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Discussion and questions related to the course How to Degree a Cam
Hi. Lets say if i already dialed in the cam on the dyno and i got the kind of power i wanted. But i want to move the powerband higher in the rpm. Right now its peak power is at 6000 rpm and i want to move it to 7000rpm. Do i just retard both cam without altering the LSA of both cam? Does it work that way?
In theory, it will move the power peak, but that's a long way to move it, and you will probably loose both power, and power under the cirve. You can also expect a drastic loss of low rpm power/torque.
As the rpm rises, there is less time to fill the cylinders, so the camshaft lift and duration becomes more critical - your camshafts may simply not have the opening, and ability to fill the cylinders, with ~15% less opening time, but it isn't just that as...
At 6k you may be in the optimum operating range of the cylinder head, intake, exhaust, and everything else - if you try moving it up to 7k, you may (small chance it won't) expect the head, etc, to become more restrictive if you're moving out of their optimised range. Less filling -> less torque -> less power.
You also need to consider the mechanical limits of the engine - the higher rpm may require a valve spring change. It may mean replacing parts con'rods/bolts and or pistons, for example - to live with the rpm required to use the 'back of the power curve', which is important as the optimum curve's chang up point is where the lower gear's falling power output is the same as the increasing in the next gear. This made worse because the lower rpm power can be expected to be lower, requiring a higher change point.
This is, of course, assuming you don't run into piston-valve interference - assuming you checked, you may have some idea how far you can move the cam' timing, but if you didn't you could have an expensive surprise.
Bottom line is, if you already have the power you want, give some very serious consideration into why you want to change things - if you really want 7k power peak, and you have the V-P clearance, valve cap/retainer to stem seal, etc, clearances a camshaft designed for those rpms would be a much better idea. Assuming the rest of the engine is up to the back of curve rpm - about the only exception would be if you're looking at pure tup speed, which won't have that concern.
Thank you for your detailed explaination. Thats what i tought it would be. Because usually when i tune a turbocharged car, the powerband can be altered easily by retarding both of the camshaft. Now im having a hard time tuning a naturally aspirated car. When i retard both of the camshaft even by the slightest bit, i lose power. Yes the engine is prepared with a lot of piston to valve clearance so i can play with the cam timing.
Engine spec is mitsubishi 4g93, stock crank, long rod setup, 84mm piston, 13.5 compression ratio, oversized valve, ported head, 304° camshaft, short runner 50mm itb. Runs on vp racing m5
Ah, not that surprised. If you have easy access to the camshaft verniers, you're going to have to just play with the timing to see where the best, overall, timing is for each camshaft. 4 (crank) degree steps should make it clear if there's an improvement, and you can then refine it - you've tried both retarded, but individually, or one or both advanced?
If you can, might be worth trying longer trumpets on the ITBs, yours may be optimised for higher rpm, depending on actual length.