General Category > Quadrajet Carb Talk and Tips
Thumpr Junk
Cliff Ruggles:
Nothing is irreversible when it comes to tuning the idle system, so it's pretty easy to reduce the idle tube size, DCR's, bypass air, etc.
Been building engines here, very successfully I might for over 40 years and I am NOT fond of tight LSA, or fast ramp camshafts for street engines.
They certainly sound good when it comes to "attitude" at idle speed, but tight LSA narrows up the power curve, and pulls power down in the rpm range. One can quickly get into trouble with detonation as well, as narrowing up the power curve and earlier increases cylinder pressure all else being equal.
I'll put up the dyno sheet of an engine we helped a customer out with. It's a 455 Pontiac at 9.3 to 1 compression. They originally installed a Comp 276HR cam and had trouble making the power they were looking for and it pinged on the dyno requiring rod bearing replacement.
They called me because we build engines and it was using a Q-jet carb we supplied all the parts for. They dyno operator was blaming the carburetor, when in fact it was the cam. We recommended a slightly larger cam on a 114LSA, the XR276HR cam is on a 110LSA.
They were nicely rewarded with more power at every rpm, decent idle quality, plenty of vacuum at idle, improved throttle response, and street manners........Cliff
novadude:
--- Quote from: Cliff Ruggles on January 04, 2017, 03:10:43 AM ---
Been building engines here, very successfully I might for over 40 years and I am NOT fond of tight LSA, or fast ramp camshafts for street engines.
They certainly sound good when it comes to "attitude" at idle speed, but tight LSA narrows up the power curve, and pulls power down in the rpm range. One can quickly get into trouble with detonation as well, as narrowing up the power curve and earlier increases cylinder pressure all else being equal.
--- End quote ---
You can't paint it with a broad brush and say "tight LSA is bad" for all engine combinations. My car weighs 3300 lbs, runs 108 mph in the 1/4, gets 19-20 mpg with no OD, and idles smooth enough for Grandma to drive it to church. This is a 9.6:1 355 SBC. With a manual transmission, I can pull it down to ~1300 rpm with no lugging, etc. It runs way better than it would with the old outdated '151' L79 cam (which I've used before), and still pulls hard above 5k rpm with only 217 @ 0.050.
There's more than one way to get it done. ;)
novadude:
--- Quote from: novadude on January 04, 2017, 06:13:18 AM ---You can't paint it with a broad brush and say "tight LSA is bad" for all engine combinations. My car weighs 3300 lbs, runs 108 mph in the 1/4, gets 19-20 mpg with no OD, and idles smooth enough for Grandma to drive it to church. This is a 9.6:1 355 SBC. With a manual transmission, I can pull it down to ~1300 rpm with no lugging, etc. It runs way better than it would with the old outdated '151' L79 cam (which I've used before), and still pulls hard above 5k rpm with only 217 @ 0.050.
There's more than one way to get it done. ;)
--- End quote ---
A Pontiac with 30 deg valve seats is likely a whole different story....
77cruiser:
Probably adds to the O/L factor with more low lift flow.
BTW my 421 seems to idle pretty good with 107 lsa & gets 18 mpg highway.
Cliff Ruggles:
I'm not painting any picture at all, just stating facts based on DIRECT testing over many decades.
A 355 with 9.6 to 1 compression would NOT like an L-79 camshaft, you need closer to 10.5-11 to 1, then it would tear your head slam off and make well over 1hp/cid. We built a 355 for a very good friend some years back and it ran into the 11's in his 68 Nova with nothing more than 4.10 gears and a mild stall converter.
We've dyno'd that cam in very well prepared 350cid street engines and depending on cylinder heads selected, quench distance and true static compression ratio will make 380-420hp.
The factory LT4-"Hot" roller cam is another very good choice for those engines, built quite a few of those as well.
We've also played around with a few of the modern "fast ramp" profiles, and tighter LSA and they fair better when the compression is a bit lower as they tend to build more cylinder pressure early in the rpm range.
Compression is your friend with cam choice, so basically you have to pic the cam based on CID and compression ratio, and also the drivetrain parameters, intended use of the vehicle, etc.......Cliff
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