questions on plenum design and injector placement

[matt4pl]

well first off im building an itb setup, to work with obd2, but i dont have any questions on that as im confident i can figure it out.

on a NA motor, a tapered plenum is best from what i understand, to provide more or less equal flow to all cylinders, so any info or links on how i would go about figuring out the degree of taper and whatnot that would be needed to make a plenum that is functional and actually aids airflow? also what is the ideal displacement of a plenum? i've read it should be roughly equal to engine displacement, but ive also read that the bigger the better?

on the gsxr throttle assembly, i can fit stock KA injectors, problem is they dont have the needle that places the tip inside the runner like the gsxr injectors do. i prolly wont mess with this idea but wondering how placing htem farther away affects performance. i know it gives better atomization of the fuel but on a KA itb setup, im not sure if there would be an adverse effect since the fuel would have a longer distance to travel to the combustion chamber, so the valve would close before all the fuel would enter, but then maybe the ecu would see the lean condition and compensate on the next cycle with more fuel so i'm not 100% sure on this

i also had an idea on a twin plenum setup for itb's but it's kinda wierd.

basically one plenum would have longer runners for torque with itb's on them, for regular driving and the twin plenum under it would have short straight runners meeting with the longer ones at the flange or thereabout. both plenums would have seperate intakes and would meet together and have the MAF and filter at the end. the bottom high-RPM plenum would be close off normally until a certain rpm would be reached at which point the throttle body or w/e mechanism i'd use to block off flow would open and allow air to flow into the high rpm plenum/runners. idealy i'd wire in a WOT switch to energize the circuit and an RPM dependant switch to actaully activate the flap on the high rpm plenum. i believe i can make it fit in a S14 engine bay, possibly relocate a bunch of stuff on the inner fender maybe. its a crazy idea. not sure if it would work and how well cause im still learning this stuff and my main concern is the high pressure pulses in the runners traveling through the wrong runner at high-rpm and WOT. dunno, but lemme know what you guys think

[deviousKA]

Youd better get yourself a BikiRom, considering what your wanting to do it is going to be the only option you have, it would be necessary.

There was a guy named Julian that did exactly this with gsxr throttles and S14 efi hardware. It was ok but had a lot more potential with some better tuning.

The saturation of fuel on the runner walls will take much longer and more fuel with the gsxr placement, some ecus can compensate for this directly but your options wouldnt be quite that broad.

You are thinking right with the improved atomization and overall advantage of having an injector location further away from port (such as gsxr location), but there is inherent drawbacks from having those injectors as primaries. I did exactly this with my sohc ITB setup, and my low load and throttle enrichment was very inefficient as far as the amount of fuel I was dumping to satisfy the engines needs (slow runner saturation). This was only an issue during most normal driving conditions such as steady highway or town driving. Driving the car wasnt even reasonable unless I was doing some WOT fun runs, getting my moneys worth.

Solution? Staged injection, 8 injectors with small primaries very close to port, and second rail different location on the runner (gsxr is excellent). This is an obvious solution from day one, but I would say you should plan for it ahead of time.

Something you might want to look into is using your 4 sequential injector circuits as twin batched sets for 8 smaller injectors. Run one rail at the gsxr location and one at the stock. You would want to use another set of high impedence injectors, ecu should handle it just fine.

When tuning, each of the 4 batches with 2 injectors each would be seen as one injector. It would not be very difficult at all to tune and would work very well.

If your running ITB, even with maf, just get yourself a good size plenum, large enough the shape is hardly relevant. The MAF will take exact care in how much air is entering, you will want a nice large cool chamber before entering the throttles. This is a good opportunity to place some water/alcohol injection.

I dont know about cutting off supply to 2 runners, maybe if you split the runner in two like Tvis, otherwise just run different length runners/stacks.

[ShionS14]

hey devious! i have been looking around to find just the throttle bodies like from twm induction but have had no luck. any suggestions

[deviousKA]

Heres a couple other manufactures of dcoe replacement type throttles I can think of.

JenveyTweakit (EFIhardware iirc)SK sanyoweber (twm)

Those may help with your seach.

[SeanC]

in relation to your longer/shorter runner design take a look at b18 GSR intake manifolds. alot of the honda guys will disconnect the secondary butterfly saying it helps but it actually hinders performance and messes with the horsepower and torque curves in the upper rpms.

[matt4pl]

i do remember seeing a cut-away view of a rxs type-s motor and it had shorter, thinner runners on the inside of the runner curve as secondaries, which is where i got part of my idea but the dual plenum came from audi's older race/rally cars. the short runners would only be active past a certain rpm only at WOT, or maybe 90% or more throttle, just because there would be no throttle body on the secondary plenum. also how would the pressure pulses work on a setup like that?

[ShionS14]

thanks devious, now here is a question for ya, the intake on the head is an oval shape for the injector ok, now if you were to make your own manifold would happen if you just used the oval and left for the plate that mates to the head? i am assuming that there would be a vacuum right behind the plate since the air is coming in from an oval and it immediately opens up to the tear drop. would this be bad or hinder performance? i ask just because making a runner that transitions would be more of a pain in arse. also what size should i use i am thinking a couple jenvey twin bore throttle bodies
Modified by ShionS14 at 5:02 AM 5/10/2006

[deviousKA]

I get that question a lot.

Oval intake manifold ports on a ka24de head is ok, if you are not running injectors anywhere in this area you can do this with a stock port without ill effect.

You could also try a light transition into the teardrop, some like doing that.

All of the "top dog" ka24de NA running custom are using oval port manifold, carbs and efi.

[ShionS14]

thanks that helps, now what about the throttle body sizes? should they be 40mm 42mm 50mm? oh yeah, what about the TPS? anything i need to know about mounting it to those jenvey throttle bodies or just mount it up?

[matt4pl]

im considering the idea of the batch firing injectors, sounds good, thing is at WOT the KA runs off the MAF on a WOT ecu map, so wouldn't it not compensate for the fuel getting dumped by both injetors? also worried about the electronic part of it and how it would work with the ecu, but anywho, it seems like i cant really do any fancy manifold stuff just cause of the limited amount of space in the engine bay. my last question would be about the intake pipe going to the plenum. isn't there a certain amount of space that has to be there between the MAF and plenum? thought i read about 18" or so but a lot of the links i got my info from i cant find anymore, oh well. thanks for the help guys, i appreciate it. devious, u rock, im gonna post up some links to stuff tomorrow too

[ShionS14]

quite honestly you are going to need some sort of tuning equipement to extract the most potential out of this setup. what i would personally do is get the greedy e-manage ultimate, set it up to use a map instead of maf, and scrap the plenum. I only say that because ITBs, again in my opinion, would work better with out having to draw air through a tube

[matt4pl]

i would if i could but i have a 96 and in my area (chicago) there are emissions so they hook up a scanner to the car and check the ecu. can't think of a way to convert to MAP and not throw codes

[ShionS14]

converting to map shouldnt throw any ecu codes, all it does is convert how the air flow is measured which inturn tells how much fuel should be added. as long as everything is in good condition and you tune to allow you to pass the sniffer i think you will be fine.

[matt4pl]

there is no sniffer, its electronic, they hook up an OBD2 scanner type thing to the car and just check the ecu for codes, readiness, and bulb illumination. if it was the sniffer test i'd have an rb25