Big Cam, Bad Idle, Why?
Discussion
Just wondering. I've got a fairly large race cam in my BMW now and idle is rubbish and power is non existent till 4000rpm.
It's not a problem in any way and i knew it would be like this but was wondering if anyone can shed some light on the physics of why big cams hurt low rev performance so much.
It's not a problem in any way and i knew it would be like this but was wondering if anyone can shed some light on the physics of why big cams hurt low rev performance so much.
With large levels of overlap between intake and exhaust valves you also get intake charge passing straight out of the exhaust when piston speed and therefore airspeed is low. As the engine speed picks up, the inertia of the inlet charge takes more of the inlet charge into the cylinder and away from the exhaust valve.
stevieturbo said:
So did you just throw a big cam in, and expect miracles ? Or have you actually dialled it in, and performed some ignition and fuelling adjustments to try and make use of this cam ??
i agree steve,maybe a milder cam would be much better than shoving the maddest one in thinking its going to do 600mph.I've done a little research in to cam profiles and the effects recently, as I'm putting a new engine together for my M3 (E30 naturally).
On your single cam engine a "BIG" cam will mean increased lift and duration on both inlet and exhaust lobes with a fixed lobe centre line (the difference between peak lift angle on both inlet and exhaust lobes). So, if your "BIG" cam has an extra 30deg duration on the valve openings, this means that the valves will open 15deg earlier and close 15deg later. This brings about two issues at lower engine speeds where there is little inertia in the gasses entering and leaving the combustion chamber.
Firstly, with the exhaust closing 15deg later and the inlet open 15deg earlier the period where both valves are open is longer and the piston has started on the inlet stroke and is some way down the bore before the exhaust is closed and trying to pull in gas down both inlet and exhaust. This reduces the amount of vacuum effort the piston exerts on the inlet tract and therefore less mixture is drawn in (and some exhaust gasses also).
Secondly, With the inlet closing later the piston has travelled further up the bore before the inlet valves makes the seal and therefore the effective compression ratio is reduced. (Effective compression being calculated from the point of inlet valve closing as opposed to the theoretical compression ratio which is calculated from BDC).
These two issues compound each other at lower engine speeds with less mixture entering the combustion chamber, being compressed less for the combustion process. The result, less torque.
As engine speeds increase then the inertia in the incoming mixture and outgoing gasses also increase and counteract the above. In the first instance the amount of vacuum created by the outgoing exhaust around TDC is greater than the vacuum created by the piston starting it's inlet stroke. This vacuum assists the vacuum created by the piston and pulls in more mixture. Secondly, the inertia of the incoming mixture creates a positive pressure in the combustion chamber which is greater that the pressure created by the upward moving piston at BDC and therefore the inlet mixture continues to enter the combustion chamber even after the piston has passed BDC.
Finally, what needs to be remembered is that if you change the amount of mixture the engine can draw in and the amount by which it is compressed, then the amount of fuel required will be different and when you want the spark to happen will be different necessitating a retune.
I hope this makes sense and is not b
ks. I'm sure I'll be told if it is.
Steve
On your single cam engine a "BIG" cam will mean increased lift and duration on both inlet and exhaust lobes with a fixed lobe centre line (the difference between peak lift angle on both inlet and exhaust lobes). So, if your "BIG" cam has an extra 30deg duration on the valve openings, this means that the valves will open 15deg earlier and close 15deg later. This brings about two issues at lower engine speeds where there is little inertia in the gasses entering and leaving the combustion chamber.
Firstly, with the exhaust closing 15deg later and the inlet open 15deg earlier the period where both valves are open is longer and the piston has started on the inlet stroke and is some way down the bore before the exhaust is closed and trying to pull in gas down both inlet and exhaust. This reduces the amount of vacuum effort the piston exerts on the inlet tract and therefore less mixture is drawn in (and some exhaust gasses also).
Secondly, With the inlet closing later the piston has travelled further up the bore before the inlet valves makes the seal and therefore the effective compression ratio is reduced. (Effective compression being calculated from the point of inlet valve closing as opposed to the theoretical compression ratio which is calculated from BDC).
These two issues compound each other at lower engine speeds with less mixture entering the combustion chamber, being compressed less for the combustion process. The result, less torque.
As engine speeds increase then the inertia in the incoming mixture and outgoing gasses also increase and counteract the above. In the first instance the amount of vacuum created by the outgoing exhaust around TDC is greater than the vacuum created by the piston starting it's inlet stroke. This vacuum assists the vacuum created by the piston and pulls in more mixture. Secondly, the inertia of the incoming mixture creates a positive pressure in the combustion chamber which is greater that the pressure created by the upward moving piston at BDC and therefore the inlet mixture continues to enter the combustion chamber even after the piston has passed BDC.
Finally, what needs to be remembered is that if you change the amount of mixture the engine can draw in and the amount by which it is compressed, then the amount of fuel required will be different and when you want the spark to happen will be different necessitating a retune.
I hope this makes sense and is not b
ks. I'm sure I'll be told if it is.Steve
stevieturbo said:
So did you just throw a big cam in, and expect miracles ? Or have you actually dialled it in, and performed some ignition and fuelling adjustments to try and make use of this cam ??
Read it again.
I have my miracles.
Top end power and torque from 4000 up is great.
It has no power or torque low down.
I knew it wold be like this.
I'm just curious as to why it's like this.
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