Intake manifold and single throttle body

[gtrkrauss]

So im thinking of building a Single throttle body intake manifold and have some quick questions.

Ive been doing some research on flow testing and CFM testing of some intake mani's on the market right now and am noticing some people saying they run lean on cylinder 6. I heard that was even a problem on our RB's stock but thats not my question. Ive looked at flow charts and noticed number 6 gets less air than the other cylinders because of distance and shape of the mani. Wouldnt that make it run rich? If you had individual injector trim you could tune this out couldnt you?

I have also heard the crap about short runners compared to long runners... I dont get turbo till 4800 rpm im not worried about the amount of bottom end im going to loose...

Who else runs a single throttle body? 80mm? 90mm?

Seems to me if built properly this would be a much better alternative to the ITB's. Perhaps pick up some top end power and some looks.....

Here is a picture of some velocity and pressure flow testing of a single throttle body intake mani. Just your basic design not ported or matched.

[caliber676]

Air is measured by the mafs or map sensor and assumes that the air distribution between cylinders is equal. I believe the problem with the stock plenum is too much air being forced to the 6th cylinder, which would cause a lean condition.
Individual cylinder fuel trim can be done to correct this but you will need a wide band and a competent tuner/ecu that can give cylinder recognition as to which one is lean and how much fuel to add instead of just dumping a little extra fuel because you hear #6 goes lean.

I'm going with a short runner plenum so I can keep the fuel system the same and not have to worry if a/m rails injectors will fit the custom injection ports. Throttle body wise, I'll take 90mm over the 80 because that just full race.

[Fred-R]

i don't think that the distance of the plenum to cyl. 6 is the problem, it's the shape of the mani on cyl. 6 that forces more air into it....

[gtrkrauss]

i don't think that the distance of the plenum to cyl. 6 is the problem, it's the shape of the mani on cyl. 6 that forces more air into it....

Number 6 is getting less flow. Why would that cause a lean issue. I can see it getting less air so running richer causing a loss in power.. Im not sure how you would ever compensate for that with the design of an inline 6.

[Fred-R]

6 is getting more air! so leaner...

[Fred-R]

go get this thing! period

http://forums.gtrcanada.com/parts-sale/59300-rb26-rips-intake-plenum-rips-custom-fuel-rail-**new**.html

[Skym]

The entry of stock GTR intake plenum is aimed at end of intake plenum, so naturally air is going to head towards runners / cylinders no 5, 6 that makes them run leaner. You can go to a cylinder head shop and get them to flowbench the intake manifold. I remember someone saying Nismo intake plenum has spots for extra injectors under intake plenum on 5th and 6th and around 5th and 6th it's shaped differently, but never looked at one in real life to confirm.

You can use individual cylinder fuel, ignition trim to adjust fueling, ignition timing to suit, but need wideband on each exhaust runner to suit (as done on high hp EVO's, etc). Most just put the highest flowing injector (cc) into 6th, next highest flowing injector into 5th, etc to help in fixing this problem, as stock ECU has no individual cylinder fuel, ignition trim. From what I can remember, with PowerFC you can tell ECU cc of each injector.

With the intake plenum design's you see above, the throttlebody / entry is aimed at the middle runners, so distributes air more evenly to all runners (I think 2-3 are slightly higher with CFM) and the shape around 5th, 6th is better.

If want a front facing intake plenum, go for the JUN intake plenum + throttlebody to suit. Many tuners copy that design. The intake plenum volume is usually changed to suit bigger turbo that pushes alot more air into engine and have to have head modified to match to make use of that extra air. But the stock throttlebodies, intake plenum is good for just over 1000hp with head to suit, so I don't see the point in changing them if you don't need to.

This R34GTR has the JUN intake plenum -

http://www.youtube.com/watch?v=IXtskdNE61k

Note the neat side throw exhaust at the back which gives a extra +10kw.

[NismoS-tune]

Nismo intake plenum fixes the #6 issue, balances all 6 better and is good for a little more power. No need to reinvent the wheel from your back yard. Here's how I see it. Tomei for example, is a top tier GTR parts manufacturer and even they did NOT design their own. Not to dissolve what little hope you have but I'd run a nice aftermarket one and call it a day!

The Jun plenum is one of the nicer ones as skym says. The longer runners helps for more bottom end I think I heard?

IMO, This engine and design has been around for 24 years. There is a darn good chance nothing the little people like us can do to make this better if the high end companies can't figure out something better. Again, no offence.

[gtrkrauss]

ohh none taken. I wasnt going to 100 percent design my own. I wanted to build my own taking in other designs and combining them. I just wanted some input on maybe some stuff other people are running and for say street driveability with a single throttle body. I know running too big of a throttle body can cause almost an on-off type of throttle. For example the RIPS plenum is almost designed like a Jun. But rips is using a 100mm throttle body, personally I think that is way too big. Im not sure what it drives like with a 100 but there is 1000hp cars running 90mm bodies.. Any more input is appreciated guys im taking all this stuff into consideration.

[Dragon Humper]

The only issue with a large TB is a more harsh tip-in. In an automatic that could be annoying, in a standard I can't see it being an issue. You can soften that a little with a variable rate throttle cable cam, large radius where the cable attaches tapering to a small radius at about the 25% open mark.

As far as runner length, low end shmo end, RB26s have no low end (ie: below 2500) and cannot make any low end so why bother trying to prop up an area in the curve that is pathetically low compared to mid range and top end. Everything below 4000 is moot in an engine like this, especially with a turbo setup capable of 700+HP.

I'm not saying you can't make good torque at 3000rpm, many have, but......NONE of the builds I have seen run a single TB, they run ity-bitty turbos and are absolutely maxed out at 600 HP creating a ton of scorching heat at that level.





Jon.