One of the reasons they aren't used on too many street cars is drivablility. Imagine whacking open that throttle and going from nothing more than soem air bleed to a big damn hole! Car is instantly going to hesitate and rebuild vacuum at low rpm. Race cars have a tendancy to be opereated with plenty of rpm all the time, so at least when the slides are opened there is a strong draw from the motor at higher rpms. Flat slide carbs were used primarily in Superbikes back in the days when teh UJM ruled the roost. (Universal Japanese Machine = transverse, air cooled, four cylinder open class bikes that EVERYBODY made.) Put a flat slide Mikuni carb on individual soft runners for each cylinder and you have the biggest allowable hole letting gas and air into your bike. Factories ALL set up like this for a while. Street bikes came with CV carbs (Constant Velocity) where there is a sliding piston to the venturi that is not connected to the throttle cable that slides open as vacuum comes up becasue of larger throttle openings and rpm. Keeps that nasty "bog - ******" from happening at low speeds from banging the throttle open too fast. Makes bikes nice and driveable, but limits airflow - not what racers want.

 

My brother in law had a Scircco, with 450 000 kms, and never chainged the settings on the MFI once. I am sure there is a way to make this system streetable, besides my carb is not working right and I only have barly any power or a lot of power.

 

i wonder if one of these could be boosted?

 

such an intelligent comment.
so if mulhullins has , then thats it . mfi is better than efi and case is shut.

hmmmmm i dont think i should be arguing with you.

 

I may have missed it somewhere but what kind of fuel governer does it have. If it has the wieghted style9like Detroit Diesel) these are fairly tunable and very accurate. but some models require a special tool to adjust depending on the model though. But if it the cat style(i think it is bosch or someone like that) the pump must b torn down to adjust. Not sure on this though. Just food for thought

 

You said that the only racers using this setup are drag racers, and I was refering to that comment. And I won't argue on the internet I'll discuss any topic, but I'll hardly waste my time argueing with anyone. I said nothing about it being better than EFI I was making a obvious point that not only drag racers use this system. LOL Learn to read LOL... I never said that it was better cause they use it LOL!!

 

Does this thing bolt on to the stock manifold, or do you need something custom built?

 

Not sure why you're talking about carbs when this is a mechanical injection setup.

 

"Not sure why you're talking about carbs when this is a mechanical injection setup." - REVHED

Well, Rev - I'll assume that even though the way you worded that seemed kinda laced with attitude that you are honestly asking for a technical description.

How fuel is delivered into the venturi is remarkably similar regardless of whether a vehicle is carbed OR injected. Since direct-to-the-combustion-chamber injection has some serious inherent limitations that currently keep it out of everything but some specific diesel models, most cars are injected some where along the intake tract. The engine doesn't know that fuel is coming from an injector or a fuel jet - all it cares about is recieving atomized fuel mixed with oxygen bearing air at a fairly narrow stoichiometric ratio. Intakes are very carefully thought out routes into the combustion chamber because they use the vacuum created by the motor to convey the mixture to where it will be ignited. First problem with that is that a motor doesn't create a nice consistent constant vacuum - it creates it in pulses caused by suction, then a valve or a port slamming shut, and the moving mixture that gets caught outside slamming into that closed door and reflecting a wave of pressure back in the other direction. Well though out manifolds can actually increase horsepower by siameseing ports so when one port closes the wave that is reflected back runs down another runner and arrives at another port just as it opens helping ram the mixture into THAT combustion chamber at slightly increased pressure. That is the origin of the term "tuning" an engine, because it is using soundwave/pressurewave theory to improve the operation of the intake phase. A similar concept is also used at the exhaust, but that is best left to another lesson. So back to the topic at hand - There are also challenges at the other end of the intake. Drivability is best when you can control the size of the aperature at the inlet end of the pipe, because it controls the velocity of the air in the pipe. Pressure, Velocity, and Temperature are all inextricably linked by the equation PV=nRT, so if you suddenly change the opening (open the venturis, whether through butterfly or flat slide) you make a DRASTIC change in the amount of pressure in the tube. Pressure drops drastically, so velocity goes right toward zero JUST when the motor wants MORE air and fuel mix. This causes the car to surge forward for a fraction of a second while it consumes the charge that was in the intake, then hesitates before the motor can create enough vacuum at low engine speeds to start drawing mixture again and pick up revs. THAT is why both carbed and injected street cars have intake control devices to avoid sudden open venturis at low engine speeds. The reason your car came with a Nikki four barrell? The much smaller primary venturis keep the air velocity high for good drivability at low rpms. When you want to take off? Wack open the throttle and the primaries open all the way - but have you ever taken a good look at your secondaries? They are vacuum operated - not linked to the throttle cable! They don't open until the vacuum signal in the intake is strong enough to open them and demand more air.

