How many rpm should the engine drop when going from park?neurtal to drive. I have my idle speed set at 700 rpm and drops down to 500 rpm. (I have a chambered exhaust system on the vehicle it sounds okay but not quite loud enough at idle fpr me) I retarded the timing and reset the idle speed to 650 rpm. (The vehicle sounded wicked, nice and much louder) The idle speed dropped down about 100 rpm. However, I did not check use my timing light to see how much I retarded the timing. (I know this is wrong) I snapped the throttle and got a nice pop and flame from the carb...Oh Oh. I then took and placed a vacuum guage at the fitting to check engine vacuum. it was at 15 inches at idle. The red area beneath the number 15 indicates late ignition timing, damn!!!! I rotated the distributor in the vicinity it was prior to retarding the timing. I put my vacuum guage back on and the needle was back in the normal range area. (The exhaust system quited down like it was in the beginning Darn, it sure sounded nice with the timing retarded) I know engine idle has nothing to do with timing. Tomorrow I will set the timing with my timing light to 36 degrees total). Back to the question, what is the proper idle speed and how much should the rpm drop when shifting into drive?

 

The proper idle speed should be 800 RPM for an automatic transmission says my manual, but it's a '71. What year is yours? Maybe it doesn't matter.
When I put it into D it goes down 200 RPM.
Recently, I learned that higner rather than lower RPM is good for an old engine - lubrication!
Hope this helps a little bit.
Were you at the Vettes at Waterside the other day?

 

Have 79, engine is modified. I will check my service manual. My engine is modified. Engine was an L-48. I will set it to the specs of the L-82 engine, even though my engine is stronger than the L-82.

 

It all depends on the cam in the car. If you have a fairly beefy cam you will need to have a higher idle. Plus you can't go by what the vac gauge says for timing becuase with a big cam you will not get nearly as much vac at idle as a stock cam would. Try setting your initial to about 10-12 and total to 36. You may have a hard time getting 36 total by around 2600-2800 rpms without new springs and wieghts in the dist. Once you do, it should be good for max performance. Remember to take your vac advance off the dist and cap it off. One more thing, the idle is correlated to the timing. If you lower the idle then you will have to advance your initial timing so that it is at the right spot. (10-12*). At least that is what I have come to understand.

 

I have an L48 that drops from 800 to 600, park to drive.

 

Check to see how your vacuum advance is hooked up, and check to see which vacuum advance control unit you are using. If the vac unit does not pull its full advance in at idle in drive, you will get a significant rpm change as the vac advance retards the timing. To test if this is happening, yank the hose off the vac advance, plug it, and re-set your idle speed. Then, drop it in drive and see how the rpm changes. Let me know the results and I can give you some suggestions.
Lars