understanding turbo rotary timing better
 

This is a big question i always had and probably most people have. what is a good base timing for a turbo rotary? i know the base answer to this that is different to every engine but really is not a base timing that can safely someone can go by? now i want to ad on this. i done research about this for a while now with some answer but seem what i'm really looking for i cant or nobody will help me. I'm been working on rotary all my life and my microtech ever since i had it. i never had anyone tune my car. i tune it in a trail and error, going to the track and driving around. i never dyno my car, is always been by the butt dyno. i always been going by the stock map or by member rx72 maps that i have saved over the years. basicly always playing with the fuel, not the timing. now the only reason i ask is because before i build my last motor (i only blew one turbo in 8 years) it felt that this new motor which is identical that the old one is seems slower. the only thing that changed over the old engine was apex seals and was build stronger. now my base timing is at 25 degree, i don't know is that too much or too little. I'm just going with what the stock map is. the second thing is that my map doesn't start retarding timing until 8 psi. i found this tread https://www.rx7club.com/rotary-car-p...-split-393114/ which was helpfull to understand some things i was doing wrong. one thing i changed was my under boost timing to -1 degree for every 2 pounds and my car got way better that before but still my base timing still uncertain. now hopefully someone help me on this or give advice. i know the best thing to have is a knock sensor but then again is a base line. this is something i ask not just for me but other forum members that i know will have this question in the future. thanks for any piece of advice

eddie

Most timing maps start life as something put together without having run on a dyno. They are based on modifying a stock/default map, or by going by a heuristic (a rule of thumb) that's been floating around.

"Increase timing with rpm"
"Retard 1 degree for each 1 psi of boost"
"Under boost, have timing dip at the rpm of peak torque"
"Retard timing if you increase compression ratio over what the map was originally for"
"Use 30 degrees for cruising"
"Set split to 12-15 degrees under boost if you want safety"

Then you have to do some trial and error on a dyno at WOT. Often part load timing can't easily be optimized except by some subjective assessment by the driver. At the OEM level the part load timing is set by the combustion chamber pressure and emissions.


For a turbo rotary, probably the most common map used as a starting point is the leading timing map that comes default on a Power FC for a 3rd gen.

Showing relative pressure:

https://www.rx7club.com/attachment.p...1&d=1442372905

absolute pressure:

https://www.rx7club.com/attachment.p...1&d=1442372807

For a safe trailing spark baseline, you can just run 15 split in all boost areas. There are other ways to do it, but you can't really go wrong with doing it that way as a starting point.

I've never seen the stock timing maps on an FD stock ECU. However we do have the stock timing maps for the 2nd gen turbo ECUs. Keep in mind that those had airflow meters, so the "load" axis is a calculated value proportional to the measured airflow divided by the rpm. It makes a lot more sense if you have ever tuned other platforms with MAF sensors, whether it's an Rx-8, domestics, or say a WRX or Evo.

Here are stock timing maps for I think the S4 T2 N332 ECU, 8.5:1 compression and wastegate spring pressure:

https://www.rx7club.com/attachment.p...1&d=1442373252

https://www.rx7club.com/attachment.p...1&d=1442373252

and the s5 T2 maps (N374 ECU), with divided turbo manifold and twin scroll turbo, plus factory electronic boost control [higher boost than an s4] and 9.0:1 compression:

https://www.rx7club.com/attachment.p...1&d=1442373252

https://www.rx7club.com/attachment.p...1&d=1442373252

also, I don't understand what you mean by base timing in this context. General approach is to set the baseline timing according to the pulley mark. Lock timing in the ECU to the pulley marks, put a timing light on there and verify that the timing mark agrees with the setting in the ECU. If it's an FD it's mostly just a verification, but if it's another engine that has adjustable crank angle sensor you've gotta mess with that and hope that your pulley's timing marks are trustworthy.

Example:

Say I have an FD. Crank angle sensors are fixed and there's a pickup on the pulley. I set my timing in the ECU to -5 Leading, -20 Trailing. Then I run the engine and look to make sure that the white mark lines up. The white mark corresponds to -20 for trailing - there's no leading mark on an FD pulley. I rev the engine and verify that the timing doesn't drift according to the timing light. Depending on the ECU, a baseline timing setting in the ECU or some crank trigger settings may need to be changed.

or, say I have an FC. Crank angle sensor drops in like a distributor. I have a crank pulley and hub set with timing marks that I trust. I stab the crank angle sensor according to factory procedure. I set my timing in the ECU to -5 leading, -20 Trailing. Then I verify that the yellow and red marks line up. If not, I may have to adjust the crank angle sensor position like I would on a stock car with stock ECU. I can also check for timing drift when I rev the engine and check with the light. Depending on the ECU, a baseline timing setting in the ECU or some crank trigger settings may need to be changed.

