High Boost and Ignition Timing

Hi All,

I have a question concerning the ignition timing on a very high boost engine. Let us consider an RB26 running say 46 PSI of boost using q16 fuel. MBT at peak torque 7000 RPM and 29 psi is around 13*, using a general guide of 2 degree less of advance for every 20KPA (2.9psi) increase in boost you find at 46 psi you are running around 1 degree of advance. From a theoretical stand point I imagine the EGTs would be quite high with such little advance. In your experience does the general rule work or is there a point at which the ignition timing sort of remains relatively fixed as the boost climbs ( and there is no knock). I have attached 2 sample ignition tables which illustrate the two scenarios for reference.

Hello,

i think you are thinking about it from just looking at your timing tables but not how actually things may work

every engine is different in timing maps.for example evo and subaru you will see that evo uses much less timing if both ran on fairly good octane,

2 degree is a rule of thumb for every 20 kpa yeh(it's just safety/starting point) but that doesn't mean that the engine will make best power at that value,you may need more or less depend on the cylinder pressure you get/piston speed at that rpm/boost,so 1-2 degree maybe right yes and maybe wrong.if your are tuning it on street watch knock.with q16 its hard to detect knock and run past mbt so you will need a dyno

building an engine with that specs and boost i am sure that you can reach a dyno to find it out :)

i hope i helped.

Thanks.

I do appreciate that every engine is different and the amount of timing the engine will accept is dependent on many variables. My question is simply from a theoretical stand point and from experience. The reason i am asking is that i recently tuned a GTR which would accept large amounts of timing up till 45 psi with no audible knock, with no dyno to confirm the changes in torque as i varied the timing i was uncertain what would be ideal. I chose not to reduce the timing too low as the EGTs started to climb significantly.

The idea behind retarding the timing 2 degrees per 20 kpa is simply an assumed safe starting point. From here it's still your job to actually optimise the ignition timing for your specific engine (and obviously a dyno is the best place to do that) If you're tuning on a good fuel like Q16 then you will be relatively immune to knock (not always so care is still always required) and can advance the timing at a specific boost level until MBT is reached. It's quite likely that you will find the timing table is much flatter than the 2 deg/20 kPa starting point.

For Q16 I'm surprised that the engine only wants to see 13 deg at 7000 rpm and 29 psi to be honest. If you're drag racing then the mph is a reasonable indicator of the effectiveness of a timing change. If you pick up mph then you're heading in the right direction. Obviously if you change anything else between runs then this goes out the window to a degree though.

If you're tuning on the road/strip/track then perhaps consider getting the tune close and then hiring some dyno time to find out exactly how much timing the engine really wants.