Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Cliff Ruggles:
I was expecting MAJOR improvements moving up to the roller. We did end up making a little more power with the HR cam, after spending the entire weekend on this testing it netted 3hp/4ft lbs better than the Crower RAIV cam we started out with.
Wow, I got a lousy 3hp/4ft lbs improvement spending well over $1000 on a cam swap...bummer.
Even though the HR cam made a few more HP it actually made it at 200rpm less than the flat cam. All of this baffled me for a while until I thought about what the engineers were doing with cams back then. They were using longer seat timing and wider LSA but not much lift. They must have known that the time the valves spend off the seat can very quickly trump shoving them around with greater authority and much further off the seats.
I suppose it all makes sense if you sit back and think about it. When we have 30-40 or even more cycles per second happening longer seat timing provides a considerable more amount of time for cylinder filling even if we aren't shoving the valve open as far or opening/closing them more aggressively.
Just some food for thought here as it's pretty rare to see anyone actually doing any back to back cam testing with no other changes. The folks selling cams ALWAYS change several items in the mix so their product wins the prize.......FWIW.......Cliff
old cars:
Wow that was a long read. Cliff was right when he said the carb should have been installed with stock calibration. Even if changes to calibration were wanted/needed you at least no where you started.
Unless you are as experienced as someone like Cliff, making more than one change at a time can be counter productive. Making single changes enables you to learn how each circuit works/reacts to changes.
.040" in the hole pistons and thick? head gaskets is a disaster for detonation/timing issues. Not sure I ever saw what the total ignition timing was. That led me to wondering why it wasn't backed off a bit.
Vortec heads do not require or like as much total timing.
Cliff's comments on ignition timing/curve, vacuum advance were on the money.
Cliff Ruggles:
Thanks. Correct about timing, and fuel requirements follow that as well.
The better you do with the engine build in terms of compression, quench distance and cam choice the LESS timing and fuel the engine will need to be happy.
If you were doing this thru the "smog" years you found out first hand how these things work.
Back in those days I owned and drove a 1970 Chevy Impala with a 350/300hp engine. It was powerful, idled smooth, great throttle response and nailed down 20-22mpg's on the highway w/o overdrive. It was stone stock with the only modification being dual exhaust, H pipe and a set of free flowing mufflers.
Those engines had flat top pistons, small cam, and excellent flowing small combustion chamber 041 heads.
I also owned and drove a 1970 GMC 3/4 ton truck with a 350 in it. Even with 4.10 gears it got pretty decent fuel economy and made great power. The engine started using a lot of oil and needed valve guides and seals. Back then I had no funds for this sort of thing. I was offered a set of finished heads from a local hot-rodder in trade for the heads from my 350 engine.
He just told me there were 1.94's and would work fine. Well I did the head swap one weekend using parts store "rebuilder" thick head gaskets. That move knocked what felt like about 100 horsepower out of my EXCELLENT running 350 engine. I couldn't believe it. What a "turd" it was with the new heads in place. Then I started looking at the heads I removed and noticed smaller chambers and much larger ports. I pulled the heads I was given back off the truck and compared them closer. The heads he had given me in trade were "882" castings.
I gave him back that junk and did a few side-jobs and got my 1970 heads rebuilt and put them back on the truck. I also used .020" thick steel head gaskets just like came off of it because I wanted everything right back the way it was. Power was restored and I logged that experience deep in my memory banks.
LOW compression, thick head gaskets and chitty flowing heads KILL power in these engines. The smog years were not good for the industry and they didn't get it figured out till almost 2 decades later........
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