Vacuum Advance Control, 1974 L82 TH400.

[Petter T.]

Hi, I have tried to find a Vacuum Advance Control online for my 1974 L82 TH400 (I think the part Number for the distributors is 1112853) but have not found any.
Does someone know where I can find one?

Br
Petter

[Mike F.]

Re: Vacuum Advance Control, 1974 L82 TH400.

I think the part number for your distributor should be 1112150 per the link below.

Page 74 of the .pdf gives you all of the vacuum specs. 0*@5-7 / 13.5*-20*@12

https://www.gmheritagecenter.com/docs/gm-heritage-archive/vehicle-information-kits/Corvette/1974-Chevrolet-Corvette.pdf

http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/Vaacuum_Advance_Specs.pdf

[Joe L.]

Re: Vacuum Advance Control, 1974 L82 TH400.

Hi, I have tried to find a Vacuum Advance Control online for my 1974 L82 TH400 (I think the part Number for the distributors is 1112853) but have not found any.
Does someone know where I can find one?

Br
Petter

Petter-------

It depends what you want------a vacuum control that has the same functionality as the original or an original vaccum control.

If you want an original vacuum control, what you want is a GM #1973448, aka Delco D1378A. It's long-since GM-discontinued but you should be able to find one on eBay or elsewhere.

If you just want a vacuum control that will perform as-original (and be very close to the original configuration), you can obtain a Standard Motor Products #VC-181, GM #88924983, or Delco D1310C. All of these are the same and will be stamped B-20.

[Duke W.]

Re: Vacuum Advance Control, 1974 L82 TH400.

1974 models, the last year before catalysts were added, have very poor spark advance maps that are setup for emissions, but severely compromise performance, fuel economy. and driveability. If you need a new VAC you have an opportunity to easily modify the spark advance map for better performance and fuel economy as long as it doesn't require field emission testing.

The first thing to do is convert the ported vacuum advance to full time, and if I am interpreting the AMA specs correctly L82/TH400 does not have TCS (which can complicate the conversion) so all you have to do is tee off the Q-jet choke vacuum break of any other full time vacuum source. Reset idle speed and mixture using the pre-emission procedure, and you will also likely have tweak the fast idle system to bring fast idle speed down to spec.

Manual trans L82s should pull about 14" at idle speed of about 800-900, but with a TH400 idling in Drive at 700 vacuum will be lower and an 8" VAC (B28, Airtex 4V1053) is likely required. Of course, the easy way to determine is simply measure engine vacuum idling in Drive.

The centrifugal is a bit short and very lazy, 18-22 @ 5000, so lighter springs will release new found performance.

The initial timing recommendation is only 8 degrees, so total WOT advance is only 26 to 30, which is way short of the optimum range of 36-40. This can be easily corrected by increasing initial timing to something on the order of 18 degrees, or with light springs set total WOT advance in the range of 36-40 with a dial back timing light at an engine speed a few hundred revs above where the light springs allow full centrifugal advance.

The greater initial advance combined with 16 from the VAC yields total idle advance of about 30, which is about right for the configuration and will yield better idle quality, lower fuel consumption, less heat rejection to the cooling system, and lower underhood temperatures.

Duke

[Petter T.]

pre-emission procedure", do you have any links on how to perform this?

Petter

[Duke W.]

Re: Vacuum Advance Control, 1974 L82 TH400.

I'm not that familiar with the TCS, but I think the input to the solenoid is full manifold vacuum. If so you could just remove the input hose and use another piece of hose to route the vacuum source directly to the VAC. If the input is not full manifold vacuum use another on the carb like teeing into the choke vacuum break. Disconnect the TCS solenoid and plug all the the inputs and outputs.

Idle vacuum and the Two-Inch Rule are used to determine what VAC should be installed for full time vacuum advance. Google my name san diego corvette and download my tuning seminar, which explains the Two-Inch Rule along with much more. With an automatic, idle vacuum should be measured with the transmission in Drive.

If you have any GM service manual (or almost any manufacturer's manual that covers idle speed/mixture adjustment of the carburetor) there will be a recommended procedure for setting idle speed/mixture (in Drive with an automatic). It will give an initial setting for the idle mixture screws, and then you adjust each in equal increments in and out in quarter to eighth turn increments until you obtain the highest idle speed and vacuum. It takes a little practice. This is the pre-emission control procedure.

Emission controlled engines generally start out as above, and then continue with the "RPM drop" method. Once optimum is attained, you lean the mixture (turn the screws in in most cases) until engine idle speed drops, typically 50 RPM. This reduces HC and CO at idle, but results in poorer idle quality.

BTW, 1974 model US cars were plagued with driveability issues. Carb calibration was lean for all modes - cold start, warmup, idle, cruise along with very retarded spark advance maps. That's what it took without catalysts to meet emission standards. The addition of catalysts in 1975 allowed richer carb calibration in most modes and less retarded spark advance maps, which combined to improve driveability and fuel economy.

Duke

[Greg S.]

Re: Vacuum Advance Control, 1974 L82 TH400.

Duke and others

Don't mean to hyjak this thread but I have some questions about a 75 timing map that may be pertinent to the original post. I also have an L82 but 4 speed and was interested in improving overall performance by remapping the distributor and vacuum can. Currently I believe everything is original, carb and distributor and vacuum can labeled 508-15. I expected the timing to be slow, all in above 4000 rpm. This is not the case. Measured with a dial back timing light I have initial timing set at 15. Spec is 12. With the vacuum disconnected at the distributor I get 30 degrees advance by 2000 rpm. I get 43 degrees at 2000 rpm with the vacuum connected to the distributor. The centrifugal advance seems to come in very early which I understand to be a good thing but not what I expect from a early emissions standpoint. The engine idles at 950 rpm with 14" of vacuum. Using a mittyvac on the vacuum can it starts to pull at 5" and is all in at 12" producing 15 degrees. So it appears the vacuum can meets the 2 inch rule. I believe the distributor vacuum hose (the four digit numbered hose in the picture) is using ported vacuum.

