Does anyone have the engineering specs on the direct injection (DI) on the new C8 LT2 engine?

I have had DI on a number of high mileage cars, and I experienced no economy or power issues with build-up of carbon deposits on the valves and intake ports (granted these were daily drivers, not performance sports cars).

DI problems have arose in others foreign brands in the past, that have required routine major maintenance every 40k, and/or a supplemental oil catch can. My 2018 Honda Accord has DI, but I've read there is a bit of a second squirt of fuel onto the valve stem area from a supplemental port injector to keep the valve area from coking up.

Any engineering thoughts or experience with DI in a high horsepower Chevrolet engine, or in the LT2, specifically?

I'll be confident that Chevrolet has considered the DI dilemma, and wondering how they have solved it....

Cheers!

 

They all have gotten better but you can do clean the intake and valve steams.

You can also run meth which keeps everything super clean

The LT1 is also DI and you don't hear much about it. The early BMW DI motors were notorious for it.

 

its and inherent design problem with DI cars. without fuel in the intake tract, any "gunk" from blow by and oil suspended can crud up the valves and make a mess

gm has done a good job with the ls and lt motors to provide decent oil separation and keep contaminants out of the intake but it's not a perfect solution

many manufacturers like gm on the lt5 have gone to DI plus port injectors which helps tremendously by cleaning off the valves

run, good oil, good gas, and probe the intake with a flex camera to keep an eye on valve deposits. You can clean the valves before they become a problem or pull the heads later

vette has been di for a long time so poll the c7 guys to see how they fare on valve deposits

 

I'm not aware of any reported problems of carbon buildup on the C7 forums. Several people have over 100K without problems. But there a lot of heated debates on the merits of catch cans....

 

I installed a catch can (Mishimoto model) on my 2017 SS Camaro's LT1. It's somewhat surprising the amount of oil it does catch between oil changes.So far in 25K I have probably caught 1/2 QT that didn't go through the engine. Can't see how a catch can hurts.

 

I put 57K miles on my 2014 C7 with no catch can, no issues, tracked regularly at VIR. Sure some oil vapor gets in there, but GM has it figured out. The LT2 has an even more advanced PCV/oiling system. I personally consider this a non-issue and will never run a catch can again. Yes, I ran them religiously on my C5s and my C5 road race car would get about 1/2 can a weekend of racing,

Plus, after seeing the C8 on Monday... doing anything to the motor from the top will be tough. I'm 6'4" and still could barely reach anything. Trying to unscrew or drain a catch can would be very tedious and annoying.

 

Had the same concern when I got my early 2014 C7 Z51 (built September 2013.) It was GMs 1st high performance engine with DI (where the intake valves get hotter than a grocery getter so more chance of baked on oil on the backs of the intake valves.) There is oil mist and some fine oil particles that are created when oil leaving the crack journals hits the spinning crankshaft, That comes into the intake from the crackcase through the PCV system. No gasoline to wash it away with DI. BMW and Ferrari were having issues with what is called "coking." Heck there were gasoline ads saying you needed "Top Tier" cleaning additives in the gas even with port injection where it did wash the valve backs! Pasted an old gasoline ad pic below from when their was gasoline washing the backs of the hot intake valves and without extra "cleaning additives" some coking occurred!

So I installed a quality "Catch Can" that collected the oil mist that came from the crackcase, condensed it into drops that had to be removed periodically. Like others I collected a few ounces/1000 miles. That was a few ounces that had no chance of baking on the backs of the intake valves. Removed it when I sold the 2014 and installed on my 2017 Grand Sport. GM said they "improved the PCV system used on the Grand Sport" and had some of that oil mist return through the valve covers instead of going into the intake. BUT there was still the same hose going from the crackcase to the intake and I still collected some oil. For all those who said it was a none issue, GM sure spent engineering time and effort to improve the PCV system for a NON PROBLEM!! However I collected only 1/2 the amount! Then saw a comment that "some oil" is a help in lubricating the valve stem SO REMOVED THE CAN!

