Why No Vacuum Advance - Team Camaro Tech
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post #1 of 10 (permalink) Old May 23rd, 14, 11:08 AM Thread Starter
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Mike
 
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Why No Vacuum Advance

Hi guys. I've got a 383 stroker rated at 493 horsepower built by United Racing Engines. The instructions implore me not to hook up the vacuum advance, saying "the distributor is set up to utilize the mechanical advance only. If you use the vacuum advance it could lead to detonation".

I've been reading about timing and vacuum advance, and am coming to the conclusion that such a set up is quite impractical for street driving (good maybe if you're much more frequently at wide open throttle), and that it will run much nicer with the vacuum advance hooked up. So I took the initial timing back to 15 before TDC (instructions say 24-27 initial timing w/o vacuum advance), and i hooked up the vacuum advance. Does anyone see any problem with what I've done? Could there be something else with how they set up the distributor (springs, whatever) that make this a bad idea? Any advice? Thanks.
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post #2 of 10 (permalink) Old May 23rd, 14, 11:29 AM
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Steiner
 
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Re: Why No Vacuum Advance

God Almighty. Do you know what the total timing was on it? You're probably going to need a stop bushing and spring kit for the distributor to recurve it for what you are wanting to do. They probably had about a 10 degree stop bushing in it so it was jumping to full timing really quickly, quicker than what you can typically get with a normal curve (lower initial) and light springs.

If that's the case, now that you've dialed it back you might only get 25 degrees total timing which will cost you a heap of hp. If it's an HEI in the engine, it probably has 20 degrees or more in the vac can which is why they would say not to use it with their timing setup. That's easier for them to do than to set it up properly. Plus typical crate engine dyno tuning is just w.o.t. tuning, not done for driveability.

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post #3 of 10 (permalink) Old May 23rd, 14, 12:06 PM
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Re: Why No Vacuum Advance

I would have left it.
You need to get a vacuum gauge and see how much vacuum the engine is pulling at idle then get the correct vacuum advance can if the motor is not stock. Get a T connection for the vacuum hose too if you want to see what the Hg is when the can is connected to the vacuum hose and gauge.

My vacuum reading with the stock can connected is 10" Hg and 11" Hg with the B28 can. My stock 70 Z28 can is a 201 15. I tried the B28 can that suppose to start at 3" or 4" and it shot my initial timing to 34* because I'm running with 16* initial and 36* total, which caused all my plugs to turn black and make the car not idle right. Forget about trying to tune the cab. I put back the 201 15. The hell with it. My car runs better with the stock can.

Mark 70 Z28 M22 3.73 Z21 Z87 - Lunati 20120121 Roller Cam

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post #4 of 10 (permalink) Old May 23rd, 14, 12:08 PM
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Re: Why No Vacuum Advance

I bet the total is around 34deg btdc and you only have a curve of 8-10 degrees in the dist. It sounds like your engine is designed and built to be at the track more than it is on the street! A lot of drag racers lock out the timing curve and run a flat advance of whatever the engine likes for total. They get away with this because the engine is run at WOT 98% of the time.

You need to know exactly what advance is built into your distributor and what total timing the engine likes. You need to know more about your engines dynamic compression, quench and cam profile. My guess is your engine is on the ragged edge of not detonating as designed by the engine builder and adding additional advance with the vacuum can will put it over that edge. Once you know a few details you can rework the distributor so the curve allows the use of some vacuum advance but you'll probably loose some performance in doing so.

Is this the URE that offers 3 yr 100k mile bullet proof warrantees and guaranteed HP and torque ratings?

...Dennis

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post #5 of 10 (permalink) Old May 23rd, 14, 12:27 PM
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Mark
 
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Re: Why No Vacuum Advance

You can always go back if you have a light that measures total timing. Your engine is the distributor machine if you don't own a Sun Dizzy machine from back in the good ol days.

Friends of mine don't use vacuum cans either on the street.


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post #6 of 10 (permalink) Old May 23rd, 14, 12:34 PM
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Re: Why No Vacuum Advance

I bought my383 from united. it dynoed at 532 511 torque.
it is very streetable. I can actually do 35 mph on the level with 3:55 gears in 4th gear without any jerking. I might add I just checked the gas mileage and it was just a tad under 13 mpg
if you talked to them before you ordered your engine they would have explained all the details to you
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post #7 of 10 (permalink) Old May 23rd, 14, 12:45 PM Thread Starter
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Mike
 
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Re: Why No Vacuum Advance

All right, I'll take off the vacuum advance and reset the initial and hope I didn't do any damage. I didnt hear any detonation, but who knows. Obviously I'm a neophyte.
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post #8 of 10 (permalink) Old May 23rd, 14, 01:06 PM
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Mark
 
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Re: Why No Vacuum Advance

You can leave the can in place just plug the vacuum line.

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post #9 of 10 (permalink) Old May 23rd, 14, 02:18 PM Thread Starter
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Mike
 
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Re: Why No Vacuum Advance

Yep, that's how I've had until recently. Plugged the "can"
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post #10 of 10 (permalink) Old May 23rd, 14, 02:45 PM
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Steps
 
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Re: Why No Vacuum Advance

Quote:
I bet the total is around 34deg btdc and you only have a curve of 8-10 degrees in the dist. It sounds like your engine is designed and built to be at the track more than it is on the street!
yes sort of
They most proberly drop an off the shelf dizzie in .. HEI ,, modern.. these are setup for modern EGR engines off the shelf... lean burn...which mean little cent and a hell of a lot of Vaccuum...
Or put another way.. they are either to damn lazy or or dont have any understanding of timing, how it relates to start motors and other components of an engine.

Stiener also explained here
Quote:
. If it's an HEI in the engine, it probably has 20 degrees or more in the vac can which is why they would say not to use it with their timing setup. That's easier for them to do than to set it up properly. Plus typical crate engine dyno tuning is just w.o.t. tuning, not done for driveability.
A Street driven engine IF set up correctly will ALWAYS have VA and if a Non egr will always have VA.. If not the engine has not been fully dialed in, most property because of a lack of understanding how time components and carb components work

basically both the carb and the dizzie have 3 separate independent functions that are dialed in separately
idle circuits, power circuits and cruise circuits.....
And what is a shame is ppl spend so much time in body detail , engine building then at the end only bother with doing the power circuit and the rest are basically 1/2 hearted patch ups.
Its like throwing panels on a car then not even bother getting them even close to straight.
Sad really.

My Spelling is not incorrect...it is creative

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