Jump to content

Why is Timing retarded as boost increases?


Recommended Posts

Just wondering why when you tune an engine you retard timing as you increase boost pressure.



Its my understanding that it takes longer for the flame front to reach the piston when you increase the cylinder pressure.



So surely that means you would need to advance it more?



I know its to avoid det, but why does it avoid det?


Edited by Weyro
Link to post
Share on other sites

It should be advanced as boost rises, for tuneing it should be advanced to the point of peek power then leveled off the rest of the rev range, if there is any det it should be retarded or some more fuel added at that point.

Link to post
Share on other sites

Get this one out the way first :


Knock comes on when the ignition timing is too far advanced (or there may be other reasons such as carbon deposits etc), for that load point. At that load point there is X amount of cylinder pressure, and also that load point may be able to take say Y amount of ignition advance. The reason it is retarded is to control the flame, the pressure in the cylinder is too great for that amount of ignition advance, hence why you would have noticed people saying "High compression engines are more likely to suffer knock" because the cylinder pressures are greater. The easy way round it is to reduce the amount of ignition advance, however this isn't the best way to tune an engine. You will find that many race cars have probably a seperate map for every track/rally/event etc because they want the torque in a specific range, ie mid range for a circuit with flowing bends but short straights. So what happens is they retarded the valve timing to make the valves open later and close later, this reduces peak cylinder pressures. Now we know that retarding the valve timing will change the powerband so we use that to our advantage when mapping for a specific event and we can increase the ignition advance in this area say from 2500-5000rpm.



Obviously theres more to it than that, the way the combustion chamber is design, piston crowns, if a headgasket is used or not all have influences on compression ratio, peak cylinder pressures, ignition advance.



The reason why forced inducted motors generally run less ignition advance is because the turbo operates within a certain window, for example you wouldn't expect a tiny ct9 to still produce 170whp at 8500rpm and also the engine is way out of its volumetric efficiency and cylinder pressures will drop off here. Knock is most likely to occur at peak torque as this is when peak cylinder pressures are.


Now looking at an N/A engine, the best way to make an N/A engine make more power is to increase the rev limit (providing there are no restrictions, intake, exhaust etc) This will be when you see ignition advance rise all the way up the rev range due to the nature of the powerband for an N/A engine.



Don't forget that the ecu will also pick up information from other area's such as IAT, TPS, so when cruising with throttle position say >35% the ignition advance can be increased for fuel economy. Also the knock properties of the fuel play a big part.


Link to post
Share on other sites

It should be advanced as boost rises, for tuneing it should be advanced to the point of peek power then leveled off the rest of the rev range, if there is any det it should be retarded or some more fuel added at that point.

If you did that you would have a very rough map with a very non-linear power curve and it would be hard to keep it in the powerband.

Try and look for an F3 race car dyno graph, they have a very linear response on the power curve.

Link to post
Share on other sites

ok thanks


but what i cant get my head around is why a later spark reduces knock, when retarded timing generates more heat?


Edited by Weyro
Link to post
Share on other sites

actually i think i get it



at TDC and full boost, the spark is unable to ignite all of the mixture in the chamber due to the high pressures, so the leftover mixture burns in a second explosion which occurs after the spark?



so you retard the timing so that the spark occurs at a cylinder pressure which will allow the whole mixture to burn at the same time


Link to post
Share on other sites

A retarded spark will reduce knock because the piston would have already gone further up/gone past TDC, so rather than the flame pushing the piston down the cylinder, the flame is following the piston down.



No that would be wrong, thats the worse thing. We only want 1 strong explosion to push the piston down the cylinder. Having more than one bang makes it harder to control the flame, hence why with poor combustion design/piston design you end up with hot and cold pockets around the cylinder.


When the plug puts out a spark, the first point to burn is the mixture that is closest to the plug tip, the flame then expands outwords.


Link to post
Share on other sites

ah right so knocking is when the mixture ignites ahead of the flame front, due to the increased pressure? but this still occurs after TDC?


so by retarding the timing you have lower pressures reducing chance of det, and have better control of the direction of the flame front?


Link to post
Share on other sites

Knocking is just a term used for violent combustion. It won't be likely to happen after TDC because the mixture will be able to increase in volume due to not being in a confined space, pressure is at its highest at TDC, and at its lowest at BDC, although it could still well happen if there are any hot pockets.


That's it, by retarding the timing the pressure is lower, but like I say depending on what your trying to achieve this isn't necessarily the best method as you can reduce the pressure also by retarding valve timing.


Link to post
Share on other sites

Yeah we do, fit an adjustable cam pulley. However thats where the E series has its limitations not being a true twin cam design, so what you do to one cam, does the same to the other. I forever hear people say that adjustable cam pulleys aren't needed unless you are running cams blah blah. But the pulley does have its uses, if your tuner knows what they are doing that is.


Link to post
Share on other sites

Hi I am still very new to all af this.. What would you say in degrees you would retard ignition timing when running 1.3 to 1.5 bar boost on a forged motor with 370 cc injectors (95 octane) ( round about 8.15 :1 compression ratio) ( normal timing is 10 degrees I think)



thanks Bertie

Link to post
Share on other sites

That would be down to your tuner to decide. Assuming you are running some form of programmable engine management system? I would expect so with that sort of boost pressure.


Link to post
Share on other sites
  • 8 months later...

What turbo and fuel are you using? I would have thought you might be able to get a few more degrees of advance in there than 24, although it is pretty much bang on for a road going turbo motor.


Link to post
Share on other sites

spark advance should decrease as VE (airflow) increases and vice versa. VE will peak around peak torque then decrease so more advance is need after peak torque. Knock is usually around peak torque. if your using 95 fuel knock is going to be present.


Link to post
Share on other sites

This guy who started this thread was pm'in me about mapping an e manage, in the end I found out he basically stolen it from a member on here, if I new that I would of gave him some good tips on how to blow an engine rather than help the tea leaf out.

Link to post
Share on other sites

More advance than that of peak torque is not necessarily meaning detonation. Depending on engine, some can go 10 and more degrees after the peak power point without det. Previous gen 911 is a typical example.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...