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Old 05-20-2008, 20:33   #1 (permalink)
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Cranking compression for a 351c 2v

Hi,

Quick question

Have a '73 cleveland 2V that has had a stock rebuild about 15k miles ago.

It has Edelbrock performer 1806 carb (650 cfm), performer intake and performer cam installed straight up, but not degreed in. At 900 rpm idle it gets 16 in vacuum.

What should the cranking compression be?

It's showing 110 psi in cylinders 1&2. Didn't check the others, but it runs pretty smoothly so I don't suspect a blown gasket or anything like that. I was expecting it to be closer to 150 psi. What should it be?
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Old 05-24-2008, 19:00   #2 (permalink)
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Re: Cranking compression for a 351c 2v

110 pretty low.......... Should be around 160+ Was the throttle wide open while cranking?
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Old 06-01-2008, 20:47   #3 (permalink)
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Re: Cranking compression for a 351c 2v

Yeah seems low. I'd do a check on all of them.......at least a few cylinders on the other side.....just to rule out any issues between cylinders right next to each other.

Are they open of closed chamber 2V's? I'd guess OC's with a stock rebuild.
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Old 06-04-2008, 14:02   #4 (permalink)
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Re: Cranking compression for a 351c 2v

Yeah, OC 2v heads stock piston rebuild with edlelbrock performer cam, manifold and carb.

Come to think of it, the throttle may NOT have been open all the way. I can't remember if I blocked it down or not.....

Looks like there is some more work to do.

Oh, I was playing with the carb's jets and managed to smash a float (not noticing it) and was pouring raw fuel down the manifold when I started it up. (Yeah, I'm a bone head...). Anyway, after doing that the idle vacuum went up to 19 in from 15-16 in. What the heck???

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Old 06-05-2008, 02:40   #5 (permalink)
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Re: Cranking compression for a 351c 2v

Was the rpm the same? If you had slightly higher rpm, this will create more vacuum.

The higher vacuum you can get on idle the better. May show it wasn't optimally tuned before. The idea is to get it as lean as you can (revs will increase) and nip it back a little on the rich side just as it starts to go rough or drop the rpm. This results in the highest vacuum. Maybe yours was too lean before.........but fix your float and play with the mixtures and see what happens.
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Old 06-06-2008, 19:11   #6 (permalink)
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Re: Cranking compression for a 351c 2v

Do it with the motor "hot" and at WOT.
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Old 06-08-2008, 19:35   #7 (permalink)
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Re: Cranking compression for a 351c 2v

Have not done much "playing" with the car (convertible) as it has been raining like nuts for the past week or so. I really hate having the top up and would rather stay home than drive it with the top/windows up.

I still need to recheck compression with it at WOT. Last fillup was 19.9 mpg. Not bad, but it should be better (has an AOD in it).

On the freeway at 60-65 mph, the throttle is so close to closed it is nearly 100% fed thru the idle (my assumption) and it goes rich. I have a sneaking suspision that the vacuum goes high and is sucking fuel out of the carb. When I lean into the throttle a bit more, it goes lean. (Narrow band A/F meter). I am also planning on a temp vacuum gage to see what it runs while driving. Will report with details as they happen.
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