RV8 3.5 SD1 9.75:1 - Compression Test results

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Prophead
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RV8 3.5 SD1 9.75:1 - Compression Test results

Post by Prophead » Fri Jun 29, 2018 12:18 pm

Further to my previous post I have carried out a compression test.
Method used:-
1. warmed engine to operating temp
2. removed all plugs
3. Ignition and fuel pump off
4. insert guage
5. cranked for a count of 10, throttle slightly open
6. Take reading - repeated for all cylinders

Results in Psi:-
1. 165
3. 165
5. 163
7. 167

2. 167
4. 162
6. 162
8. 140 (ran test 3 times removing and reinserting gauge - consistent reading)

As per the procedure I was following I injected 10ml of fresh oil into cylinder no. 8 and re-ran the test - 190psi. My interpenetration of this is that the valve seats and bore are ok but the rings on cylinder 8 are worn. All other cylinders appear to be in good health.

I have a few questions about this:-
1. Is this the most likely cause of my blue smoke?
2. What could be the cause of just one cylinder / rings to wear faster than the rest?
3. Am I likely to cause more significant damage if I continue to to drive the car i.e. score the bore? It's not a daily, I just want to be able to take it to some shows and events over the summer months, and the occasional evening cruise. I'm assuming that track-days would be a bad idea.

Thanks


Porsche 924 GTv8 Track-day car project
Porsche 944 2.5 Lux gone but not forgotten

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Ian Anderson
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Re: RV8 3.5 SD1 9.75:1 - Compression Test results

Post by Ian Anderson » Fri Jun 29, 2018 5:30 pm

Hi

First did you try any of the other cylinders with oil in too?

I have also been told compression I test is on fully open trhrottle to get constant results.

If your combustion chamber is say 32cc and you ass 10 cc of non compressible oil you will also get a higher compression. Hence the reason to get a standard adding same amount of oil in the other cylinders.

But given you think the rings are a problem get a bore scope and look at the cylinder walls via spark plug holes and compare to other cylinders.

Ian
Owner of an "On the Road" GT40 Replica by DAX powered by 3.9Hotwre Efi, worked over by DJ Motors. EFi Working but still does some kangaroo at low revs (Damn the speed limits) In to paint shop 18/03/08.

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Prophead
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Re: RV8 3.5 SD1 9.75:1 - Compression Test results

Post by Prophead » Fri Jun 29, 2018 6:29 pm

Hi Ian, thanks for response.

The purpose of adding the oil and redoing the compression test is to rule out certain things, the absolute reading is not relevant, what is relevant is whether the compression significantly increases as a result of the addition of the oil. If there is a significant increase in the compression result this indicates that the oil has 'sealed' worn rings. If the low compression was as a result of a valve seat issue the oil would have no effect and the reading would still be low. Equally, if the rings are broken or the bore badly scored the oil would have escaped past and given a low reading. So by deduction the oil test indicates the reason for low compression being worn rings on that cylinder.

I guess the next step would be to be absolutely certain by getting a leak down test done.

But in the meantime my questions still stand:
1. Is this the most likely cause of my blue smoke?
2. What could be the cause of just one cylinder / rings to wear faster than the rest?
3. Am I likely to cause more significant damage if I continue to to drive the car i.e. score the bore? It's not a daily, I just want to be able to take it to some shows and events over the summer months, and the occasional evening cruise. I'm assuming that track-days would be a bad idea.
Porsche 924 GTv8 Track-day car project
Porsche 944 2.5 Lux gone but not forgotten

DaveEFI
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Re: RV8 3.5 SD1 9.75:1 - Compression Test results

Post by DaveEFI » Sat Jun 30, 2018 8:35 am

Of course anything is possible, but rings don't normally break for no reason. They can be broken when installing. Or if the piston is very worn indeed. Or possibly though detonation. What is possible is the could have rotated so the gaps are in line. Or sticking. Which could show up as low compression at the very low cranking speed of a compression test. That low speed is not a speed the engine ever runs at - hence many being very wary about taking action on the results of it unless it also supports what could be reason for poor running.

But have you already fitted new valve seals?
Dave
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Prophead
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Re: RV8 3.5 SD1 9.75:1 - Compression Test results

Post by Prophead » Sat Jun 30, 2018 7:19 pm

DaveEFI wrote:
Sat Jun 30, 2018 8:35 am
Of course anything is possible, but rings don't normally break for no reason. They can be broken when installing. Or if the piston is very worn indeed. Or possibly though detonation. What is possible is the could have rotated so the gaps are in line. Or sticking. Which could show up as low compression at the very low cranking speed of a compression test. That low speed is not a speed the engine ever runs at - hence many being very wary about taking action on the results of it unless it also supports what could be reason for poor running.

But have you already fitted new valve seals?
Hi Dave,
No not fitted new oil seals yet. Actually the car had me very confused last night but I think today I have figured out what has been going on.

The past 4-5 weeks since i've had the car back on the road i've been running it on Super unleaded, where as previously I had been using normal. The track session when my troubles started would have been the first tank of Super. The engine was running fine but felt 'flat' and wasn't pulling as well as I know it can, at the time I just put it down to the hot day. At the track a couple of pals pointed out that they thought it was running rich. This was before the puffs of blue smoke started.

A couple of drive outs later the engine was definitely running a bit rougher, not really wanting to pull, and there were a lot of sooty deposits out the exhaust. I felt like it was ignition rather fueling so I checked everything over, replaced plugs, cap & rotor, and did the relocation of the module (it was on the side of the dizzy. Nothing really improved. Until yesterday when I realised that I needed to advance the timing a bit for the Super unleaded, which I did. Engine sounded much better, idle was stable, couldn't detect it missing a beat, so I went out for a drive. I didn't come back for over an an hour as I was having so much fun. It pulled really strongly through the range, in fact stronger than I'd felt it before especially above 4500rpm, no sign of a misfire or being one cylinder down. I got home with a big smile on my face and feeling confused. On top of which I didnt notice any smoke blue or black, which I had been seeing in the mirror previously. Basically, it felt like a completely different engine!

I did some more research and came up with this hypothesis:-
1. The engine was running rich because I had switched to super from normal and not adjusted the timing to suit.
2. The combination of the hot and hard running on track and running rich either washed cylinder 8 of its oil lining, or gummed up its rings.
3. Doing the oil injection compression test either re-established the cylinder wall oil lining and /or freed up the rings.

So for now I will continue to use the car as intended and monitor the situation, I'll do another compression test at the end of next month after I've had a chance to use it a bit more and get some miles done.
Porsche 924 GTv8 Track-day car project
Porsche 944 2.5 Lux gone but not forgotten

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