Need to pick sum compression, how much can I shave my AFR heads?
04-25-2009, 12:28 AM
#1
Need to pick sum compression, how much can I shave my AFR heads? My compression ratio is way too low after switching to my new AFR heads, I got a good deal on 72cc's so I went ahead and got them. How much can I take off from resurfacing to pick up some compression without weakening the deck without making the head too weak for boost? I like to pick up one point in comp.
Is that possible? Whats the smallest chamber size I can go before I start having trouble?
Is that possible? Whats the smallest chamber size I can go before I start having trouble?
04-25-2009, 01:53 AM
#5
I had a turbo, I started out going for 8.5 with my old 63cc Patriots, now with the 72's it gives me 8.0. I would like to go up to 9.0/9.2CR if I could for the blower i now run. I dont want take off too and make the deck too thin, and dont want to run into detonation problems due to too small sized chambers. this is 100% streetcar with 91 octane
04-25-2009, 02:04 AM
#6
What are you using for pistons? You must have a huge dish to be at 8.0:1.
I used the Piano Prodigy DCR calculator and came up with the following:
4.000" bore
3.622" stroke
0.006" positive deck clearance
63cc combustion chamber
25cc piston dish
.060" gasket
8.52:1 compression
Changing to 72cc heads, I get 7.90 compression
I'm not so sure your compression is as low as you think it is, unless I've overlooked something, or your dish volume is really that large.
I used the Piano Prodigy DCR calculator and came up with the following:
4.000" bore
3.622" stroke
0.006" positive deck clearance
63cc combustion chamber
25cc piston dish
.060" gasket
8.52:1 compression
Changing to 72cc heads, I get 7.90 compression
I'm not so sure your compression is as low as you think it is, unless I've overlooked something, or your dish volume is really that large.
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04-25-2009, 02:45 AM
#8
TECH Senior Member
04-25-2009, 02:48 AM
#9
Ok, that confirms my math wasn't too bad 
Using the .006 per 1cc rule, you need to take about .080" off your heads to get to 59cc and reach 9.00 (same calculator). A lot of material, more than AFR would recommend, I'm sure.
If it was a really good deal on the heads, then you won't terribly mind passing them on and selling to someone who has a better combo planned for it. You can only make so many concessions before you end up way off your mark, and right now, your heads are just too damn big for the dish in your pistons. Since those are easier to swap than pistons, send them on their way. Start with smaller 67cc heads, or swap to smaller dish pistons, but you will have a tough time getting there with your existing parts.
Best of luck!
Using the .006 per 1cc rule, you need to take about .080" off your heads to get to 59cc and reach 9.00 (same calculator). A lot of material, more than AFR would recommend, I'm sure.
If it was a really good deal on the heads, then you won't terribly mind passing them on and selling to someone who has a better combo planned for it. You can only make so many concessions before you end up way off your mark, and right now, your heads are just too damn big for the dish in your pistons. Since those are easier to swap than pistons, send them on their way. Start with smaller 67cc heads, or swap to smaller dish pistons, but you will have a tough time getting there with your existing parts.
Best of luck!
04-25-2009, 03:41 AM
#10
TECH Senior Member
Yep, that is in the ballpark, too much mill for the compression you desire. Even by running .040 gasket it is still too much.
Best get rid of heads and use money to get another set. 243 casting would work well and would only take .035 mill to get you above 9.0
Best get rid of heads and use money to get another set. 243 casting would work well and would only take .035 mill to get you above 9.0
04-25-2009, 12:20 PM
#11
thats what I was afraid of, its hard to give up AFR for 243's tho
But believe or not the car drives really well with the set up I got even with the low compression, even NA its was pretty responsive.
But believe or not the car drives really well with the set up I got even with the low compression, even NA its was pretty responsive.
Last edited by SELLSZ06; 04-25-2009 at 12:42 PM.
04-27-2009, 06:50 PM
#13
im building a stock bottom end ls1 with comp cam .581 .581 200 224 cam for a toyota mud truck i got a set of 799 heads for it. how much can i take off them with stock gm gaskets and not have ptv clearance issues? want as much compression as possible. but mite put a 150 shot on it too.
