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Old 03-25-2010, 10:01 PM
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Yet another vacume advance question...

Hi everyone

First off, thanks to everyone for your previous answers to my posts, it seems like in New Zealand no-one understands or cares about old school cars anymore, so this site is really my only source of informed information. And thanks in advance for any replys to this post, I don't get to check on the site too often so if it seems I haven't thanked for your reply its probably cos I haven't had a chance to check back for a while.

Anyway, get on with it...

68-69 327 chev, 3608 pound car, 350 turbo trans, very mild cam, Barry Grant Demon Jnr Carb 625, Edlebrook Performer RPM manifold, crappy 882 heads (75cc? low compression?) running either 95 or 98 octane, HEI, recently reconditioned (would have been better and cheaper to buy a crate motor but thats another story...)

I have read and tried in practice all the posts regarding vac or ported distributor advance. I am completely sold on the idea that ported was an emissions thing, and for optimum fuel economy, idle cooling and cleaning, and drivability that manifold vac is the best option. There is no way of knowing the original setup, as it had twin tunnel ram, with twin point non vac advance dizzy when I got it.

So, I have tried setting my total timing to 36*, all in by about 3k. The problem I am having is that just off idle, if I accelerate gently, about 11-1200 rpm, i get a little light rattle (if I give it a bit more throttle it seems to not do it or push thru it faster). It also happens when I come off throttle, but still have a few rpms on. Apart from that it drives nice, responsive, doesn't detonate under load or full throttle, and cool at idle. Also seems to me to be under very little load, slow creep from idle to get it moving, and even less load when I come off the throttle.

I have played around with this for about six months, every possible combination of springs, initial timing etc, even tried barry grant and they suggest richening up the idle, which works to an extent but clearly not optimal as smells rich, and uses more gas, and eventually fouls plugs. Tried higher power valve on the theory that maybe it was leaning out during that portion of the rev luck.

Rather than damage the engine, I eventually put on ported source, which meant I could use lighter springs to bring in centrifugal advance in faster, but I just don't like driving around knowing I could get better MPG, cooling etc if it was 'the right way'.

I originally had points dis, changed recently to HEI on the theory that the points one may have been an emission distributor, and just not suited for manifold vac operation. It made no difference. Which raises one question, is there any difference between ported or manifold distributors, as I guess that because most mechanics here want to hook to ported vac, that maybe the design is different, and the vac source is not interchangable?

I also thought because the HEI had an adjustable vac can, that I could taylor it. It seems I can only change the amount of advance, not when it comes in. So this didn't help.

I eventually found one chap, just one, in the town I live that agreed manifold vac should be used for auto trans cars, and he added that ported vac is used for manual cars. Didn't care about the last bit, at least I found one person who was willing to set it up with manifold vac. He had a distributor machine, and so I saved up, and took it too him, thinking my problems were over.

Indeed it came back with no rattle,..... and no performance. What he did was set the timing exactly the same as I originally had, but put on the heaviest available springs. I can see how this would work, as the centrifical advance probably never comes in anywhere near its full amount. To me this is not optimum, either for performance, or fuel economy, seems that in order to solve one small problem in a small rpm range, he has degraded the whole range.

To rub in salt, the MPG is the same as if I just hooked it into ported vac in the first place.

So thats where its at now.

To give you an idea of my driving style, its not performance orientated. I have a vac gauge installed, and drive to economy 98% of the time. Occassionally I need to do the odd wheely, but thats not the goal, which is primarily to be able to afford to drive it.

So I am determine to sort this manifold thing. To give you an idea of the milage, I will convert from kms/litre....

ported, and the current setup, mixture of city and higway but mostly city, it gets 12.27 mpg.

manifold, if I ignore the slight pinging, I can get 16.36 mpg.

Which is the second question, is this reasonable milage for a car of my weight, engine etc with a 3.08 rear end? Maybe I am dreaming that I can get it any better, but regardless I would rather have 16 than 12 mpg.

Coming to the end now, please hang in there....

My latest theory is goes something like this. As I can only adjust the amount of the vac advance, maybe to run on manifold I also need to adjust the amount of vacume that activates the advance?

With manifold vacume source, I run approx 20 inches of mercury at idle, about the same at cruise. With my driving style vacume rarely drops below 14-15.

So I'm thinking that if I could adjust the amount of vac required to activate the can, I could have it so that at 20 inches all vac advance is in, but at 18 or 19 inches it all drops off, which might get me through the 11-12 rpm range without rattling. Would not really help with part throttle economy though, so maybe even if it were possible that wouldn't be the best solution.

There must be a solution for this. If there was someone in this small but pert country to take it to and say 'fix it', at this stage I would do it. However, as mentioned above the solution of all the local mechanics is 'plug to ported vacume'.

