Tech talk: Why 2v heads need more advance than 4v heads.

[5mall5nail5]

Was talking to a buddy the other day about timing figures (he has a supercharged LS1). We were just bouncing timing values off of one another and, as can be predicted, he was saying he runs around 20 - 25 degrees of advance where we run 14 - 16 degrees of advance (considering other factors moot). We both agreed it's due to the combustion chamber efficiency mostly but I couldn't think of a good way to visualize why. I was browsing through other images I have saved and found it - thought people might find it interesting and it might give some guys a visualization of why M20/M30 maps have so much more ignition advance compared to 4 valve motors.

The goal is to time your ignition so that the flame propagates in time near the end of the compression stroke. The mixture takes some time to burn and produce the explosion, so too soon and you risk sparking the mixture while the crank is still coming up, and too late and you blow fire out of the exhaust manifolds.

So why is it that a 2v head is "less efficient" than a 4v head? This image makes it clear:



What this image is showing is not necessarily 2v vs 4v chambers. It's showing flame propagation based on spark plug location (which is driven by valve count and placement). So, for instance, the first row is degrees in crank angle after a spark in a head with an offset spark plug. You can see that 25 degrees of rotation after the initial spark and the flame has still not fully propagated.

The second row shows more swirl in the chamber with the same offset plug position. Swirl is important on an indirect injection motor. The fuel is injected and vaporized ahead of the hot intake valve and is sucked in by the engine. Combustion chamber geometry can induce a swirling action by which the mixture is distributed more throughout the chamber. Swirling and port design can also increase the velocity by which the mixture is ingested. You can see that in the second row of images the flame has propagated more. The 3rd row induces more swirl and the flame is propagating faster yet. The first three rows represent an offset spark plug in a chamber.

The 4th row is pertinent to all with a centralized plug. Granted, some combustion chambers in a 4v central plug motor are created more efficiently than others but this is a generalized explanation. You can see that the mixture lights up really fast compared to the off-center plug chamber.

Finally, at the bottom, is a multi-plug motor. Some manufacturers choose to use a 2nd plug in a relatively "inefficient" head to maximize the ability to burn the fuel faster and complete. Porsche engines do this frequently. 2 plugs per cylinder have both advantages and disadvantages. The advantage is you can stagger plug fire with EFI solutions or vary plug fire sync throughout the powerband based on velocity through the chamber. Disadvantages are 2x the number of plugs, wires, etc. which generate more ignition noise and involve more maintenance.

So the reason why a 4v motor might only want 20 degrees where a 2v motor might want 35 degrees is because the flame moves slower through the chamber in a 2v motor and that delta of 15 degrees (hypothetical figures used). So to time it such that the flame is at its most complete state at the end of the compression stroke the initial spark needs to occur that many degrees in advance.

Just thought some one might find this visual explanation useful.

[BobRoss]

[5mall5nail5]

This thread also kind of serves for someone who may or may not have loaded a 2v timing map into an ECU for a 4v turbo motor...

[wazzu70]

You mean i can't use a timing map from another motor? Dang

I have seen the flame front image before, its quite cool. I read a book long ago about some of the IC testing done at MIT. Some very cool stuff. Lots of studies on combustion chamber shape, timing advance, plug location, complete burn, flame speed, ect.

Too bad it was an older study with a 2v head.

[bawareca]

I dont know what M30 guys are running,but I rarely go over 20 degree on a full load with M20.BMW engines have been always significant for their combustion chamber shape and optimised combustion process(each of it's era).That is one of the reasons why they are so friendly to FI out of the box.
There is no doubt 4 valve engines are much more efficient but the difference is not that huge.You 2D example is interesting,but it doesnt show the complete picture.If you imagine the combustion chamber in 3D you will see that the spark plug is still close to the wall,not in the geometrical center of the chamber.Much better than the 2 valve design,but not 2 times better.
Also the head and the piston are shaped in a way to "move" the chamber close to the spark plug.The furthest end has almost no volume in it.

