Compression

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highwaystar

highwaystar

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How much compression is too much for the street, before detonation presents a problem? I am putting together a new shortblock and with the W-2 heads @58 CC I will be running a 11.5:1 compression using a .051 cosmetic head gasket. Now I have always run 94 octane with a mix of octane booster, or even possibly some racing fuel occasionally. I generally only drive the car 600-1000 miles a year. Should I take steps now to get the compression down to 11:1 or 10.5:1 for a more streetable combo?
Thanks for the comments,
T.J.
 
i would.11 to 1 will act like 10 to 1 with aluminum heads quenched properly
 
Rule of thumb I was always told to be able to run 87 octane without pinging was 9.6:1 with Steel Heads and 10.6:1 with aluminum heads. Each setup is different, I know I built a 440 with final about 9.7:1 final CR, and had no pinging on mid-grade. Ran quite well at Mile high (Bandimere) 13.95.
 
It depends on how big your cam is as longer duration and wider lobe centers bleed off cylinder pressure. There's a good dynamic compression ratio calculator on the KB website. You will need to know your true measured static CR to get an accurate number though. Altitude also plays a part, but I see that's not a factor in PA.

Piston top and chamber finish as well as head material play a part too not to mention quench.

As you can see, there are many varibles in choosing the correct CR for your application and a good engine builder will do this before even sending the block to the machinist. Give us some more info as requested below.

True static comp ratio. Don't forget to include the gasket thickness and bore size plus the amount the piston is below deck in your calculations.
Cam specs.
We already know your chamber size and head material.
Polished pistons and chamber?
Special coatings on the above?
Piston material? Forged, cast or hyperutectic?
Good new radiator?
Carbed or fuel injected?
 
ramcharger said:
It depends on how big your cam is as longer duration and wider lobe centers bleed off cylinder pressure. There's a good dynamic compression ratio calculator on the KB website. You will need to know your true measured static CR to get an accurate number though. Altitude also plays a part, but I see that's not a factor in PA.

Piston top and chamber finish as well as head material play a part too not to mention quench.

As you can see, there are many varibles in choosing the correct CR for your application and a good engine builder will do this before even sending the block to the machinist. Give us some more info as requested below.

True static comp ratio. Don't forget to include the gasket thickness and bore size plus the amount the piston is below deck in your calculations.
Cam specs.
We already know your chamber size and head material.
Polished pistons and chamber?
Special coatings on the above?
Piston material? Forged, cast or hyperutectic?
Good new radiator?
Carbed or fuel injected?
Click to expand...

The pistons are 4.060 bore, J.E. srp pistons, no special coatings, forged, 351 stroke, aluminium Radiator, carb, the piston is out of the deck 0.09, figuring on using a .051 compressed head gasket. cam specs are still open at this point, other than looking @ a solid roller in the 550-575 lift area, but I am not stuck on any profile at this point.
T.J.
 
Are those flat top pistons and do they have cc rating? I'll do the calcs tomorrow, my district manager has been in town all week and he's been piling on the work. I need to get some sleep.
 
ramcharger said:
Are those flat top pistons and do they have cc rating? I'll do the calcs tomorrow, my district manager has been in town all week and he's been piling on the work. I need to get some sleep.
Click to expand...

Hi ramcharger,
They are flat tops semi-symettrical, I believe 8 CC, not sure box is at machine shop. The machine shop has run the #;s, and 11.5:1 is what were getting, I can flycut the pistons to 0 deck height and get it down to 11.2:1 or go a little thicker on the head gasket and drop it alittle more, but I want to know for what I drive it is 11.5:1 too much? Or is it not worth the trouble with possible detonation.
Thanks for all and anymore comments.
T.J.
 
I believe the valve pockets are symmetrical and 6ccs. If it were me, I'd be taking the pistons down to be zero deck and run the Felpro .039 gasket. That should be around 11.2:1 with a quench of .040ish. With a cam around 255°@.050 you will be fine on pump 93. Also on the cam, I'd go another .030-.040" lift on that roller. The offset intake rocker loses some lift, plus you have to take out the lash figure.
 
moper said:
I believe the valve pockets are symmetrical and 6ccs. If it were me, I'd be taking the pistons down to be zero deck and run the Felpro .039 gasket. That should be around 11.2:1 with a quench of .040ish. With a cam around 255°@.050 you will be fine on pump 93. Also on the cam, I'd go another .030-.040" lift on that roller. The offset intake rocker loses some lift, plus you have to take out the lash figure.
Click to expand...

Moper,
I am thinking the same, 11.2:1 seems like a little more streetable. I was a little hesitant about flycutting a good set of pistons,(kind of ruins them for other future builds.) I was thinking of just going to a thicker head gasket, the machine shop says it has a quench of 40ish now. I got to make a decision in the next few days, so i can get the assembly rotated. Our you saying .030-.050 more lift than the 550-575 lift I was thinking about?
T.J.
 
On the pistons.. I'd rather have the rings down a little and zero deck than have the dead gasket space around each piston for liquid fuel to get trapped and cause problems (detonation starts in such areas). The head gasket is aband aid.. Truthfully, any future build would have the block square decked anyway... So the pistons are not unusable. Just a hair shorter.
On the cam, yes, I'm talking about something in the area of .580-.590 lift or more. Because (a) the heads flow well up there, and (b) the lash and angles take away about .040" of lift on a solid flat tappet. With the 11.2 you wont have any issues with a softer low end power level...lol.
 
moper said:
On the pistons.. I'd rather have the rings down a little and zero deck than have the dead gasket space around each piston for liquid fuel to get trapped and cause problems (detonation starts in such areas). The head gasket is aband aid.. Truthfully, any future build would have the block square decked anyway... So the pistons are not unusable. Just a hair shorter.
On the cam, yes, I'm talking about something in the area of .580-.590 lift or more. Because (a) the heads flow well up there, and (b) the lash and angles take away about .040" of lift on a solid flat tappet. With the 11.2 you wont have any issues with a softer low end power level...lol.
Click to expand...

Thanks again Moper for the good info. I went to the machine shop today and there going to mill off the .09 off the pistons(this will give me zero deck) and balance the assembly next week. The cam profile sounds good, I want to be able to ease out of the hole with my 4 spd, 4:30 gears and make pulling power, with out side stepping the clutch @ 3500 and breaking **** like I have been. I know I won't get peak E.T. out of it, but I am thinking 1800-2200 out of the hole and let the motor do the work. I know this will have to be done in the cam profile, and I think the lower the duration like you said the more capable I am going to be able to do this. any more suggestions?
T.J.
 
I dont know what else you have for a package...lol. so I dont know what I'd do different...lol.
 
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