LT1-LT4 Modifications 1993-97 Gen II Small Block V8

Highest CR on pump gas?

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Old 05-23-2013 | 08:27 PM
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This is why I love this forum, the wealth of information is amazing. The more I think about it the more fun a HCR 355 is sounding to build.
Old 05-23-2013 | 08:56 PM
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Originally Posted by BoostedDSM
This is why I love this forum, the wealth of information is amazing.
Exactly why I ask the hard questions here!
Old 05-24-2013 | 12:05 AM
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Bigger cam changes the timing of the cycle. if you close too early you leave a bunch on the table. To a point, the pressure is still there it just changed the rpms that it might happen at. As far as lift goes your heads will dictate that, and your springs will try to keep up.
Old 05-24-2013 | 11:56 AM
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Originally Posted by fex77k
Bigger cam changes the timing of the cycle. if you close too early you leave a bunch on the table. To a point, the pressure is still there it just changed the rpms that it might happen at. As far as lift goes your heads will dictate that, and your springs will try to keep up.
Cam timing and the valve relief depth in the piston will dictate max lift you can get away with....along with valve stem length yada yada.
Old 05-25-2013 | 03:39 AM
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Originally Posted by FASTFATBOY
Cam timing and the valve relief depth in the piston will dictate max lift you can get away with....along with valve stem length yada yada.
Yes, that's physical limit of the valves. Some heads aren't big enough to need it but if you are looking for every last bit and the springs can handle the valvetrain then get as close as you dare. How long do you need it to last?.
Old 05-25-2013 | 07:15 AM
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Originally Posted by fex77k
Yes, that's physical limit of the valves. Some heads aren't big enough to need it but if you are looking for every last bit and the springs can handle the valvetrain then get as close as you dare. How long do you need it to last?.
Forever!!!!
Old 05-25-2013 | 07:46 AM
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Originally Posted by fex77k
Yes, that's physical limit of the valves. Some heads aren't big enough to need it but if you are looking for every last bit and the springs can handle the valvetrain then get as close as you dare. How long do you need it to last?.
...or go too far and have to flycut the pistons
Old 05-25-2013 | 08:11 AM
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Originally Posted by Puck
...or go too far and have to flycut the pistons
We're considering doing this to my dome pistons to lower compression.... But I'm hesitant
Old 05-25-2013 | 05:58 PM
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Originally Posted by VAformula
We're considering doing this to my dome pistons to lower compression.... But I'm hesitant
I had to do it to my flattops, but its because of my valve spacing and aggressive cam.
Old 05-25-2013 | 06:40 PM
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Lift doesn't have a big effect on valve-to-piston clearance. When the valve is at max lift the piston is about half way down in the hole.
Old 05-25-2013 | 07:27 PM
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do you need high compression pistons to achive mid 12 CR's or is it all depending on the combustion chamber in the head and headgasket?
Old 05-25-2013 | 07:36 PM
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as very few N/A setups flow better than atmospheric pressures, or atleast to my knowledge they're arent, how could cam selection make a difference (other than valve lift and timing with the piston)?
Old 05-25-2013 | 07:52 PM
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Google Pat Kelly compression calculator. It'll explain the details. Compression does not happen until the intake valve closes. The longer the duration on the cam, the later the intake valve closes, the lower the dynamic compression for a set combo. There are charts that tell you max safe dynamic compression for octane and temperature. Aluminum buys about a half point of headroom.

Static compression is just the ratio of the combustion chamber size with piston at BDC to chamber size with piston at TDC. Piston design, head chamber, bore, stroke, gasket thickness and bore, and deck clearance set that. Dynamic compression is your true running compression.
Old 05-25-2013 | 08:04 PM
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Originally Posted by Ed Wright
Lift doesn't have a big effect on valve-to-piston clearance. When the valve is at max lift the piston is about half way down in the hole.
Correct.

My PTV issues were because of the late EVC and 60/40 valve spacing. Closest point to pistons is not at max lift.
Old 05-25-2013 | 08:36 PM
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Originally Posted by SSellers
Google Pat Kelly compression calculator. It'll explain the details. Compression does not happen until the intake valve closes. The longer the duration on the cam, the later the intake valve closes, the lower the dynamic compression for a set combo. There are charts that tell you max safe dynamic compression for octane and temperature. Aluminum buys about a half point of headroom.

Static compression is just the ratio of the combustion chamber size with piston at BDC to chamber size with piston at TDC. Piston design, head chamber, bore, stroke, gasket thickness and bore, and deck clearance set that. Dynamic compression is your true running compression.
Very well explained thank you!
Old 05-25-2013 | 09:44 PM
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Originally Posted by Puck
I had to do it to my flattops, but its because of my valve spacing and aggressive cam.
How well did this go?? My builder has experience doing it and said he can if it gets down to needing that.
Old 05-30-2013 | 09:23 AM
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Bumping this up!

Decided to just go with race gas and high compression, instead of risking using pump gas, I'll barely drive it so it'll be fine.

Thanks for your help guys!
Old 05-30-2013 | 11:44 AM
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Id like to have a little more compression in my stock bottom end motor without changing too much around. Could I just swap to a .026 head gasket and be fine or would I need to make some other changes at the same time? I was going to get the head surface milled down just a touch as well as change the gasket but I dont know what compression just changing gaskets would put me at.
Old 05-30-2013 | 12:33 PM
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Originally Posted by SSellers
Google Pat Kelly compression calculator. It'll explain the details. Compression does not happen until the intake valve closes. The longer the duration on the cam, the later the intake valve closes, the lower the dynamic compression for a set combo. There are charts that tell you max safe dynamic compression for octane and temperature. Aluminum buys about a half point of headroom.

Static compression is just the ratio of the combustion chamber size with piston at BDC to chamber size with piston at TDC. Piston design, head chamber, bore, stroke, gasket thickness and bore, and deck clearance set that. Dynamic compression is your true running compression.
So how do bigger cams with later closing intake valves make more power than smaller cams with earlier closing valves? If the dynamic compression ratio was related to how an actual engine works in the real world you would want a cam that closed when the piston is at the bottom of the stroke. According to your calculator a stock cam would have a higher compression ratio than an aftermarket cam which means there's more air in the cylinder whould would equal more power.

Those calculations assume the engine is operating at 100% ve with no scavenging going on or anything like it. The whole thing just started so internet experts had another talking point to try to make themselves look smart.
Old 05-30-2013 | 02:44 PM
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Originally Posted by Ed Wright
Depends a lot on camshaft also.
Yes.

Running high compression on a street engine (pump gas) requires that you have proper engine quench (ideally .035" to .040"), optimal dynamic compression (about 9:1 w/the LT1/4)....this is where the camshaft specs come into play....and a spot-on tune.

12.5:1 static compression is very doable when your build has been done correctly!

KW

Last edited by KW Baraka; 06-04-2013 at 02:28 PM.


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