compression question
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Tourer
Joined: Nov 2005
Posts: 282
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From: Georgia
compression question
So how do I know which pistons I have.
I have found8.4:1, 9:1,and 10:1 pistons with 3 3/16" bore available. If I have 9:1, I'd like to go with 10:1 if it's safe. If I have 10:1 right now than I assume it's correct or OK. This is just in case I need to re-bore due to wear on cylinders. Not looking to "bore and stroke", just what's nessessary and maybe a bit extra. According to the shop manual "3A" denotes xl and xlh. Assuming xl would be 9:1 and xlh would be 10:1, which do I have and is it ok to step it up from xl to xlh. I know I've repeated myself a bit but info on harleys always seem to be criptic, miss-leading, or incomplete.
Rich
I have found8.4:1, 9:1,and 10:1 pistons with 3 3/16" bore available. If I have 9:1, I'd like to go with 10:1 if it's safe. If I have 10:1 right now than I assume it's correct or OK. This is just in case I need to re-bore due to wear on cylinders. Not looking to "bore and stroke", just what's nessessary and maybe a bit extra. According to the shop manual "3A" denotes xl and xlh. Assuming xl would be 9:1 and xlh would be 10:1, which do I have and is it ok to step it up from xl to xlh. I know I've repeated myself a bit but info on harleys always seem to be criptic, miss-leading, or incomplete.
Rich
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
RE: compression question
Rich....
The XLH and the XLCH use the same piston.....9:1....and that is what came standard in both.
The only way to determine this is to measure the 'displacement amount' of the piston dome. Complicated and not worth doing.
You have 9:1 and you need to replace the pistons with 9:1.
The octane rating is so low today (compared to days of old) that you would be happy with an 8.5:1 set of pistons if you can find them. High compression ratio was great in the old days of 101 octane gas, but those days are long gone. Don't even consider 10:1 pistons........pg
The XLH and the XLCH use the same piston.....9:1....and that is what came standard in both.
The only way to determine this is to measure the 'displacement amount' of the piston dome. Complicated and not worth doing.
You have 9:1 and you need to replace the pistons with 9:1.
The octane rating is so low today (compared to days of old) that you would be happy with an 8.5:1 set of pistons if you can find them. High compression ratio was great in the old days of 101 octane gas, but those days are long gone. Don't even consider 10:1 pistons........pg
Intermediate
Joined: May 2007
Posts: 27
Likes: 0
RE: compression question
PG
Years ago I had my clyindersbored out .10 over sizeand I replaced the pistons. My memory is shot and I don't remember the specifics but based on your reply to Rich, does that mean Ihave 10:1 pistons?
Jim
"78 XL-original
[IMG]local://upfiles/33563/F365393245E44F1D8F64CC3E25F30D7C.jpg[/IMG]
Years ago I had my clyindersbored out .10 over sizeand I replaced the pistons. My memory is shot and I don't remember the specifics but based on your reply to Rich, does that mean Ihave 10:1 pistons?
Jim
"78 XL-original
[IMG]local://upfiles/33563/F365393245E44F1D8F64CC3E25F30D7C.jpg[/IMG]
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
RE: compression question
Jim....
No, the 10:1 compression ratio has nothing to do with overbore size.
The ratio is related to the size of the dome on the piston and the area of the head recess that receives the compressed air.
. A higher dome will result in a higher compression ratio, all other things being equal. By that I mean the head has not been altered inside the compression chamber area........pg
No, the 10:1 compression ratio has nothing to do with overbore size.
The ratio is related to the size of the dome on the piston and the area of the head recess that receives the compressed air.
. A higher dome will result in a higher compression ratio, all other things being equal. By that I mean the head has not been altered inside the compression chamber area........pg
Road Warrior
Joined: Nov 2006
Posts: 1,450
Likes: 798
From: Heart of Dixie
RE: compression question
Most of the replacement pistons I've seen fall into two categories - 9:1 cast pistons and 10:1 forged pistons. The cast ones (stock style) are cheaper and a little more quiet. The forged ones are stronger and a little more noisy when cold (because they require more clearance). For a mildly modded engine, the 9:1 cast pistons are fine. I think using 10:1 pistons on an Ironhead without dual plugs is asking for detonation trouble with today's crummy gas. If you use the forged 10:1 pistons, a 1982 head gasket is thicker and will lower the compression a little, maybe to around 9.7:1.
Trending Topics
Club Member
Joined: Feb 2008
Posts: 18,575
Likes: 4
From: Norfolk, VA
RE: compression question
I agree with pinion on the compression. Stay with 9:1 because it's not worth it if you gas up with 93 on the road with 10:1 pistons and the motor starts knockin' shortly after. Those domed pistons are unforgiving with high compression and standard 93 pump gas. You won't notice much difference between 9:1 vs 10:1 performance wise, but your starter sure will! Your right leg will also tell you so if you're kickin' it to life.
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