[jmckeefery]

Porsche 968 1993 engine knock.



I Love the car but it has always had a persistent engine knock. The cylinder compression has always been very high. I am the third owner and do not know of all the work that may have been done to the car. I have the full set of Porsche shop manuals and the compression spec is 8 bar and over for a new motor and 6.5 bar is considered low enough for a rebuild. I have 17.5 bar.



In PSI they are as follows.

#1 = 245 PSI

#2 = 250 PSI

#3 = 256 PSI

#4 = 238 PSI



The engine knock is worse with lower octane fuel and with higher engine temperatures. Adding octane boost helps but not much, and not nearly enough.



I was worried about lean injectors, so I checked the fuel rail pressure and it is good at 45 PSI.



The plugs look sooty but even across all four. These indicate too rich or too cold. This makes no sense, I would have guessed the opposite in both cases. Oil burning is not seen and not likely with compression like that. A poor spark could cause all of the above conditions by allowing so much carbon buildup that the compression is increased.



I have wired into the engine computer (I am an electrical engineer) a bright blue LED that flashes when the knock sensors detect a knock. Hard acceleration is always fine. Cruising at 3200 RPM with light load is where the knocking is. If I hard accelerate to 5,500 RPM and let it sit there as the car stabilizes speed the knocking is extreme.



So my hypothesis so far are:



1. Bad spark, lots of carbon, increasing compression, then pre ignition due to carbon hot spots and high compression. The solution to this may be to buy a higher energy ignition coil and see if the condition improves. I also need an additive that you would recommend to get the carbon out of the cylinders.



2. Either the timing belt or the cam to cam chain is mis-adjusted, causing very high compression. There is no fuel available that can run at 18 to one compression ratio. The solution to this is to remove the timing belt cover and valve cover and do a lot of measuring. It's a lot of work. I am hoping to avoid this.



Thanks for you help,

James McKeefery

408-946-8550

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[TheMirror]

My first questions would be what type of compression gauge are you using, who owns it, and when was it last calibrated? Those are bizzarre numbers.



[quote name='jmckeefery' date='Apr 12 2006, 02:23 PM']Porsche 968 1993 engine knock.



I Love the car but it has always had a persistent engine knock.  The cylinder compression has always been very high.  I am the third owner and do not know of all the work that may have been done to the car.  I have the full set of Porsche shop manuals and the compression spec is 8 bar and over for a new motor and 6.5 bar is considered low enough for a rebuild.  I have 17.5 bar.



In PSI they are as follows.

#1 = 245 PSI

#2 = 250 PSI

#3 = 256 PSI

#4 = 238 PSI



The engine knock is worse with lower octane fuel and with higher engine temperatures.  Adding octane boost helps but not much, and not nearly enough. 



I was worried about lean injectors, so I checked the fuel rail pressure and it is good at 45 PSI. 



The plugs look sooty but even across all four.  These indicate too rich or too cold.  This makes no sense, I would have guessed the opposite in both cases.  Oil burning is not seen and not likely with compression like that.  A poor spark could cause all of the above conditions by allowing so much carbon buildup that the compression is increased.



I have wired into the engine computer (I am an electrical engineer) a bright blue LED that flashes when the knock sensors detect a knock.  Hard acceleration is always fine.  Cruising at 3200 RPM with light load is where the knocking is.  If I hard accelerate to 5,500 RPM and let it sit there as the car stabilizes speed the knocking is extreme.



So my hypothesis so far are:



1.  Bad spark, lots of carbon, increasing compression, then pre ignition due to carbon hot spots and high compression.  The solution to this may be to buy a higher energy ignition coil and see if the condition improves.  I also need an additive that you would recommend to get the carbon out of the cylinders.



2.  Either the timing belt or the cam to cam chain is mis-adjusted, causing very high compression. There is no fuel available that can run at 18 to one compression ratio.  The solution to this is to remove the timing belt cover and valve cover and do a lot of measuring.  It's a lot of work.  I am hoping to avoid this.



