Inliners International Forums General Discussion Blown and Injected Intake Plenum volume?

I am just about to start making an intake for a turbocharged 230 cu.in I6. I am wondering if there is a magic formula for figuring out plenum volume or not. My thoughts are that if it is a boosted motor, it should be less critical then on a naturally aspirated motor,taking into consideration that to small and the possability of running front and rear cylinders lean is present.
Any help would be appreciated, thanks.

Not sure about a magic formula, but 1.5 times runner volume or 50 to 75 % of displacement is a good rule of thumb. Also, add 20% if manual trans.
Z

Generally a plenum volume of approximately 80% of engine capacity for naturally aspirated engines and 150% of engine capacity for turbocharged engines works best.

But I have seen as much as 220% of engine volume used in high hp / high boost turbocharged applications, in that one case a 140% engine volume plenum lost top-end power in comparison to the 220% one, I would not say that the "150% is best" is written in stone..

When I was road racing we did a lot testing on plenums and here are some of my experiences.

The plenum can be tuned for lower rpm torque peak (helmholtz). But the effects can diminish with long duration cams.

Larger plenum volume:
Pushes resonant frequency (and tuning rpm) lower down the rpm range.
Top end power increases due to better cylinder to cylinder distribution and larger reflected wave amplitude.
Throttle response can decrease with larger plenum if no changes are made to engine inertia (with throttle before plenum).

Current N/A and turbo designs use main plenum with what I call an "air knife" feeding into it along the length of the plenum. Air knife can be a divergent flat cone or a tapered cone along the length of the plenum.

Resonant frequency of plenum can be sensitive to plenum shape and plenum excitation can be influenced by runner spacing.

$UM FUN, is that 80% of engine capacity (all the cylinders), or 80% of a given (each) cylinder? Or any of the percentages you mention for that matter.

Thanks.

Yes all cylinders (i.e. 100% for you is 230 cubic inches). I agree with what Sum Fun said, but if your not road racing, 150 % could cause a little lag on an inliner "driver" setup.
Z

The set up I described earlier is for a race application and it should be adjusted for a street application. The traditional way of feeding a plenum on an inline engine has been from one end of the plenum. The big problem in this is that the cylinder close to the throttle body will run lean and the one furthest away will run rich. This can also be seen in the EGT readings if you do them for each cylinder. Another effect is costing hard earned HP

This is why the plenum shape is very important in getting the cylinders to balance out. Take a look at any of the modern plenums that are used in racing from the inline stuff to the Audi diesel race engine. They all have a slot (almost a restrictor to feed the plenum) and they are fed on the top or bottom and are usually curved around with the throttle body behind the head or in front of the head.

In this scenario it is important that the plenum is big enough. This also kills the velocity of the incoming air a bit, resulting in a higher and more equal pressure in the "main" plenum. I've tried making inlets with a shape closer to a flat/wide funnel, so the air would "ram-feed" the runners,i.e. keeping the velocity...but never found any hp in it, when the plenum volume was cut back the "straight-hitting ram-feed" did NOT compensate and the result was a net hp loss.

There are a couple of ways to size the slot. If you've got your intercooler tube and throttle body is the correct size, then you can base it on the intercooler pipe i.d using a chart. Plenums that have a simple slot just cut into the sheet metal will have a Cd of 0.6 Better designs will be higher, up to Cd 0.8.


Or you can get an approximate area in sq-in by using the formula:

Area = (cfm * 144)/(Cd * 4005 * sqrt(deltaP))

Where:
Cd - coefficient of discharge
deltaP - allowable pressure drop inH2O (0.5psi ~ 14")

And you can then check the approximate fps through the slot with:

fps = (cfm * 2.4)/Area

Thanks for the answer, that's what I had heard from most, but I had also heard it as the volume of 1 cyl as well (kind of important to know which ).

Ok I have a silly question here because this Just doesn't sound right to me.( The big problem in this is that the cylinder close to the throttle body will run lean and the one furthest away will run rich.) Now how is it possable for the closest cylinder will run Lean?? and the furthest one run rich???

I would think it would be the other way around. Or is this only a problem when Under pressure?

Lean and rich flip depending on how much boost.

High boost blowing past first runner.



Red lean. Blue rich.

Best way.


Z

To a certin degree, the third drawing above is how my manifold will look. I am running a draw through type turbo off of a Buick Regal, so I don't have an intercooler. I know it hinders performance, but this is to be a mild street car, not running down the drag strip all the time. It may find its way there once in a while, but the car as a whole is not set up for it, so I'm not worried about it.

Thank you for all the help, it is most appreciated.
When I get around to building it, I will try to post some pics of the manifold and my car.

Larry, The scenario I was talking about is high boost. The explanation that Z33 has given is right on the money. In his diagrams you can see what happens at high boost. The third diagram is the optimal. In my case I cant do that, because I have a stock GMC 302 head and the headers get in the way. Instead I have opted for the style of putting a slot on top and feeding the air through it, very similar to the Audi Diesel Lemans engine. I will post pictures of it soon.

Z33 thanks for posting the diagrams, much better explanation that I could have done.

Ok now that i see the photo That were not there at the time I posted my question. So ok I can see and Understand. I kinda figured it had to do with the amount of boost. and Plenum set-up