If you actually LOOK at the picture of the system that he put up earlier, you will see that it is intake trumpets connected to a flat slide throttle (used to control the size of the aperature at the intake end of the tract, and therefore the manifold pressure and velocity) and below that the injector nozzles that atomize fuel into the intake stream. Mechanical fuel injection pumps are dead reliable, and can be set up for the same kind of fuel maps that their more evolved EFI cousins have, but those values are set in the shop and cannot be modified by sensor input like EFI. In most modern EFI setups, the lines to the injectors are kept charged at high pressure whenever the car is running and an electrical impulse tells the injector when and how long to fire. In a mechanical injection set-up like this one, the pump sends fuel pressure impulses down the injector lines and out through spring loaded injectors that are NOT controlled by electronic "firing orders" - they are simply nozzles that supply the pressure delivered from the pump. On race cars, we just don't care that much about low speed drivability, or cold running and startup cycles. It doesn't matter that the injection system isn't as tunable over a wide range of situations. It can be set up primarily for open and closed throttle, high rpm, max power operation and no one worries that it wouldn't get very good milage in 5th gear at 1500 rpm, becasue it is never going to have to be operated there.

There endeth the lesson - next time ask nicely and I'm usually happy to share any of the hard accumulated technical knowledge that I possess. Since I give it for free, the price is definitely right!

 

Mine usualy did not open for a while, then I did some tinkering and now they open... but I question their operation.

I like the price of your information, and I would like an answer to my question. I know its dumb but do you have to get a custom manifold for this type of setup, or does it bolt straight on?

 

Will somebody please hurry up and pay $800 for this non-complete paperweight?

I have some old "Racing" spark plugs for sale too. $100 each. Get them while they are hot.

 

way old school and not much of it there either, they make for a solid set up though.
efi would work out cheaper i think and more tuneable but old school rocks, give it nos.

 

Boswoj, are you saying that because of large TB openings, regardless of carb or FI, the car will stumble with a sudden application of WOT, and this is an untunable property?
I assure you that this is not the case with FI, even with ITBs.
What do you mean by "direct-to-the-combustion-chamber injection"? Something like ITBs? The 13bs from 84+ all used primary injectors directly on the housings, is this what you're referring to? I'm just not sure what are these "inherent limitations" you're talking about..

I believe people have very good success connecting the secondaries to mechanical as opposed to vacuum on the stock nikki.

So far as this ebay item goes, if you're looking for performance on a stock or mildly ported motor, I would look elsewhere. The cost to performance ratio is just not there, IMO. Great collector's piece and would be very interesting, but you're better off with a modded nikki, I think.
Unless you have a PP of course, then it probably is an excellent choice, but most of us don't so that's why I say that.

 

good point. I am thinking about jerry rigging mechanical secondaries, and removing rats nest... I wonder how much that would damage my Fuel Economy?

 

Well - I don't think that anywhere in anything that I have written did I even suggest that the properties I was discussing weren't tunable. Drivability is a really big issue to about 90 percent of the people who buy and operate cars. True car nuts who like to fiddle with old cars (last first gen brought into this country by normal channels was manufactured over 20 years age) are rare. You may like fiddling with a car that would be considered finicky and tempermental by modern car owners, and so do I. Here is a bit of heresy that I have posted on this site before: within very limited parameters fuel injection is no better or worse than carbueration. Where it gets it's advantage is it's adjustability! It can sense and adjust the fuel mixture for a variety of temperatures, loads, rpms, fuel quality, and driving styles. I can tune a carb and a fuel injection system to make the same max power at 7000 rpm at the same atmospheric, fuel and load conditions. What I CAN'T do is then make that same carb operate at max efficiency at every other set of atmospheric, fuel, and load conditions while the fuel injection can through fuel map adjustments based on sensor feedback. I love the Weber IDA 48 on my race car, but I have no illusions about it's drivability if I put it on an everyday driven street car. No chokes and huge venturi openings means it is best suited to a warm engine, on-off switch throttle operation, and high rpm wide open running. Most people that wire open the secondaries on a Nikki are people who drive in a fashion that keeps the engine higher in the rev range. At higher rpm's, the motor is able to produce a "draw" that can create adequate stable vacuum across a large opening. It means slipping the clutch a little more to get away from a stop, and shifting higher in the rev range so as not to always be falling into the iffy mixture that comes with low venturi speeds. For me, and maybe for you too, that is one of the things we LIKE about these cars. For the average driving public - it is a different story.

