I Need More Power!

[xirdneh]

Timing lights are evil. I posted awhile back that my wagon was showing 30-40 degrees BTDC (if I remember right). I tried this with 2 different timing lights. When I set it down to 10 degrees (as shown by the light), the car had no power and I could barely drive down the street. I ended up setting it manually instead. Cleaning the battery posts made the timing lights read a bit closer to what it should. The same timing lights seemed to work fine on my '81 Tercel. It makes me wonder how people fine tune their timing (eg: 8 degrees instead of 5 or 10) when the lights seem so randomly unreliable.

it would do that if you tested it without disconnecting the vac adv hoses

[Mark]

By the way, I don't think the vacuum advance on the distributor has any effect at idle. I think it's meant for part-throttle conditions (maybe 2000 rpm?).

By the way, I don't know what I was thinking when I wrote this. The manifold vacuum advance operates at high vacuum (idle and when the throttle is only slightly actuated). I think the centrifugal weights take over at rpm's over 2000 or so (I'm probably making that 2000 number up again).
Anyways, whenever I post anything always look it up somewhere else to verify it before taking it as gospel, otherwise your car might blow up.

[lannvouivre]

centrifugal weights

What are these? All I can think of is "crank throws."

[dlb]

centrifugal weights

What are these? All I can think of is "crank throws."

in the very bottom of the dizzy are the weights for the mechanical advance. when the RPM reach a certain point, the centrifugal force of the spinning weights advances the timing. this is a separate advance mechanism from the vacuum advance, which uses vacuum from the carb and manifold to pull on diaphragms that are attached to an arm that advances the timing.

[lannvouivre]

Got it. *writes to self: "wow, distributors are some weird stuff."*

[Mark]

Speaking of weird distributors, I checked the timing with a light on my '81 Tercel this evening and it showed 10 degrees BTDC. My distributor (which was originally from a 2nd-gen '83 Tercel) only has one vacuum hose on the diaphragm. I didn't think this existed after the 1970's. Here's the picture to prove it:

distributor.jpg

When I removed or replaced this vacuum hose, the timing didn't change and the idle didn't change either. Usually when I unplug a distributor vacuum hose, the engine idles like crap unless I plug the hose. I didn't feel any vacuum from this hose with my finger. traced the hose back to the carb and it's plugged into the right port (I think) according to the vacuum diagram under the hood:

carb1.jpg

emissions diagram.jpg

I unplugged the hose from this port while the engine was idling and there was no change in the idle and I didn't feel any vacuum with my finger at this port. Is this port manifold vacuum? I assume a single hose distributor diaphragm would use manifold vacuum. Usually when I disconnect a vacuum hose on my wagon and put my finger over it, I can feel a vacuum.
My '81 Tercel gets noticeably less fuel economy compared to my wagon. I wonder if the lack of vacuum advance at the distributor could account for this.

[dlb]

that's ported vacuum that comes off the carb ('carb vacuum' would be a better name for it). manifold vacuum comes off the manifold. the single diaphragm vac advances only have the ported vacuum advance, which advances the timing by about 12 or 13* at part throttle for cruising conditions. it's the manifold vac advance that advances the timing by 8* at idle AND cruising conditions so it's normal that the timing didn't change when you unplugged that vac hose.

[Mark]

I know there are different views on which port to draw from for distributor vacuum advance on aftermarket systems (manifold vs "timed advance" or whatever they call it), but since my distributor and carb are mismatched (from different generations), I'm thinking or trying to draw distributor suction from the manifold vacuum so the timing is advanced at idle. Most of my driving in my '81 is in town so a lot of time is spent at idle. I think the "timed advance" (part throttle conditions) is more for improving emissions and the old fashioned manifold vacuum is for improving combustion at idle. I think that would be more beneficial for me. As far as I know, the second gen single hose distributor may have been designed to operate on manifold vacuum and the 1st gen points-type distributor may have been designed to be hooked up to "timed advance" vacuum as per the vacuum diagram. I'm babbling now and probably mixing myself up. I'll try switching to manifold vacuum tomorrow and seeing how it runs.

