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Discussion Starter #1 (Edited)
Hi,

I'm back again. It's almost embarrassing, but I have become confused on my vacuum set up. I have a (I believe its 7101) RPM Performer intake with one tap for a vacuum and a Holley Avenger carb. I have a non-vac MSD HEI Distributor. A long time ago I had the car tuned at a speed shop. They connected a 90 degree from the intake to the PCV breather in the passenger side valve cover. The driver side has open breather. They ran the front vac off the carb to the RS head lights. They ran the brake booster off the rear of the carb.

The car ran pretty much fine. The car was parked for several years and I have recently woke her up for a rebuild. I wanted to see how much vacuum I'm pulling to determine if I needed a hydro-boost system and to check engine health. I've read you should not take the vac test from the carb if can help it and it should not be off a runner if taken from the intake. The only intake port looks to be on the #8 runner.

A friend of mine, knowledgable in mechanics, but not American pushrod muscle, has his BB Buick with an Eldelbrock B-4B intake PCV'd from raised center port off back of intake to back of carb. The two lowers on his intake run elsewhere. I thought at least it would have to be running from valve cover to carb cause both the carb and intake create vac. Am I wrong about that? This created a huge debate last night.

1)Will my disconnected RS create a vac leak to make test invalid?
2)Where should I connect vac tester?
3) What is best way to run vac lines for booster, pcv, et? (I will be blocking off RS since it does not work. Maybe didn't work due to connection. Possibly moving to Electric RS anyway)

I've been all over the net without getting a concise answer. Thanks
 

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Use the intake manifold port to test vacuum. Typically the brake booster is connected to the intake manifold port which is often a 2 size "T" fitting so for automatics the smaller side of the T can connect to vacuum modulator of automatic trans and the larger side connects brake booster vac hose. Stick cars don't don't need to have vacuum for trans.

The nipple for the automatic trans is small so a good size to plug in a vac gauge. Or use the "manifold" (read not ported) nipple on front of carb also for a vacuum gauge

Usually the manifold vac port on front of carb is used for vacuum advance on dizzy but you have a non vac advance HEI...something you should consider changing to a vac advance type. Thats another topic though. The 3/8 vac nipple on front, or back of carb can be used for PCV

IDK where RS vacuum operated headlights normally plugs into. I would think manifold port though
 

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Discussion Starter #3
OK. But the rear intake is on the runner. I took a reading with vac gauge and it was "wagging" between 5-10 until it warmed up. Then fluttered at 8-10 once warm. Was told it fluttered due to be on the runner. Took reading from front and it read steady 10. Is the van low because I have opening on RS line or leaky therm housing? Is it fluttering because it's on the runner or do I have a valve issue? What about friend who has connection from intake to carb?\
 

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if its steady at carb on "manifold" port vs "timed" and you have any other vacuum port & lines plugged and its steady at 10hg...your power brakes will not work very well and thats where a hydro boost comes in.

I assume you have a fairly big cam

If you had a VA on dizzy you would have more timing at idle and thus more vacuum. Non VA dizzy are ok for WOT use but street cars really want VA. More idle timing from VA will get you more idle vacuum

But you need to set up the VA right and limit how much it does advance. This is where VA stop plates come into play. There are several threads about this.

With that said if you are happy with the way the car drives otherwise than a hydro boost is what people do with motors that make low vacuum because of big cams
 

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Discussion Starter #5
Did the compression check. It flutters 9-12. I raised RPM to 2000 and held. The gauge read steady 20 and blipped up when released throttle.Then back dow to 12 and slowly flutter down to 9-10 and continues to flutter. I did test off of intake, rear of carb, and front of carb. Same all around for the most part. I am trying to get a compression test but am having difficulty screwing in tester. Engine is a 383 and is supposed to have enough vac to run accessories. I can't tell if I have a leak, valve issue or too much cam. Plugs were completely covered, when I first pulled them, but could've been valve cover leak and sitting for so long. I "cleaned" them up and they looked ok when I pulled them after the test. Advice, insight, diagnosis.
 

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Well nothing wrong with doing compression and leak down tests to see if there are issues. I replied to your other thread abut that

I mentioned running more timing at idle via going with a VA on your dizzy.

Ideally you want 20-24 degrees at idle but if you did that just as base timing it would be stupid high with mechanical added.

