Hey Guys,

So I've read that for best power on a 12a you want your ignition timing to be 16* and 24* BTDC at full advance.

I followed the tutorial on Paul Yaw's site to make a pulley with timing marks where I want them (I actually made marks at exactly 16* and 24* instead of the "Every 5*" that he did) but now I'm wondering something.

Sure it said that you want to set that at full advance (4krpm ish) but afterwards do you want to leave your advancing mechanisms hooked up? I mean, with all this trouble of setting it at full advance, is it really going to make best power with the vac advance and mech advance still in place or should I disable both (by uncapping the vac pots and removing the springs inside the dizzy)

Is this recommendation meant to make full power with those hooked up, or do you make better power with those disabled?

And before anyone says it, yes I understand that without them gas mileage goes to hell in a handbasket. At this point, I already will need a second job to pay for gas, but there's no point in driving a car that *hasn't* had every last bit of performance wrung from it. If I'm going to do that I might as well drive a sunfire or a civic.

Jon

 

i have a question about timing, i know how to time my car, and i know that btdc means before tdc but how do i measure that? i'm confused on that, i can time my car to stock timing but i wanna get the most power possible out of it by doing the time advance or w/e its called lol.

 

ok, so what is the difference between 0 & 10 @ idle VS 16 & 24 at full advance.

 

I cant stand the fact that so many people rate at 4K vs idle - so difficult and it doesnt have to be that way...

 

so setting it at idle is the correct way then right? and if so whats the best settings?

 

The 18/24* advance provides the best power curve once the vacuum and mechanical advance have run thier full course. The purpose of having both vacuum and mechanical is to bump up the timing for smoother/quicker acceleration. Vacuum advance has run it's course by 1000/1500 rpm, mechanical is at full advance by 3500-4000 rpm.

Since we don't drive/run our engines at 4k+ all the time, there needs to be some balance for those times when we drive under 4k and the ability to get there smoothly when needed. Disabling the vacuum and mechanical advance should only be needed for boosted applications. That's why the 12AT dizzy is sought after as it compensates for boost along with vacuum.

I am curious where the leading and trailing timing fall at idle when set to the 18/24* specs at 4k. Seems no one has ever posted those numbers.

 

well there are 4-5 different distributors with slightly different advance curves, so it could be done but the starting number would depend on the pullies you're using and the dizzy you're using. its easier to do it at 5k

 

the vacuum advance is ONLY there to give more timing at part throttle, at full throttle vacuum should be 0. so it doesnt affect full power, just part throttle and mpg, so leave it.

the mechanical advance is fully advanced over about 4500 (it varies a tad with different dizzy's) over 4500, you're getting full power, under 4500, thats debatable. its really common in the v8 world to tune the dizzy so it advances faster, as the oem curve has to pass smog etc etc.

 

"Mr T" is the only paying attention to my question??? I will wait for others.

 

I've often wondered where the timing would be at idle, after someone jumped thru hoops to set it 4-5K.

Personally, I just set my timing at idle, using the stock 20* split, then adjust the dizzy for best idle/ throttle response. Must be doing something pretty close to right. Last tank of fuel in the car, 200 miles of 75 mph highway driving, and an autocross in the middle, still averaged 18.9 MPG.

 

http://www.yawpower.com/pultime.html
No hoops required.
step 1: unplug vacuum hoses from dizzy
2: plug hoses to prevent leak
3: rev to 4k
4: set leading timming to 26* BTDC
5: back at idle set the trailing to what ever you want. the closest I've ever been able to get the split is 8*
6: hook vaccume lines back up.
The whole process took me 10 minutes yesterday. Literally 10 minutes. i did it in my work parking lot...
At full advance my car is 26btdc L and 18*btdc T
at idle with just mech advance it's 2*btdc L and 6*atdc T.
I don't use the vaccume advance right now. The mechanical advance advances the leading and trailing the exact same amount so T and L advance 24* from idle to 3500 RPM. If you watch the advance curve mechanical advance really stops between 2500 and 3200 rpm. 4k is just a safe point to set from.
I personally do not like a split in my timming and neither does Mazda. The 26b ran no split at WOT so does the FC and 13b MSP timming maps. I'm working on modding some stuff to get more control. I love my carbs, but this shitty dizzy crap is making me want to go FI...

