Oval Track Dizzy… Locked Advance or Not?

[Krooser]

I value the opinions here so I have another question.

We are a few days away from being ready to fire up and break in the engine for my son't dirt B-mod… SBC, 355, HEI dizzy, compression somewhere north of 9.5-1… you know the drill. I think we ought to run an advance curve on the dizzy… the PO of the car sez 'no!' (of course he no longer owns the car)… My engine builder sez it's Ok if we want to… not sure we do.

Cam King did the cam… I think Mike advised against locking it out...

What do YOU think?

[sanfordandson]

locked! absolutely no reason for advance in a circle track car. one more thing to break.

[Krooser]

I've never run one that was locked. I've always been under the impression that a oval track car that runs at varied rpm's can benefit from a curve in the distributor especially on restarts.

When I put my early blown Hemi together several years ago I had a vintage Cirillo Mag for it but sold it since it was a locked timing deal… I know street and oval track are different…

I never had any issues with the advance mechanisms on the Ford, Mallory and Accel units I've run in the past.

[sanfordandson]

ignition advance is normally fully on my 3000-3500rpm. you MIGHT restart below that rpm but probably not. you will be at full advance all the time while racing.

i've seen many springs/weights break over the years!

[CNC BLOCKS]

I build a lot of circle track engines and run locked dist. With the dist. locked you create the most vacuum and the boosters get a better signal to drop fuel.

On the Chevys the timing will actually retard when setting the time at the higher RPM's from cam twist, That why I am a big believer in BBC cam cores in the SBC's seems to work from what I have seen.

So far over the years no problems.

The ones that have a small amount of curve in the timing is all in around 2000 RPM.

[rfoll]

A distributor can be set up with a curve that comes in at very low rpms and will help starting/starter issues when everything is hot.

[BigBro74]

A distributor can be set up with a curve that comes in at very low rpms and will help starting/starter issues when everything is hot.

^^This is one consideration, especially if its with a newer driver.

I feel like cars that have locked advance DO have the advantages Carl stated, to me they also seem a bit sharper on restarts.

On the reliability side, I have seen everything break, including locked advance mechanisms.......

one other problem I have seen (it is very condition dependent) is on a very tacky track, if there is a lot of compression (which yours is not what i'm talking) and fuel that is a bit more prone to pre-ignition (e85) then you get a big deal in a hurry...if the driver gets to lugging the car down in the bottom of the corners real hard...

so I guess im on the fence--I usually set them up for a short, quick advance, all in by 3 grand or so. almost everyone else locks them though.....

[sanfordandson]

On the reliability side, I have seen everything break, including locked advance mechanisms.......

i've seen this too. especially with the aluminum lockout plates. we weld all of ours.

[billet]

Save the trouble and lock it, I'd say 95% of all modifies have the advance locked.

On a side note about starting: Make sure you have a separate ignition switch and spin the engine first then flip on the switch to start.

[BigBro74]

On the reliability side, I have seen everything break, including locked advance mechanisms.......

i've seen this too. especially with the aluminum lockout plates. we weld all of ours.

Actually I have seen welded ones break too, but like I said I think I have seen darn near everything break. They fixed the welded one pretty quick

[victorymachine]

Save the trouble and lock it, I'd say 95% of all modifies have the advance locked.

On a side note about starting: Make sure you have a separate ignition switch and spin the engine first then flip on the switch to start.

The southern sportmod I sponser has the dizzy locked, as does everyone else here I'm aware of.
The seperate ignition/starter switches make a big difference. Hit the starter and get the motor spinning good then flip on the ignition, they usually come right to life....

[BigBro74]

Save the trouble and lock it, I'd say 95% of all modifies have the advance locked.

On a side note about starting: Make sure you have a separate ignition switch and spin the engine first then flip on the switch to start.

The southern sportmod I sponser has the dizzy locked, as does everyone else here I'm aware of.
The seperate ignition/starter switches make a big difference. Hit the starter and get the motor spinning good then flip on the ignition, they usually come right to life....

I agree with this , but a newer driver spins out they can often forget to shut the ignition off before cranking

[CNC BLOCKS]

When welding the dist. have it in the advanced position.

[Krooser]

Good points.

My son and I were at the carb shop tonite picking up one of our reworked/rebuilt carbs. We talked about this locked/unlocked deal. Al, my carb guy, was on the fence. A slow restart may favor a dizzy with an advance. A faster restart would favor a locked advance.

I think we'll weld the darn thing 'advanced' (as Carl pointed out) and give it a try.

[Tuner]

You can make a useful curve with a genuine GM parts HEI using a 375 cam and 41 weights with the two big springs which come on MSD billet distributors out of the box, (approximately) .046” wire diameter, .286” OD, 4 and 1/2 coils. Trim a bit of the end of the eye loop off each end, about ¼ of the loop and close the loops up until the inside free-length of the springs is about .010” or .015” shorter than the length you measure on the outside of the weight pivot pins and cam mounting pins, the outside distance of the spring mounting pins, so the springs are pre-loaded equally

---- It is very important that these heavy springs are pre-loaded equally ----

The curve will be close to this, use 18° initial, start: 1100-1200 RPM, 26°-28° @ 2000-2200, 32°-34° @ 3000 RPM, 34°-36°@ 5000 RPM, 36°-38° @ 7000 RPM, 38°-40° @ 8000 RPM. This is contingent on the weights and pivot pins are not worn or loose and the cam shows no wear.

Again, it is very important the springs in an HEI advance mechanism are exactly alike and installed with exactly the same preload. Measure the pin span and the relaxed springs’ eyes internal lengths to be sure the springs will be stretched equally when installed.

This type of advance curve with the flyweight and spring mechanism in control of the timing at all RPM, instead of bottoming the mechanism on a mechanical stop by welding the slot or using a screw or roll pin in the rotor mounting bar, it isolates the torsional noise – oscillation - in the distributor shaft caused by driving the oil pump. This is especially useful when a high-volume or high-pressure pump is causing spark scatter because the advance is welded solid or bouncing on and off a solid stop.

In restricted classes, such as circle track where everybody has the same size 2-bbl carb, using this curve with the initial set to wherever makes the best power, depending on carb size 350, 500 or 390, etc. but typically advancing from 36 at 3000 to maybe 44 at 8000, is almost an unfair advantage in a field of engines which are “all in at 2500” because the locked or all in early timing retards as RPM increases (because the electronics in modules or CD boxes have a constant slew rate) and engines actually need more timing because of the high intake vacuum at high RPM. This curve compensates for the retard and allows timing for best torque and then advancing to best power at high RPM.

Each engine’s best timing will depend on intake vacuum at peak RPM which will depend on carb size and engine power. Engines with small carbs which have a large increase in intake vacuum from peak torque to peak power may want more timing at peak power than is best to avoid detonation or is even safe at peak torque. This advance curve allows satisfying both requirements at each end of the RPM range.