Mech vs vac advance....

« **on:** June 30, 2015, 04:32:24 PM »

Ok guys I'm interested in hearing the pros cons and anything in between on dizzys w mechanical advance and vac advance. Can u take a vac advance off a distributor? What modification s are necessary? I've heard boats aren't usually vacuum advance but would that hurt performance if it did?

« **Reply #1 on:** June 30, 2015, 04:43:36 PM »

I don't run a vacuum advance, and they do make a vacuum advance delete kit, believe it's made by msd and super easy to install.

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« **Reply #2 on:** June 30, 2015, 04:47:09 PM »

In a car the vacuum advance is hooked to the trans and obviously jet boats don't have a trans so maybe that has something to do with it, I'm no genius tho

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« **Reply #3 on:** June 30, 2015, 04:57:15 PM »

Quote from: che77y on June 30, 2015, 04:32:24 PM

Ok guys I'm interested in hearing the pros cons and anything in between on dizzys w mechanical advance and vac advance. Can u take a vac advance off a distributor? What modification s are necessary? I've heard boats aren't usually vacuum advance but would that hurt performance if it did?

While a vacuum advance won't actually "hurt" the performance, it also won't help at all.

The way the vacuum advace works is advances the ignition timing under low engine load.

It is very difficult to do that in a jet boat application.

There are delete kits, you can also just pull the line off.

GT

« **Reply #4 on:** June 30, 2015, 04:59:07 PM »

Quote from: nordic_scandia on June 30, 2015, 04:47:09 PM

In a car the vacuum advance is hooked to the trans and obviously jet boats don't have a trans so maybe that has something to do with it, I'm no genius tho

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Sorry, but this is fundamentally incorrect.

The vacuum comes from the intake manifold.

GT

« **Reply #5 on:** June 30, 2015, 05:02:39 PM »

Well crap...

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« **Reply #6 on:** June 30, 2015, 05:10:25 PM »

Quote from: GT Jets on June 30, 2015, 04:59:07 PM

Sorry, but this is fundamentally incorrect.

The vacuum comes from the intake manifold.

GT

maybe in a Venezuela built ford escort?

« **Reply #7 on:** June 30, 2015, 05:11:13 PM »

Hey if I just pull it off, won't that affect my timing as the weights and springs haven't been played with?

« **Reply #8 on:** June 30, 2015, 05:29:34 PM »

It might affect your timing CURVE but not your total timing, and it won't affect the curve by much. Jet boat motors spin so freely the weight is doing most of the adjusting anyway.

« **Reply #9 on:** June 30, 2015, 06:24:22 PM »

My dizzy has vacuum advance I just left it alone boat runs about 65 at 4800 rpms no problem

« **Reply #10 on:** June 30, 2015, 07:05:26 PM »

Quote from: che77y on June 30, 2015, 05:10:25 PM

maybe in a Venezuela built ford escort?

What?

The vacuum signal comes from the intake manifold.

Where else would it cover from? Sometimes on a smog controlled car it will be switched by the carburetor step port. The vacuum still comes from the intake manifold.

GT

« **Reply #11 on:** June 30, 2015, 07:13:05 PM »

Quote from: che77y on June 30, 2015, 05:11:13 PM

Hey if I just pull it off, won't that affect my timing as the weights and springs haven't been played with?

It won't effect the timing on a jet boat engine.

By removing the vacuum hose and running it the way it is, it just won't advace on a low engine load. When do you have a low engine load? Anything under about 1,800 rpm. So why not just advance the timing base?

The timing setting that matters most is above 2,500 rpm.

Your current distributor likely has 10-12° of advance built into it.

Check the timing at 2,500 rpm and shoot for 32-36° BTDC. This number will vary depending on many variables.

As the engine slows, it will drop to about 20° which in most cases is low enough to start. It will idle better with more base timing.

GT

« **Reply #12 on:** June 30, 2015, 07:19:17 PM »

Quote from: crazyjim on June 30, 2015, 05:29:34 PM

It might affect your timing CURVE but not your total timing, and it won't affect the curve by much. Jet boat motors spin so freely the weight is doing most of the adjusting anyway.

Not correcting you, just stating it in a different way.

The torque curve on a jet boat engine is linear, meaning the faster the pump spins, the more torque is required.

An automotive engine has an inverse torque curve, meaning it needs the torque to "pull a gear", once the engine starts to speed up, the torque requirement drops.

This is what the vacuum advance is really for. It is detonation protection for this load. It can also increase the fuel economy because it generates more compete combustion under extreme low engine loads (like cruising down the freeway at 3,000 rpm). Your boat won't do that.

GT

« **Reply #13 on:** June 30, 2015, 07:26:17 PM »

Quote from: GT Jets on June 30, 2015, 07:05:26 PM

What?

The vacuum signal comes from the intake manifold.

