Peripherl Port intake runner length.
 

Hey people,

I have done a bit of looking about as as always people are nice and guarded about specifics. You may have seen my project in the N/A forum.

Quick out line,
13B N/A Port.
51mm ITB's and 4x RC injectors

Now the question is the intake. I may take this chance to get it more finely tuned as the rotating assembley is going off to be balanced so the block will be apart. ANYWAY:

My Exhaust primaries are 93" long.
Intake length is:
13" to the butterflies in the throttle bodies / 22" to the tip of the trumpets

Can anyone shed some light on wether i should take this oppotunity to make these shorter? As i understand shorter runners more peak power.... BUt does this lead to narrower powerband?

Any info appriciated.

the competition prep manual says the PP intake is 155mm/165mm, or about 6.4" (its not equal length), a weber IDA from the top of the stacks to the base is about 7"

so 13.5" is "stock" for a carbed 12A P port

the later engines use a telescoping intake to get more low end.

i would go dyno a few lengths and see what the engine likes

sounds like a plan to me. i may make them shorter now and the do a couple of longer versions for the dyno to see.

Man, this is really interesting to me. I have been running a 4 port ITB setup since 2007 on my 1/2 BP 4 port 13B. I worked it all out and came to the same 13"/ 7" concept, but chickened out and made them an inch longer as I wanted a bit more low end grunt as the car is running a 4:10 rear and stock box. The rules also said that I had to weigh in at 2650 with driver.

My car pulls hard from 4,500 rpm to 9,500 rpm with no let down. The car is very flexible and can spin the r-comps in second coming out on to the mock grid. With all this discussion, I may have to try a shorter ram and see if I can pick up any top end. Here is a shot from the first day we started the car. It has since been tidied up and a cold air box fabricated. Only issue is varying brake booster vacuum depending on the length of the straight.

Eric

23Racer, where did you get your velocity stacks? They look nice.
You could redyno with no velocity stacks. The whole power curve will be down on power, but it will still be evident where the power peaks. Using straight tube is a cheap way to tune runner length without spending money on various lengths of stacks or on modifying the actual runners. Once you find a length that makes a curve you like, make a velo stack to get back to that length. Just dont get discouraged at first at the loss in power without the stacks. Here is a picture of different stack designs and the flow increase they provide.

I am running Hayabusa throttle bodies, so I went to a place called Orient Performance who build Suzuki Superbikes. They had them for sale online. Work very well and they take trumpet socks okay.

Eric

I have a set of 50mm Stacks that I'm using for a the inside of a Intake manifold for a Turbo P-Port thats in the works.... I got them from a dude that used to dyno tune my old 95 Gixxer 750 for me after I built the motor.

Mikuni Makes them... theyre cheap... Like $75 for a set of 4 AND they make them in All sorts of different lengths... from 15mm to like 85mm in length.

Yeah i think your right a run up on the dyno first will be the best way forward see where lines on the graph go and work from there. I have a feeling i'll end up using 2x 60mm ITB's and then tapering them down into the PPorts at at the block.

We'll see.

I looked at the mazda 787b 4rotor sae paper, measured some things out, and for the 787b the following length worked best (measured from the tip of the trumpets to the rotor)

6000rpm - 585mm (23")
6500rpm - 560mm
7000rpm - 535mm
7500rpm - 485mm
8000rpm - 435mm
8500rpm - 410mm (16,1")

These won't match your car exactly offcourse but should give an indication.