205 Low Compression on Cylinder 1

neilc said:
After putting rings and bearings in my Gti I ran for the first time at a sprint meeting. While the car went ok it was blowing smoke under acceleration down the main straight which I put down to an overfilled sump.

On return home checked compression cylinders 2,3 and 4 - 240 psi cylinder 1 - 80 psi
I am using a competitive .027” head gasket

I have since removed head couldn’t see any visible signs of damage on head gasket or head. Inverted head and poured petrol into combustion chamber of cylinder 1 the valves seemed to be seating properly as no petrol leaked out

What would be the liner protrusion expected with seal installed, I know it is normally measured with seal removed but I dot want to disturb liner if I don’t need to.

Where to from here is it possible to remove piston without taking engine out?


Any suggestions welcome
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240 psi is a lot , DFZ pistons with DKZ head? I have had split cylinder liners with that sort of compression. I think just a failed piston.
 
That is a bloody train wreck for one meet?
The top edge of the top ring land doesn't appear to be distorted, so I don't think it is detonation related.
What were the oil ring rails like?
I'd almost think one of the oil ring rails has come up the side of the piston to allow that much carbon build up in so little time.
Did number 1 piston tap down easily when you put it together?
It's also hard to imagine the bore survived intact after seeing the piston and ring damage?
 
The bore looks good I can see the cross hatching and no scores. I will try to take photo tomorrow

I will check oil ring

We did crack test pistons before reassembly

What sort of compression have you seen Graham on your cars?
 
Have seen 240 and as I said the liners cracked but due to pinging. With that much compression you have to retard the timing so much that you lose power, better to run lower compression and more timing I think.
 
I’m with Greenpeace on lower oil ring rail distorting on assembly going by wear around centre oil return hole looks like a wear “ramp”at inner portion of groove as if lower oil ring rail has twisted and eaten into piston groove.this would have seized oil expander ring creating excess pressure on ring lands causing them to fracture.may be wrong but that be my take on it..jim
 
There's a bit more to whether it will ping or detonate than just cylinder cranking pressure.
Other than the inherent engine design characteristics, vehicle weight, final drive ratio, fuel grade, engine temperature, air temperature, fuel temperature, altitude, etc.
I'm not familiar with the specific vehicle in question, but certainly if it's a light weight, stripped out track car, predominantly operating at high rpm through the gears, it is going to be more tolerant of higher cylinder pressures than a full weight road car lugging through the gears with the AC on.
I've certainly run pressures in the mid 200 range on cars and bikes on both super leaded and premium unleaded fuel, without pinging or detonation issues.
My Reliant has 200 psi cranking pressures, then has 10 psi of boost added on top of that from a positive displacement supercharger, and it tolerates 95 octane unleaded.
However the vehicle weight isn't much more than a packet of chips.
As has already been said, a longer duration cam will reduce cranking pressures at low rpm, however when the engine is "on the cam" those cylinder pressures don't have the time to bleed off like they do at low rpm.
 
I've found that a Euro spec 1.9 GTi engine (9.6 to 1) is extremely critical of timing to prevent pinging, even on 98. Most of the ones I have had were grossly retarded when I got them , with poor performance. The 16 valve engines are better.
 
I have a cam made by Tighe Engineering with a 292 degree duration and peak valve lift of 11mm

I found a photo of the piston before I removed rings oil control ring looks fine
 

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That's a huge cam. What is your calculated static CR? Back in the day I had a 272 degree cam on about 11:1 and it did run okay on 98. I don't think an 8v can handle much more compression than that though (on 98).
 
When I published the cam card options I had from Tighe on the forum (205 GTI Sprint Car - way forward) Peter T said it would be the one to use

I calculated 11.18 if I did correctly
 
That all seems reasonable assuming good tune.
 
I can't see the spec is any different from a Group A, other than the pistons aren't forged. 240psi is very normal for that cam and 11:1. Plenty of folk in the UK have built similar engines. I think it's just bad luck, from a dud piston. The issue is, what are you going to do moving forward? I'm currently building a very similar engine using XU9J4Z pistons & rods.
 
My plan at this stage is to replace piston get the tune checked and see how it goes.

What are the benefits of using XU9J4Z pistons and conrods? Are there many differences between XU9j4 pistons and rods and DFZ rods and pistons. Are they readily available?
 
neilc said:
I have a cam made by Tighe Engineering with a 292 degree duration and peak valve lift of 11mm

I found a photo of the piston before I removed rings oil control ring looks fine
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Oh I see what's happened, sorry, I'm only on my phone, so images are a little small.
Those oil rails look like they might have little whoopsies in them?
Are you sure one didn't sneak out a little bit under the ring compressor when the piston was being knocked in?
Looking at the first photo the ring land break has started right in the center of the oil hole and radiated upwards and outwards.
I reckon an oil rail may have snagged the edge of the cylinder during assembly, and unfortunately dead in line with an oil hole.
From what you indicated in the first post, it was blowing smoke from the get go, which would point to a pre-existing condition.
 
neilc said:
My plan at this stage is to replace piston get the tune checked and see how it goes.

What are the benefits of using XU9J4Z pistons and conrods? Are there many differences between XU9j4 pistons and rods and DFZ rods and pistons. Are they readily available?
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They're about 150g lighter and have a full floating pin. A much nicer design. I'd just stick another DFZ piston in though for now, as I suspect they're a little thin on the ground now.
 
PeterT said:
They're about 150g lighter and have a full floating pin. A much nicer design. I'd just stick another DFZ piston in though for now, as I suspect they're a little thin on the ground now.
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DFZ or J4Z? I've got a few J4Zs kicking around. Also got a set of new 11.8 to 1 Wossners and 83.5 mm sleeves for 16 valve if anyone is interested.
 
GRAHAM WALLIS said:
DFZ or J4Z? I've got a few J4Zs kicking around. Also got a set of new 11.8 to 1 Wossners and 83.5 mm sleeves for 16 valve if anyone is interested.
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Your reply is somewhat ambiguous. I'm not sure what you mean. I was suggesting XU9J4Z (DFW) pistons are hard to come by.
 
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