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Mk1 TT 225 track day car, Wossner piston failure

desertstorm Apr 7, 2017

  1. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    Final piece in the puzzle before it gets out on track at Bedford on the 15th, pretty much the ultimate brake set up for the fronts bar a stupidly expensive carbon ceramic disc setup.
    BTCC spec front brake set up. These are the same calipers used by the current BTCC cars, 6 pot CP9660 and the discs and pads I have are used BTCC items.

    The top teams only run the discs and pads for one race or practice etc. They get through a huge amount of these so they are available at very knock down prices used. The discs are AP Racing 368 X36 mm that are specifically designed for the BTCC cars as the largest brake they can squeeze under the 18 inch rims they use.
    The 3 sets of discs I have are all 35.8- 35.9mm thick against 36mm thick when new. Pads have 18mm of friction material when new, second hand pads typically 16-17mm. The discs on the car ATM were on Andrew Jordans car last year.
    They were redesigned 2 years ago to improve them with heavy duty Bobbins for the floating disc and S shape vanes for cooling, the standard pad is a Raybestos ST45, They are an American company and make lots of stuff for American racing series like Indy Car. I have watched the touring cars racing and have always been impressed by how quick the stop, I now know why. You only have to wave your foot across the brake and the speed starts disappearing. Normal heavy brake pressure application just results in a lock up at what ever speed you care to be doing.
    Only one real issue fitting these and that was caused by the fact I have MK2 TT bottom ball joints. These allow a lot more camber than standard ball joints. However the knuckle on the bottom ball joint is very close to the disc. Requires some attention with an angle grinder.
    The complete brake system disc, bell, bobbins, caliper and pads as well as caliper adapters weighs in at 14.9Kg. I weighed a standard 312mm system a few years ago at 13.6Kg.
    The 6 pot calipers are well matched to the TT with the piston area only being about 10% bigger than the standard calipers.
    Loads of clearance on my rims.


    used pads


    weight of all the parts combined.


    The bottom ball joint does foul the disc. Needs some attention with an angle grinder.



    Back side of the disc that picked up a small score from the ball joint. The float is in the discs and not the bell, so the bells don't wear out.

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  3. Seanchadwick2k3

    Seanchadwick2k3 Audi A6 C6 2.0tdi 2011 Sline Special Edition

    I had a garage tour about 4 years ago at Silverstone with pirtek and can just about spot those calipers in my pictures. They should do you well!
  4. RyanJonS4

    RyanJonS4 Drive It Like You Stole It

    There’s not many threads I follow outside of the b8 section but this is 1 of them and what a build this has turned into!

    Ap brakes are in a different league than anything else, I can’t overheat mine no matter how hard I try. Be very careful of coming to a complete stop though when they’re hot, I ruined a set of Front discs with pad transfer and could have only had my foot on the brakes for a second of 2. A couple of cool down laps and even a few laps round the pits is advisable.
    desertstorm likes this.
  5. jojo

    jojo Looking for Boost! Staff Member Moderator

    Stopped pretty well the last time I was in it, I don’t think I even used the old setup to its potential because it was damp? This should rip my face off now! Lol
  6. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    You would be quite shocked just how much stopping power these have in terms of pedal pressure required. These calipers even have anti knockback springs on the pistons so very little pedal movement required . Be interesting to see how it goes on Monday on the Bedford GT circuit. With a 1 Km straight it should get some speed up. Modified the exhaust a little to try and help with the noise limits they have there.
    jojo and Blu-iTT like this.
  7. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    Took the car to Bedford for it's first track day this year. Never been to the track before and it's a slightly unusual track as they don't actually race here. Probably the longest circuit of any track in the country the 3.8 mile GT circuit has a straight just over 1Km long. Some really quick cars turned up, three Mclarens along with many Porsches, M3's etc etc.
    Had some issues with the car that I never really worked out till I got home and looked at the photos that were taken on the day. I got the aero set up completely wrong with this new rear wing , and the splitter angle on the front was wrong as well.
    You can see the rear squatting down and the front splitter stuck up in the air, instead of being parallel with the ground.

    Upshot of this is the handling was very poor, the car would not hold a line in a corner and the understeer / non existent turn in was very noticeable.
    Two much rear wing, probably positive lift being generated from the front splitter meant no front end.
    I was hoping to hit 150 on the straight but the best I managed was 149.4mph . The BTCC brakes were absolutely epic, stops so quick and no sign of fade at all. Very little pad use
    This track has the tightest noise restrictions in the country with an 87.5 dB at 20m limit. I did get pinged in the afternoon so had to deploy the home made exhaust turn downs which did make a big difference. Noticeably quieter in the car I could go flat out past the microphones and set my quickest times with these on.



