L 71-72 Ported Vacuum

[Keith M.]

Okay Guys we convert the L 71-72 vacuum what impact will it have on PV? Thanks, Keith

[Dick W.]

Re: L 71-72 Ported Vacuum

Keith are you talking about a L-71 tripower or a late '71

[Dick W.]

Re: L 71-72 Ported Vacuum

Keith are you talking about a L-71 tripower or a late '71

[Keith M.]

Re: L 71-72 Ported Vacuum

Reba, Sorry about the confusion. You an Dick probably know my mind runs in only one direction when it comes to Corvettes. Yep,I am talking about 427's 425 and 435 cars. Keith

[Keith M.]

Re: L 71-72 Ported Vacuum

Reba, Sorry about the confusion. You an Dick probably know my mind runs in only one direction when it comes to Corvettes. Yep,I am talking about 427's 425 and 435 cars. Keith

[Duke W.]

Re: L 71-72 Ported Vacuum

It should not have any effect, but you have to run through the idle speed/mixture adjustment procedure after the change because if you just switch from ported to full manifold vacuum and do nothing else your cold fast idle and hot idle speed will increase by several hundred RPM.

See your service manual for proper cold fast idle and warmed up idle speeds and adjust the carb as necessary to meet spec.

The air cleaner will cover the evidence if you teed into the choke vacuum break line, but if you're clever you can make the mod inside the carb and it will be invisible!

Remember, the PV judge is checking for proper opeation of the car including proper cold start/fast idle/warm up, and warmed up idle speed/characteristics.

If one checked two identical engines back to back, one might be able to tell the difference. The real clue is to have them both idling side by side. The ported vacuum advance set up will have stronger exhaust pulses, but checking exhaust pulses next to an identical engine configuration that has been verified to have the OE ported vacuum advance is not part of the PV process.

Duke

[Duke W.]

Re: L 71-72 Ported Vacuum

It should not have any effect, but you have to run through the idle speed/mixture adjustment procedure after the change because if you just switch from ported to full manifold vacuum and do nothing else your cold fast idle and hot idle speed will increase by several hundred RPM.

See your service manual for proper cold fast idle and warmed up idle speeds and adjust the carb as necessary to meet spec.

The air cleaner will cover the evidence if you teed into the choke vacuum break line, but if you're clever you can make the mod inside the carb and it will be invisible!

Remember, the PV judge is checking for proper opeation of the car including proper cold start/fast idle/warm up, and warmed up idle speed/characteristics.

If one checked two identical engines back to back, one might be able to tell the difference. The real clue is to have them both idling side by side. The ported vacuum advance set up will have stronger exhaust pulses, but checking exhaust pulses next to an identical engine configuration that has been verified to have the OE ported vacuum advance is not part of the PV process.

Duke

[Jack H.]

Re: L 71-72 Ported Vacuum

I believe your advice is wrong, Duke... During PV, the proper function of the factory original CEC system IS checked/verified (and also during most Operations Check sessions during Flight Judging). Judges specifically want to see the 15-30 second initial engine start timer functions as designed, the transmission spark control portion of the CEC system works as designed (shift between low gears/neutral and high gears and verify the tach registers the appropriate RPM change) and that the temperature control side of the CEC functions as designed (engine RPM AUTOMATICALLY drops when the temp gauge climbs from cold to normal heat range).

Plus, if the '71 is A/C equipped, the 1-year only automatic deployment of the A/C compressor on engine shut-down is verified. So, changing the car's distributor vac advance routing (wire around the CEC to provide full time, non-ported, vac) will result in vehicle operation failures.

While changing the engine's emissions control profile might make for more efficient engine operation, it'll cost you judging points during NCRS scrutiny...

[Jack H.]

Re: L 71-72 Ported Vacuum

I believe your advice is wrong, Duke... During PV, the proper function of the factory original CEC system IS checked/verified (and also during most Operations Check sessions during Flight Judging). Judges specifically want to see the 15-30 second initial engine start timer functions as designed, the transmission spark control portion of the CEC system works as designed (shift between low gears/neutral and high gears and verify the tach registers the appropriate RPM change) and that the temperature control side of the CEC functions as designed (engine RPM AUTOMATICALLY drops when the temp gauge climbs from cold to normal heat range).

Plus, if the '71 is A/C equipped, the 1-year only automatic deployment of the A/C compressor on engine shut-down is verified. So, changing the car's distributor vac advance routing (wire around the CEC to provide full time, non-ported, vac) will result in vehicle operation failures.

While changing the engine's emissions control profile might make for more efficient engine operation, it'll cost you judging points during NCRS scrutiny...

[Dick W.]

Re: L 71-72 Ported Vacuum

Jack, Mr. Majure was talking about pre 1970 vehicles, 435's and 425's. I agree with Duke and unknowingly, over the years, probably have performed PV's on some vehicles configured this way.

[Dick W.]

Re: L 71-72 Ported Vacuum

Jack, Mr. Majure was talking about pre 1970 vehicles, 435's and 425's. I agree with Duke and unknowingly, over the years, probably have performed PV's on some vehicles configured this way.

[Jack H.]

Re: L 71-72 Ported Vacuum

Thanks for the shoulder tap, Dick! I saw '71-72 and dove too quickly to conclude the thread was about the model year vs. engine option....

[Jack H.]

Re: L 71-72 Ported Vacuum

Thanks for the shoulder tap, Dick! I saw '71-72 and dove too quickly to conclude the thread was about the model year vs. engine option....

[Dick W.]

Re: L 71-72 Ported Vacuum

Yah, I had to ask the question also

[Dick W.]

Re: L 71-72 Ported Vacuum

Yah, I had to ask the question also