atten Ray : plenum sizing

[HotStreetVw]

Ray

Since you seem to be the expert on the forum conserning plenum designs I am going to run mine by you and maybe you will be able to give some input.

Im running the cb end castings, similar to stock but with the injectors. Currently I have a t-section consisting of two 1-5/8" pipes merged into a 2.5" pipe that is about three inches long. Very little plenum volume.


What im working on now is a 3' pipe that ties the two endcastings together. The ends are about 1" long of 1-5/8, and the center, about 13.5" is 3"dia. Its designed to fit behind the fan shroud(non doghouse), by swaping the endcastings. I notched the 3" tubing around the fan shroud ring, and on the bottom side because of a case flange.

There will be about 10" of 2.25" pipe to the throttle body. The volume of the 3" and the 2.25" will be close to 2200cc. The 10" length can be adjusted to change the volume as needed. Since the runners are so short, about 1/3 a cylinder volume, how much bigger should the plenum be?

I tried to minimize the distance between the endcasting merge and the plenum to make it close to individual runners. Do you think that will have any effect? There is only about 2" length where the runners are confined to the 1-5/8 tubing.

The main question is how much bigger the plenum volume needs to be because of the short runners?

[raygreenwood]

What size engine is this going on? Bore and stroke? What size is your intake valve? Ray

[HotStreetVw]

1915 94x69

40mm

[raygreenwood]

Let me mo0ck this up in the mind and I will drop some suggestions in tonight. Ray

[MinamiKotaro]

Marking this post because I would like to see the answer to this question, too. I want to add a plenum to my intake and I have a similar engine.

[HotStreetVw]

Well I went ahead and finished the plenum. The total volume is 2300cc, 400cc larger then the current motor, but hopfully that will help resolve any issues with the shorter then ideal runners.

3/8npt Iat fitting, the other side is 1/8npt for the vac signal.

I completed the intake this weekend and also got a wideband(innovative lc-1) and it runs like a champ. I dont know if the intake or the tunning is responsible, but its making atleast 20 more horsepower. Tunning the megasquirt with a wideband is so unbelievable easy. The VE map is so much flatter now, before it was all over the place. I had to remove the fresh air on the drivers side so that the throttle body would fit. I have a 36horse shroud on the way so there wont be any interference. My plans are you build a header similar to turbobob's (turbo under the apron on passenger side) and have room for an intercooler across the top of the engine. Everything under the decklid without hacking up the body.

[lowemp]

http://homepage.mac.com/mplowe/PhotoAlbum5.html

This is a plenum I just put together for a 2615 type 4 Raby is putting together for me. It is roughly 2.3 L volume which includes the stub pipes that cover the runners. Please post any opinions about design. I will rebuild it with properly bent tubing instead of the mitered angles to improve flow if it passes the Raby once over.
Excuse the welds, I'm a TIG novice.
Michael P. Lowe

[raygreenwood]

A couple things to think about:

One of the most important details will be volume.
Short of just using math to say "how big of an intake tract will engine of "x" displacement need?"
There are issues of restriction, vacuum signature, velocity and reversion.
Reversion...will not really be addressed here...because you can't. It is in all of the details, not just the math. It is diameter of runner, valves, cam, TB size, exhaust, porting, etc.

In general, the runner volume should be right at one cylinder volume. yes...it can give or take, but then it must be compensated for.
Unless you "know" your configuration needs either more velocity at the valve, or less....then its a reasonably safe bet to try and keep the intake runner diameter for each cylinder...within 1-2mm of the intake valve head diameter....minus the stem area. So if you have 42mm intake valves with 7mm stems, a 34-36mm ID runner tube should be pretty close. The object is to have no restrictions to the plenum in the intake runner......and no restrictions to the runner at the valve.
Bear in mind.....that a runner volume of one cylinder "lungful"....is pretty commonly used in many successful cars. So on a 1915....478.75cc would be it.
Now....this may cause some problems...trying to keep both the length and the diameter correct while getting the pipe to lead where it needs to.
Generally you have to compromise diameter to get the correct length pipe. Generally...it gets smaller.
In a perfect world.....if you are building your plenum from scratch...make the pleum smaller in cross section (while keeping the same volume)...to "reach out" to your correctly sized runners.

For your plenum volume....in standard street rpm engines...about 2.5 time runner volume is good for the plenum. There has to be some overlap in the available volume of the plenum ...or else the vacuum signature at the TB gets very peaky. It also can restrict the engine and cause you to use too big of a TB to compensate. That makes the system "peaky".
In a very high rpm engine that will be processn gair at a high rate...perhaps 3.0 to 3.5 runner volumes. For yours....3 runner volumes should be fine. That is 1436.25cc for plenum and and 1915cc for the engines 4 cylinders= 3.35 liters of total plenum and runners. 2.3 liters of total intake tract....I think...will be too small....and require a very largeTB to compensate. I think it may make metering difficult.

I also say watch out for the runner diameters. Try not to make them smaller than teh open area of the valve at max lift...because you have a long stroke engine. Depending on what cam you have, this will create high vleocities in a runner system that is too small. Yes....thats a good thing in one respect....but is creates high vacuum...and will also create more violent reversion (if reversion is even an issue on your engine).

Yes all intake systems have reversion, but....contrary to "theoretical belief"....its not always an issue. If the plenum volume and spacing of ports in that plenum are good, reversion waves get cross ventilated into adjacently opening valves on other runners. Cam is important here too

What for a TB? You should be able to get away with something in the 55mm range. If all else is good in sizing, i would rather have a TB that is 1-2mm too small than one that is 1-2mm too big. Over-ventilation of the plenum stalls velocity and makes the throttle praky and creates flat-spots.

