Hello all,

I'll be adding a supercharger to the car as my winter project. I'm still researching a bit, and still undecided whether I want the torque of a roots style blower or the mid to high range power of a centrifugal blow-thru setup. The way I understand it, very little (if any) modification is needed on a roots style setup, but when going to a centrifugal setup the carb will need to be modded.

My Q is, how difficult is it to setup and tune a blowthru carb? I presently have a Pro-Systems 750 DP on it now, which is less than a year old and still looks brand spankin new. Even though Pro-Systems claims to have "the quickest" blow-thru carbs on the market, would I be better off just giving them my setup specs and have them modify my carb for me, or just do it myself? They advertise that they will "upgrade" carbs (at the tune of 350-400 bucks), or should I just modify my existing carb myself? I've been told that either way, I'll need to do some tuning.

Is buying a already modded for blow thru carb from one of these companies worth it? I'm looking for minimal tuning, but if I end up having to do major amounts of tuning whether I buy a off-the-shelf modded for blow through carb or modify mine myself, I'd just assume mod mine myself and save the 350 - 400 bucks for a wide band fuel/air ratio meter so I can tune it right. So that being said, how good are these pre built blow-through carbs in general?? Anyone have experience with them? I'd also love to hear from anyone that has modded there carb themselves for blow through also.

Also, for the second part of the questions, if I decide to go with a roots style blower, what's really involved with setting up a carb for that? I've heard you need to have some kind of "boost reference" port so the power valve does not sense boost pressure, but vacuum instead? - or something like that.

Any wise words of advice appreciated!

PS - Please don't try and talk me into slapping a turbo on it - I already know they're more efficent, but require a lot of fab work which I don't want to do.

 

There's a fundamental difference in the operating environment of a carb on a roots blower vs. a blow-through centrifugal.

On top of a roots blower the carb still functions just like it would on a N/A engine. It never sees boost. It only knows how much air is flowing through it and how much fuel to add into that airflow. Tuning changes are typically minimal (mine runs almsot exactly the same calbiration as it did on top of my previous N/A engine).

In a blow-through setup the carb actually sees boost. THink of boost as being the equivalent of a HUGE change in altitude- like running your engine 3 miles under sea level. Unfortunately, the way carbs work, changes in altitude (or boost) is one thing a carb can NEVER compensate for by it's design. It is basically "blind" to altitude/boost. So when you go into boost they go lean becuase they can't compensate for the increased DENSITY of the air going into the engine. It's sees the VOLUME of air, but not the DENSITY of that air.

To compensate, typical mods start with slightly richer jetting and then go quickly into the realm of drilling out the PVCR (Power Valve Channel Restriction) so that it feeds a LOT more fuel than it ordinarily would as manifold vacuum drops to 0 and then climbs into boost (when the power valve opens on a Holley). Some of these changes are of the "big" variety- such that the carb could never run a N/A engine very well at WOT, but feed the fuel needed at WOT/under boost for a blow-through application perfectly.

And that's just a brief and incmplete overview- there's a lot more to it than that. If you've never done a blow-through motor carb you might well want to seek help from the aftermarket for your blow-through carb application.

 

Of course you can aways put a box around the carb.

And the PV needs to be referenced to the manifold with a Roots style.

 

Thx Damon -
I understand the fundamental differences between the roots and centrifugal type blowers. I guess what I'm trying to ask is should I go buy a made for blow-thru out of the box bolt on carb, or modify my own.
So you pretty much answered it right there! I've read about having to drill out the PVCR holes like you were saying, to a pretty huge diameter it seems. And unless you're really savy at reading spark plugs (and I'm not), I think a wide band A/F meter would really be required to tune it and get any kind of MPG - wouldn't you think? I can call and ask around at Pro-Systems or Quick Fuel and see how much tuning their carb would require if I bought one.
5-7,
If I get a Vortec blower, they come with a box. But I'd still need to mod the carb right? I mean, at least the special bowl floats right?
I've also read about the PV reference for roots blowers, but what does that really mean? How is a Power Valve referenced?

Thx again - keep the replies comin! I might be able to get this right the first time and avoid costly mistakes!

PS - Damon - I take it from your reply that you have a roots blower on your car? How do you like it, and what kind of blower do you think I should get to make meaningful boost on a 400 - like mine for example.

 

With the box, you can't use brass floats, from what I understand.