Thanks for you help,

James McKeefery

408-946-8550

[right][post="19066"]<{POST_SNAPBACK}>[/post][/right][/quote]

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[jmckeefery]

[quote name='TheMirror' date='Apr 12 2006, 02:43 PM']My first questions would be what type of compression gauge are you using, who owns it, and when was it last calibrated? Those are bizzarre numbers.

[right][post="19068"]<{POST_SNAPBACK}>[/post][/right][/quote]



I race a Legend car with my son and this gauge was compared with the track tech official's calibrated gauge and found to be the same, or even 7PSI less than, the officials gauge in 200 PSI range of that car. So if anything my actual cylinder pressure is higher.



I just spoke with Bob Larson on the phone and he feels it is in the cam timing. I am going to give Pete Fitzpatrick a call and see what he says.



James

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[flash]

yeah - short of the head being shaved, or other pistons in there, i can't think of what else could cause those numbers, having already calibrated the gauge, tested with all plugs out and throttle body open (thereby eliminating between cylinders accumulation), checked for water/oil contamination, and checked the plugs - it would take a boat load of carbon to make the numbers that high



that's why i pointed him to pete - he's the cam guru



anybody else got any thoughts?

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[frank moran]

[quote name='flash' date='Apr 12 2006, 05:03 PM']yeah - short of the head being shaved, or other pistons in there, i can't think of what else could cause those numbers, having already calibrated the gauge, tested with all plugs out and throttle body open (thereby eliminating between cylinders accumulation), checked for water/oil contamination, and checked the plugs - it would take a boat load of carbon to make the numbers that high



that's why i pointed him to pete - he's the cam guru



anybody else got any thoughts?

[right][post="19073"]<{POST_SNAPBACK}>[/post][/right][/quote]

I agree, check the valve timing before you go buying a bunch of stuff. It's really not that hard on these engines, more time is spent removing the lower cover and front air filter housing. Also check the cam timing at the chain and tensioner, there are marks on the cam and colored links on the chain.

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[jww968]

I'm not sure I understand why valve timing would increase compression as severely as you have seen. Your numbers would tell me that I had a lot of carbon buildup, flooded cylinders or a decked head. I guess another possibility would be an extremely hot engine. If your readings were taken with a hot engine, then you may want to check your temperatures. If they were with a cold engine, then something is lowering the dome volume above the pistons.

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[TheMirror]

Same here, I can't get a rope around how cam timing can increase compression. Can anyone shed light on that? Compression readings are taken at TDC, correct?



[quote name='jww968' date='Apr 13 2006, 10:11 AM']I'm not sure I understand why valve timing would increase compression as severely as you have seen. Your numbers would tell me that I had a lot of carbon buildup, flooded cylinders or a decked head. I guess another possibility would be an extremely hot engine. If your readings were taken with a hot engine, then you may want to check your temperatures. If they were with a cold engine, then something is lowering the dome volume above the pistons.

[right][post="19097"]<{POST_SNAPBACK}>[/post][/right][/quote]

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[jww968]

The only scenario I could come up with for timing to increase compression was if the intake valve normally started opening just before TDC and the timing was changed and caused it to open at or after TDC. But I do not see that adding 50 to 60 psi.

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[RS Barn]

Those numbers floor me. They're 20% too high. I can't think of anything you could do to raise the numbers that much. The cam timing could be off so there is less overlap but I don't think that would raise the compression that much. When you have a ton of carbon on pistons and head the valves usually don't seal that well and bleed off comp.

First try another guage then at least look at cam timing visually with valve cover off to make sure cams are in correct relation to each other at compression. Exhaust should be at 2 o clock and intake at 10 o clock. There is a template you can make from servive manual.

Any questions give me a call.

