General Category > Quadrajet Carb Talk and Tips
Thumpr Junk
novadude:
--- Quote from: Cliff Ruggles on January 04, 2017, 01:30:36 PM ---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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Can't argue with that. The higher the CR, the better a wide LSA cam will work. For a 9 to 10:1 SBC, tighter is usually better though. You might want to check out some of the many dyno tests David Vizard has performed.
My cam is a hyd roller with 270/278, 217/225 @ 0.050", .495"/.500" on a 108 LSA. Not very far off of LT4 HOT cam specs except for LSA. In my 9.6:1 engine, I can't think of any good reason to spread the lobe centers. It has great manners as-is, and it is not the least bit "peaky" compared to other similar engines I've built on wider LSAs.
Vizard's got the dyno tests to support the tighter LSA choice on a ~10:1 SBC. I was skeptical based on all the wives tales about "bad" behavior with tight LSA cams, but I figured I'd give it a shot. Glad I did, as I believe it pulls much harder in the mid-range as compared to a wide LSA cam. With a Muncie, my car spends a lot of time in the 3800-5200 band in a typical 1/4 mile pass.
Wide LSA isn't always the right choice, but tight LSA isn't always the right choice either. No way would I have picked a small 108 LSA, 217@0.050 deg cam if I had an 11:1 engine.
Cliff Ruggles:
270 degrees seat timing is smaller than a stock replacement 350 camshaft, so it will enjoy tighter LSA in a lower compression build.
Tight LSA narrows up the power curve and increases VE at lower rpms. Combined with the roller lobe profiles that cam would have excellent cylinder filling abilities to offset the lower compression and make good low and mid-range power.
I build engines for a living, and dyno all of them, so we have pretty extensive experience with camshafts. Cylinder head flow is also a BIG player with camshaft selection, and the better the ports flow the less duration you can get away with and still make respectable power.
Roller camshafts also can get away with less seat timing as they supply a much larger "window" for air flow once the valves are off seat. A flat cam that small would not delivery the same results you are seeing with the roller cam, it would need to be 10 if not 15 degrees bigger to accomplish similar results.
Folks don't take any of that into consideration, and the current trend is to lower compression down around 9.5 to 1 for pump gas, then install smaller cams on tighter LSA's to bring the power back.
Here I build SBC engines with very tight quench, usually around .025-.035", excellent flowing heads, and shoot for 10.5 or a little higher SCR (iron heads). Combined with a longer duration cam like the CS-179R or something similar you will enjoy decent idle, strong/broad/flat power curve, and excellent upper mid-range and top end power. This will also happen on pump gas without running hot, overheating, detonation, etc.......Cliff
Cliff Ruggles:
(Continued from the last response had too many characters)
Kind of interesting but when folks call here, and this happens dozens of times each week, they absolutely HATE Thumper, Mutha-Thumper, Comp XE, and similar cams on 110 or tighter LSA. The complaints are ALWAYS the same, shootty idle quality, stinky exhaust, poor throttle response, "reversion" at low rpm's, and just not very user friendly anyplace. When I start asking questions about "quench distance", heads used, head flow, true static compression ratio, camshaft installed ICL, etc, most don't know any of the answers. Right to start with IF they don't know anything abut the heads other than they were 1.94 intake valve size, they don't have a CLUE as to how well that engine is going to work with the total combination of parts used. Chevrolet made some excellent factory heads, OK factory heads, and complete GARBAGE when it comes to performance potential. Many are nothing more than "door stops" but I continue to see them used in many "performance" engine builds despite the many and readily available EXCELLENT heads out there these days. This tells me that the machinist, engine builder, or most of the folks involved in the engine build really don't know what they are doing, aside from poor camshaft selection for the engine based on the parts involved and intended use, etc.
In a lot of cases when I ask about why a certain cam was used, I get nothing more than "my engine builder said bubba used one in his engine and absolutely loves it". Problem is, Bubba's engine may have completely DIFFERENT parameters as the one in question, hence very poor results from the next poor soul who tried to use the same cam.
In this deal I consider compression my friend, as it opens up choices for larger cams and a LOT more engine power over a broader rpm range. Most of the Internet jargon you read in that regard will tell you to avoid high compression and use "modern" camshaft to bring back all the lost power, so for sure PLENTY of ways to skin that cat/opinions on the subject......Cliff
429bbf:
excellent discussion for a carburetor forum . I've seen more people blame the carb for there problem than anything else . they take a 8.5 c.r. engine stuff in a big cam , put on a set of headers. and say that quadrates are junk .my advice to them is give me the quadrajet and install an 850 holly that way you truly have a gas guzzling pos.fwiw
novadude:
--- Quote from: Cliff Ruggles on January 07, 2017, 03:54:58 AM ---so for sure PLENTY of ways to skin that cat/opinions on the subject......Cliff
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Agree with this.
My 9.6:1 355 with Vortec heads and tight LSA runs 108 mph in a 3300 lb car, gets ~19 mpg with no OD (cruising at 3200 rpm), and has a great idle quality and drivability with a Q-jet and only 217 @ 0,050. Lots of bigger engines aren't running that kind of mph. I don't regret not going for more compression and wider LSA.
My quench distance is 0.044-0.046 (depending on which hole I measure), and I don't believe there is much to gain by running it .010" tighter. I did pay attention to all of the details on this engine.
Everybody's got their own opinions, but I am very happy with the performance of the combo I've designed.
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