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

[Matt Cramer]

Not really. When you're relying on mechanical advance, be it via a vacuum pod and/or centrifugal, you have a fixed timing curve to work with, all you can do is move the whole curve up or down by turning the distributor body.

That data lists the distributor PN precisely because they all have different curves.

Trying to compare them based on a single operating point is meaningless.

You could potentially tune it by changing the springs in the advance mechanism as well. There's only so many curves you can get this way, but you do have some adjustment. At least, if there is some measure of support from whoever built the distributor...

[Captain Bondo]

You could potentially tune it by changing the springs in the advance mechanism as well. There's only so many curves you can get this way, but you do have some adjustment. At least, if there is some measure of support from whoever built the distributor...

But that's not my point. What I am saying is that you cannot compare two different timing curves based on a SINGLE operating point.

[5mall5nail5]

Yeah a vacuum or spring / fixed distributor can't be compared to an EFI controllable spark. Matt knows this though, he's just sayin'.

[BoostedE21]

Here's my m30 at 20psi. It was set to about 15* before tuning and it
was a complete turd with that low of advance.

A little OT, but can someone please explain to me how to read that map above? I mean I know what it means, X advance at X RPM. But the poster said the timing was at 15*, I don't see where on the table is equals 15*. Unless there is another parameter where you set the amount of retard at a certain MP. (this is how SDS does it, you have base timing then tune the MP ign. ie pull timing under boost).

Sorry for the noobish question.

[5mall5nail5]

A little OT, but can someone please explain to me how to read that map above? I mean I know what it means, X advance at X RPM. But the poster said the timing was at 15*, I don't see where on the table is equals 15*. Unless there is another parameter where you set the amount of retard at a certain MP. (this is how SDS does it, you have base timing then tune the MP ign. ie pull timing under boost).

Sorry for the noobish question.

That's the issue - whenever you hear someone like bawareca say "See, the engine runs at bawareca at 100 kPA" you have to instantly know that they're just picking one arbitrary point. The timing map on a proper ECU should have 3 scales - RPM, Timing Advance (in degrees, radians, whatever), and a load site. The load site can be throttle % for alpha-N, or it can be +- PSI, or it can be kPA, etc. or it can be lb/min or whatever the unit is you want to represent load.

So, for instance, in Yellowbed's post when he says "it was at 15" we're usually assuming under like peak boost around the torque peak. Like if I were to verbally describe my map I'd say "Yeah around 30 psi I run about 9 - 10 degrees". On my map, at the 300 kPA line around 3000 RPM I am sure its higher than 9, but I can't run the car there and it isn't a real useful figure anyway. It's still vague, but it's a brief way to describe. Factory Motronic 3.1+ ecu's ramp the timing up after 5000 - 5200 RPM or so. They might be 19 - 20 deg in the WOT 5000-5200 range but will ramp up to maybe 24 - 25 degrees by 7000 RPM. Bawareca kept saying "ha, see, the M5x doesn't run less timing!" but that's because he's looking at the ramp up at redline which is almost irrelevant. You really need to look at the timing around 5000 - 5200 RPM to get an idea of how the motors compare.

[Matt Cramer]

A little OT, but can someone please explain to me how to read that map above? I mean I know what it means, X advance at X RPM. But the poster said the timing was at 15*, I don't see where on the table is equals 15*. Unless there is another parameter where you set the amount of retard at a certain MP. (this is how SDS does it, you have base timing then tune the MP ign. ie pull timing under boost).

Sorry for the noobish question.

It looks like you're reading this correctly - he's got a single 3D table with MAP (presumably - it could be MAF) on the Y axis and RPM on the X axis. And unless something like an air temperature correction curve that pulls timing when the air heats up too much is kicking in, his engine won't be seeing 15 degrees. I think he was saying he HAD been running 15 degrees under boost, only to find it wasn't enough timing when he tuned it and came up with a table that always gave him more advance.

[5mall5nail5]

It looks like you're reading this correctly - he's got a single 3D table with MAP (presumably - it could be MAF) on the Y axis and RPM on the X axis. And unless something like an air temperature correction curve that pulls timing when the air heats up too much is kicking in, his engine won't be seeing 15 degrees. I think he was saying he HAD been running 15 degrees under boost, only to find it wasn't enough timing when he tuned it and came up with a table that always gave him more advance.

Yep he was implying he had 15 deg and it woke up by putting it up closer to proper timing.

[bawareca]

................

You really need to look at the timing around 5000 - 5200 RPM to get an idea of how the motors compare.

