[70bird]

What do I need to do to modify my Holley 4150 (Vacuum Secondary) to work on a blower (if it can be done)?

[stoneshrink]

Let's start with a basic principle. Blowers make motors more efficient by putting more air into the motor. Simply put, if you only put more air into a motor without more fuel; the motor runs more lean and bad things happen like burning valves, blowing holes in pistons, etc (good if you want to build a new motor, bad if you have to explain the expenditure for another motor to your wife).

The good news is: it's pretty easy to tune the motor for the blower. Upsize your jets by 2 numbers as a starting point, then run the car and read the plugs. If you don't know how to read plugs, here http://www.garbee.net/~cabell/SparkPlugData.htm is a good site to give you the basics of plug reading.

For the vacuum secondaries. Start with your lightest spring and work backwards. With a non-aspirated motor, you go the other direction but with a blower too much fuel is the better choice rather then too little.

I run a Holley 870 with 74 jets in the front and, I believe, 68s on the back (secondaries). I use the lightest spring possible and have found it works pretty good. Interestingly, it was the setting (except for the light vacuum spring) from the factory.

I am changing to a 750 (flows 790) Holley DP carb next because I am moving to experiment with E85.

I hope this helps. good luck....

[70bird]

My thoughts are that the power valve will not open on a roots system b/c the carb base will always be under vacuum.

I'm thinking that elimination of the power valve and an increase of primary jet size may be in order.

Holley has not responded to my inquiry on this issue.

[stoneshrink]

Now you're making me think. (Watch for smoke)

First experience - The power valve works on my car and the boost gets up to 9 lbs.

Now the thinking part - I see what you're asking, but with a positive displacement blower, it has vacuum well into the powerband. One of the things I hadn't thought about when I put the blower on the car was that boost is only developed at about 3500~ below that you have up to 15 lbs of vacuum. Again, as long as it is not a blow through design the carburetor would react simply by whatever pressure it reads between the butterfly valves and the rotors. Interestingly, I now realize why putting a 1" spacer on my set up worked so well - it increased the signal the carb receives during throttle position changes.

[70bird]

Does vacuum decrease at WOT at the base plate with a blower as it does w/o one?

[stoneshrink]

If I understand you correctly, at idle you have around 15 lbs vacuum at the manifold (under the blower). At ~3500 the blower starts developing positive pressure up to your redline - in my case at ~6000 I am developing 9 lbs of positive pressure (boost).

At the carb, the signal is the same as if the blower wasn't below it..... again keep in mind the "only" difference the carb (at the base of the carb) sees is the volume of air passing through it.

What blower are you using?

[tweaked]

Holley emailed me directions to boost reference the carb. I made alterations to the carb cause they said I needed to. Afterwards I did my own test and put a vacuum gauge at the intake and watched what it did while I nailed it under boost. The result was I never needed to ever mess with the carb. Keep in mind this was a 144 on a 355 with 7lbs of boost.

[Augusto]

I have a 4160 atop a B&M 144 blower huffing over a 355 chevy, the only mod I made to the carb was a jet adjustment to increase fuel, I have the brake booster connected to the PCV fitting in the carb and everything works fine, the carb does not care about the blower, it just thinks there is bigger engine underneath. I used the same powervalve as normally aspirated.

between the carb and blower there is vaccum just like in a normally aspirated manifold, under the blower things change, don't hook up nothing there, just a boost gauge.

if you run a vaccum brake booster connect it like I did using the PCV port in the carb, if you run a pcv system then make a new fitting to connect.

I also run a spacer but mine is 2 inch, does not make much difference but looks cool.

it took a while to adjust the opening of the secondaries and the timing curve, but the carb does not need anything special, I also tried a q-jet and I just loved the sound of it at wot, too bad this carb was too old and had to put the holley back.

Augusto.

