PDSIT vacuum retard and idle.

[Frank Bassman]

Hello all, I am back at it again experimenting and I had a thought. I know the type 4 motor with PDSIT solexes runs smooth. This is in part due to the balance tube and probably the volume of the intake manifold ports. (I speculate)

But also if we talk about the 2 cylinder idle of dual single barrel carbs, we notice that the charge robbing effect is apparent at idle but diminishes with rpm. I wonder if this is by virtue of actual RPM or by virtue of more flow going through the carbs at idle?

If it is flow related, I suspect that the retarded idle timing on the type 4 with DVDA, coupled with the larger throttle plate holes on bus 34 PDSITs (to raise the idle up) ALSO help with smoothing the idle on the type 4 with dual carbs. Flow, not RPM.

Even with a 34 pict, I have noticed it is easier to get a seriously rock steady idle with a DVDA than an SVDA at times.

If this logic is correct, I wonder if adding a DVDA distributor to PDSIT's on a type 1 with a very mild cam further smooth out the idle characteristics of the engine?

My intention is to get some german bus plates re bushed and try them on CB performance PDSIT's. Has anyone tried similar? Or just bus carbs with the DVDA on type 1? Notice any idle improvements?

-Frank

[Alstrup]

Ahh, the fun of dual sgl barrel carbs.
You are on the right track, but you overlook one thing. That is the volume of the plenum in the type 4 manifolds. In most aftermarket manifolds for sgl barrel carbs on type 1 engines the plenum is too small. In most cases they should be enlarged by 20 cc. Sometimes more. The larger plenum along with a properly located bleederhole for the balance tube works wonders. That said, there are some engines, with some more cam duration that does NOT want the balance tube. They just want a larger plenum.

The use of a DVDA was done on later type 4´s, but mainly to take care of exhaust temps (emission) at idle. The ignition alone does not help much as such. But then again, because if maniufold vacum drops the timing advances and the engine picks up, when vacum increases the timing retards and the engine settles down. VW a.o´s. soon found out that there was too many downsides with this approach, so they abandonned the DVDA and went "electronic" with the TZH ignition system. There they used the engine rpm to determine whether the timing should be advanced or retarded. If you look at the timing on a CU or the later WBX engines you will notice that the timing at idle can be all over the place form -5 to +15 degrees as long as the idle stabilizer is enabled. If you disable the idle stabilizer and set the timing to say 8 degrees BTDC you will notice that the same engine will roll a little bit.

Wrt the 2 cylinder iudling issue diminishing at rpm. That is due to port flow increase. When the port flow reaches a velocity where the reversion gets compåresses so much so the pulses do not go as far as the "end of the isolated runners" practically spoken, the engine pulls even on all 4.

If you want to try your luck with the DVDA you do not need throttle plates from PDSIT´s. you just need to make the balance tube right and then drill a hole to take the port vacum for the retard side. The larger bypass in the butterflys are only interesting if you run more displacent like 1800 cc or so.

[Frank Bassman]

Thank you Alstrup! You've confirmed my suspicion that this is flow related and not necessarily rpm related, which explains why larger engines are easier to tame with the singles.

So my next question is, I have two sets of intakes. Steel Brazil VW "tall" manifolds and slightly taller aluminum scat manifolds originally from their 35 pdsit kits. (I was going to put the 35's on but linkage options suck due to the plates opening in the same direction)

The scat manifolds have larger ports than the Brazil manifolds, but no balance tube provision. They also I THINK have a larger plenum aread but I suppose they can also be easily enlarged and or spacers added.

Which manifold route would you take on this one? I think the Brazil manifolds may be a bit restricted but if you have experience with them you would know.

Thank you again!

-Frank

[Alstrup]

I don´t think I have ever come across a set of steel manifolds for the 32/34 PDSIT´s. There was a cast manifold back in the days, typically from Varga. They were rather good and had a nice big plenum too.
The Scat ones ought to be good too on that account due to "long" isolated runners. But I almost always needed to enlarge the plenum in those to make the engine run its best.

[FreeBug]

I just don't get it. Are these heavily modded engines? On stock type 3 and stock type 4 with 32 and 34-PDSITs, and the rpm drop is the same with each of the cylinders, when you unplug it.

This seems to not be the case when there is a lot of late intake closing, maybe also overlap. But to my mind, the cam plays as much a role in good idle mixture distribution as the carbs/manifolds.

Maybe I'm saying that wrong, I just think that carbs, manifolds, AND cam play a part in eliminating or reducing charge robbing.

Just my two scents...

[Alstrup]

Absolutely, which is why I refer to the cast manifolds as being good. I don´t know if you have ever noticed that a 16-1700 cc engine with type 3 intakes and balance tube runs as you say, or at least close. But the same engine with all manifolds has a tendency to run just a little rough idle. That is exactly why I´m talking about plenum volume. For comparison, look at the 412S manifolds, the plenums are huuge.

[FreeBug]

Thanks, that helps clear up some stuff for me...is there a ratio of manifold plenum volume ccs to 1 cylinder's displacement? I have NOT tried different manifold brands and types on an engine such as you describe.

I will accept the answer that there is a magic ratio....but you don't want to share it... .

There are steel manifolds for the 32-34 PDSIT...on single ports .

as for balance tubes..they evolved on the type 3, from one on each side, crudely placed high up and perpendicular to the flow, to having one tube per cylinder, angled into the manifold at a tight angle, pointing down to the intake port. Did type 4 balance tubes evolve the same way?
Both these stock setups also have bigger diameters than a lot of after-market balance-tube set-ups.

[Alstrup]