Does exhaust affect low end torque?

What about your car? You bypassed that ?

 

"Bolt together" ? So you whittle your parts out of unicorn bones and sprinkle fairy dust on it?

 

Yawn.

Keep trying.

You've already made yourself look like a fool, I need not point anything else out.

You can try to redeem yourself though by addressing the points instead of trying to play your grade school playground games.

 

alrighty now this is like a carb showdown the bigger the motor the more fuel it will feed , so does anybody understand that most Rochester carbs cfm rating was either 750 or 850 its all in the motor capability to pull the vacuum Is'\nt it??

 

Back to exhaust flow...
I wonder what quad pea-shooter 1.250" pipes would do?
2 sides of double barrel fun!

Mandrel bent of course and no pressure wave termination box needed.
The velocities exiting the pipes will be so great it will fold the throttle blades right over!
(thanks to whomever I stole that concept from!)

 

this made me grab one of my old carb books to see on the different size qjets. So there were 750s and 800s. Its interesting the rest of the paragraphs are in agreement with what has been said here. Doug Roe, Rochester Carburetors by HP books, 1981. page 124

 

what would the actual effect be on low rpm torque and 3 to 4 second full throttle acceleration from a dead stop?

 

You know I was joking, right?
I'll use the sarcasm font next time :moonu:

I'd be happy to set up a back to back comparison, with road testing and dyno verification at various throttle angles, all at your expense of course.

Kidding aside, everyone has a good point here in that for drive-ability, max cfm's on the induction side isn't really the focus.
Too big with either induction or exhaust can hurt things.
At part throttle, engines use restrictions rather nicely to keep velocities up and improve vaporization.
I think that's the point Gary is illustrating.

Anyone skilled can make either approach work with intended results.

 

Get a 750 double pump Holley and lock out the back two and you will now have your 2 bbl and larger than the Q-Jet on the primaries. :eek2:

 

I'll try to iron this mess out.

Two cores were made for the Rochester Quadrajet, the main difference between the two was the primary venturi. The 800 was a bit larger than the 750. The rear holes were the same on both in terms of CFM potential (this term is important to remember). Primary CFM flow (at 1.5") was 178 for the 750, and 228 for the 800. The Qjet got the rest of its potential CFM from the secondaries, which fed the engine based on air/fuel demand, which wasn't the full CFM potential the carbs were rated at unless air volume was increased, either by higher RPMs on a smaller engine, or from simply adding more CID (big blocks).

What does this mean to a stock Buick 350? It means simply that the engine will not use the full potential of the carb rating size for low-mid range operation, which is one of the reasons the torque band is raised in RPMs vs a 2 barrel, because it has to rev in order to get that CFM for more power.

Not exactly conducive for a tire-burning torquer at lower RPMs with a highway gear and heavier car, particularly with lower compression. Can the Qjet accomplish a burnout? Absolutely! ...but the 2 barrel will do it much easier. It can also be the difference between a burnout and no burnout.

Remember also that the Quadrajet was used for ALL GM vehicles and engines (many as a more expensive option, coupled with a higher numerical gear ratio (and higher compression in the earlier years) to compensate for reasons explained above). It is an extremely versatile carburetor, because the secondaries can give very little CFM above what the primaries flow (or maximum CFM), making it suitable for engines ranging from the 252 (4.1 litre) v6 all the way to Cadillac's massive 500.

It is a great carb for cruising and gas mileage over many other carbs, due to the efficient atomization the primaries give, even when opened up further. Reasons for this include the fact that they're so small, atomization would be virtually effortless, particularly on larger CID engines.

Alternatively, larger bore primaries (or 2 barrels) require less opening for similar CFM flow, permitting efficient atomization at those throttle positions for cruising. (there's debate on which is more efficient, but for the purposes of this post, I won't go into too much detail here).

The end result is ultimately how much throttle is required to keep the car moving under any particular load (uphill, towing, acceleration, etc.), which the Qjet tends to need more because of the smaller size primaries (especially on larger CID engines).

This is where the 'partial throttle performance' is increased with other larger primaries/2 barrels vs the Qjet, and is where you will spend the vast majority of your driving time.

Getting back to your combination, with taller gearing, heavier weight, and lower compression, the Qjet is at a disadvantage at RPMs lower than 3000, and has no real advantage from 3000-4500 RPM over a larger 2 barrel carb. Beyond 4500 RPM, the stock cam doesn't have much breath left for the Qjet to really outshine it (on a 350 CID engine) --UNLESS you open it up more on the intake and exhaust side with some head porting and headers, then the Qjet will give significantly more, but only at higher RPMs (and does not apply to your combination or goals). This is a large 2 barrel vs the Qjet, not a small 2 barrel vs the Qjet. Anyone who says different is ignorant, I don't care how many engines were built or cars owned. Some people will just never get it, and even if they do, will refuse to admit they were wrong.

Anyone who believes they know it all or have all the answers is short-changing themselves. With a closed mind, there's no room for learning and expanding their knowledge. I feel sorry for them. You can lead a knuckle-dragger to knowledge, but you cannot make him think.

In your situation, I firmly believe (based on evidence and science) that a larger 2 barrel and a high velocity exhaust that enhances low RPM scavenging will be your answer to improving your low-mid range torque.

You can get a large base 2 barrel intake for free, since no one thinks it can do anything other than bring a hefty scrap value. Videos posted earlier in this thread show quite the contrary.

