Technical > Matt Shaff's Engine Shop

Short or tall stacks

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gasserx:
I finally got my engine this week! Its a SBC with Hilborn injection (alky), and shorty stacks.
Short stacks will probably work good also, but i would like the taller ones for a little more nostalgia look.
I scored some 12 metres of 2.5" alu tubeing for almost free, and was thinking of making another set of taller stacks for it. What would be a good length to try?

Frontenginedragsters:
 Stack length is another tuning parameter that is different for your engine than mine.
A lot of things affect the RPM range that a engine operates in for maximum torque.
The short story is long stacks make torque in the lower RPM ranges and short stacks let the engine
make peak HP at a higher RPM range.
 Check out Pipe Max software. I do not own it but a engineer friend of mine does a lot with it.
I know it has formula's for header tubing and I think it also works with intake length.

Matt

JrFuel Hayden:
Matt is right, long stacks = lower RPM power, and short stacks = higher RPM power.
So if your cam/ motor combo is higher power at high RPM you'll run better with short stacks.
It's no secret that SBC's will make more power the higher you rev it, but in order to run higher RPM's you need a better lower-end and better valve train. So your questions should be what RPM should I shift at and what RPM thru the lights.
Now my guess is you don't have a 10,000 RPM Chevy, because if the tubing you just bought is 2.5" ID, then your Hilborns are no bigger the 2.5" . Most of the 10,000 RPM Jr fuel engines are using 2 7/8" injectors unless they are racing the 14° heads and are JrFuel"A" class that the rules allow 3 1/8" injectors, where a couple of teams are running 10,500 to 11,000.
My JrFuel "B" SBC 9700 to 9800 RPM is running as short a stack as we can get to fit on the injectors.
One of the JrFuel "C" teams running at around 9,000 RPM is running 4" or 5" stacks, on 2 3/4" Hilborn's. You could call Don Enriquez at Hilborn, that's what he runs.
So you need to find-out what RPM your engine is making the best power, and set-up your stacks, torque converter,shift points, and rear gears/ tires size to LET your engine run in the best RPM range. You'll have to dyno your engine if you can't find out what you need from your cam guy or engine builder.
A tuning tip about stacks is if it turns out you want 8" stacks, and you are racing a track that has a crappy start line, put your short stacks on because they will kill some of your torque/ bottom end power so you can get off the line. 
Here is a picture of my 2 7/8" Hilborns, but now I run angled Kinslers with sort stacks, I can post pictures of them too, but I figure you'd like to see my Hilborn's.

Good Luck with your combo
Jon

gasserx:
Thanks for your replies guys. I dont know what type of cam the motor has yet, but im about to disassemble it these days so hope to find out.
I can say for sure its not going to shift at 10.000 RPM  ;D

Those are real short stacks Jon! Thanks for helping. I`ll see if i can manage to do a dyno on the engine before spring.

dusterdave173:
STOP!!! That engine ran some low 5.20's like it is--it does not need taller stacks--now for looks...maybe you prefer taller--I can go with that but.... don't second guess a Well Tuned combo you just shelled out for--That is one heck of a SBC engine for what you are doing--get it running--drive some THEN screw around with it if you want but you need a well tuned ready to go combo--messing with success is not the way to start out--Learn the car, learn to drive then mess with it otherwise you will spend all your time fixing things that were AOK and not get to run as much. Just my two cents worth

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