[stoneshrink]

Augusto, you bring up a couple really good points.
1) the carb doesn't care about what is under it. B&M got the number for their blowers by the number of cubic inches the motor "grows" when you put their blower on it.... thus a 144 makes your 355 act like a 499 cubic in motor, and a 177 would make a 400 act like a 577.
2) You can put a quadrajet on the blower without any modifications, if you like the huge secondaries - Holley made a carb (I'd get the number if I wasn't too lazy to walk to the garage and look) for GM that they put on the big blocks in the late 60s and early 70s - they go by the name Spreadbore v. Squarebore.

My first round with my 144 blower was on a 358 (.040 350) - it really needed more CFM then the 650 Edelbrock that was originally on it. You can gain effective CFM with spacers - figure 50 cfm per 1", but even after changing to the 870 I found the spacer really helped increase the signal the carb "saw".

[stoneshrink]

I need to make this absolutely clear. I run a blower that is NOT a positive displacement blower!!!! Therefore, the power valve operates normally and with the 1" spacer I have added insurance.

If you have a 6-71 or larger GM style blower you do need to boost reference the carb. It is done pretty easily, and any Holley tuning book will tell you how to do it. If you attempt to run a positive displacement blower without making the modification you could burn your motor up by running lean at idle.

http://www.holley.com/TechService/FA...ategory=Blower
there is Holley's take on the situation.

[70bird]

I'm planning on putting a 177 on the 355 in my '76 K5 Blazer.

I thought all roots blowers were positive displacement but apparently I was mistaken.

What defines a positive displacement blower?

"don't hook up nothing" under the blower?.....So what do I hook up under it? LOL

Just giving you a hard time Augusto. The use of double negatives cracks me up.

[stoneshrink]

Positive displacement builds boost at idle (6-71s and those normally make boost at idle). Your 177 is efficient enough that you will likely have to do the modification below... normally speaking 177s are used on 400 plus cubic inch motors so you're likely to build more boost earlier.

Your question has created quite a discussion among my friends and it appears I need to give more explanation.

The power valve operates by manifold vacuum. In the presence of low to zero vacuum the power valve opens and provides more fuel to the motor. On a blown motor the blower itself does not allow as strong of signal to come from the motor because of the volume of air that it is pulling even at idle.... By experience I've found that a spacer (plus the 870 carb) does the same thing as running a line for manifold vacuum. The discussion in real time pointed out that while the immediate result of opening the butterfies would be an instant of zero vacuum, the blower would almost immediately change the volume of air coming through the carb and thus close the power valve..... I continue to argue that the spacer and larger carb on my set up more then offsets that and while it could lean the motor out for a milisecond the jets sizing would make much more of a difference.

The modification discussed by Holley simply takes the power valve manifold from below the blower rather then above. It is a simple conversion where you plug a hole in the bottom of the carb plate, drill a hole to the power valve circuit and run a line from below the blower to the power valve.

[70bird]

I understand how the power valve works, that's why I was thinking they would not open b/c the base plate would always see vacuum, even at WOT.

I would think that if the power valve circut is plumbed to the manifold it would never close b/c there is never any vacuum in a boosted app resulting in being rich even at idle.

[stoneshrink]

At idle, with the B&M/Weiand/Holley blower you have quite a bit of vacuum under the blower. (this was a surprise to me when I first fired it up - but it makes sense). I suppose with a small enough pulley (underdrive) you'd have the same situation with the GM -71 series.

I suppose the easy way to tell is to plug a vacuum guage at the vacuum port that is directly under the throttle blades. If no or little vacuum, you have to run a line....

Racers get away without the bypass by oversizing the jets.

[Augusto]

I've been living happyly with the vaccum under the carb that powers my brake booster with no problems, that I have failed to think about the power valve, I burned a piston once and I never knew why, I blamed the gasoline back then, now I'm pretty sure that there is a powervalve issue, it must have been closed all the time and this created a lean condition that led to the failure, I'm gonna run a line from the manifold to see how it works, I'm sure there is some hidden performance rigth there, thanks for uncovering it.

I wonder how hard is to make the same mod to the q-jet.

Augusto.