 

Well according to the FSM and my measurements (not very precise) - there are 20 degrees mech advance (give or take on trailing).

0/-10 (I believe you meant -10 - FSM spec) idle equates to 20/10 at 4K

24/16 at 4K is 4,-4 at idle

Hopefully I did my math right.

Scott

 

Kinda weird how all this **** works. I hooked up the vacuum advance and at idle there is about 35-40* of advance. it was idleing at about 37-42* btdc L and 29-34* btdc T. What a crazy curve. I went for a drive and didn't notice a difference in driveability at all. I'll fill it up tomorrow and see if gas mileage gets any better with the vacuum advance hooked up. I watch mine pretty well and I average 16mpg city and freeway.

 

To answer Wacky's question. It will be different on every car. You first have to understand how your distributor advances the timing. There are a set of little weights attatched with springs inside your distributor. As the engine rpm goes higher the weights start to swing outward due to centrifugal force causing the timing to advance. At idle the weights are not yet moving thus the timing is set were you set it at idle. When your engine is at 6k the weights are swung all the way out and advancing the timing as much as the distributor will allow. Each distributor will vary because of the wear and tear on the springs. Worn out springs or weaker ones' will cause the timing to advance much sooner and more and stiffer springs later and less. Alot of racers will change springs to allow a custom timing curve for their application. This is why for performance reasons you want to adjust your timing at 6k because that's at full advance and were you are going to run and make the most power. After setting your timing at full advance you can then check it at idle. Each distributor will give a full advance a little different from another just do to wear and tear and the spring condition so that's why you want to set your timing at 6k. And this is why most racers will weld the weights from moving so that the timing is constant and can't advance too much due to a worn or broken spring which can cause a motor to detonate and grenade.

So lets give an example for Wacky question. Lets say you set your timing at 0 degrees at idle. You then rev your motor to 6k and check it(without touching and advancing) and it reads 15 degrees. That means your distributor advances a total of 15 degrees. So now knowing this if you advance your timing at idle to lets say 5 degrees your timing at 6k will be 5+15=20 degrees. Get it now? I always set mine and check at full advance at 6k. Who cares what your timing is at idle.

 

Too much reading there J. No need to waste your time as Im very familiar with the process

so answer the question. where is your timing mark set when you release the accelerator cable

 

+1 Wack

 

Thanks guys.

I'm going to install the pulley I made with the 16* and 24* marks on it tomorrow and set my full advance timing. I'll re-hook-up my vac advance after and we should be good to go

Jon

 

Quick question...I'm running an old Rx2 ported block, no nest and all that. Should I connect the dizzy nipples to the manifold/carb, plug them, or leave them open?

 

Reading through an old Rotary Engineering catalogue that I have, they don't recommend connecting the vacuum advance to the dizzy when using either their Weber DCD, IDF or Holley carbs. If there are ports on the carb or intake, those need to be plugged, the ones on the dizzy must be left open.

 

This engine is running a modified Nikki, not a weber (I know, what am I thinking!...gas mileage ATM.)

 

Then I would go ahead and hookup the vacuum lines. Believe it or not, I'm running a 1/2 SP, 1/2 BP 12A with RE's DCDs that have the larger venturi for thier 13B setup. I can pull 24 mpg on the highway, but once I put my foot in it, the mpg quickly goes down the crapper.

 

so becuase im running a mikuni side draft carb setup i shouldnt use my vac advance?

 

That's what all the old school tuners said, but I have vever seen one single reason why not to. Better gas mileage and emmissions just for running 2 vaccume hoses. Why not... I don't understand why you wouldn't run it. Vacuum advance only effects the car at idle and part throttle situations.

 

With the vac advance I got more heat and less of that chainsaw smell, had to rich up a bit in the carb for idle. I get 18-25mpg, butt dyno says good down low, but something missing up top, felt the opposite before. I left it setup for the mpg, and have not messed with it for a few months. I premix with quicksilver, different premix's gave me different characteristics, that might be relative to power and burn.

 

That was for the noobs not you silly.

And to answer this question that depends on if you put it in nuetral or not. Your timing will be determined by your engine rpm.