Blow-Thru/vacuum advance????

[bill Dedman]

I am running a Vortech S-Trim V-1 on a Dodge 360 Magnum in an old Valiant sedan and I drive it on the street a lot, with an occasional trip to the strip for some Bracket racing.

It's a blow-thru setup with a Holley 750 double-pumper, and the distributor is a Mopar Performance stock-type magnetic pickup unit that has a slightly modified curve in it.

It also has a vacuum advance diaphragm that was factory-calibrated for ported vacuum.

I am curious as to whether I might be able to access the ported vacuum signal on this Holley 4bbl and use it to increase spark advance at low-throttle (cruise) conditions.

I know that normally-aspirated engines, under cruise conditions, run cooler and get better gas mileage with an operating vacuum advance unit.

What I don't know is this: Will the ported vacuum in the fitting at the base of this blow-thru carburetor "go away" if the throttle blades open and the boost that is already in the supercharger air delivery pipe begins to enter the intake manifold, or, will there be some residual vacuum in this line at the fitting, since it's ported and not manifold vacuum???

It's a mystery to me, but somebody who knows Holley carburetors should be able to tell me the answer to that, I am sure.

If it's possible (yea, "feasible") to use a vacuum advance for cruising, yet have the vacuum signal DIE instantly, as soon as the throttle blades open, I'd like to try it.
But, if that vacuum signal doesn't go away imediately (if, indeed, there is EVER a vacuum signal at the ported fitting), then I think it's not going to be do-able.

Anybody got solid information about this tantalizingly confusing question???

If nobody knows, I guess I can put a vacuum gauge on that fitting, and run a line to the cockpit, and watch it as I drive the car.... but, I thought I'd ask, first.
Any information will be appreciated!

Thanks!

[speedytang]

I have run Dual Carbs with Dual Paxton VR4s in the past on Cobra kit cars and a Speed Boat. The vacuum will drop as soon as the throttle opens and the supercharger starts building boost. This works great for a vacuum distributor because it will not have the advance so one less thing to do deal with. Now the PROBLEM. You will end up with a dead spot and a dull throttle response when boost hit because you dropping so much timing its like a on and off switch. I went with a mechanical that I adjusted for the total spark I wanted at WOT. The method I used on the boat was a micro switch on the throttle arm on the carb and used a ignition retard box that retard the spark so many degrees when I went WOT.

[bill Dedman]

Thanks for the quick reply!!!

I will have to experiment with that setup, then, because right now, the throttle response coming into boost is really, really good.... amazingly good, as a matter of fact. It pulls like a freight locomotive; no dead spots anywhere.

I was just curious, after I got to thinking about it, as to whether (since the carb is always under pressure from the blower/hat) if it would EVER see any vacuum at that "ported vacuum" fitting.

Where were you picking up your vacuum signal from, Speedystang? I THINK the ported vacuum fitting on this 4150 Holley double pumper is on the passenger side, just above the base plate, toward the front. I'm going to put a gauge on it and see what I get, driving down the road.... I obviously don't know enough about Holley carburetors to be doing this.... I hate being in "over my head" all the time.... and it seems like I am.

Thanks again or the response!!!

[speedytang]

Was this carb modified for Blower applications? If the carb is converted for a blower they use a different power valve and a couple different changes that allows the vacuum port only to register vacuum and not pressure(boost). The truth is if you have enough timing with the weights you will not have a problem but then how much timing are you pushing with out the vacuum advance. You might still be giving the engine 30+ degrees of timing. As long as you keep that WOT timing in the back of your mind and give the engine enough octane to handle you will be OK..

[bill Dedman]

Speedytang,

Yes, it is a 4150 750cfm double pumper Holley that I bought new from the Carb Shop in Ontario, CA for the measley sum of nine hundred dollars about two and a half years ago. Just now getting my act together with the Vortech. It's a blow-thru modified carb that was supposedly run on their test mule engine on a dyno with 15 pounds of boost. I hope they knew what they were doing....

It has a 6-5 power valve. 6.5? 65? Anyway, the power valve has a 6 and a 5 stamped on it, and it's not blown (no hole in the diaphragm).

