Ok - the 331 (30 over) for my 66 chevelle convertible is complete, broken in and roughly tuned. I'm running a 216/228 with 107/112 separation, ported 416 heads and flat-tops. My quench is poor which put me in the 9.5:1 static and 7.8:1 dynamic ratio. As I was tuning it I had 14" manifold vacuum at 800 rpm and 11 deg initial advance without the distributor. As I rev it up it takes an initial vacuum drop to around 12" then she builds vacuum up to around 24" at about 3500 rpm. Does this sound right? The only reason that i ask is that with my previous 283,my vacuum was the highest before i cracked the throttle and would continuously drop until I let off of it. The 327 is the first performance engine that i ever built so i don't really know how they will react. I haven't got to drive it yet because the the road to my house is a little muddy and i don't want to get it muddy.

Clyde

with some tuning I bet your idle vacume will go up to around 16-18, What carb is on the motor, headers? stock ignition curve?

Originally posted by Clyde_Maston:
As I was tuning it I had 14" manifold vacuum at 800 rpm and 11 deg initial advance without the distributor. As I rev it up it takes an initial vacuum drop to around 12" then she builds vacuum up to around 24" at about 3500 rpm. Does this sound right?
Clyde Clyde,

First off, congrats on getting it running!

I think I understand what you are saying - when you first crack the throttle, the vacuum drops for a split second. After that split second the vacuum stabilizes at a that speed and the vacuum is higher than it was at the previous rpm? If so, that is normal and the way that it should be.

How much vacuum do you have at 1,000 rpm with the vacuum advance hooked up? If you have 18-21 you are in great shape.

Mike

Badbob - I'm running a stock HEI, 4BBl quadrajet, performer RPM and headers. The timing is 11 deg initial + 10 mechanical + 14 vacuum.

Uncle - the vacuum at 1000 with vacuum advanced is about 17-18". I was just concerned because it was so different from my previous engine with stock cam.

Clyde

Badbob - maybe it was wishful thinking but my intake is a performer, not a performer RPM.

Clyde

If you're just winging the engine in neutral you will get a very high vacuum reading because the throttle blades are basically closed and the engine is trying to pull in alot more air. Think about putting your hand over your mouth while running, your hand is sucked to your face. When you're driving down the road you will have less indicated vacuum as your throttle blades are opened a little more to power the car down the road. At full throttle, accelerating the car, you would expect to have near zero indicated vacuum as there is no restriction ( or very little ) to the incoming air.
14" at idle with your combo sounds pretty good. Should run really sweet.

I think you need to add about 17 more degrees of mechanical advance, unless the 10 degrees you mentioned is in distributor degress not crank shaft, you need about 38 total not including the vacume advance. The vacume advance disapears as the throttle is opened closer to WO.

Clyde,

Are you hooked to ported vacuum at the carb or are you looking at manifold vaccuum. It sounds like you are looking at manifold vacuum which should behave different than ported vacuum. If the carb it tuned correctly, you should see little to no ported vacuum at idle. Perthaps you are already aware of this but wanted to make sure.

TJC hit the nail on the head if you are looking at man. vac. You aren't seeing the true picture becuase there is no load on the motor meaning the throttle plate is not open very much.

Troy.

Thanks for the input guys. I am looking at manifold vacuum with my vacuum gauge, not ported. However, I am using ported vacuum for the vacuum advance. Clear as mud right? The total timing right now is 35 (11+10+14) deg. The 10 init was without the vacuum advance hooked up and the port plugged. I understand the principle of manifold vs ported vacuum. It sounds like I'm in good shape. I guess that means the old 283 wasn't right as it would loose vacuum under the same conditions as you opened the throttle. I can't wait until it dries up some so i can take it out for a drive.

Clyde

Clyde,

Total timing is just two numbers - initial + centrifugal (no vacuum advance at WOT). Those two should add up to ~36 or so.

Mike

Can you get 27 degrees of mechanical advance out of a stock HEI? I've read everywhere that the initial should be between 10-12 degrees so the rest must come from the HEI without vacuum. Will i have to enlarge the holes in the HEI mechanism where the posts for the advance weights poke through?

What about setting the timing without the vacuum at 36 degrees at 3500 rpm or so and then see where it is running at idle?

I'll try it out tonight. What is your opinion of ported vs. manifold vac for the vacuum advance. I've heard it both ways. I assume (whoa! don't throw things)that the general had them set up to work on manifold vac.

Clyde

Originally posted by Clyde_Maston:


What about setting the timing without the vacuum at 36 degrees at 3500 rpm or so and then see where it is running at idle? You could do that, if it starts okay warm, then you are okay. Best bet is to get the correct amount of centrifugal advance though.

Originally posted by Clyde_Maston:

I'll try it out tonight. What is your opinion of ported vs. manifold vac for the vacuum advance. I've heard it both ways. I assume (whoa! don't throw things)that the general had them set up to work on manifold vac.

Clyde If you don't have to pass an emissions test manifold vacuum is the way to go.