potatoe in exhaust sciene (bernoulli effect) ?
Discussion
mighty car mods did a somewhat joke peice where they stuck/floated a rounded potatoe in the cars exhaust and gained some bhp on the dyno.
about 4-5% across the whole rev range.....which is nothing to sniff at.
ive been looking into why it may of worked in reducing backpressure, and ive come across the science experiment where you can blow into a cone/funnel and it sucks a ping pong ball back into the cone in instead of blowing it out.
why the above experiment works is as the air goes round the ping pong ball it speeds up...creating low pressure air and this suction is pulling the ball in.
somthing to do with the bernoulli effect and the coanda effect....im not smart enough to get too deep into it.
my thoughts are, if you could suspend a ball (potatoe) at the right position at the exit of the exhaust, it too creates this low pressure section around the ball, creating a vacuum which sucks the exhaust gasses out better, reducing backpressure.
ive tried with a funnel and a long tube i use to fill my pc water cooling....and i can notice a drop in blown pressure (my lungs) if a get my finger in the funnel at the correct position.
another wierd thing....my mate put one of those silly car turbo whistles on my exhaust.....didnt make a noise. i only noticed it after a week as i saw it.
again, no noise....but i did notice some "pep/responsiveness" had gone after removing it. this too places a roundish object in the centre of the exhaust exit path (and my exhust tip is a slight internal cone shape)
thoughts on all the above?
about 4-5% across the whole rev range.....which is nothing to sniff at.
ive been looking into why it may of worked in reducing backpressure, and ive come across the science experiment where you can blow into a cone/funnel and it sucks a ping pong ball back into the cone in instead of blowing it out.
why the above experiment works is as the air goes round the ping pong ball it speeds up...creating low pressure air and this suction is pulling the ball in.
somthing to do with the bernoulli effect and the coanda effect....im not smart enough to get too deep into it.
my thoughts are, if you could suspend a ball (potatoe) at the right position at the exit of the exhaust, it too creates this low pressure section around the ball, creating a vacuum which sucks the exhaust gasses out better, reducing backpressure.
ive tried with a funnel and a long tube i use to fill my pc water cooling....and i can notice a drop in blown pressure (my lungs) if a get my finger in the funnel at the correct position.
another wierd thing....my mate put one of those silly car turbo whistles on my exhaust.....didnt make a noise. i only noticed it after a week as i saw it.
again, no noise....but i did notice some "pep/responsiveness" had gone after removing it. this too places a roundish object in the centre of the exhaust exit path (and my exhust tip is a slight internal cone shape)
thoughts on all the above?
jimmyhackers said:
mighty car mods did a somewhat joke peice where they stuck/floated a rounded potatoe in the cars exhaust and gained some bhp on the dyno.
about 4-5% across the whole rev range.....which is nothing to sniff at.
ive been looking into why it may of worked in reducing backpressure, and ive come across the science experiment where you can blow into a cone/funnel and it sucks a ping pong ball back into the cone in instead of blowing it out.
why the above experiment works is as the air goes round the ping pong ball it speeds up...creating low pressure air and this suction is pulling the ball in.
somthing to do with the bernoulli effect and the coanda effect....im not smart enough to get too deep into it.
my thoughts are, if you could suspend a ball (potatoe) at the right position at the exit of the exhaust, it too creates this low pressure section around the ball, creating a vacuum which sucks the exhaust gasses out better, reducing backpressure.
ive tried with a funnel and a long tube i use to fill my pc water cooling....and i can notice a drop in blown pressure (my lungs) if a get my finger in the funnel at the correct position.
another wierd thing....my mate put one of those silly car turbo whistles on my exhaust.....didnt make a noise. i only noticed it after a week as i saw it.
again, no noise....but i did notice some "pep/responsiveness" had gone after removing it. this too places a roundish object in the centre of the exhaust exit path (and my exhust tip is a slight internal cone shape)
thoughts on all the above?
