[TNHARLEY]

I did the HD big bore kit with 203 cams with stock heads? What compression ratio am I running now. Thanks mike

 

[springer-]

There technically isn't enough information to answer your question. You would also need to know your deck height, head gasket thickness and actual combustion chamber sizes.

But on a guess, assuming 85cc combustion chambers, a deck height of 0 and stock gaskets would be about 9.1:1

Assuming larger than spec combustion chamber (typical) around 8.9-9.0:1

This is static compression.

 

[adlerx]

There technically isn't enough information to answer your question. You would also need to know your deck height, head gasket thickness and actual combustion chamber sizes.

But on a guess, assuming 85cc combustion chambers, a deck height of 0 and stock gaskets would be about 8.6:1

Assuming larger than spec combustion chamber (typical) around 8.4-8.5:1

This is static compression.

Springer, doesn't the big bore kit come with SE flat tops? I thought those were listed at 9.4:1 so maybe a little less cuz of the combustion chamber size? School me oh wise one.......

 

[springer-]

Springer, doesn't the big bore kit come with SE flat tops? I thought those were listed at 9.4:1 so maybe a little less cuz of the combustion chamber size? School me oh wise one.......

Wasn't paying attention ... numbers are corrected to reflect a 95"

 

[springer-]

Springer, doesn't the big bore kit come with SE flat tops? I thought those were listed at 9.4:1 so maybe a little less cuz of the combustion chamber size? School me oh wise one.......

Here's the thing... :roflback:

I can't make HD numbers come out right based on their published specification. For example....

Stock engine says 8.9:1 CR

If you use 3.75" x 4.00" (bore x stroke)
Head Gasket 3.85" x .052" (bore x stroke)
Deck height = 0.00"
Piston head volume = 0 cc
Combustion chamber volume = 85 cc

I come up with 8.63:1 CR. Add to that, the pistons have valve reliefs and the combustion chamber typically run larger. You could be adding on upwards of 6 cc's !!! That brings it to around 8.2:1 CR. Also keep in mind that when the engine is hot it grows slightly and that reduces the CR also.

The stock head gasket after crush measures about .045 and barely makes a difference in the CR.


I can't get any of there Compression ratio's to come out right, they're all lower than what HD claims.

If I am wrong in figuring this out, maybe someone (Doc) can correct me. But I think my numbers are right. :dunno: :dunno:

 

[HIPPO]

Here's the thing... :roflback:

I can't make HD numbers come out right based on their published specification. For example....

Stock engine says 8.9:1 CR

If you use 3.75" x 4.00" (bore x stroke)
Head Gasket 3.85" x .052" (bore x stroke)
Deck height = 0.00"
Piston head volume = 0 cc
Combustion chamber volume = 85 cc

I come up with 8.63:1 CR. Add to that, the pistons have valve reliefs and the combustion chamber typically run larger. You could be adding on upwards of 6 cc's !!! That brings it to around 8.2:1 CR. Also keep in mind that when the engine is hot it grows slightly and that reduces the CR also.

The stock head gasket after crush measures about .045 and barely makes a difference in the CR.


I can't get any of there Compression ratio's to come out right, they're all lower than what HD claims.

If I am wrong in figuring this out, maybe someone (Doc) can correct me. But I think my numbers are right. :dunno: :dunno:

Your numbers are correct, it's Harley's that are a figment of their imagination.

 

[adlerx]

Here's the thing... :roflback:

I can't make HD numbers come out right based on their published specification. For example....

Stock engine says 8.9:1 CR

If you use 3.75" x 4.00" (bore x stroke)
Head Gasket 3.85" x .052" (bore x stroke)
Deck height = 0.00"
Piston head volume = 0 cc
Combustion chamber volume = 85 cc

I come up with 8.63:1 CR. Add to that, the pistons have valve reliefs and the combustion chamber typically run larger. You could be adding on upwards of 6 cc's !!! That brings it to around 8.2:1 CR. Also keep in mind that when the engine is hot it grows slightly and that reduces the CR also.

The stock head gasket after crush measures about .045 and barely makes a difference in the CR.


I can't get any of there Compression ratio's to come out right, they're all lower than what HD claims.

If I am wrong in figuring this out, maybe someone (Doc) can correct me. But I think my numbers are right. :dunno: :dunno:

OK, so I'll assume what you and Hippo say is correct. By your calculations, what would the Hi Comp Cast ones come in at with stock (bigboyz) heads. Hope you see where I am going, don't really wanna mill my heads but still trying to get to that magic 9.8-9.9:1 comp ratio. I know without ccing my heads it is hard to figure. I just don't want to put the hi-comps in there and end up with the SE claimed 10.25:1 but from your calculations (I flunked math) it sounds like i'd be fine running the hi-comps with my 37g cams. Guess I should have had bean cc and match my heads when I had em there but I really knew absolutely nothing(even less than I know now if that's possible) about any of this stuff when I sent em in last spring. I already have the flat tops but that's like the cheapest part of the build, I can get the hi-comps and put the flat tops on ebay.
Any input welcomed
thanks, plm

 

[springer-]

OK, so I'll assume what you and Hippo say is correct. By your calculations, what would the Hi Comp Cast ones come in at with stock (bigboyz) heads. Hope you see where I am going, don't really wanna mill my heads but still trying to get to that magic 9.8-9.9:1 comp ratio. I know without ccing my heads it is hard to figure. I just don't want to put the hi-comps in there and end up with the SE claimed 10.25:1 but from your calculations (I flunked math) it sounds like i'd be fine running the hi-comps with my 37g cams. Guess I should have had bean cc and match my heads when I had em there but I really knew absolutely nothing(even less than I know now if that's possible) about any of this stuff when I sent em in last spring. I already have the flat tops but that's like the cheapest part of the build, I can get the hi-comps and put the flat tops on ebay.
Any input welcomed
thanks, plm

I have to make a couple of assumption, piston head volume is unknown, deck height = 0, combustion chambers = 85 cc, stock head gasket. Using reverse calculations I can only assume the piston head volume is around -8.5 cc (can anyone confirm this?).

Using the numbers above yields 10:1 CR

If I increase the combustion chamber volume to 87 cc I get 9.8:1 CR

I would think you fall somewhere in there.

With the flat tops, I would guess you are around 8.9:1 and if you use the cometic gasket around 9.1:1

Actual measurements is the only correct way to determine exactly what you have. These are rough guesses but should be in the ballpark.

 

[adlerx]

Thanks alot springer I'll take my chances. Ballpark is close enough for me, I could live with 10:1, and from everything I've read here the chamabers are usually a little larger than the 85cc so likely I'll be a little below 10 which is perfect! I just really don't wanna be all the way up to 10.25.
Thanks a million for the input!

 

[TNHARLEY]

SPRINGER
with the build that I have now if I add the SE heads this winter will it help much. Mike

 

[springer-]

SPRINGER
with the build that I have now if I add the SE heads this winter will it help much. Mike

Yes, but IMO your stock heads can be made to out perform the SE heads for half the cost.

Click down there.
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[bubbarosa]

Another option would be the SE Forged Domed piston. Smaller dome than the cast ones and with a .030 head gasket should yeild you 9.7 to 9.9 compression.

 

[springer-]

A cast piston it a better choice for a street engine that isn't going past 6200 RPM's and not over 10.5:1 CR. IMHO

For more info checkout this thread http://www.v-twinforums.com/forums/showthread.php?t=57196