80FB

Evening

Im not a huge poster on here but was wondering if some one can help? Im having a J Port 12a built for me with 1pc carbon apex seal's

and was wondering if there is some one help me develope my nikki to cope with fuelling? i a mechanic by trade so have sorted most of the rest of the car and happily removed and rebuild everything but this carb seams a little trick to modife i would normally stick a webber on it but i thought i would try and give the nikki a chance

i dont mind sending it to a specialist to sort out, but living in the UK not many nikki specialist

many thanks guys

liam

PercentSevenC

J-port, huh? Don't think you're going to be able to build a Nikki that'll feed that engine satisfactorily. Even the mighty Sterling Nikki only flows 465 CFM. I'd suggest either a 600 CFM Holley double-pumper on a Racing Beat Holley intake manifold (the 12A bridge-port one) or a Weber 48 IDA.

81WideMariah

iTrader: (8)

J-Port is going to need some serious flow.... I doubt a 48 will keep up.... I'd recommend a 51 IDA.... they cost a little coin though.

trochoid

iTrader: (6)

If the engine is J-ported in the secondaries only, find a dual Weber DCD set up. There's one for sale in the 1st gen parts section. It will drive like a street port on the primaries until the secondaries open up. Then hang onto your ***.

PercentSevenC

What kind of airflow are we looking at for a 12A J-bridge, anyway? I guesstimated somewhere in the 600 CFM range, and the 48 IDA will do 620 with the right venturis. I wonder if a 51+ mm carb would be worth the extra cost.

81WideMariah

iTrader: (8)

really depends on the HP goals and the intended use... also powerband will come into play... CFM at 8k is alot different than CFM at 10k....

ultimately he'll get the most power from a 51... but driving manners may be better with the 48

80FB

im looking for sterling's email (??) to see what he can do and a price for his work. was wanting to keep it a stock looking as possible but if its not possible then i will have to spend out for a bigger carb/inlets

power and intended use are fast street with a weekend drag. would love to keep the nikki to then whip a weber on the strip if possible.

orion84gsl

iTrader: (5)

Sterling isn't taking any orders right now, and may not for quite a while. However there is a lot of good info at his website, www.sterlingmetalworks.com. You can also speak to him on his forum that's linked there as well.

80FB

is there any reason for him not taking any order's?

84stock

iTrader: (5)

It's a hobby business for him. Built one for himself, then another, then another for someone else, and soon the demand got out of control due to the success of his work. He has a career and a family and chooses his obligations well.

j9fd3s

iTrader: (3)

you're basically building an IMSA RS engine from the early 80's. very old combo. the stock carb limits power a little. the airflow mods were all developed for that.

in the 90's paul yaw worked out the drivability, and sterling made it under license.

the basic mods are to pull the venturis out, and machine them larger. the other mods are actually pretty obvious once you look in there, the throttle shaft screws are way too big, etc

the jetting is the hard part, sterling machines the air bleeds and such to take holley jets, which is pretty precise work.

not knocking sterling at all, btw, our local shop teacher showed me his modded nikki and not only is it about the same as a sterling, but he's been building em for a couple decades.

his shop car since you ask, is a rx3, bridgeport 13B with a modded nikki. they have an FB setup the same...

Siraniko

iTrader: (13)

we used modified RX-2/3 nikkis on bridge back in the days BUT there is a difference between running at the track versus street driving. And tuning is a bitch also. its easier to just run a 51-IDA (of course, holley 600 or 650 but tuning is not easy also).

Cost wise, it will be between $150-$250 to enlarge a 48IDA to 51.5. Same mods that I do to my IDAs but the casing gets machined and you need venturies that are made for a 51. Although you can use a venturies from a 48IDA, there is a big difference in performance.

Crispin38

iTrader: (3)

I would strip your nikki first and try it and then go from there. I have a monster port 12a with a modded/stripped nikki and I'm even running vacuum secondaries and i love it. Even over my weber.
To each is his own, I'm just saying that you should try it completely stripped first and go from there. Just because we might have huge port work and the carb won't flow nearly enough doesn't mean it won't run damn good and be damn fast.

Siraniko

iTrader: (13)

waste of time IMHO

ultimatejay

iTrader: (1)

Wow you're still around? I thought you sold all your rotaries and went to build some Hondas or something.

What's up Wackoff?

80FB

have you got a build thread? or some pictures of the carb?

cheers

Siraniko

iTrader: (13)

wahts up Ultimategay!!!

Crispin38

iTrader: (3)

Hey 80FB.. Send me a PM and I'll try to give you all the help I can with pics and stuff of my nikki!

vipernicus42

iTrader: (4)

The first thing I'd ask is what class you're racing in. They probably have very specific requirements for carburetors, airboxes, exhaust, etc...

I assume you're building this for racing because a J-port with carbon seals is going to have really poor around-the-city-style driveability no matter what carb you put on it. From what I've read, carbon seals don't seal well at low rpms, and a j-port isn't going to idle under 2,500rpm anyway because of the overlap. When it comes to fuel, it'll drink more than an Irishman on St. Patrick's Day!

But to answer your question (since I'll assume you've done your research just based on the fact that you know what a j-port and carbon seals are), I'll echo what folks have said here about Sterling being the undisputed authority on Nikki carbs. The knowledge found in his posts here and on his own website and forum is incredible. I don't know if a Sterling is "suited" for that motor, but I don't see why not. No matter what size port you have, you can't flow more air than the displacement of the engine. Your volumetric efficiency doesn't exceed 100% except in very rare situations (and forced induction) and 465cfm will flow enough for max VE on a 12a, so I'd say go for it.

But since Sterlings are currently unavailable, if you're comfortable with Weber carbs they're definitely available for the 12a. Grab one of those and go nuts.

Jon

PercentSevenC

That CFM formula doesn't work for carb sizing. It's been proven time and time again on the dyno. If it worked, no one would ever see any gains from a carb larger than 600 CFM on a 350, which clearly isn't the case. The problem is that carb airflow ratings typically assume a 1.5" Hg pressure drop, which is actually quite large, even for a street car. If we calculate the Sterling Nikki's airflow at 0.5" Hg, a more typical value for a racing engine, it only flows about 268 CFM. Hmm.

Also, for the record, VE for a large-ported rotary can and often does exceed 100% peak VE.

80FB

no class racing pure street car and run what you brung machine, i dont really understand your arguement on the nikki, i can fix cars all day long but my knowledge of the theory behind carb's, air flow, air pressure aint brilliant sorry.

j9fd3s

iTrader: (3)

i read all the same things and then actually BUILT an engine with carbon seals and mine is a P port, but close enough right?

well IF the seals don't seal as well, its not obvious, the P port starts right up, just like your stock 12A, and its actually harder to flood.

also the P port drives quite well too, it'll idle as low as 750rpms. although the WEBER is happier @900

the "bad driveability" thing comes from the people taking the race engine and putting in the street car. the road racers don't care about anything under 5000rpms, and except that it idles.

turns out if you spend a little time tuning the carb, it runs surprisingly well

Crispin38

iTrader: (3)

Doesn't matter what kind of porting you have, if you spend a little time on the carb you'd really be suprised how 'Streetable' a 'race' motor can be..
My monster Port wont idle close to 900 rpm, in fact it'll barely do double that even with my nikki. But, it takes off without even giving it any gas, and is very streetable. And suprises everyone when they go for a ride. That pretty much sums it up.. There's alot of 'variables' to streetability.

80FB

thats nice to know i was looking at the having an engine idle nr 1500rpm, but if can idle lower then that would great, will be really intereseting in seeing how the nikki handles the port