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Is fuel washed cylinders an issue with CIS?

Ever had a moment where you thought about something you never considered before?

Well, today I bring you Deep Thoughts by Tim™.

In today’s episode, I was thinking about the nature of CIS with emphasis on CONTINUOUS.

That made me wonder something. I don’t think I can recall anyone ever mentioning fuel wash as a particular issue with CIS. And that made me wonder if it is an issue or if there is something in the 911 engine design that mitigates the effect of fuel wash.

My understanding of fuel wash is that if too much fuel is being delivered, the fuel washes away the oil lubrication and in severe cases this can lead to premature failure.

Since CIS is constantly spraying fuel even through the power stroke, this seems like it would be different than with batch or sequential EFI where the injectors aren’t firing all the time.

The duty cycle of the FV in CIS equipped with lambda can affect the fuel delivery rate so maybe that is a way to deal with it.

And I also assume that as rpms increase, the differences between CIS vs batch vs sequential are minimal because the injector pulse width in EFI is so long it’s almost continuously open too. (maybe EFI is still more efficient though)

Does the fact that CIS is constantly spraying make a difference in other ways? For instance does it help cool the valves?

Anyway, just pondered that a bit and was generally wondering out loud.
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Last edited by tirwin; 07-11-2019 at 10:10 AM..
Old 07-11-2019, 08:00 AM
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I once experienced this with an Alfa...having started and stopped the car a few times to move it around the driveway without properly warming it up. The car had SPICA mechanical injection, analogous to Bosch mechanical (pump) injection on an early 70's 911E. It was immediately apparent: the engine spun REALLY fast on the starter motor and the crankcase was filled with a mix of engine oil and gasoline.

I've never heard of this with CIS, but I am (happily) not omniscient! John
Old 07-11-2019, 08:08 AM
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Way less of a chance with CIS in good condition. Meaning no leaky injectors or out of spec adjustments/pressures.
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Old 07-11-2019, 08:17 AM
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37 years, 245,000 miles of CIS fuel wash here on my un-opened motor. Get about 20 M.P.G.
Old 07-11-2019, 08:17 AM
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but it sprays at the back of the valve so the fuel is not inside the cylinder.

think about how much fuel is being dumped on a 930 under full boost. its a LOT.
so the 911 has nothing to worry about compared to the 930.

where you have an issue is when the plunger gets stuck and the engine wont crank, or you have leaking injectors that wash the oil away while sitting, then you crank it.
your other worry there is hydro lock. the cylinder can get too much fuel and it acts like a rock in there and not letting the piston come up.
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Old 07-11-2019, 09:27 AM
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Quote:
Originally Posted by jjeffries View Post
I once experienced this with an Alfa...having started and stopped the car a few times to move it around the driveway without properly warming it up. The car had SPICA mechanical injection, analogous to Bosch mechanical (pump) injection on an early 70's 911E. It was immediately apparent: the engine spun REALLY fast on the starter motor and the crankcase was filled with a mix of engine oil and gasoline.

I've never heard of this with CIS, but I am (happily) not omniscient! John
Thatís it exactly. I donít think Iíve ever heard any stories on here where someone had a bunch of fuel in the oil. Maybe if the rings are bad and the injectors were over-fueling.

I was just curious why it doesnít seem to be an issue with CIS.

Maybe cars with neglected maintenance are more likely to fail lean rather than rich too.
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Old 07-11-2019, 10:18 AM
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Originally Posted by T77911S View Post
but it sprays at the back of the valve so the fuel is not inside the cylinder.

think about how much fuel is being dumped on a 930 under full boost. its a LOT.
so the 911 has nothing to worry about compared to the 930.

where you have an issue is when the plunger gets stuck and the engine wont crank, or you have leaking injectors that wash the oil away while sitting, then you crank it.
your other worry there is hydro lock. the cylinder can get too much fuel and it acts like a rock in there and not letting the piston come up.
That makes sense. So if too much fuel was being dumped while the intake valve was closed, the result would be unburned fuel in the exhaust which would be noticeable.

Iím not familiar with the 930 version.

So really the reason is the fact that itís a pressurized injector delivering the fuel. Could be more of an issue with MFI or carbs ó basically any way that has greater potential to dump a lot more fuel in.