    Surprised the plastic centres in the wheels didn't melt they were that hot. But the brakes were epic.


    Modified exhaust outlets looked daft but did the job. Microphones are on the infield on the passenger side of the car hence pointing exhausts to the driver side.


    Not quite 150mph , never got passed by anything on the straights but never came across any of the Mclarens.


    Didn't have much luck with videos the camera stopped working on my phone in the afternoon because it got too hot and the Go Pro I had was set up wrong so completely over exposed.
    An early lap still learning the circuit and trying to wonder why the car was understeering so much.

    An alternative view out the rear. 1min 32 watch the s2000 spin . Had a bit of a run with a Caterham, Definitely quicker than me in the corners, he was taking much better lines through some of the corners.If the car had been handling as it should would have just left him.

    Short video running down the straight, Using the push to pass to up boost from 1.8 to 2 bar on the longer straights.

    Blu-iTT and 45bvtc like this.
  8. Bluepower

    Bluepower Registered User

    I'm pretty sure you had a fun day, Karl!! Nice driving, but there is still a lot to gain with better cornering(but like you said: aero setup is not well, so that will need some addressing)
    Next up is to invest in a higher positioned shifter, like the CAE or Coolerworks
  9. Gazwould

    Gazwould Registered User

    Great stuff , was that a ". uck" at 1:20 going by your timer ?
  10. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    I was trying to get my head around how good the brakes are now. Stopping from 140+ mph that quick. And it would stop quicker if the grip was there. I have now replaced the 6 year old Yoko A048 235/40/18 tyres with some wider stickier 255/35/18 Nankang ARI's. I have another trackday booked for Bedford end of July so hopefully have the aero issues sorted by then.
    RyanJonS4 and Bluepower like this.
  11. Sean Wheatley

    Sean Wheatley Registered User

    0F627206-A795-49C0-9142-71F4EF3E529D.jpeg Karl, I’ve been running the AP9660 on my TTRS for over a year now and they really are great calipers; much better cooling, easy pad changes, more pad choice, less weight. I dropped over 15kg in unsprung weight by moving to these calipers and a AP floating disc!
    The first two times on track with my car were at Bedford, chosen because it’s wide open with little to hit if it goes wrong. I think it’s a fantastic circuit and underrated by many. Here’s a video from my second visit once I’d got some proper brakes; running around 2:43 on Pilot Sport Cups

    here’s my first visit all stock apart from MSS springs and street tyres running 2:49

    I would really like to go back but the noise limits, even with my oem exhaust, make it tricky. 81BE5B22-9793-4996-861A-97A5EC78131E.jpeg
    desertstorm likes this.
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  13. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    @Sean Wheatley looks like you were having fun. I have booked another track day there on the 30th and with some better tyres and the aero sorted looking to hopefully go better than the first day. Noise is an issue there one guy in a GT3 Porsche only did 4-5 laps in the day because he got black flagged twice for noise. You should look into some second hand BTCC pads. The ST45 pad compound is great, no squealing , great stopping power when stone cold and works really well on the track. I picked up two used sets of pads for £120. New pads are 18mm thick they were 17mm thick .
  14. Sean Wheatley

    Sean Wheatley Registered User

    Thanks for the pad recommendations; where did you source them? I’ve heard the BTCC have a choice of three pads, the st45, a Ferodo 3:12 and a Project Mu pad.
    I got black flagged on my first lap the last visit to Bedford for noise (Stock exhaust) but ended up with only a warning luckily. Gt3’s hardly ever go there for that reason.
    Have fun on the 30th!
  15. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

  16. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    Well it happened just over a week ago last Monday on the way to work. The Piston and rod in number 4 decided that they had suffered enough abuse and exited the engine taking a large piece of the block with them. Fortunately I have a fire extinguisher in the car as it went on the rear of the block and covered the very hot manifold in oil and coolant. Otherwise the resultant fire may have finished the car off.
    After removing the engine and dismantling it looks like another Wossner piston failure. Several people have had issues with these pistons cracking and then failing causing the same results I saw. If the piston is measured they actually shrink ???.
    So before you rebuild a 1.8T think twice before fitting Wossner pistons. This engine has only done just over 3K miles since it was built.
    I have picked up an AUM block and head that will be the basis of the new engine. Exactly the same block and crank as a BAM but without the cost .
    Ignitron logging caught the moment the engine failed 6600 RPM, timing , fuelling and boost all spot on. The engine was still timed up perfectly when I checked.

