PercentSevenC

LOL, nope. Higher vacuum advances the timing. Ported vacuum is VERY different from venturi vacuum. Only real difference between ported and manifold vacuum is at idle: ported vacuum places the vacuum port such that it's effectively above the butterfly when it's closed, but sees full vacuum as it starts to open.

Here you go, straight from the FSM (for a 12A dizzy, but it doesn't make a difference):



Vacuum goes up, advance goes up.

rwatson5651

iTrader: (5)

I very much agree.

I had been running without the vacuum advance on my 4-port 13B with twin webbers and after I figured out how it worked and how much advance/retard it provided I hooked it back up, advanced my timing (from where I had it set without the vacuum advance) by the same amount that the vacuum "advance" would retard the timing @ wot. End result was the same amount of timing @ wot but more advance @ idle and @ cruise. The milage went from about 14 up to 20-21 and the driveability at cruise increased significantly.

Rogue_Wulff

Still doesn't change the FACT that as the throttle is opened further, manifold vacuum decreases. Yeah, once the RPM is high enough, manifold vacuum does start to come back, but never to the same level as it is at idle/throttle closed. Likely explains the need for a vacuum check valve in the brake booster.......

I'm not gonna go into the arguement that "ported" vacuum comes from the port being exposed to manifold vacuum, as even that theory proves that the vacuum advance needs no vacuum at idle, and increasing vacuum as the throttle opens (where manifold vacuum drops).
The stock vacuum port for the vacuum advance in the carb is designed in a manner to use the venturi effect to pull a vacuum based on the speed/density/volume of air rushing past it, not the amount of manifold vacuum that is left below the throttle plates. There can be 15+" vacuum in the vacuum advance system, with under 5" vacuum in the intake manifold. Explain that without totally supporting the venturi effect.........

PercentSevenC

At WOT, the vacuum advance should NOT be advancing the timing at all. The only advance should come from the centrifugal advance at that point. Take another look at the TII timing map I posted earlier, and you'll see that at low engine load (i.e. high manifold vacuum), the timing is at its most advanced, and at high engine load (i.e. low manifold vacuum), it is less advanced. You don't want timing to be advanced as the airflow across the venturis increases, because that would lead to the advance being higher at high load (i.e. low manifold vacuum) and lower at low load (i.e. high manifold vacuum). That's the opposite of the intended function of the vacuum advance mechanism.

Again: ported vacuum is not the same thing as venturi vacuum!

Rogue_Wulff

TII timing map is rather irrelevent here. First and foremost, that's a boosted FI system, with ECU controlled timing. With boost, you cannot have the timing advance as far under higher engine load as can be done in N/A form. As boost goes up, so does the dynamic compression ratio, along with the possibility of detonation.

You're right, venturi vacuum is not the same as what everyone called ported.
True venturi vacuum comes from the airflow thru the venturi's of a carb, and is used to suck the fuel out of the fuel bowl.
On the other hand, the "ported" vacuum used for vacuum advance is similar to venturi vacuum, but only in that it uses the same basic principal. Just like those fancy little suction guns that use air pressure flowing thru them to suck the liquid from a container and spray it out with the air. The air flowing thru the internal passages creates a vacuum, just like the "ported" vacuum in a carb.

PercentSevenC

I don't even... fine, okay, here's a stock RX-8 map if that makes you happier (only because I couldn't find a stock NA FC timing map):



The vertical axis is RPM, the horizontal axis is load. Higher number means higher load means lower manifold vacuum. Guess what? Advance still decreases with increasing load! Whether the engine is boosted or not makes no difference. All timing maps look like this - rotary, piston, NA, turbocharged, it's all the same.

The ported vacuum used for vacuum advance is, in fact, ported vacuum, not venturi vacuum. That is, once the throttle is cracked, it's essentially seeing manifold vacuum. Advance decreases with load, it does not increase with load. I'm really not sure how I can make this any clearer.

And anyway, this thread is getting off track, so I'll let it go at that.

Rogue_Wulff

Study that map table closer. At lower RPM, the timing backs off as load increases, but not as the RPM increases. The lower RPM timing retardation is to fight detonation, as the RX8 engine has a much high static CR to begin with.

