I preface with stating I have very good skill mechanically, Ive built and tuned plenty of motors over the years, but this one is just really not making sense. I purchased a 71 with a 454 and a 4-speed. It appears to have low gears, as I am doing 3000+ when going 65-70. The motor was rebuilt as a stroker motor topping out at 496CI. It does have a smallish cam (Comp 11-430-8, which is 224/224@.050" .566 lift), but it does have stock cast iron manifolds, 2 1/2 exhaust, and Magnaflow mufflers. It has Brodix heads, a Brodix intake, and an 850 Holley DP. Plugs look fine, and it has 200-210 compression, and the CR is 10.5:1. Ignition is stock distributor and coil and a Pertronix conversion kit. I've checked voltages to that and adj gap; it's fine.
In troubleshooting this, I took it out this morning after a while of not working on it. It's got some power, but going around a corner in first, I punched it, and it barely chirped the tires. I would expect it completely sideways until I left off - nope... Even with the small cam and small exhaust, I would expect this combo to be around 500HP and 550 torque, more than enough to nuke tires, which it can barely do. The only way to describe it is that it feels like it's in a 5000 lb Cadillac or I have the rear brakes on.

Some things Ive looked at and tried

• Bumped timing to 20 initial, 36 all in by 2800
• Verified throttle is opening all 4 barrels (I had to buy a new bracket and cable holder)
• Verified 12V at the pertronix, and re-shimmed the gap
• Re-adjust the valves to verify the roller rockers have the correct preload
• Measured compression (200-210)
• Floats look good on carbs
• Plugs look of maybe a tad rich. (I need to verify what jets and power Valve I assume whatever they came from Holley with new)

Just doesnt pull like other big blocks I had in Camaros, and they were only 396. A 496 feels like my stock 270 HP 350 in 71. Since I didn't build it, I cannot verify that the cam is installed correctly. A severely retarded cam could have these low-end torque symptoms, but the cranking compression wouldn't be 200+. The next thing I am looking into is getting a hotter coil and an MSD 6A, and getting that out of the way, as 200PSI is pretty high compression, and if ignition isn't up to it could cause issues.

 

What size runner heads ? Dual plane or single plane? What rpm does it stop pulling at?

Disconnect the secondarys and how does it feel ?

I would expect that thing to be wheezing by 5000rpm with such a small cam. Maybe it isn't a 496 ?

 

Gear Ratio.

Those rear gears sound like 3.36.
And my 72 BB came std with them as well as a 2.20 1st gear Muncie trans.
Only 7.4 gear reduction in 1st gear, makes it feel like a dog.
1st gear revs to 50 mph in first right?
Compared to my LT-1 with much better gearing, this thing stock felt like it was starting out in 2nd gear. Yuk!

That cam is actually too big for that gear combo, the cam picks up well at 2000 rpm, and you are already moving almost 20 mph by then.
That gear set needs a stock cam that makes power from idle. Or a lot of clutch slipping. Or really just more gear.
Basically you do not enough enough rear gear for it feel like a BB.
It is so choked down by gearing it feels almost like a SB right?

That engine should be close to 550/550. So that is not the problem. Say 50 less with iron manifolds.

You need at least a Autogear wide ratio gear set with a 2.52 or so 1st gear., or a TKX with a 2.87 1st gear.
Don't change only the rear gears because that alone won't cut it. A 336 to 370 is only a 10% improvement. Ho-hum.
A 2.52 WR Muncie and 3.55s has the same 1st gear as a 2.2 CR Muncie and 411s. BTDT. Those combos ran terrific back in the day. The trans makes more difference, unless you change the rear a LOT.
A 252 WR trans is 15% more in first gear. That and the 355s or 370s would really wake it up with 21% or 27% more grunt than stock.
That will feel like an extra 150HP! And 150 more tire smoking TQ!
2.52 x 3.70 = 9.4 in first gear vs 7.4 now.

Plus you will need the autogear Italian gears strength to keep the trans together under the 496 TQ. A stock Muncie will not last.

Plan B is a 2.87 TKX 5 spd O.D. will give you 9.6 in first gear overall, with the 336s, and it will highway cruise at 2000 vs 3000 rpm, and handle the TQ.
That is my combo.

And more gear than that with the TKX will make it feel like a rocket, at least until the IRS starts breaking.
There is a reason all the BBs came stock with 2.20s and 3.36s, even L88s. Less IRS breakage under warranty. They did not put that much more TQ to the half-shafts with those gears.

I know at least 2 people with a TKX, warmed over BBC, and 411 diff gears, and both of them easily broke IRS parts.

Past 400 HP you have to be gentle with the IRS!