General Category > Quadrajet Carb Talk and Tips
cam selection
Cliff Ruggles:
Are you saying that adding manifold vacuum to the advance increased timing at idle from 7 to 13 degrees?
That's only 6 degrees if my math is correct. Most engines will like a bit more than that at light engine load and some like it at idle speed.
The BIG myth with all of this is that all engine like, want, respond well to and run better with a lot of timing at idle speed.
In reality the better we do with compression, squish distance, cam selection, etc, the LESS timing the engine will want everyplace. They will also require LESS fuel as they are more efficient.
Tuning for a living I get to see all sorts of troubled engines combinations. The trend in recent years is to LOWER the compression ratio then install camshafts with advanced intake lobe positions, shorter seat timing, and tighter LSA.
The thought process there is to bring back some of the lost compression, power and efficiency from lower the static compression ratio. Sounds good in theory but doesn't work well in actual use. At least not if you are trying to make optimum power on pump gas with these N/A engines.
I run higher compression instead, then use cams with more duration at the seat, later intake closing and wider LSA. The engine result is an engine that idles well without a lot of timing in it, and has a broad/flat/strong power curve (torque).
Combine that with good flowing heads and the end result will run as good as or better than a lower compression engines with smaller a smaller cam, then reward the end user with a LOT more upper mid-range and top end power. I'd also mention that even with more power output octane requirements will often be less than a lower compression engine doing it the other way.
This simply happens because efforts to increase dynamic compression early in the RPM range spike cylinder pressure high even if you have a relatively low static compression ratio. SO basically having peak VE occurring lower in the RPM range increases the chance for detonation/ping all else being equal. For these engines I like to push peak VE up higher in the RPM range which makes detonation less likely as the events are simply happening much quicker so not so high of a "spike" in cylinder pressure.
If you don't think this works well below is a clip of my car at the track. Notice how well the engine idles with only 10 degrees timing in it. The static compression ratio is 11.3 to 1 and it runs fine on currently available pump fuel with zero issues anyplace..........Cliff
https://www.youtube.com/watch?v=6zVdoLR-VzM
Too Fast:
My question is where to source the springs/weights for the HEI? The distributor I am running I don't remember where it came from, I put it in because the Mallory Unilite was burning up modules suddenly when I had no issues the previous 10 years, and it left me stranded multiple times. The HEI has been reliable for 3 years now except the mechanical advance now needs help.
I don't want the cheapie spring kit. With my Kauffman 74cc heads/041 cam/406 Pontiac/4 tubes/Pypes 2.5" with X-pipe the information I have found says my timing should be around 32* by about 3000 RPM. I found out I was only 26. I had to advance my base timing to 14 to get my desired 32. I have ported vacuum advance that adds another 10.
Kenth:
I´d say you´re right on the money with your timing.
Remember, Pontiac used 15° initial for the RAIV.
Only thing i would add is a stop for the centrifugal advance so the weigths don´t override max advance.
Cliff Ruggles:
Good advise. Installing a positive stop for the advance in an HEI is not optional...IMHO
IF you install lighter springs the weights can and will ride out past the "flats" on the center cam and add more timing at high RPM's. I discovered this fact on the dyno decades ago following an engine built here with a timing light to verify total timing on a dyno "pull".
There are several different methods folks recommend to provide a positive stop. I use the MIG and a round chainsaw file here. I can also closely control the exact amount of advance added with this method......Cliff
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