2014 C7 Chevrolet Corvette

[serialk11r]

Originally Posted by ZDan

You *can* run a lot leaner than 11:1, but there's little to no drawback to running richer under WOT conditions to cool down combustion chamber temperatures and stave off detonation. You can run a higher compression ratio with a richer mixture (within reason), which benefits both power and fuel economy under cruise conditions.

Cats run HOTTER with a richer mixture as they burn off excess fuel that didn't get burned in the combustion chamber.

Factory tunes being rich has nothing to do with preserving the cats.

Cats are catalysts, they do not change what the reaction is. You need oxygen to burn fuel, at low AFR the oxygen content in the exhaust is very low. Dumping all that fuel into the cylinder cools down the cylinder, but it also cools down the cat in this case. arghx7 at some point said this is the primary purpose of running ultra rich at high load for the OEM, NOT preserving the engine, which is easy to see because at lower rpm the temperatures can run even higher yet they don't try to run it rich like that.

[Dimman]

Hmmm, I highly respect @ZDan 's and @arghx7 's technical opinions, plus @serialk11r is pretty damn smart himself. There are probably some operating conditions misunderstandings or assumptions here.

[LSxJunkie]

Originally Posted by serialk11r

Depends on circumstance. At full load factory tunes run very rich because that prevents the exhaust gas from directly frying the cat, that's why the twins are like 10.8 AFR at the top end. Otherwise, yea you run stoich if the cat temperature is safe.

That also prevents your high compression motor from knocking and/or melting a piston. If they're anything like LSxs, at WOT they run open loop. AFR is commanded and you pretty much dump fuel. Dumping fuel blows out cats. I know this because I've seen enough of it on blower cars. Hell, stock LS2 GTOs were tuned so damned rich that there were guys who blew out their stock cats within 20k miles.

[arghx7]

Funny how this forum has the little "mentions" thing which basically begs you to jump into some thread.

Originally Posted by ZDan

They run rich at WOT as a hedge against detonation and to run cooler, and because it doesn't cost much if anything powerwise. The cats certainly aren't troubled by leaner mixtures within reason.

Factory tunes are rich to preserve the ENGINE, not the cats.
Cats will run hotter as you go richer, anyway, as they burn off more unburned HCs. Running RICH is the primary cause of converter meltdown.

So what I'm hearing here is a debate: does a richer mixture under heavy load preserve the cat? This is more complicated than you think, especially when you get overlap involved. There are two things we're talking about here. One is the temperature of the feed gas for the cat, and the other are exothermic reactions inside the cat. It depends on the catalyst spec, but the rule of thumb is, on a ULEV engine (basically, engine not trying to meet special California emissions standards) you want the midbed temperature of the catalyst to not exceed 850C. To measure the midbed temperature, you drill a hole into the catalyst shell and insert a thermocouple inside the actual brick of the cat.

After the cat has warmed up, it will usually be hotter than the rest of the exhaust due to exothermic reactions (heat-producing reactions inside). So there are two main things you have to do to keep the cat from burning up:

You limit the feed gas temperature. The way to limit this is to adjust your compression ratio, spark timing, and AFR. Higher compression ratio=more thermal efficiency, which lowers exhaust temperature. More compression ratio also increases knock tendency, meaning more retarded spark which raises exhaust temperature. Richer mixtures lower catalyst feedgas temperature. But here's where it gets tricky, where @ZDan has a good point. The exothermic reactions will heat up the cat. This happens when you load up the cat with LOTS of oxygen and CO. How does that happen? Two main ways.

Deceleration fuel cut will fill up the catalyst with oxygen, and under certain conditions you can have reactions occur as a result of that loading. The cat has an oxygen storage capacity based on the amount of cesium inside. This is what's modeled when the ECU is trying to figure out whether to throw a P0420 (Catalyst Efficiency) code. Where it really gets confusing, and where I think @ZDan meant to go, is when you have high O2% and CO% (in terms of ppm concentration) in the feed gas under load. When does that occur? Scavenging operations.

When you have a lot of overlap under the right conditions, intake pressure exceeds exhaust pressure. Unused air gets blown out the exhaust valve as you evacuate spent gases. What this does is raise your O2% in the exhaust. This might make you have an engine-out lean mixture, but the ECU can target a richer tailpipe mixture and "mask" the extra oxygen with more fuel. Rule of thumb is, when O2 concentration exceeds 1% you are going to have a lot of oxygen that can cause exothermic reactions.

