Vacuum reading question - Chevelle Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 15 (permalink) Old Oct 2nd, 19, 12:06 PM Thread Starter
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Bobby
 
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Vacuum reading question

Is something awry if my 402 BBC with rectangular port heads and Holley 750 DP is only pulling 10 inches of manifold vacuum at 700 rpm? Cam specs attached. Intake is RPM air gap, no headers, 4 speed, if any of that matters.
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post #2 of 15 (permalink) Old Oct 2nd, 19, 12:11 PM
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Re: Vacuum reading question

What's your timing set at? Vacuum advance hooked up to full manifold? What distributor? Compression? Do you have a PCV valve? Power brakes? Is the needle steady or bouncing?

Rectangle port heads on a 402 usually isn't a recipe for a good street engine.

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post #3 of 15 (permalink) Old Oct 2nd, 19, 1:43 PM
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Re: Vacuum reading question

I have the rectangular 208 heads and a 282 "Saturday nite special" cam, 700 dlbpmp, on my 396. She gets down the road pretty good for what she be.

I have 11hg at 850rpm

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post #4 of 15 (permalink) Old Oct 2nd, 19, 2:22 PM
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Re: Vacuum reading question

I'll let engine guys critique the overall build of the engine. That's not my strength. Boyd says his rectangle ports work good and I believe him. He has first-hand experience and I don't.

What I can help with is getting the most out of what you have tuning-wise but I'll need to know what sort of tune you're working with currently. I think you probably should have a little more vacuum but it's hard to say without more info.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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post #5 of 15 (permalink) Old Oct 2nd, 19, 2:28 PM
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Re: Vacuum reading question

Quote:
Originally Posted by boyd66k20 View Post
I have the rectangular 208 heads and a 282 "Saturday nite special" cam, 700 dlbpmp, on my 396. She gets down the road pretty good for what she be.

I have 11hg at 850rpm
Seems like lots of folks think they are not the greatest compared to today's technology, and of course they are right. Did I hear boat anchor at one point ?
It 's just an original combination that is date correct, if that means anything.

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post #6 of 15 (permalink) Old Oct 2nd, 19, 2:38 PM
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Re: Vacuum reading question

A quick search indicates that those 208s were only on '65 Z15 375 HP Chevelles and 425 HP Corvette 396s. I have no idea how well they work but they're definitely worth some money to the right folks.

Sounds like they work just fine for you!

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt

Last edited by Tommy the Cat; Oct 2nd, 19 at 2:58 PM.
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post #7 of 15 (permalink) Old Oct 2nd, 19, 2:56 PM Thread Starter
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Re: Vacuum reading question

22 initial, 36 total, all in by 2800. No vacuum advance because it does not work. I backed it down to 12 and hooked the vacuum advance up to full manifold vacuum. The timing stayed at 12. I'm thinking it is not enough vacuum to work the can.

Distributor - brand new Davis Unified HEI.

Compression - not sure. I inherited this engine and have never had the heads off. Thinking 9 or 9.5 because even at 22 degrees initial it does not ping and turns over easily.

Yes I have a PCV valve. Breather on the other side.

Yes I have power brakes. New 9 inch booster. Brakes are ok but I wouldn't take it to Nuremberg.

Needle is steady at 10 inches.

I agree it is not an ideal combo, but it came this way, so I am trying to get the best out of what I have.

I think it runs fine without any vacuum advance but most people on this forum seem to think running without vacuum advance is a bad idea.
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post #8 of 15 (permalink) Old Oct 2nd, 19, 3:12 PM
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Re: Vacuum reading question

Quote:
Originally Posted by esko ss View Post
22 initial, 36 total, all in by 2800. No vacuum advance because it does not work. I backed it down to 12 and hooked the vacuum advance up to full manifold vacuum. The timing stayed at 12. I'm thinking it is not enough vacuum to work the can.

Distributor - brand new Davis Unified HEI.

Compression - not sure. I inherited this engine and have never had the heads off. Thinking 9 or 9.5 because even at 22 degrees initial it does not ping and turns over easily.

Yes I have a PCV valve. Breather on the other side.

Yes I have power brakes. New 9 inch booster. Brakes are ok but I wouldn't take it to Nuremberg.

