Vacuum vs centrifigal

[crvc]

I put a Compuserve system in the centrifigal distributer in a 1967 1600cc engine. The carb is an Empi for the 1967 engine I bought less than two years ago. Yesterday while cleaning out the garage I found a vacuum advance distributor. What's the difference, if there is any, between the vacuum and centrifigal's performance or economy?

TIA,

kevin

[Bugfuel]

There is a lot of discussion on this... run a few searches and read up on it.

Here's my take on it and I believe this represents the opinion and experiences of most people who know what they are talking about:

There are basically three main types of distributors:

Vacuum only
Mechanical only
Vacuum & mechanical combo

The vacuum only distributor does not recognize engine rpm. It only sees changes in the vacuum signal from the carburator. (engine load).

The mechanical only distributor does not see the engine load because there is no vacuum signal. It only sees engine rpm and adjusts itself to that pretty much linearly and blindly. These need to be adjusted to max advance at 3000rpms, or whenever it's "all in". You set it to the highest value that is determined SAFE under ANY conditions. It's a compromise, and it means it will not be optimal.

The vac&mech combo is a combination of these features, giving you the best of both worlds so to speak. The vacuum reacts faster than engine rpms, and will thus help the engine react faster to you pushing the gas pedal. Less likely to bog down during take-off, and quicker throttle response, smoother shifts etc..
When cruising at steady speed, the vacuum signal is again present and will give you some extra ignition advance, which translates to better fuel efficiency.
The mechanical system is still giving you the "safe under any conditions" -base advance, but when the conditions allow it, the vacuum unit will give you some extra, without risking anything. When the conditions don't favor extra advance, you fall in the arms of the ever so safe, yet dumb mechanical curve.

So... the combination type will give you the best fuel economy, safety, and performance.... with two conditions:
1. IF the overall advance curve suits your engine
2. IF your engine is able to provide suitable vacuum signal.


#1 can be adjusted in several ways to suit your engine's needs

#2 is dependent of the carburator type you are using. The carb body needs to have a vacuum fitting in the right place, and the vacuum signal needs to be of correct strength and smoothness to suit the whole package. If it's not, you can modify the distributor again to adapt it to the signal available. In case of dual carbs, you can also tee both carb vacuum lines together.

If you are not able to provide a vacuum signal, you will be stuck with the centrifugal-only distributor and it's compromises. It will work "fine", but usually it requires internal tweaking to get it right.
These are simple solutions for highly modified engines with dual carbs and missing or totally unsuitable vacuum source.

[Lingwendil]

Gives us the model # of your carb and diz, if it's an empi 30 pict-1, I've had good results with using a 205K vacuum only distributor with it. If you have a h30/31 pict you may be better off with the mechanical only diz, as the 30/31 doesn't give enough signal to pull the vacuum advance mechanism properly.

[daylight]

howdy so where do you find vac/mech dist? will it fit on a type 4 ?

[Marc]

The most popular vacuum/centrifugal is the type found on 49-state 4-speed 1974 Type I. It has a centrifugal curve that's "all in" by ~3500 RPM (~1000 RPM higher than the curve of the centrifugal-only 0 231 178 009) and vacuum advance only, leading to it being commonly referred to as an "SVDA" - single vacuum, dual advance. The DVDA type used on most other `71-up ACVWs has a vacuum retard as well, there to pull down the timing at idle speeds to reduce HC emissions. When the latter is used with the retard side disconnected (and the initial timing therefore bumped up from 5°ATDC to 7½°BTDC like the SVDA) it has nearly the same characteristics as the SVDA.
There's another SVDA that was used on early AutoStick cars, it's not what you're looking for.


Here's a pretty comprehensive listing of the various distributors used in ACVWs:
http://www.oldvolkshome.com/ignition.htm

[daylight]

howdy ok are the dist still made? and if so where can get one?

gil

[crvc]

The centrifigal # is 0 231 478 009

Thevacuum # is 0 231 137 035

The original carb # is 113 905 205 T. The model is H30/31 PICT.

I replaced it with an EMPI clone but can't see any numbers on it.

The car starts then dies and won't restart. There is a strong gas smell but I don't see any spills or wetness on the motor.

[Marc]

0 231 478 009 is meaningless to me, but 0 231 137 035 is a `68-~`70 vacuum-only unit.
I know nothing about EMPI carburetors.
But the "dies and won't restart" complaint is not likely to be due to distributor choice - regardless of the advance curve characteristics, if it will start in the first place it should at least continue to run in the driveway, odds are the problem is fuel-related.

[crvc]

When I was having the problem of gas mixing with the crankcase oil I replaced the fuel pump. When that didn't solve the problem I ordered the cheap EMPI carb. That didn't solve the problem either. It sounds like I need to go back to the stock carb and give the vacuum distributer a try. I'll probably go back to points/condenser so I can at least time it with the continuity tester. That way I don't have to buy a new timing gun.

Thanks for the help,

kevin

[Lingwendil]

Since you do have a h30/31 I wouldn't bother trying that vacuum advance dizzy with it, those two tend not to work well together. However, if you can get your hands on a 30 pict-3 they'd work very well.

[Bugfuel]

make sure your fuel pump is working. There are two types, which each take a certain length pushrod. You can mechanically mix them but the pump would either not work (rod too short) , or it would likely break (rod too long).

[aussiebug]

Since you do have a h30/31 I wouldn't bother trying that vacuum advance dizzy with it, those two tend not to work well together.

Unless you get an SVDA from www.aircooled.net and tell them you are going to use it with an H30/31...they will set the distributor up to use the weaker vacuum signal from the H30/31.
Try this link
http://www.aircooled.net/new-bin/viewpr ... 1093211472

[Stray Catalyst]

Kind of a thread hijack, kind of not...

I have a 1600DP, stock except for 009. It has the 34PICT-3 carb. I've been having a variety of the usual problems with the 009 vs. the 34PICT-3 not playing nicely together. As I can't afford to upgrade the carburetor, I picked up a rebuilt SVDA dizzy. There are a few places I can run the vacuum line from the intake to the distributor - which one should I use, or are they all the same?

Stray

[Bugfuel]

Kind of a thread hijack, kind of not...

I have a 1600DP, stock except for 009. It has the 34PICT-3 carb. I've been having a variety of the usual problems with the 009 vs. the 34PICT-3 not playing nicely together. As I can't afford to upgrade the carburetor, I picked up a rebuilt SVDA dizzy. There are a few places I can run the vacuum line from the intake to the distributor - which one should I use, or are they all the same?

Stray

The distributor advance vacuum comes from the left side of the carb, a fitting that is almost at the same height as the throttle plate. All unused fittings must be capped off. Some carbs have a vacuum fitting pointing straight back, that's for distributor retard for those distributors that have it (a dual fitting vacuum canister to provide retard and advance).
No distributor uses vacuum from the intake manifold. Some manifolds have a small vacuum fitting just below the carburator, that's for the stock air cleaner box.

[hemicat]

Sorry for the excitement. Realized my input/ questions were not exactly where this topic was going. Sorry