Where else would it cover from? Sometimes on a smog controlled car it will be switched by the carburetor step port. The vacuum still comes from the intake manifold.

GT

haaaha I'm aware bro usually metered from carb sometimes w a restricted valve in between. the vacuum modulator is the trans . I hear ya I'll unplug that suckered and put a plug on my holley. I prolly shoulda said in Portugal they put it in the trans..... LOL btw I threw in a 4.5 pv and upgraded to pertronix 2 ignition what a difference man can't wait to do more stuff to it

« **Reply #14 on:** June 30, 2015, 07:31:41 PM »

Quote from: GT Jets on June 30, 2015, 07:19:17 PM

Not correcting you, just stating it in a different way.

The torque curve on a jet boat engine is linear, meaning the faster the pump spins, the more torque is required.

An automotive engine has an inverse torque curve, meaning it needs the torque to "pull a gear", once the engine starts to speed up, the torque requirement drops.

This is what the vacuum advance is really for. It is detonation protection for this load. It can also increase the fuel economy because it generates more compete combustion under extreme low engine loads (like cruising down the freeway at 3,000 rpm). Your boat won't do that.

GT

I'm confused? I was referring to the timing curve, not the torque curve. I was trying to explain to him, the rate at which his timing advances to total advance might change a HAIR, but probably not much if at all. Since there's little to no load on an engine spinning a jet up to around 2500-3000 rpm. And your motor is likely already at full advance just based on spinning the dizzy by that point, even without a vacuum advance hooked up.

« **Reply #15 on:** June 30, 2015, 07:35:38 PM »

Quote from: che77y on June 30, 2015, 07:26:17 PM

haaaha I'm aware bro usually metered from carb sometimes w a restricted valve in between. the vacuum modulator is the trans . I hear ya I'll unplug that suckered and put a plug on my holley. I prolly shoulda said in Portugal they put it in the trans..... LOL btw I threw in a 4.5 pv and upgraded to pertronix 2 ignition what a difference man can't wait to do more stuff to it

The modulator in the transmission sees manifold vacuum, I don't care of its in Cuba, Chile, Afghanistan, or Siberia.

Also not restricted. A delay valve can be used, but again, that is a SMOG thing.

Our stuff should be compared to a car from the forties to the early sixties.

Edit:

Is crazy what those small changes will do.

Sounds like your getting a handle on it.

GT

« **Reply #16 on:** June 30, 2015, 07:39:59 PM »

Quote from: crazyjim on June 30, 2015, 07:31:41 PM

I'm confused? I was referring to the timing curve, not the torque curve. I was trying to explain to him, the rate at which his timing advances to total advance might change a HAIR, but probably not much if at all. Since there's little to no load on an engine spinning a jet up to around 2500-3000 rpm. And your motor is likely already at full advance just based on spinning the dizzy by that point, even without a vacuum advance hooked up.

I see what you are saying.

All I was trying to convey was that the advace curve would be polar opposite of what is desired (with the vacuum pot hooked up).

GT

« **Reply #17 on:** June 30, 2015, 07:42:07 PM »

I have a vacuum advance on my mallory dizzy in my 56 302sbf. I run a mech Mallory on my jet 460 bbf. The vacuum works off carburetor to dizzy not mani or trans....

« **Reply #18 on:** June 30, 2015, 07:44:41 PM »

Quote from: GT Jets on June 30, 2015, 07:39:59 PM

I see what you are saying.

All I was trying to convey was that the advace curve would be polar opposite of what is desired (with the vacuum pot holed up).

GT

« **Reply #19 on:** June 30, 2015, 08:00:39 PM »

Quote from: Your Mom on June 30, 2015, 07:42:07 PM

I have a vacuum advance on my mallory dizzy in my 56 302sbf. I run a mech Mallory on my jet 460 bbf. The vacuum works off carburetor to dizzy not mani or trans....

My '64 Chevelle was originally off the carburetor when I got it. Changed it to the intake.

This was a 500 horse small block and when I sold it was getting 22 mpg.

It's done both ways. The Chevelle had a pretty lumpy cam and putting the vacuum on the intake really improved idle quality and power brake operations.

GT

« **Reply #20 on:** June 30, 2015, 08:17:49 PM »

Quote from: nordic_scandia on June 30, 2015, 05:02:39 PM

Well crap...

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I was just teasing you bout the escort no one else thinks it's funny tho

imma delete that thing. I like topics like this because we all want to gain every last drop out of our engines.... mines a stock 68 455 dizzy w the vac pot. I'm seeing no need for it now and hey if Itl run a lil better then I'm all for it. Ive built a ton of cars mostly chevy and I caught the boat bug 3 yrs ago without sites like this it would've been a major trial and error session over here. Too bad I'm in sonoma county or I'd come see u guys at the lakes you go too

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