You should, as a test, just set your base timing at 22 degrees, adjust idle back to where it should be and then measure your engine vacuum. I bet its higher that 12.

Shop that put my engine in did just base 14 degrees with VA plugged and aside from part throttle problems, crap MPG it had maybe 12hg of vacuum and my brakes were not as good. I connected VA and vacuum was 17hg but I then had to much timing at idle so did the Crane adjustable VA mod. They make one for HEI and Points dizzy

If the 22-24 degree "test" at idle resolves your issue (it should) You should consider doing the VA for your dizzy

With that said if you feel your motor is hurt than certainly do the compression and leak down tests but if its just a low vacuum issue causing weak brakes, do the timing "test" (at idle) adjustment and see if your vacuum gets to where it needs to be.

link to my thread about the Crane kit. Dave Ray on this forum has a full detail on how to do the "modification" to the VA mounting the $9 detent plate (read don't do per Crane instructions mounting the plate)

Crane VA/spring kit? | Team Camaro Tech (camaros.net)
 

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Mike
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I agree with SoCal805.
If you're considering adding Vacuum Advance, you may be able to update your current HEI (coil in cap) distributor.
Remove the cap and rotor and see if there is a lock-out arm connected to the pick-up advance plate (see image below - Red Arrow).
269460

If so... it could be as easy as removing the lock-out arm and installing a vacuum advance unit into your current HEI.
If you go this route, don't forget about adding a VA limiter plate to control the amount of additional vacuum advance. There is a lot of info that has already been posted about this modification... and tuning.
 

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Discussion Starter #8
Thank you. It going to be a couple days before I can do the test, but will try the timing test and go from there.
 

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PCV hose should be from the FRONT large vacuum fitting on the carb base plate, UNDER a Holley fuel bowl.

What is more disconcerting is this, "I have a non-vac MSD HEI Distributor". I have never seen an MSD "non vac HEI" EVER. And, no vacuum advance on a street engine, NO chance of getting timing right.
 

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Discussion Starter #10
I'm running an MSD 6Al, Blaster 2 coil, MSD Pro Billet Dizzy part number 85551. I bought the car when I was in my 20s. Replaced the 355 with this 383. Never took a baseline on the engine. Let a speed shop and magazines steer my performance direction, etc. The car ran well enough, though it burned rough. By which I mean, driving or standing near the car, and you walk away stinking of exhaust. Younger me took that as part of owning early model car with HP motor. I've learned a lot since then and even more today. Let me know what you think of this dizzy. It can not accept a vac canister. Am considering going to Sniper. Will this cause a problem?
 

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Moto

Rich smell with a motor not using VA at idle does not have enough timing to burn the fuel completely...which is another reason you want VA on a street car. yeah magazines and speed shops will steer towards WOT performance where VA is not a real factor. Just whatever the biggest HP dyno # or fastest 1/4 mi time

IDK about your dizzy on if it will accept a VA can. Maybe someone else can advise or you could call MSD tech and ask

EFI is never "plug & play" aka "self learning" set up. If you want to go through the learning curve or have someone qualified "tune" it fine but that can be a deep pool to jump in in terms of getting EFI really dialed in

IMHO getting your current dizzy set-up or new dizzy with VA, including possible carb adjustments, with a VA that has limiting ability (stop plate) like the Crane adjustable VA kit and timing settled will go a long way to making the motor run right assuming it is not hurt otherwise. Once your base & idle with VA and "all in" timing are settled you can make valid concise carb adjustments if needed. This may include metering rod and or spring changes and or jets.. Modified motors take a little more detail in getting them dialed in but once done the car will run as if it came from factory that way
 

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Mike
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I'm running an MSD 6Al, Blaster 2 coil, MSD Pro Billet Dizzy part number 85551.
Well... that explains the non-VA distributor.

Start your engine and advance your initial timing to 22°- 24° @ idle (as SoCal805 suggested in post #6).
Recheck you vacuum level and "exhaust smell" at idle with this advanced timing setting. If you notice an improvement, it is an indicator that a distributor with VA could be beneficial to your engine performance.
 

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Discussion Starter #13
Ok. For clarification. I do not need to be at operating temp. Check my initial timing on startup. If it's not at 22-24 @ idle advance and check vac. If this improves vac, bring IT back to whatever it was set to and get a VA Dizzy and set it to 22-24 IT. What is the best way to figure what total timing should be for the car? 33-36 is what I've seen as a base for SB 350.
 