If the "ported" vacuum is "seeing manifold vacuum", then how can the "ported" vacuum be higher than the manifold vacuum? Manifold vacuum is result of the engine trying to pull air in, but is being restricted by the throttle plate. There is no restriction above the throttle plate(s), therefore no vacuum. Hence, the need for the venturi in the carb bore to create a means of sucking the fuel out of the bowl.

Starfox07

So on a stock GSL-SE engine, the distributor gets manifold vacuum to advance the timing, is that correct? Is there perhaps a spacer for a weber DCOE carb that has provisions for manifold vacuum, IE vacuum between the throttle plate and the intake port? If not, could I drill/tap the intake manifold for a vacuum fitting?

PercentSevenC

There's some weirdness in the mid-load, high-RPM parts of that map, I'll admit (also, notice the timing drops off as RPM rises toward the upper end of the range), but if you look at the overall trends, my point stands. Advance is still higher at low load than it is at high load. I challenge you to find an ignition map, any ignition map, done by a professional tuner, that has higher advance at WOT than on decel. It would make no sense to do it that way, since flame propagation is slower at low load than it is at high load.

Some more reading for you:
http://en.wikipedia.org/wiki/Ignitio...timing_advance
http://www.corvetteactioncenter.com/...m-advance.html
http://www.gofastforless.com/ignition/advance.htm

Okay, I'm done for real this time. If you're still not convinced, I really don't know what to tell you other than, "You're wrong."

On the 12As, at least, it gets ported vacuum, which I believe is controlled by the solenoid rack, though I'm fuzzy on the specifics of how the emissions system works as I've (fortunately) never really had to deal with it. Manifold vacuum will work just fine for your purposes. I've never had the opportunity to examine a GSL-SE LIM before, but I'd wager there's a vacuum nipple on there somewhere that you could use.

Starfox07

Front:



Back:


I see no vacuum sources on the LIM :/

Also, something I had not thought of:

Does the OMP connect to the UIM? Is there any way to solve this without converting to pre-mix?

j9fd3s

iTrader: (3)

yes. http://www.jimrothe.com/mazda/84_tra...raining_4b.pdf

there is a long section about how the vacuum advances work. they are hooked to manifold vacuum with the solenoid in between, so that they can turn them on and off.

basically the vacuum advances are OFF at idle and high throttle.

mechanical advance advances timing with RPM, and the vacuum advance increases timing at low loads.

you can go verify this with your own car, if you want BTW. is fun to hook the vacuum advances to a boost gauge and go for a drive, but you could pull the hose off the advance and put your finger over it too...

if you do drill and tap a fitting (i would but after the fuel is right, unless the carb has a fitting) without the ECU timing with basically just be advanced at idle over stock, which is fine. the -5 stock timing is for emisssions, before that timing was more like TDC

Rogue_Wulff

Yeppers, even more fun to use 2, one in the vac advance and one to manifold. On an old school carbed application, you will see times where the vac advance sees more vacuum than the manifold vacuum could possible supply.
Try it sometime. I have.......

PercentSevenC

j9fd3s: Interesting. So it's not using ported vacuum but rather manifold vacuum, using a solenoid to emulate ported vacuum? Are there conditions where vacuum advance is turned off, where it would still be active using ported vacuum?

The oil injectors go in those two holes near the top of the LIM, in the intermediate runners.

Starfox07

That's right, I was thinking those were the fuel injector bungs.

82FanTC

iTrader: (9)

OK wow I have read this thread twice and I'm not sure a conclusion was ever made. I have always been under the assumption that you should just run the vacuum advance open on the dizzy.

I have 2 nipples in my manifold for vacuum; one for brake booster and one to run through a PCV on the crank case.

Is it beneficial to hook that another source straight up the the vac advance? Does this affect where timing is set? I use the standard 8* split at 4000 RPM

I certainly would appreciate better gas mileage if that is what it gives you!!!

Starfox07

Ok so I went and had a look for myself, took some pics, and made some diagrams.

For the SE knowledgeable, please let me know if I made any mistakes:


This picture is the top side of the intake manifold. From what it appears after following the vac lines, the vacuum advance goes through a solenoid first, and then into the upper intake plenum, as pictured.


This picture shows the lower part of the manifold system, and where the brake booster vacuum line feeds in. It appears to connect to the LIM and there is another vacuum line right beside it.