I richened up the primary main jets from 72s to 78's in an attempt to get it to run rich enough to quit backfiring through the intake when I'd open the throttle. I also replaced the squirters with much richer ones (went from .46 to .72) and reduced the size of the holes in the adjustable air bleeds from .32 to .26's (smaller is richer, in this case) and after doing all that, found out that I had a BAD DISTRIBUTOR.... REAL BAD!!! Replaced the distributor with a new M-P "curved' one (with a vacuum cannister) and ALL my tuning issues went away.... just like that!!! Runs like Bentley Continental GT, now... well, not quite! LOL!!!

Now, I'm going to UN-do all the stupid things I did to try to richen this thing up when it was backfiring and I thought it was too lean. Back in will go the original Carb Shop jets, squirters and adjustable air bleeds, and it will run like it's supposed to. Not that it's running bad right now.... it pulls REALLY HARD right up to 6 grand (my self-imposed redline.)

I just thought that the engine MIGHT like some vacuum advance at cruise...
I'm only going to run 30 degrees total mechanical timing, and have an MSD "BTM" that I can dial retard under boost with. I figure 22 degrees at WOT; I am running 8 pounds of boost, so I'll set ot for 1 degree for every pound of boost... 30 minus 8 is 22.... Is that a good starting point for 93 octane pump gas with the SnowPerformance BoostCooler staring to spray at 3.5 pounds?

So, anyway, I'll put a vacuum gauge on this port that I THINK is a ported vacuum fitting, run the tubing into the cockpit and see what I get at idle (should be nothing), cruise (should be some vacuum), and WOT (should show (0) or maybe some boost; I don't know about that one.

Thanks a lot for your help and advice. I am new at this and need all the help I can get!

[bill Dedman]

http://tinyurl.com/bg9cem

Speeedytang,

I cancelled the order for the non-logging Prosport a/f meter and ordered a F.A.S.T. logger model # 170401 from Summit. They gave it to me at the price listed above, which is their "we wil not be undersold" deal..

Thank you SO MUCH for the good advice; you helped me dodge an expensive bullet with your timely tip!!! I owe you...


Bill

[speedytang]

I just used one of those this weekend on a Talon TSI. It is a excellent unit. Great Buy.

[bill Dedman]

The folks at ProSport were not at all reluctant to cancel my order... very gracious, as a matter of fact. I did cancel it first thing Monday morning, five minutes before they "opened," actually, so they hadn't shipped it, so nobody was out any re-stocking or shipping. An unusually pleasant experience, given the circumstances.

Some places are not so easy to deal with. They will get my business, down the road.

For the money I saved, I could have gotten the dual-sender unit, but I think it will be pretty easy to just put a bung in each collector and make a run with the sensor in each one, consecutively, and compare. I can't imagine that there will be much difference, side-to-side, since there's no stagger-jetting in the carb, and I'm not running much rpm or boost ( 6,000 max @ 9 psi).

Sunday night I should have a timeslip or two for this thing and can find out where I'm at with regard to horsepower output. Hoping for a 10.00-sec., 1,000-ft. @ about 100 mph. That's about a 7.62 @ 91mph eighth-mile, I think.

The problem will be hooking it up, if the weather is cold. VHT doesn't work well if the temerature is much below 50.... and, this IS February.... :(

We'll see. I have several "traction aids", but nothing as effective as Cal Tracs.
I've got 90/10s in the front...
I have three spring clamps on the front "half" of the spring, an air bag on the right side for preload, and an adjustable pinion snubbber with a 1/2" air gap... and some 26"-tall, BF Goodrich 8"-wide T/A Drag Radials (D.O.T. tires). A 2,500-stall (that's unblown) converter with stock 2.45:1 1st gear T-Flite (904) and an 8.75" 3.55:1 Sure Grip (new.) I have a 4.10:1 SureGrip for later...

Wish me luck; I'll be running av-gas, so I can advance the timing, somewhat. Open exhaust, too...

If the head gaskets hold (stock, composition, .040"-thick), I may be okay....

Wish me luck; this is her maiden voyage with forced induction!

And, thanks again for the heads-up on thje logger situation; I am really ignorant about this stuff; you saved me a lot of grief!!!

Bill, in Conway, Arkansas