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Discussion Starter #1
Let's start this off in a new thread so the info can be found a little easier.

The following is in responce to another thread...

In my experience, higher RPM's have less detonation than low RPM's (regarding your statement about high static compression ratio's). Actually the place that is most likely to have detonation is peak cylinder pressure, ie peak TORQUE. Above that RPM cylinder filling is not as productive per engine revolution (that's why the torque curve drops after peak).

Also, compression ratio increase will directly effect the torque curve across the entire rev range. The limiting factor in the dynamic compression vs. torque argument is the ignition timing. There is a fine line between having enough dynamic compression and timing, and having to much timing for the compression. Increasing the anti-knock values of a fuel (comonly but incorrectly refered to as octane), will raise this threshold, at the expense of efficiency on the top end. Keep in mind that the amount of cam's overlap (between intake and exhaust cycles) also directly effects the dynamic compression ratio of an engine. As overlap increases the engines threshold for static compression increases.

In all, the zetec's ability to take advantage of higher static compression is limited by the lack of camshaft overlap (Solved with more aggresive cam(s) and adjustable gears), and the factory timing curve (again solvable with chip/ ecu changes.

Thank's for letting me state my opinion,

Brian
 

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Yes, most engines will show a decreasing tendency to knock at higher rpms. It is not only because of decreased cylinder filling however. It is also because there is less time for the process of detonation to occur. Yes, detonation is a process, and it takes a certain amount of time to occur. At higher RPM, there is less time in which it can happen.
 

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Well yes that does occur but look at his peak torque.... where doe it occur... 5000 rpms as i remember... this area has the highest risk of detonation and there is no way possible on this four cylinder naturally aspirated engine to gain 30 horsepower from porting the head alone. Outside of stroking it or just going insane on power levels. There is a point where the combustion chamber limits valve size and flow because of diameter and the head simply cant flow anymore.. I believe you would definatly reach this before you got 30 horsepower out of a port job without compression increase....
 

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Discussion Starter #4
Your claim re: 30hp. gain from porting is somewhat valid. It would be difficult, but not impossible to see a gain of 30hp. from porting alone.

Think about this scenario,

2.1L Zetec
16:1 Compression
Prototype SVT head
stock cams (both)
Cam gears (tuned)
Race header (4 into 1 side exit)
50mm. Throttlebodies
Pectel T6 ECU
made 225hp. on the engine dyno.

Same head with more extreme porting, and larger cams made 260hp to the wheels. Everything else stayed the same. That's a change of 60+hp, with just a porting change and cam change!

Again, it is possible, but on the street it would be fairly hard to attain.

Brian
 
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