K-Tune

iTrader: (26)

dashpot?? new word to me.

CrazyJoe12a

Well I didn't mean to come accross as a smartass, I just disagree that popping flames mean the car isnt tuned right....

Gregs

the only time my rex shot flames was when i revved it to 6k and killed it while parked...

shot a goot foot and a half flame and sounded like a 12 guage

scared the **** outta me cause it didnt happen till i was outta the car and walking behind it... lol
good thing i was on the driver side..

-greg

hornbm

Well, the Renesis is currently making horrible gas mileage. About 14-17MPG Alot of people say worse than some FD's. I guess it works in theory, but in real life, I guess not.

Sterling

I, uh, (heh-heh) blew my home-made exhaust dump apart. I also blew the pipe apart.

...Guess I should've had it welded.

rotary emotions

Hmm, first thing I hear about that. Doesn't sound good.
We still don't have RX8's here, so I can't judge. Testers claimed a seriously improved milage, so I believed them.

Anyway, they probable don't shoot flames

I still think it's so silly to try to make flames. I just want the best compromise between milage/power/reliability, and if that means flames, so be it. If it means no flames, that's ok aswell.
And again, don't compare racecars with street cars.

rotary emotions

EFI race cars blow fuel through just aswell. They actually do it on purpose. By keeping the intake gas mixture rich on decel, they avoid it getting poor when accellerating again. For them decel is not important, but leaning out would be. So they keep pumping fuel in. Because of the velocity of the intake gasses and the huge valve or port timing overlap they go straight out again on decel, burning inside of the hot exhaust. There's no reason for us to do the same though.

Narcisse91

So on an ITA car, with the stock port, a Yaw carb, and a straight through exhaust, the flames they'll shoot is because the overlap is huge? I watched one do it all weekend shifting from 3rd to 4th on the back stretch at WGI.

Metallic_rock

I was watching a video put out by Re-Amamia... all FD's Everysingle one of them popped a 2-3 foot flame every time they shifted. They were all street/track cars.

rotary emotions

A straight through exhaust hasn't got any backpressure, so lift of throtlle creates a vauum and sucks the incoming mixture out. This is indeed ONLY because of the nature of a stock ported aswell as a race ported engine. A Mazda rotary (pre-renesis) has the exhaust as a PP, while intake is side housing. Ever taken a look at how many degrees of overlap that gives? The exhaust port is serveral cm wide, yet only a 2 or 3 mm apex seal has to close it! As long as the apex seal is not entirely past the exhaust port, intake and exhaust are connected. Because of free flowing exhaust and intakes, more gasses come in then usefull, and go out again.
When shifting the rpm goes down when you clutch (unless you keep your feet on the gas), slowing down the engine. But the velocity of the exhaust gasses suck the rest out of the engine.
This is part of the reason why Rotaries are a bit thirsty.
PP engines are even worse, and if you look at them, you clearly see why...
I cannot say anything about Paul Yaw carbs, as none of us uses them here. We all go for Weber, Dellorto or Mikuni.
But that wasn't the question of course. The question was: how can I make my 12A spit flames. And I still stay it's plain idiot to even ask that question. WHO wants flames??? 12 year olds? So I claimed that on a stock (more or less stock) engine the exhaust shouldn't spit flames. If it does you can get rid of them by adjusting ignition and carburation.
That's all there was. You guys start comparing your little street car with a racer... well, then also know that race engines don't last that very long. They do make power though...

Sterling

Your opinion regarding others' facination with exhaust flames was never asked for. You seem unreasonably mature for your age.
You must really be alot of fun.

To even concider the rotary because you, "just want the best compromise between milage/power/reliability..." is silly in itself. The rotary engine, Renisis or no, is not likely to become a very fuel efficient engine without the use of some recycling component attached to it.

I also am not quite certain that you are quite certain that what you speak of is in fact true with regards to 'flame shooting', Nor do I myself claimed to be the be-all and end-all of this topic.
However, it is this statement that leaves me guessing as to just how deep your vast well of knowledge really is;

"EFI race cars blow fuel through just aswell. They actually do it on purpose. By keeping the intake gas mixture rich on decel, they avoid it getting poor when accellerating again. For them decel is not important, but leaning out would be. So they keep pumping fuel in. Because of the velocity of the intake gasses and the huge valve or port timing overlap they go straight out again on decel, burning inside of the hot exhaust. There's no reason for us to do the same though."

It is my understanding that fuel conservation during a race is rather important. It is also my understanding that fuel injection utilizes great fuel pressures, and that there should be nothing to worry about with regards to "avoid(ing) (the mixture) getteing poor when accelerating again.". It is also my understanding that fuel injection is controlled by a computer (rather quick, those things tend to be).

You mention over and over the overlap that causes this exhaust combustion, but you never say anything regarding how the actual mixture itself is regulated at the times that the engine is ingesting it.

Overlap may very well be the primary reason for flaming exhaust on decel on fuel ie racing cars, but the question was put to the 12a carburetted engine. The port overlap has much less to do with an overly rich mixture during decel on these cars than the carburetor idle circuit seeing an undo amount of vacuum signal.
The decelleration dashpot is a device much like a shock absorber that a tongue on thew carburetor primary shaft linkage makes contact with upon decelleration. As it makes contact, the piston in the dashpot compresses slowly to avoid the very situation I'm talking about.

For all of you who want to shoot flames out of your headered exhaust, go remove your decelleration dashpot, install larger fuel jets, smaller airbleeds, a larger fuel pump and regulator, and have some fun!

If a flaming exhaust is a problem for you in a street car, then perhaps the dashpot is in need of adjustment, along with the carb mixture screw.
But remember, when you let off the throttle and that dashpot is'nt there to let it down slow, your engine is at high RPMs and is acting like a vacuum pump. Only if it were completely sealed would it behave as though it were in a vacuum (air free) environment. But because the idle circuit is exposed to the engines demanding signal, it will slurp fuel right in.
And if you're running lean in the mid or up top where you spend most of your time, then you're heating up your exhaust just right for the rich idle mixture to hit those pipes when you let off the pedal to shift. Remember, the brakes are vacuum operated, so they are relieving some of that signal; as well as the fact that while braking on a curve you are often using your engine to slow the tranny, too. Backfire is less likely at this point.