Your post leads me to believe that you don't understand fuel injection very well either. I'm assuming that by TB, you mean Throttle Body, and ITB maybe Injected Throttle Body? The throttle body is the butterfly or flatslide area that controls the aperature at the intake end of the tract. It is nowhere near the combustion chamber. The main problem with "Direct Injection", which is fuel sprayed directly into the COMBUSTION CHAMBER (you know, where the top of the piston or face of the rotor comes close to the spark plugs Einstein) is that it is then exposed to the combustion process. This leads to it being coated with soot, and experiencing high temperatures and shock waves. Since their sole job is to precisely meter fuel, they usually stop doing that pretty quickly in that environment, so they aren't used there very often. In second gens, I belive one of a rotaries advantages is that the direct injector can fire fuel into the compressing air charge then the seal can pass over it REMOVING it from the combustion chamber before it fires. Just my "opinion" I suppose, but I learned my mechanical engineering at WSU..........

For no name - That system was originally on a peripherally ported race car. It is set up for Mazda Motorsports factory peripheral port housings, and those curved runners go straight into some big ol' holes right in the side. It would be a pain in the azz to adapt to something like an IDA manifild, but it could probably be done with the right fabrication skills.

 

First of all, thanks for the "lesson."

Second of all, your technical knowledge doesn't seem to be all you think it's cracked up to be.

I agree that intake tuning and velocity plays a big part wether you're talking carbs or injection but if you fail to see the differences between the two you're a very confused indvidual.

Huh? Injection uses venturis? I must have missed something. Unless you're mistakenly refering to the THROTTLE PLATES as venturis.

With a carburettor, the pressure drop caused by the air passing through the venturi/booster venturi causes fuel to be sucked from the fuel bowl through the fuel jet, through the emulsion tube and into the air stream. With injection, the fuel is forced into the intake tract under HIGH PRESSURE whenever the injector is open. That's a big difference.

Yes, but the carb requires airflow through the venturis to draw fuel from the float bowl. The engine falls flat on it's face because it's starved for fuel until the rpm's rise and the main circuit starts working properly. Injection does not have this issue!

Really, I must have missed all that years ago when I rebuilt and modified the Nikki on my first Rx-7. After that it was IDA's and now EFI.

BTW, the secondaries actually get their signal from the venturis in the carb. There is no intake manifold vacuum at WOT. Thanks for trying though.

Thus endeth the lesson, next time don't be such a ******.

 

Thanks for the info... I think I lernt the most out of this one. Thanks guys

 

Ahhh ... obviously I have hurt you tender widdle feewings! HAHAHAHA

ALL vacuum is created by the motor - a carbuerator is physically and mechanically unable to "create" vacuum by itself. There is ALWAYS vacuum somewhere in the tract or the fuel mixture doesn't actually move! Pressure differential created by the intake phase of the moving piston or rotor is the root cause of the whole thing. Vacuum is much less at wide open throttle because the aperature at the intake end ( throttle bodies ) are open to atmospheric pressure, but it is still there - so you are actually agreeing with me, although your anger management handicap appears to be blinding you to it. Suggesting that you somehow understand the physics behind the operation of a carbuerator because you have rebuilt one is quite a lot like claiming to be an electrical engineer becasue you know which button turns on the TV. I spend a lot of time spinning wrenches as a hobby maintaining my racecars, so I don't look down on others who do the same, but if you want to talk theory you are going to need to hit the books a bit.

I will restate - at two inches down the intake from where the fuel is introduced , you cannot tell whether a car is injected or carbed. As I am married and have quite an attractive and willing wife I will leave the solitary genital self manipulation to you ........ ;-)

 

hah.

 

Jerry Jerry Jerry

 

they're at $930 now!!!

 

Are you actually going to refute anything I said or just spout bullshit and try to make yourself feel good?

What does two inches down the intake have to do with anything? The issue is how the fuel is introduced into the intake tract.

How about a forced induction carb setup? There is no vacuum there yet fuel somehow magically rises out of the float bowl. It's because the venturis create a pressure drop. This is one of the fundamental operating principals of a carby and if you don't understand that there is no point talking to you.

I obviously know a lot more about the operation of a carburettor than you do so I think you might have to hit the books.

 

I totally agree with REVHED! I have been the driver of a 1974 Nova with methanol mechanical fuel injection for 10 years now. With a barrel valve to control the fuel "pressure" whether you slowly apply the throttle or stand on it there is no hesitation. Sure the motor or "pump" if you will provides the vacume. The fuel pressure provides the fuel. The jets in the injector throats atomize the fuel. It is a totally different setup than a carb!! After 10 years of running this setup the only tuning I have had to do is change the main jet in the barrel valve to compensate for air conditions and atmosphere changes. No accelator pump is needed as there is constant pressure in the fuel system. I wish I had the $$ for that setup. As I think this would be a sweet system to play with. OOPs I forgot to add that through the use of lowspeed and highspeed bypass valves you can easily tune the drivability of a mechanical setup.

 

Keeps going up. Huh, must be a real piece of **** with a huge market for it for absolutely no reason at all...

 

Winning bid was $1,146.45
I should got the other one last year for $800.
Oh well.