mech or vaccum adv???

[jasonsnova]

ok guys upon pulling the motor out my nova last weekend i found out that my msd dist was getting oil up inside the cap( worn) this is a old msd ,pre billet design mech adv that was a spare out of one of my dads drag cars. anyway im gonna pick up a new one , so should i go with the same mech adv or get a vaccum adv unit. the car has a msd digital 6 box and blaster ss coil, it has a 355 w/bowtie heads,holley hp 750 mech 2nds, solid cam with arpm range of 3500- 7000. the car is mostly street driven...with a occasional trip to the drags each year. so any ideas on which one ....with some reasoning behind it! lol! seems like the mech would be easier to tune and set up....maybe im wrong??? suggestions.....

[68Formula]

Well both have mechanical advance, so really you mean with and without vacuum advance.

Vacuum advance is useful for part throttle acceleration and cruise. When properly tuned, will see better part throttle driveability with it and improved fuel mileage.

It's not much more work to tune. Just tune WOT with it disconnected, and then hook it up and tune part throttle and cruise while listening for pinging (some pinging under light throttle will not damage you engine). It's adjusted using an allen wrench inside the vacuum housing. Very simple.

[6'9"Witha69]

amcmike is correct. If the car is mostly street driven then a vacuum advance is definitely needed. The car will run much smoother and feel better under the 3000 RPM range.

[jasonsnova]

ok vaccum it is...right now the car hates anything below 3000rpm....and with a 6 speed im gonna be luggin it down on the highway!

[LS6 Tommy]

Well both have mechanical advance, so really you mean with and without vacuum advance.

Vacuum advance is useful for part throttle acceleration and cruise. When properly tuned, will see better part throttle driveability with it and improved fuel mileage.

It's not much more work to tune. Just tune WOT with it disconnected, and then hook it up and tune part throttle and cruise while listening for pinging (some pinging under light throttle will not damage you engine). It's adjusted using an allen wrench inside the vacuum housing. Very simple.

"Some pinging under light throttle" is not really acceptable. The noise that sounds like ping you hear from too much advance is called Preignition. Preignition is very similar to Detonation and it starts LONG before you can hear it and it's not good. At the least it causes lost power and at worst it beats on your bearings, although nowhere near as much as Detonation.

Also, not all vacuum advance pots are adjustable. If it doesn't have a hex shaped area by the hose nipple or if it has a number like "10" stamped on it, it's not adjustable. If an engine is bone stock connect the vacuum advance to a ported source, and to manifold vacuum if it's modified. Never use a stock HEI vacuum advance on an aftermarket distributor. Stock HEI's from smog motors have very little initial advance (some are even set AFTER TDC) and can have over 30* of vacuum advance in them.
Generally, a Small Block Chevy likes around 36* of total mechanical advance, "all in" by about 2500 RPM and about 10* of vacuum advance. Initial ( or static) timing should be 10-12* for cams with duration of 220* or less@ .050, 14-16* for duration less than 240* @ .050 and 18-20* for duration of less than 260* @ .050.

Tommy

[jasonsnova]

thanks tommy, it currently has another msd mech adv only in it and the car always seemed to like 38 degrees and was all in by 2800 rpm if i remember correctly(been awhile).

[68Formula]

"Some pinging under light throttle" is not really acceptable. The noise that sounds like ping you hear from too much advance is called Preignition. Preignition is very similar to Detonation and it starts LONG before you can hear it and it's not good. At the least it causes lost power and at worst it beats on your bearings, although nowhere near as much as Detonation.

Also, not all vacuum advance pots are adjustable.

Tommy

I was referring to some pinging during the part throttle tuning process, not after it's tuned and certainly not during heavy throttle and WOT.

A vacuum gage should also be used to keep tabs on the efficiency during this part of the tuning process. Higher vacuum readings indicate better efficiency.

It's also good to note that the carburetor should already be tuned prior to this.

Most aftermarket systems use an adjustable canister. If you are attempting to use a stock HEI, an aftermarket canister can be swapped relatively cheaply.

[MonzaRacer]

Well as for hooking it to ported vacuum WHY ?
this will advance your timing only under acceleration.
For proper use of vacuum advance in a non emission application hook it to manufold vacuum.
Set your intioal and tune your centrifical for best performance.
Then you hook up the vacuum advance and reset your curb idle in drive and dont forget to readjust the mixture too as moretiming makes and engine "see" a richer mixture.
I found it works much better to tune your base+ centrifical doesnt run more than just enough to pull hard.
This being said tune it up to a slight "ping" under hard load then back off 2 degrees and try to drive it hard again. I like to tune for the cheapest ggas possible and then see if it still runs good.
I like to tune the engine for hard running THEN do up the vacuum advance.
Generally most distributors I buld come up with a total in the 48 to 60 degree range.
I ran one of my Blackwidow distributors in my 402 BBC and it had 10.5 to one pustons, closed chamber heads and they had been milled beforew I bought them.
That engine easily handled 54 to 56 total depending on time of year (cool winter air let me add a couple degrees).
Figure this the engine made good power, pulled like a ox and never spark knocked on 92 octane(I used 94 most of the time) and in a 4000lb+ car with 3.55 posi gears and 3 spd trans I drove it daily and got anywhere from 14 to 16 mpg.
I use a simple chart and check my timing with weither a timing tape or an advance timing lightStart at idle out of gear and make sure your weigts arent advancing at idle, too light of springs will do this, and you can mix and match advance weight springs to taylor it.
I prefer to use stock weights and curve cams at first and swap springs for best performance then you can use that weight curve and change the actual hard parts for curve delineation. The stock weights are heavier and let you get a good curve then you can try to speed it up in the curve.
90 percent of the cars I have tuned, even supposedly high rpm race engines driven on the street have had very good characteristics with stock weights and swapping springs.
On your mechanical your tuning for power and acceleration try to have it all in by your non overdrive cruise speed, I have found that if you tune it to OD engne speed you dont get the best effect, besides if you have a little mech adv lose you can get it back with Vacuum.
Ported vacuum will put your advance opening under acceleration till you hit WOT and can cause plate bounce under high rpm driving.
Mt 78 C10 I currently drive has an HEI and here are my specs for the curve with World SR torker heads, tall Holley Street Dominator intake and 800 cfm 4011 Holley and the LT4 HOT roller.
Idle 875-900 18 deg
1000 21
1250 25
1500 29
2000 31
2250 34
2500 37

This is with out Vacuum advance.
I use adjustables on track born engines but for the most part I have found that the Echlin advances from NAPA with great success and if I use adjustables I start them tight and loosen it up in 1/4, 1/2 or full turns till I see no improvement in mileage or drivability.
Honestly either the Echlin or one from Wells work great and never had a problem.
Big trick is take it in steps and make your self a tracking book and use as many variables (mileage, drivabilty, how it behaves traffic, etc.) I had to learn tuning in tech school and perfected how I do it over the last 20 yrs,,, now very few cars have distributors or carbs. Also tuning with EFI is just as important but you have na electronic"ear" helping look for spark knock.
Good Luck
Lee