Needle is steady at 10 inches.

I agree it is not an ideal combo, but it came this way, so I am trying to get the best out of what I have.

I think it runs fine without any vacuum advance but most people on this forum seem to think running without vacuum advance is a bad idea.
Hopefully your vacuum can is adjustable. There may be a screw inside the nipple, take a peak. Vacuum advance would definitely help. Do you have a vacuum pump you could hook up and see if/when it advances? If the needle isn't moving your engine is probably sound. What happens to the vacuum if you plug the PCV and brake vacuum individually? Running without vacuum advance isn't "bad' but you're leaving fuel mileage and driveability on the table by not using it.

Also, what's your altitude?

Quote:
Originally Posted by boyd66k20 View Post
Seems like lots of folks think they are not the greatest compared to today's technology, and of course they are right. Did I hear boat anchor at one point ?
It 's just an original combination that is date correct, if that means anything.
Ok, Boyd...you got me. You know exactly what you have! I had to Google it. That was a dirty trick and I'm keeping an eye on you going forward.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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post #9 of 15 (permalink) Old Oct 2nd, 19, 3:23 PM Thread Starter
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Re: Vacuum reading question

No vacuum pump. I will plug the draws, pull the cap, and report back.

700 feet above sea level.

Thanks for all your suggestions. I appreciate your expertise. I have followed other advice you have given in other threads always with positive results.
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post #10 of 15 (permalink) Old Oct 2nd, 19, 3:47 PM
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Re: Vacuum reading question

Quote:
Originally Posted by esko ss View Post
Thanks for all your suggestions. I appreciate your expertise. I have followed other advice you have given in other threads always with positive results.
Thanks for that! I'm not an "expert" and this isn't my profession but I have quite a bit of experience with exactly what you're working with. If you read between the lines that means I've made enough mistakes to know what not to do. I'm pretty sure we can improve what you've got, though.

A hand operated vacuum pump isn't very expensive and is a great tool to have around if you can swing it. Not a "must" but it would be helpful to set up your vacuum advance.

70 Camino...Roller

69 Malibu Stock 307, Reverse Manual TH350, 4,500 Stall Edge Converter, 4.56 Spool 35 Spline 12 Bolt

76 C20 VortecPro 496 628HP 655 lb/ft TH400 Edge Converter, 4.10 14 bolt
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post #11 of 15 (permalink) Old Oct 2nd, 19, 4:07 PM
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Re: Vacuum reading question

10Ē is right on for that cam. In fact thatís pretty good. Surprised it will idle that low (assuming thatís accurate).

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post #12 of 15 (permalink) Old Oct 2nd, 19, 5:36 PM
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Re: Vacuum reading question

Sell the heads (3856208) and use that money to get a set of AFR's and some ceramic coated headers.

John
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post #13 of 15 (permalink) Old Oct 2nd, 19, 5:48 PM
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Re: Vacuum reading question

Quote:
Originally Posted by 69-CHVL View Post
10Ē is right on for that cam. In fact thatís pretty good. Surprised it will idle that low (assuming thatís accurate).
Up the idle a bit & find a vac. can that will give max. adv. at 8 inches.

Jim

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post #14 of 15 (permalink) Old Oct 2nd, 19, 6:40 PM
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Re: Vacuum reading question

Did you try another vacuum port for your vacuum advance ? Not all vacuum ports read full manifold vacuum or give the same vacuum reading and depending on how far open your base idle is open, that port may not be working fully.
Also once the dist cap is off there are only 2 screws holding the vacuum can onto the dist.
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post #15 of 15 (permalink) Old Oct 2nd, 19, 7:23 PM
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Re: Vacuum reading question

"No vacuum advance because it does not work." HORSE HOCKIE.

"22 initial". And, here wqe have it, ABSOLUTELY NO COMMON SENSE IN TIMING SPECS, YET ANOTHER TOP FUEL ENGINE ON THE STREET, let me guess, dyno hero set it up.

"Distributor - brand new Davis Unified HEI." YUP, THERE'S THE REAL REASON.

So easily fixed, it isn't even funny. It would be totally hilarious, but for it being absolutely sickening that this mental illness about ignition timing keeps on happening.
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