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Effect of increasing plenum size?

Effect of increasing plenum size?

Effect of increasing plenum size?

I have an engine that I've done the following on-

ported head
high flow cats

This has brought the HP/TQ at the wheels from 201/198 stock with a redline of 6500 to 249/232 currently at a redline of 7300.

There are no other commercial parts available to get anything more out of this engine. The porting of the head gained nothing in real life even though the flowbench #'s seemed very promising.

The only other thing I can think of to do is modify the intake manifold.

The intake manifold is plastic with a normal plenum feeding long runners and a valve that opens a secondary short set of runners to another plenum/resonance chamber at around 4300.  

This 2nd set of runners/resonance plenum doesn't bring in air from anywhere, it just changes the pulse of existing air that still comes through the long runners.

I've picked up a spare manifold and the only modifications that can be done to it due to design and fitment would be to cut the tops of the plenums and increase the chamber size.

Any thoughts on how much larger I should go VS the factory considering the engine is making 25% more power and reving 800rpm more?  

Should I just enlarge the main upper (real) plenum or also include the resonance chamber?

RE: Effect of increasing plenum size?

Tuned length runners will by design favor one or possibly more than one narrow rpm bands at the expense of causing torque "valleys" elsewhere.  If your goal is peak HP, the length can be tailored to promote this, based on the expected peak HP rpm.  If you want a broad, flat torque curve, you're better off with very short runners and a decent sized plenum.
The trade-off of increasing plenum size is slower throttle response.
Depending on the means at your disposal you can either experiment with hardware, or model the engine in software, and iterate to find the solution you are seeking.

RE: Effect of increasing plenum size?

Hi Hemi,

Yes I am aware of runner length and diameter tuning, but as mentioned above due to packaging there isn't anything I can do to that.

To simplify it, basically I just want to know if enlarging the plenum sizes alone will have a positive benefit.  I am OK with slower throttle response and possibly a TQ loss at one RPM as a trade off for a gain elsewhere..

RE: Effect of increasing plenum size?

I think there is a good chance that the plenum volume is already OK - simply because if more torque or power were available just by increasing its size the OEM would have done it already.

Quick suggestion, put a small housebrick in the current plenum and see if it degrades the performance. If not then a small change in plenum volume is not significant.



Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Effect of increasing plenum size?

Actually you can change the runner length and diameter usually on your setup if you are willing to go through the work involved.  With your current setup and the fact that you don't feel you picked anything with the porting, you should probably look at the camshaft at this time.  Increasing runner volume is just going to move the power band higher in the rpm scale.  What kind of engine are we talking about here?  With the dual runner lengths, it sounds like you could be working on a SHO Taurus.  The different runner lengths should be controlled by vacuum on your setup depending on your throttle position.


RE: Effect of increasing plenum size?

the first thing to do is check that the manifold is passing
on the air flow to the head, we have done a fare amount on airflow work on plastic intake/plenum and found as much as
20% restiction to flow so it may pay to test yours before you get to plenum volume.or make any radical change

RE: Effect of increasing plenum size?

bigpaul is right on his statement.  That is why you should always test the actual parts you are going to be using when flow testing the cylinder head.  It is not unusual for airflow to drop by 20 to 50cfm when you test a head with the intake manifold attached vs. a radiused flow entry plate.


Larry Coyle
Managing Partner
Cylinder Head Engineering, LLC
CNC Porting
De Soto, KS 66018

RE: Effect of increasing plenum size?

A bigger plenum goes hand and hand with shorter runners/higher RPMs for one thing. However, because most stock manifolds are not even close to being fluidynamically matched to the valve flowing into the cylinder you will almost always have manifold flow restrictions. Even upon the application of a flow radius. As long as the short sides and long sides of the port are unequal in length at the entry of the runner you will not get any closer to the best possible results.

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