The standard Holley PV vacuum source comes from the baseplate to the intake plenum. With a pull-through blower, you need to run a line from the manifold, which will be under pressure, up to the power valve port. Otherwise, when under boost, the carb base will be seeing low pressure and lean out the mixture.

 

You still need to do the carb mods whether you do the box or a carb hat + sealed-up carb. Either way, the fuel bowls see boost but it still doesn't address the extra fuel you need when you jump into a boosted condition.

Roots blower you can run without a boost reference line to the power valve- if the engine is mildly cammed and you can pull good vacuum at the carb at mild RPMs. I run a QJet on my mild roots-blown 383 and there is no way to boost reference it. Fortunately, I have a very mild cam and I see plenty of vacuum at the carb just cruising around. At heavier throttle, vacuum at the carb drops near zero and the power valve (power piston actually in a QJet) opens and fattens up the mixture before the engine gets into boost (about 5" of vacuum at the carb = beginning of boost at the intake on my combo). So it still functions almost exactly like it would on a N/A engine.

Boost referencing is more important when you get into wilder setups with big cams that rarely draw much vacuum at modest RPMs and with bigger roots blowers that don't make much boost at low RPMs. You don't want to be driving around with the power valve open most of the time. Boost referencing to the intake manifold helps in these situations.

 

The problems with a blow-through carb go deeper than just calibration...

Consider just the simple problem of keeping it full of fuel. We all know fuel (or anything else) flows FROM a place of HIGHER pressure, TO a place of lower pressure. So.... what happens when you seal the carb in a box and subject it to, say, 15 psi? How are you going to get 7 psi fuel in there? The only way, of course, is with a high-pressure fuel pump, and a boost-referenced regulator.

The brass float issue is familiar to anyone in freezing climates... brass floats can be compressed under pressure. A crushed float doesn't float, it sinks.

Then there's the PV issue. The PV is simply a valve that opens when the manifold pressure drops to a value some preset amount BELOW atmospheric pressure (that being what "vacuum" is....). It has a diaphragm, and a little port that exposes one side of it to manifold vacuum, and the other side is exposed to the pressure in the fuel bowl (normally, atmospheric). It doesn't know what to do when the pressure in the manifold is HIGHER than atmospheric; or, as would happen in a sealed-box blow-through system, when the whole relationship between the 2 pressures is constantly changing, and no longer is a "signal" of the engine's output demand. You have to either re-manufature the point that the PV looks at, and select one that's a compromise of the possible correct values over the range that it will see during normal operation; or, block it off and run without one. Neither is really attractive for a typical street car.

The "extra fuel" thing is another issue. Carbs respond to the VOLUME of air passing through them, not to its MASS. The problem there being, the carb's calibration implicitly ASSUMES that a given volume of air has a given mass (contains a given # of molecules of oxygen). But as we all know, that isn't the case; anyone who has driven a carbed car from sea level to 10,000 feet knows that. With a blow-thru setup, you have the same problem, except in reverse: each volume of air contains MORE molecules, instead of LESS like the altitude problem. In other words, in orde rto meter fuel properly, somehow the carb has to EITHER be equipped with some method of measuring, and compensating for, changes in pressure (like the little aneroid thing in some Q-Jets does for altitude); or, it has to be preset to ASSUME some range of boost values, and be equipped to meter an amount of fuel that's within some sort of reasonable range for all such possible values.

IMO any blow-through setup for a carb is little better than a hack and a patch. The fundamentals of how a carb works, make it a VERY difficult thing to ever get right. The Roots type setup works much better, with the carb at atmospheric pressure, untroubled by whatever else is going on; that way, it can just go happily along doing just what it always does, without all of those pressure-related hacks. All it needs, is to be dialed in to the engine's new air/fuel ratio requirements, that are somewhat different with a blower than without. A much easier and more stable thing to accomplish.

But if you DO decide to do the centrifugal/blow-through thing, you're probably better off just buying a carb from somebody that has already seen as many as possible of the ways motors get popped from having the carb set up wrong, and avoiding the EXPENSIVE trial-and-error process of learning.

 

Thx 5-7, Damon and Sofa - Great responses and informative! Gives me more insite on which way I should go with supercharged carb setup. After reading your replies, it would seem I'm basically at a crossroads here. If I go with a blow-thru setup, most likely a custom built carb will be required with alot of tuning - and I think less streetable overall unless tuned perfectly. The resources cost $$.....Seems programmable FI would be the way to go if using blow-thru to me. You could program whatever fuel curve, ignition advance curve etc... it likes, and you have feedback to know how the engine's really running.
Again MO $$$....and if I drove the car alot, I'd go that way possibly - but I don't.

