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Exhaust thrust

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Exhaust thrust

Postby srq » Tue Mar 19, 2013 10:29 pm

Hi guys,
I tried searching but couldn't find any info but maybe somebody could get me in the ball park. Can anyone get me a scientific wild ass guess (SWAG) as to about how much push might be coming out of a 600 cui inch engines open exhaust headers at oh say around 6000 rpm. I rember reading some where on this forum someone talking about dragsters etc using them to clear smoke away from their tires and then someone starting touching on how much thrust there was but then it didn't go any farther. I mean are we talking 50 lbs thrust, 100 lbs more? I'm curious because I could possibly put it to good use in the marine industry.
Thanks in advance
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Re: Exhaust thrust

Postby MadBill » Wed Mar 20, 2013 11:14 am

I intend some day to get real data by running the exhaust of a stout dyno mule through some flex pipe into a 90°elbow pointing vertically and resting on a weigh scale...

It may be many hundreds of pounds on a TF car, but my SWAG is 15-20# for 500 HP.
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Re: Exhaust thrust

Postby TMSJoe » Wed Mar 20, 2013 11:18 am

I know on TF it is significant thrust. Enough to push the car sideways when one cylinder loses fire. Pretty sure I saw a video of a Funny car losing the header off one side and it lifted the car.
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Re: Exhaust thrust

Postby rklessdriver » Wed Mar 20, 2013 3:36 pm

Even on non Fuel Cars the exaust thrust can be signifigant.

Take a look at X275, ODR and Outlaw 10.5 cars. You will see the "Ram Horns" we run comming out of the side of the front bumper (some guys still use the fenderwell).... For the past 2 yrs we have been putting them on V-Band connectors so we can rotate them to make minor adjustments for track conditions....

I don't have any data to say how many lbs of downforce it makes but I can tell you how the ram horns are oriented makes a big difference in wether our car will go down the track or just rip the back bumper off.
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Re: Exhaust thrust

Postby af2 » Wed Mar 20, 2013 3:59 pm

rklessdriver wrote:Even on non Fuel Cars the exaust thrust can be signifigant.
Will


Yep even blown Alcohol will try and change lanes with a cylinder out just not as fast and most don't notice but the driver...

I am amazed at the single turbo cars with the pipe out one side or the other how they contain it??? I would asume the sterring wheel is always to the exhaust side when going down the track...
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Re: Exhaust thrust

Postby srq » Wed Mar 20, 2013 6:24 pm

I'm amazed. One cylinder out is enough to make a car change lanes??? That is crazy ! I had no idea but then again blown alcohol is a bunch more wicked. Interesting stuff about the outlaw circle track guys too. Theres potential here but don't know how I want to harness it yet.
The engines I have in question are 600 cui gas n/a. Some offshore boats run tractor headers which sound conterintuitive after hearing these replies. Most run straight out the back in efforts to reduce the low presuure area behind the transom . I have other ideas.
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Re: Exhaust thrust

Postby srq » Wed Mar 20, 2013 6:35 pm

One quick question and hopefully not to far off the track but if one cubic foot of air goes into a gas engine and complete combustion occurs how much expansion/gas comes out the exhaust? Without trying to go into a lot of math I thought someone might know the approximate expansion ratio off the top of their head.
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Re: Exhaust thrust

Postby DaveMcLain » Wed Mar 20, 2013 6:40 pm

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Re: Exhaust thrust

Postby BrazilianZ28Camaro » Wed Mar 20, 2013 6:52 pm

Ok here is my SWAG:

A blown engine making 1000 HP burn 1300 hp worth of fuel so, theoretically the blower is drawing 300 hp to pump the air to feed this engine.

Now this same engine with a pair of --more efficient-- turbos, makes 1100 hp. This means the turbos are drawing 200 hp at least to drive the turbines to "pump" X pounds of air into the engine.

The 200 HP+ of force is coming from the exaust gasses expansion, in first place.


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Re: Exhaust thrust

Postby srq » Wed Mar 20, 2013 8:21 pm

I guess I was asking one cubic foot of fuel and air ignited becomes about how many cubic feet of exhaust at ambient pressure.
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Re: Exhaust thrust

Postby MadBill » Wed Mar 20, 2013 9:08 pm

The governing principle is Newton's Third Law of Motion: "Every action has an equal and opposite reaction", so the force of the exhaust being directed upwards causes a reaction force driving the exhaust pipe downward.The equation is F = MA, so what's needed to compute it is the exhaust mass and the acceleration (from zero vertical velocity to whatever). Mass will be ingested air plus fuel, or 14/13 x CFM x 0.076 lb., but acceleration is a little trickier as it depends on exhaust gas velocity, which in turn depends on density and exit pipe diameter. For example, compared to a typical collector, a proper rocket nozzle would increase thrust at the expense of power lost to backpressure.

I have a policy of not doing math in public, so someone else can take it from here... :)
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Re: Exhaust thrust

Postby rookie » Wed Mar 20, 2013 9:16 pm

It clearly makes enough that some teams in F1 is using it to create down force by blowing it over diffusers.
http://www.formula1-dictionary.net/diffuser_blown.html
More here.
https://www.google.com/search?q=exhaust ... =firefox-a
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Re: Exhaust thrust

Postby crazyman » Wed Mar 20, 2013 11:36 pm

Motorcyle racing teams use this principle to help their cause too.
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Re: Exhaust thrust

Postby BlackoutSteve » Thu Mar 21, 2013 6:31 am

Ask Don Garlits.. He found out how much downforce/traction zoomie headers were worth back when they were running just under 200..
In fact, I think the zoomie header was one reason they got to that milestone.
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Re: Exhaust thrust

Postby Cubic_Cleveland » Thu Mar 21, 2013 7:22 am

rookie wrote:It clearly makes enough that some teams in F1 is using it to create down force by blowing it over diffusers.
http://www.formula1-dictionary.net/diffuser_blown.html
More here.
https://www.google.com/search?q=exhaust ... =firefox-a


I think F1 was using exhaust gas to help the diffuser create downforce, not directly for downforce. From what I understand, it was of most benefit in corners where the driver was off the throttle (or at most part throttle), and the throttles are held open, with the motor only acting as an air pump to help the diffuser. Spark is cut to some cylinders to reduce power. You could hear the difference from 2011 to 2012 seasons when it was outlawed. But I could be completely wrong :oops:

I do find exhaust thrust extremely interesting! I think I remember Darren Morgan talking about it on here in relation to a salt lake racer project.
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