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- Aug 18, 2008
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- 4,081
I've been havin' a little thinky about this subject, and something puzzles me about the way we tune our Hondas with regards to cams, and the crazy compressions we run. This is only a little theory, to promote a bit of discussion on the matter, so feel free to shoot holes in it/me.
The basic way a cam is selected for an engine should depend on it's capacity, it's head flow characteristics and the target power band. The stock cams are a blend of midrage and top end power, with a bias towards midrange on the B18C and one towards top end on the B16B.
When we stick an uprated cam in, what happens? Well assuming we increase both lift and duration by an equal amount, we lower the dynamic compression ratio (DCR), and raise the point at which the flow requirements of the engine outstrip the supply from the cams. The lower DCR reduces midrange power until the point where inlet port velocity overcomes the port reversion allowing the cylinder pressures to gradually increase with engine speed, from this point the increased valve open time increases power and holds it further up the revs. So you lose some midrange, gain some top end and extend the power band a little, sounds logical right?
So now lets look at some 'm4d tizzle' cams, proper gnarly race spec stuff. They can feed your stock B18C to 11000rpm, but have dropped your DCR to the low 9s. Your midrange will SUCK with this setup. So then, the usual reccomended solution is to go and drop in some 13+ CR pistons to return to the stock engines mid 10 DCR. In theory this is great, we have enough flow for 11k, and a stock DCR, so it'll drive around town like a factory car and tear chunks out of ex BTCC cars on track.
Except as far as I can see, it won't...
Can anyone tell what flaw I am looking at here?
The basic way a cam is selected for an engine should depend on it's capacity, it's head flow characteristics and the target power band. The stock cams are a blend of midrage and top end power, with a bias towards midrange on the B18C and one towards top end on the B16B.
When we stick an uprated cam in, what happens? Well assuming we increase both lift and duration by an equal amount, we lower the dynamic compression ratio (DCR), and raise the point at which the flow requirements of the engine outstrip the supply from the cams. The lower DCR reduces midrange power until the point where inlet port velocity overcomes the port reversion allowing the cylinder pressures to gradually increase with engine speed, from this point the increased valve open time increases power and holds it further up the revs. So you lose some midrange, gain some top end and extend the power band a little, sounds logical right?
So now lets look at some 'm4d tizzle' cams, proper gnarly race spec stuff. They can feed your stock B18C to 11000rpm, but have dropped your DCR to the low 9s. Your midrange will SUCK with this setup. So then, the usual reccomended solution is to go and drop in some 13+ CR pistons to return to the stock engines mid 10 DCR. In theory this is great, we have enough flow for 11k, and a stock DCR, so it'll drive around town like a factory car and tear chunks out of ex BTCC cars on track.
Except as far as I can see, it won't...
Can anyone tell what flaw I am looking at here?
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