MILD THUNDER

iTrader: (3)

Seems like theres alot of questions lately regarding this topic. Things like how much timing to run, what AFR to look for, whats a good EGT, etc. These are my thoughts, which may or may not be correct. And some of it may go against what I previously though. A long time ago, when I worked in a shop, the old timer mechanic who was my mentor back then, told me something that rang true. He said "Joe, you cannot fix a problem, if you don't understand how it works. Do you want to be a parts changer, or a technician in this field?" He was certainly right, and I enjoy learning the meat and potatoes of this stuff.'

Anyhow, I'll first give my thoughts on EGTs. Exhaust gas temperatures, are a byproduct of the combustion effect. Of course ignition timing, plays a big role, in what we see when monitoring EGT temperatures. As we know, advancing the timing, typically results in lower egt's, and retarding the timing, typically increases it. One may then think, to keep adding timing because the EGT probe is showing a cooler number. BUT, its not quite that cut and dry. While the temperature moving past the exhaust valve, is lower, you did not necessarily cool down your cylinder. All you did by advancing the timing, was move the combustion process away from the exhaust valve. The peak temperature inside the cylinder, likely did not change, may have even increased, but it just took place at a different time. So what happened, is the spark plug may have took some of that heat, the intake valve, piston top, cylinder wall, etc. Peak combustion temperatures are much hotter than what the exhaust gas prob is seeing. Just because the probe is seeing 1300*, the temps during combustion, are much hotter. While EGT is a nice gauge, in my opinion, it is not a tuning gauge for a marine engine. You want your spark timing, to be the most optimal for efficiency and power, not to cool or heat your exhaust ports. '

This is why, we run Inconel, super alloy valves, and quality valve seats. Because their simply is alot of heat to deal with. Rather than crutch the temperature of the exhaust valve by adjusting spark advance, you simply put in a component that can handle the thermal stresses under peak operating conditons.

MILD THUNDER

iTrader: (3)

Moving on with air fuel ratios and spark advance.

There is one job of the spark plug. Thats to ignite the intake charge. The job of the ignition system, is to ignite the intake charge, at the right time. When is that time? Well, theres tons of variables. Engine speed, intake charge density, spark plug location, piston shape, fuel type, and of course, fuel mixture or AFR .

Its common in the car world, to combat spark knock, with a richer air fuel ratio. A guy throws a few degrees of timing at the engine, and picks up some knock. Richens the AFR, and knock goes away. What he did by adding a richer fuel mixture, was he slowed down the burn. Why it gained power, was because by slowing down the burn, it moved peak pressure angle during combustion, closer to the optimal point relative to the crankshaft angle. However, the same thing could have been accomplished, with a leaner fuel mixture, and less spark advance. Often leading to less fuel consumed and less wasted energy. Todays engines are all about fast burn. Piston designs, chamber designs, spark plug location , multiple spark plugs, and so on. All in an effort to get a quick, and efficient burn of the intake charge.

So, when a guy asks, "what air fuel ratio should I shoot for", or "how many degrees of timing should I run?", its really a loaded question. '
'
A supercharged engine, does not need tons of timing advance down low. The compression ratio of the engine basically goes up as soon as the throttle blades are opened, and the intake charge is extremely volatile and fast burning. It doesnt need that spark lit off very early. And if you light it off too early, not only are you wasting power because the piston is still coming up the bore while you ignite the charge, you're also creating an enviroment for knock due to the increased pressure and temperature (that isnt doing you any good for power).

Now, on a big cammed naturally aspirated engine, the low speed intake charge entering the cylinder, is not very dense, due to several possible things. Cam timing, port speed, and so on. Therefore, the engine responds very well to advanced timing, as it needs to get that slow burning charge going early, so combustion happens when the piston is at the ideal relationship to the crankshaft, or peak pressure angle.

Gasoline itself, typically burns the fastest at 12.5:1 Air fuel ratio. This is why generally, most engines make the most power there. That being said, I have seen a few supercharged engines make their best power , with a air fuel ratio in the 11's on the dyno. Back when this engine was on the dyno, we kept adjusting fuel , till it started losing power. We left timing at 34* if I recall. Looking back on this, it would have been interesting to see, where the engine made its best power, if the fuel ratio had been kept closer to 12-12.5:1 , and timing adjusted up/down vs fuel.

