small bridge 12a/stock nikki

[sen2two]

a friend of mine has a 1st gen he wants a simple rebuild on. and since it will be open, he wants a small half bridge. no porting to the actual 4 ports. and minimal porting of the exhaust to handle the half bridge...

how will a stock untouched nikki handle this? any home mods i can do for the nikki to handle this? I told him he shouldnt do a bridge without upgrading to a holley/webber or similar. but hey, its not my car...

so, will it bo ok?

[RotaryProphet]

The bridge isn't going to increase flow very much on the intake side (generally the bridge is a last step after opening the ports up as much as possible), but it is going to move the intake open -way- forward, and without a completely open exhaust to keep backpressure to ~zero, it's going to ruin driveability pretty much everywhere. He'd be much much better off with a street port, as even a very large street port will handle these conditions better than a stock/bridge combination, and the stock nikki could be modified to run it very well.

[sen2two]

i understand the difference of the bridge/street port differences. (this is not my car)

its a drag car a friend has. he drives it to the track with the exhaust, and runs open header at the track. so back pressure will not be a problem. he wants me to show him how to rebuild a motor. and in the process, learn a bit about porting technique. hes dead set on a bridge.

my questin is simply how to make the stock nikki handle it... you say it can be, so how do i do it?

[PercentSevenC]

Strip it down, get a wideband, and play with jets and air bleeds. It's all you can really do. Really need a bigger carb to take advantage of the bridge.

[RotaryProphet]

My real point is that a bridge on a stock port is going to be slower -and- much less drivable than a large streetport. Why is he dead set against porting the stock ports?

[sen2two]

Originally Posted by RotaryProphet

My real point is that a bridge on a stock port is going to be slower -and- much less drivable than a large streetport. Why is he dead set against porting the stock ports?

well, hes a rookie to rotarys. and like most rotary new comers. they all want the brap, brap, brap sound...

im going to open the runners for him and do a decent exhaust port. nothing wild. Im pretty sure the drivability will be there and there will be gained power. im just thinking his stock nikki will have a hard time flowing the fuel it needs.

[sen2two]

Originally Posted by PercentSevenC

Strip it down, get a wideband, and play with jets and air bleeds. It's all you can really do. Really need a bigger carb to take advantage of the bridge.

what do you mean by "play with the jets"? there larger jets for the nikki?

can any porting be done to the carb body/Venturis to increase its flow?

[Rogue_Wulff]

Originally Posted by sen2two

well, hes a rookie to rotarys. and like most rotary new comers. they all want the brap, brap, brap sound...

im going to open the runners for him and do a decent exhaust port. nothing wild. Im pretty sure the drivability will be there and there will be gained power. im just thinking his stock nikki will have a hard time flowing the fuel it needs.

The stock nikki will not be able to flow enough air to feed even a mild street port to it's fullest potential. A sterling modded nikki can.
The bridges won't help much unless the intake ports are opened up for more flow. The port timing on stock ports can flow more than the port runners allow, but that's not a big issue on a stock 12A, since the exhaust ports are the main bottleneck.
Using a 4bbl carb, a 1/2 BP will idle without the brappy nature, since it runs off the primary ports until the carb secondaries open. On the other hand, a webber-type 2bbl will have the diminished idle and driveability commonly associated with a BP engine.

Personally, if not opening up the existing ports to at least a moderate SP size, I'd say doing a bridge cut would be a waste of time. Any gains that could come from the bridgeports, would be lost to the limited stock ports. Especially if the exhaust ports aren't opened up enough.

[PercentSevenC]

Originally Posted by sen2two

what do you mean by "play with the jets"? there larger jets for the nikki?

Yes, Mazdatrix carries them. Dunno where you get air bleeds, though. Sterling is the man to ask about Nikki tuning.

Originally Posted by sen2two

can any porting be done to the carb body/Venturis to increase its flow?

Yeah, there are things you can do. Removing the choke, grinding down the throttle shafts, and so forth. Again, get Sterling's advice. Probably be easiest to just buy one of his carbs if this friend is dead-set on a Nikki. It'll help to port the intake manifold, too (but don't make the mistake of port-matching the manifold to the engine).

Even with all that, though, it's going to run out of breath faster than you can say something that doesn't take you very long to say.

Originally Posted by Rogue_Wulff

The stock nikki will not be able to flow enough air to feed even a stock port to it's fullest potential.

Fixed that for you.

Originally Posted by Rogue_Wulff

Using a 4bbl carb, a 1/2 BP will idle without the brappy nature, since it runs off the primary ports until the carb secondaries open. On the other hand, a webber-type 2bbl will have the diminished idle and driveability commonly associated with a BP engine.

You'll still get some brappage with the half bridge with a 4bbl, just less than you would with a 2bbl.

Regarding the ports themselves, here's what I'd do: keep the intake closing timing stock or very mild, cut the bridge in the secondaries if you must, and extend the opening timing of the primaries as far as you can while still supporting the corner seals. I bet you'll get a nice power curve that way. Obviously open up the runners as well.

Definitely spend some quality time with the exhaust ports, too; as has been said, stock 12A exhaust ports suck and high-overlap ports need to breathe. I'd port the closing edge at least out to the stock sleeve, and port them to the side as well. Keep the opening close to stock, maybe with a slight curve. A healthy exhaust port will actually help drivability at the cost of increased noise.

[sen2two]

[QUOTE=PercentSevenC;106187]
Even with all that, though, it's going to run out of breath faster than you can say something that doesn't take you very long to say.

^that made me lol...

Regarding the ports themselves, here's what I'd do: keep the intake closing timing stock or very mild, cut the bridge in the secondaries if you must, and extend the opening timing of the primaries as far as you can while still supporting the corner seals. I bet you'll get a nice power curve that way. Obviously open up the runners as well.
QUOTE]

^good advice there...