Draw-thru it is!! Just seems (to me) to be the most trouble-free way I could go -using a carb and being really streetable. I won't be asking the carb to operate "against the grain" as with a blow-thru setup, and realistically, I think I could handle the carb tuning on a draw-thru....seems to be much easier.

So now I need to figure out what blower/cam combo I'll need for a 1500-6K rpm powerband torque monster! I'm hoping I can get around 500 RWHP and keep it mild....on a 400 is that asking too much you think?

Again, great info...this could be a "sticky"....

 

Suck-through carb tuning is pretty straightforward; you shouldn't have too much trouble with it. The main thing to remember is, the carb is "seeing" an engine that's like 1.5 to 2 times whatever the actual cubic inches is.

Easiest ones to buy and work with are probably the Weiand ones http://www.holley.com/division/Weiand.asp

A typical blower cam is VERY exhaust-heavy (since it's pretty easy to move material past the intake valve by way of The Force, but somehow it's all still got to get back out after it's done its work). And, one of the most undesirable outcomes is a backfire into the intake (tends to blow the blower right off the motor), so they tend to be small on the intake side for that reason, as well. I'd suggest talking to cam mfrs directly, and see what they've had good results with.

 

Agreed, the wide split pattern is good for a blower setup, and also nitrous if you so desire. Stick with the widest LSA possible to keep overlap low.
I think a 142 or 6-71 might be a bit small for your setup. I'd look at the 8-71 or something

 

500 Rear wheel HP? Not with a little 142, I can tell you that. I'm just shy of 400 at the wheels with the little Weiand 142 on my 383 with AFR heads and a bit of a cam, and I'm one step shy of maxed-out on pulleys.

The 177 won't get you there either, in all likelyhood. The slightly larger 256-series blower...... maybe.

 

The Weiand 177 won't even be big enough?!? And it seems the 256 series is only a measiley 2 inches shorter than a 6-71 or 8-71 series. If I were building a '57 Belair, it wouldn't be an issue, but this is a Thirdgen!

I'd really LOVE one that would fit under a 3" cowl hood if possible (that's air cleaner included) - is that asking too much? I'll quote myself:
Others have told me that making power on a blown car is like "falling off a log" and the Weiand literature says gains of 30-75% power increase can be obtained (and that's with a 142 model). It seems I can't "have my cake and eat it too" once again. It also seems I'll need to either #1 - lower my expectation of 500 RWHP and go with a smaller blower, or #2 -put up with two carbs and a huge 8-71 blower sticking out of the hood blinding my vision on the street, and losing any "sleeper" potential. -And I don't see #2 as a very good option.

I'm not giving up yet though - I'll call Weiand (Holley now) - just to see what model of blower will at least get me close to what I'm looking for, and I'll post back up.

 

The 142 just can't move enough air to make 500 RWHP (which would be 600+ at the crank with the usual drivetrain losses). You can only spin an old-design roots blower so fast before it's efficiency falls off a cliff and you end up doing little more than heating up the air to get a boost reading on the gague. If it was possible, I would say so, but my experience is that you'll fall way short of it. 500HP at the crank seems to be about the limit before it's all pooped out. The little 142 was designed a long time ago and was built to make a 250-300HP engine into a 350-400HP engine. When you get up into the 500HP range at the crank it's pretty much all done.

The 177 might get your near there, but again, you'll fall short in all likelyhood. Too many RPMs at the blower, not enough airflow.

The 256 can get you there because it's big enough and can be pulleyed up for enough airflow without spinning the blower out of it's efficient range. And yes, it's about as long as a 6-71, but shorter (smaller lobes). A 6-71, by comparison, moves 411ci of air per revolution where the smaller blowers are rated directly by their displacement (142 = 142, 177 = 177, 256 = 256).

Here's a very helpful article you might want to read about roots blowers:

http://www.holley.com/data/TechServi...ech%20Info.pdf

NONE of the blowers will fit under a stock hood- not even the 142. You're talking about a 4" cowl hood for the 142 and only bigger from there with the larger blowers (big Outlaw-style hood or a hole cut in the stock flat hood).

That's one of the reasons why centrifugals are so popular- you can fit it under a stock or near-stock hood.

 

here is something to look in too.

http://www.carcraft.com/featuredvehi...d_350ci_chevy/