gasket. I would go with the 0.040 if someone made one that was more forgiving. This engine went 15 years and 120k with graphite gaskets and they have been a pain to remove to say the least. I'm not disassembling the bottom end so I don't want to take any abrasives to the decks and it obviously can't get decked. It was slow going with a razor and permatex gasket remover, aircraft remover, and lacquer thinner. I'm not convinced a cometic gasket would be happy with the surface. That being said, I'm ok with going a little more conservative on compression to stave off the increased chance of detonation with the larger quench.

Using the gm gasket I'll have around 45 quench. What would be the max dynamic compression I could get away with with that quench and my cam? According to the calculator I'm using, shaving 0.016-0.017 off would put me close to a 62cc chamber and 8.4:1 dynamic and 11:1 static. Could that be made safe on 91 with the right tune? Would you push it any more?