There are many variables that determine the rpm drop and there is really no correct answer unless you are referring to a bone stock engine, trans and torque converter. Once any of these change, the game is off. A larger cam, change in ignition timing, change in torque converter stall speed, etc. You get the picture.

If you ask others with modified engines and transmissions, you will learn that the rpm drop may drop as little as nothing, to as much as 500 rpm (although not too common) depending on the entire recipe.

IMO, 350 drop is not completely uncommon, unless of course we are talking about the bone stock scenario.

 

Original engine and powerglide. It doesn't sound or feel like a larger cam. Engine was rebuilt many years ago from what the previous owner told me and don't know about the originality of torque converter.

 

Is the car new to you? Have you driven it much? How does it run/accelerate/up shift? Is this concerning you for any particular reason? What is the initial timing set at?

 

Had the car since January. After discovering a vacuum leak today, It really runs pretty good at acceleration, cruise, and when I step on it. (Despite the Holley Carb that is running fat). Timing is set at 8 deg. New metallic core wires, new plugs, points, etc. I don't like the way it kinda slams into gear..not real bad..but still annoying. I drive it at least once a week.

 

Could be a number of things, but the vacuum modulator sounds a likely suspect.

 

I seem to have found the problem. Luckily the manual for the Holley 600 carb that is now on the car came with the car when I bought it. I noticed the vacuum advance line was plugged into the full manifold vacuum source. I pulled it out and stuck it on the ported vacuum source. I read that when the car is at idle there should be no suction on the ported inlet and that is what I should be using. One of my mechanic friends told me the ported inlet is not for a GM. I told him maybe not, but it fixed the problem and runs great. Do you all agree that I should be using the ported inlet?
By the way, only 150 rpm difference between neutral and drive now. I also advanced the timing to 12 deg.

 

In a stock or near stock configuration that slamming into gear and big drop in rpm is a sign of not enough initial advance or the need for a good tune-up because the engine is acting anemic or weak at idle. In a performance build it's usually a sign of needing a higher stall converter to deal with the cam in the engine but more idle advance often will help here too.

I'm glad you are seeing better results, advancing the timing probably solved your problem but I see a conflict in the things you did. When you switched from manifold vacuum to ported vacuum it should have lowered your idle timing. Then you adjusted your timing up from 8 to 12 degs btdc which countered the effect of moving the vacuum advance to the ported source.

Are you removing the vacuum source and plugging it when you set the timing? You should even if the vacuum advance is on the ported source... Also be careful when setting initial timing because they extra 4 degrees you added also gets added to the total.

Vacuum advance is supplemental advance that increases and decreases based on the amount of vacuum the engine is pulling. Your Camaro when new came with the vacuum advance on a ported source. It was that way for emissions along with an initial timing of about 4 degs btdc. One of the best things you could do is move the vacuum advance to a manifold source and let the vacuum advance supplement your initial advance. Nothing else changes by doing this, the vacuum advance still works just like it does on a ported source, it just helps the engine at idle and usually helps the big rpm drop with an automatic when put in gear.

 

I did have it on a manifold source until this morning and the idle was rough. I varied the timing from 8 deg to 12 to see what worked better but that never solved the idle roughness and rpm difference between neutral and drive.

I called a tech at Holley this afternoon and he verified the ported vac is the application to be used on my car especially to alleviate the rough idle. He told me the hot rodders used to use the manifold source.

 

Glad you have it running better. If you like to tinker or are looking for a little more read this post from JohnZ.

http://www.camaros.net/forums/showthread.php?t=150344

 

Thanks for that link!

 

FYI: If the 327 in your 67 has a Holley carb then it's not exactly stock (Rochester 2v or 4V would be stock) although it is a common upgrade. I assume it's an 1850 as you state it's a 600 cfm version. Does your engine also have an aluminum intake manifold?

 

Congratulations on getting your RPM drop to only 150 when shifting from neutral to drive. You don't have to do anything else now but enjoy your car.

 

Well, IMHO, it should also bark the tires.

 

Holley model# 4160. Sorry, I guess that does constitute an upgrade. Also, original manifold is still intact, no aluminum.
I do have the original Roch. Quad that I will have rebuilt in a couple of weeks. I would like to try it out for a while.
I drove the car around today and it is really running good.

 

Jeff,

Glad that you sorted it out and it is running well. :hurray:

 

Thanks Steve. All the suggestions were great. Appreciate all of the support from everyone!