If I could buy a two way adjustable vac can (when and how much advance) I would, but they just are not available in NZ. All the available ones are adjustable only in the 'how much' aspect.

Sorry for the long post and thank you to those who have managed to hang in to this point. I am hoping that by giving all the information in the first post it may click in someones head, and they will say 'ahh, thats the problem, your an idiot, but this will fix it'.

From all the other posts, suggestions have been just try one or the other, and see whats best. Well, manifold is best in economy, cooling and drivability but will ping the engine to bits, ported sucks with every other aspect, but probably doesnt harm the engine...

Could it be something to do with the rubbish heads? Maybe the 882 combustion chamber shape? Something to do with fuel? (although I would have thought with low compression heads I should be able to run 91 octane).

Please please give me your thoughts, I am pulling my hair out, I want this car to run as efficiently as possible, on the lowest (cheapest) octance fuel possible, and performance is definately a secondary issue.

Thanks for hanging in there, I look forward to any replies, and appreciate any in advance.

Cheers Rob
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Old 03-25-2010, 10:35 PM
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See if you can find a Crane Adjustable vacuum advance kit. It came with a Adjustable can AND a small notched cam-shaped piece of steel that is about the size of a quarter, with notches around the outside like a bycycle sprocket. This piece has one off-center screw hole and is bolted in under the back screw that holds the advance can in the distributor. The notched portion fits up against the rod from the advance can. Each notch was 2 preload added(removed?) to the vacuum advance travel limit, allowing you to tailor the amount added along with being able to adjust the can for the amount of vacuum to start movement.

I don't know if it is still being made, be a shame if it isn't, glad I have a couple stored away. I would love to find a source for this little stamped cam, I'd buy a dozen.

Just took a look on-line, package is still available as PN #99600-1, just the little cam-shaped Limiter plate is 99619-1, glad I took the time to look one up.

I think 16 mpg is pretty good for that small an engine and that heavy a car, you might be able to find another 1 mpg or so.

Last edited by ericnova72; 03-25-2010 at 10:52 PM.
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Old 03-26-2010, 01:53 AM
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There are literally dozens of different weights of the HEI mechanical advance that all have differing amounts of total advance and differing amounts of weight location that alters the rates along w/the springs.

The aftermarket kit weights are often dismissed as junk- and usually they aren't what's needed. Instead, the OEM weights are more often reused along w/the kit's springs.

So several possibilities exist for your situation.
  • Try to get your mitts on some different OEM weights and substitute them for the ones you're using, along w/going through the different springs.
  • If you had another set of weights like yours you could lighten them a bit at a time. This allows a lesser amount of change than a spring change, and can "fine tune" a spring change.
  • The aftermarket weights can be used instead of the OEM weights to see if they have a spring combo that will help matters.
I'm suspicious of why- w/a spring change- that the ping remains at the same RPM, as well as on deceleration. This makes me think there could be a hot spot, carbon or a sharp metal edge of the chamber that's acting as a glow plug, causing preignition.

Look closely for a plug (or all of them) to be too hot of a heat range, or one or more plugs to show signs of oiling in their cylinders- oil is VERY low octane!

If the engine has any intake gasket leakage, the cylinders affected can run lean, causing the same type problems.

Plug wires can have induced currents causing cross firing if they aren't routed correctly and in tip-top shape. The distributor cap itself needs to be crack-free and clean, w/o any carbon tracking that also causes cross firing. You can look under the hood at night w/the engine running to see if you can detect any sparking where it shouldn't be if, you look closely and move the wires around. Just don't get shocked and jerk your hand into the fan or exhaust manifold/header!

Ultimately, if nothing you do w/the ignition will quell this ping, you should try a richer air/fuel mixture. Even though that's counter intuitive for good gas mileage, you can't have the engine pinging, either.

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Old 04-08-2010, 05:49 PM
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Hi guys thanks for your replies, sorry for the delay in replying.

Ericnova, great idea the crane adjustable plate thing, I think that will certainly help the problem giving more flexability to taylor the curve. Unfortunately we don't have em here in NZ anywhere I can find so I might have to order one from over your way. Might have to wait for next pay tho! On the milage thing, I can get 16 mpg but thats with the ping, can obly get around 12 mpg with no ping :-(

Cobalt, thanks for those suggestions, I don't have any other weights at the moment but might be able to get some at the wreakers over the weekend. And I also have an oil control problem, and if this lowers the octane then that could be part of the problem. I am trying to get the mechanic to look at it under warrenty, and then I might be able to go down a heat range in the plugs if they aren't getting oil fouled all the time.

Thanks for the replies and suggestions I will come back and let you know how I get on once I have tried the various ideas.


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