pistons

[pig pin]

i have s.e. pistons #22851-99 witch calls for 9.4:1. compression. i have 26gs in now. if i use 37gs andrews how much compression can i go? Can i go up to 9.8:1 with these pistons?

[springer-]

The pistons are flat top pistons and I wouldn't go by the HD published compression ratio. You can use the pistons but you have to measure everything and mill the heads to achieve the desired compression ratio.

You can use our compression calculator to help figure everything out .. http://www.bigboyzheadporting.com/TwinCamComp.htm

[pig pin]

Originally Posted by springer-

The pistons are flat top pistons and I wouldn't go by the HD published compression ratio. You can use the pistons but you have to measure everything and mill the heads to achieve the desired compression ratio.

You can use our compression calculator to help figure everything out .. http://www.bigboyzheadporting.com/TwinCamComp.htm

ya springer these are flat tops. and i have used your calculator.. thanks. i just wasnt sure if the pistons would hold up to that much compression. (9.8:1)? have you every heard of someone using these pistons for 37gs going that hight of compression?

[springer-]

They are SE Cast pistons and are good to at least 10.25:1 static compression. No problem running them to 9.8:1.

If you have a bunch of time to kill and you want to read about pistons ... http://www.v-twinforum.com/forums/v-...ine-95-bb.html that will keep you busy for a while. Otherwise you are good to go.

[pig pin]

Originally Posted by springer-

They are SE Cast pistons and are good to at least 10.25:1 static compression. No problem running them to 9.8:1.

If you have a bunch of time to kill and you want to read about pistons ... http://www.v-twinforum.com/forums/v-...ine-95-bb.html that will keep you busy for a while. Otherwise you are good to go.

thanks for that springer. i started reading other forum it has my interest. i am going back to finish it. just wanted to say thanks.

[pig pin]

springer you mention the cast pistons in the explanation of, cast over forged. you talk about 10.25:1. these pistons are pn 22661-99A mine are 22851-A witch are 94:1 now i am not sure you meant i could take the 94:1. to 10.25 :1 or you though i had 10.25:1?

[springer-]

Originally Posted by pig pin

springer you mention the cast pistons in the explanation of, cast over forged. you talk about 10.25:1. these pistons are pn 22661-99A mine are 22851-A witch are 94:1 now i am not sure you meant i could take the 94:1. to 10.25 :1 or you though i had 10.25:1?

A compression ratio for a piston is not a limit. A compression ratio defines what the compression would be if you use that piston with predefined other parts like 85cc heads. If you use a 9.4:1 pistons with a head of smaller size such as 82cc, then the compression will be higher than the 9.4:1 advertised compression.

Cast pistons are made from cast aluminum. The cast aluminum will hold up to a certain amount of abuse. Cast pistons will hold up to an including 10.25:1 compression. Once you exceed that compression it make more sense to use a forged piston because it will take more abuse. There are trade offs with each type of piston. In your case you are only looking to go to 9.8:1 compression. The pistons you have till do that without a problem since they are cast and will be good to 10.25:1.

[pig pin]

Originally Posted by springer-

A compression ratio for a piston is not a limit. A compression ratio defines what the compression would be if you use that piston with predefined other parts like 85cc heads. If you use a 9.4:1 pistons with a head of smaller size such as 82cc, then the compression will be higher than the 9.4:1 advertised compression.

Cast pistons are made from cast aluminum. The cast aluminum will hold up to a certain amount of abuse. Cast pistons will hold up to an including 10.25:1 compression. Once you exceed that compression it make more sense to use a forged piston because it will take more abuse. There are trade offs with each type of piston. In your case you are only looking to go to 9.8:1 compression. The pistons you have till do that without a problem since they are cast and will be good to 10.25:1.

thank so much for clearing this up for me. you guys will be getting my heads soon.

[rebel78]

Originally Posted by springer-

A compression ratio for a piston is not a limit. A compression ratio defines what the compression would be if you use that piston with predefined other parts like 85cc heads. If you use a 9.4:1 pistons with a head of smaller size such as 82cc, then the compression will be higher than the 9.4:1 advertised compression.

Cast pistons are made from cast aluminum. The cast aluminum will hold up to a certain amount of abuse. Cast pistons will hold up to an including 10.25:1 compression. Once you exceed that compression it make more sense to use a forged piston because it will take more abuse. There are trade offs with each type of piston. In your case you are only looking to go to 9.8:1 compression. The pistons you have till do that without a problem since they are cast and will be good to 10.25:1.

Hey Springer, can you plz explain a bit more about the trade off for forged vs cast pistons? Thanx

[springer-]

Originally Posted by rebel78

Hey Springer, can you plz explain a bit more about the trade off for forged vs cast pistons? Thanx

Read post #4 ... http://www.v-twinforum.com/forums/v-...ml#post1508237

Thanks

[route66paul]

The KBs are made with valve reliefs that will accept up to 2.02 intake valves. There has been some trouble with certain SE and other brands in this regard. i would advise anyone to clay it fit anyway, but the KBs are said to be trouble free in this area.

[Hillside]

Originally Posted by pig pin

i have s.e. pistons #22851-99 witch calls for 9.4:1. compression. i have 26gs in now. if i use 37gs andrews how much compression can i go? Can i go up to 9.8:1 with these pistons?

The 37 will run good at 9.8.
That can be established with those pistons.

[HarlessR]

Just wondering if you start out with say, 9.8:1 with a new build (37G cams) and after a few thousand miles you start to build up carbon, does that effect the compression ratio? Can you get too much compression due to carbon build up? Just wondering.

[nw_guy4_fun]

Yes and No
When the mixture is properly set and there is just normal oil consumption and good motor health (low blow-by) there will be minimal buildup. With buildup there will be pinging at even conservative compression ratios.