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390 Compression

August 5 2013 at 8:36 PM

  (Login 66birdman)
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Just wondering what compression should be at ?? I'm getting about 135 PSI in cyl 1..? Just had it rebuilt and so far not happy with performance, Werby figured about 375hp and 425ft/lb but doesn't feel close to that..

 
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(Login A671BlownMustang)
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lots of factors involved and

August 5 2013, 8:41 PM 

The engine needs to be warm , ALL plugs out, Carb Wide open , and Battery fully charged / on a charger , that said 135 is what My blower motor was at 6.75-1

 
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(Login 66birdman)
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Re: lots of factors involved and

August 5 2013, 8:56 PM 

Ahh ok... I'm taking it to a local speed shop this week here in Edmonton, AB that came highly recommended so hoping he can figure it out and get her tuned up..!!

 
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(Login hotrodfeguy2)
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With what Faron said

August 6 2013, 6:50 AM 

Don't just let it hit one pulse on the tester. Most of the time 3-5 will show a good average where it starts to flatten out. And alot depends on CR what pistons you selected, what heads, head gaskets all played a little factor. That would give a better ballpark number for your PSI.
I for one in my "truck 390" start at 9.5 CR pistons. And when I put a serious load on it IE my Minneapolis Moline tractor 7500# then I splash in the 93 octane.

 
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(Login werbyford)
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Try a stopwatch before&after

August 6 2013, 6:31 AM 

Gord,
Well remember thats a heavy Bird, so Gonkulator still just says 91-92mph trap speed.

If you dont have a Gtech, try some stopwatch times before & after tuning if your speedo is fairly accurate.
My cool Iphone has a laptime feature, so you could in theory get
0-30
0-40
0-50
0-60
0-70
0-80mph
all in a single blast.
Before & after tune will tell you a lot (even if the speedo is off in a relative sense)

Yes that 135psi seems low to me too, since you have a pretty small cam in there, about a 428cj cam.
Was that just 1 cylinder or all 8?

 
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Dave Eames
(Login DEames)
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"Compression" figures just tell how efficiently

August 6 2013, 7:33 AM 

the rings and valves are sealing. Who did the build, ask him/them, could be a broken ring, leaking valve to seat(s), etc. A cylinder leak-down test will tell the tale. It should hit 75psi. on the first kick; what are the rest of the readings?
Eames.

 
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(Login 66birdman)
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Cyl check

August 6 2013, 9:13 PM 

No I just tried it on #1 because its the easiest one to get at in that tight engine bay.. But don't you think I should be able to roast the tires if I wanted too..?? Is there a better cam that I could look into ? I think I do want more power..!!

 
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(Login werbyford)
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Just a "chirp". Stopwatch or Gtech would tell the story

August 6 2013, 9:37 PM 

Roast the tires depends -
We had our Galaxie setup much the same as your Bird once, 3.50 gear and about the same cam and weight.
Just a chirp is all we'd get now and then with the stock converter.
Then again the Galaxie is already almost 50/50 weight and has good transfer.
I've never had a 60s Bird so not sure how they are on traction.
The stopwatch times would give an idea, even approximate, if it is about right or way off.

As for compression, you can even have a valve not seated just right or corroded from sitting the night before, it will clear up but if one reading is all you have it doesnt tell much. Also as noted be sure its wide open throttle, all plugs out, good battery, do 5 or 6 hits on the gage then call it good, same number each time. I test em cold (well, 60F or so anyway), they probably read a little lower but better consistency.

 
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Dave Eames
(Login DEames)
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"Corroded" from the night before;

August 7 2013, 7:28 AM 

you've got to be kidding, right?
Eames.

 
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