After found a lot of differences in answers, would somebody with tech. understanding, please (at least try..) to give an answer on : What would give best lowend torque, long runners with carb "far away" from the head.....Or, as close as possible,short runners? What about plenium size? bigger or smaller?
The longer the runner, the more top end (tunnel rams are all top end). Open plenum's are generally better for top end but the smaller the open plenum the more torque they produce. You'll get better low end torque from a 180 degree intake. I performed back to back to back testing on my O/T Stock Eliminator car with the GM Q-jet carb isolators. When I use the four hole, it e.t.'s better. When I use the open one, I lose some e.t., but gain mph. It does pick up on both ends with 2 isolators stacked.
This is actually a very complicated question to answer as many factors need to be considered. I will say that "IN GENERAL" higher rpm, lower torque engines require a shorter intake runner. I am presently building a JR Fuel SBC that runs between 9000 and 10500 and it has a stack injector with extremely short intake runners. According to Bill Jenkins, plenum volume has more to do with '"driveability". Hope that helps.
something to think about is the GM tuned port intakes, in stock form they are long and small, they are all low end torque and die out on top end, if you buy aftermarket larger runners then you get more HP at a higher RPM, study up on those intakes and you will find your answer, also look at a duel plane intake, long runners, single plane have short runners, now with that info look at the rpm range of those intakes on intake manufactures web sites.
My understand is, the longer runners are for small RPM engines ie..Flatheads..Long runners small RPM, short runners high RPM.. Duane.
We dyno test these things a bunch.. and every engine combination is different,, could be header configuration, camshaft specs.. cam timing, cylinder head volume,flow..etc.. but every engine we dyno with a tunnel ram makes insane low end torque. also, think about the long ram chrysler engines, the are a perfect example of a long runner,and they made crazy torque, with very poor top-end rpm power. not saying that you need a tunnel ram for low end torque, just using it as an example.. there ya go, that's my 2 cents.
Yes, long runners for low end torque. Ever seen the specs on the Chrysler 413 with the long ram intake? By long ram, I mean 30 inches from carb to intake valve. Motors that will pull the butthole out of an elephant at 2800 RPM. Theory here: http://www.chrysler300club.com/uniq/allaboutrams/ramtheory.htm Big ports and short runners work for high RPM.
Years ago, Hot Rod mag did a shootout using a mild sbc. All things equal, the Victor Jr made the same torque but more hp as a dual plane. They were surprised that the street tunnel ram made decent torque for the street. The tri-power was down across the board and the crossram had a terrible reversion problem. That being said, I favor the air gap dual plane type manifold for both torque and hp. Strip, I prefer the Victor Jr. Hoodless, anything goes. I like multi carbs for the "look".
So, in a Chevy six inline, intake runners w/carbs stacked on the fender of a 40s car (if that length is what the best calculated ram air is) should give even more lowend grunt..... The question came up when begin to plan a new intake, and found that most of people think about max. hp, but in a street driven car, the torque at lower end should be better. I know its a lot considerations on sizes,etc. Found that dynotests on a Ford six give approx. 10% gain in hp when lowered the carb to the straight top of plenum, rather than sitting atop of the raised carb mount. And fastest style 6 inlines uses sidedraft Webers straight on the head. A lot did know,but answer about the torque got raised too didnt came up..... Thanks for clearing this out! For my car, yet the plenum size is more of interest, since its gonna be supercharged in some month, and here also found some differences. Turbo Injected engines is said to get plenum size as you want, bigger was better, but on a carbed roots supercharger (blower)? I belive size doesnt matter much on the street, its pressurized anyway, and the roots style as a Eaton, dont make much pulsed pressure, and not in a need of a plenum to even them out, or do storage , right? If wonder, engine is a 250 Chevy inline 6 in a -50 Chevy
I owned an early 60's ( can't remember the year) GMC , 4x4 , had the carb down in the hole between the valve covers. Very short intake manifold. V6. As I remember. Tons of truck torgue! But I don't know the tech. stuff.
For torque, long runners and a small plenum. Short runners make for distribution issues at the lower rpms with cylinders that are located side by side and fire consecutively. On the Y-Block Ford, short runners creates some reversion issues at cylinders 1 & 2 but lengthening these runners while also maintaining a tapered design to the runners solves this problem.
Here is a link that has info on the different types of intake manifolds. Interesting read.... http://www.grapeaperacing.com/tech/inductionsystems.pdf
This is the closet to the correct answer. Short exhaust and short intake runners in general produce the best high end torque. Longer runners in general will produce more low end torque. bigger plenums are a little more forgiving but with a bigger plenum you get bigger air requirements, you got to keep a plenum full if you want to maximize its effeiency. That is over simplified, there are a lot of variables to take into account but it is the basic rule of thumb.
as i understand things were talking about how to get the largest volume of air/fuel mixture into the cylinders, in an intake with long, small runners the velocity of this mixture reaches its maximum efficiency at a lower RPM, the mass of air/fuel moving along the runner has enough momentum to keep entering the cylinder even when the valve is beginning to close and the piston is starting to move up, a kind of ram air affect, this same effect does not happen with short large runner intake till higher up in the RPM range, the velocity is not there at lower RPMs, as RPM increase so does the velocity of air/fuel movement and so the ram air effect on filling the cylinders can happen.
I have a Q? That has to do with this same matter? Im building a offroad buggy with a ford 460. It has upright headers with straight thru race mufflers and a short runner manifold? But I put a 7" tall carb/plenum spacer on it. If Im reading this correctly it should increase my lowend torque ? Correct ? Or false ?
Not if it's an open spacer. 7"?! WOW. Look at it this way. Torque is all about engine efficiency...filling the cylinders with as much of the correct air fuel mixture as possible. This requires port velocity to keep the fuel in suspension, which in turn requires long, small diameter runners. Think about sucking soda through a straw. A small straw is much more efficient (requires less "suck") than a larger straw, but won't flow as much Coke...err, soda...as the larger straw. Your 7" spacer isn't adding any effective length to the runners if it is open. Your 460 doesn't need much help making torque... Now prepare to get reamed for the O/T car question.
hey there salt man... the 7" spacer would be plenum volume,and not really runner length, not sure what that would do, never dynoe'd 7" spacer....wow i have tested several engines, with up to 3" spacers,and they either love it, or hate it. some engines respond with 1" spacer. if i had to guess, a 7" spacer would be a turd on any engine.
To help confuse things further, look at some late model Ford factory setups. The 300 six had intake runners that went all the way over the top of the engine to throttle bodies on the opposite side. The three liter Yamaha SHO Taurus engine had four valves per cylinder. It used the long runner intake port up to 4300 RPM, then the short one opened and ran up to the factory setting of 7300. At about 4500, it felt like a JATO bottle had been set off. For a car that big to go that fast with only 181 cubic inches is amazing.
When you start injecting the fuel close to the valve (Taurus SHO), instead of trying to keep the fuel mixed with the air through the whole process, everything changes. With port injection, I suspect the faster you can get the air moving, and the more you direct that air towards the intake valve before you add the fuel, the more effective the process becomes. Throttle body injection is closer to a carb setup, depending on where the throttle body is located compared to the intake valve. Gene
Smaller diameter longer runners will increase the intake charge velocity, which will improve the cylinder fill, which will increase compression. This is why you want smaller carb primary venturi to increase velocity at low RPMs to get better vacuum and better MPG. These guys have a good summary here: http://ftlracing.com/intake.htm
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