Compression Problem

[Archdeacon]

I just completed a 120, TMan Thumper heads with stock style castings, 660SM cam, SE 58mm throttle body, Supermeg exhaust and compression at 10.7/1.  After running heat cycles I checked the compression and it was 185-190 which is where the calculator said it should be at for my elevation.  Put a few miles on it and took it to the tuner.  I didn't think it ran that great but assumed it to be the tune even though I had v-tuned it in the lower rpm ranges.  He ran a baseline run as delivered and it made 110/110 which sucks.  He checked the compression on the dyno and told me it was 145 on both cylinders.  I brought it back home and let it sit while running errands.  When I got back I checked the compression and had 190-195.  I do plan to leak it and check with another gauge tomorrow however this seems like a huge discrepancy just between gauges.  Although it did use some oil the first 250 miles, this trip to the tuner was about the same and it used 0 oil.  No obvious blowby, plugs look good and the oil is still clean.  Have any of you experienced similar issues before?  Is it possible for a roller rocker to hit only when hot and hold a valve open?

[Ohio HD]

The likelihood that some mechanical issue effects both cylinders and reduces the compression the same in both, is very unlikely. Stock heads, meaning 110 heads? ACR's? If yes, did the tuner disable them before the compression test? Try borrowing another gauge and test at home, see is it the same as your gauge. If the compression were 145, I doubt you'd see 110 hp.

[Archdeacon]

103 style heads, no ACR. Yes you are right a mechanical thing affecting both the same is very odd.

[Archdeacon]

I should have a leak down tester tomorrow to use as well.

[1FSTRK]

I would call the tuner and ask what the engine temp was when he did the ccp test and where he took the temp reading. Also if he had the throttle open, basically you want to replicate his test to duplicate the results before jumping to any conclusions on the engine. 

[Archdeacon]

Yes I plan to do more testing myself to see what I think is going on.  I do technical/troubleshooting on Cat engines so know most of the drill albeit apples/oranges.  You guys are a huge help on the details though.

[les]

Your hot cranking compression should be higher than your cold cranking compression measurements.  Make sure to do both at home with your tester.

[tmwmoose]

Its hard to find a good comp tester these days at least in a store forget it there all Chinese junk. But you being a Cat Tech you know this. Been away from the tool trucks for some time have no idea what they pedal and where there made. Craftsman is Chinese and I settled for that . Guess what I'm getting at is its going to be hard to find a couple different shops gauges to be calibrated the same

[PoorUB]

Its hard to find a good comp tester these days at least in a store forget it there all Chinese junk. But you being a Cat Tech you know this. Been away from the tool trucks for some time have no idea what they pedal and where there made. Craftsman is Chinese and I settled for that . Guess what I'm getting at is its going to be hard to find a couple different shops gauges to be calibrated the same

Find the local "tool truck", give him a call and find his schedule and meet him. I don't buy much Snap-On any more, but when I need it I look him up. They will sell to anyone with a good credit card or green cash.

[Ohio HD]

If you want to buy Snap On, just press "add to card".

https://store.snapon.com/Compression-Gauge-Sets-Set-Automotive-Compression-Gauge-P650760.aspx


My Mac gauge set is withing 1 to 2 pounds of my buddy's Snap On set. A 50 pound difference is simply one is way off, or the process on one attempt was wrong.

This is my Mac set.

Mac Compression Gauge Set

[Archdeacon]

Well my compression tester took a dump yesterday ironically.  Seemed to be accurate per the calculators for compression but it is stuck on 120 now.  SnapOn comes tomorrow morning so I may have to bite the bullet and get one. 

[Archdeacon]

Yes 50 pounds difference is crazy. My 103 compression on my gauge before it broke was also reading as it should and the gauge was hardly used at all.  I got a leak tester yesterday and am going to leak it tomorrow. I am beginning to think the combo just isn't going to work as far off as it seems to be.

[Ohio HD]

Are you sure the guy tuning it is up to the task? If the compression is right, that cam should do better.

[kd]

   Even with the lower 10.7:1 compression, you should be hitting high 130's or 140 ish with the parts you list.

[Archdeacon]

No I don't so I am going to take it to Trask in PHX next time.  I want to make sure the engine is healthy before I do however.

Yes I am looking for another 15/15 out of it.

One thing I was told by a vendor here that refurbed the heads that there could be reversion problems with the heads so I have that stuck in the back of my mind.

[1FSTRK]

No I don't so I am going to take it to Trask in PHX next time.  I want to make sure the engine is healthy before I do however.

Yes I am looking for another 15/15 out of it.

One thing I was told by a vendor here that refurbed the heads that there could be reversion problems with the heads so I have that stuck in the back of my mind.

So these are no longer Tman Thumper heads as delivered by Tman performance ?

[Archdeacon]

Just changed the guides, springs and a valve job.