Snake oil.about 4-5% across the whole rev range.....which is nothing to sniff at.
ive been looking into why it may of worked in reducing backpressure, and ive come across the science experiment where you can blow into a cone/funnel and it sucks a ping pong ball back into the cone in instead of blowing it out.
why the above experiment works is as the air goes round the ping pong ball it speeds up...creating low pressure air and this suction is pulling the ball in.
somthing to do with the bernoulli effect and the coanda effect....im not smart enough to get too deep into it.
my thoughts are, if you could suspend a ball (potatoe) at the right position at the exit of the exhaust, it too creates this low pressure section around the ball, creating a vacuum which sucks the exhaust gasses out better, reducing backpressure.
ive tried with a funnel and a long tube i use to fill my pc water cooling....and i can notice a drop in blown pressure (my lungs) if a get my finger in the funnel at the correct position.
another wierd thing....my mate put one of those silly car turbo whistles on my exhaust.....didnt make a noise. i only noticed it after a week as i saw it.
again, no noise....but i did notice some "pep/responsiveness" had gone after removing it. this too places a roundish object in the centre of the exhaust exit path (and my exhust tip is a slight internal cone shape)
thoughts on all the above?
Placebo effect.
Is it still half-term?
i just find it weird that the video they did is 8 years old and no ones tried to debunk it or replicate it yet. only this kind of talky speculation were currently engaging in
there are lots of possibilities asto what happened in the video:
1. they fudged the dyno numbers
2. that specific car actually preferred more backpressure than even the oem/original exhaust provided (unlikely)
3. over subsequent runs (hard pulls upto temp) the "italian tune up" on that old used car freed up some ponies
4. its legit and they've stumbled across some new "magic trick" completely by accident.
have a look at the bernoulli effect demonstation videos with the cone/funnel and the ping pong ball.
vaccum is created at the end of a higher pressure pipe exiting to atmosphere.
if your exhaust tip could generate a vaccum, wouldn't that decrease backpressure?
theres a fair amount of written proven science about exhaust scavenging, harvesting exhaust pulses to be beneficial to one another, im just wondering if this could be another peice of that science puzzle that isnt fully understood yet.
there are lots of possibilities asto what happened in the video:
1. they fudged the dyno numbers
2. that specific car actually preferred more backpressure than even the oem/original exhaust provided (unlikely)
3. over subsequent runs (hard pulls upto temp) the "italian tune up" on that old used car freed up some ponies
4. its legit and they've stumbled across some new "magic trick" completely by accident.
have a look at the bernoulli effect demonstation videos with the cone/funnel and the ping pong ball.
vaccum is created at the end of a higher pressure pipe exiting to atmosphere.
if your exhaust tip could generate a vaccum, wouldn't that decrease backpressure?
theres a fair amount of written proven science about exhaust scavenging, harvesting exhaust pulses to be beneficial to one another, im just wondering if this could be another peice of that science puzzle that isnt fully understood yet.
jimmyhackers said:
if your exhaust tip could generate a vaccum, wouldn't that decrease backpressure?
No, because that "negative pressure wave" is about 3m from the exhaust port.Just accept that there is a reason why nobody has tried to harness the effect.
Or at least nobody has documented it, it won't work.
ive learned the dan qualye meme now 
i cant spell either 
there is quite some documentation about harnesing the venturi effect in exhausts, and a fair few practical applications.
just theres none as a ball in the middle of exit flow way, and not many used on cars in the way im on about.
aparently the spitfire plane used specially shaped exhaust exits to aid extra outside air being pulled along, increasing overal thrust.
there is a duramax exhaust that draws in extra outside air to cool the exhaust gasses. i think more for emissions than anything else.
there is something similar to the duramax where an outer exhaust sleeve is connected to a ram air vent, so at speed, outside air is going into the exhaust exit stream creating lower exhaust pressures.
the rest seems to be about rocket nozzle designs.
i suppose youre right, if it was a thing more people would do it
i cant spell either there is quite some documentation about harnesing the venturi effect in exhausts, and a fair few practical applications.
just theres none as a ball in the middle of exit flow way, and not many used on cars in the way im on about.
aparently the spitfire plane used specially shaped exhaust exits to aid extra outside air being pulled along, increasing overal thrust.
there is a duramax exhaust that draws in extra outside air to cool the exhaust gasses. i think more for emissions than anything else.
there is something similar to the duramax where an outer exhaust sleeve is connected to a ram air vent, so at speed, outside air is going into the exhaust exit stream creating lower exhaust pressures.
the rest seems to be about rocket nozzle designs.
i suppose youre right, if it was a thing more people would do it
jimmyhackers said:
aparently the spitfire plane used specially shaped exhaust exits to aid extra outside air being pulled along, increasing overal thrust.
