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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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 Posted: Mon Feb 23, 2009 10:23 pm Post subject: Cranking pressure (was Connecting rod length) |
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I could open this one. Thanks Ned. I have that manual. I didn't think to check it. I've become too dependent on the internet.
I checked the cranking pressure vs the leak-down pressure on two separate Tigers today
Tiger 1 (1600 TTSMOH)
72/110
75/110
76/112
75/125
Tiger 2 (140 SFNEW)
79/130
78/130
79/130
79/130
Compare this to a Lycon O320 and 140 to 145 pounds. This Lycon dynoed at 188 hp at 2725 rpm.
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Gary
AuCountry Aviation
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Discover
Joined: 26 Feb 2007
Posts: 429
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| Posted: Wed Feb 25, 2009 4:16 pm Post subject: Cranking pressure (was Connecting rod length) |
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I've been trying to find an old article about a couple of A&P's that do compression testing and keep records to compare from annual to annual. I can't find the article right now but I think it is a great idea.
We used to check out car engines this way. Seems like you could expect pressures around 17 to 20 times the compression ratio. I sure would like to find what Lycoming or Continental says you should have. It is a little different than just the compression ratio times the ambient pressure because of the effect of temperature during compression and valve overlap etc.
For the Tigers you tested below it is interesting to note that for 8.5:1 you could estimate a pressure of 8.5 x 30.2 X 0.4912 psi/inHG = 126 psi
(where 30.2 was the barometric pressure around 3pm yesterday http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KCAAUBUR7&month=2&day=24&year=2009)
On Tiger 1 did you do a leak down test to see if it could help tell where the wear was?
Seems like we could tell whether the rings were worn with a compression test by doing it wet after the dry run. Wet meant addding a teaspoon or 2 of oil in the spark plug hole and doing another test. If the low cylinder psi went up then it was thought that the rings were the source of the leak instead of the valves or a worn cam. But I haven't done a compression test in a long time...
Do you have any data or rules of thumb as to what the pressures should be in a Lycoming?
ned
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Discover
Joined: 26 Feb 2007
Posts: 429
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| Posted: Wed Feb 25, 2009 4:18 pm Post subject: Cranking pressure (was Connecting rod length) |
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Would running a compression test with the throttle closed be a good way to find the cylinder that might have an induction leak?
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Wed Feb 25, 2009 4:42 pm Post subject: Cranking pressure (was Connecting rod length) |
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there are a lot of equations and supposition regarding computing the cranking pressure vs compression, but, if I use any of them, they don't correspond to what I've been measuring.
I did the leak down and cranking pressure tests as part of an annual.
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Gary
AuCountry Aviation
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Wed Feb 25, 2009 4:43 pm Post subject: Cranking pressure (was Connecting rod length) |
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I've done it both ways, there is no significant change. THere is too much volume in the induction system to have any significant effect.
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Gary
AuCountry Aviation
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Discover
Joined: 26 Feb 2007
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| Posted: Wed Feb 25, 2009 7:38 pm Post subject: Cranking pressure (was Connecting rod length) |
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I found some stuff on the net:
Direct readings should be proportional to compression ratios. Expect high compression engines to be 140-170 PSI, 80 Octane engines 120-140 PSI and Turbo-charged engines 100-130 PSI. Engines with equal direct readings tend to run smooth. A 20% spread between low and high cylinders is considered normal
http://www.littleflyers.com/engcomp.htm
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Wed Feb 25, 2009 11:09 pm Post subject: Cranking pressure (was Connecting rod length) |
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Once again, I'll reiterate: These formulas don't work. What is High Compression? Turbo charged engines, by design, are low compression engines (does not apply to standard engines that are turbo normalized).
Like I said before, I've seen an O320 with 8.5:1 pistons with 145 psi. And an O360 with 8.5:1 pistons with 130 psi.
I've seen 7.5:1 O320s with 100 psi and 8.5:1 O360s with 120 psi
There are too many variables involved to make blanket statements regarding the relationship between compression and cranking pressure. The biggest affect on cranking pressure is lobe separation angle and the closing time of the intake and opening time of the exhaust.
I'm in the process of building a database and determining what I should expect with a given engine.
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AuCountry Aviation
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GrummanDude
Joined: 15 Jan 2006
Posts: 926
Location: Auburn, CA
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| Posted: Wed Feb 25, 2009 11:33 pm Post subject: Cranking pressure (was Connecting rod length) |
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8.5:1 SOHC 8 valve 4 cylinder engine: Desired 178 psi. Service limit: 125 psi.
7.8:1 turbo engine, DOHC: Desired 164 psi. Service limit 114 psi
9:1 2 ltr 4 cyl: Desired 192 psi. Service limit 145 psi
9:1 1.8 ltr 4 cyl: Desired 185 psi. Service limit 131 psi
From another chart. 7.5:1 VW engine. Desired 139 psi. Service limit 115 psi.
8:1 VW engine. Desire 152 psi. Service limit 125 psi.
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