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I do not need to be at operating temp.
I would probably be best to do the timing advance test when the engine is at operating temp.
Check my initial timing on startup. If it's not at 22-24 @ idle advance and check vac.
Set timing to 22°- 24° while also maintaining your normal idle rpms. As you advance your timing, your rpms will rise... so the engine rpm's will need to be lowered as you are adjusting/increasing your timing. Keep adjusting both timing and idle rpms to achieve the desired results.
If this improves vac, bring IT back to whatever it was set to and get a VA Dizzy and set it to 22-24 IT.
After the test, return your current distributor to the original initial timing that was previously set.
If you get a distributor with VA, you would set your initial "mechanical only" timing to about 12°-14°@ idle with the VA disconnected (and vacuum source plugged)... and then hook the VA canister to a full time/manifold vacuum source and limit the amount of VA provided to about 10°-12° with the addition of a VA limiter plate. This will provide the 22°- 24° initial timing @ idle (12°-14° initial + 10°-12° VA) with the VA connected to a full time vacuum source.
What is the best way to figure what total timing should be for the car? 33-36 is what I've seen as a base for SB 350.
Total "mechanical only" (no VA) timing of 34°-36° for a sbc seems to work well for most.

If you haven't already, please read the "Timing and Vacuum Advance 101" post in the link below. A lot of useful info.
 

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22 to 24 INITIAL degrees, no vacuum advance, NOT gonna work well.

And, (I just KNOW this answer, "USING THE DIAL BACK ON THE TIMING LIGHT), to get the idle degrees. WRONG, WRONG, WRONG. Mark the balancer, leave the dial back off, and read the timing that way. You DO NOT have a digital timing curve in an EFI system on the distributor in your engine, dial back is not workable for anything but those digital timing systems, NOT ones with mechanical/vacuum advance.

269574


269575


269576
 

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22 to 24 INITIAL degrees, no vacuum advance, NOT gonna work well.
David

The comment on setting his base timing to 22-24 degrees without VA is ONLY to TEST if he gets improved vacuum (along with making temporary idle adjustments to carb). If that resolves his current low engine vacuum at idle (along with rich condition) as he currently has a non VA dizzy than he can concentrate on dealing with putting one on his dizzy, or replacing it with one that has VA, and not going to a hydro-boost deal.

Repeat...it's just a "test".
 

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Discussion Starter #18
I'm about to go out and do the test today or tomorrow. I reached out to MSD for a VA dizzy recommendation and they told me that I did not need to get a VA Dizzy, but instead upgrade my coil from the Blaster2 to part #8207 and to change the weights and springs in the dizzy.
 

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MSD tech is wrong regarding not using a non VA dizzy on a street car. They just want to sell you WOT use parts

Changing weights and or springs is a tuning thing if you want total mechanical advance to come in sooner (RPM). That type of thing typically is for WOT use and on a VA dizzy car one of the last things you "may" need to do but a properly set up VA dizzy generally does not or if any for fine tuning to get full advance in sooner you start just changing 1 spring with the heaviest of the springs that come with the kit, including the Crane adjustable VA kit, and test to see if it brought in full advance sooner where you may want it. If not change the 2nd spring, test, repeat using incrementally softer springs until desired timing at x rpm is achieved.

Your issue is low vacuum at idle resulting in poor power brakes. You have a non VA dizzy so your base timing is likely around 12-14 degrees so your motor makes low (less than 14hg) vacuum with a cam at idle. You want idle advance 20-24 (12 base +10-12 degrees of VA). Mechanical advance generally about another 12 degrees so that gets you to 35-36 at 2500-2700 rpm which is where SBC want to be

Your gas mileage will improve with a VA dizzy and you will not have a pig rich smell at idle (if carb is set up right)

Just do the quick test of advancing your timing at idle to 20-24, drop idle back down on carb to where you want it (likely around 750 rpm) and check your vacuum. Don't drive it around with this TEST timing as with mechanical added it will be stupid high. I suspect it will increase to 16+ which will be fine for the brake booster. If this happens then you need to get a Dizzy with a VA and do the stop plate mod.
 

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OK, lets waste volumes of time and effort on "TESTS", instead of getting right down to it, and fixing it the right way in the first place. shall we.
 
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