Those are very high numbers what compression are you shooting for? If you are say at 11 to 1 you might want to confirm your valve closing events. Pup gas I think is around 210 or so max cranking compression. YOu have pretty short duration cam in here or what is the cam you are running?

 

Yes the build was supposed to be around 11:1 CR and the cam is the Summit Big Truck Torkinator 212/218 .600 int./.585 ehx. lift. I CC'd the heads as accurately as I could with a 100cc syringe and a piece of plexiglassa and came up with 65 so I calculated it to be at 64cc chamber size just to be safe. The valves were lapped and leak checked with gasoline :-) Im not opposed to taking the heads off and putting thicker gaskets on it to be safe or am i going to have to get new heads... Learning lesson test **** before getting it milled lol.

https://www.summitracing.com/parts/SUM-8728R1

Give it to me raw man did I screw up overshooting compression or is that the good layer of oil that I left in the cylinders before closing it up along with the lifters not being pumped up for proper valve timing?

 

I think the issue is the cam. That cam was designed to build max low end cyl pressure in a 4.8-6.0 w/ 10.0:1 compression and you are pushing that in a 11:1 408. I think that cam is small in a stock 6.0, much less a stroker build. Double06 is right, pump gas is out with that type of cranking comp. I would re-cam it. I don't think dropping the compression with a gasket is going to be enough to make the combo work.

 

If the lobes are symmetrical, then the .006 cam numbers put your dynamic compression ratio at 9.22:1.

I think I did that right.

If I recall, 8.7-8.8 is about peak for pump gas.

 

I am finding a good amount of mixed information and a good amount of it pointing towards not entirely being worried about a high pressure during a wet test with unbroken rings and a good amount of that info saying to run it and do a hot compression test after breaking in the Rings which happens relatively fast on the first run as long as it entirely goes through a heat cycle from my understanding.

 

The most cranking compression I have ever seen on pump gas was like 210 and that was a specific dedicated build. Cranking comp is not everything, but your combo is all wrong. Not enough cam, too much compression. If you believe all the internet bs that it will be fine, all I'm gonna say is video the first dyno pull because the cranks going to go flying out of this thing from the detonation. Do yourself a huge favor, call Summit, Cam Motion, Comp, BTR, GPI, basically any if the can companies and get a second opinion.

 

What is this going in with a 200 or so at .050 that is like for a tow truck or something like that? If this is in a car and you want some performance you want like in the 220s duration at .050 and that will bleed down the cylinders. For more all out performance a cam in the 230s at .050 is more in line but you begin to loose some street manners.

 

Not sure what the intended use for this engine is but that is a tiny cam for a 408. Being in TX you should be able to get 93 octane everywhere which will support 12:1 compression.

 

I am not looking for top end power as it will be going in my 5k Lb silverado with a 4l70e and 4.1:1 rear end, I want it to be a mild build with plenty of head room for more power later just had not expected for the CR to be so high with the short valve events.

 

You can pull the heads off, and grind the quench pad down a bit. That will lower the compression. You can also grind out the sides of the combustion chambers right next to the valves. That gives the air more room to get around the valves, and makes the combustion chambers bigger (lower compression). I'm not sure if it will be enough though. That cam is really small for that engine.

 

I definitely did take some material around the valves and curved it all back up to the old gasket I was using as a guide. Taking them to 76cc's was the worst idea ever. It worked though and lowered the compression to the low 200's psi and gave me somewhere to start and also showed me that wet compression tests can be higher than the "after first run" compression test which turned out to be 190-195 all around if I remember correctly. I will also add a part I have been leaving out which is the fact I had .045"~ thou worth of extra material/2layers on top of the .054" for a total of nearly .100" worth of head gasket just to play it safe. With all the warnings thrown my way it felt right as I feel comfortable doing such things that others consider a risk and also considering 100k+ miles I have drove on altered head gaskets without an issue.

I did receive my other heads back late wednesday and got the truck started Thanksgiving day. The 68-70cc stock chambers with the head gasket cushion did not let me down. I could tell immediately the truck was happy and had a consistent idle finally. I was so happy with it I tore the heads back off friday and put a brand new .054 gasket in and now the throttle response is so crisp it is crazy and I am probably asking for knock but with the cold weather and a tune with the timing backed off like I have it running on 93 should do it fine as long as I watch the temps. Anyone who says increasing displacement without other mods doesnt give much gains is crazy. My truck has so much power down low compared to the handful of Silverado's I have driven over the years. I love the way the cam added all the power 1K-5k just my type of setup I LOVE IT!!! My luck the second oil change change will look like glitter cause of some noob mistake haha jk I think it is running just fine. I never did another compression test, I will probably do one tomorrow after looking at the oil just to be safe but I think it will be in the 210's.

This will probably be the last time it gets 23.5° of timing XD I have since backed it off 3°.
I don't know how I left this out but the truck even managed to get me into trouble today. A random wack job lied and said I was doing donuts in the middle of one of our town intersections. Must have been someone I passed on the way to the store right after pulling out my place as the guy told them where my vehicle came from leading them to my place right after returning from the store XDDD

 

This is the stuff I like to hear at the end of one of these builds.



The world is full of fun haters...

 

So the oil looked pretty clean compared to the first change and the compression test came out to 220-225psi. The calculators I used are suggesting my calculations are off somewhere so I even over estimated the deck clearance at -.020 and the chamber size at 68cc to give a 10.82 SCR. That combined with the wallace racing DCR calculator at 57° degrees IVC gives the motor an estimated 9.98 DCR??? Did I do that correctly? Sounds like more head gasket thickness or a bigger cam is needed or I am right at the fine line of don't let the IAT's spike or get a bad batch of fuel?

Also why does this calculator give an effective stroke smaller than 4"? Wouldn't the rod be parallel with the furthest reach of the crank at BDC and
TDC or is LS geometry wonky?

 

I think your deck clearance is off. The negative sign is ok, but I'm sure it's not -.020". The most I have seen above deck is about .006". If the piston is below deck, use a positive number.

 

When trying to make sense of the high compression numbers compared to my calculated DCR I overshot the negative deck clearance further above deck so it would make sense on paper. I do think the the actual deck clearance was closer to -.006-.010