Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

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Cliff Ruggles:
The worst cam you will ever put in a big CID engine with moderate compression will have short seat timing and tight LSA.

Sure they will pack some punch and wake things up by yanking the power down some, but you can very quickly run into detonation issues even at relatively "low" compression ratios.

Case in point below with an engine I got involved with:

The engine builder (owns a speed shop and dyno) called Comp for a cam, and they sent him an XR276HR for his 455 engine with 250cfm head flow and 9.3 to 1 compression.  Sounds like a great choice, right?  Not much more seat timing than a 350SBC 300hp cam, 224/230 @ .050", 110LSA, etc.  It didn't make chit for power and pinged pretty hard on the dyno resulting in rod bearing replacement.

I was contacted as everyone at the dyno facility right down to the guy taking out the trash was blaming the Q-jet for "not keeping up with their engine".

Instead of messing with carbs I suggested a cam change with more seat timing, 114LSA and ICL at 110.  The engine builder thought I was nuts!

He did it anyhow, no other changes and a couple weeks later called and told me that the engine was a completely different beast.  It made nearly 90 MORE horsepower on the dyno, idled smoother, and no detonation whatsoever.

Once the engine reached the vehicle the owner reported that it was very "mild" at a glance, but excellent street manners and more power than any type of street radial could ever hold!.......Cliff

old cars:
those are some pretty vague statements. Not sure if you understand seat timing

Cliff Ruggles:
What's not to understand?

I helped a customer who couldn't make 425hp with a 455 CID engine at 9.3 to 1 compression because he trusted the tech and Comp to supply his cam and took him to nearly 520hp with more power (torque) at every RPM and it quit pinging on pump gas.

The engine builder and the owner also were remarkably surprised at how well the engine idled with the larger cam in it and absolutely love the end result once it made it into the vehicle......FWIW......Cliff

old cars:
Maybe there was more wrong with the initial build then the builder told you.

Cliff Ruggles:
No, they only made ONE change.

The engine builder actually owns a machine/speed shop, and dyno.   He specializes in Big Block Chevy and Ford engines, but this was his first Pontiac build.

He bought a complete rotating assembly to stroke a 400 block to 467CID, zero decked, tight quench, ported the factory iron heads to 250cfm, etc.

The missing piece of the puzzle was the camshaft, picked it too small and LSA too tight.  One has to realize that a 455 Pontiac is a poor design from the start, although it is capable of big power.  The stroke is long, piston speed high, heavy internals and fed by cylinder heads with no more cross section in the intake runners than a decent small block Chevy head.

They are excellent "mid-range" engines if you know hot to exploit them, but using tiny cams on tight LSA's just makes them too good at what they already do best, which is making all the power right off idle.

I suppose the XR276HR cam may work OK in a 454 Chevy built with peanut port or PASS round ports on it, but it's not nearly enough cam for a 455 Pontiac build, or at least far from ideal as the dyno chart and results we got with a much larger cam on a wider LSA clearly show.......FWIW......

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