Need help with this vacuum reading

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Have you done a leakdown and/or compression test on this engine yet? It may be just too big of a cam for the static compression ratio.
 
ramcharger said:
Have you done a leakdown and/or compression test on this engine yet? It may be just too big of a cam for the static compression ratio.
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I did a couple compression tests.......seems fine there.....pretty consistent across all cylinders...two were a little higher....as I recall I was up around 150-160 warm. As far as leakdown, all I did was let the comoression gauge sit and watch the needle for 30 sec or so...didnt see any drop....not sure if thats the right way though...
 
shadango said:
I did a couple compression tests.......seems fine there.....pretty consistent across all cylinders...two were a little higher....as I recall I was up around 150-160 warm. As far as leakdown, all I did was let the comoression gauge sit and watch the needle for 30 sec or so...didnt see any drop....not sure if thats the right way though...
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150 to 160 warm is just fine, so that's not a problem. That's not the way a leakdown test is performed, but it looks as though you don't have a burnt valve which will cause a vacuum fluctuation.

That cam is too big for your application and is indicative of the driveablity issues we often mention when suggesting the correct cam for the application. If you have power brakes, I imagine they don't work worth a darn either.

It's time for a cam swap.
 
7 in of vacuum in Park? I agree with a few on here that you have a little big of a cam for what remainder of parts combo you have on your engine, especially compression psi. My 360 has about the same vacuum reading with a 274xe cam, but I also have 200-205 psi per cylinder to handle that cam.
A bit smaller cam would definitely do you some good.
 
I agree with Red.

Just to be clear Shandango, when I said that 150-160 was fine, I meant that there's no mechanical damage, not that it's high enough for that cam. :)
 
No, no manual brakes luckily. standard drums all around.

I was thinking a cam change may be in order...but I think that will need to wait til winter.

She isnt dying out anymore, so far, so the low idle in drive is the worst of it...and she stays running in traffic/at lights unless its a really long stop....at which point it seems like teh car wants to lung forward a bit....so I shift in neutral and all is well....

I have been driving her around and she does pretty well....like I said, low end is not great, but the 2.76 rears have something to do with that too....when she winds up, she really kicks in strong 2500 and up.....

I know that rear end ratio has a lot to do with what cam you get, and I know I am hoping to upgrade there a bit so wanted to wait a bit and see what happens......

With the J heads and eddy carb/manifold I have, what do you guys think I should be looking for cam-wise?
 
That lunging at the light is from having too tight of a converter for that cam. Those gears are way too low numerically too.

Let us know what gears and converter you plan on running and we can give you a good suggestion on the cam.
 
I would say look for a cam that is around the 256 xe by comp as a guideline to which to look for. The lunging forward tell me stock converter with big cam combo.....and yes this cam will work just fine with say a 3.23 rear gear as well.
 
The lunging doesnt happen at first.....only after sitting at the light for maybe 30 seconds or so.....if I let up off the brake and drift a bit and then reapply, it os ok again for a bit.....over all, street manners are OK enough to drive around. The vibration bugs me a bit though...that coudl be a convertor issue too I guess....

Someone on another forum suggested just starting fresh with a different engine , build it the way I want and then go from there....But for now, not an option.....

I am sure you guys are right...I have a cam in it.....that coupled with the stock convertor and I have the issues I have.

Can I drive it this way for a bit without damaging anything? I can live with the driveability issues for now.....

Yeah I think 3.23 or 3.51 (??) is what I was considering....nothing too crazy.

I would also like to confirm what I have in my rearend now.....the tag says 2.76...but who knows if the previous owner tinkered with it and left the original tag on, ya know?

Can someone refresh me as to how to figure that out? Do you turn the wheel and count the number of rotations versus the number of driveshaft rotations? Whats the math involved?

Not sure as far as a convertor......I know that it, the rear end and cam all work together.......I guess the cheapest/fastest thing would be just to swap out the big cam for something smaller......but ideally a rear gear swap and cam swap at the same time would be best....so maybe thats something that will need to wait a bit....

As its stands, I get around 9-10 mpg it seems......so at least thats a good thing....LOL,...compared to what I thought i would get......
 
Go with the cam red suggested or a Lunati 60401. Both will work with what you presently have. You won't really hurt anything driving it the way it sits. Your mileage will be much, much better with the aforementioned cams.

The rear end can be eyeballed by jacking up the rear and turning the rear tire until you have one driveshaft revolution. Just count the number of turns and fraction of turns. If you turn the tire 2 and 3/4 turns it's a 2.76 rear gear.
 
after reading all of this, 2 things.

[1st] raise the initial timing higher like 24* and limit the mech advance so it only adds 8*-9* giving you around 32*-34* total @ 1800-2200rpm.
By doing this one of the benefits is that you will now have the throttle blades where they need to be for sure, which will help with the hunting it does 'sum'.

[2nd] with 7inches vac in gear you need at least a 'real' 2800 stall convertor.

Mainly like others mentioned convertor would greatly help/fix.

Compression aint that low for the cam, I've run worse with no issue.jmo
 
for gear try a minimum of 3.73.

If you had a manual trans you could get away with less but you don't.