GM is learning and probably has reduced the amount of oil vapor & mist going to the LT2 intake. Since there were no significant issues reported with the LT1, doubt there will be any with the LT2.

PS: Managing "Blowby" past the piston rings and oil vapor/mist in the crackcase has been an issue since the ICE was invented. Up until the 1960's there was a "Road Draft Tube" that just put the "stuff" into the atmosphere! Had a "roadcraft tube" on my '56 Chevy small block. The EPA rightfully said you can no longer dump that "stuff" into the atmosphere- burn it in the combustion chamber with the gasoline. The addition of a Positive Crackcase Ventilation (PCV) system started. Up until DI, gasoline passing over the intake passages and backs of intake valves (with extra cleaning additives, i,e, Top Tier etc gas) washed most of it way.

Old Ad When Port Injection Had Gas Washing The Intake Valve Backs! Still Needed Extra Cleaning Additives. Top Tier gas never gets to wash the backs of the valves in a DI engine! It's injected directly into the combustion chambers.

 

In my new 2020 Corvette C8 owners manual it states that it's important to use Top Tier Detergent gas and to add ACDelco Fuel System Treatment Plus to the fuel tank at every oil change or 9,000 miles - this "will keep your engine fuel deposit free"....
So now all your wondering about how Chevrolet solved the DI dilemma can finally be put to rest ;>)

 

You realize DI sprays directly into the combustion chamber right? There is no fuel to clean the back of the intake valves. That said, valve deposits have proven to not be an issue on the C7 LT1s even with well over 100K miles.

 

Statement has nothing to do with coking on intake manifold side of the intake valve on the outside of the combustion chamber! The only thing that hot intake valve sees is air and crackcase "stuff" from the PCV system. It's that crackcase "stuff" that causes "coking." Yep, Top Tier in a port injection or even the 850 Holley carb on my street rod helps significantly reduce coking. Here is a pic of valves with "port injection" (C6 and prior Vettes) with various amounts of Top Tier type "extra" cleaning agents as gasoline buy itself is not enough!

PS: Now before anyone gets apoplectic, GM spent a lot of engineering time and effort to improve the dry sump in my 2017 Grand Sport from my 2014 Z51. They didn't do it for fun! I collected 1/2 the amount in my catch can of crackcase "stuff" in the Grand Sport and IMO for normal street driving a catch can was not needed. Expect they improved the LT2 even further. Tadge said in a forum post they find that the coking that occurs is cosmetic. Perhaps - I'm sure everyone believes everything Tadge says! Particularly when it fits their agenda!

I add a container of Techron, made by Chevron, about every 3 months. It was one of the first Top Tier additives. I do that partly to make up for when I don't use Top Tier gas and mostly to help protect the fuel sender from sulfur compound build-up on fuel sender parts, quoting:
Technical service bulletins and the National Highway Traffic Safety Administration state--sulfur in the fuel can cause inaccurate fuel gauge readings. What is happening? The silver in the sending units is tarnishing just like your good knives, forks and spoons. That is why silverware comes on those lovely boxes lined with sulfur-free cloth. Silver reacts readily with sulfur compounds to create that tarnish--silver sulfides. Though it is fairly easy to clean off (remember cleaning your mother's silverware?), it is difficult to keep it from returning. Fuel sending units suffer the same fate.

General Motors has an additive to help keep the tarnish at bay. Chevron Techron Concentrate Plus is the product GM dealers use (under their brand name.)



Good accurate ad with C6 and prior Vettes with Port Injection- useless for Direct Injection where gas never sees the backs on the intake valves!

 

That's correct and the irony of the Owner's Manual statement is not lost on me. That's why going to the track a couple times a year and redlining the engine is both very helpful & a lot of fun too.
Quick question Rapid, how did you determine that valve coking on the C7 LT was a non-issue?

 

Valve coking in DI engines hhas been a non issue for GM. They have been delivering cars with DI engines since 2007. They have millions on the road and there haven't been any complaints about valve coking. A lot of complaints about V6 DI engine timing chain issues, nary a word about valve coking. None of the domestic automakers seem to have a problem.

