Don Terrill’s Speed-Talk

i am trying to find out about plenum volume. how does it affect rpm range and power

is there a rule of thumb of volume to engine size or running rpm

the engine i am working on is a dogde ram 360 gas engine. the plenum has a volume of 404 sia this does not count the
runners volume. this much seems like a power loss to me at low rpms.

thanks for any insite Dewey

If this is a moderate HP application, you may want to experiment with an RPM Air Gap dual plane style intake manifold. At $200, they're cheap for what they do and can have a positive overall effect. I've had a number of applications where low rpm torque is dramatically increased with a minimal loss of top end power.

In general terms, the large or larger plenum volumes increase torque and horsepower - provided the carburetor can handle it. Often, people get bad results when adding spacers or running large plenum chambers, but in most or many cases it is due to the carb design and calibration.

You haven't given enough information for me to comment on this engine in particular.

Troy Patterson TMPCarbs.net TMP Carbs

I'm not sure what's going on in TR applications.
The total carb CFM, even at 1" Hg, sometimes appears to be at or above 100% VE at the peak RPM, and yet the manifold is 50% of engine displacement.
Where is the power increase coming from?

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.

Yap, it's all fuel curve. A small or conventionally sized carburetor in many if not most or even all cases does not allow for a good balance between main jet and high speed air bleed, nor does it allow the possibility of a really good fuel curve.

It's not about cfm, it's about fuel curve and it's about reducing the velocity of the air and fuel exiting the carburetor

Troy Patterson TMPCarbs.net TMP Carbs

i am sorry i did not say this is a fuel injected engine with a throttle body

it also spends most of is life from 1500 to 3000 rpms does not pass 4000 rpms

i was thinking of reducing the plenum volume, i think this mite help the engine pull better at 1500 to 3000 rpm
am i wrong i have had a hard time finding any info on this deal.

to me the volume is to large and takes all the air energy like a sponge.

thanks

This is a factory intake manifold and plenum? If so, I doubt the factory missed the mark, but...

Troy Patterson TMPCarbs.com TMP Carbs

Troy Patterson wrote:Yap, it's all fuel curve. A small or conventionally sized carburetor in many if not most or even all cases does not allow for a good balance between main jet and high speed air bleed, nor does it allow the possibility of a really good fuel curve.

It's not about cfm, it's about fuel curve and it's about reducing the velocity of the air and fuel exiting the carburetor

Troy Patterson TMPCarbs.net TMP Carbs

SO you want to put a 1250 dominator carb on a street/strip 350 and think it will work better than a 750? That will certainly slow down the air and fuel exiting the carb.

Troy Patterson wrote:This is a factory intake manifold and plenum? If so, I doubt the factory missed the mark, but...

Troy Patterson TMPCarbs.com TMP Carbs

Yes it is a factory maifold. the maifold has been called a kegg or a kegger manifold it is big. I will work on getting some pics up

I would like to have a understanding for this plenum volume. to have an expantion this big between the throttle body and the intake runners
seems like a bad thing to me ( a loss in air velocity). I work with mostley single cylinder engines (atv/motorcycle) in a port from the valves to the carb if there was a expantion of this size in a single cyinder it would be a very bad thing. Does plenum volume relate to this at all .

thanks Dewey

Way back years ago when most here were pups, E-brock brought out their SP-2P dual plane economy both 2 and 4-barrel intakes because emissions crap had just came out and compression, mileage, and power were in the toilet.

An SP-2P has very small paired siamesed runners that connect into the plenum together so actually it is a very good low rpm torque producing intake because the runners are so long.

What killed E-brock's marketing strategy was that when guys bought them, they also bought smaller so-called, "Mile-a-More" cams and also installed small 4-tube, long headers and I'm talking like 1-3/8" and 1-1/2" diameter jobbers.

An engine combination like this sucked so good that it increased the dynamic CR so high that stock CR'd smog engines set up this way went into detonation!!

This and the fact that better gas came out shortly later so E-brock stopped selling them.

I have two of them, a 4-barrel SB and a 4-barrel BB so have looked 1st hand at their designs.

Now, bttt, you can crutch a stock dual plane intake by doing no more than add a 1/2" to maybe 1" tall spacer and then "bullnosing it from the top down as far into each runner that you can get to with say a 1/2" bullnose here.

This allow's the air/fuel mixture to turn the "corner" easier so feed's the heads better!

And fwiw, the old early cast-iron Q-Jet L-48 type intake is the best stock dual plane made for power/torque up to say 34 to 3500 rpm, bar none! Been proven and you can wake one up bullnosing the top of it like I mentioned.

pdq67

Here is pic of what i am working with

this a pick of the inside of the plenum the runners are 14.5 in long
the volume not including the runners is 404 sia plenum area only

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

In-Tech wrote: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.