Compression
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6th Gear
Joined: May 2006
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Compression
HI I have a 1996 flhr and it has 33500 miles on it. I did a compression test and got 150 front ,151 rear? Could there be engine work done? Looks like inner primary and case has been split!
Tourer
Joined: Feb 2006
Posts: 429
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From: Southampton, Pa
RE: Compression
Compression like you have means that you have an engine that the rings are seated and you don't have any valve or gasket problems. As far as if it's had work done it's hard to tell. You should be good to go. my.02
Tourer
Joined: May 2006
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RE: Compression
Sounds good to me. That's well within the recommended range for an engine with stock or slightly higher compression. My EVO is at 195-200 psi, but I have 10:1 compression and an EV27 cam. The actual compression ratio AND the cam both affect cranking pressure.
Tourer
Joined: May 2006
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RE: Compression
If it's got adjustable pushrods, then there's a good chance that it has a non-stock cam. You can always pull the cam out to see what it is. It's easy, and all you'll need is a new gasket. I don't know why they don't put the cam's ID on the outside where it can be easily seen..... ????? You should be able to tell if the pistons are flattops by looking through the spark plug holes. Of course, the compression could have been raised by shaving the heads... which would also require adjustable pushrods.
Road Warrior
Joined: Dec 2005
Posts: 1,950
Likes: 2
From: Colorado Springs, CO
RE: Compression
Well, I'd say there's been some engine changes, not necessarily good ones. And if the cases have been split, is it a stroker? Here's what I would do:
Look in the cylinder and see what the pistons are like; dished, flat tops, or raised. Then you can guess at the approximate compression ratio. dished = 8.9, flat tops=9.5, and raised = 10.25. There are some other variables that may affect that (like .030 head gasket), but that estimation should get you in the ball park
Take that info to the RBR racing web site. Plug in info and see what you get. Start with the intake valve closing at 40*. Adjust that valve closing # until you get a dynamic cylinder pressure around 150 psi.
Once you know the intake valve closing point, you can take a WAG at the cam you have. Not the exact grind/part number, but what kind of cam it is. If it's <40*, it's a torque cam. If it's >40*, it's a high rpm high hp cam - I suspect this is what you have. Like I said, that's a WAG.
The only other way to tell is to remove the cam and maybe it will have some markings on it. But often times they do not, so you have to try and mic it out and see what you get.
At your altitude, you want some pop up pistons and and a closing point of 35-40*'ish. That will give you good compression and good torque on mid grade or premium. I live at 6,500' ASL, and I use low octane fuel with that combo.
Look in the cylinder and see what the pistons are like; dished, flat tops, or raised. Then you can guess at the approximate compression ratio. dished = 8.9, flat tops=9.5, and raised = 10.25. There are some other variables that may affect that (like .030 head gasket), but that estimation should get you in the ball park
Take that info to the RBR racing web site. Plug in info and see what you get. Start with the intake valve closing at 40*. Adjust that valve closing # until you get a dynamic cylinder pressure around 150 psi.
Once you know the intake valve closing point, you can take a WAG at the cam you have. Not the exact grind/part number, but what kind of cam it is. If it's <40*, it's a torque cam. If it's >40*, it's a high rpm high hp cam - I suspect this is what you have. Like I said, that's a WAG.
The only other way to tell is to remove the cam and maybe it will have some markings on it. But often times they do not, so you have to try and mic it out and see what you get.
At your altitude, you want some pop up pistons and and a closing point of 35-40*'ish. That will give you good compression and good torque on mid grade or premium. I live at 6,500' ASL, and I use low octane fuel with that combo.
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6th Gear
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Road Warrior
Joined: Dec 2005
Posts: 1,950
Likes: 2
From: Colorado Springs, CO
RE: Compression
Not a pain at all. You're looking for the advanced dynamic compression calculater.
RBR Racing - Calculations
RBR Racing - Calculations
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