Well he didnt say Q jest specifically. What he basically says that dual plane intakes ony have access to 1/2 of the potential Cfm of the carburator, so the carb only has access to the vacuum of 4 cylinders, so low cfm carbs tend to choke even quicker than they chouls with low dual planes. I have seen alot of enthusiasts end up not pulling the ETs that they would like with the typical Edelbrock Performer intakes, rebuilt Qjets, Cam, exhaustwork and somtimes heads. I have faith in the q jet to pull up to 500 hp of cfm without choking, but we never seem to see that, or I never do. I'll give some thought to any concept that seems to have merit and research behind it, and this guy seems to know hit stuff. It was in PHR btw, if you think you may have read somethign about it.

What are peoples thoughts on this?

 

better for what?

 

Better on top end AND torque, more ETs, throttle response, no choking ect... better for the Q jet in general.

 

Probably not.

The Q-jet was carefully designed for operation on dual-plane intakes. I don't think that whatever your engine builder said, can be applied to Q-Jets.

It is however true that air flow through a carb is not a smooth, uniform, continuous flow, but rather a series of pulses. The smaller the plenum of an intake is, the stronger these pulses are, since there's less damping effect from a volume of air for the motor to draw from. Therefore, in general, any given engine will require a larger carb tahn the CFM formula would tend to indicate, if used on a small-plenum intake, becaus the carb must be sized in accordance with the flow that exists at the peak of the pulses, not the average flow. This is why, for example, even though a 350 engine should only flow about 625 CFM at 6000 RPM, it will still run quite a bit better with a 750 CFM carb, when used on a dual-plane intake such as a Performer RPM for example, even though the usual narrow interpretation of the "formula" indicates that the larger carb is unnecessary.

The Q-Jet comes in 2 variants. The small one (about 99.99% of all Q-Jets in existence) flows around 750 CFM, except for the ones of that model that have the little stopper tang on their air valve (easily ground off). The other model, which was only used on a few models of early 70s 455s and similar large motors, has larger primary throttle bores that give it a flow capacity of around 850 CFM.

I think there's a whole bunch of confusion and misinformation and over-generalization in this post. Little or none of the subject matter at hand actually applies to the Q-Jet.

 

The difference between a dual and single plane intake is basically the same as the difference between a long and short runner EFI intake. The dual plane and long runner intakes, both have more distance for the fuel/air to travel before it enters the head. This also gives a volume of fuel/air in the intake, waiting to be sucked into the engine. This gives good low-end torque, and throttle response. However, at higher RPM, this long distance becomes a restriction. This is where you would want a single plane (or tunnel ram), or a short runner EFI intake. This will give you less low end torque, because there is not a 'charge' of fuel/air in the intake waiting to be shot into the cylinders, but there is little resitriction/short distance from the carb/tb to the head. This allows for air to flow through the intake as fast as possible, which is what you want for high rpm. There is really no right or wrong intake for a Q-Jet, or for any carb for that matter. It depends on the application. If you're car is street driven, you wont really need to rev to 8500 rpm, so you'd be better off going for a setup that will produce low end torque. However, if you're building your car for racing, you're going to want to rev, which is where a single plane would come in handy.

 

a dual plane will generally give you more throttle responce and better low end being that that vacuume signal is a lot stronger to the carb and the plenum isn't slowing the air down too much going into the runners

problem is as the rpms build up with that small plenum the air starts to send a vac signal that is too strong to the carb and that is where the single plane kicks in

it kinda dampens that signal out a little bit

something like that
don't hold me to that
just what I heard though



and there is more but fuzzy brain is killing me

 

Thats a pretty good simple of dual vs. single plane. Basically, the idea in the first post about "the carb only gets vacuum from half the cylinders" is a skewed idea. It's really half the carb gets half the cylinders, which gives you a much better signal at low RPMs.

Like RB said, air doesn't flow into an engine constantly, it moves in pulses/waves. At low RPMs those pulses are very distinct from each other. With a single plane, each pulse gets divided by the 4 barrels of the carb, so each barrel only gets 1/4 the signal = weak. With a dual plane, that same pulse (actualy stronger due to smaller plenum as well) only hits 2 barrels so they get a much better signal. So while a single plane has more potential up top, a dual plane should always give better low end into midrange and streetability.

As for why cars don't run to their potential, i'd say that probably has more to do with a lot of people's inability to match a combo, assemble it properly and tune than anything else. When you see a guy with a lumpy cammed 350 running 14s, his problem IS NOT that he needs a dual plane intake. Much more likely he slapped it together in a field one night and hasn't turned a screw on it since. If he swapped to a single plane, odds are the car would slow down because there is no way somoene can screw up tuning a dual plane and then nail the tune on the single plane. Getting the single plane to have street manners is a little more involved than the plug and play capability of a Qjet/dual plane.

 

A smaller single plane is fine for street use, I use them all the time but the higher you go the more problems you will face. A weiand X-celerator (#7546) would be the most ideal single plane for street use, in fact I'm changing back over to one in the next couple days. Ive noticed no loss of bottem end grunt with it and it flows alot free'er in mid to upper RPM range. It's actually recomended for 1500-6500 rpm the same as most performance dual planes. Of course I only have standard transmissions, it helps. A q-jet wont bolt up to one of these intakes unless you dont want the secondarys to crack open more then 1/8 of an inch you might just want to stick with a spread bore RPM.

 

Okay... maybe I did overgeneralize. But I have the article right in front of me now.

He goes on talking about how the runner pulses on the Edelbrock Performer are far superior than anything before (a little bit of brand pushing) and that these stronger pulses require more cfm as RB and Ed said, and these pressure crests in the wave pulsed for the cylinders is flattened out and not at it's peak efficiency. Then he talks about pulling better nubers from a Performer Rpm than a Victor using a 1020 Dominator.

Obviously it is impossible for us to run 1020 Dominators and still stay emmisions (and computer) compliant without a ton of tuning and work, and I would think a vacuum pump. So he wasnt saying that the single is better than the duel, he was just saying that the single makes more use of the carbs Cfm and cause most of us need or want to stick with the Q jets that it would make thebest use of the 750 cfm we have to work with.

The engine builder is David Vizard, and the article is Mine Your Own Business in the June 03 issue of Popular Hot Rodding.

I am still very interested and the thoughts and ideas expressed here and would like to see more.

 

EDIT: Srry... had something posted but was getting to side tracked on air pulses and the speed of sound.

Ignore this one.

 

Was looking up the Weiland Accelorator...



Is the divider plate to divide the driver and passenger sides of the intake? I see the slits on the plenum, is that what their for?

EDIT: O sweet... look at this puppy

http://www.holley.com/HiOctn/ProdLin...AMSM/7525.html



---

Sorry for all the junk here, I dont want to deture from the topic at hand here which is single vs. dual planes. My apologies if I got side tracked.