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Compression results before Rings seat....

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Compression results before Rings seat....

Postby DOHCPower » Sun Nov 19, 2006 8:31 pm

Hello all, i just fired up my 2 liter Honda engine today, fresh with new CP Pistons and rings. I ran her at a constant 2k rpm for about 15 minutes, then shut her down, drained the oil, replaced the filter, and filled her back up with oil. I then did a compression test just for kicks and got about 180psi all the way across. I was expecting a tad more (roughly 12:1 compression) but this is the first time ive ever done a compression test so early in the life of an engine.

My question is, how much can i expect the compression to go up once the rings are fully seated? I was looking for compression of somewhere in the neighborhood of 230-250psi. Thanks.
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Postby #84Dave » Sun Nov 19, 2006 9:33 pm

It depends primarily on the nature of the cams you're using and where their centerlines are sitting. In the end, assuming the assembly is sealed well, it's the dynamic compression ratio that'll win races for you. At least as far as the engine is concerned.

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Postby DOHCPower » Sun Nov 19, 2006 9:37 pm

I understand the concept of Dynamic Compression, and its importance in power output, especially at the high end of the rev range, most of it being dictated by intake valve closing. However, im just looking for an answer as to how much the rings actually 'seal' once they have seated.
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Postby DOHCPower » Sun Nov 19, 2006 9:43 pm

oh and for what its worth, im running around 102 lsa. These are with the cams 'straight up'
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Postby MadBill » Sun Nov 19, 2006 10:10 pm

First, there a lot of discussion on the right way to break in rings, (it's a little late, but do a SpeedTalk search or check out: http://mototuneusa.com/thanx.htm ) but I think there will be good agreement here that yours has been anything but so far! Rings need pressure, limited lubrication (NOT synthetic!) and a variety of operating conditions to get friendly with the bores. (I know, what you did is recommended for flat tappet cam break-in, but...)
That said, with a little luck, yours haven't done much of anything yet. At this point I'd just drive it hard: Short periods of full throttle, soon extending the WOT bursts towards the red line. Lots of decelerations and load changes and anything but steady part throttle. Plan B. would be to wait for a few more suggestions from this forum, then pick one, keeping in mind that: "The man with a watch knows the time. The man with two is never quite sure..."
Next, a leakdown test is a far better indicator of ring seal than cranking compression, especially on an new or unknown combination. A relatively small bore like yours with advanced tech rings should show maybe 5% brand new and less than 2% seated. Ideally, this will translate to no more than a 10 psi increase when sealed. Check it now, hammer it for 500 miles and verify.
Lastly, JIC you'd like to verify what the cranking compression should be, here's one of the better sites re DCR, which includes altitude in it's cranking compression estimate: http://www.wallaceracing.com/Calculators.htm
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Postby DOHCPower » Sun Nov 19, 2006 10:35 pm

Thanks for the response, madbill. I am planning on fully seating the rings on the dyno, and only got it up to operating temperature today so i could ensure she was ready for the dyno session (no leaks, timing in check etc. etc.) Im planning on 'sneaking up' on the redline, meaning, make sure a/f is good at idle, then pulling to 4k, cleaning up the maps a bit, then pull to 6k, etc. etc. until full redline is realized. By this time, im hoping the rings will be 95% seated.
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Postby Wolfplace » Sun Nov 19, 2006 11:37 pm

My opinion is if the bores were done correctly & you did not drown the thing on assembly the rings are damn near seated on the starter :wink:
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Postby DOHCPower » Sun Nov 19, 2006 11:49 pm

uh oh, we're treading into controversial territory here! ;)
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Postby Cobra » Mon Nov 20, 2006 1:18 am

Mike makes a good point. Most piston rings come pre-lapped and are essentially preseated when combined with a proper plateau finish in the cylinder. The cylinder finish should approximate a broken-in engine and will minimize wear on new rings. A profilometer reading of around 20 microinches RA is typical. Follow piston and ring manufacturers specs as different ring materials will require unique handling. I would like to see a longer intial run time on a new engine then what you applied.
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Postby DOHCPower » Mon Dec 18, 2006 2:35 am

Just a little update for anyone interested...

Towed the engine to the dyno facility, and didnt have much time, and wound up with 238hp at 8000rpm. This is with the camshafts zero'ed and the ignition maps untouched. The plan was for 9000rpm but we ran out of time, and left it at that. I drove it home and performed a compression test, and was greeted with 250psi across all four cylinders.
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Postby Larry Sockwell » Mon Dec 18, 2006 10:14 am

180 up to 250?

I'd be asking my machinist a few questions and changing the oil, NOW.


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Postby DOHCPower » Mon Dec 18, 2006 8:58 pm

Larry Sockwell wrote:180 up to 250?

I'd be asking my machinist a few questions and changing the oil, NOW.


Larry


elaborate please.
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Postby Larry Sockwell » Mon Dec 18, 2006 10:17 pm

A 70psi change in cranking compression before and after one dyno test implies that you had an ill prepared bore finish. I've never seen more than a 15psi difference before and after ring seating and that was on a re-ring job with a hand drill and ball hone about 15yrs ago.

70psi is a huge and unacceptable difference. Lots of bore/ring material had to be ground away to make that kind of difference. Not good stuff in a new engine.


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Postby Ed-vancedEngines » Mon Dec 18, 2006 10:53 pm

Larry,
I am wondering if possibly one test was made with it cold and the other with it warmed up. Also I am wondering if there was any valve adjusting done between the two testings.

If there was an even test at same engine temp and with throttle open in both tests and no other things were changed, I am with you. He could have a filter and pan full of metal. That makes no sense if both compression tests were done in equal conditions.

I did think of something else that is not good. If his cam is going away that would allow it to show more compression.

I doubt if anyone moved any cam inbetween the two compression tests to change anything there.

Also This does not make sense to me.
Towed the engine to the dyno facility, and didnt have much time, and wound up with 238hp at 8000rpm. This is with the camshafts zero'ed and the ignition maps untouched. The plan was for 9000rpm but we ran out of time, and left it at that. I drove it home and performed a compression test, and was greeted with 250psi across all four cylinders


You towed the engine to the dyno facility?
And then you drove it home?

If I can read right you did not say that you towed your car. You said you towed your engine.

Then you drove it home?
Obviously you are not driving an engine home. It would be tough to steer. :)
So the only thing I can see is that you towed the car to the chassis dyno and then drove it home?

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Postby DOHCPower » Tue Dec 19, 2006 12:26 am

im sorry guys, i typed that waaaay to fast!

I towed the car, engine and all, to the dyno facility, made sure she had a nice safe a/f ratio and then we started doing pulls, starting from 5k and working our way on up.

Ed-vanced you must be a mind reader! The first compression test was with the engine around 100 deg F coolant temp, and the throttles shut, the second with a fresh valve adjustment, 140 deg F coolant temp, and the throttles wide open.

Im draining the oil tomorrow into a fresh clean clear container, and inspecting the filter. If im alarmed by what i see, ill be dropping the pan, and maybe sending an oil sample to Blackstone Labs. Thanks for the replies.
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