This is so far one of the best answer I had. I appreciate Arghx. What I meant as a base timing is once you lock the timing in the pulley what goes next. Like for instance my base timing is at 25 degrees at 3k rpm. Now that goes up and down with load (vacuum or pressure) now is that a good start for anyone?

Do you have a lot of experience with mechanical distributors? That's a bit different frame of reference, where timing is baseline value + centrifugal weights' advance + vacuum advance - boost retard (on the few turbo cars that came with dizzy + boost retard from the factory).

So, distributor curve for a 12A (I think from an FB service manual):

https://www.rx7club.com/attachment.p...1&d=1442414857

Boost retard on a very very old Chevrolet Corvair Monza turbo (carb'd boosted 4 cylinder boxer that GM sold a few of in the early 1960s):

https://www.rx7club.com/attachment.p...1&d=1442415611

Getting back to ECU tuning:

If the mark on the pulley agrees with the timing in the ECU, and you trust the pulley, and you can rev the engine with a fixed timing setting and it doesn't drift, your "base timing" in the popular parlance is set. Maybe it's the word "base" that we're tripping over here. With tuning electronic spark advance it's usually used to refer only to that initial pulley/software comparison I mentioned in the previous posts.

So when you use the word "base" timing at 3k rpm, it's not a conventional use of the word in this context. "Base timing" is just something you deal with during initial setup of the ECU and engine. Once you have that out of the way, everything goes by maps.

If 25 degrees is the most spark advance you have at 3k rpm (say at 20 kPa absolute pressure, high vacuum condition), then as my manifold pressure increases to 30, 40, up to 100 (similar to WOT on a naturally aspriated engine), up to 200 or more, timing advance should decrease (retard spark as manifold pressure increases). How fast should it decrease though? How do I know if I need to progress from 25 degrees at 20 kPa to 5 degrees at 200kPa? or 10?

For a normal enthusiast with a project car, well you don't know really. You look at the fuel you are running and the hardware you have then go off some rules of thumb you believe to be safe, based on the accumulated wisdom of the community. After that you go on a dyno and move up and down in 1 or 2 degree increments to see if you pick up any power. Then you have to make a decision as to whether adding that spark was worth it.

Typically, the further off you are from optimal, the more torque/power you lose at an incrementally increasing rate. Modern stock ECUs model engine torque, and can calculate the effect of spark on engine torque with a "spark efficiency curve"

https://www.rx7club.com/attachment.p...1&d=1442417185

This is an example of a spark efficiency curve you find in stock ECUs (it's for a piston engine, but the principle applies). The exact shape of the curve varies, but you can see that at the upper left is our optimal/ideal spark for the engine speed and load. As we move right, engine torque incrementally drops at a slow rate. Then it increasingly drops at a faster rate, until it becomes insensitive again.

When we're tuning timing we want to be at that upper left corner of the curve if the fuel, intercooling, etc allows it. We want to be where the engine is mostly insensitive to adding additional spark, where 1 degree of spark advance gets me hardly anything. Then I can pull 2 or 3 degrees from best output spark advance and enjoy a nice safety margin.


Knock sensor is a whole thing altogether... they can be hard to trust.

dam i still need to learn more. i had to read your write up a few time to understand what are you explaining still not 100% but i keep reading it. there's defenally a lot more to it that just retarding and advancing. i have a fair amount of experience with distributors and at one point i had those 12at ones in my old GSLSE turbo/carb setup. by any means i'm not a expert on it. so now you mention alot the timing marks on my pulley and the trust worth to them. i have a RB double puley and i have my timing locked at 0 degrees. is that ok to do or should i still find -5 degree like stock? should i trust the RB marks? is theres any other way to actually check timing?

I don't know much about the RB pulleys, but there are plenty of horror stories about 2nd gens with mismatched pulleys and hubs from somebody's junk pile having problems. I guess the RB timing marks are fine. Somebody else could chime in.

I actually bought a brand new pulley hub combo from Mazda years ago and followed the service manual procedure.

the RB mark is at 0, so just set it there. all it does is tell the ECU that 0 is zero. we just run -5 with the stock pulley because that is where the mark is.

One of the most informative and better write ups I've seen on this forum. Thanks!!

hey guy, sorry to revive an old thread, but trying to get some knowledge on self tuning. basically ive got a AP PFC on my fc right now. running stock turbo with like a 2.75 in dp, a cat and a knightsports dual catback. anyways, im gonna be going hybrid v trim (so like 55x70 compressor) and no cat and ill be doing the tuning adjustments hopefully by myself. its professionally tuned with whats seemingly a pretty safe tune based on AFR (running like 10.1 under full boost, according to my AEM UEGO). cant say much about my timing map cause me and my car aren't in the same place right now.
what kinda adjustments can I expect to have to make for this new setup? Ive read the rules of thumb above and in other places. but for instance: when people say retard 1 degree for every psi is that across the whole rpm range after 0psi? like what I don't understand is: when my tuner did my map, he tuned it to like 10psi; however, the map on PFC goes all the way to 24k for pressure (19.5ish psi). why aren't the values that are in there for like 15psi ok for my new setup?


** actually, I thought about it and I guess its because my new compressors "10psi" isn't the same as my stock compressors"10psi". its bigger so itd be more air of course. but I left that part in there just in case its not clicking for someone like it wasn't for me or if theres more to it that I don't understand.


and things like: is trailing timing always based on desired split in comparison to leading timing? will all the non boost parts of my map stay the same? is a positive number on a timing map "BTDC"? is advancing timing heading more towards btdc? sorry, I feel like im missing some fundamentals, but I have tried a little self research.


thanks in advance,
Carrington

If you're keen to learning about how timing in the rotary works, I suggest you read this book - Street Rotary by Mark Warner.

Now to address some of your questions.

The thing is you can't really move towards BTDC, because BTDC (Before Top Dead Center) is a range the key word being 'Before'. Now if a given timing point is in BTDC and you advance it, then you are moving towards the TDC. And if you pass TDC you're in ATDC range (After Top Dead Center). Now if you advance a timing point which is in ATDC, you are moving farther from TDC. Hope this makes sense.


Yes (if I understood the question correctly). The trailing timing always follows the leading one. So e.g. if you retard the leading timing by 2 degrees, you should do the same for the trailing as well so they keep having the same relative split.


The values beyond 10psi on your PFC might be because of the base map generated by the PFC (but I'm not sure). You can just leave it untouched, until you make more than 10psi boost in the future, in which case you should properly tune (dyno tune) it.

thanks man, so moving in the direction of ATDC is advancing, and moving in the direction of BTDC is retarding? and ATDC is represented by a negative number while BTDC is represented by a positive number, correct?


any insight into some of those other questions?


Thanks,
Carrington.

sorry just figured i should've said retard instead of advance in my previous post. So it should be: if a given timing point is in BTDC and you retard it, then you are moving towards the TDC. And: Now if you retard a timing point which is in ATDC, you are moving farther from TDC.

Yes.

haha yeah, I was thinking about it today actually and figured itd make more sense for something happening 'before' a point (TDC) should be represented by the word advance. but I was like, "whatever, what do I know.." haha
thanks for clarifying though.

advancing your timing does move the ignition event closer to before tdc, or further before tdc if you're already at tdc, or before it.

The terms 'advance' and 'retard' are specifically referring to the timing of the spark event in relation to the motion of the piston or combustion face on its specific stroke towards the peak of compression.

so retarding the timing will delay it, or move it towards after tdc, or further after tdc.

advance = sooner
retard = later

All timing charts I have seen show the numbers in BTDC (Before Top Dead Center), so 20 will be 20 degrees before top dead center, advancing that value 5 degrees will make it 25 degrees before top dead center. When you see a negative number in the timing chart, for example -5, read it as -5 degrees before top dead center, which is 5 degrees after top dead center (ATDC)

You will often see to check for timing at -5 and -20, if you follow what I wrote above, that will be -5 BTDC and -20 BTDC, which will be 5 ATDC and 20 ATDC.

However, if you have a tuner, you will have to see what the 0 is actually on the engine. I have heard of some meaning 0 degrees on the timing chart, is actually the base timing of -5 degrees on the engine, so the actual timing values in your chart are 5 degrees higher than they actually are.

Good point. I now admit that it is easy to get things swapped around.