Aussie Gazz":jtiagih3 said:
I am trying to work out what the cam rev range and max rpm will be with the CSC-280-hsp-08 on my 250 2V using 1:65 ratio yella terra roller rockers....
Anyone know the formula for working out cam specs ,engine rpm etc?
Yep, engine anaylser, but even that is no way of allowing for the variables from the engine 'insitu'.
Peak rpm is different from where maximum power is, and both peak power and torque varies according to exhast configuration.
And 280 degrees from one cam doesn't mean that it will have the same stats on rpm as it does from any other 280 degree cam. Lobe centre, net lift at 20thou, and the lift when both vlaves are 30 thou open, and what the real overlap figures are vary massivlely.
Generally, a 250 2V with 280 degree cam nad headers should be very ,very close to the 250 2-BBL SVO engine Dick Johnston discussed in 1987. Some upgrades of the stock intake manifold and carb or EFI were publicly reported by Micael Stahl to yield 196 hp peak flywhee power. Any alloy head flows 206 hp with the lift and cfm the Brisbane Engine centre used. Genie exhast headers were to be used, worth 20% more power over a stock iron exhast and single 51 mm pipe.
From all the info I've found from that era (Modern Motor, Wheels and Streetmachines later expose in Hot Sixes in 1990). SVO 250 Rev range would have been extended to 5000 rpm or perhaps 5300 if the pushrods were upgraded to suit. The peak power rpm would have been 4600 rpm, and torque should have been about 3200 rpm. Rev limit was down to 4500 on the stock 1987 EFI engine, yet in 1979, a stock 250 iron head was able to hack 5300 rpm. Power for 4.1 XF EFI was at 4000 with the EFI's 264 degree cam, torque at 3000 rpm
As for some other other matters noted. Even a dyno tested engine will not have the drivability and engine tuning sorted out on a custom cam, there are too many variables. Its nothing to walk away from a dyno tuner with an extra 40 rear wheel hp on a V8, and not notice any difference in idle quality.
The problem is the background variables are just mind bending.
Any 280 degree cam could be any thing. Even a split duration cam, via 270 degree intake at lash, 290 degree exhast at lash, and could have 205 degrees duration at 50 thou intake, yet 215 degrees of duration at 50 thou on the exhast. If it's a solid cam, it will be more savage than a hydraulic, and if its automatic, it could use a wider lobe separation than a manual. If its compression ratio is too low, the 1.65:1 rockers will hurt low end torque. If the car doesn't have enough cold cranking compression at idle or too much, the cam timming will have to be advanced or retarded to suit. Valve lash may have to be varied to suit.
This cam could therefore idle like a stocker if insitu tuning is perfect, or run like a hairy dog if its just bumped in over the weekend. A driver may hate the low end performance soo much, he'll want the engineer to retard tha cam, even if it looses performance, just so he can drive it around town.
The technical method is to mathematically benchmark an engine with the carburation, same piston size, port area, bore and stroke ratio, compression ratio, piston cc's, chamber cc's and fuel. Then consider the drive train (diff ratio, gear ratios, does it have a torque converter and if so, what stall ratio, does it have power steering, whats its weight), and then what % of race driving will it have in proportion to daily driving.
The engine anaylser is a better bet, because it does most of this for you.
