2 cylinder Suprex - warning sign?

[Thrumbleux]

If this is the case then can you please explain why the 700cc engine in the Roadster is proven to do 150,000-200,000+ miles without lunching it's piston rings or valves?

Different cam and gearing push the operating rpm range upwards. This impacts on a host of factors - piston side thrust, detonation limit, etc.

[Buckster]

Considering the motor has a a turbo, it produces very little power for and engine of its size.

[Thrumbleux]

Considering the motor has a a turbo, it produces very little power for and engine of its size.

Back at launch circa 1999, around 100hp/litre was about as much as anything else had outside of anything with real performance (or competition homologation) aspirations. The focus was biased toward fuel efficiency and it's taken until now for downsized blown engines from others to catch up. Nearly 20 years ago, it wasn't too bad an effort - and they have been persuaded to deliver a lot more.

[Buckster]

Without the turbo it is producing 36-40hp, the same as a 600cc motorcycle engine from the 70's

[Thrumbleux]

Without the turbo it is producing 36-40hp, the same as a 600cc motorcycle engine from the 70's

True, but beyond the 70's motorcycles began using much higher rpm to achieve their outputs. The Suprex with Pmax at 5250rpm is a slogger. Take the revs up to 14000 and 120hp is there. Three times the power at nearly three times the rpm.
Having had a wastegate stick open I've driven a smart with 36-40hp. Absolutely horrible it was. The turbo boosting the midrange is what makes these motors.

[Buckster]

I am just saying that the turbo is where the good power comes from, the actual engine is a fairly soft state of tune, it is over square as well for torque coupled with a cam to make the most of turbo hp. With the right care this should be a 200k engine, the problems seem to come from questionable and variable build quality and subsequent poor service from dealers who are not used to working on small engines, from my experience of motorcycle engines and fitting of small turbo units it is all familiar ground. These cars really are a triumph of design.

[Buckster]

Back to the misfire issue, are you sure it was a misfire and not the wastegate dumping suddenly, it can do this if one of the pipes comes off the actuator.

[Thrumbleux]

I am just saying that the turbo is where the good power comes from, the actual engine is a fairly soft state of tune, it is over square as well for torque coupled with a cam to make the most of turbo hp. With the right care this should be a 200k engine, the problems seem to come from questionable and variable build quality and subsequent poor service from dealers who are not used to working on small engines, from my experience of motorcycle engines and fitting of small turbo units it is all familiar ground. These cars really are a triumph of design.

My theory on what goes awry is - suspect oil control rings and high piston skirt loadings (high cylinder pressure and low rpm) allows oil into combustion chamber (as rings progressively seize with carbon) which burns and the Lambda sensor detects a rich mixture and leans off the fuel which creates a fuel mixture with a reduced octane rating. Thus more prone to detonation when detonation occurs the ECU retards the ignition timing to suppress it. Prolonged running with retarded timing increases the heat flow over the exhaust valve which subsequently burns and causes compression loss. That's my theory at least but there is another that says carbon deposits from burned oil get trapped between a valve and its seat and cause it to burn - a theory which has some plausibility.

[Buckster]

Piston rings are certainly suspect.

[Thrumbleux]

Back to the misfire issue, are you sure it was a misfire and not the wastegate dumping suddenly, it can do this if one of the pipes comes off the actuator.

It's a misfire alright - no doubts there! EML is also on - which it wasn't when I had a sticky wastegate.
Starting on a plan to get the unit out and rebuilt. I'll wait for the compression test result (waiting for adaptor) before diving in though - just in case....

[Thrumbleux]

Still waiting for compression tester adaptor (previously ordered one not now available 'till Jan 2017 - another ordered, expected any day now) but given it looks like the motor is coming apart I'm trying to source the bits and pieces that I'll obviously need. Beginning with sealant.
What is recommended as sealant for the timing chain cover and cam box? I generally hate sealants but given I have little choice but to use, what is best? Do MB/smart dealers stock the same as was used in the original build, and if so should I go with that - or is there better? (Having had it fail on my sump and seen numerous reports of leaks from the cam box (mine also I suspect) is the MB stuff any good?)

[bob-in-dav]

My personal preference is Dirko for timing cover and sump - cam cover has a rubber seal/gasket

[Thrumbleux]

Thanks BID. I knew there was one previously mentioned and highly rated. I'll get hold of some.
I already have a rubber sump gasket from Evil which I'm expecting/hoping to re-use (unless I'm told it's a one use item) as it hasn't been on long and was well greased when fitted so should come off in one piece. Is the cam box gasket available from MB/smart dealer? Or is the existing one to be reused (I think mine is shot though) or is sealant the way to go?

[tolsen]

Best sealant IMHO is polyurethane PU40 only £4.04 for a 300 ml cartridge from a Toolstation depot. http://www.toolstation.com/shop/p85566?table=no

Available in both black and white. I have used this on sumps and all sorts of engine covers. Even good resistance to petrol. Perfect for sealing holed petrol tanks. Also perfect for repairing splits in drive shaft boots.

[Buckster]

Smart tune do a rocker gasket and I have seen them in other rebuild kits advertised elsewhere as well, I am pretty sure the timing chain cover is sealant though.

[Thrumbleux]

Cheers for sealant and gasket replies.
A bit of investigative work today reveals compression pressures of 50, 35, and zero psi. Cyl#1 is the one lacking any pressure and the only cylinder where disconnecting the coil pack has no impact on running whatsoever. A bit surprised to see such low readings. Unless the closed throttle affected the readings (unlikely) then I have to take them as they are.
Anyway, the motor is coming out ASAP and I'll see what has gone wrong inside the critter. The vibration present just prior to it failing is still a concern. I wonder if I'm going to find broken rockers. Good news is that I filtered the removed oil (through muslin) and no sign of shiny bits. More tomorrow...

[10:45PM]

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