    Ignitron log engine failure .JPG



    Cracks in all 3 remaining pistons, on 2 pistons there are multiple cracks.




    Gudgeon pin was in three pieces.


    Have several pieces of piston this size.



    Engine is being rebuilt with JE pistons and tool steel pins. Hopefully it will last a bit longer this time.
  17. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    Finally after the best part of 2 months the car is back to life. Been a bit of a mission getting the engine rebuilt. Big thanks to Bill Brockbank for sourcing the pistons, rods,exhaust valves, and getting the block cleaned and bored for the JE pistons.
    Got 60 miles on the car today just running 1 bar actuator pressure and 6000 RPM rev limit it's quite perky at that making around 280bhp according to Ignitron. Will be tweaking the boost and rev limit up over the next 300 miles or so.
    Happy to say no piston slap or knock from the JE pistons, The job done on the block was excellent.
    Took the opportunity to relocate the Turbo coolant pump repurposing the original hoses. Seemed daft the way it was run from the factory I suspect it was put there to try and get some cooling air so have left it out in free air. Saved a bit of weight. Also rerouted my oil cooler hoses which now go down the side of the rad than over the top. So able to take about 2.5ft out of the length.
    Also now able to drain the oil cooler without taking the front bumper off. Fitted the NA exhaust cam as well as the NA intake cam I was using before. In fact used the APG head I got the cams in so car has a N/A head and cams now. No longer a BAM with an AUM block and crank.
    Hopefully make a little more power than before.






    RyanJonS4, ADs3, baliszoft and 2 others like this.
  18. bobby singh

    bobby singh BSR Gold Supporter VCDS Map User

    Nice work Karl
    What the compression ratio now?

  19. Bluepower

    Bluepower Registered User

    Glad to see it up-and-running again mate!!

    Got a request: if you can ever get another set of those BTCC brakes for me, would you please let me know??
  20. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    JE pistons are 9.25:1 the Wossners were 9.5:1 . When I did a compression check on the Wossners the readings were 210-215 PSI. There was very little in the way of a bowl in the piston, certainly compared to a BAM stock piston. The JE pistons now I have run them in are showing 190-200 PSI .
    For reference the stock OE pistons with about 90K miles on them measured 175-180 PSI and they were supposed to be 9.0-1.
  21. desertstorm

    desertstorm Moderator Staff Member Moderator VCDS Map User

    Bit of an update on the car, Had an issue with the gearbox after putting about 250 miles on the new engine and starting to ramp up the boost and revs. I suspect it may have been damaged when the engine blew up. With such a catastrophic failure on number 4 this could well have created a shock load in the transmission input shaft. Only a theory but pretty much from the get go the gearbox was noisy when lifting off at high RPM after a gear change. Quite a noticeable and distracting noise, after searching this quite often appears to be due to end float on the input shaft and happens as adjacent gears touch.
    After pulling away quite smartly off a roundabout and accelerating onto a dual carriageway selected 4th around 70 mph and met with a rather large bang and much grinding / general noise.
    Quick dip of the clutch and determined the engine was OK.
    Pulled over in a layby and no sign of holes, oil or other issues. Managed to drive another 5 miles or so but getting very noisy and almost impossible to select a gear.Not good sitting in stop start traffic at 16.30 on a Friday afternoon. On the AA recovery truck again, rapidly becoming their best customer.
    Found a replacement gearbox on Ebay about 30 miles away, £100 DQB the same as what is on the car and picked it up Saturday afternoon. They are pretty heavy with the transfer box around 90Kg so decided to take the engine out. Sunday morning 09.30 started and by 16.00 I was putting the engine back in the car with the new box on.
    Fortunately it seems to work well so changed the oil and got on with running the engine in.
    Very common issue with teeth shredding off 4th/6th gear on the input shaft, there are some mods that can be made to improve them but long term a DSG gearbox is probably going to be the answer.

    Video of the noise, in neutral it's silent, lift the clutch it makes a lot of noise.

    Secondhand gearbox.


    Some of the damage.


    ADs3 and RyanJonS4 like this.

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