Savington

iTrader: (31)

Margin of error is 0.3% on a Dynapack. My results were 10x that.

I did.

I don't mean to sound like a complete dick here, but I want to hear you explain this in detail, because from where I'm sitting it sounds like you don't understand what MBT is. Read post 66 before replying.

Yeah, I changed the correction factor on the dyno between pulls.

The air temp was basically the same. I had been getting small 1-2whp changes as I approached MBT at 12.0:1, then I made this change and saw 5whp.

Reasonable, but doubtful since it was a recently calibrated Innovate LC-1 that was producing the readings.

y8s

iTrader: (8)

I dont think I saw a clear answer to this question:

You found MBT at 12.0:1 and subtracted 2 degrees.

Did you find a new MBT at 11.6:1 and subtract 2 degrees when you measured at that AFR?

Savington

iTrader: (31)

No, I don't think I bothered, but isn't that irrelevant? There's no way I could have adjusted the timing at 12.0:1 to make any more power than I had already made. All I could have done was make MORE than 5whp difference if I were to start adjusting the timing at 11.6:1.

Faeflora

iTrader: (2)

Why do we care about 5 measly HP on a 350hp car? I gain more power than that by taking a **** before going for a drive

y8s

iTrader: (8)

I know dyno time isn't free, but it seems like if you're going to bother finding MBT anywhere, it's at the conditions you're going to run.

And in the back of my head I keep thinking that in theory, your MBT-2 degrees or whatever at 12.0:1 was actually closer MBT at 11.6:1. (as you mentioned earlier, as AFR goes down, MBT is less advanced to keep peak pressures happening around the same time)

In other words, if you advanced 2 degrees at 12:1 would you gain back 5hp OR if you retarded 2 degrees at 11.6:1 would you lose 5hp?


And so I too can use the "not to sound like a dick" disclaimer, some article I ran across on some hotrodder site (or maybe in Heywood, I forget) said that you can't really reliably find MBT on a chassis dyno because of the wide range of variability. Which is why the term MBT has coalesced (and is used in Heywood) to mean "Maximum Brake Torque" timing. That is: Torque produced on a brake dyno (same like brakehorsepower) measured at the flywheel.

found the link: http://www.daytona-sensors.com/tech_tuning.html

the key is the shape of the torque vs. timing curve. as you approach MBT, the torque change per timing change decreases to almost nothin.

JasonC SBB

I notoiced that at high boost, the % torque change with timing changes is much greater, than at low boost. For example, at 200 kPa, 1* retard from MBT makes a noticeable change. At say, 130 kPa, it takes more like 2* to make the same % change in torque. At part throttle like 70 kPa, you can change it 3-4* and not see much change in torque.

1slowna

iTrader: (1)

Y8S"Torque produced on a brake dyno (same like brakehorsepower) measured at the flywheel."
I had always assumed brake hp was measured at the hubs and base hp was measured at the flywheel.

Savington

iTrader: (31)

I said that to prove a point - it's not true. MBT moves earlier in the cycle (more advanced) as you add fuel, because richer AFR mixtures have a slower flame front speed. Slower flame speed = you have to light them earlier to get peak pressure at the same time.

Even if it WERE true, it wouldn't matter for the sake of this argument - if more fuel accelerates the flame speed (again, it does NOT), I would then retard the timing at 11.6:1 to get closer to MBT, which would FURTHER INCREASE MY GAINS.

What part of "I found MBT at 12.0:1" is not making it through the fray here? If you find MBT at an AFR ratio, you cannot magically dick with the timing to find an extra 5whp. It doesn't work like that.

I would expect to lose power with reduced timing at richer AFR levels, yes. That's pretty obvious IMO - unless you are being willfully idiotic with the timing map by advancing past MBT, less timing = less power.

Is the confusion that I was losing 5whp by being 2* back from MBT at 12.0:1? I was leaving like 1, maybe 2whp on the table by backing the timing off that far. Way, way less than what I found by adding the extra fuel.

I know this. You add timing until it results in no torque change, then you back it off to the point at which you last saw a change. This is the minimum timing advance for best torque, or what we universally refer to as MBT. Then you back it off 2 more degrees, because presumably that last 2 degrees was worth virtually no power, so the risks of the added timing advance outweigh the rewards.

For example:
14deg - 240whp
16deg - 248whp
17deg - 251whp
18deg - 252whp
19deg - 252whp

At 19 degrees, you know that MBT occurred at 18 degrees. You would then go back to 18, then subtract 2 additional degrees for safety.

Savington

iTrader: (31)

This test was performed on a ~155whp car at 6psi.

1slowna

iTrader: (1)

I agree that 11.6 AFR will defiantly make more power. I have seen higher HP cars pick up much more then 5whp from adding some fuel from 12.0.

JasonC SBB

Sav, was said 155 whp car running ginormous injectors, or normal wee injectors?

JasonC SBB

As I understand it the reason different AFR's have different max torque at their MBTs is the burn rate. The fastest burn yields the most torque. And the fastest burn will produce the lowest MBT value.

I've seen the theory that fastest burn is at ~13:1, and 12~12.5:1 only gives up a little while providing a lot more cooling (and thus more detonation resistance).

