Lets Talk About A/f Ratios

[Super Bleeder!!]

well we obviously run our cars richer than the n/a guys for safetys sake. and i believe stoich is 14.7:1, meaning the greatest amount of power can be made at that ratio, but the heat is just rediculous so shit melts and welds to cylinder walls.

so are the the (piston, cyl wall) materials heat resistance the sole reason for not leaning out to rediculous amounts? do diesels run the same a/f ratios as we?

[1ViciousGSX]

The max cylinder temp for any engine is about the same, if not exactly the same, but the ratio changes due to the extra heat produced by a supercharged engine. Leaner is faster. We run lower ratios (more fuel) to cool the internals of the cylinder and piston which reduces the chance of detonation and meltdown. A normally aspirated engine can run leaner to achieve the same 900 degC we see on a turbo engine.

All engine internal materials are heat resistant until you reach the point of meltdown. :P

[Super Bleeder!!]

but there should be some materials that have a higher meltdown point then. especially if you get all coaty-like

[Shane@DBPerformance]

14.7 is about where you want to be at idle and cruising on NA or boosted cars. It's good for fuel economy and emissions and bad for power. On an NA car best power at WOt is going to be around 13.2:1. On a lightly boosted car you might run a 12.0-12.5:1 A/F on pump gas. On a turbo car you usually want to be in the 11.0-12.0:1 range on pump gas. On race gas you can go a bit leaner for more power, but you want to monitor the knock and make sure you have some good gas. Most DSMs that I tune on pump gas are happiest in the 11.0-11.5 range. On race gas I will bring it up closer to 12.0:1. On Mara's I run it as lean as 12.5:1 on race gas at high boost and like 11.0 on pump, her car hated pump gas with her somewhat crappy sidemount and boost creeping Mutt turbo. Nash's was usually tuned aggressive on race gas also, but I wouldn't push the average persons car that far.

[MustGoFaster]

On the subject of diesels, I know they don't run the same EGT's. I was reading a forum for 6.5l turbo (chevy) and one guy mentioned keeping EGT's under 1K *F. As far as A/F ratios, I think it would be really hard to regulate A/F's with out a throttle plate. In fact the dam things run off detonation. Low octane fuel, and from my understanding 14:1 CR is on the low end for them.

[1ViciousGSX]

Originally posted by MustGoFaster@Feb 11 2004, 09:51 PM
On the subject of diesels, I know they don't run the same EGT's. I was reading a forum for 6.5l turbo (chevy) and one guy mentioned keeping EGT's under 1K *F. As far as A/F ratios, I think it would be really hard to regulate A/F's with out a throttle plate. In fact the dam things run off detonation. Low octane fuel, and from my understanding 14:1 CR is on the low end for them.

Diesels run on the properties of detonation. Air flow into the engine stays the same, no throttle body. As more fuel is added power goes up, the richer the mixture, the more power it makes.

[1ViciousGSX]

Originally posted by gixxer@Feb 11 2004, 06:38 PM
but there should be some materials that have a higher meltdown point then.  especially if you get all coaty-like

There are. Forged pistons are stronger and have a higher melting point than a cast piston. Coated pistons help. Stainless steel valves are less prone to burning, etc. Anything Titanium has a higher melting point.

[ACRucrazy]

My recent "learning" is Inconel...... As in valves...mmmmm

[JET]

Haha, try machining inconel though! That stuff is a bitch!

[LightningGSX]

My thoughts on this....
Raising the melting temperature of engine components is pointless, from a power standpoint that is, not from a safety standpoint.There is no reason to exceed the melting temp of the stock components, more heat doesn't equal more power.The adiabatic efficiency of most engines is poor(and I would assume past a certain point it decreases even further as cylinder temp rises), so past a certain point, most(if not all) energy produced as heat is wasted.

Stoichiometric(14.6-14.7:1 for gasoline) is where an ideal chemical reaction takes place(enough air to completely burn all fuel), theoretically this should be where you get the greatest power and emissions, because all available energy from the mixture is released.But in reality its more like 12-13:1, which is where I would assume the adiabatic efficiency peaks.I would also assume the reason turbo cars like 11:1 or so, is because heat increases with pressure(via the ideal gas law), which in turn makes the adiabatic efficiency peak at a lower point.

[Super Bleeder!!]

i guess i got own3d by physics, good reply!