[bmmrr]

if we retard too much, we loose power , reduce chamber heat but get more egt.
i wonder , Will that egt is enough to burn exhaust valves or damage some parts

[e30tomo]

Not really deep in this like you, but I've heard that some VW guys with early 4 valve engines have some crazy ignition advance numbers... Was talking with some guy who drives an Opel C20LET powered car, and he was like what is that with VW's, they can use much more advance than his engine and BMW's... But C20LET and M/S 50/52 combustion chambers look very similar, and all of them, including VW are with centered spark plug, but VW's chamber is a little different...

VW




Opel




And, guess which




However, it can be 17 deg and 30 deg, it's good till it makes pauwa and your engine is still in one piece

[Captain Bondo]

\
So the reason why a 4v motor might only want 20 degrees where a 2v motor might want 35 degrees is because the flame moves slower through the chamber in a 2v motor and that delta of 15 degrees (hypothetical figures used). So to time it such that the flame is at its most complete state at the end of the compression stroke the initial spark needs to occur that many degrees in advance.

Just thought some one might find this visual explanation useful.

Great post.

Keep in mind though, it's not that the flame necessarily moves slower, it just has further to move in order for the burn to be complete and so the burn takes more time. Subtle but significant difference.

Worth noting mainly becuase flame speed itself is its own variable, based mostly on AFR.

[YellowBed]

I use 25* on an m30 (old bmw engine from a distant past) and raise it up to 27 after 6k.

What do m50s like?

[Lurker27]

I use 25* on an m30 (old bmw engine from a distant past) and raise it up to 27 after 6k.

What do m50s like?

At what MAP pressure, though?

[SiGmA]

YellowBed - Mid-teens like Jon said.

[RK-Tunes]

It depends on the set up.
here is what ive gained from my recent experiences--

17 psi at 515whp on a m52 at 9* mid range and 13 at redline I saw no knock and the car could have taken 2-3* more timing easy. This a a efficient set up.

on a less efficent set up at 20psi with a pt61 the car would have slight knock at over 9* in the midrange.

With 630whp on a 3.2 it was at 20psi it was at 8* mid range no knock.

At 14psi with 450whp 13-14 degrees in the midrange and 20 up top doesn't knock. This is with a efficient set up. With a cast manifold/ smaller intercooler expect 1-3 degrees less timing.

[BoostedE21]

I forget what I run through my powerband, but I can post my "map/tune" for my timing if you like (yes from my gameboy EFI )

Also, great post. Definitely good insight I never thought or knew about.

[GG///M3]

I usually go for 29 * of timing everywhere when using forged internals from unicorn pre-mix parts.

[5mall5nail5]

I dont know what M30 guys are running,but I rarely go over 20 degree on a full load with M20.BMW engines have been always significant for their combustion chamber shape and optimised combustion process(each of it's era).That is one of the reasons why they are so friendly to FI out of the box.
There is no doubt 4 valve engines are much more efficient but the difference is not that huge.You 2D example is interesting,but it doesnt show the complete picture.If you imagine the combustion chamber in 3D you will see that the spark plug is still close to the wall,not in the geometrical center of the chamber.Much better than the 2 valve design,but not 2 times better.
Also the head and the piston are shaped in a way to "move" the chamber close to the spark plug.The furthest end has almost no volume in it.

I personally disagree - the difference is huge. Having tuned my own car (when back on a holset) and a buddies M20 on a similar holset, the power bands were completely different with the M50 being much stronger. I don't follow you close to the wall statement - the spark plug on an M50 motor is equidistant to the cylinder wall from any other point on the wall. It doesn't much matter whether it has significant volume or not - we're talking about a compressed combustible mixture - it still takes time for the flame front to engulf the entire combustion chamber on the off-set 2v motors.

if we retard too much, we loose power , reduce chamber heat but get more egt.
i wonder , Will that egt is enough to burn exhaust valves or damage some parts

Yes - excessively retarded timing figures will shake and bake the out side.

Not really deep in this like you, but I've heard that some VW guys with early 4 valve engines have some crazy ignition advance numbers... Was talking with some guy who drives an Opel C20LET powered car, and he was like what is that with VW's, they can use much more advance than his engine and BMW's... But C20LET and M/S 50/52 combustion chambers look very similar, and all of them, including VW are with centered spark plug, but VW's chamber is a little different...

