Which do you prefer, and why?


I'm using manifold vacuum, but I believe that the sudden drop in vacuum when I floor it is retarding the spark too far. I don't know anything though.

 

Ported will do the same thing except make it idle worse.

 

because it's not as advanced? I know that advanced timing is good for fuel economy (right?) and efficiency.

 

On the 396, I prefer manifold because the Proform main body isn't drilled to support ported.

For decades, the factory used manifold vacuum. They started using ported when emissions controls were being imposed. However, with the advent of computerized feedback controls, the timing curves started looking like manifold again.

If you don't have enough advance when you floor it, it's because you don't have your mechanical system set up correctly. You want somewhere in the neighborhood of 34 to 36 degrees total mechanical-only advance at high RPMs (2500-3000), with 8-12 degrees initial advance at idle. If you use manifold vacuum, that goes on top of that. If, with those setting you get pinging at part throttle & under load, you need an adjustable vacuum advance that lets you back it off a little.

 

Ported won't make the idle worse, necessarily... Depends on the engine. Bigger cam, yea, manifold will give better idle. Less duration cam, manifold perhaps.

It's more of a "what your engine likes best"

I'm basically pure stock, ported for me. If I run with manifold it'll idle much higher then I prefer. Once I start modifying the engine, i'll use manifold.

If you're having performance "bogs" like you say, we could discuss that.... What is your initial timing set at? Your vacuum? mechanical advance? distributor? etc....

 

I've set my timing all over the place. it's around 14-16 degrees advanced base right now. it's a GM distributor, stock.

 

Ported, the only different between manifold/ported is at idle, and my car idles perfectly on ported.


When I did run it on manifold I didnt' like the transition from no throttle to full throttle. I think Fbird88 explained it before that if vaccum advance is being used to maintain inital timing (example. 8* initial timing plus 8* from VA at idle using manifold = 16*, when you go full throttle, VA goes away, and your timing retards to 8*, which may or may not cause problems.)

I've got 14* initial, 10* VA, and 20* Mechanical, I get 20MPG on the highway and it runs perfectly, has great response, no dead spots, no off idle bogs or hesitation, no dieseling at shutoff.

 

I have to agree with stealthelephant (in all regards, my setup is virtually identical, with identical results.)

Token - 14* initial? stock dist? stock vacuum advance and mechanical advance? stock weights/springs? I hope not...


Have you revved it up to 3k rpm or so to check if the mechanical is coming in right? (disconnected vacuum advance?)



Either way, what I did, and this may help you:

-Set your idle below 800RPM
-disconnect vac advance
-set timing to around 14* say
-rev engine to 3k rpm, should get around 34* total
(I used an MSD timing tape, which is now long gone... going to have to superglue another one on...)
-reconnect vac advance to manifold vacuum, set idle back down to 800RPM or lower (it'll probably go up when you set on manifold vac)
-check timing now, should be around say 24* (24-14initial = 10 vacuum)


That means you're good to go. However a stock dist won't get you these results, I used a Crane kit to readjust, came with new springs and an adjustable advance can.
Without this, your mechanical advance would come fully in much later, and vacuum would be higher... overall screwing up your results so you couldn't run 14* initial....



Clear as mud?

 

Just make a mark 2 3/8" from the line on the balancer. That'll be 34 degrees on an 8" balancer.

 

Always helping me Apeiron, thank you. But I'm pretty sure I have a 6.75" balancer, if that sounds right for an LG4....


Token, I just realized you have another thread about this, since i'm straying into the territory others are already helping you about, might as well just move to that thread and leave this one...

 

2" on a 6.75" balancer.

 

I know the F-bodies aren't as particular to torque production off idle but my 1983 G20 definately ran better and got better mileage with manifold vacuum. Also the idle was much less prone to RPM variations when loads were applied. With ported and my idle rpm in drive set to 600 it would increase to 900 when the transmission was put in park/neutral. On manifold it could be set at 550 and only increased to 700 when put into park/neutral. I do have to admit manifold vacuum tends to mellow out that cam sound that we all love. That didn't really matter to me becuase of how much better it came off the line. Lots of tirespin versus a nasty bog.

The comment about the idle being too high is pretty irrelevant in my opinon. Just adjust the idle speed. The reason it picks up so much RPM when it is hooked up is because the spark is hitting at a more appropriate time and the engine is more efficient.

Ported was NOTHING more than an emissions band-aid.

 

Again, if the engine wants 20* initial timing, then give it 20*initial timing, 16* mechanical for 36* total @ WOT.

If your engine wants 20* of timing to idle correctly, and you run 10* initial, and 10* VA for 20* timing at idle, when you go WOT, all vacuum advance goes away, and for a second your timing retards to 10* until mechanical advance starts to kick in. It just makes no sense to me, if your engine wants X amount of timing, how does it makes sense for that timing to momentarily drop out when you go WOT.

Again, some people run manifold and swear by it, so if it works for them great, but there is nothing wrong with running ported. Emissions bandaid or not, doesn't mean it's bad.

 

Only thing is most SBC and even big blocks for that matter like to idle with about 25-30* of advance. Don't believe me look in a TBI or TPI engine .bin file. There is no way you are going to get that much advance from initial alone. It just kicks back on the starter way too hard. I like the factories way of 12* initial, 20* centrifical, and 20* vacuum. Makes 42* at about 2,500 rpm when cruising which is perfect(varies some with load and RPM). It also gives you 32* @ WOT which is perfect for the fast burn of the 193s I was using.

If you are that worried about the fraction of a second of timing loss when you step on it, put a vacuum delay valve inline with the vacuum advance. That will give you about a 1 sec delay before the timing retards to cover up the hole. This was used on my LE9 from GM and was retained with all of my different combinations. Every carbed engine I run uses manifold vacuum.

You would not believe the number of cars running ported vacuum that ping when the throttle is opened slightly. This is due to trying to advance the initial timing to make the engine idle right, off-idle the vacuum advance comes in and really advances the timing too much. I have also seen power loss on a dyno due to this. Manifold vacuum just produces a smoother torque curve.

This applied on my stock 305, my XE-274 cammed 305, my Mellings MTC-1 cammed 350, my LT1 cammed 350, my friends 70 Monte Carlo 350 as well as the ECM in my TBI.

 

The amounts are good, but the rates of advance on the stock components are terrible. 20 degrees of centrifugal doesn't do all that much good when it doesn't come in until 5k RPM.

 

That is why you change the springs. My centrifical advance was all in by 1,400 distrbuter rpm or 2,800 on the crank. My advance is still all in by 2,800 but is now more refined in the EPROM.

 

And the advance can.

 

The stock advance can works fine.

 

I installed looser springs and my 2000-5500 acceleration is much better.

 

So, your real "issue" had little or nothing to do with your question.

And most of this chest pounding debate had nothing to do with it, either.

At least the required information was contained within.

Hopefully we can all learn (another) lesson from this.

 

Yeah, sorry, I suck
I should have researched more before swapping to carb