Forum » General Tuning Discussion » External velocity stacks and the Bernoulli principle

External velocity stacks and the Bernoulli principle

General Tuning Discussion

Discuss all things tuning in this section. News, products, problems and results. 

= Resolved threads

Page 1

Somthing I have oftern thought about and would be intrested in others opinions

When running velocity stacks out the bonnet (which I think is awesome!)

is there a chance the wind speed up over the bonnet is counteracting air flow into the VS and trying to pull air back out of inlet with the Bernoulli principle?

If the tuning is spot on while setting up on the dyno is it possible to see a richer mixture on the track because there is not as much air going into the motor?

Most definitely you will see a large difference in mixture, either positive or negative depending on the trumpet's inlet surrounding pressure area between stationary and real driving with this setup. To get as close as possible, you would have to elaborate a speed dependent compensation table.

I had always wondered this for the old mechanical injection V8's at the Drags with the big long trumpets... but some faced more towards the front of the car and others towards the rear so In that case I thought it would be different for every cylinder

I guess the main point would be how fast would the air needs to be moving over the top of a velocity stack for it to start to act on it and pull air out.

but at 100kmph + surely that is moving quick enough

It's pretty common to see the intake air either positively or negatively influenced when the car is being used at road/race speed.

A bigger issue than velocity stacks like those in the video however is when the car is fitted with a forward facing airbox that can provide some degree of ram effect at speed.

Use an empty trumpet, welded from one side and source map sensor from the bottom of it. Put your hand out of the window with it at speed and see how much you could boost) I guess at 100 kmh the effect would be around zero) Some redneck "tuners" in Russia fit electric motor from the vacuum cleaner to the intake and think that the car became boosted)

Probably of little concern at pedestrian speeds, however I've seen positive pressure measured in both a land speed record car I tuned that ran at Bonneville, as well as in the intake of my drag car. The speeds were in the 150-240 mph and the pressure was minimal but it's a real consideration, particularly on the LSR car which was mapped off TPS.

A vacuum cleaner though... Hadn't heard that one before! Wow.

It's clear that there is a speed treshold when effect becomes positive. On aircrafts the Pitot tube is the main source of gauge speed.