camronforney

Hello all I am new to rotary motors and have been building a pro4 late model to race in the northwest. The fords have been dominating and I want to build something different. The rotary powered cars used to do well in the Boise area but have disappeared. I race with the northwest pro 4 alliance and will race at tracks ranging from 1/4 mile to 5/8 mile.

The rules state that a 12a non ported must weigh 2000 lbs 12a ported must weigh 2100 lbs and a non ported 13b must weigh 2200 lbs. intake must be Holley two barrel or Weber side draft carb's no forced induction or other funny business. I believe a peripheral port 12a would be my best bet. Im thinking I should be able to get to 275hp with a pp 12a. Is that a reasonable expectation? What mods can be done to a 13b or 12a without porting that can improve hp and what could I expect for numbers?

The fords are making in the ballpark of 275hp with the svo ford head and have to weigh 2100 lbs and are spending close to 16 to 20k to do it. The stock head engines are making about 245hp and weigh 1950 lbs. So that's what I'm competing against. Do you think I have a shot at being competitive with a rotary on a smaller budget or am I looking at spending the same or close to it?

peejay

I agree, looking at those rules, a ported 12A would be your best bet. Do they define what size sidedraft carb is allowed? Single sidedraft or two?

camronforney

Heres the rules.

Holley 2brl. /Webber Side Draft and Mikkuni Slide/Side Draft carburetors will be the only
carburetion permitted. No down drafts, Predators or Holley 4brl.

Rotary Motors
1. 12A Stock block must weigh 2000 lbs.
2. 12A Ported block must weigh 2100 lbs.
3. 13B Stock block must weigh 2200 lbs.
4. All rotary powered cars must use a two barrel carburetor with 38mm chock and meet all
noise rules.

peejay

The 38mm chokes will kill the power. I don't think you will be able to see 275hp with that. I know 38mm chokes were used in ITA to limit rotary power, and people found that you could modify a Nikki carb and manifold to flow more than that (involved completely redesigning the airbleeds/fuel circuits and running an open plenum under the carb)

camronforney

What do you think I could expect with the 38mm choke? I emailed the promoter of the class for clarification on the carb rule. I believe that they allow at least the holley 500cfm 2 barrel. What could i expect with that for carburetion? The ford guys dont seem to have a problem getting to these numbers with the holley.

camronforney

Heard back from the promoter and he said disregard the carb rule for the rotary. Carb rule will be same as everybody else. Holley two barrel or mikuni/weber side draft allowed two or four venturi any size.

peejay

Ooo... then you can make 275 no problem. You'll have to shoot for more than that so that when you quiet it down to acceptable levels (whatever those are) then you could still be competitive. Bigger ports hate restrictive exhausts so it will be a balancing act.

The subtext I am getting is "We need more people so we will look the other way if you don't meet the rules as written" but if you start winning hard then things may change. Which IIRC is the reason why rotaries are usually banned from circle track altogether.

j9fd3s

iTrader: (3)

i believe the ITA guys were running, well it would have been a 6 port 13B, with a 48IDA and 38mm chokes, and they were around the 210-215hp level

if you built a 13B peripheral engine, to the MFR specs, it should start out at 300hp@9000rpm this is flywheel, i believe that is with 42-43mm venturi's, in a 48IDA. the 12A was rated at 265hp@9000, with wet sump and a carb, the dry sump injected engines were 280…

i guess what i'm saying is that 275hp from a 38mm choke seems a little bit of a stretch…

on the other hand, i don't see how you could spend 20k building a rotary, even if you bought all the fancy aluminum side housings and everything else was new from Mazda you'd still be under 10k.

camronforney

He said disregard carb rule it will be same as for all others and they dont tech them. They just look and say yup its a holley or yup its a weber or mikuni so any size weber is legal and I can run two. As long as its one of those they don't care.

They also said I can use narrowed 13b housings to make it a 12a since the 12a housings are hard to come by. Any porting allowed so peripheral port is ok. Fabbed intake is ok.

Noise rule is 90db at 100 feet. Is it going to choke it down too much? They don't really tech this either but I don't want it to be so freakin loud my ears bleed in the car.

I hope to find a decent 12a complete that I can rebuild and do the peripheral port at my friends machine shop. He said it looks super easy. Just need to know where to put the hole for timing and if I should go straight in or at an angle. Straight in seems to make a longer period of time that the intake is open and further from the exhaust the less over lap. Im thinking less overlap will make more torque less peak hp same thing with angling the intake ports down slightly. Am I correct in my thinking on this?

Thanks for the help guys!

j9fd3s

iTrader: (3)

my 12A peripheral is under 93db on a clear day, the important part was the air cleaner. the exhaust is just a 3" with a borla in it, nothing special at all. when i redo the exhaust, i will probably go for a larger bodied muffler. the other thing that works well is to split the 3" pipe at the end into two smaller tips. i have this resonated tip i cram in the exhaust when i move the car around, and it makes it quieter than my turbo Rx7.

the air cleaner makes a HUGE difference in the noise level, i run the stock 12A one, which is made from thick steel, and when i start the car without it the neighbors come out of their house holding their ears.

Re the P port. Mazda has tried a few port timings over the years and here is the skinny. straight in makes more power. the aftermarket JDM housings use a straight tube, and then the housing casting actually does the rest, so your milling is for two sizes. i can get pics if you need em.

the factory 12A engines ran a 44mm intake port, and the 13B ran a 46mm.

Mazda did some studies of the port timing, and their findings agree with your suspicion. the port timing got milder later on and power went up

the late engines ran a port timing of 80Io/80Ic and i think 73 Eo and 55ec. if you look you'll find the early port timings which are something like 100io/75ic and 75 or 80 eo/65ec. they found that the rotor blocks the intake port, so opening it that early doesn't add power.

you should play with intake length on your specific engine, when you get there, but went from 400mm to 370mm, and then they built a telescoping system, which could add i think 400mm?

the FD engine clearances and specs are the same as the race engines, although you might want to add a little rotor tip to side housing clearance.

i'm not at home right now, but we have collected all the SAE papers and some of the parts catalogs for the race engines

Mazderati

iTrader: (3)

j9 dropping the knowledge as per usual. Awesome.

mustanghammer

iTrader: (3)

I think you guys are referring to EP cars not ITA cars. In EP they currently get 42mm chokes in the 2bbl carbs or a Nikki with enlarged primaries. In ITA they have to run stock Nikki carbs on a stock, de-smogged manifolds.

I'm with Peejay, the carb size restrictions return when you start kicking piston ***!

For mufflers on the race cars we run 3" magna-flo mufflers with the packing replaced by lava rocks. They meet SCCA/NASA sound and the packing lasts a long time.
kis.