Rotating heads?

[perelet]

I'm putting my 3.6 engine back together. Today I was replacing valve guides and (no surprize) #3 and #6 showed more wear and surely undergone more temp stress.

All heads are machined to exact same height by groups. I thought why dont I swap #1 and #3 and #4 and #6. They are in same groups, so no obvious crime that I can see.


Thanks!
Oleg.

[911pcars]

If all guides replaced, don't see the advantage despite your approved swap of interchangeable parts.

Sherwood

[perelet]

Besides guides 3.6 heads have ceramic coating on exhaust ports. 6 and 3 surely have more stress on coating compared to 1 and 4.

[911pcars]

Do previous 911 engines also have these concerns or just 3.6 air-cooled engines?

Added stress on specific cylinders should be confirmed by studying a sample of EGT, AFR, historical piston/cyl. wear, broken head studs, etc. data. Rather than rotating cylinder heads at intervals, perhaps there are mitigation steps to provide increased airflow to those cylinders.

Perhaps those who do this for a living (many engine rebuilds, racers, etc.) can chime in. Where's MotorMeister when you need them for expert testimony?

Sherwood

[Lapkritis]

I'll chime in on mitigating steps as I have experience with hot cylinders on other engines...

If you're using a programmable EFI such as Megasquirt 3 then you have the capability for individual cylinder trimming of the fuel injector pulsewidth. On the hot cylinders, add a bit more fuel. You would need individual cylinder EGT and wideband measurements to dial it in perfectly. I won't go in to COP individual ignition advance/trimming as I believe you could achieve the desired result with fuel adjustments alone.

[911pcars]

Originally Posted by Lapkritis

I'll chime in on mitigating steps as I have experience with hot cylinders on other engines...

If you're using a programmable EFI such as Megasquirt 3 then you have the capability for individual cylinder trimming of the fuel injector pulsewidth. On the hot cylinders, add a bit more fuel. You would need individual cylinder EGT and wideband measurements to dial it in perfectly. I won't go in to COP individual ignition advance/trimming as I believe you could achieve the desired result with fuel adjustments alone.

Those with turbo engines also use fuel trim (more fuel) to help reduce combustion temperature. Perhaps a better method is to use water injection since this merely lowers combustion/EGT temps w/o disturbing the A/F ratio that's been hopefully preset for best power.

Sherwood

[Lapkritis]

With water injection, you're going to be impacting other cylinders that are otherwise operating optimally. I would trim the fuel for the hot cylinders only - it's not that you're cooling the charge with fuel so much as correcting improper tuning that is in place. As you know, EGT is a product of the combustion mixture (fuel and air) as well as ignition timing. Too lean, it's hot EGT. Too retarded on spark, it's hot EGT. If two cylinders are showing indicators of hot EGT then tune them if you have the capability to isolate them.

Water-meth would be on my list of recommendations in addition to the fuel trimming on boosted vehicle. Even on boosted, I would start with treating each cylinder as it's own miniature engine that must be tuned properly to match the others before all else.

[911pcars]

Assuming the OP has those issues, your suggestions sound fine. However, his theory is that cyl. 3 and 6 valve guides have worn due to insufficient air flow. This is yet to be confirmed. I suggest it's better to correctly identify the source of the issue rather then devise mitigating solutions that may only indirectly apply.... or to rotate cylinder heads to eventually subject all heads to the same operating conditions.

An analogy would be one corner of the car with incorrect camber/toe in. Instead of identifying the real source and correcting it. the chosen solution is to rotate tires more frequently to even out the eventual wear.

I assume if air flow to a few cylinders is an issue, there's an appropriate engineering solution. If not, then you go with the next best alternate solutions, but not as a first choice.

Sherwood

[Lapkritis]

Cylinder trimming is very common to tune EGT and control things like accelerated valve guide wear.

[911pcars]

Originally Posted by Lapkritis

Cylinder trimming is very common to tune EGT and control things like accelerated valve guide wear.

If that is the next best solution to mitigate valve guide wear, ok, but only after:
- checking for sufficient valve guide clearance
- checking for adequate valve guide length (some builders trim overall length)
- checking valve seal efficiency
- confirming compatible valve stem material
- confirming best material available for valve guide longevity
- confirming sufficient oil film strength and access
- confirming accurate valve train geometry

[perelet]

I have stock 3.6 engine in 964 car, 78kMiles. I took ot apart because of broken delivar stud on #6. Those things just run hot on 3 and 6.

Here are pics of mine before cleanup, but pretty much every 964 tear down thread have very similar pics. #3 and #6 are where the black stuff






Valve play at 10mm lift was 0.4mm on all exhaust valves but #3 and #6 had 0.6 play. Funny thing factory manuals for 964 and 993 both say that acceptable play is 0.8mm.

[911pcars]

Sorry to hear that. Those appear to be the next to last version of Dilavar studs; black coating on unthreaded shank. Yeah. It happens. 993 or equivalent case studs are probably the solution. Heat isn't the only reason why Dilavars break.

What are the other signs of suspected overheating? e.g. Piston or cylinder scored? Show us the plug comparisons, combustion chambers.

A couple of piston tops look shiny as in excess oil suckage or a just-completed wet compression test? How was compression on all cylinders?

[Lapkritis]

Originally Posted by 911pcars

If that is the next best solution to mitigate valve guide wear, ok, but only after:
- checking for sufficient valve guide clearance
- checking for adequate valve guide length (some builders trim overall length)
- checking valve seal efficiency
- confirming compatible valve stem material
- confirming best material available for valve guide longevity
- confirming sufficient oil film strength and access
- confirming accurate valve train geometry

Originally Posted by perelet

I have stock 3.6 engine in 964 car, 78kMiles. I took ot apart because of broken delivar stud on #6. Those things just run hot on 3 and 6.

Valve play at 10mm lift was 0.4mm on all exhaust valves but #3 and #6 had 0.6 play. Funny thing factory manuals for 964 and 993 both say that acceptable play is 0.8mm.

Sounds like all confirmed on a stock engine...

If this is an across the board issue with those two cylinders... I maintain that trimming the fuel to those cylinders to tune for proper egt is necessary.

I agree - a quick plug read might tell a story if you perform the shutdown pull properly. That is a relatively crude method. Otherwise, 1 egt and 1 wideband sensor per exhaust port and tune it.

Compare this to the VW VR6 engine where it is far more complex for tuning. First, you have unequal length intake and exhaust ports for the two cylinder banks. Add to that, one bank has a cool intake manifold hanging off it and the other has a hot exhaust manifold hanging off it. Coolant flow across the head is exit on the exhaust side to make matters worse. The engines are notorious for the rear bank (cyl 1,3,5) running too hot and cracking ring lands. On these, I add 3-5% trim for the hot bank. Issue resolved. Emissions aside, EGT tuning is just as important if not more so as lambda. Lean air to fuel doesn't do damage, heat does.