BoostedDSM

iTrader: (6)

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.

VAformula

iTrader: (8)

Exactly why I ask the hard questions here!

fex77k

iTrader: (2)

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.

FASTFATBOY

iTrader: (23)

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.

fex77k

iTrader: (2)

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?.

VAformula

iTrader: (8)

Forever!!!!

Puck

iTrader: (17)

...or go too far and have to flycut the pistons

VAformula

iTrader: (8)

We're considering doing this to my dome pistons to lower compression.... But I'm hesitant

Puck

iTrader: (17)

I had to do it to my flattops, but its because of my valve spacing and aggressive cam.

Ed Wright

iTrader: (1)

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.

jrwilliams95z28

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?

jrwilliams95z28

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)?

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.

Puck

iTrader: (17)

Correct.

My PTV issues were because of the late EVC and 60/40 valve spacing. Closest point to pistons is not at max lift.

jrwilliams95z28

Very well explained thank you!

VAformula

iTrader: (8)

How well did this go?? My builder has experience doing it and said he can if it gets down to needing that.

VAformula

iTrader: (8)

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!

BSmiff

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.

speed_demon24

iTrader: (6)

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.

KW Baraka

iTrader: (7)

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