Rochester 2g carbs are dirt cheap, and you can get a reputable builder to mod one to flow 500-600 CFM, or you can opt for the Holley 500 CFM 2 barrel. You can keep your existing dual exhaust, just splice in an "X" pipe or the "Y" and dual out splice. Make sure your head pipes are 2" or 2 1/8" and mandrel bent up to the merger pipe. Coat your exhaust manifolds inside and out, as well as the head pipes once welded together.

Ignore the pea shooter exhaust comments. It was intended for humor and a jab at me. We'll see who laughs last.

It'll cost you more money, but not too terribly much vs what you've already spent.

OR if you're mostly satisfied with what you have now, you can try to work with it by putting a better gear ratio under the back, which will help the Qjet get the RPMs it needs (which will be more expensive than the aforementioned induction/exhaust mods, unless you can luck out and find some used stuff).

So there it is.


Gary

 

Quite a difference with a large 2 barrel vs the Qjet. The Qjet won't even be at its full torque yet, while the 2 barrel is already there (or even past it), with a much wider band. The torque band is moved further down and is spread out wider.

This comment is aimed at the 2 vs 4, not the ridiculous exhaust comments.

A 500 CFM 2 barrel carb shows peak torque to be flat between 2400-2800 RPM (literally flat--same numbers between these RPMs), with not much less than this from 1500-4000 RPM. The Qjet doesn't turn on until 3000 RPM, with a higher peak on torque, meaning unless you're at this RPM or higher, the Qjet will be a dog vs the 2 barrel.

This is why even GM put higher numerical gearing under cars equipped with Qjets.


Gary

 

Gary, my contribution to the pea shooter comment was for humor only.
If there was any personal attack in the original comment, I ignored that.
I did think it was funny though. :beer

You have a compelling technical scenario that seems to fit under the paradigm of a stock Buick 350, along with the appropriate vehicle equipped.
I trust your personal observations and cannot comment to any technical background you might have with developing either stock or improved over stock applications.
Therefore I trust you are speaking with integrity and personal experience, as always.
I'll also agree that in the vast majority of cases where people with less than expert tuning skills begin swapping parts onto a stock engine, that things are rarely improved upon.
I think people's comments that show different opinions than yours are coming from the mindset that the better than average tuner CAN exceed stock performance, mileage, longevity, reliability, etc.
I also trust my own as well as others' observations and experiences to come from integrity.

A complete paradigm shift breaks rules every time. (I know, that isn't the topic of this post).
Some people stay close to stock. Others run far away from it.
Some stay close to stock for budget reasons, fear of getting too complicated, the myth of longevity being shortened, restoration reasons, etc.
The vast majority ask for more power though, despite budget.
I think most are also offering technical based comments based on what they perceive that the OP's want, even between the lines of the text.
It's hard for the newbie to ask for stock performance when there aren't such marketing pressures to sell OEM parts, backed up by magazine articles, dyno tests highlighting the improvements of stock parts over aftermarket (followed by an ad on page xx), dyno or engine building contests supporting the same mantra of stock.....ad infinitum.
There's no $ chain there. Few new jobs created.

I always respect and appreciate those that do a lot with a little, whether stock or not.
I also dislike seeing when people get so misdirected and misinformed.
There's more than one way to skin the cat. It's just how you choose to do it.
There's plenty of low hanging fruit regarding stock vehicles.
Changing one or two items without the ability to steer things definitely sets a person back away's.

 

some have also suggested a 1" spacer between carb and manifold to increase velocity and thus low end torque? Do you agree?

also in the case of the 400 chevy sb, 2bbl, the rating was 265hp and 400hp but that was measured at the crank, did they measure this in the car with exhaust or on a stand in the lab with what exhaust?

 

I know you are asking Gary, but 1" spacer would reduce the venturi signal and lower torque.
It would reduce velocity in most cases.
It is sometimes used as a measure to help an engine breathe when it's carb restricted near peak hp, or to add plenum area when needed.
In some cases it helps ease the direction change into the intake when they have a "smashed" design to fit under low hoodlines.

[editing post to add that I ASSumed an open spacer was queried. The open spacer allows the signal to see both sides of the carb, reducing some of the pull through the boosters. A 4 holer would maintain the isolation better (ignore the base gasket space for the moment) by keeping only 1/2 the throttle bores "seen" at a time, just like a dual plane already does]

 

thank you and everyone who is taking the time to really get into this subject which has been bothering me for a while. All the magazine talk about is hi loud reving engines, loping idles and racing stuff. I just want my car to be hard launching, thrilling 3 or 4 second stop light runs, and just generally fun to drive, decent gas mileage etc. I appreciate everyone taking time to comment.

 

For a second there, I thought I was back at the Pontiac boards.

 

why what happened over there?

 

Nothing too dramatic. One person who posts here on occasion posts over their quite frequently, very knowledgeable and very helpful. A relative newcomer(to the board, he had been building engines for some time) kept on "throwing shade" saying things could be done a different way. Thing was, he had built all of two or three Pontiac engines and didn't have any dyno sheets, etc. to back up his statements. He was helpful at points but overly confrontational on other points. He had a few adherents/others who agreed, but again no one had any documentation to back up the points being made. Eventually, he was suspended.

I don't think he was wrong, just needed a little proof to go with what he said. But it did create quite the drama over there for a week or two.