You can talk as much $$hit about me as you like,I dont really care,but ridiculous statements like that come to show that you really know this engine stuff backwards

I am leaving you to dominate again all the internets and all threads

[BoostedE21]

That's the issue - whenever you hear someone like bawareca say "See, the engine runs at bawareca at 100 kPA" you have to instantly know that they're just picking one arbitrary point. The timing map on a proper ECU should have 3 scales - RPM, Timing Advance (in degrees, radians, whatever), and a load site. The load site can be throttle % for alpha-N, or it can be +- PSI, or it can be kPA, etc. or it can be lb/min or whatever the unit is you want to represent load.

So, for instance, in Yellowbed's post when he says "it was at 15" we're usually assuming under like peak boost around the torque peak. Like if I were to verbally describe my map I'd say "Yeah around 30 psi I run about 9 - 10 degrees". On my map, at the 300 kPA line around 3000 RPM I am sure its higher than 9, but I can't run the car there and it isn't a real useful figure anyway. It's still vague, but it's a brief way to describe. Factory Motronic 3.1+ ecu's ramp the timing up after 5000 - 5200 RPM or so. They might be 19 - 20 deg in the WOT 5000-5200 range but will ramp up to maybe 24 - 25 degrees by 7000 RPM. Bawareca kept saying "ha, see, the M5x doesn't run less timing!" but that's because he's looking at the ramp up at redline which is almost irrelevant. You really need to look at the timing around 5000 - 5200 RPM to get an idea of how the motors compare.

It looks like you're reading this correctly - he's got a single 3D table with MAP (presumably - it could be MAF) on the Y axis and RPM on the X axis. And unless something like an air temperature correction curve that pulls timing when the air heats up too much is kicking in, his engine won't be seeing 15 degrees. I think he was saying he HAD been running 15 degrees under boost, only to find it wasn't enough timing when he tuned it and came up with a table that always gave him more advance.

Yep he was implying he had 15 deg and it woke up by putting it up closer to proper timing.

Okay that makes sense. Thanks for the help!

[5mall5nail5]

You can talk as much $$hit about me as you like,I dont really care,but ridiculous statements like that come to show that you really know this engine stuff backwards

I am leaving you to dominate again all the internets and all threads

I am not talking "$hit" on you, but I am highlighting to a tuning newbie that you saying an engine "runs at ____ degrees" makes no sense.

Yeah I really suck at this engine thing. My history tuning them and such is pretty terrible.

[Alchemee]

I have some experience with tuning ls motors as I've had 3 different sets of heads on my car na, and now FI.

If we look historically at GM motors starting back in the early 90s with the lt1, they ran best at close to 40 degrees of timing. GM refined the combustion chamber in the lt4 heads and guys were seeing the bets results in the mid to low 30s.

Along comes the ls1 in 97 and now it's 28-30 degrees, then in 2002'ish GM made the ls6 heads which were again an improvement over the ls1's and max timing was lowered to the 23-24 degree range.

I picked up these a few years ago:




They are an aftermarket casting from MAST with revised valve angles and NA I saw the best power at 20-22 degrees of timing. Ask any of the 4-valve mustang guys and they'll tell you that they run in the mid to high teens for timing because the chambers are that efficient.

I think some of the attitude in this thread is coming from confusing efficiency with power or performance. The ls motors were making 330rwhp and juast as much torque from the factory in the late 90s and still got mid to high 20s gas mileage and were actually a low emissions vehicle.

Just interesting food for thought.

[Lurker27]

^ I think that's the nub of it - 2valve guys are taking a FACT of flame propagation and getting their feelings hurt. It's clear to anyone paying attention that it's pretty easy to make huge power on 2v motors, and all the ancillaries (turbo, fueling) are going to consume way more of your attention and money in any build.

[gath]

Bringing this from the dead coz I have a question. What will be needed to run a 2 valve m20 efficiently using a m50 NV engine management. This will include the ECU, harness, injectors, MAF, COP, and all the rest of m50 stuff now hooked onto the m20. How much retard is needed in the m50 maps to make it run well in the m20?

[m20powered]

Interesting thread.

Bringing this from the dead coz I have a question. What will be needed to run a 2 valve m20 efficiently using a m50 NV engine management. This will include the ECU, harness, injectors, MAF, COP, and all the rest of m50 stuff now hooked onto the m20. How much retard is needed in the m50 maps to make it run well in the m20?

This could be tricky because Motronic uses airflow as a load index. A stock M20 could flow less air than an M50 in certain RPM ranges (because of lower VE), and this would cause it to calculate MORE advance (because it would be interpreted as lower engine load). I'm going to say that you might need some retard in the 3760-5400RPM rows and possibly around 1500. This is based on looking at M50NV EPROM dumps and my old M20 Megasquirt table.

