overrun

[woodyracing]

Can someone explain engine overrun, when you see racecars blow out flames from the exhaust?

and is it a good thing or just a waste of fuel ?
and why is it more common in turbo cars ?
also why dont most road cars produce it ?

thanks

[RWC]

I read about this just recently.Forgot the exact details but got the general idea.For turbo racecars they burn fuel IN the exaust.The sole purpose of this is to keep the turbo spinning when the throttle is closed so that it is ready to do it's job when the throttle is opened again.So it cuts down turbo lag.I can't remember though if they inject the fuel into the exaust or just keep the injectors going with the throttle closed.Can't remember why F1 cars do it though.

[woodyracing]

yeah thats the anti lag system in turbos, but turbos without anti lag and engines without turbos still get the engine overrun. I heard it was something todo with the fuel pump or non return valve or something ?????

[THR]

the reason turbo cars do it is different to other cars i think.

its advantagoues to see flames out the rear of the car. i cant remember y, but it is!

[KC]

I think a lot comes from the amount of valve overlap the cam has. When the engine is running hard it is consuming a steady amount of fuel. If the throttle is lifted off abruptly, the fuel flow does not reduce as instantaneously as the engine slows down and unburned fuel is expelled into the exhaust system. The unburned fuel may be set off by ultrahot exhaust pipes or maybe by the glowing exhaust valve. Its common in almost every race car I have seen in person, from Ferrari 333SPs to 360 cu in late models on dirt tracks.

[woodyracing]

so if I get some race cams, my road car could do it ?

[chunder]

It is especially common in turbo cars because you have to run the mixture so rich to compensate for the turbo.
I have pictures of John Welch's Opel Xtrac rallycross car that flamed out so much it caught fire.
THe exit was just before the rear wheels and it was a cool day making the charge even more repsonsive.
]John's door cauhgt alight and he had to try and finish the race, needless to say the fire got the better of him and he stopped, causing a lot of damage.
With N/A cars I think the reason is pretty much the same. Trick fuel and silly valve timing coupled with ich mixtures will make any highly tuned engine flame out.
The more the merrier I say, looks cool

[KC]

The LeMans winning Mazda 787B suffered a similar problem. The exhaust temperatures from unburned fuel in the resonators would debind the carbon fiber and eventually set the rear of the car afire. The rotary engine was bad about exhaust detonation.

Woodyracing, if you geta cam with sufficient overlap between the intake and exhaust you can get overruns pretty easy. Ford small block V8s used in the Mustang are notorious for this.

[woodyracing]

wicked, i think it looks cool. I have on video some Japanese guy lighting a cigarette off someones exhaust, mad!

Fuel does ignite in my exhaust coz it makes small banging noises but it only ignites at the front and not out the back so you cant see anything.

I will ask about some lumpy race cams this weekend.

[Lola]

At Rockingham last year my mate who drives a F3 Ralt in the monoposto championship blew the rear end of his exhaust of whilst testing because of fuel build up. We walked the entire circuit looking through the grass looking for the missing part to no sucess, but i did find some interesting stuff....filler caps,wheel nuts ,one way fuel valves etc. All good handy stuff.

[Red Dog]

Definitely the valve overlap causing it. The inlet valve is well open before the exhaust valve shuts. Exactly why it causes the flame out? I'm not sure but the throttle snapping shut must cause a sudden pressure variation in the cylinder, perhaps preventing ignition occurring there and allowing a bit more unburnt fuel mixture into the exhaust. It certainly helps to have a short straight through exhaust system. So you might have to get the hacksaw out, woodyracing!

Beware of very long period cams though. If you stick in a cam with 285 degrees or over and don't do something to seriously raise your compression, you are going to struggle in the low and mid range. Reason is, long valve opening effectively lowers the compression ratio, because as the piston is squeezing the mixture, the inlet valve is still open and some of the mixture is pushed back out. At high revs there is far less time for the mixture to escape and it generally doesn't.

Anyway, the net result is a gutless motor until you build some revs. Then you reach the red line and you've gotta change gear! But it's the flame out that you want so who cares about the power output?!

[DAVID PATERSON]

Would you believe you can actually buy a kit, that has a spark plug to mount in your exghaust to ensure any unburnt fuel is ignited at the tailpipe. I have a client who brings in a lot of lowrider gear from the States and he says it's the latest thing in that market.

