ITB size for Road Racing
 

Looking at ITB setups and not sure what I should be looking at for size. I see everything from 40mm to 62mm and not a whole lot of information for racing application.

I do NASA sprint racing, looking to do some car updates this season, go to a 5.5" clutch and allow higher RPM. Engine is currently an NA frankenwankel 4-port, rotors are setup for high RPM but not crazy high (FD rotors with scallops, balanced).

Thoughts?

assuming you're looking at a single (twin throat), i would say 50 or 55. any porting?

all of the 80's Mazda race cars were 48mm, so 48mm to 55mm is probably about right.

if you wanted to get scientifical, the carb cars used the same venturi size as the runners, and then the throttle plate is bigger, so @wot it is probably a lot like a straight pipe. the 12A was 43mm venturi and port runner, with a 48IDA. the 13B is 46mm venturi/runner, and either a 48 or 51 throttle.

Dia: There is a medium-to-large streetport on the setup and the rotors are scalloped so it works out to be a large streetport. Its torquey for a rotary but the intake is choking it at higher rpm. It builds power up to 7200 and then falls like a rock, I think its intake related.

j9, Do you know if Mazda was running bridgeports or P-Port? My only knowledge of early Mazda history is they learned of those two tricks quite early.

They used both. The MFR engines were peripheral port. The Group A rally engines were bridge port. The 70s race engines were bridge port.

The original "combi port" engine from the 60s was a bridge port, if the drawings are anything to be believed.

If you think a street port is torquey, you need to try a bridge :)

Interesting, did the TB size change for bridge vs periph?

Oddly enough I have been around many different rotaries but never a bridge port, I think the scallops is best compared to the half bridge but the on the correct side of the advantage trade-off.

nope! they all either ran a 48IDA or the factory mechanical fuel injection setup.

since you're a flabby* side port, you could actually find a 4 barrel TB, like a holley, like PJ did, that is the same venturi size as the runners, and then a throttle that is bigger....


*all side ports are flabby, peripheral is just better everywhere

The advantage of bridge and peripheral is that you can get way more port area without having to close super late. Really, IMO you shouldn't ever close later than 60, maaaybe 65 degrees ATDC. The gains at the top end don't offset the losses at the bottom end. My ports close around 65 and if I was going to do it again, I'd close them not much later than stock. It pulls WAY beyond where I feel safe revving it. I make useful torque pretty much anywhere over 3000rpm but best times at the dragstrip are with shifting at 10k mainly because I am afraid to shift higher than that. I already have enough torque that I can't use the Mazda rears anymore, am in the process of putting a Ford 9" wih 31 spline axles in it, which just means I will be free to make more oopmh and so I can start breaking those :) So if I could close the ports around where stock is, I could make even more axle-shredding torque and also get the RPM band down into a range where I'm not worried about the clutch sawing the car in half.

Did I ever tell you guys about my experiences driving RX-8s? By the numbers, my bridge port makes the same power. By feel, RX-8s feel like my engine at half throttle. Just totally gutless and not very zingy at all.

I stand corrected, based on that sentence scalloped rotors have more advantage then I thought. :egrin:

I think you misinterpreted - RX-8s feel like if I was driving MY CAR at half throttle...

I read you loud and clear, my statement stands.