Plenum Volume

[Mudgey]

Hi guys, I am currently trying to find a formula for the correct plenum volume for a V8 with cross plane crank, ie so i'll have to have all 8 leading to a common plenum rather than twin manifolds.
I have read numerous forums and everyone seems to have different views on the correct size plenum chamber.

Apparently for a plenum thats not taking advantage of resonance and is fuel injected then you can run as big as you like, however as I am using a single throttle plate the bigger you go the less throttle response u get, tho i dont know what size you need to get to before this is noticeable?

Can you guys confirm what ive stated is correct and what i should aim for in my design?

This is a WIP, but is where i have got to so far, as it stands i want to raise it at the front so the lid slopes towards the rear to promote flow to the rearmost cylinders, tho whats the best way to avoid a pressure drop towards the rear most cylinders?

My design incorporates a perspex/acrylic type plastic lid, has anyone had much experience with this idea and are there any associated problems? Ive seen this done on a couple of V8 engines and it certainly looks awesome!

The engine is around 385-400bhp with a port cross sectional area of 1571mm^2 (at the head/header mounting face), i have decided to run 48MM bellmouths which have a cross sectional area of 1808mm^2, am i right in saying its best to have a gradual decrease in cross sectional area towards the valves on the runner rather than it being the same all the way along? The other option is to use 45mm trumpets with a closer matching area of 1589mm^2, does anyone have any ideas on what would be most suitable?

Any do's/dont's/suggestions to aid my design are welcome

Regards,
Mudgey

 

[Jim Cowden]

No takers, Yes one of those questions that when you answer you get your arse blown off.
Oh well
My rule of thumb and having said that you cant always get what you want in real life due to space ect.

99000 divide by max rpm will give a runner length to the valve seat in inches.
You can play with this length depending what you are looking for in the engine.
Whatever the valve size is lets say 50mm then I would go aprox that at the entry of the runner.
2/3 of the tapper will be done in the first 1/3 of the runner.
Plenum volume is = to all the runner and port volumes.

You may end up tight for space with the TB on the front depends on engine position and chassis .
Maybe 2 singles on either side, but they may not look as tidy as your layout.
2 may also help with feeding the plenum equally.
If the plenum roof is to close to trumpets it will effect the Helmholtz effect at lower rpm.

I use this to start then the compromises come into play.
Let me know what thickness you use for the lid so it does not pulse with the vacuum signal as I am going to do the same project for my modular.
Are you going to use the existing lower section of the manifold or are you planning on making the lot.

Jim

 

[Mike Pass]

Interesting idea. It might help to have a throttle body at each end to equalise the flow to all cylinders and maybe then you could use a smaller plenum size as it would not have to flow to the far end to maintain even feed. The throttle bodies could be easily linked with a rod running under the plenum to ensure smooth and easy throttle action. How much space do you need above the engine? I would guess that inch thick acrylic would be stong enough particularly if you mould it with curved edges or fix flat panels into a steel frame.
A good guess now will help when it hits the dyno and you find out what works and what doesn't!
Good luck
Cheers
Mike

 

[mark b.]

go over to Speed Talk they'll give you a big man-hug, there's been discussion on the 180 deg. crank and i'm sure you can find anything else or just ask.

 

[Mudgey]

Thanks for your responses guys, it seems i have even more researching to do!!! I think i'm going to step away from a common plenum chamber idea and go with individual throttles per cylinder.

Ive favourited that website, there is a significant amount of information available there!

Many thanks,
Mudgey