[BVHOG]

A couple things stand out to me here. First off, what brand dyno, secondly, T-man heads are not T-man heads once someone else has worked on them and it can be misleading and downright detrimental to  TR to post this without stating that up front.  And "just a valve job" can be the difference from a just ok head to a great head.   You also have a supermeg, did the tuner tune the pipe for the build? if not then you could have a ton of power left to find.  Was the throttle blade open for the tests, were comp releases involved in any way?  What air cleaner?  Did you clearance properly for the roller rockers?

[1FSTRK]

A couple things stand out to me here. First off, what brand dyno, secondly, T-man heads are not T-man heads once someone else has worked on them and it can be misleading and downright detrimental to  TR to post this without stating that up front.  And "just a valve job" can be the difference from a just ok head to a great head.   You also have a supermeg, did the tuner tune the pipe for the build? if not then you could have a ton of power left to find.  Was the throttle blade open for the tests, were comp releases involved in any way?  What air cleaner?  Did you clearance properly for the roller rockers?

I agree BV all good points and that is where I was headed with my questioning.
They are no longer Tman heads once someone else has worked on them.

[Archdeacon]

Yes you are correct and that is my mistake and thanks for the correction.  They are not as from TR so I should have made that clear up front.

The guy only pulled it once, saw the lines were smooth and the AFR was reasonable, told me that it had 145 psi compression and took it off of the dyno.  The 145 psi began my thinking something was amiss in the engine and wanted to confirm by leak test before taking it to a different tuner.  I did clearance for the rockers on the valve end per the SE rocker instructions however I later saw that the pushrod side needs clearancing also but haven't had the covers back off yet to check for witness marks.

[kd]

If they are hitting you will hear it.

[Archdeacon]

If they are hitting you will hear it.

I would think you would too. I did however find this in an old post in the dyno forum.

If you have roller rockers , make sure they are clearances enough . I had a 124 that looked the same .
Once the rockers were clearanced , it went 146 hp .
You couldnt hear it , and a compression test wouldn't show it . Cranked 200 ccp
Leak down test showed a leak .

[Ohio HD]

If you get another gauge, and the reading isn't what it should be, it would be relatively easy enough to remove the rocker covers and try again, see what the compression is. You never know, strange things happen. Cam timing could also cause lower compression, and power output.

[BVHOG]

Yes you are correct and that is my mistake and thanks for the correction.  They are not as from TR so I should have made that clear up front.

The guy only pulled it once, saw the lines were smooth and the AFR was reasonable, told me that it had 145 psi compression and took it off of the dyno.  The 145 psi began my thinking something was amiss in the engine and wanted to confirm by leak test before taking it to a different tuner.  I did clearance for the rockers on the valve end per the SE rocker instructions however I later saw that the pushrod side needs clearancing also but haven't had the covers back off yet to check for witness marks.

not likely to be hitting on the pushrod end, if they were they would keep the valves closed not open. If there was not enough clearance then it is highly unlikely it would have even the 145 lbs  The numbers tell me it's the throttle blade not being open for testing causing the problem at the dyno shop.  Find a shop that will tune that Supermeg!!!

[Archdeacon]

Yes you are correct and that is my mistake and thanks for the correction.  They are not as from TR so I should have made that clear up front.

The guy only pulled it once, saw the lines were smooth and the AFR was reasonable, told me that it had 145 psi compression and took it off of the dyno.  The 145 psi began my thinking something was amiss in the engine and wanted to confirm by leak test before taking it to a different tuner.  I did clearance for the rockers on the valve end per the SE rocker instructions however I later saw that the pushrod side needs clearancing also but haven't had the covers back off yet to check for witness marks.

not likely to be hitting on the pushrod end, if they were they would keep the valves closed not open. If there was not enough clearance then it is highly unlikely it would have even the 145 lbs  The numbers tell me it's the throttle blade not being open for testing causing the problem at the dyno shop.  Find a shop that will tune that Supermeg!!!

That's the only reasonable answer I can think of or he simply didn't want to spend the extra time it took to get the bike tuned correctly.  Do you have an idea where to start disc wise?  I've got 20.

[BVHOG]

I would be starting at 12 with an open end cap

[kd]

An open end cap is equal to 8 to 10 discs according to SuperTrapp. I don't know how the relates to the difference between 3 1/2" or 4" discs but it is a start point to understanding the effect of the open end cap..

[Archdeacon]

Thanks guys.

[koko3052]

I would be starting with the compression discrepancy...one issue at a time or you will never get it running properly.

[motolocopat]

Well, what happened?

[barny7655]

A simple leak by test on front then rear will provide a answer,  before a hot comp test

[jjdalynh]

crummy comp tester.  that Chinese junk seems to work ok up to maybe 100 psi, then the gauge goes off.   I tried a couple different ones last year, then finally got an OTC (supposedly they make them for Matco as well?), and it was right on.  Chinese gauges said I was at ~175.  OTC said 210, which is right about what it should be.