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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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 Posted: Mon Oct 26, 2009 7:25 pm Post subject: cranking pressure |
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Cliff, here's one for you to chew on.
I've been doing both leak down and cranking pressure tests now for about a year. Tigers generally run 130-135. One Lycon Tiger was 145 across the board. Cheetahs run about 125 to 130.
I did a cranking pressure test on a Tiger last weekend with
(1) 125 psi
(2) 123 psi
(3) 142 psi
(4) 120 psi
I redid the #3 4 times and got the same value. Something is changing the valve timing events enough to cause this difference. I didn't go any further into it. The leak down tests were 79/78/78/78
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Gary
AuCountry Aviation
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flyv35b(at)minetfiber.com
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| Posted: Tue Oct 27, 2009 5:45 am Post subject: cranking pressure |
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I've have lots of other stuff to chew on but here is a thought without giving it much thought! You obviously are aware of dynamic compression ratio which is affected by valve timing and probably more significant that static CR. I would GUESS that something is different on the #3 cylinder valve timing like you mentioned. Short of a camshaft that is maybe not to spec I would suspect that an intake lifter is bleeding down rapidly or the dry tappet clearance is out of spec, resulting in an early intake valve closing, less overlap, and therefore higher dynamic CR and cranking pressure. I would wonder if the difference you are seeing would result in a rough running engine. How smooth does it run?
Also there could be other factors like the actual combustion chamber clearance volume and therefore static CR. On my Porsche engine a .010 2 change in piston deck height changes the static CR by 0.25 points. A small change in combustion chamber volume changes it even more. I could imagine some manufacturing variables with the cylinder which could at least partially account for the difference in cranking pressure. Seems like the easiest thing to check or try would be to check the dry tappet clearance on at least #3 and #4 and swap the intake hydraulic lifters and retest the cranking pressure. That might tell you a lot. Having the tester that Bill Scott uses to test the leakdown rate would be very informative possibly.
BTW, I assume the cranking pressures you mentioned for Cheetahs is with the stock engine with 7.0:1 CR?
Cliff
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Tue Oct 27, 2009 10:19 am Post subject: cranking pressure |
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Funny you should ask about the Cheetah compression ratio. I haven't seen a stock compression ratio Cheetah in years. Those values are for the HC pistons.
This particular engine with the weird #3 cranking pressure had the #1 cylinder replaced a few months ago by Pacific Air Care in Watsonville, CA. My thought is, the plane doesn't get much use and there is a good chance the lifters are all gummed up. I wish I had seen the plane to replace the cylinder. I'd know what shape the lifters were in.
When James Courtney had a cylinder fail over San Francisco bay I had a chance to see the lifters before I put everything back together. They were carboned up to the point they did not collapse at all. We ended up doing a complete top end overhaul. The cylinders had about 1100 hours on them and I didn't trust that the other valves would fail. Besides, you know I recommend new cylinders at 1000 hours.
Whatever is weird with the #3 it's either causing the cylinder not to scavenge the exhaust or the intake event is closing early. Either way, leak down didn't show anything.
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Gary
AuCountry Aviation
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jamey
Joined: 10 Jan 2006
Posts: 124
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| Posted: Tue Oct 27, 2009 5:35 pm Post subject: cranking pressure |
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I remember freeing the lifters with you at San Rafel to install the
temporary cylinder from Lycon to ferry the plane to Auburn. It was sticky!
Hopefully the new owner is enjoying a trouble-free engine. I still greatly
appreciate your excellent customer care in coming down to San Rafel to get
things straightened out!
Jamey
From: owner-teamgrumman-list-server(at)matronics.com
[mailto:owner-teamgrumman-list-server(at)matronics.com] On Behalf Of
teamgrumman(at)aol.com
Sent: Tuesday, October 27, 2009 11:18 AM
To: teamgrumman-list(at)matronics.com
Subject: Re: cranking pressure
Funny you should ask about the Cheetah compression ratio. I haven't seen a
stock compression ratio Cheetah in years. Those values are for the HC
pistons.
This particular engine with the weird #3 cranking pressure had the #1
cylinder replaced a few months ago by Pacific Air Care in Watsonville, CA.
My thought is, the plane doesn't get much use and there is a good chance the
lifters are all gummed up. I wish I had seen the plane to replace the
cylinder. I'd know what shape the lifters were in.
When James Courtney had a cylinder fail over San Francisco bay I had a
chance to see the lifters before I put everything back together. They were
carboned up to the point they did not collapse at all. We ended up doing a
complete top end overhaul. The cylinders had about 1100 hours on them and I
didn't trust that the other valves would fail. Besides, you know I
recommend new cylinders at 1000 hours.
Whatever is weird with the #3 it's either causing the cylinder not to
scavenge the exhaust or the intake event is closing early. Either way, leak
down didn't show anything.
--
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Tue Oct 27, 2009 6:54 pm Post subject: cranking pressure |
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Hi Jamey,
The new owner, Dave, loves the plane. Every year he comes up here and does an owner assist.
Gary
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Gary
AuCountry Aviation
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