intake plenum volume

[MadBill]

MY general "rule of thumb" is 1.5 cu in of plenum vs cu in of engine. That's a good starting point anyway.

What does carb cfm have to do with plenum area?

I'm not sure why people keep claiming a particular size carb is needed for a particular application. It's very hard to find a bolt on carb that will work.

The key is fuel curve. I'm not trying to sell Troy's carbs but I've also found more overall and average hp using as big a carb as possible by fixing the fuel curve. Tuning a carburetor can be just as intense as efi if you have the ability to change the circuits.

Sorry, I messed that up. I meant 1.5 square area of plenum per cu in of engine displacement, but again, it's just my opinion through a minimal amount of testing.

You were right the first time; plenums are a volume measure, not area.

[Dewey]

The way I understand it is runner length effects engine operating range and where torque is optimized. Like the Chevy TPI set up, the Dodge Magnum kegger intake is made for max torque in low to mid rpm ranges and sacrifices top end power. I think the large plenum is good for throttle response. More learned members here can clarify. I doubt the volume is causing any problems and is most likely helping in a stock truck based FI application. Carbed engines need correctly sized plenums. This isn't a concern for EFI which introduces fuel near the intake valve.


The 360 with a kegger is factory rated for 100 lb ft more torque than the 318, which comes in around 325 lb ft or so.
If this is a basically stock truck application, try some Summit Shorty headers, get a new, or port your own throttle body out to 52MM straight through eliminating the factory 48 to 52 MM hourglass shape. If you can find one, install a Mopar Performance PCM OBDI to '96, or get a tuner, OBD II 96 up. I removed my center divider, don't think it did much, also recently installed a Comp shelf cam designed for these engines (206-210 @.50, 112 LC) and a set of 3.90 gears. Tuning is very limited for OBD I systems.


Search Dakota-durango.com or dodgetrucks.org

You are correct about these fourms there is a little info there. I have not seen any one explain or seem to have an undersdanding of how runners work, not saying they do not know they just dont show it. It just look like they are shorting the runners to turn more rpms. some are cutting the runners down 2 inches and I do not want to raise the rpms in my truck.

The volume may not be a problem but i would like to understand it. It may be why the engine is responsive, and gives the fealing as if a clutch or converter is not working properley or slipping.

runner harmonic pulse stock intake runner is 14.5 and the head runner is 5.5 long so total runner is 20.0 long. I shotened my runner to .5in so total runner is 19.5 i did so to improve the runners curtain area.

----- 3rd pulse is ------ 97000/rpm ------- to find a lenght you want to use.
--------- or------ 97000/lendth-- (20in)---- to find what is in use. so this happens at 4850 rpm

----- 4th pulse is ----- 74000/rpm ----- to find a length you want to use
---------- or----- 74000/ length-- ( 20in) ----to find what is in use. so this happens at 3700 rpm

so if you shorten the runner 2 inches you will raise the running rpms by about 400 rpm and torque curve up 400 rpm also.
I do not have all the specs on this engine to plug this it to PipeMax and get a better piture.

thanks Dewey

[MagnumTPI]

The way I understand it is runner length effects engine operating range and where torque is optimized. Like the Chevy TPI set up, the Dodge Magnum kegger intake is made for max torque in low to mid rpm ranges and sacrifices top end power. I think the large plenum is good for throttle response. More learned members here can clarify. I doubt the volume is causing any problems and is most likely helping in a stock truck based FI application. Carbed engines need correctly sized plenums. This isn't a concern for EFI which introduces fuel near the intake valve.


The 360 with a kegger is factory rated for 100 lb ft more torque than the 318, which comes in around 325 lb ft or so.
If this is a basically stock truck application, try some Summit Shorty headers, get a new, or port your own throttle body out to 52MM straight through eliminating the factory 48 to 52 MM hourglass shape. If you can find one, install a Mopar Performance PCM OBDI to '96, or get a tuner, OBD II 96 up. I removed my center divider, don't think it did much, also recently installed a Comp shelf cam designed for these engines (206-210 @.50, 112 LC) and a set of 3.90 gears. Tuning is very limited for OBD I systems.


Search Dakota-durango.com or dodgetrucks.org

You are correct about these fourms there is a little info there. I have not seen any one explain or seem to have an undersdanding of how runners work, not saying they do not know they just dont show it. It just look like they are shorting the runners to turn more rpms. some are cutting the runners down 2 inches and I do not want to raise the rpms in my truck.

