I've heard 10% to 25%. My engine, with the smaller carb, made 490 HP and 551 lb/ft torque. That was with the fuel pump, water pump and open headers. I changed the camshaft from a 230/236 to a 236/245, that had a projected loss of 5 lb/ft torque and a gain of about 5 hp.
Last Saturday the club in Sidney had a dyno session at a shop in Victoria, 15 cars, two pulls each to get a base line for further tuning. To try to get a closer comparison, I removed the Q-850 and installed the 800 EPS, The car has manual fuel pump, water pump, power steering, oil cooler, tubular headers, 2-12" dual exhaust with flow through mufflers, twin 30 amp fans, a 140 amp alternator, manual transmission, steel drive shaft, 69 Slomaro c-clip posi rear, 255/60R15 tyres on 15 inch Torque Thrust II wheels, all taking there share of power.
The car made 389 HP and 428 lb/ft torque. I know it's not an exact comparison as I did not engine dyno with the new camshaft, but I'm positive the projected are within five units.. The chassis numbers are 79-80% of an engine dyno. I would have liked to engine dyno it with the new cam and the Q-850 but money ran out long ago.
Rather coincidental both the torque and power numbers are Pontiac engine sizes, 389 and 428!
I should have included that. Engine dyno with 800 eps, 95 secondary, 110 primary, 65/47 rods: Torque - 551.8 @ 4200 rpm; Power - 490 @ 5200 rpm. Power leveled off at 5200 and we shut down there. Chassis dyno same carb: Torque - 428.2 @ ???, didn't get a print out just graph, looks like it's about 4000 rpm; Power - 389.5 @ 6200 rpm, Still climbing as we approached 6000, our agreed rpm shut down, I said he could try to 6500 next pull, we only gained .5 hp in 200 rpm so he shut it down there.
Timing was 12 initial and 29 total. A/F was a bit rich in these tests, leaning out 3/4 a point may improve results. If I can scrounge up the $$ I will do a few more with a Q-850 and a handfull of jets and restrictors.
Engine dyno with 750 Holley HP 73/73 jets: Torque 561 @ 4200 rpm; Power 496.8 @ 5400 rpm.
Didn't have time to tune the Holley to the engine or test the 850 HP. Dyno operator thought another 15-25 HP with a properly tuned 850 HP. 1.5" of vacuum at WOT with the 800 eps. Should be 0.5" or lower.
We tried from 26 to 36 degrees of advance, got the best torque and power at 29 degrees. Typically aluminium heads can use less advance than iron. These have fast burn chambers. I also worry about detonation with the 10.5:1 CR and high cranking cylinder pressure.
As I'm always behind the wheel when It goes over 6000 I didn't realize what it sounded like. That chassis dyno is in a high ceiling bay with the exhaust pointing to an open overhead door. The Dart and the Super Bee went up to just over five, you could hear the screaming all the way down the street. Way louder than at the track. When the operator got the 428 going I started to get a bit worried about the hold down straps.
I haven't calculated the force on the crankshaft when the piston changes direction but I most likely don't want to know. I don't know anyone who's broken one, have a few friends who've snapped BBC cranks by spinning them like a small block. A lot of stock fifty year old Pontiacs out there still going strong. Too bad they still didn't make them.