In Sav's case in that 155 hp car, his test implies that the burn rate at an *indicated* 11.6:1 was faster than at 12:1. The possibilities include:

- as Fae mentioned, perhaps the wideband was a bit off
- that particular engine really likes 11.6:1, maybe because it has ginormous injectors so the squirt is very short and intense, and doesn't mix well with the air
- one injector is very lean and pulling the rest back in terms of torque output at an indicated 12:1, and at an indicated 11.6:1, the best compromise is reached

y8s

iTrader: (8)

Andrew, you know this is all just to make your head explode.

my bad. i forgot that burn speed peaks at 1.1-1.2 lambda (depending on fuel) and slows down as you go richer or leaner from there.

But MBT does indeed change with AFR. My point was to speculate that if there were some error in your measurements to find MBT at 12:1 and then you retard the 2 degrees for safety, it's plausible that you were ending up with timing better suited for 11.6:1 AFR.

Just saying it's PLAUSIBLE. For example if your dyno runs showed gains due to measurement error beyond MBT timing.

And yes, I'm familiar with Occam's Razor and the more I search the more I see people saying 12:1 TO 11.5:1 is best power for boost. But shutting down the discussion based on internet lore is no fun.

You said you removed 2 degrees of timing after finding MBT timing at 12:1. Would you consider adding back in those 2 degrees "dicking with"?

I understand the process to find MBT and agree with what you describe.

What I don't agree with is that you can do it on a chassis dyno and expect measurement accuracy within 2%. For the example above, MBT could be anywhere between 16 and 20 degrees. If your example continued on to show power drop back down to 245 hp at 20, 21, 22, etc then you could potentially solve the curve mathematically even without knowing the true power the timing values at or around MBT timing.

Imagine a hump-shaped curve with a missing top flat area. I made a graph. You measure til you get a reliable drop in power and then the midpoint-ish between equal power points is roughly MBT.

Savington

iTrader: (31)

I'm going to kill you all.

Plausible, fine. Believable? No way. If the suggestion is that I didn't really find MBT at 12.0:1 because the dyno was giving me false readings, and causing me to add timing advance beyond MBT, then this entire debate is based on pretty much nothing.

No, and here's why:
1. We both agree that as AFRs decrease, timing advance for MBT increases
2. Thus, I was further away from MBT at 11.6:1
3. Thus, if I had set the car exactly at my measured MBT point @12.0:1, then added fuel, in theory I would have been able to FURTHER advance the timing and find additional power at 11.6:1, resulting in a larger gain.

Like I said, I lost ~1whp. If I had lost 5whp, then fine, I could have made the same power with less fuel and more advance. But, that's not what Ben said - he said that I could have made more power with less fuel and less advance, which makes absolutely no sense at all.

How? Dyno margin of error? Like I said, if we're going to argue that the dyno readings are wrong, then I'm not going to play the game anymore.

Why would you do that, though? I don't care about getting the timing dead nuts center in the range of maximum brake torque - I care about getting it to the absolute far left edge, where I can make ~99.5% of maximum brake torque with as little timing as possible. If I can leave 3 or 5 degrees of timing on the table and only lose 1whp from peak, then that's what I will do.

In my real-world example, I can't see a way to make the same power with timing alterations. The only way that would be true is if the dyno was giving false readings, in which case the entire debate is totally pointless.

y8s

iTrader: (8)

I think you've answered all my lingering questions and curiosities.

I guess the ultimate resolution is that, sure, a brake dyno might be more accurate to know exactly what 100% MBT is but you were looking for 99.5% and a few degrees margin of safety and achieved that at 12:1 AFR and then added fuel and made more power.

I am still (academically) curious what minor changes in timing would determine MBT timing at 11.6:1 AFR but it's clear you're happy with 5hp more than your optimized power level at a slightly leaner mixture.

Also I wouldn't blame you for not wanting to remove a motor from a car just to find a couple horsepower... maybe.

Faeflora

iTrader: (2)

Dude injector being screwy is a great sexplanation jason. i like it.

Ben

iTrader: (33)

No, I said you may have made the same power with a little less fuel and a little less advance. I stepped away from the thread because it kind of looked like you wanted to measure peepees instead of have a conversation.

My previous conclusion is spot on though, and really is what should be the lesson from the thread more-so than anything else (and it's essentially the same argument that you're making): If you're just tuning for "best" AFR on the dyno, you're doing it wrong.

A little less fuel means a faster burn, so you don't have to start it as early.
Jason also brings up a valid point concerning unequal fuel distribution between cylinders. We see that fairly often, and it's why the highest end applications tune per cylinder trims.

Savington

iTrader: (31)

So you're saying that if I had run a little less advance at 12.0:1, I could have made the same power as I made at 11.6:1?

y8s

iTrader: (8)

JasonC SBB

True, IF, the new AFR has faster burn.

Again the theory is that there is an AFR that corresponds to the fastest burn, and MBT at that AFR is lowest, and make the most power.

The reason is that later spark (at MBT) means less work goes into compressing the charge up to TDC as the burn begins to raise cylinder pressure.

Fast burn combustion chamber design is THE main reason more modern engines make more BMEP (see my other thread ) then older (e.g. 1960s) designs. The old-school "hemi" were bad (with domes pistons and thus an orange-peel shaped combustion chamber), later design pentroofs with narrower angles between the valves and flat-top pistons for the same compression ratio, were much better. Swirl also improves burn rate.

Again for the particular motor Sav was tuning, fastest burn may have been at said indicated 11.6:1, due to perhaps an off-cal AFR gauge, lopsided injector imbalance, or huge injectors injecting short squirts at a non-optimal point in the hcycle.