This leads me to this assumption, tell me if I'm wrong: The brake booster and vacuum advance work from the exact same vacuum, IE manifold vacuum.

So if this is correct, is there any reason I couldn't 'T' off the vacuum line near the brake booster and use that for vacuum advance?

Also too, is the advance solenoid itself dependent on vacuum from the UIM/TB? Or is it ECU controlled? In other words, would taking off the UIM totally f*ck up the solenoid system and cause it to not function all together?

edit: I guess what I'm trying to ask is whether the vacuum advance will work the same without all of the other emissions components. From taking a brief look at the FSM vacuum diagram, it all appears to be tied into each other. :/

PercentSevenC

Your "unknown vacuum" line in that pic should be manifold vacuum, so you could use that. Otherwise, you could tee off the brake booster line, but I kinda prefer the idea of my brake booster having a dedicated vacuum source. No idea how the solenoid system works, maybe j9fd3s will chime in on that.

In any case, not having the solenoid hooked up won't cause any problems. It'll just make your idle timing a bit more advanced than it would be normally, but function the same otherwise. It seems Mazda retarded the timing at idle for emissions purposes anyway, so it doesn't matter.

Starfox07

Ok awesome. The unknown I believe is actually for the (non functioning) windshield washers.

craaaazzy

iTrader: (9)

so maybe a dumb question...what will make more power, all things being the same other than carb versus EFI (and their required supporting mods)? why not go standalone instead of carb? i would assume a standalone could achieve just as much power unless the intake design is superior on the carb.

Starfox07

$$$

PercentSevenC

*cough*MegaSquirt*cough*

craaaazzy

iTrader: (9)

ok, remove $ out of the equation as well now. what is able to be tuned appropriately to make more power?

Starfox07

I don't really understand the question.

An EFI system with an ITB setup would produce similar results but with better driveability and probably a bit more power.

Even with megasquirt it would be more expensive.

Intake $150
ITB ~$400
Megasquirt ~$500
Tuning ~$500
Injectors ~$500
Fuel Pump $??
Wideband 02 + Gauge ~$300

Rough total of big parts: ~$2,000

vs

Intake $150
Carb $400-$500
FPR $100
Wideband 02 + Gauge ~$300 (not really necessary, but helpful)

Rough total of big parts: ~$700-800

And then with a standalone, I'd probably want to convert the ignition system to be able to fine tune the ignition curves, so that's even more money.

PercentSevenC

Those aren't really equivalent lists.

You're tuning the carb yourself, so you shouldn't compare it to a professional MegaSquirt tune (also, with things like VE Analyze Live, MegaSquirt is capable of basically tuning itself once you've got an AFR target table set up).

You already have injectors and a fuel pump.

You don't need the intake and ITBs. They might get you more power, but you'd already be tossing the biggest restriction: the MAF sensor. You could also pick up a stock S5 NA intake very cheaply if you're looking for further flow improvements.

The wideband is optional in both cases, but in both cases makes life far easier.

Ignition is easy: grab an FC CAS for next to nothing and file two teeth off of it. You can then use the MegaSquirt II unmodified. Or you can add a second VR conditioner to the MegaSquirt and use an unmodified CAS.

If you're on a tight budget, a basic MegaSquirt I would cut the cost down by $180 even if you buy it pre-built. You'd have to either use a distributor or add a VR conditioner to it, though. But you'd be using a distributor with the carb anyway.

Also, you forgot the cost of jets for the carb. Those add up fast.

I mean, yeah, a MegaSquirt setup would probably end up being a bit more, but not by all that much. It mostly depends on which set of issues you'd rather deal with. But if you can solder or are willing to learn, the MegaSquirt route is definitely cheaper than the carb route.

craaaazzy

iTrader: (9)

i'm looking at it very simplistically between the 2 options. as mentioned, take $ out of the equation for now...can a tuned EFI system out perform a carb system?

then we can start discussing what it takes to get an equally tuned and powered EFI and carb system. with either systems, the restriction seems to be the MAF, no? going carb, you'd be eliminating this. going standalone, you would also eliminate this.

sorry to highjack your thread...i should probably start one myself but i think it's good discussion, especially if someone is considering converting (which I would consider myself as well).