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tips, technology, tools and techniques related to vehicle driveline components

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Johan3500
Johan Van Wyk

(9 posts)

Registered:
03/06/2009 06:17AM

Main British Car:
Plenum Mods
Posted by: Johan3500
Date: March 25, 2011 04:47AM

I read that one can improve breathing on a SDi by shortening the 8 standard tubes inside the plenum by 10 to 25 mm. Is there anyone who could shed some light on this? And also, what will the pay-off be since one cannot always gain without a down side? The engine is a 3.5, 9.35:1, Viper hurricane cam and fully ported and flowed heads on a manual 'box in a very light car.
Could a well sorted Holley 390 outperform the old flapper injection system?
Thank you,
Johan


Moderator
Curtis Jacobson
Portland Oregon
(4577 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Plenum Mods
Posted by: Moderator
Date: March 25, 2011 11:24AM

Well... I researched that topic and included some of the results when I wrote this article:
Service and Troubleshoot Rover 14CUX Electronic Fuel Injection

Quote:
The lengths and diameters of runners in the intake system, and also the volume of air in the plenum from which they draw air will affect the performance characteristics of the overall system. Although we lack the resources to comprehensively and scientifically evaluate the performance effects of modifying these components, we can make a few physical measurements and see how they fit "rules of thumbs" published by others.

The inside diameter of each trumpet is 3.81cm. The cross-sections of the runners in the cylinder heads are significantly smaller, but in the intake manifold the runners quickly transition up to round cross-sections of about 3.8cm diameter each. The average total length of all eight runners measured from intake valve to open plenum is about 38.75cm. (Runners 1 and 8 are approximately 41cm long. Runners 2 and 7 are approximately 39cm long. Runners 3 thru 6 are approximately 37.5cm long.)

Based on these measurements, and on articles written by engine guru David Vizard describing the behavior of Helmholtz Resonance induction systems, we can calculate that the Rover induction tract seems to be tuned to achieve peak torque at approximately 5130rpm. Furthermore, according to David Vizard's explanation and formulas, if the runner length is shortened the induction tract's tuning will be shifted to achieve peak torque at an even higher engine speed. (If the shortened runners average 37.95cm, the system will be tuned to provide peak torque at about 5310rpm.)

A quick-and-dirty check revealed that the total air volume of the plenum (base plus chamber to the throttle plate, minus the trumpets) is in the ballpark of 3250cc.

I know other people on this board have first hand experience with this and with the other questions you asked (carb etc.) - I hope they'll comment.

Johan, you didn't comment on how your car will be used. Real racecars operate in a relatively narrow rpm band, and a carb can be tuned well to suit that car for a particular weekend of racing. It's harder to tune a carb to suit all the traffic, weather, and road conditions you'll experience with a street car. That's where EFI shines.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(6470 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Plenum Mods
Posted by: BlownMGB-V8
Date: March 26, 2011 10:39AM

That plenum volume is close to the displacement of the engine, runner volume is about half of cylinder volume I believe. Some theories hold that runner volume equal to cylinder volume yields maximum torque, I think this was the theory behind the old Red Ram intakes on the early Dodge Hemi. I think you have some room for tuning but the system is already optimized for street use.

JB
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roverman
Art Gertz
Winchester, CA.
(3188 posts)

Registered:
04/24/2009 11:02AM

Main British Car:
74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L

Re: Plenum Mods
Posted by: roverman
Date: March 26, 2011 12:53PM

Johan, look on-line at the mod's done by the specialist companies, and the dyno results, for each particular build. Bigger trumphets = more flow, not shortening. Velocity is relatively low at entry. You want to build velocity constantly, towards the intake valve. What does "Real Steel" say ? Good Luck, roverman.


Moderator
Curtis Jacobson
Portland Oregon
(4577 posts)

Registered:
10/12/2007 02:16AM

Main British Car:
71 MGBGT, Buick 215

authors avatar
Re: Plenum Mods
Posted by: Moderator
Date: March 26, 2011 02:18PM

Just to put things into perspective...
http://www.britishracecar.com/ScottHughes-McLaren-M8F/ScottHughes-McLaren-M8F-BC.jpg


Johan3500
Johan Van Wyk

(9 posts)

Registered:
03/06/2009 06:17AM

Main British Car:
Re: Plenum Mods
Posted by: Johan3500
Date: March 29, 2011 07:47AM

WOW!!!
I am impressed and thank you all for the contributions! I should have explained my reasoning...I was considering the mod only to allow more "turning space" between the lip of the tube and the roof of the plenum box..just like when one cuts the choke tower off a Holley to create a bigger gap between the roof of the air cleaner and the carb body.
To answer some questions...the 3.5 is in a small Triumph Dolomite on a manual LT77 box with a Ford Capri LSD rear axle, ratio 3.08:1.
Now...another question please...what has the experience taught you guys...to balance-pipe or not...??
I "upgraded" the dual exhaust system laste year and put 2" pipes and silencers into car because it failed a 100dB track day sound test and the old silencers had very little guts inside. The gent who made up the new system used internal fits at joints...in other words wherever there is a joint in the pipes the diameter reduces....I thought if I fit a balancer pipe I effectively "open up" another route to help lessen the restriction to flow. Ha-ha, but it really feels as if the car has lost some of its grunt. The balancer is also about 2" in diameter and is quite close to the rear of the engine.
Any comments on that?
I have a similar system in my Triumph saloon in South Africa which works really well...or so I thought...now I am playing devil's advocate here...if my Dolomite got slown down like this, maybe I should chuck the balancer out of the SA car...hmmm...

Thank you again to Curtis, Jim and Art


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