However, it can be 17 deg and 30 deg, it's good till it makes pauwa and your engine is still in one piece

Yeah I was more or less comparing the BMW engines. The older 16v heads from the VWs are a little odd because their chambers are so flat.

Great post.

Keep in mind though, it's not that the flame necessarily moves slower, it just has further to move in order for the burn to be complete and so the burn takes more time. Subtle but significant difference.

Worth noting mainly becuase flame speed itself is its own variable, based mostly on AFR.

I don't mean speed as in velocity so much as I mean how distant the initial spark must spread to engulf the entire mixture.

It depends on the set up.
here is what ive gained from my recent experiences--

17 psi at 515whp on a m52 at 9* mid range and 13 at redline I saw no knocked and the car could have taken 2-3* more timing easy. This a a efficent set up.

on a less efficent set up at 20psi with a pt61 the car would have slight knock at over 9* in the midrange.

With 630whp on a 3.2 it was at 20psi it was at 8* mid range no knock.

At 14psi with 450whp 13-14 degrees in the midrange and 20 up top doesn't knock. This is with a efficent set up. With a cast manifold/ smaller intercooler expect 1-3 degrees less timing.

You are running timing figures that I run at 30 psi on pump gas w/ methanol. Extremely soft.

I forget what I run through my powerband, but I can post my "map/tune" for my timing if you like (yes from my gameboy EFI )

Also, great post. Definitely good insight I never thought or knew about.

Post it up!

[good & tight]

I posted this awhile ago, this is my timing table for the M30 turbo running E85.

Keep in mind a top fuel dragster is a 2V engine and makes around 6000+hp.
Show me a 4V engine that can do that

[5mall5nail5]

I posted this awhile ago, this is my timing table for the M30 turbo running E85.

Keep in mind a top fuel dragster is a 2V engine and makes around 6000+hp.
Show me a 4V engine that can do that

Otis - you're confusing this topic. It's not about what can make more power. It's about timing and why a 2v requires more than 4v.

At 300 kpa M50s like 12 advance... roughly half of what you're running.

[bawareca]

Otis - you're confusing this topic. It's not about what can make more power. It's about timing and why a 2v requires more than 4v.

At 300 kpa M50s like 12 advance... roughly half of what you're running.

How do you determine what it "needs"?Do you say that if you go 15 degrees it will go down on the power?
I dont have much experience with 24 v motors and I am curious.Best bet will be to see some factory maps deciphered.

[5mall5nail5]

How do you determine what it "needs"?Do you say that if you go 15 degrees it will go down on the power?
I dont have much experience with 24 v motors and I am curious.Best bet will be to see some factory maps deciphered.

If I run an M30 and M50 at the same press and RPM the M30 might require as much as 2x the amount of timing advance (ignoring a lot of minute details, so work with me).

[good & tight]

Otis - you're confusing this topic. It's not about what can make more power. It's about timing and why a 2v requires more than 4v.

At 300 kpa M50s like 12 advance... roughly half of what you're running.

Not confused, just sayin.

The limit I found on 93 octane was 19* at 230kpa, after that I switched to E85.

[5mall5nail5]

Not confused, just sayin.

The limit I found on 93 octane was 19* at 230kpa, after that I switched to E85.

I assume that was without water/meth? My M50 was well within limits on 93 octane + 50:50 water meth at 9 degrees at 299 KPA. I went to 11 degrees only to slip the clutch worse.

[bawareca]

Your comparison with M30 may be correct,but i can assure you that M20 and M30 are different beasts.
And you didnt enlighten us how did you get to the magical number 12.

[5mall5nail5]

Your comparison with M30 may be correct,but i can assure you that M20 and M30 are different beasts.
And you didnt enlighten us how did you get to the magical number 12.

An M20 is not a different beast. Look at a timing map for each.

How I got to the magical number 12? It's called tuning my friend.

[good & tight]

I assume that was without water/meth? My M50 was well within limits on 93 octane + 50:50 water meth at 9 degrees at 299 KPA. I went to 11 degrees only to slip the clutch worse.

Correct no water/meth. 19* was it's max limit (showing light signs of det), I bumped it 2* more and lifted the head.

[5mall5nail5]

Correct no water/meth. 19* was it's max limit (showing light signs of det), I bumped it 2* more and lifted the head.

Heh - ouch.