BTW not having enough gear for a given converter will cause the trans to have 'heat' issues, at least with most cause chances are you'll be slipping all over town till you get on the freeway. You wanna try and get your cruise rpm into the converters lock up band, if that makes to you, by lowering or numerically raising the gear ratio.
 
1wild&crazyguy said:
for gear try a minimum of 3.73.

If you had a manual trans you could get away with less but you don't.

BTW not having enough gear for a given converter will cause the trans to have 'heat' issues, at least with most cause chances are you'll be slipping all over town till you get on the freeway. You wanna try and get your cruise rpm into the converters lock up band, if that makes to you, by lowering or numerically raising the gear ratio.
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Ok, now you are scaring me a bit.

I havent noticed any slipping or anything.....pulls out ok from lights etc....feels pretty "normal" to me......shifts real firm, etc....

The tranny was rebuilt by a shop, and I had them put in a transgo kit, billet rear servo, some other stuff.....

I have had to mess with the kickdown just a bit...setting it all the way back makes her shift late......so I backed off the Lokar kickdown cable a bit and she "seems" to shift ok....sometimes I have to back off the gas just a might to get her to make the upshift to 3rd.....but I figured that was more kickdown adjustment needed?

Every once in a while I will notice a short delay in shifting from neutral into drive.....but I assumed that was me jumping the gun a bit....

Should I be worried here????

My cruise rpm at 60.....dang it, I had made a note of that somewhere....I think its around 2300 rpm or so? Doesnt seem too high or anything....but if the stock convertor lockup is 1400 or so then I am out of that range for sure....
 
Shadango: Sounds like you need more "in person" opinions. Maybe put a shout-out to members in the Pittsburgh area?
It does sound like a cam, timing, convertor, gear ratio thing. A big cam could also be opening the power valve sporadically at idle, and the carb may need idle circuit changes, at the least.
That vacuum reading also could be a dead cylinder.
Ramcharger: You got it bass-ackwards! Turn the tire one revolution. Count the number of turns the driveshaft makes. 2 3/4 turns of the driveshaft equals 2.76. This works for a sure-grip or spool. Open diffs are "diff". :) Maybe someone else knows.
 
SpeedThrills said:
Ramcharger: You got it bass-ackwards! Turn the tire one revolution. Count the number of turns the driveshaft makes. 2 3/4 turns of the driveshaft equals 2.76. This works for a sure-grip or spool.
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Lol! Yep you're right! :-D
 
shadango said:
Ok, now you are scaring me a bit.

I havent noticed any slipping or anything.....pulls out ok from lights etc....feels pretty "normal" to me......shifts real firm, etc....

The tranny was rebuilt by a shop, and I had them put in a transgo kit, billet rear servo, some other stuff.....

I have had to mess with the kickdown just a bit...setting it all the way back makes her shift late......so I backed off the Lokar kickdown cable a bit and she "seems" to shift ok....sometimes I have to back off the gas just a might to get her to make the upshift to 3rd.....but I figured that was more kickdown adjustment needed?

Every once in a while I will notice a short delay in shifting from neutral into drive.....but I assumed that was me jumping the gun a bit....

Should I be worried here????

My cruise rpm at 60.....dang it, I had made a note of that somewhere....I think its around 2300 rpm or so? Doesnt seem too high or anything....but if the stock convertor lockup is 1400 or so then I am out of that range for sure....
Click to expand...


What I'm saying is, don't buy a cheap convertor that stalls a considerable amount higher rpm than the rpm you cruise at with the gears you have. you want to be cruising at or above the actual stall. jmo
 
Ok, so can I in theory run what I am running (a stock convertor) with what I have until I figure out the gears and such that I want to go with without hurting anything?

Sorry guys....I am really a newbie at this stuff....learning as I go.

I probably picked the hardest combination I could have picked as far as figuring our what works --- an "unknown" engine that has been tweaked and massage, with no way of knowing how other than dismantling, and a car that came with "high way" gears....LOL

Thanks for bearing with me.
 
SpeedThrills said:
Shadango: Sounds like you need more "in person" opinions. Maybe put a shout-out to members in the Pittsburgh area?
It does sound like a cam, timing, convertor, gear ratio thing. A big cam could also be opening the power valve sporadically at idle, and the carb may need idle circuit changes, at the least.
That vacuum reading also could be a dead cylinder.
Ramcharger: You got it bass-ackwards! Turn the tire one revolution. Count the number of turns the driveshaft makes. 2 3/4 turns of the driveshaft equals 2.76. This works for a sure-grip or spool. Open diffs are "diff". :) Maybe someone else knows.
Click to expand...

Must be different indeed.....if I try this method on the car, the tire goes around one full turn and the shaft moves 1.25 turns!
 
Ok...found the answer on a.....:pukel: CHEVY site! LOL

You turn BOTh rear tires at the same time.

I made a mark on each tire, referencing it with the backing plate...then made a mark on the oke, referenced to the chunk.

Laid under the back of the car and rotated each tire at the same rate, slowly and together and I got 2 and 3/4 turns of the shaft to one turn of the wheels.

SO I definately have a 2.76. Hey, at least I know that for sure now! :cheers:
 
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