The final results is, the wideband misleads you. It says some AFR, but really the exhaust is loaded with oxygen (from the scavenging) and depending on the in-cylinder AFR, lots of CO. So you get reactions in the exhaust and exothermic reactions in the cat which degrades.

The only way you'll ever really see this is in an engine dyno lab with thermocouples and pressure sensors, all through the exhaust, a $250k gas analyzer tapped into multiple points on the engine, and a wideband with backpressure compensation.

TL;DR Rich can keep the engine-out exhaust temps down. If there is a lot of Oxygen in the exhaust (which can't always be detected by a wideband) you can get reactions inside the cat which degrade it.

[ZDan]

Originally Posted by serialk11r

Cats are catalysts, they do not change what the reaction is. You need oxygen to burn fuel, at low AFR the oxygen content in the exhaust is very low. Dumping all that fuel into the cylinder cools down the cylinder, but it also cools down the cat in this case.

The excess fuel ultimately burns in the cat, causing it to overheat. This is the #1 cause of catalytic converter damage in improperly "tuned" engines. The factory rich mixture isn't rich enough to do this, though.

arghx7 at some point said this is the primary purpose of running ultra rich at high load for the OEM, NOT preserving the engine, which is easy to see because at lower rpm the temperatures can run even higher yet they don't try to run it rich like that.

Factory tunes are rich at WOT because rich is safer for the engine and allows for lower-restriction intake and exhaust without going too lean. Those mods would only tend to lean out the mixture at higher rpm, as stock intake/exhaust aren't restrictive at lower rpm. So it would make sense for the conservatively rich factory a/f to be richer at higher rpm. If you have a stock FR-S dyno plot with A/F shown I'd be interested to see how much richer it is up top.

The purpose for factory tunes being rich is all about saving the engine from detonation, accounting for possible fitment of lower-restriction intake/exhaust, while allowing as high a compression ratio as possible. It is NOT to preserve the cats.

[Mikem53]

I owned 4 vettes in the last 14 years.. My last one was a 08 with Z51 and NPP option. Loved the car and hated it.. Loved the power.. Pulls like a freight train down low and keeps pulling at high rpms too.. Never stops pushing u back in the seat.. Comfortable and good looking.. Hated the run crap tires. I ditched them early on for some normal performance rubber.. The run flats got better with each vette I owned.. But I still prefer normal tires..
The vette is very rigid.. It gives you that one piece feel you need in a sports car.. But I always found the corners too heavy.. The damn wheels and tires are too big and heavy.. I always had road trammel going on.. And I never liked that in normal driving.. The damn thing is a target too.. Cops, Honda civics.. They all want to either give you a ticket or race you.. And parking it was always a hassle.. Not the best grocery getter... But it had lots of cargo space and one of the best hwy cruisers ever.. I found myself driving slower in the vette than in my beater car.. You can't play with it as much cause its too fast.. Put ur foot down for 2 seconds and your breaking speed limits and other laws..
I went with the FRS because it makes more sense on the street.. I can push it and have more fun with its low grip, low power all day long.. And I like the narrow tires MUCH better for everyday use.. While I miss the power of the LS3 and that V8 rumble.. I like driving the FRS more.. And being able to wind it out all the time.. It also feels more nimble than the vette.. Sometime less is more.. And driving a slow car fast is more fun than driving a fast car slow..

[arghx7]

There is a knock preservation aspect to having a richer in-cylinder mixture. However, engines do exist that run stoich almost all the time. The current Audi 1.8L, found only in Europe, is turbo with port + direct injection and basically runs 14.7:1 under WOT.

[jdm-az-fuk]

i am liking it a lot

[Sport-Tech]

Needed to hear that, thanks - thoughts of dropping $60k on the C7 have been dancing through my brain lately.

