88 twincam compression ratio?
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88 twincam compression ratio? - 5/15/2008 7:54:14 PM
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23champ
Posts: 18
Joined: 5/24/2007
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Anyone know what the stock compression ratio is on a twin cam 88? Have thoughts of purchasing the Andrews tw37b cams. Was wondering if stock comp ratio was high enough to really make much of a diff?
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RE: 88 twincam compression ratio? - 5/16/2008 6:41:15 AM
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mtclassic
Posts: 345
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in a stock 88 with no head work (lowering the head cc's) and without a thinner headgasket the cmpression will be too low for a 37 to perform well. I know 3 or 4 people that put 37's in thier stage 1 bikes and were disappointed. Harley might claim 9 to 1 but in reality it is probably closer to 8.4 to 1. If you want to change cams 21's or 26's will be a better choice in a stage 1 88.
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RE: 88 twincam compression ratio? - 5/16/2008 12:59:57 PM
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23champ
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how much would have to be taken off the heads for it to run properly? I'm having head work done right now. Where do you get a thinner gasket?
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RE: 88 twincam compression ratio? - 5/16/2008 1:13:15 PM
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mtclassic
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http://www.bigboyzcycles.com/TwinCamComp.htm here is a great resource to see what cc your heads need to be for your desired compression. Cometic is the supplier of the .030 head gasket. any dealer or indy should be able to order you one or you can order direct. If someone is doing your heads they should have the knowledge of what your heads should be set at to run with 37's if they don't have the knowledge then you might want to find another head porter.
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RE: 88 twincam compression ratio? - 5/19/2008 4:17:56 PM
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23champ
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so are you guys lowering the cc by milling the heads down? If so, how much are you taking off, like 10 or 20 thou? How much valve clearence does that leave you with stock pistons?
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RE: 88 twincam compression ratio? - 5/19/2008 5:29:46 PM
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mtclassic
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The amount you shave off is totally dependant on how big the volume is to start with. If your heads are 89cc's (some are mine were) then you might have to take 40 thou off to get to 83cc. If you are staying 88 you might need to go smaller than 83cc.
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RE: 88 twincam compression ratio? - 5/19/2008 6:00:30 PM
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23champ
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well, I've got other issues now. I had already ordered the tw37's before anyone replied to this thread (not a big deal, it's not really that big of a deal to pull the heads off). Unfortunatly I have already assembled the motor and fired it up, only to discover that something is not right with the rear cylinder. The exhaust sound is a little on the strange side. sounds like its echoing in the pipe??? Very strange, dont really know how to describe it. Front pipe sounds good. If you put your hand over the end of the pipe (on the rear cylinder, while running) it wants to suck on your hand, instead of blowing it back. Front pipe is good, blows your hand back like normal. I thought at first perhaps the cam timing is off...I checked it, and it was. One tooth off from the crank shaft to the rear cam. After correcting that problem, and checking to see it the dots on the cams lined up (which they were). I fired it back up with the same results, funky sounding exhaust sound out of rear cylinder. So now I'm thinking I have a bent valve. Plug a compression gauge on it, and come up with a little over 150psi on each cylinder. That should mean the valves are sealing correct? Whats going on here?..Got me scratchin my head.
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RE: 88 twincam compression ratio? - 5/20/2008 5:09:45 PM
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23champ
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no suggestions?
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RE: 88 twincam compression ratio? - 5/20/2008 7:30:53 PM
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Colonel Sanders
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It sucks instead of blows? I'm just speculating but that would sure have me wondering if the cam were somehow installed incorrectly or (since you say it's not) that it was ground incorrectly. Strange. Or....maybe the exhaust valvespring is broken? I take it that cylinder is not firing so perhaps with a broken spring on the exhaust side it's sucking in on the combustion stroke.
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RE: 88 twincam compression ratio? - 5/21/2008 4:08:55 AM
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BVBOB
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My guess would be that you still have the cams installed incorrectly, if you think about it though, if the outer gears are correct, the rear cannot be off because of the fact that the front cylinder cam is driven off of the rear, so if the inner cam drive is the problem it would seem as though the front cylinder would be the odd one. Go back in and check the cam to cam alignment, if you have chain drive be sure and check the dots with the tensioner released, I had a bike in once with this problem and I looked at the dots for 15 minutes and would have sworn they were correct, after releasing the tensioner to take the slack out of the chain it was easy to see they were off a tooth, and yes, it had an odd exhaust note, also when examining SERT data log the MAP was around 50 instead of the 35 range like it should have been. Doubt you hurt the valvetrain or you would not have compression, 150 sounds about right for 37's with no other work.
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