Also, be prepared to have several sizes of TB handy to slave on...to see what the tuning is best at. TB...is not just open area. Each one flows a bit differently...and more importantly creates different turbulence in the plenum. Many times, just changing the type/brand of TB even within the same size can make noticable difference in throttle response.

Also...I have not seen any results in from using two small plenums...with a connecting pipe (like a balance tube)...with a centrally mounted TB. That seems to be a way to keep teh runner lengths vs diameter consistant. Don't be afraid to try it. Ray

[HotStreetVw]

I estimate the total volume of the runners and plenum to be in the 3100cc range. It has great throttle response. I can mash on it and away it goes, no stumbling like before. This is with the volvo throttle body(about 55mm).


I am very pleased with the way its running. It makes driving alot more fun.

[KDF Lad]

A couple things to think about:

In general, the runner volume should be right at one cylinder volume. yes...it can give or take, but then it must be compensated for.
Unless you "know" your configuration needs either more velocity at the valve, or less....then its a reasonably safe bet to try and keep the intake runner diameter for each cylinder...within 1-2mm of the intake valve head diameter....minus the stem area. So if you have 42mm intake valves with 7mm stems, a 34-36mm ID runner tube should be pretty close. The object is to have no restrictions to the plenum in the intake runner......and no restrictions to the runner at the valve.
Bear in mind.....that a runner volume of one cylinder "lungful"....is pretty commonly used in many successful cars. So on a 1915....478.75cc would be it.

Forgive my ignorance but I am thinking out aloud here, but when talking of runner diameter wouldn't that be determined more by port diameter in the head than anything else?

If you had a nice set of 044 ultra mag plus heads (aka "oval ports"), wouldn't the runner have to be match ported to the head. This would eliminate the more critical issue of turbulance and restriction, rather than trying to acheive the "within 1-2mm of the intake valve head diameter....minus the stem area" theory?

I'm just wondering if we are prone to getting too wound up over "critical" measurements?

[uberbug]

Now I am getting concerned that my setup isn't "tuned" for my engine.

I am using a 1915 engine with Engle 110 cam with the Mexi FI setup.

What is the symptom of "reversion"?

I do have a rough MAP signal and a whistle with the throttle just barely open. Is this a sign of mis-sized plenum and runners?

[mschilling]

It's probably more the fault of the 110 cam, I've got the same one.

[raygreenwood]

Kdf lad....yes...good observation. The Intake runners should be sized to the intake tract in the head (the port lead in before the valve)....but what so many are missing...is that a restriction is a restriction is a restriction.

I have ranted long about this very subject. I find many people just slapping tubes onto whatevr head they have...with little regard to what happens with sizing. If you have a runer tube that is larger than the port in the head....it will be a restriction.
Now..as there is a good conversation between Len and Jake going on about match porting away ledges on FI.....there may some merit....on some engines...to creating a bit of turbulence close to the valve for mxing purposes of incoming air and injected fuel.
Not match porting though...is a far cry from gross mis-matching of port sizes. If you have a 35mm runner ID and a 31mm port ID.....that WILL cause a restriction and piling up of intake air......unless...your specific eengine is either so anemic in breathing characteristics that evlocity is very low....or the engine displacement is so small that both the head port and runner are grossly oversized...even though they may be mismatched. The latter will cause por velocity and running anyway.

By the same token.....and this is why the intake valve size should be heeded.....if runner to port can be a restriction.....port to valve (in the case of too small) can be just as much of a restriction.

I have noticed looking at hundreds of different engines with FI.....that most manufacturers are careful to keep open cross-section area of intake valve at full lift....(and sometimes even at half lift on very short stroke engines).....very close to the cross section of the port....and hence the port should be very close, if not identical, to the cross section of runner.

The odd compromises come with trying to keep total runner volume...vs runner length....vs cross section to match the port and valve.....in synch to each other.....within a confined space.

This is where manufacturers get creative. Sometimes....especially with lal the cool aluminum castings they make these days....you will see runners that start out either "oval" in cross section....or slightly reduced...in cross section......eventually going back to exact match to port by the time they get to the head. These are excercises in controlling volume vs length vs cross section vs port match shape.

There are even a few runner cross sections out there, that have "keyhole" areas cast into them.....like very small parralell runner tubes that start at the plenum ports and dump back into the main runner at the injector or beginning of the head port.
Hard to say...whether they are used to control velocity, turbulence....or volume. Ray

[Theo]

Now I am getting concerned that my setup isn't "tuned" for my engine.

I am using a 1915 engine with Engle 110 cam with the Mexi FI setup.

What is the symptom of "reversion"?

I do have a rough MAP signal and a whistle with the throttle just barely open. Is this a sign of mis-sized plenum and runners?

Uberbug,

I had the same whistle; it's the ported vacuum line that is right at the throttle plate. I plugged the line (both air control and the ported vacuum lines) and removed the air control valve. It whistled really bad and would not idle well at all. I drilled the throttle plate to allow more air in at idle and it helped a lot. My MAP is fairly steady and it idles great when warm. Still rough when cold. I don't have the f-idle hooked up but if I unplug the line it will idle cold just fine. I think the Mexican system is really well designed. I have no problems rolling from 900 RPM in 3rd, no stutter or bog. My air fuel ratios are steady and reproducible.

Sorry for the hyjack

[Guest]

Thanks for the advice Theo. I am not sure I totally understand which to block but I'll start a discussion in another thread when I have some pics of the system to "point" to what is whistling...

(end hijack)