Specs.
New bridgported 12A
Webber 48 DCOE
110 octane race fuel, non oxigenated
Direct fire, DLI, the Jeff 20b system.

Running SCCA GT3.

How much advance?

Some have told me stock, others up to 20*

Where should I go with this?

 

I would contact Mazdaspeed Motorsport Development at 949-222-2652, they have been a wonderous help to me this year to get my bridgeport going.

 

Anyone telling u to use stock timing on a bridgeport should be ignored because they don't know what a race ported rotary wants.

My 12a Jbridge made peak hp with 30*L 22*T total advance measured @ 400rpms but I didn't like that much timing so backed it down to 28.5*L and 20.5*L

Althought I used the Yawpower Direct fire at the time I have since gone back to stock wiring with exception of MSD6AL on the leading. Many peoples say Direct fire makes less peak hp on a race ported rotary.

It doesn't need 110 octane. 87 pump gas works fine.

 

What about large street ports w/ very delayed port closing?

 

On my 12A & 13B peripheral port motors (using Racing Beat style PP rotor housings) I run 24 deg leading, 16 deg trailing, as measured at about 4000 rpm. I use the exact same timing settings on my 12A street port motor.

On a rotary, the combustion chamber is sliding sideways past the spark plug very quickly. There is a lot of turbulence going on, so it is best to get combustion over in a big hurry, instead of slow combustion. High octane fuels burn slower than low octane fuels; this is why naturally aspirated rotaries like low octane fuels.

Several other racers loose power using 110 octane gas on a naturally aspirated race rotary.
With my 12A pp motor, 9.4:1 rotors, I made more power with 87 octane fuel.
With my 13B pp motor, 9.7:1 rotors, it seems to be stronger with 93 octane fuel.
With my 12A street port motor, I run 87 octane fuel.

All engines have Weber IDA carbs (various venturies and jetting as required)

 

With lower octain fuels and advanced timing isn't detonation a problem?

 

No. You only have 9.4:1 compression ratio in your motor. Detonation is not a problem at the 24 deg total advance at 4000 rpm specification I gave.

The timing numbers I gave are total spark timing; this is measured at 4000 rpm.
Total advance = static advance + centrifugal advance + vacuum advance

The timing numbers listed in a Mazda factory service manual are measured at idle; this is without centrifugal advance, this is also called static timing. If had a stock 12A distributor ignition system, and you set your timing by the stock Mazda method at idle, Then if you revved up your motor you would see the timing advance even more because the centrifugal advance would then kick in around 2000 to 2500 rpm.

With your race engine, the amount of spark advance at idle is not very important. The amount of spark advance when you are running wide open at high rpm is very important. This is why with a race engine you set the timing at 4000 rpm, after the effects of any centrifugal advance system have already kicked in.

Also, do not use any vacuum spark advance system with a naturally aspirated race engine.

 

On your race rotary, do not use a timing light with the degree advance **** on back. If your timing light does have an advance **** on back, set it at 0 degrees.

The degree numbers on those timing lights are not correct for a rotary, and they will make you over-advance your spark timing, and you will end up with detonation.

 

There must be an association b/t the advance timing and the **** on the light. Since a piston engine turns twice per ignition and the rotary once, the factor must be a 2. If a piston engine fires #1 every 0.1 sec, then for the same rpm, the rotary would fire twice, or Leading #1 every 0.05 sec. Dare we assume that the advance on the light is actually twice the actual advance?

 

I awoke this morning thinking of the waste spark. I think I need help. If you add the waste spark to the above thoughts, that would make the factor 4, not 2 - since the leading plug fires twice per rotation.

This would make the advance on the light 4 times that of actual.

 

Go to the max of 30* advance. It doesn't hurt the non turbo bridge, and it prevents leak over explosions to the fresh intake gas, which is done by getting the firing of the leading plugs done earlier.

 

How about on a BP Turbo? Would you recommend around 28* until boost hits & then retarding back to around 10* on the leading ?

 

i ran gt3 for 6 of my 22 race seasons. my motors were built by daryl drummond and roger mandeville and were Bridgeported. both ran 25 degrees advance and made 259 hp. power curve 7 to 10,000 rpm. daryl's motors were dry sumped and rogers were not.

oh, and i just LOVED running 87 octane while my competitors were paying 4X for race gas.

25 degrees worked for me. during the 6 years i had track records at road america, midohio, brainerd, blackhawk and irp with wins at the June Sprints etc. at the runoffs i had a 2nd, 2 thirds and 2 4ths. ('can't win'm all i guess....)

good luck in gt3,

howard coleman

 

howard,

quite a resume! what did you use for exhast system? what size pipes, collector location, muffler.

and

how do you get a 12a jbridge quiet enough to run @ laguna seca and still make ok power?

 

23.5 inch primary exhaust runner. mazda speed stainless muffler.... 103 db at 50 ft. since my last race i imagine that the noise restriction noose has tightened.

it is interesting that you make the point about bridgeports and noise. i had a discussion w my engine builder judge ito in april about what port setup to run on my new engine and he said something i will probably remember which was....

"by the time you get enough sound surpression into your street exhaust system to make a bridgeport liveable on the street you will have lost the power advantage over a raceport."

the raceported motor he built for me has large ports and is ( actually was since i threw on a RB quieter catback) completely insane as to how loud it was...

howard coleman

 

I don't know what your resume reads like, but I would think it would have to do more with the heat content of the low octane fuels than burn speed. Of course, I'm just guessing at that since the LeMans pits smelled oddly of Kerosene.....

(although Kerosene would tend to burn quickly, too, so that doesn't end the discussion)

 

I think this thread has proven that there is no one right generic setup. Every engine is different. Minor port shape differences, carb sizes/types/jetting, altitude, temperature, exhausts, gas type, etc, all affect what works best for any combo. It does seem that it falls somewhere between 24 to 30 degrees total advance though and around 8 to 10 degrees of timing split. Pick a spot in there and get it on the dyno. Then start trying things to see what works best for yours.

 

No, the difference is 1.5.
And it is a direct product of the relationship between the rotor gear and the stationary gear.

 

Would those timings work for idle as well I’m trying to get mines to just brap or will that be a whole different Timing.

 

Holy thread bump!! No you cannot set the timing at 24-26 degrees at idle because once the centrifugal advance kicks in you will be waaay too advanced. Shoot for those numbers at 4k rpm.

 

Racing rotaries with a distributor typically run locked distributors for reliability. No vacuum advance, no centrifugal retard.

If it doesnt idle at 25 degrees at, you raise the idle rpm.

 

It's very easy to recurve a distributor with different springs to alter how fast or slow the advance comes in, bushings to limit how much advance there is, and so forth.

That said I like to idle bridge ports at 0 degrees or so. Makes them a lot tamer to drive on the street.

Stock ports, I just remove the advance springs. Low advance when cranking but as soon as the engine fires, it's got 22 degrees.