Bill

 

Because people have 100K plus miles on them and there are zero posts about it on any forums or any social media. I put 57K miles on my 2014 and never had an issue.

 

Only buy gas from brands that meet Top Tier specs (for detergents) and you'll be fine:

https://toptiergas.com/licensed-brands/

https://www.consumerreports.org/car-...h-extra-price/

And no, Top Tier specs are not a gimmick.

 

Maybe the GM valves just run cooler so the stuff doesnt flash bake

 

Hmm, year old post BUT doubt the large intake valves in the LT1/LT2 run cooler than the 4 valve/cylinder BMW engines that had significant coking and where some cars required walnut shell blasting to remove it at ~50,000 miles.

SIDEBAR
Intake valves are exposed to the >2500 F combustion temps and a major amount of the required cooling (stainless steel not strong at those temps and getting close to melting) occurs at the valve seat. The large valves have a longer heat conduction path to the valve seat than two smaller valves that coked in BMWs etc. . However valve temps are also function of power used. The large 6.2 Liter Vette engines (in normal use) are operating at a lower rpm and with less stress of the typical European performance engine where coking was an issue, BMW, Ferrari. In fact it was mostly an issue in "high performance engines" not grocery getters. So GM's first use in a high performance engine was 2014 in the C7 as the C6 did not use DI. So GM no doubt learned from competitive prior high performance engines

Also, although it was said "there was no issue with the 2014 LT1," GM spent a lot of engineering time and effort to improve the dry sump PVC system in my in my 2017 Grand Sport from my 2014 Z51. They didn't do it for fun! I collected 1/2 the amount in my catch can of crackcase "stuff" in the Grand Sport. IMO for normal street driving a catch can was not needed in the later design dry sump. Expect they improved the LT2 even further and for street driving a catch can is not needed in the LT2. The key, IMO was reducing the oil and oil mist being pulled into the intake manifold. They said they accomplished that in the later dry sump design that was much different and more complex that the one in my 2014 Z51.

Tadge said in a forum post they find that the coking that occurs is cosmetic. He didn't say it didn't exist, just that the buildup was not a "performance issue." The larger intake passages in the single valve engine may be another factor allowing more build-up without a significant performance degradation. GM knows but has not discussed.

Before I added the catch can removed from my 2014 to my 2017 Grand Sport I investigated the changes in PCV system best I could to be sure I did no harm. Installed but once I saw I was collecting ~1/2 the oil I removed it as it wasn't needed and a comment in another technical article said "some PCV oil is useful to lubricate the intake valve stem." No doubt they improved the LT2 further.

Defined the entirely different routing of PCV lines etc in my 2017 Grand Sport from my 2014 Z51. Among other differences was their was no line bringing filtered fresh air into the crackcase in the 2017 versus 2014. Also lines were coming from both valve covers.


Installed a pressure/vacuum gauge in the one line going from the dry sump tank to the air intake tube after the air filter. It appears to perform the the function of the 2 lines in the 2014. It takes air pulled into the dry sump tank by the oil/air scavenge pump that has to be feed back into the intake AND provides sufficient air so a separate line, required in the 2014 was not needed.

 

Same here, installed the same can at 200 miles, next 200 miles told me it was a good move. DI maintenance is a good move, Think of it as cancer prevention. But I keep keep my corvettes , bought new, for decades. Your plans might be give the next guy the problems.

 

"Sure some oil vapor gets in there, but GM has it figured out.", that is funny in so many ways.

 

Can you show me a 200,000 mile LT1 that needs a walnut job on the valves? I can't find where anyone has ever needed to walnut shell the engine.

 

Some Corvette owners spend more timing dreaming up problems their car could possibly someday maybe have than they do actually driving it

I sold my 2014 Z51 @ 56k miles and the intake valves looked just fine anytime I swapped intake manifolds throughout the years.
My 2-cents, BMW's design flaw sparked this aftermarket craze for catch-cans for anything DI. Throughout my years of modify and tuning cars, I've seen more problems created by catch-cans than solved by them.