Preferred method to lower compression

[Philv6]

Given a short deck 9.50, 6.50 rods (7.147 to the top of the rod), and a 3.59 stroke crank. Add in the piston thickness @.250 with .020 rod/piston clearance and there is not alot of material that can be moved to lower the c/r. That being said, what would be the most preferred method? (need to order pistons)

- Thicker head gaskets .060 or more vs .039
- Larger gasket bore dia (4.015 cyl) 4.090 vs 4.020
- Drop the upper ledge of the piston .020 down in the hole or more if possible vs zero deck

What would be the advantage / disadvantage, or max to shoot for each method?
Head cc's not included, since that usually can't be changed much

[KendallF]

Given a short deck 9.50, 6.50 rods (7.147 to the top of the rod), and a 3.59 stroke crank. Add in the piston thickness @.250 with .020 rod/piston clearance and there is not alot of material that can be moved to lower the c/r. That being said, what would be the most preferred method? (need to order pistons)

- Thicker head gaskets .060 or more vs .039
- Larger gasket bore dia (4.015 cyl) 4.090 vs 4.020
- Drop the upper ledge of the piston .020 down in the hole or more if possible vs zero deck

What would be the advantage / disadvantage, or max to shoot for each method?
Head cc's not included, since that usually can't be changed much

All three of these options suck. You want minimum quench of about .040" (deck height and gasket combined) to avoid crevice volume that doesn't promote flame travel.

You will have enough room for a dish to get your compression at least in the 8.5:1 range, as long as your head chambers are at least 40 ccs or more. I have a 9.501" deck, 6.5" rods and a 3.632" stroke crank and I was able to get 8.66:1 compression with a D shaped dish and retain about .180" thickness over the pin (head chambers are rather large at 54 cc).

Let's do some math: 7.147" +.030" clearance for the pin, say..plus 1/2 of 3.59" leaves .528" thickness for a flat top piston over the pin. Say you want to retain .200" thickness; you can have a dish .328" deep. If it's a round dish with about 3/4" rim around the dish (an approximation, I know they're sloped to the bottom) you could have a 3.155" cubic inch dish, which is about 52 ccs. This would result (http://www.rbracing-rsr.com/compstaticcalc.html) in an 8.44 CR with a 40 cc chamber head (and I hope yours are bigger). You can opt for a round dish and have a bunch of thickness over the pin, or probably even a D shaped dish as I did and retain adequate thickness.

The only other thing to worry about is the ring locations; most turbo pistons should have the top ring at least .300" down. You will probably end up with a bridge rail because the pin will go through the oil ring. It's not a big deal.

Just call somebody up and order 'em... :)

[EightSecV6]

You can have the piston sitting furter down in the hole or use a thicker head gasket to lower compression when using a D shaped piston because you will still get adequate flame travel and a larger quench area than a standard full dish piston. Ideallly you want the piston at zero deck but for most guys it aint happenin on a S2 head due to the fact that most have small chambers. You should only opt to do any band-aid if you have no other choice! A shorter rod will work allowing more of a dish, open the chambers up, etc. The thicker head gasket will blow easier. I really dont think youd see any major difference either way, if it costs you 100hp to set the piston in the hole, turn the boost up!!!!! Who wants to run less than 9:1 compression anyway? :add_wegbr