engine and drivetrain Velocity stack interchange

vinntagejoe · June 11, 2016

This is probably a simple Q, but I'll ask anyway: can I run two sizes of velocity stacks on my dual 40's? I wasnt to run two "normal sized" 2" stacks in the rear carb and longer 4" stacks in the front. The reason for the difference is that I want to use unifilters and by using longer stacks in the front, I can access the oil stick without rubbing on the filters. Make sense? Will this be a problem?

M3M3 · June 11, 2016

No. Stack length changes the carb flow and tune. All should be the same.

TobyB · June 12, 2016

Yes. You can.

Stack length changes the carb flow and tune.

So you'll have a different torque bump from intake resonance- you might want

to tune 2 of your exhaust runners to compensate. And you could even have a cam ground

with different profiles to match your differently tuned intake and exhaust.

Then you get 2 cylinders optimized for torque, and 2 optimized for horsepower. Or something like that.

Full- race engines have tried it in an effort to widen the power band when gear ratios are limited/restricted...

Practically speaking, though, you may have jetting imbalances if you mix your trumpets.

Like Marshall, I wouldn't do it either, unless I was just horsing around.

t

tisalover · June 12, 2016

The velocity stack shapes the air induction charge. Classically longer stacks help with mid range, and shorter stacks are utilized with higher revving race motors. You do not want to have different airflow , Which will affect the fuel air mixture and Jets, in the anteriort and posterior halves of your motor. Tuning would be quite Bizzarre.

vinntagejoe · June 12, 2016

alright then, it's settled - thanks for the insight and quick replies.

avoirdupois · June 12, 2016

21 hours ago, vinntagejoe said:

This is probably a simple Q, but I'll ask anyway: can I run two sizes of velocity stacks on my dual 40's? I wasnt to run two "normal sized" 2" stacks in the rear carb and longer 4" stacks in the front. The reason for the difference is that I want to use unifilters and by using longer stacks in the front, I can access the oil stick without rubbing on the filters. Make sense? Will this be a problem?

I take a slightly different view regarding your mix and match question. Yes, I agree that all things being equal, symmetry is desirable so that each cylinder should receive the same intake and exhaust treatment as every other cylinder. That includes the same carb air horns, intake runner lengths, intake plenum diameters, jetting, valve sizes, etc. This is why, in virtually all side draft applications, the air horns are uniform. However, pure theory is not always achievable, or at least compromises occasionally creep into the mix.

On a test stand, all cylinders seem to be treated equally. In an engine bay, equal treatment can take a back seat to reality. Often a header pipe will take an unintentional bend to clear steering or some other equally important structural impediment. The same thing occurs when non-stock intake manifolds, carburetors and air filters are fitted to otherwise original street cars. M10 and M30 applications are notorious for minimal space because of brake servo placement. This is the major reason the smaller tii booster seems so popular with the side draft crowd. On occasion, the lack of space causes some mechanics to become creative with placement of side draft air horns and filters. It is hard to say with certainty that such creativity is always rewarded with improved results.

Often despite being identical in appearance, air filters and/or air socks placed over air horns may not all function the same, depending upon where and how they are crammed in the engine bay. Just like the weather, not all of the areas under the hood receive the same air pressure and air temperature. This affects air flow to each air horn. Arguably, those air horns closest to the radiator may receive air at a different temperature than those farther removed from the heated air. This is one reason why intake air is often ducted away from radiators and exhaust manifolds and many with the original downdraft air filter housing love those snorkels (or think they do). There is also the so-called ram-air effect that may come in to play at higher vehicle speeds. More to the point, contorted filters filter the incoming air but, depending upon placement, may be more restrictive than their identical twin sitting in front of other carburetors and defeat some of the reputed benefits of such stacks. So, even though the science seems unsupportive of using different length trumpets, if you are running filters, some of which may likely be more restrictive than others, why not give the different size trumpets a try? The worst that happens is an engine with less-than-optimal performance, that can easily be remedied by swapping out two horns. Who is to say you wouldn't notice any difference in engine performance - plus you get to use the dip stick the way YOU want to use it!

side draft clearance