Compression

Classic short-frame models

Compression

Postby Otis » Sat Oct 31, 2020 3:18 pm

I recently rebuilt my 66 xlch and compression was low in the front. Turns out it had a piss poor bore job and getting it honed again seems to have solved the issue. I found that testing compression with a kicker gave low inconsistent results so I tested it with a set of rollers designed for starting racing bikes. Now I’m getting 160 psi front and back. Just out of curiosity what is the standard range for a 900 with stock pistons? Seems like 115-125 seems about right and I’m wondering where 160 is coming from.
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Re: Compression

Postby Ferrous_Head » Sat Oct 31, 2020 7:45 pm

140 is about right for a well built stocker.

At 90 PSI the things will barely run and smoke like a chimney.

An engine showing 115 is mathamatically under 8 to 1.

I always check my cranking pressures with rollers. Kicking them, especially "cammed" engines won't give you an accurate number. Unfortunately, that's what far too many people do.

I'm running 215 in my current race engine which is a little over 14 to 1. (We can run Methanol)
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Re: Compression

Postby Otis » Sun Nov 01, 2020 6:56 am

So is 160 psi out of the norm for a stock build. I know it’s not a bad thing.
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Re: Compression

Postby Ferrous_Head » Sun Nov 01, 2020 9:28 am

The last engine I built that was close to stock was stock bore and stroke. A little port work, XLR valves and Hustler cans.

That engine went 165. It was more than I expected.

From memory it had no base gaskets or head gaskets. (lapped in cylinders).

But I'm very careful with boring and honing operations as well as a rigorous break in regime. Most people do not go to the trouble I do in that area.

I am not sure that the heads weren't decked before I got to them. I had suspected that they had. Valve to piston clearance was only 060.

I suspect if your has gone 160 you should be pretty happy. If that's the first test you may find it picks up another 5 after some use.
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Re: Compression

Postby panic » Sun Nov 01, 2020 10:16 am

Never mind
Last edited by panic on Mon Nov 02, 2020 7:28 pm, edited 1 time in total.
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Re: Compression

Postby hayleyl » Mon Nov 02, 2020 7:20 pm

Hello all - here is my two cents worth...you can't conclude or suppose any exact connection between test pressure and compression ratio without knowing the cam profile. If you take the pressure indicated on the compression tester and divide it by 14.7 which is atmospheric pressure (I think) you have actual compression ratio. If you take volume at bdc and divide it by volume at tdc you get a theoretical compression ratio. Also gauge pressure reading will generally be lower than absolute pressure. So if your gauge is showing 160, then the absolute pressure will be higher. You may experience issues with some pump gas. Hope this makes sense. Something around 8:1 -8.5:1 on a stock XL is OK. Cheers Hayley.
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Re: Compression

Postby Ferrous_Head » Mon Nov 02, 2020 9:24 pm

I only use the ratios as an proximate guide. I always check the cranking pressure and then if someone asks me I'll work it out as a ratio but a lot of people just go by what the piston is SUPPOSED to provide.

The stock Sportster 900 is listed as having 9 to 1 compression. That would be appropriately 135 PSI but the numbers can vary due to so many things. Ambient temperature, altitude above sea level, leakage, cranking speed.

The actual numbers are that important to me as the limiting factor for so many engines will be the fuel your using. All my race engines for the past 10 years have run on Methanol so I can run very much higher pressures.

I'm not sure how good the gas you guys are getting is but ours is not great. It doesn't matter in the newer engines that have exceleent (comparative) combustion chambers designs.

But our poor old hemi's in cast iron need a little help.

The 900 I ran on gas I ran on Avgas with a touch of benzene. Not pump gas. Even then I holed a piston when I had the timing out a couple degrees.
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Re: Compression

Postby hayleyl » Mon Nov 02, 2020 11:07 pm

Yes Ferrous, the fuel we get here in Australia is less than impressive, particularly from chain petrol stations owned by syndicates from the sub continent that have been known to 'shandy' their products. I add a dash of Octane Supreme (tetra ethyl lead) every tank full to my old scooters. (37 OHV Norton, 1980 Shovester) even my 2005 XL1200R appreciates it. Not sure what similar products are available in the US. I've found BP is the most consistent sound our parts. Cheers Hayley.
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Re: Compression

Postby Otis » Tue Nov 03, 2020 7:18 am

I have been blending 50-50 non ethanol 90 octane with 100 octane non ethanol low lead airplane fuel. I should be covered with that mix but the 100 octane is hard to source and with a 2 gallon tank I can’t go far before needing pump gas.
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Re: Compression

Postby Ferrous_Head » Tue Nov 03, 2020 4:45 pm

It's a problem or our poor old Ironheads I'm afraid. The quality of gas varies everywhere. It's volatile and goes "off". So buying from a country station that only gets a new tanker of gas every second month has it's own problems. When it's marginal to start with ......
Maybe just buy a Honda to ride and have a Harley to work on ?
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