Pete

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[flash]

my thought was not an instantaneous one - i was pretty much out of ideas, as he had done all the standard stuff, including calibrating the gauge



what led me to thinking it was possibly cam timing was a comment he made, that came from the guy who just did the belts - he said he couldn't get a fix on where the drive pulley should be - this told me maybe he had that cam out of whack, and should look at that



but, yeah - i'm pretty stumped - my first thoughts were also decked head or pop up pistons



pretty weird for sure

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[flash]

ps - pete - i told him to give you a call at the shop, because you would know about cam overlap and such, and whether or not that could do this

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[jmckeefery]

Pete,



The gauge is good. The engine was warm, not hot. I have gotten similar reading from different gauges over the last 3 years. I know with my race engine in the legend car (that has an adjustable intake cam gear) that if I advance that relative to the exhaust that I can go from a cold compression of 180 PSI to 210 PSI. We also got the intake cam off 2 teeth and advanced one tooth from the crank once and got over 220 PSI. But even that is only 22% and we are more than that. Add carbon to the mix and maybe.



I have a boroscope. I will try and take pictures. Maybe one of you can tell me if the carbon looks extreme or the pistons look like the after market pop up type.



I have the shop manuals and have seen the cam template you refer to. I will need to buy a set of valve cover gaskets before I open that up. Any recommendations there? I have seen oil on the plugs, so I need the "O" rings too.



Thanks for all of your help, this has been great. I appreciate the time and thought you guys have put into it so far. Solving the mystery will be fun, and being able to drive the car hard on days other than cold and rainy will be great (with high air density the car has no issues and lots of power). I have also done other tests where I forced the mass air flow sensor to believe there was more air, causing a richer air fuel mixture, and the knocking was significantly reduced.



James

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[jww968]

I used Pelican for the gaskets.

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[bob blackwell]

Compare prices with Sunset Imports in Oregon. www.porscheparts@sunsetimports.com Good luck, Bob Blackwell.

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[Kim]

I am watching this carefully as I, too, have persistent engine knock at 3500 rpm and above. I am really hoping that after Monday my mechanic will tell me that the timing belt was put on incorrectly. I have had it checked for tension 4 times but never for correct installation. Otherwise, I am running out of ideas. Octane boost fluid did absolutely nothing at all.

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[jmckeefery]

    I set up the boroscope and shot some photos and video of the piston tops to check for carbon and "pop up" pistons. They look to be stock pistons and the carbon build-up looks fairly normal. Cylinder 2 has the worst carbon build up but even it has 50% of the aluminum showing and the carbon thickness is small, maybe .010" maximum.



I have the gaskets on order and will check the cam timing next.



James

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[jmckeefery]

I have just completed a full timing belt analysis, and yes, the cam gear was advanced TWO teeth from the crankshaft gear at TDC on cylinder #1 as measured with a dial indicator on top of the piston through the spark plug hole.



See photos of setup and cam position before the belt position was corrected. This situation created a compression of 240 to 260 PSI.



James

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[jmckeefery]

These next two photos show the corrected timing belt position. It was fairly easy to put a wrench on the crankshaft pulley nut and turn the crankshaft while watching the dial indicator. Always go forward, never reverse rotation or the readings will not be accurate. I slid the belt off the camshaft pulley, held it from slipping on the crankshaft pulley, and then rotated the exhaust cam with a wrench by two teeth, and reset the belt back on the camshaft pulley. Then two full forward rotations on the crankshaft to check my work and take these photos.



The car runs great now, no knocking at all, and there is a nice growl to the motor on hard acceleration that was not there before. It used to be more turbine like, taking time to spool up but ran well above 4 grand. Now the power is at any RPM and definitely has a better launch in first gear than when the cam position was wrong.



Where I used to have to baby the throttle to avoid the dreaded engine knock, now I can thrash it all I want and it never complains. The engine also runs a lot cooler in hot weather.



James

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[flash]

sweet - VERY happy to hear that you got it worked out

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[jmckeefery]

Just to complete this thread, after driving it for a month with the cam timing belt correctly aligned, the compression was checked again and here are the results:



In PSI they were as follows with timing belt advanced 2 teeth.

#1 = 245 PSI

#2 = 250 PSI

#3 = 256 PSI

#4 = 238 PSI



In PSI as they are now with corrected cam timing.

#1 = 180 PSI

#2 = 185 PSI

#3 = 190 PSI

#4 = 180 PSI



James

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