scathcart

Who cares what you think?

No one asked what you think.

They asked what you know.

Which, apparently, is nothing on the subject.



So, if you don't know anything on the subject, keep your yap shut.





Dry vs. Wet.



The reason I prefer wet is for several reasons.

Dry shots increase the fuel delivery via an FMU, a fuel management unit. Basically, to get the injectors to pump in more fuel, they jack up the fuel pressure. This would be alright, except that, as fuel pressure rises, fuel flow from the pump decreases. Even with an upgraded pump, this can become an issue on big shots. Throw a fuel pressure gauge on the car with any big hp shot, and watch the fuel pressure skyrocket. Over 100 psi of fuel pressure is not unheard of.



Eep.



This excessive fuel pressure is not at all what we're after. Sure, it adding more fuel, but HOW is it doing this?

Have you ever noticed that the static rail pressure of pretty much every EFI car out there is 35-45 psi? Think there's any reason for this? You know the manifold reference vacuum line? Its job is to maintain this pressure differential.

Below this pressure level, atomization suffers. The fuel just dribbles out like an old man taking a ****. Above this pressure level, atomizaton suffers. Fuel just winds up spraying directly onto the opposing runner wall. The overall combustion is less efficient from the poorly atomized fuel at this high fuel pressure, and the runner walls are literally getting soaked in fuel... this presents a LARGE problem of nitrous backfire.



Eep again.



Laslty, our cars have very high charge velocity travelling through the runners, and with our rotors constantly drawing an air charge (as opposed to a 4-stroke engine's intake valve remianing closed for 3/4 of the crankshaft rotation), we are much less like to see fuel puddling at our valves. And with our VERY long intake runners, we can give the fuel plenty of time to atomize, and completely avoid corners. Most piston engines HAVE to put the foggers right next to their intake valves.

With our such gradual runner corners and our very long intake runners, fuel puddiling is not an isue at all. Nitrous backfire is a non-issue with properly placed foggers.

Notice I said properly placed. All large shots should have the foggers mounted AFTER the TB. The TB, especially in stock form, does tend to collect some fuel, especially with the sharp turn at the throttle body elbow all of the stock systems present.



To sum it all up: Dry shots suck b/c they lower the pumping volume, which can cause you to run lean, and they cause excessive fuel pressures which lead to fuel puddling. Wet nitrous, when properly placed, offers much better atomization and power gains, with minimal risk.

RONIN FC

Not defending the FMU by any means.

But can you clarify this a little better To me it sounds like your saying the pumping volume is related to running lean in this situation.



And these two statements sound like a contradiction;



To me it sounds like everyone who has run into problems with a dry kit has no fuel mods other than an FMU and a pump. Pulse rate is still at a max of around 60% (+ -) at full duty, and your not effectively adding fuel where it needs it most with this method.

scathcart

I agree, they do, don't they?

Let me clarify.

When running an FMU, with the higher rail pressures, atomization suffers because the fuel injectors, which face 90 degrees against the air flow through the runner, physically force the fuel to be sprayed onto the opposing runner wall. This is what I meant about FMU's.



The other statement about our high charge velocity was meant to be a comparison to 4 stroke engine's, not to the rotary itself.

Wet systems are known to cause nitrous backfire in piston engines. With our higher air charge velocity, this is never an issue with properly placed foggers.



Make sense now?

One was a descriptive of our fuel systems, the other was a comparison to short-runnered piston engines.





I agree, to a point.

Zex kits are the most common dry kits, mostly due to the simplicity of their install. THIS is what causes so many problems. If you made a bolt-on turbo kit for an FC that only included the turbocharger and adapters to fit the AFM, exhaust, and turbo outlet to the TB, hundreds of people would blow their engines.

Heck... look at FCD's. How many engine's have those killed?

When you install a product, you should always know how it works, and to ensure it is working properly. Dry kits are so simple to install that it requires no understanding of how nitrous works, and without ensuring the system is operating safely, many problems follow.



Ignorance is the #1 cause of all car problems.



Here's the flip side of my point:

Dry nitrous kits work great for 50-75 hp shots of nitrous. Big power shots... the fuel spraying onto the opposing runner walls becomes too much of a problem.

I always reccommend wet. No problems, period.

RONIN FC

Still not adding up, Ok, lets say the pump is cavatating but supplying 100 psi. At what point do we get low pressure or volume? When the RPMs go up and consume more fuel reducing the cavatation?



I keep getting double talk, "The fuel cant puddle" "fuel will puddle if..."



Or "Pressures too high!!". then "you cant make enough pressure and run lean"



I guess I will never know why people condemn the system.

scathcart

Pumping efficiency goes down as pressures go up. A pump which supports 400 hp at 40 psi may not support 400 hp at 100 psi. If we're using a massive pump, that's one thing, but most people are merely dropping in a walbro and calling it a day. At such high rail pressures, the walbro could fall short.



If the engine is basing its fuel calculation at 100 psi, we need 100psi of fuel or we will run lean. If the total volume of injected fuel is higher than the total volume pumped, the rail pressures will drop and the engine will run lean.

All pumps will pump less fuel at higher rail pressures.







We want a ~40 psi pressure differentil all the time between the injectors and the manifold. This is why the pressure drops under engine vacuum, and why it increases under boost. At 10 psi of boost, the rail pressure will increase by 10 psi. This is because the boost in the manifold will also be pushing the fuel back into the fuel rail. At 40 psi of boost, we need 80 psi of fuel pressure to maintain this pressure differential of 40 psi. If we only had 40 psi of rail pressure at 40 psi of boost, the engine would get zero fuel, even with our injectors at 100% duty cycle.

If we jack up the fuel pressure, but don't jack up the opposing force (boost), we end up killing our atomization by spraying all of the fuel onto the opposing runner wall.



We want to avoid excessively high rail pressures.

1. Its hurts fuel atomization and promotes puddling in any engine.

2. It decreases the flow volume of our fuel pump.





When I am saying the fuel can't puddle, you have to realize I am talking about completely different scenarios.



If we jacked up the fuel pressure and ran a wet system, we'd still have fuel puddling issues. What I am saying is that:

On rotaries, with properly placed wet foggers nozzles, and standard fuel pressures, fuel puddling will not occur due to our very high air velocity. This a comparison to 4-stroke engines, where using a wet shot can cause fuel puddling at standard rail pressures.

Our very high air charge velocity can NOT overcome the puddling problems caused by excessive rail pressures.



The entire problem is the excessive rail pressures. Avoid them!



I condemn the system b/c I have seen the failures it has caused on these cars, and witnessed several tests demonstrating what happens to the air/fuel mixture as the fuel pressure differential increases.

petex

s4 or s5 block? stock s4/s5 block can take 300hp no problem, it's been done zilion times with PP setup running alcohol.

RONIN FC

With this chart your supplying, and the generally reguarded 0.5lbs/hr of fuel per horse power @100 psi you should have no problem supplying fuel to a rotary with most anything done to it. Do the math , I think you will be surprised..I think you should be condemning the FMU like I have many years ago. There has got to be better ways than to jack up pressures to fuel the system..

rowtareh

how much of that s4/s5 block do you think is stock?

scathcart

I have always hated FMU's (and RRFPR's). This is nothing new.

We're on the same page when it comes to FMU's.