1conic

Hi guys,

My '79 L-82 currently has a Q-Jet with tired throttle bushings and I'm possibly considering going the Holley 4150 route in the future.
I did this convertion before on 350 L-48 1980 I owned. I used the original manifold, and a squarebore to spreadbore adaptor, it worked fine.

But, now a few years on, and aware of ported take-offs, I'm thinking I may have connected my dizzy vacuum take -off incorrectly, to mainfold vacuum:o

Does anyone know for sure that the Holley 4150 have a ported vacuum take-off so I can use my std '79 GM HEI dizzy?

thanks guys

bashcraft

You unknowingly did the best thing.

73StreetRace

Ported vacuum was only introduced for emission control systems, not for performance...
MOST engines, but not all, will idle better and have better performance with vaccum advance connected to manifold vacuum.

Ganey

It would be better to change intakes or use the Holley spreadbore.

mr.beachcomber

The ported vacuum takeoff in the 4150 Holley is on the metering block while the manifold vacuum port is on the aluminum base in the front.

1conic

many thanks guys,

Have got something wrong here?

1/ I should use ported vacuum take-off with a std '79 HEI (or any HEI) with std curve as they are designed this way, to act with a vacuum switch for idle and cruise conditions.

2/ But, if I connected to manifold vacuumm my idle will then have way too much advance.

3/ My understanding is that the ported vacuum take off means no/little vacuum at idle for the advance capsule and when the throttles are opened then this uncovers (opens) the port and I get more vacuum such when on the cruise thereby advancing my ignition?

cheers guys for the help so far

73StreetRace

You always check timing with vacuum advance hose disconnected and plugged.
When your timing is correctly set and you reconnect the vacuum hose to the distributor,
your timing will increase by about 15° at idle ( if you're using manifold vacuum ).

BUT DON'T MIND. THIS IS NORMAL AND YOUR ENGINE CAN HANDLE IT

Jeff_Keryk

A lot of advance at idle (like 28 or more) is great for off-idle transition, among other benefits. I wish I could run 15* of vacuum advance, but crappy 91 octane CA premium will make it ping like crazy during low load conditions.

1conic

thanks for the info'

If I set my timing with vacuum tube disconnected and my HEI 'static' timing is 12', and reconnect my vacuum tube then I end with 27' at idle using manifold take-off, this is way too high for small block 350 isn't it?

I thought, please correct me here ifm I'm wrong, that 12' for idle, 36' for WOT and 52' for cruise would be ideal?

cheers guys

Wrencher

This comes up pretty often and there are some misconceptions out there.

First off, 90% of all cars and trucks before about 1970 had either full mechanical distributor advance using a system of centrifugal weights and springs, or a vacuum advance diaphram running ported vacuum, or most often both.

You are correct that ported vacuum at idle is minimal or zero as it is supplied above the trottle plates, and increases as the throttle is opened. With good, high octane fuel, this was optimum as the vacuum side came on quickly and then the mechanical side took over up to maximum advance.

Direct, or manifold vacuum became popular as fuel degraded, mileage became more of a concern and emissions packages were developed, especially EGR systems. The advantage is lots of vacuum advance under LIGHT throttle as oposed to foot-to-the floor acceleration when it drops off to almost zero.

So it depends how you drive as to how you want your advance to work. I've set up many cars using manifold vacuum that transitions pretty early to mechanical to provide adequate advance at higher RPM and load, yet still keep a lid on detonation. I run into problems occasionally with light throttle 'pinging' (mainly on Cleveland style Fords), but a well set up EGR system solves that.

Most of the applications I tune are for max performance with race fuel or fuel addatives to 91 octane pump fuel. All of them are setup with either full mechanical, full electronic or a ported/mechanical combo.

There is no one 'right' way to do it. It depends on what your driving conditions are, engine type and what you want to achieve. Hope this helps.

Hans

Mike Ward

You've got the two types of manifold vacuum reversed. Full vacuum was the standard until emission became a concern, at which time ported vacuum became popular.

Mike Ward

You're mixing apples and oranges. Forget throttle setting or vehicles speeds. Forget which year your car is or what type of ignition you have, it's irrelevant.

You want 36' approx with full mechanical advance, but with vacuum disconnected. To achieve full mechanical advance, the engine must be run at a high enough speed for the weights to hit the stops. This might be 2500-3000-3500 RPM according to how the dist. is set up. Whatever initial timing you end up with is just fine.

Now plug the vacuum advance back in [U]to the full time port[U]. At this point the total timing will increase significantly but will be the ideal advance for you engine.

Now close the hood and stop over thinking the issue.

1conic

Thanks Mike....but one last thought before I close that hood, and go for an Indian....Lamb Korai, Vindaloo and a few beers...and now I have the attention of timing guru

These HEI's are meant to have real soft advance curve to give cleaner emissions, as in low overall advance with regards both vacuum and mechnanical, with full WOT mechanical advance not coming in until 5000 rpm, so if I set 36' at say 3K rpm I'm get be too high at 5K....or am I just getting myself even more confused?

cheers

Mike Ward

Yes you are. Once again, forget throttle position. Stop guessing the characteristics of the advance curve. Find out for sure by measuring it yourself with a dial back timing gun. I'd be surprised if it's 5K rpm. If so, you've got two choices, re curve the dist or just set the timing to stock specs and close the hood.

Either way, enjoy your curry.

1conic

Cheers Mike,

this site is simply great, helpful, funny, great advice, a goldmine of info'...if sometimes the opposite of what you know/think you know, always good to question and make one think?

....just found a link to guy called 'Lars' who also is a timing anorak....he has written a HEI primer and also seems to think timing is very soft on HEI's with max advance rarely seen @ before 5k which seems to tie up with what a guy @ Rousche Research told me when I had them as a customer for AFR systems, I will drop Lars mail and chimeback the result.

many thanks for the help guys

hmm, curry and Kingfisher

73StreetRace

My timing is 37° total, and idle is set at 16° ( w/o vacuum advance ).
With vacuum advance, I get 16° + 15° = 31° at idle ( and 52° at cruise ).
Off-idle response is great this way, idle is much smoother and coolant temperature is lower.
As soon as absolute manifold pressure increases, vacuum advance decreases, preventing pinging... assuming your fuel has enough octane.