X flow compressions

[Midnight]

My car has begun to blow smoke from the exhaust when either accelarating hard or under sustained high revs (ie flat out in top) I did a compression check on the 1700 Xflow and it looks like I have a large variation in the readings, which are :

1. 250 psi - 2. 220psi - 3. 240 psi - 4 200psi So much more than the usual + - 5psi varience

A drop of oil into each cylinder sent the readings off the gauge on all cylinders (The gauge is a Gunson Hi-test and readings on it stop at 300psi).

So it looks like I've some work to do this winter, I know its hard to tell, but in view of the relatively high readings whether I'm wondering if I could get away with with just new rings or whether an engine out and a rebore and new Forged pistons are called for. Any advice appreciated.

Ps what is also strange is that the oil catch tank never fills up, yet after parking up a patch of oil appears under the breather to road exit pipe. car does use a little oil as well.

[Graham Perry]

Those numbers seem very high for a 'sick' engine My 12:1 compression ratio xflow when fresh produced numbers like that

[DW]

How did you do the test? Throttle needs to be open when you're doing it....

 

David

1700XF: click here

[DaveMorris]

If throttle fully open then the pressure values would be higher than if closed - so I presume that you did this when throttle fully open.

 

Concur with Graham that the numbers are v.high - would expect something around 180 for standard(ish) crossflow running normal CR.

But the actual numbers are not too important - as you say it's the variation between them that matters. On that basis it doesn't look too good, especially with the symptons you mention aswell.

Is the engine a typical caterham s/sprint or has it already been upgraded to "real" 1700cc with forged pistons?

How many miles ?

 

Dave

 

X-flow(er) powered

 

Ooops - now read your post properly so can see it's got standard pistons. Would re-check pressures with another comp tester before deciding that just a re-ring is the route forward.

Oil catch tank will only collect any oil which condenses in it. Unless you have something to help it condense in there then this is unlikely to happen (especially if there's a fair amount of breathing going on). The few drips you see are likely to be stuff that has condensed in the vent pipe and then dripped out the end whilst parked.

 

Edited by - DaveMorris on 18 Dec 2006 18:25:26

[Roger King]

I would re-iterate Dave Morris' suggestion about even figures being more important than the absolute value.

 

Ultimately, the absolute value is important, but unless you spend a lot of money there is a HUGE variation in readings between gauges. If the test was done with open throttle it is likely that the gauge is over reading. If the test was done with closed throttle, throw the gauge away!

In fact, if a little oil down the plug holes causes figures over 300psi, it is almost certain that the gauge is rubbish.

 

With an accurate gauge, I would expect even figures between 180psi and 230psi on a healthy Crossflow tested with open throttle. Factors such as compression ratio, cam duration, cam timing, carburettion, etc, will cause differences so there is no absolute right answer.

 

It is quite common with standard Crossflow pistons for the oil ring performance to deteriorate to the extent that oil is allowed onto the compression rings and this can actually cause the figures to get better. The fact that there is visible smoke under the circumstances described makes it likely that rings are a problem. Valve guide wear is also a possibility. Both guides and rings are a problem on a Caterham Supersprint because less than ideal materials were used, although the origin of this engine isn't mentioned.

 

It is ill advised to re-ring the pistons because they are notoriously weak and there is a strong chance of a breakage in the ring land area if they've done any sort of mileage. Ideally bore the block out and fit forgings (these will be more reliable and also release around 8bhp thanks to lower weight and running friction), but at the very least fit a new set of castings.

 

If the breather is exiting from the cylinder block onto the road rather than into a catch tank, you are simply not seeing how much oil is being chucked out, because believe me, a sick Crossflow WILL chuck it out.

 

None of the above is really the piston's fault. It is simply being asked to perform under much more demanding conditions than it was designed for.

[Midnight]

Hi Guys and thanks for the advice. The engine is not standard Caterham Supersprint, and already has forged pistons fitted into a complete brand new AX thickwall block engine (at the time). The engine was built by Steve Parker with a lot of upgraded internals including a cam ground to Steve's specification and even now at 30.000 miles has been reliable, (across America without a hiccup) Has an extremely hard cutting exhaust sound that many people have commented on as being different from other X-flows, and is an absolute corker. Rolling road at Marshalls of Cambridge a few years ago after the engine got its last rebuild gave slightly under 158BHP top power reading. Performance wise, the only thing I've really noticed is the smoke. and the oil, about a two inch wide patch after each run coming from the oil catch tank breather pipe (although the tank remains empty bar a half inch of oil in the bottom. If a rebore is needed then its new forged pistons at what cost. Seriously wondering if I should bite the bullet and go for a Zetec but I would miss this engine and its character.

[Roger King]

If you already have forgings, the situation is different.

 

You should crack-test them and if they are OK (very likely), a simple re-ring job should suffice. I am assuming miminal bore wear here, but the AX block is much harder wearing than the 711m so you have a good chance.

 

Whatever you do though, have the bores properly glaze-busted and left with a plateau honed finish. This makes a huge difference to ring performance.

[Midnight]

Thanks again Roger, your advice is much appreciated!