This is topic compression results, question in forum Tech Talk at Northern California Ford Owners  .


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Posted by shade-tree (Member # 298) on :
 
Just wondering if you guys thought this was ok or weird or anything.
Mainly, I'm wondering if 'possibly' I'm hanging a valve open ever so slightly, or if the reduced compression #'s would be attributed to
1) cam bleeding a little compression +
2) increased combustion chamber of TFS TW heads (61cc + mild smoothing porting that probably added 1cc to each combustion chamber) vs. the iron GT-40 heads on factory 93-95 cobras which are supposedly milled ever so slightly for 58-59cc sized combustion chambers.

well, the #'s
#5 150
#6 150
#7 158
#8 152
#1 165
#2 152
#3 165
#4 152

These are all 'cold' readings.
IIRC , cold readings previous to swap where 160+ on all 8 with former GT-40 iron + stock cobra cam.

Previous 'hot' readings were all over 170.

Just curious/paranoid.

(cam stats, Anderson N-41 222/230 @0.05, 110 LSA installed 105.5 ICL, .512/.512 lift)

I also wedged a big screwdriver in backwards in the throttle body to improve air intake during compression testing. Spun it over 4-8 times until maximum reading was obtained.
 

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Posted by shade-tree (Member # 298) on :
 
triple post
 

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Posted by st5150 (Member # 51) on :
 
For what its worth... here are the numbers I got from my car 3 weeks ago:

#5- 150 psi
#6 149 psi
#3- 143 psi
#1- 145 psi
#4- 145 psi
#2 144 psi
#8- 145 psi
#7- 149 psi

(listed in the order tested)

Your numbers look okay to me... but I'd test # 1&3 again.
 

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Posted by shade-tree (Member # 298) on :
 
yeah but I was 160+ cold on all 8 like 2 months ago, I was just wondering if I can attribute the drop to the cam + the drop in compression from the bigger combustion chamber... (which I don't have exact measurements for :/ )

also #1 and #3 are repeatable!
 

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Posted by st5150 (Member # 51) on :
 
Do all cylinders "keep" their pressure?

What I mean is.... 5-15 seconds after you stop cranking, does the compression guage needle start to drop quickly or does it stay put at 160?
 

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Posted by shade-tree (Member # 298) on :
 
The gauge I use stores maximum value :/

'course harbor freight (or is it summit?) has like a $30 leak-down tester. That would settle the 'hanging' exhaust valve question...
 

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Posted by Yellow94GT (Member # 431) on :
 
I'm not even going to share the numbers fomr my last compression test, (yes, they are that bad )

Lets just say, be happy with what you have
 

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Posted by 90LX (Member # 1487) on :
 
This is off the topic but shadetree do you still have those GT-40's. If you do, do you want to sell them
 

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Posted by shade-tree (Member # 298) on :
 
ok this is the third or fourth time I've been asked that question in 2 days.

*attention*
The GT-40 irons on my former combo are going to be stage 0.5 ported (by me) and dropped off for a 3-angle valve job and then they're going to be dropped in a junkyard 302 and dropped in my brother's former 6 banger '65 coupe (aka rustbucket)
*attention*

thanks

(I think that 2700# car will see 12.1s @ 111 w/4.10s and ET Streets, said GT-40 irons,junkyard 302, 650 carb, edelbrock perfomer rpm carb intake, and a gearbanger riding a fox T5)

[ January 29, 2003, 02:39 PM: Message edited by: shade-tree ]
 

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Posted by 90LX (Member # 1487) on :
 
Sorry for asking!
 

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Posted by shade-tree (Member # 298) on :
 
no need to be sorry just trying to clear things up!
 

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Posted by 93PONY (Member # 60) on :
 
Hey Joe, GT40 irons come stock with 65.5cc chambers.
 

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Posted by shade-tree (Member # 298) on :
 
searched the corral archives, that's the size from the motorsport offerings, OEM 93-95 GT-40 irons were in fact milled...
 

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Posted by 93PONY (Member # 60) on :
 
Are you telling me I have a 9.6-9.7 to 1 compression motor!?!
I shoulda' cc'd the things while they were off...
 

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Posted by TheBrain (Member # 35) on :
 
Joe,
D.F.W.A.I !!!!

Cold compression readings mean absolutely DICK and are unreliable

For what it's worth, the above initials stand for Don't Fucking Worry About It

Later
Brian
 

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Posted by jmcclesk (Member # 1355) on :
 
what the brain said
 

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Posted by CobraSVT2001 (Member # 2309) on :
 
Do you still have those GT-40 headds and if so do you want to sell them.. j/k

If the #'s are all close together.. don't worry about it.. Unless they are really low.. They look about right.
 

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Posted by st5150 (Member # 51) on :
 
For what its worth... I've done before and after tests with a warm and cold motor. Almost a 10 psi difference across the board.
 

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Posted by street_illegal_stang (Member # 1554) on :
 
if it makes you feel any better i got 60 psi with the number 8# cylinder on my stang
try checking it when its warm..
 

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Posted by 93PONY (Member # 60) on :
 
IMO compression tests are not worth the effort. A leak down test is MUCH more informative.

The heads/cam profile play a big factor in your readings. Basically, your heads/cam combo don't flow shit at such a low rpm. (starter cranking speed) Low air velocity through the motor.
 

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Posted by Blown Mystic (Member # 476) on :
 

quote:

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Originally posted by 93PONY:
IMO compression tests are not worth the effort. A leak down test is MUCH more informative.

The heads/cam profile play a big factor in your readings. Basically, your heads/cam combo don't flow shit at such a low rpm. (starter cranking speed) Low air velocity through the motor.

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Yeah, that and they are usually depressing
 

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Posted by shade-tree (Member # 298) on :
 
I appreciate all the replies.

However, I stand by a quick compression test to verify a proper valvetrain rocker adjustment relatively quickly. I mean either your valves are hanging open or they're not.

I just fixed my vacuum leak (solid 13" at idle now) and of course it runs beatifully. With a vacuum leak and 5" of idle I needed to do a sanity check on the rocker adjustment stuff.
 

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Posted by 93PONY (Member # 60) on :
 
You have more trouble in the garage then anybody I know.

I've never had problems with vacuum leaks. Well, not big ones like that! Sure, I work slower then some, but I check/double check everything. The worst I had on my car was a hole in the oil pan & a leaky valvecover seal which were vacuum leaks.....Yes, vacuum leaks on my car! Don't ask, it's an original setup.
 

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Posted by shade-tree (Member # 298) on :
 
well, nothing in my car is original except for parts of the wiring harness and the alternator so I'm probably not doing too badly.

(I did the rear, suspension, control arms, tranny, clutch, motor swap, multiple head/cam/intake packages, etc.)

anyways, I suck. oh well. Just a hobby...
 

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Posted by 93PONY (Member # 60) on :
 
Hehehe, I remember meeting you at the track.

Me: "Hey bro, your smog pump is about to fall off. It's hanging the timing pointer."
You: "Huh? (checks) I hope it falls off."

Hahahahah!
 

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Posted by shade-tree (Member # 298) on :
 
thanks for pointing out the smog pump, seriously. I had forgot about that obviously.

anyways, I'm spinning crank+alt+H20 only right now with the smog bits put in the shed till '04
 

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