Velocity Stack mounting

[cheezoto(OU-FSAE)]

Is it better to have the rolled lip of the stack flush with the bottom of the manifold or slighly raised up off the surface.

University of Oklahoma
FSAE Vice President
FSAE Engine Systems Leader

[cheezoto(OU-FSAE)]

Is it better to have the rolled lip of the stack flush with the bottom of the manifold or slighly raised up off the surface.

University of Oklahoma
FSAE Vice President
FSAE Engine Systems Leader

[Scott Wordley]

Good question, and something I've been thinking about.

I'm inclined to think that if the lip is properly rolled it will allow the runner to breathe from 'behind' itself which is a good thing. This is also probably good for packaging and is easy to do if you have an ally manifold.

On the other hand I would be inclined to mount the bellmouths flush with the plenum IF I were using a side entry plenum. You cant stick the stacks out into the flow coming from the side and explect it to remain smooth with the flow for all cylinders coming past through a reasonably small cross section. Problem is you probably have to do the bellmouths/plenum in composites.

If anyone has done some manifold flow vis(Fergus?), flow bench or CFD work on this topic I'd be keen to hear about your experiences.

This year we will use a carbon side entry plenum with flush mounted bellmouths.

Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae

[woollymoof]

We haven't done any CFD or any sort of testing as of yet but, just going on gut this is what I feel....
Having the bellmouths flush with the bottom of the plenum and all of the necessary tapers and such this would give you the best flow rate.
But,
engines aren't just about flow rate, pulsing is an issue as well. If the bellmouths are flush with the plenum the change in area will be gradual, for a good strong reflected pulse you want a sudden (and large) change in area which is what you have if the trumpets are risen up into the plenum a bit.
Just another note if they are risen up into the plenum a way (for a side fed) air will be able to flow under and around the trumpets and make its way to the rear of the plenum, perhaps this will help prevent starvation of the rear trumpets and cylinders. Just a thought

[Scott Wordley]

Interesting what you say about reflection strength.

I agree with you, we found that more sharp, definitive bellmouths do give a sharper tuning peak. The question is, is this what you want?
Maybe for an engine that operates in a very narrow RPM band but we find with our gearing we pretty much drive the event in 2nd using a very wide RPM band (7000 to 12000).

Which is exactly why our bellmouths have a longer, gradual diffusion for a wider, softer tuning peak and flatter torque spread.

Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae

[woollymoof]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR> <HR></BLOCKQUOTE>Which is exactly why our bellmouths have a longer, gradual diffusion for a wider, softer tuning peak and flatter torque spread.

when you say this do you mean that if you had sharper bellmouths that the peak would shoot out of the torque curve, somewhere, and not effect the rest of the curve or do you mean other areas will be sacrificed as a result?

[Kevin Hall]

The peak is spread out, as in the basic torque curve is somewhat constant, and with a sharp bellmouth, the peak wil be added as a narrow high amplitude peak. If the bellmouths are gradual, the peak will be added as a general increase over a broad band of RPM. I would say that gennerally the baseline torque curve is defined by runner lengths, areas and plenum design. Just my 2 cents.

[Scott Wordley]

I think you've go it right wooly.

For our engine with sharp bellmouths you can see the peak jump out of the torque curve and then drop off again within about 1000RPM. We were able to get close to 80hp with this type of bellmouth tuned to the correct(read: high) RPM point but obviosly the rest of the range suffered. We ended up going with the big bellmouths tuned for around 9000 which gave pretty much between 50 and 60Nm from 5-10000RPM

Regards,

Scott Wordley & Roan Lyddy Meaney
Monash FSAE Wingmen
http://www-personal.monash.edu.au/~fsae