The volume may not be a problem but i would like to understand it. It may be why the engine is responsive, and gives the fealing as if a clutch or converter is not working properley or slipping.

runner harmonic pulse stock intake runner is 14.5 and the head runner is 5.5 long so total runner is 20.0 long. I shotened my runner to .5in so total runner is 19.5 i did so to improve the runners curtain area.

----- 3rd pulse is ------ 97000/rpm ------- to find a lenght you want to use.
--------- or------ 97000/lendth-- (20in)---- to find what is in use. so this happens at 4850 rpm

----- 4th pulse is ----- 74000/rpm ----- to find a length you want to use
---------- or----- 74000/ length-- ( 20in) ----to find what is in use. so this happens at 3700 rpm

so if you shorten the runner 2 inches you will raise the running rpms by about 400 rpm and torque curve up 400 rpm also.
I do not have all the specs on this engine to plug this it to PipeMax and get a better piture.

thanks Dewey

BTT

Are you having problems with a vehicle you are now driving? I Doubt Intake volume is the problem, if anything I bet the plenum volume helps. Some aftermarket TPI Intake systems tend to increase plenum volume and shorten runners VS. stock, to raise operating range. I had a 355 TPI in a camaro and currently have a 94 Dak 4X4 5 speed with a 5.2 Magnum with some mods. Compared to the TPI system (including when it was on the stock 5.0) I like the Dodge Magnum long runner system better, it has the torque of a TPI system but seems to have a better top end range, I don't know if it is because of runner length or plenum volume. Modern Hemi's and LSX motors also have intakes that resemble the Magnum Intake dating back to '93. As stated before the design is probably right on for the intended use.

The Keggers weak spot is the sheetmetal bottom cover with the stock paper gasket. Leaks here will cause oil consumption and spark knock.

Can anyone here explain the advantages of a large plenum on a stock late model EFI truck motor, if there are any?

[Dewey]

The plenum gasket was bad other than that the truck is in good shape.

The plenum volume may not be a problem, I would like to undestand it better.

I have a two intake manifolds i am working on. I would like to see the engine pull better below 3000 rpms.If the maifolds work or not i would like
to find an understanding of what I have changed to made it work or not.

the manifold on now is stock i am installing ai manifold with the runners cut .5 inch and the divider cut down 4 inchs

the next manifold will have the dividers cut .5 inch and will fit a 2.5 inch pipe inside the plenum about 16 inchs long so outside air will flow throught the center of the pipe. so not to get a heat gain form filling the plenum with somthing solid.

thanks Dewey

[1989TransAm]

I have highly modified my TPI intake system using EA Pro and Larry Mieux's Pipe Max for guidance. As you know the TPI system has 3 components, that being the plenum, runners and base. I might get some flack for this but I see the LSx intake system being a modernized repackaged version of the TPI. The LSx uses the 3rd harmonic wave versus the 2nd harmonic wave of the TPI.

My TPI is made up of the factory plenum, SLP runners and the First Intake Base. I had the First intake base flowed out of the box and one of the worst port flowed 301cfm. I wanted peak horsepower to be at 6500 rpm and followed the guidance of the above programs. The total intake runner track was calculated to be 12.5" long. I think mine wound up being 13". I had to fully siamese my SLP intake runners into one big paired oval save for the last 1/2" for the transition into the base that looked like an hour glass. IMHO the now ovaled runners actually turned what was runner into plenum volume. The reason I believe this is the peak horsepower indeed turn out to be in the 6400-6500 rpm range.

The factory plenum was opened up to match the runners and to accept a monoblade throttle body. The First intake manifold was open up to the dimensions suggested by the programs and the head end of the manifold was 2.2 square inches. It then tapered up to an inlet diameter of 1.85". So I incorporated the suggested cross sectional areas, taper and total length of the runners. Plenum volume was increased by default.

So I would suggest puchasing one of those programs and enter the parameters of the goals you want to reach. I think EA Pro has a free trial that might be enough time for you to come up with the parameters you will need for your motor. Here are some pictures I found with a couple of in progress pictures showing the modifications I made to match dimensions suggested by PipeMax etc.

[crazyman]

I cut the divider out and knocked off a half inch from the runners (enough to radius them nicely) on a Dakota 3.9 keg, and I lost almost all torque and 2mpg.

[Dewey]

I cut the divider out and knocked off a half inch from the runners (enough to radius them nicely) on a Dakota 3.9 keg, and I lost almost all torque and 2mpg.

Do you know why you lost torque? Did you change any thing else at the time you changed the manifold.