Originally Posted by Mikem53

I owned 4 vettes in the last 14 years.. My last one was a 08 with Z51 and NPP option. Loved the car and hated it.. Loved the power.. Pulls like a freight train down low and keeps pulling at high rpms too.. Never stops pushing u back in the seat.. Comfortable and good looking.. Hated the run crap tires. I ditched them early on for some normal performance rubber.. The run flats got better with each vette I owned.. But I still prefer normal tires..
The vette is very rigid.. It gives you that one piece feel you need in a sports car.. But I always found the corners too heavy.. The damn wheels and tires are too big and heavy.. I always had road trammel going on.. And I never liked that in normal driving.. The damn thing is a target too.. Cops, Honda civics.. They all want to either give you a ticket or race you.. And parking it was always a hassle.. Not the best grocery getter... But it had lots of cargo space and one of the best hwy cruisers ever.. I found myself driving slower in the vette than in my beater car.. You can't play with it as much cause its too fast.. Put ur foot down for 2 seconds and your breaking speed limits and other laws..
I went with the FRS because it makes more sense on the street.. I can push it and have more fun with its low grip, low power all day long.. And I like the narrow tires MUCH better for everyday use.. While I miss the power of the LS3 and that V8 rumble.. I like driving the FRS more.. And being able to wind it out all the time.. It also feels more nimble than the vette.. Sometime less is more.. And driving a slow car fast is more fun than driving a fast car slow..

[Mikem53]

Originally Posted by Sport-Tech

Needed to hear that, thanks - thoughts of dropping $60k on the C7 have been dancing through my brain lately.

I'm sure it will be a great car.. With super car performance for a reasonable price.. But with great power.. Comes great responsibility... Lol..
Seriously.. I enjoyed all my vettes... But I'm finding that you don't need all that to have an enjoyable driving experience.. I do miss my 0 to 60 blasts in the vette.. But the FRS has many virtues that can be enjoyed more often..

[LSxJunkie]

Originally Posted by Mikem53

I owned 4 vettes in the last 14 years.. My last one was a 08 with Z51 and NPP option. Loved the car and hated it.. Loved the power.. Pulls like a freight train down low and keeps pulling at high rpms too.. Never stops pushing u back in the seat.. Comfortable and good looking.. Hated the run crap tires. I ditched them early on for some normal performance rubber.. The run flats got better with each vette I owned.. But I still prefer normal tires..
The vette is very rigid.. It gives you that one piece feel you need in a sports car.. But I always found the corners too heavy.. The damn wheels and tires are too big and heavy.. I always had road trammel going on.. And I never liked that in normal driving.. The damn thing is a target too.. Cops, Honda civics.. They all want to either give you a ticket or race you.. And parking it was always a hassle.. Not the best grocery getter... But it had lots of cargo space and one of the best hwy cruisers ever.. I found myself driving slower in the vette than in my beater car.. You can't play with it as much cause its too fast.. Put ur foot down for 2 seconds and your breaking speed limits and other laws..
I went with the FRS because it makes more sense on the street.. I can push it and have more fun with its low grip, low power all day long.. And I like the narrow tires MUCH better for everyday use.. While I miss the power of the LS3 and that V8 rumble.. I like driving the FRS more.. And being able to wind it out all the time.. It also feels more nimble than the vette.. Sometime less is more.. And driving a slow car fast is more fun than driving a fast car slow..

And then you have folks that go the other way. A bunch of my friends have C6s. 2 of them have LS3 cars with Novi 1500s, another has an LS2 car with a cam, a T-Trim, and meth.

These cars aren't daily drivers, but they are DRIVERS. They go out in the rain, in the winter, on 250 mile road trips.

[zigzagz94]

I've been reading and watching every review with awe. This car is amazing but I wish GM would come out with a mini Corvette. Something that's 3/4 the size, weight (2600lbs), power (345), and 2/3 to 3/4 the price (35-40k). It could be a Nissan Z, BRZ STi, future 16X RX-? competitor.

Yes I know the Camaro ocupies that price bracket but it's a heavy muscle car not a true sportscar. Two different demographics that occasionally overlap and cross shop but there's enough room in their line-up for both cars to co-exist.

[n2oinferno]

Just remember this when you're telling your significant other that you would like to purchase a fun, economical car that you can also take to the track on weekends.

[meadowz06]

Originally Posted by LSxJunkie

And then you have folks that go the other way. A bunch of my friends have C6s. 2 of them have LS3 cars with Novi 1500s, another has an LS2 car with a cam, a T-Trim, and meth.

These cars aren't daily drivers, but they are DRIVERS. They go out in the rain, in the winter, on 250 mile road trips.

+1. I drive my LPE supercharged C5 everyday. Rain or shine. 560+hp.