[Mark]

I just did a quick search for more info. Here's a link to an article on the vacuum advance topic by some random guy I don't know on the internet. He has strong opinions so they must be true:http://chevellestuff.net/tech/articles/ ... nifold.htm
Basically, he says that ported advance was a lame stop-gap measure to improve emissions and it made your car run like crap and gave you bad fuel economy. Manifold vacuum advance was invented by Jesus and is so awesome it will make you weep with joy as you idle at stop lights while saving fuel.

[ARCHINSTL]

@ Mark: Loved your last sentence!
It also makes your pants wet...
Tom M.

[Petros]

what this guy says is false, vac advance existing before there were any emissions systems on any cars, the first 4 cars I owned had no emissions sytems at all, some had vac advance, and some did not.

when the engine is at a part throttle cruise condition, like driving at hwy speeds at anything less than full throttle (which is most of the time unless you want to loose your license), the mixture is both lean and at a lower atmospheric pressure (the throttle works by lowering air density in the intake manifold). the flame front in the combustion chamber, initiated by the spark, will travel slower in a lean, low density mixture than at full throttle. by advancing the spark at part throttle the peak pressure in the combustion cycle occurs at a more optimum time relative to the piston/crank angle position, and you improve efficiency. The idea is to advance the spark for the part throttle conditions to improve part throttle efficiency.

the vacuum advance is there to improve part throttle efficiency, not because of emissions. On later years there was the use of a vacuum advance/retrad to alter the and lower the peak pressures to reduce emissions, but this was not in all operating conditions, and not on all models of cars nor is a true across the board. If your car system has a vacuum retard for emissions reasons, removing that system will improve economy. But as a blanket rule it is not true.

The car will run okay without the vac advance, it just means there is a slight reduction in economy at part throttle cruise conditions.

I suggest you do not waste any time reading what he has to say because he does not know what he is talking about.

[Mark]

I think he was saying that manifold vacuum advance was the original method of advancing timing and the ported vacuum advance was brought in for emissions reasons. My "modern" '84 wagon has vacuum drawn from both sources so vacuum advance is happening both at idle (manifold vacuum) and part throttle (ported vacuum). My '81 Tercel only had one hose on the distributor and it was routed to ported vacuum so there was no advance at idle. This evening I switched it to run off manifold vacuum (through the gas filter). I set the non-advanced timing for 8-10 degrees BTDC (it was at 10 before). when I connected the hose (from manifold vacuum) the timing advanced to 20 degrees. It idled the same as far as I can tell compared to when it was on ported advance. I took it for a quick drive to the grocery store and it seemed to accelerate a bit quicker. Before it seemed to have a slight "flat spot" as I accelerated, but the improvement so far might be my imagination. My reasoning for the switch is that since I drive this vehicle mostly in town, I spend a reasonable amount of time at stop lights and coasting to a stop. I'm hoping the vacuum advance at idle will improve my overall fuel economy. This '81 sedan gets in-town mileage in the low to mid 20's mpgs. My 84' 4wd wagon (which weighs more) gets in the high 20's in town. The only difference between them (except for things like engine wear) is that the wagon has the timing advanced at idle. I'll drive the '81 for a week or two and see if the switch to manifold vacuum makes any difference in fuel economy.

[dlb]

it's interesting that the '81 gets worse mileage. with the same engine, lighter weight, and 2wd, it should be better than the heavier 4wd wagon.

i don't think there's anything wrong with hooking the advance up the way you just did. i don't think it will make enough difference to improve the fuel economy but if it runs smoother, that's reason enough.

[Mark]

I don't really get why the in town (highway fuel economy is pretty much the same) fuel economy is worse in the '81 either. It has much lower kms (100,000 vs 260,000) and I checked for vacuum leaks in the various components (I found one in the AAP and disabled it). Both cars idle nice and smooth. Maybe the gearing ratio is different (I'm sure it is, but I haven't looked up what it is). I assumed the points-type distributor might have been an issue, but I replaced it with an electronic one with no difference. I'm hoping the switch to timing advance at idle might make a difference.

[lannvouivre]

I assumed the points-type distributor might have been an issue, but I replaced it with an electronic one with no difference. I'm hoping the switch to timing advance at idle might make a difference.

How is the electric one different from the points-type?