Alex-7

A good freind of mine of mine back home in KS is building a four port 13B for his '84 GS out of his existing 12A and one of my old six ports. After hearing his plans on the phone last night I had a couple questions.............



1. will his 12A intake manifold work on a 4-port 13B?


2. He is planing on using a Sterling modded nikki to fuel this beast. Can you guys make a carb that'll flow enough for this motor (it will be street ported)? and if not what carb setup is recomended?




he has already ordered a rebuild kit and a header, I just thought that these were a couple of things that he haden't put enough thought into.......any advice is appreciated, I'll pass it his way.



--Alex

Siraniko

iTrader: (13)

3 things.

1. got to modify a 13-B intake for a 12-A carb
or
2. use a 12-A to 13-B intake manifold adapter

3. send him a PM.

BMS2004

according to my half-assed calculations the sterling modded carb flowed 480 cfm....yes this is way way off of what it really is i'm sure. the way I came up with this number was took mazda's displacement and multiplied it by the highest rpm the engine ever saw (12,000)
so 1146cc= ~.04 cubic feet. then I multiplied by ~12,000 rpm to get 480 cfm. it was likely closer to 12,500 scary. this was on a streetported engine that regularly saw 9-9.5k on the track. it pulled nice all the way up to it so i wouldn't hesitate to use it on a 13b SP personally.

there's a story behind 12,000 rpm. one i'd like to forget.... =)

edit: hope the banana doesn't shoot me for false advertisement.

DAVID GRIMES

Question #2: I believe the answer was Yes if stock ported and No if street ported or more. I specifically asked Carl and Sterling and that was the answer I got.

BMS2004

david when you ask them? we got our carb from them spefically for the 12a SP. maybe theres something different between the normal "sterling" and the one we got. well i know they did some "special" things but for sake of business i won't elaborate. the answer is yes they can build you a carb for a SP.

DAVID GRIMES

A couple of weeks ago, but the question at the start of this thread was can they supply a crb for a 13B SP !!! I'm pretty sure they can for a SP 12A, but that wasn't the question.

BMS2004

touche' I thought you meant you asked them if they could for a 12A sp thats why I jumped on your case sorry man

DAVID GRIMES

np

Alex-7

Thanks for the replies. So the manifold will fit the 13B block? or does he need to get a four port B specific intake manifold? in which case he should probobly probobly just go with a different carb (webber ect.)







Thanks again,

--Alex

Sterling

The Sterling Carb flows 452 CFM, which is enough to feed a 13B. Weather it will perform well on a 13B SP has yet to be seen. It would probably be beat on the track but kick *** on the strip.
-True- it's all about application.

BMS2004

hey i was only 30 cfm off woo hoo not bad for my shotty calculations

Sterling

You can't calculate the flow of a carburetor. It must be measured on a bench.
You can only calculate the estimated volume needed for an engine @ a given RPM & VE.

Blake

Yes, exactly. And, even a restrictive carb can take you to obnoxious RPMs because the CFM requirment is lower than it is at peak HP.

Peak VE (Volumetric Efficiency; how effectively the chambers are filled) is at peak torque. In other words, when you cram more air/fuel in the chamber, you get a bigger 'bang' and thus more torque. VE falls off after that, but generally slower than the advantage created by RPM (smaller 'bangs' but more of them). Peak power is the point where VE falls off at a rate greater than RPM can make up for. Everywhere after that, CFM is lower because the engine can't swallow it...if it could, you'd just make more and more power.

A 12A engine is 1146cc or 69.9ci. CFM at 100% VE can be calculated: (CI x RPM)/1728. If your torque peak is, say, 6K and we presume 100% VE, then the requirement is 242.7 CFM. Now, let's say the peak power is at 8K and VE is now 90%. The CFM would be 291.2 -- more than at peak torque, even though VE is marginally lower. Because it's the power peak, VE must drop off significantly after this point. Let's say VE at 9K has dropped to 75%. Even so, the CFM is still higher than at peak torque: 273 cfm. But, by 12K RPM, the VE would be in the basement and even with those RPM, the CFM would be very low. Imagine 50% VE at 12K. That would be back to 242.7 cfm...same as at peak torque. So a carb that is too small for a 12A at peak power is still big enough to flow the CFM requirement at 12K (presuming an optimistic 50% VE) without being "restrictive" at that point.

I hope that make sense.