Has anybody swapped the weights and springs in a vaccum advanced HEI? I swapped the CC Dist. for an HEI this weekend. I've set the advance at 10*, and my low end and midrange are incredible. The high end is somewhat of a let down, but still ok.

I was thinking of changin the vaccum canister out, to make sure that there isn't a problem there...

I also see a set of weights and springs for sale in summit for real cheap...has anybody used these and how does it work?

I would certainly like to keep the low end improvement, but would also like to improve the high end....

please advise.

 

The vacuum can should have no effect at WOT.
The aftermarket springs & weights are to allow you to adjust the advance curve.
The smaller weaker springs allow you to reach full centrifugal advance quicker.

Before you go changing your curve around, first you need to know how much centrifugal advance you are getting now with your HEI (they are not all the same).

You will need to use a dial-back timing light to get this information.

*with your vacuum advance plugged off and your idle @700rpm or lower, check your intial timing with the light (dialback set to zero).
you may have around 10-degrees of initial advance.

*then there are a couple of ways to use the dialback light.
The first way is to have someone rev the engine to about 4000rpms (or until the advance stops increasing) and look at your damper with the timing light. Twist the dial-back **** to about the 20-degree setting so that the mark on your damper once again is lined up with the 10-degree mark on your timing tab.
Once the damper mark is back to reading 10-degrees @4000+ rpm you have found your centrifugal advance. Just look at where the dial points to (maybe 25 degrees) and that is your centrifugal advance built into the distributor.
Add this number (25) to your initial advance (10), and that gives you your total advance at WOT (35 degrees).

*The other way can be used if you have a large calibrated timing tab.
You twist the dial on the light (at idle) until the mark on the damper is equal with the -10 (negative ATDC) mark on your timing tab. Then you rev the engine until it stops advancing and note which timing-tab mark the advance stops at. If the mark on the damper stops at 15-degrees, then you have 25-degrees of centrifugal advance, and 35-total.

The weakers springs will allow you to reach your total advance quicker (at a lower RPM). You will be able to see this by using the dial-back timing light. Your timing may stop advancing at 3000 rpm or less with the weaker springs.

Contrary to popular belief (due to Hot-Rod magaizine, etc) the exact shape of the centrifugal advance curve is of no importance when it comes to WOT performance.
Many drag racers do not run any kind of advance curve whatsoever. They just lock the advance plates together and run all of their advance as initial. This increases bottom end response and consistency.

A good rule for the street is to run at least one of the weak springs and have your full advance all in by 3000 rpms. Small-block chevies generally make best power at around 35 to 37 degrees TOTAL advance at WOT. If your car is running best at a higher total advance like 40+, then that means your air/fuel ratio is way off and you need some tuning before you go to find your best timing setting.

A good place to start with power tuning is to set your total timing to 37 degrees and go leaner on the carburetor until you see your trap speed drop, feel lean misfire, or hear pinging. At that point you can drop the timing back a little more and play with the A/F ratio until you are at the lean-limit and maximum power.

Your high-end power is a function of TOTAL timing and A/F ratio.
Your low-end response is more a function of initial advance & how quick your centrifugal advance comes in.

If you have any other questions, or need some more advanced tuning methods let me know. There is much more to power-tuning than I've written here.

ODB


[This message has been edited by The ODB (edited February 19, 2001).]

 

Couldn't have said it better myself.

You can pick up weights and springs from other HEI's and try swapping them around to get your total advance. You need the movable weights and the cam in the center also. However all HEI distributors are not the same. From 74 to somewhere around 81 or so, GM used a variety of different styles of distributors. The better ones were in the late 70's that used bushings in the weights. The bushings didn't wear out as easily as the metal pins did and you can buy replacement bushings for the ones that did wear out. You can even drill out the small hole weights and insert the bushings but even a titanium bit will dull unless the weights are heated up cherry red first.

The weights will change the amount of advance. If your original initial timing is 10 degrees with a total of 36 then the weights are allowing 26 degrees of advance. If you advance your base timing to 15 degrees then your total is now 41 (too much). You need to change the weights so that the total is back down to 36 (or whatever total works best for your engine).

The springs just change the curve. Lighter springs allow the advance to occur sooner. Factory stock springs may limit full advance until 4000-4500 rpm while performance lighter springs allow full advance to come in by 3000 rpm. Having the advance come in sooner will increase performance but will also make the engine run a bit hotter.

Like OBD said, racers don't usually have advance curves for their distrubutors. Mine is locked out at 36 degrees. My ignition does a 20 degree start retard so the engine can crank over and start. Once it's started, the ignition goes to full advance. Since a race engine rairly operates at part throttle, (driving around in the pits), advance curves and vacuum advance are not needed. Race engines are designed to operate above 3000 rpm so there's no need to have an advance curve below it and above that it should be at full advance.

The vacuum canister gives an additional 10 degrees of advance but is only used on part throttle applications.

The best way to set up your distributor is to take it into a shop. From your car and engine setup they can easily set up an advance curve on a machine. You can do it yourself with a timing light like ODB said but it takes time to set it up properly.

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Follow my racing progress on Stephen's racing page
and check out the race car

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
461 Big Block installed and ready for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

Changing the weights and springs in my distributor gave me the biggest "seat of the pants" feeling of all my mods. It totally changed the way my car drives especially around town.

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Cosmik Debris