[gath]

Interesting thread.This could be tricky because Motronic uses airflow as a load index. A stock M20 could flow less air than an M50 in certain RPM ranges (because of lower VE), and this would cause it to calculate MORE advance (because it would be interpreted as lower engine load). I'm going to say that you might need some retard in the 3760-5400RPM rows and possibly around 1500. This is based on looking at M50NV EPROM dumps and my old M20 Megasquirt table.

I am running this setup now and am very impressed with the progress. After starting the vehicle it hunts for idle then settles at 600 rpm, runs a bit rich and CEL comes on after getting to 60 mph.. but drives really well. My only issue is that i have no tuning skills, it is a nightmare..

[Binjammin]

I am running this setup now and am very impressed with the progress. After starting the vehicle it hunts for idle then settles at 600 rpm, runs a bit rich and CEL comes on after getting to 60 mph.. but drives really well. My only issue is that i have no tuning skills, it is a nightmare..

Well, considering the 2v nature of the m20 and the 4v of the m50, your ignition timing alone is going to be a ways off. Also, not sure but someone may be able to confirm if the m20 and m50 have different crank offsets? You'll also be getting a code for a lack of cam sensor, as the m20 doesn't have one of those either...

[e30kid89]

Well, considering the 2v nature of the m20 and the 4v of the m50, your ignition timing alone is going to be a ways off. Also, not sure but someone may be able to confirm if the m20 and m50 have different crank offsets? You'll also be getting a code for a lack of cam sensor, as the m20 doesn't have one of those either...

They have the same trigger offset. I'd say the main issue would be satisfying the cam signal somehow with the m20. Doable but will be challenging

[gath]

Cam sensor is installed on a custom cover plate and seems to be working well. I think the the main issue is the timing/ignition maps.. am running the 403 maps which needs adjusting but that is the challeng.

[5mall5nail5]

An M50 motronic on an M20 would be about 12-15 degrees off (retarded) on average - it'll make very very poor power. Wouldn't recommend it unless you have some way to tune the dme.

[digger]

most of the reason why 4V makes more power than 2V is in valve curtain area and absolute flow potential rather than combustion efficiency. this allows use of shorter duration and potentially wider lobe centre cams which tend to broaden the power curve. plus modern 4V engines have other bags of tracks like vanos and so forth that also allow use of smaller cams

when you look at custom design race heads of 2V nature they give nothing up to the 4V in peak hp and peak torque which is evident in the BMEP they are achieving, though wont have the broadness in the power curve. this is not much use to any of us when you start off with a OEM casting decades old where power and torque weren't the primary focus but nevertheless it should be noted.

saying that 4V is more efficient than 2V is not absolute IMO, it certainly applies to OEM castings but is not an absolute

in addition to this most aftermarket m20 pistons lose the good features of the stock pistons killing the squish and hardly anyone does a good job with the cylinder head. there are a couple of NA m20 in the range 290-350rwhp from 2.9 and 2.8L respectively you dont see many US spec S50 or M50/52 near there.....

[bawareca]

in addition to this most aftermarket m20 pistons lose the good features of the stock pistons killing the squish and hardly anyone does a good job with the cylinder head. .

It is amazing how no one aftermarket M20 pistons "engineer" cares about the squish,but no one cares about M20 anyway and everything is good in the world.

[Quicksilver328i]

It is amazing how no one aftermarket M20 pistons "engineer" cares about the squish,but no one cares about M20 anyway and everything is good in the world.

It is amazing how few aftermarket pistons are designed with combustion chamber shape in mind at all.

There are a lot of lessons that can be taught by looking at the shapes of OE parts that aren't.

One of my favorite "puzzles" to throw people is how "messed up" looking an LSx combustion chamber is compared with most modern DOHC combustion chambers, yet they still seem to work incredibly well in a variety of environments still...

[digger]

It is amazing how no one aftermarket M20 pistons "engineer" cares about the squish,but no one cares about M20 anyway and everything is good in the world.

you can get them by JE and even ross do them now, all of them would do it if you sent them a mould of the chamber and a sample piston but most customers seem to like any piston as long as it is shiny, forged and CNC'd because it must be good.
the ones that use it seem to be better overall for it and have less tune issues......

[DarkSideofWill]

It is amazing how few aftermarket pistons are designed with combustion chamber shape in mind at all.

There are a lot of lessons that can be taught by looking at the shapes of OE parts that aren't.

One of my favorite "puzzles" to throw people is how "messed up" looking an LSx combustion chamber is compared with most modern DOHC combustion chambers, yet they still seem to work incredibly well in a variety of environments still...

If you know what you're looking at, the LS chamber doesn't look messed up at all... And has WAY more quench pad area than any 4V chamber.

[Quicksilver328i]

If you know what you're looking at, the LS chamber doesn't look messed up at all... And has WAY more quench pad area than any 4V chamber.

Exactly, which is why its fun to show it to people who don't know what they're looking at