DOHC power delivery

[EuroDude]

Hi quick question about DOHC - ive searched to no avail.

U know that boost in performance when you hit ~4500RPM in a DOHC (non-VTEC), is that how the ECU is programmed? Or is that simply because at those RPM's the valves are most efficient?


I ask because it seems strange that from 1000RPM to 4500RPM the performance is very consistant, then suddenly when I hit ~4500RPM, the instant performance difference is very substantial - it almost feels like a turbo kicked in.

Wouldnt the power delivery be consistant all the way through the rev range, with a higher performing curve after 4500RPM?

Or is this because the ECU is programmed to be granny-efficient below ~4500 and then pump more fuel+air above 4500RPM?



In other words, if I get a chipped ECU, can I recover the lost power under 4500RPM?


[EDIT] To make things clearer, i'm talking about the sudden ~10kw boost/kick from 4500 to 4600. I'm not talking about the whole 4500 -> 9000 RPM range.



btw the engine is a D16A8 in an Si Eg Civic.

[bennjamin]

are you talking of when the peak torque is produced ?

[iamhappy46]

Basically:
DOHC camshafts are designed for best delivery above a certain rpm.
DOHC engines can supply better hi-rpm power and airflow.

Most non vtec sohc 4 cyl motors are built around 'long piston stroke' torque engines. Designed for granma's getting the groceries and driving to church in cars like ford lasers

[EuroDude]

@Ben, not the peak torque (thats around 6800 I think), I'm talking about the rapid change in torque when hitting 4500RPM

@imhappy, yes I already figured that, but why the sudden performance change?


Does the ECU control the amount of fuel to inject according to a graph?

It just seems that the ECU is holding back some power below 4500 for emmision reasons or drivability in slow traffic. Does the ECU enter some sort of performance mode when it hits 4500?

[locote]

i feel same with my gsi.
the sound of the motor changes!!
sounds alot angrier after 4000 rpm, and thats when u feel the power.
i find this more so with the B series engines.
i remember my sr20 had a more liner power curve same as my mate N/A sss pulsar bottom end his feels gruntier but after 4000rpm i leave him

[iamhappy46]

The camshafts are what makes it perform better... It is like running a VTEC motor around on the 'HI LOBE' profile all the time, it would end up gutless down low. The low lobe of VTEC fills in the gap from idle to VTEC switch point.

A 'chip' will not suddenly make you car scream from idle to redline. The ECU is just fulfilling the engines demands for fuel/ign timing when the camshafts are working at their best.

Fitting adj cam gears changes that, you can gain 'torque' at low rpm but trade off power at hi rpm.

The internal combustion engine is always a compromise.

[locote]

i imagine its the cam profile and tunning of the motor that plays the major part of this.

[EuroDude]

I see, so the ECU really doesnt have anything to do with the sudden performance kick, its just the cam profiling, power efficiency etc.. of the actual engine physics. Basically everything "comes together" when it reaches 4500RPM


I would have thought the power delivery would be smoother like in the first pic, instead of a VTEC-style "kick" like in the second pic:
http://img74.imageshack.us/img74/3594/dohc1fz.gif

...Because the DOHC CAM's are fixed, unlike VTEC which is variable 50/50

[KB]

what you on about willas?

[EuroDude]

Talkin about Different "Strokes" at Different "RPM", Arnold :P

[iamhappy46]

VTEC actually has a 'dip' not a 'kick' when the camshafts change over. The power curve will drop and then start to rise again(which can feel like a kick) once running properly on the hi lobe profile.

It depends on the camshafts of the individual DOHC engine as to where the power is made. I had adj cam gears on my original N14 SSS Pulsar SR20DE(R.I.P) and playing with the cam gears, I/H/E and ECU mods made 88Kw at the wheels at 8000rpm but made the engine an absolute pig to drive in traffic.