We look back into the old days, and we hear things , and often mentally note them to be gospel, and the only way. Things like, "never run more then 30* of timing with a blower!" "Air fuel ratio has to be in the low 11's or it will eat pistons"

While most of this is a collection of crap I've read about, I can say this for certain. The last few top speed runs I did with my boat, the air fuel ratio on my NGK powerdex, was fluttering at 12.0-12.2 at 6000rpm. The little angel on my shoulder kept yelling in my left ear, "its too lean back out of it", while the little devil on my right shoulder said "quit being a puzzy, if you burn it you burn it". I held it at 6000 for those runs for a good amount of time. No 10 minute chit, but a solid 60-120 seconds making passes. Pulled plugs, they looked fantastic. Did a compression check, all good there. Maybe the key isn't flooding it with fuel and timing advance after all.

I been following some of the Hellcat / Mopar tuning stuff lately. The 700HP Supercharged Hellcat in stock form, run in the 12's at WOT for air fuel ratio. Engine making almost 2hp per ci and almost 12lbs boost with 9.5:1 compression. They definitely arent throwing fuel at it to keep it alive. But they also arent throwing much timing at it either.

Any thoughts on this topic?
'

Unlimited jd

iTrader: (5)

The first thing you stated about you can't fix it without knowing how it works was taught to me by my grandfather at a very young age. His first step to fixing anything was "how does this normally operate?" Then he'd go on to figure out why it isn't.

When i I started drag racing and hanging around with guys at the track I started working with a team that was "melting" down a motor every couple races. 509" motor, 12.5:1, 36 jet in the fogger, pulling timing back to 26* and air fuel in the low 11's. Safe right?? WRONG
Fuel was puddling on the rings and blowing the ring lands off not melting the pistons. The safe spot on tune up ended up being 12.9-13.0:1 air fuel and 29-30* timing depending on the weather.

Crude Intentions

iTrader: (1)

Great write up. To relate it similarly to what I do no one thing is end all be all. In my case we have aids to navigation this includes nav buoys, plotters, radars, gps, paper charts, vhfs among other things. Not one of these components should be trusted alone. But the cumalitve is how to safely operate the vessel.

Very similar to what your saying in that not one of those things is the end all be all to tuning. You should use all to tune for optimum performance and monitor as much as you can. I read a lot where people say "fat is safe" "add fuel it'll live" but just as unlimited JD pointed out that isn't always the case either. That's why I think dyno tuning is a huge benefit regardless of the actual number as we all know every dyno will give a different number but it helps to use all those factors for the best performance.

Lee

iTrader: (4)

This is an excellent book to learn exactly how an IC engine works.

MILD THUNDER

iTrader: (3)

I have a crazy amount of books. Books on carburetors, cams, ignition systems, superchargers, cylinder head porting, etc. My girl has caught on, and when she goes to the used book store, she comes home with books she thinks id enjoy. Just wish I had more time

getrdunn

iTrader: (7)

Some good info joe. I'm sure I've mentioned before on OSO how EGT's were in a sense giving me false positive readings on the dyno 454/420 MB'. Temps showed extremely high so I kept throwing more jet at it. Plugs indicated very rich. In a nutshell I was blowing the flame to far out the port. One of my first dyno sessions and the AFR was broke day one. I kept leaning it down and watched the temps come down. Amazing how fat it was initially set up.

Even with all the protection devices we have available I still go back to good ol years of checking plugs. That never fails.

getrdunn

iTrader: (7)

that's a good woman!

ICDEDPPL

iTrader: (1)

What about the blown Cadillac engine management throwing AFR`s in the 10`s ( or 11s) at WOT? Thought they did that to cool the pistons.
I only have my personal boat to compare but if I am to believe my knock sensors the engines do like fuel. or maybe they would like less timing but then again that seems to kill power.

Can you read detonation off spark plugs?

MILD THUNDER

iTrader: (3)

I don't know the reasoning why they did that with the North star engine. Or should I say , why they needed to do that. Heck , your LS3, my Hemi, they don't run much spark advance at all compared to a big block. That's a good thing.


absolutely you can see detonation on a spark plug. If it'. Bad enough you'll see physical damage , like closed gap, cracked porcelin, melted aluminum from pistons , etc. Remember my plug on the dyno when he forgot to turn the fuel pump on and air fuels went dead lean at end of the pull?

Of course we know not everything that applies to cars applies to boat engines. My biggest fear with our types of engines, running in that 12ish afr range, is our fuel distribution sucks azz. Port efi and symetrical port heads, prob a whole lot better than ours .There's a good chance we have a cylinder or two in the danger zone . Without an 02 in each cylinder, it's a guess. We need a dyno at the house Dan ��