[woodyracing]

ive not heard about them yet, whats next bolt on flame throwers?

Red Dog, my exhaust is straight through (well from the CAT back) but it is the standard length.
I wouldnt want to lose any mid range or power though. My local tuning garage will advise me, though they would probably recommend fast road cams.

[Slowcoach]

Actually Woodyracing yes !! You can buy flame throwers for your car in South Africa - they are legal and not outrageously expensive. (In actual fact they fit either side of the car and at the touch of a keypad you can make somebody toast). They are really aimed at stopping carjacking but they might make the opposition think twice about getting to close on the track...........

[av8rirl]

What is the point of all the work and expense to see a poxy flame come out your exhaust??? Drink some beer, eat some beans and light a fart. Cheaper and nearly the same effect!!!

[woodyracing]

Thats a bit extreme Slowcoach, wouldnt want one on my car.

I was just wondering what caused the flames but i do think they look cool. A set of cams wouldn't cost too much and Ive read it can give me another 16bhp.

[KC]

I don't think it is the latest thing to mount spark plugs in one's tail pipe. My dad did his '40 Ford when he was teenager back in the early 60s. Just rev up the engine to build up unburned gas in the exhaust and flip a switch to set it off. He said it was hard on the mufflers!

[Alpina]

When anti-lag systems were all the rage it could be quite hazardous being a a stop marshal as some cars had side exit exhausts and they could set your trouser legs on fire!

[Robbieracer]

These days of processor controlled fuel injection, flame outs can be programmed....simply fiddle the fuelling map at closed throttle/ maximum inlet vacuum according to revs, and viola

[imull]

to get an engine to rev to high levels (F1 etc) the valves must be open for longer. There is also a longer time when both valves are open (overlap). This works to clear out the smoke etc and put in the clean and new charge for teh best burn. Some of this fuel will burn in the exhaust.

Anti lag is a system where petrol is injected onto teh impellar to keep the turbo speed up and thereofr ehave it constantly on cam.

Someone asked if they can put this onto their road ca. You can but you will never get an MOT certificate again. THe emmisions go sky high as you are pumping out neat hydrocarbons and a mix of CO2 and CO.

My housemate was running the highest spen cams on a Mk1 1600 Twin Cam escort and he put it in for an MOT. The emmissions were 12 times the limit before is sent half the engine up through the bonnet!!!

[woodyracing]

thanks everyone for the info.

i think flame-outs look cool on any car,
im not going to pay loads of money just to get the same effect on mine though. I got a quote for some cams and it was more than twice the price i expected and for only 16 bhp extra. I think ive done enough to my engine already

[Barrie]

Rallycross cars are by far the biggest flamethrowers Ive seen. With there 800+bhp and anti lags. Chunders story on John(fire)Welch's car reminded me of the flames that used to shoot out of his Opel, awesome. Only matched by Sven Lestander's brutal Quarttro S1, J.A Iversons RS200E, and by far the most impressive, Martin Schanche's Escort Xtrac from the early 80's.

[Bloggy]

It may be worth your while to look at the Ferrari's in race trim when they were having trouble running a long first stint compared to Mclaren.
Ferrari had a smaller tank and on the overrun they ran the engine lean so as to improve the economy. I havn't looked but you'll probably find that during those races there was little flame-outs, comparativly speaking. Of course it may be hard to spot against the scarlet paint.

[Truckosaurus]

I was reading in Dave Walker's 'Engine Management' book that if the fuel mixture is too weak then you'll get flames out of the exhaust as the mixture isn't strong enough to ignite in the combustion chamber as intended...

[Chris Griffin]

Surely if the mixture is too weak to ignite in the combustion chamber it's unlikely to ignite in the pipework, if anything the mix will get weaker?

A few years ago a friend of mine used to race a Jag XJ12 Mod Saloon. He created flame out as a by-product of extreme injector timing (duration).

As an aside, the best case of flame-out I have ever witnessed was marshalling at Donington in the Mid 80's when the European (Mid Engined) Renault 5 Turbo boys came over for a one-off round. At the end of Wheatcroft straight literally every one of them, one after the other, created massive flame-out. Under a dark gloomy sky it was a great sight. Put a smile on my face each time they came out!

Chris