The car idles great even with air conditioner on (loses about 200 rpm), runs and responds well to throttle inputs but seems a little weak on torque and flattens out about 5200 rpm under WOT. Red line is 5600. What should I do to improve torque and upper rpm pull? Or should I leave well enough alone?

[Duke W.]

Re: Vacuum Advance Control, 1974 L82 TH400.

Did you compare your data to OE specs in the CSM or AMA specs? If you did you should know that the centrifugal meets OE spec, but the VAC does not, though I think the VAC specs in the AMA specs may be wrong because they are just too weird.

So with only 16 total centrifugal even 15 initial provides only 31 total WOT advance, which is well short of the 36-40 optimum range.

Note in your photo that the weights do not retract fully to the football since the weight ends interfere with each other. Remove the springs and lift one weight enough to manually retract them all the way to the football. Then use a protractor and eyeball the degrees of total rotational freedom, and remember crank advance is double what you measure at the distributor. Cross check by measuring how many degrees the weight will retract to hit the football. Since total crank centrifugal is only 16, in its current configuration there should only be 8 degrees rotational freedom, so the sum of the additional retraction plus 8 times two is how much total centrifugal is available in distributor degrees if the weights are allowed to retract fully to the football.

Also with the bottom weight fully retracted install the spring and determine whether it still has some tension.

Even though the VAC doesn't meet the weird OE spec, what you measured indicates it's a good match for the engine when converted to full time vacuum advance.

As far as top end power is concerned the big problem is the effective single exhaust and OE bead bed catalyst. This generates 10+ psi backpressure that generates huge parasitic exhaust pumping power that kills the top end. (For every psi of exhaust back pressure BMEP is reduce by one-plus psi.) By comparison the full dual 2.5" exhaust system on previous models generates only about 3 psi back pressure at peak revs with a SHP engine. A modern monolithic cat will help, but retrofitting a previous full dual system will unleash a lot of wild horses... as long as your car doesn't have to be field emission tested and keep all the OE parts.

BTW the centrifugal mechanism has some mild surface corrosion. Clean it up with a Scotch Brite pad and mineral spirits, then dry and wipe down with WD-40. Do not use any kind of oil or grease on the mechanism. Also you will probably find that the plastic weight pivot bushings are worn and need to be replaced, so do this before you begin the measurements. IIRC the bushings are available as BWD DG-13, and they are likely available under other brands/part numbers.

Report you results, but please start a new thread as your engine is quite different from the OPs due to different model year, transmission, catalyst, and HEI.

Duke

[Greg S.]

Re: Vacuum Advance Control, 1974 L82 TH400.

Did you compare your data to OE specs in the CSM or AMA specs? If you did you should know that the centrifugal meets OE spec, but the VAC does not, though I think the VAC specs in the AMA specs may be wrong because they are just too weird.

So with only 16 total centrifugal even 15 initial provides only 31 total WOT advance, which is well short of the 36-40 optimum range.

Note in your photo that the weights do not retract fully to the football since the weight ends interfere with each other. Remove the springs and lift one weight enough to manually retract them all the way to the football. Then use a protractor and eyeball the degrees of total rotational freedom, and remember crank advance is double what you measure at the distributor. Cross check by measuring how many degrees the weight will retract to hit the football. Since total crank centrifugal is only 16, in its current configuration there should only be 8 degrees rotational freedom, so the sum of the additional retraction plus 8 times two is how much total centrifugal is available in distributor degrees if the weights are allowed to retract fully to the football.

Also with the bottom weight fully retracted install the spring and determine whether it still has some tension.

Even though the VAC doesn't meet the weird OE spec, what you measured indicates it's a good match for the engine when converted to full time vacuum advance.

As far as top end power is concerned the big problem is the effective single exhaust and OE bead bed catalyst. This generates 10+ psi backpressure that generates huge parasitic exhaust pumping power that kills the top end. (For every psi of exhaust back pressure BMEP is reduce by one-plus psi.) By comparison the full dual 2.5" exhaust system on previous models generates only about 3 psi back pressure at peak revs with a SHP engine. A modern monolithic cat will help, but retrofitting a previous full dual system will unleash a lot of wild horses... as long as your car doesn't have to be field emission tested and keep all the OE parts.

BTW the centrifugal mechanism has some mild surface corrosion. Clean it up with a Scotch Brite pad and mineral spirits, then dry and wipe down with WD-40. Do not use any kind of oil or grease on the mechanism. Also you will probably find that the plastic weight pivot bushings are worn and need to be replaced, so do this before you begin the measurements. IIRC the bushings are available as BWD DG-13, and they are likely available under other brands/part numbers.

Report you results, but please start a new thread as your engine is quite different from the OPs due to different model year, transmission, catalyst, and HEI.

Duke

i will do as you advise. May take a little while to report back. Will start a new thread. Forgot to mention that this car has been converted to dual exhaust which is one reason I expected more top end performance.

[Duke W.]

Re: Vacuum Advance Control, 1974 L82 TH400.

With your current spark advance map you are a good 8 degrees short of optimum WOT spark advance, which is probably costing at least 5 percent of peak power potential and several hundred revs of useable power bandwidth.

Once you get the spark advance map optimized you should notice a SOTP improvement.

Duke