I see alot fo people boring the throttle bodies to 50mm or to 52mm i think this a bad thing. My thoughts are that a plenum of this size must have
good or high velocity at the TB to produce torque at lower rpms.

on the maifolds 1998 and newer the manifold was cast without the divider. Why would Dodge remove this ?

thanks Dewey

[MadBill]

A bigger TB has no downside. (other than new air cleaner plumbing and perhaps overly-touchy response) It's just like opening the throttle more, say to 125%, although there are of course diminishing returns as it's made larger and larger.

[MagnumTPI]

The factory TB has an hour glass shape. As Mad Bill said this helps with the part throttle control, softens it a little. Without it, the pedal is more touchy, you get the feel of improved power which does happen at a smaller throttle angle. I can't prove better WOT power but I believe it is there. Straight bores on the TB will make the cruise control to pulse fast and slow by 1 MPH every 2 seconds or so because it is calibrated for a slower response by the engine caused by the hourglass shape. Very noticable with a manual trans. Will make you want to loosen the cable but you can't. I don't think air velocity through the TB is a big issue with a port injected motor if it is sized correctly. Might matter on a TBI motor with a wet flow dual plane intake and central injectors.

There are many speedtweaks found online for these engines like relocating the MAP vacuum source to the manifold from the TB, Increasing the base TPS voltage to .7V, Playing with the fuel sync, drillling and relocating the crank sensor to advance timing, etc. You'll have to research these and decide what you want to try and if it is an effective mod or not.

I have tried therm all most are back to stock except Fuel Sync, (need a snap-on or dealer scanner) and MAP sensor Vac. source.
I have the summit shorty headers, straight through TB (home ported stock unit, smoothed top), Comp Cam with Hughes springs & retainers, MP PCM, KN FIPK 14" round air filter (from my old S-10, really nice countoured base design), empty cat, and knock off Flowmaster (flowsound) muffler. I tried a Walker Super turbo single 2 3/4" system before that, no change going to the fake flowmaster and back to a single 2.5" pipe. I also removed the divider but left the runners stock length. I think the runner mod works best on 5.2's, they are cammed a little more agressively than the 5.8's. I removed the divider while I had it off to fix the plenum plate gasket, I have read that it is there up to '96 to assist in even EGR distribution which supports why it is not found on newer models, they weren't equipped with EGR. Can't say there was a power difference because the oil leak was messing things up but It ran really good after all was done (port matched the intake to the heads also).



Going from a 3.20 to 3.90 axle ratio cost a mile to the gallon but the truck will haul 2000 lbs. with ease. At it's best I was getting 18.1 mpg on combination city/highway driving, I'm now in the low 16's which is what it did bone stock, The gears and the cam were the best mod's and had the biggest impact on economy.

[panic]

A friend with some Chrysler sources says that 1/2-way attempts to improve the beer keg have been mostly failures, with no clear pattern of mod X produces result Y.
I suspect that this may have Helmholtz characteristics, in which the volume, inlet pipe size, runner length, static CR are all inter-related and synched - modifying one throws it off the cliff.

[Dewey]

A friend with some Chrysler sources says that 1/2-way attempts to improve the beer keg have been mostly failures, with no clear pattern of mod X produces result Y.
I suspect that this may have Helmholtz characteristics, in which the volume, inlet pipe size, runner length, static CR are all inter-related and synched - modifying one throws it off the cliff.

Did he say what a 1/2-way attempt is or say what will work . Is this thing like opening a can of worms and should be left alone.

thanks

[panic]

That was the impression I got, and he wasn't too helpful after that.

[crazyman]

volume, inlet pipe size, runner length, static CR are all inter-related and synched - modifying one throws it off the cliff.

That's the impression I got after messing with my intake.

[panic]

Well, just idle speculation on my part, but how many of the variables Vizard identifies can be quantified?
Z the tuned RPM at which the effect takes place
c the speed of sound, in meters per second (348 is a safe value here, estimating box temperature at 80° F)
A the intake port area, in cm2
L the intake port length, in cm (multiply inches × 2.54)
V the engine displacement, per cylinder
C the compression ratio, static or nominal

Z = π × 200 × c × (A ÷ L × V)^.5 × (C-1 ÷ C+1)^.5

[SchmidtMotorWorks]

That formula doesn't appear to consider runner taper in the volume.

My free online calculator computes Helmholtz considering runner taper and changes the computation depending on whether the port has pushrod choke or not.

User hint, push the calculator button once before entering any values.

http://www.schmidtmotorworks.com/engine_designer.html

http://www.schmidtmotorworks.com/enginedesigner.aspx