For the tuners

[jezza323]

Generally - in an n/a engine, if you were to stick slightly longer duration cams, with slightly more lift in, what would you expect to be doing to the timing? adding more in (advancing) or taking out (retarding) from where it was on the previous set of cams?

[Jester]

My understanding with inserting larger Cams is that they increase in flow, valves opening further for longer allows more air in and quicker passing of the gas out, with timing you may need to advance it a little more so as to make sure that the valves are closed when the mixture is ignited.

By setting the timing you're really just setting where the piston is at the point of ignition, with the piston down more you are able to burn the entire mixture more effectively as it is not so dense, thus why advancing the engine will generally be a better ignition point and produce better combustion, power and/or torque.

Depending on what engine your using and the performance your after, I'd say about 10 degrees advanced would be good, however others on here have a far more better understanding than me so I'd be waiting for a few more posts before you take my advice to the bank.

EDIT*
Advancing tht timing ignites the fuel while the piston is on the compression stroke going up, the further advanced it is the earlier the spark and thus the further from the top the piston is.

[Jt_70R]

if it was more effective to burn a less dense mixture why wouldnt engine designers just drop the static compression and leave the timing at TDC?

[Jester]

Was just my understanding for setting the timing going by what little i learnt from having to do it recently, i said i might be wrong and others would know more, just offering what help i can.
i understand the physics of it in my head but struggle to express it in words.

[jezza323]

im not talking about setting the base timing, im talking about adjusting the timing map on my ECU to suit a change in cams, hence why the thread is titled, for the tuners, as in people that know how to tune a car

and dont worry, i understand the physics of it fine, but im not so sure you do. the reason you fire the spark BTDC is that the burn is not instantaneous, its a controlled (hopefully) ballooning of gases inside the cylinder, the idea is to have the peak pressure just after TDC.

[YelloRolla]

If changing the cams for longer duration, then you are going to be moving the rpm point where maximum cylinder fill is achieved until later in the rev band. This means that your "dynamic compression ratio" is going to be reduced; essentially you will be trading low rpm torque, for a little bit more torque higher in the rev band.

Therefore you will normally need to advance the ignition timing early on.

Ideally you would increase the static compression ratio to suit the new cams, keeping the dynamic compression ratio very close to what it had.

[jezza323]

cheers rolla, thats what i thought, more timing down low

[roadsailing]

start at standard timing, adjust on the dyno to shift torque where you want it.

[Jester]

My Bad, will be more careful next time and keep to myself to avoid further embarrassment

[jezza323]

i dont have access to a dyno, cos im a tight ass and i enjoy playing with the maps myself and learning something without spending $100+ for dyno time while i work it out

[Chrisso]

If changing the cams for longer duration, then you are going to be moving the rpm point where maximum cylinder fill is achieved until later in the rev band. This means that your "dynamic compression ratio" is going to be reduced; essentially you will be trading low rpm torque, for a little bit more torque higher in the rev band.

Therefore you will normally need to advance the ignition timing early on.

Ideally you would increase the static compression ratio to suit the new cams, keeping the dynamic compression ratio very close to what it had.

Given the amount of lift has also increased I assume, happy to be corrected, that this will increase cylinder filling characteristics and tone down the effect of the longer duration. That is - longer duration = max output at higher RPM; higher lift = moderation of duration effect.

Is there a means of measuring the impact, or is this another case of string length = <?>?

Chris