The "extra outside air" is already being pushed past the exhaust by the propellor, which would be travelling faster than the exhaust gasses, so I can't see that at all.ETA. Though if the air speed is faster than the exhaust gas speed, the resultant low pressure at the exhaust tip MAY help exhaust port scavenging at a small range of air speeds.
Even if the exhaust gasses were going faster than the propelled air they would then be slowed down by the slower moving air giving no net positive result.
Just my 2p.
Edited by E-bmw on Monday 20th April 11:03
i see you're starting to come around to the idea 
in some some weird irony, dyno tests are "static"
i know some dynos use a big fan up front for extra cooling. but this airflow isnt quite real world air flow volumes.
i could imagine 100mph+ air in the real world and not on a dyno, if directed correctly, could also add a scavenging suction effect to a correctly designed tailpipe.
i know intakes sometimes use hermholzt resonant chambers....but it doesnt seem a thing for exhausts. most "tuning" stops at headers for four stroke engines.
i really need to set up a smaller scale model with pressure gauges at various points to see if there is any validity to all this conjecture.
in some some weird irony, dyno tests are "static"
i know some dynos use a big fan up front for extra cooling. but this airflow isnt quite real world air flow volumes.
i could imagine 100mph+ air in the real world and not on a dyno, if directed correctly, could also add a scavenging suction effect to a correctly designed tailpipe.
i know intakes sometimes use hermholzt resonant chambers....but it doesnt seem a thing for exhausts. most "tuning" stops at headers for four stroke engines.
i really need to set up a smaller scale model with pressure gauges at various points to see if there is any validity to all this conjecture.
Sporky said:
jimmyhackers said:
i really need to set up a smaller scale model with pressure gauges at various points to see if there is any validity to all this conjecture.
Gosh yes.Do report back once you've done it.
Sporky said:
I am intrigued to discover whether the results change with different starchy root tubers.
Baby new potato vs a trimmed-down King Edward. Parsnip vs swede. Charlotte vs pink fir apple. The possibilities boggles the mortal mind.
We should also investigate potato gnocchi and pierogi.
Carrots probably have an interesting aerodynamic effect.Baby new potato vs a trimmed-down King Edward. Parsnip vs swede. Charlotte vs pink fir apple. The possibilities boggles the mortal mind.
We should also investigate potato gnocchi and pierogi.
found these things in my googlings
https://www.dkcustomproducts.com/thunder-torque-in...
very similar to my concept/idea
also found these velocity tuning cone insert things, they go inside exhaust tips. same venturi/vacum/bernuoilli effect.
https://www.youtube.com/watch?v=v4TRgirb2GQ
concept seems valid...
vacum generation at exhaust tip = good
https://www.dkcustomproducts.com/thunder-torque-in...
very similar to my concept/idea
also found these velocity tuning cone insert things, they go inside exhaust tips. same venturi/vacum/bernuoilli effect.
https://www.youtube.com/watch?v=v4TRgirb2GQ
concept seems valid...
vacum generation at exhaust tip = good
jimmyhackers said:
found these things in my googlings
https://www.dkcustomproducts.com/thunder-torque-in...
very similar to my concept/idea
They have an effect because those exhausts are incredibly short and open, a bit of back-pressure helps the scavenging. Very common on bikes to fit baffles in the exhaust and change the power/torque response - very often you gain a bit of mid-torque with a baffle to restrict the system at the cost of a couple of hp up top. It can also help smooth out throttle response.https://www.dkcustomproducts.com/thunder-torque-in...
very similar to my concept/idea
jimmyhackers said:
also found these velocity tuning cone insert things, they go inside exhaust tips. same venturi/vacum/bernuoilli effect.
https://www.youtube.com/watch?v=v4TRgirb2GQ
concept seems valid...
vacum generation at exhaust tip = good
It's not generating vacuum, it's just changing the back pressure and resonance at particular frequency ranges (which is where the raspyness comes from). OEM exhausts have these types of baffles entrained in them already, he's put them in the bin with the silly 3-inch exhaust which is basically a straight pipe with tiny resonator and a cheap back box, then fitted one back in.https://www.youtube.com/watch?v=v4TRgirb2GQ
concept seems valid...
vacum generation at exhaust tip = good
Vs the OEM 350Z system which is narrower and with more adequate silencing and resonance control
Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff