Well guys, I have a question for you. I read in another part of the forum that our cars have 12 degrees of timing as a base, and then the computer changes timing on top of that depending on....well..lots of things.

Fair enough.

Now the question, is why?

With the computer controlled ignition systems, why not just have the base timing on the car set to 0, and have the computer control everything? Is it possible for the car to go below the base timing? Like, can you have 8 degrees of total timing?

Is it basically just a carry over from the distributer days where they had the base timing and then the mechanical or vacuum advance, or is it something totally different?
Come to think of it, did I just answer my own question about going below the base timing?

Just curious.


As always, your knowledge is appreciated.

Brian

Just as a clarification, I'm talking about strictly electronic ignitions with NO distributer. I understand why you have a base timing with a distributer. But for cars with no distributer, why not just build the engine with 0 degrees total timing, and have the ECU make it all up?


Brian

Some engines without distributors do have adjustable base timing. The 4G63 for example, electronic DIS type system with an adjustable cam/crank sensor. You can adjust the base timing anywhere from -5 to 10 or 15 degrees or so (can't quite remember the entire range for that sensor).

To answer your main question, there's no need to go below 12 total timing with the 420A... so the factory set that as the base. The ignition can't be timed lower than that, and there's no reason to. There may be some more specific reasons that the factory chose 12 degrees that I don't know about, but it doesn't matter because there's no good reason to have the timing that low.

Total ignition timing on a highly boosted 420A is normally in the wide range of 25-31 degrees.

I hope I answered your questions... it's early in the morning for me and I don't think well in the morning. :)

*edit: Also, if you ever locked an engine's total timing at a low value (0-10 degrees)... then you'll know exactly why there's no need for the base timing to be below 12. The engine needs to be timed AT LEAST 12 degrees BTDC in all conditions imaginable.

I'm assuming that you are talking about when the spark occurrs. 0 degrees would cause problems. I believe that the piston needs to have moved down a bit before the fuel mixture is ignited.

Yes, we're talking about BTDC. And no, the piston needs to be on its way UP when the mixture is ignited, BEFORE Top Dead Center.

Right, if the piston was on its way down, and THEN you ignited the mixture, you wouldn't gain a lot of power out of the process as the flame front would "chase" the piston down the bore. It would also make for very high EGT's since a lot of the heat would be coming out on the ensuing upstroke.

Doug, you did answer my questions, thank you for your time. I'm intrigued that the 4g63 allows for adjustable base timing, that's very interesting, and I do understand that the engine must have a certain amount of timing into it to run efficiently. The motor couldn't run on 0 degrees very well (if at all) for the same reason I stated above.

So the answer is basically, the engine will never need less than 12 degrees total timing, so the factory sets that as its lowest amount.

I suppose the lingering question is, why not just have the computer limit the timing to no less than 12 degrees? Are they possibly afraid of people messing with the computer and making the total timing less than 12, thus pushing up the EGT's, and maybe toasting the exhaust valves?

I guess my point is, why not do it all electronically, instead of partial electronic, partial mechanical?

I'm sorry to be picky, I guess my curiosity is getting the best of me.

Thanks again,
Brian

Originally posted by ForceFed420a
I guess my point is, why not do it all electronically, instead of partial electronic, partial mechanical? I'm sorry to be picky, I guess my curiosity is getting the best of me.

I'm thinking that, on the 420A, anyways, it is all electronic. The 4G63s just hav that weird manual adjustability for some reason. I mean, it's a 20 year old engine, right? :P

it has everything to do with the burn rate of whatever fuel you're using as to when you need to ignite the spark. That rate is much more constant than the rate at which the engine turns. The faster the motor spins, the earlier you need to light off the A/F mixture so that peak cylinder pressures continue to occur at the point where the most torque is transfred to the crank. With out motor and head design, and the octane of the fuel it was designed for, there's no need to have timing less than 12*BTDC, because anything earlier would create inefficient torque transfer to the crank, and anything later would be wasted as heat out the exhaust valves.

Sound right, Doug?

The 4g63 probably has adjustable base timing to help account for different fuels uin different countries where the motor is used. Just specualtion, because i only have minimal experience with them and one with USDM stuff.

Aah... My bad.

I get it now.

Maybe I should ask this question before continuing. I've always assumed (possibly incorrectly) that the base timing due to the mechanical design of the engine. The base timing is "cut" into the crankshaft at 12 degrees BTDC. Is this an incorrect statement? Is the mark on the crankshaft actually directly at TDC, and the computer then limits the timing knowing where TDC is?

Again, this is probably the first question I should have asked. Sorry for the confusion, and thanks for all your responses.

Brian

PS. Yeah, I understand the idea of lighting the mixture before TDC to give time for combustion, and this lighting needs to occur more and more before TDC as the RPM's increase. I believe I read somewhere that the ideal peak pressure in the cylinder should be at or around 15º of crankshaft rotation AFTER TDC to allow for maximum transfer. I'm sure though that that number changes depending on the length of the rods, R/S ratio....etc

Originally posted by DarkOne
...there's no need to have timing less than 12*BTDC, because anything earlier would create inefficient torque transfer to the crank, and anything later would be wasted as heat out the exhaust valves. Sound right, Doug?

Actually, there is a need to spark earlier earlier than 12*BTDC (that's where advancing the timing comes into play)... but not any later (that's why 12* is the base). If our base was 5*, then we'd just advance the timing electronically. However, the manufacturer has determined that the timing will never need to be later than 12* BTDC. I think that's really what you were trying to say though. And yeah... it's all about the fuel.

On a side note, differences in base timing between different engines is the reason that timing advance values don't mean a lot, it's the TOTAL timing the matters. What I'm trying to say is that just because a friend with the same engine is running 22* of advance, doens't mean you can run 22* of advance. His base timing may be much lower (or higher) than yours. Don't ask me how I figured that out (I do have the pistons to show for it). However, us 420A guys don't really have to worry about that because the 12* base timing is set in stone.

Another interesting example: when I tune a standard 16G-powered 420A engine with FMIC (1.0 BAR boost), the advance at WOT usually ends up around 17 degrees... but when I do a 4G63 with the same equipment, it usually ends up with about 22 degrees of advance. That's an advance difference of 5 degrees. I always set the 4G63 base timing at 5 degrees and the 420A is locked at 12. That's a base timing difference of 7. The 420A will spark at 29 degrees BTDC and the 4G63 will spark at 27 degrees BTDC. It's always seemed to me that the 420A is slightly less likely to detonate (likely due to intake/head/combustion chamber differences). The point of this example is to be very careful when comparing timing advance numbers without knowing what the base timing is set at.

Originally posted by dougie2
I think that's really what you were trying to say though.

Yeah, i was thinking backwards. You said what i was trying to say.

Forcefed brought up a question i've been thinking about as well, is the 12 degrees built into the crank notch pattern? If not, then how do we know base timing is really 12 degrees? In other words, how did you find out it was 12 degrees? Just curious..

Yes Matt, that's the root of my question.

#1 IF the base timing of 12 degrees is governed by the mechanical design of the crank (ie. the notch pattern)--then my original question still remains: why do the choose the mechanical/electronic way of setting the base timing, when it would seem easier to do option 2.

#2 IF the crank is designed so the CKP sensor is picking up dead on TDC, and thus the computer knows TDC and compensates accordingly (without going below 12 degrees total timing) then my question is answered.

I guess this opens the larger question: On distributerless ignition system, is the base timing set through a mechanical/electronic system, or simply all electronic? Are all motors like this?

Brian

Matt- Our base timing is triggered by the falling edge of the last tooth on the crankshaft's trigger wheel (I also confirmed this with James, who created the 420A decoder).

ForcedFed- Alomg other things, the base timing also serves as a "limp home" mechanism incase other components fail. The engine would not be able to run with 0 degrees of timing. Base timing is never "electronic".

Originally posted by dougie2
Matt- Our base timing is triggered by the falling edge of the last tooth on the crankshaft's trigger wheel (I also confirmed this with James, who created the 420A decoder).

Yep, thats what I was thinking..just wanted to be sure.. Cool :)

Excellent. Therein lies my answer.

Thanks Doug.


Brian

No problem... great questions.

moved to Advanced, because this is why it exists.

i have a question doug...on a 420a have you ever had to pull out timing to prevent detonation? and if so how much timing did you take away? just a few degrees? my car melted a set of forged je pistons in less than a week on 8 psi. it was not running lean, and i never heard knock. i started getting blow by, so i tore it back apart think one of my rings never seated, ended up having had detonation on all 4 pistons and melted the intake side of them off. ever heard of that happening before?

Check you PM inbox. I already told you once about bumping old ass posts.

well one of my buddies is only running 8 degrees advance on his sr20 on 15 pounds of boost. 30 degrees sure seems like alot to me on a highly boosted 420a. unless your running e85 i dont think it should take near that much time to ignite the fuel

Originally posted by raptor2323
well one of my buddies is only running 8 degrees advance on his sr20 on 15 pounds of boost. 30 degrees sure seems like alot to me on a highly boosted 420a. unless your running e85 i dont think it should take near that much time to ignite the fuel

Stop bumping 4 and 5 year old threads.