Good discussion. Thanks.
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Old 07-11-2019, 10:41 AM
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Quote:
Originally Posted by T77911S View Post
think about how much fuel is being dumped on a 930 under full boost. its a LOT.
so the 911 has nothing to worry about compared to the 930.
Just for giggles, a quote related to me by someone who attended factory 930 training back in the day:

"if there isn't raw fuel running out of the exhaust, a 930 is not too rich"

He also said the factory would refer to the 930 as "fuel-cooled", LOL.

These days, guys tend to tune for "safe" AFRs in the range for best power. But even with the pig-rich state 930s used to run, they don't contaminate the oil (or even foul the magic, extremely cold - and eye-wateringly expensive - Bosch W3DPOs).

Usually. I think a CIS car with fuel in the oil has a different problem. Like an injector stuck open.
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Old 07-11-2019, 10:57 AM
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the 930 just puts out WAAAAAY more fuel than the 911 when it is under boost. same principle for the CIS as a 911.

if it is putting too much behind the intake valve there would be a point where the engine wont run. up to that point I would think CIS is no different than anything else that is running too rich.
so its a matter of how rich can it go before it wont run and what happens to the unburned fuel.
once it wont run hopefully the FP shuts off.

just my 2c
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Old 07-11-2019, 11:00 AM
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Granted this is a BMW motorcycle engine running at 14k RPM, I don't think the fuel being delivered to the backside of the intake valve on a CIS engine that's not on the intake stroke hangs around too long.



Old 07-11-2019, 11:22 AM
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Quote:
Originally Posted by SCadaddle View Post
Granted this is a BMW motorcycle engine running at 14k RPM, I don't think the fuel being delivered to the backside of the intake valve on a CIS engine that's not on the intake stroke hangs around too long.



Right that was my speculation. People make a big deal out of batch vs sequential EFI injection but as the rpms increase, the differences that can be more easily seen at lower rpms go away. If there is an upper bound on rate of fuel delivery, then you have to start injecting the fuel sooner as rpms increase to get the same volume of fuel. The duration that the injectors are firing turn out to be close to the same. And in that regard, CIS probably isnít that much different either.

So I was thinking fuel wash, if it did happen in CIS, would only happen at lower rpms.

Itís just something you never hear about with CIS and I was just wondering why.
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Old 07-11-2019, 12:05 PM
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Thinking too much can be dangerous. It can cause dementia if left without intervention. I consider this board a form of intervention!
Old 07-11-2019, 12:35 PM
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I've been having deep thoughts as well -

CIS vs Motronic

The "Constant" fog of fuel just struck me as an inefficient fuel delivery method. Then I realized that the Motronic system, batch fired, squirts the back side of the valve 6 times per combustion stroke.

Apples, Apples?

BTW, that BMW valve train video is causing some extra deep thoughts. Very impressive...
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Old 07-11-2019, 12:47 PM
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Thinking too much can be dangerous. It can cause dementia if left without intervention. I consider this board a form of intervention!
Well... truth be told my dad is in the hospital and I’m sitting with him. I’ve run out of other stuff to read so I’ve been entertaining myself by reading about engine management.
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Old 07-11-2019, 01:40 PM
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Sorry about your dad. I think about this on and off too. What started me on this topic was reading about the L-jetronic system for the 76 912E, which does 4 discrete squirts per 720 degrees rotation for each ignition. The CIS was the next step in the evolution- maybe a worse system, maybe better? I believe that some of the CIS fuel lands on intake surfaces and then evaporates off again when the intake valve opens. I don't think it just sits there as a cloud of fuel mist. It would be interesting to hear from a Bosch person on this subject.
Old 07-11-2019, 01:59 PM
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Quote:
Originally Posted by tirwin View Post
People make a big deal out of batch vs sequential EFI injection but as the rpms increase, the differences that can be more easily seen at lower rpms go away.
A couple of points:
The video shows an engine at around 14,000 rpm. I guess most of our engines spend their time at much lower speeds where port wall wetting is an influence.

The greatest benefit of sequential is that you can then start to optimise fueling for each individual cylinder. Add direct fire ignition that allows individual timing is another step up. Then add knock control that can manage individual cylinders then youríre at current OEM levels. So sequential opens some significant doors.
Old 07-11-2019, 02:47 PM
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Quote:
Originally Posted by tirwin View Post
That made me wonder something. I donít think I can recall anyone ever mentioning fuel wash as a particular issue with CIS. And that made me wonder if it is an issue or if there is something in the 911 engine design that mitigates the effect of fuel wash.

No deep thoughts here but ... given the emphasis on an even injector spray during injector tests, I would conclude that the fuel stays somewhat atomized and not pooled at the intake.
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