Back on the road

[Matt H]

Hi all,

At long last, I am back driving a BX again

Has been a nasty few months with out a car, but I finally got my 14 back from having the head gasket done. The head needed skimming, and I have to say it seems quite a bit quicker as a result. I will have to run it on 99 Octane now of course, but I don't mind. When I get a twin choke carb on it, I imagine it will really fly... In relation to other BX 14s that is

I wouldn't mine testing it up against a 16 of some description. I bet there wouldn't be much in it.

It's late and I'm tired, so I'll be back tomorrow, maybe with pictures if I wash it, and the rain holds off... although I doubt it will.

[DavidRutherford]

I will have to run it on 99 Octane now of course

How do you come to that conclusion?

[Tim Leech]

Well done matt welcome back!

[Matt H]

I will have to run it on 99 Octane now of course

How do you come to that conclusion?

Well, with the bit of 95 that was left in it, it was pinking... filled it up with 99 and it was fine.

Would probably be okay on 97, but I live so close to Tesco, putting 99 in doesn't really make that much difference in price.

[BX Bandit]

Interesting fact: The higher the octane rating of petrol, the slower it burns! I expected the opposite!

[DavidRutherford]

Well, with the bit of 95 that was left in it, it was pinking... filled it up with 99 and it was fine. Would probably be okay on 97, but I live so close to Tesco, putting 99 in doesn't really make that much difference in price.

Sounds to me like the timing hasn't been set correctly after the cylinder head has been removed and refitted. The increase in static compression ratio caused by skimming a head is incredibly small. So small, there's no way it could possibly have changed the characteristics of the engine that much. The difference between a high and a low atmospheric pressure day will have a massively greater effect.

I'd get the timing re-set and use normal unleaded petrol. The timing being out as it is may be causing (inaudible) high engine speed pre-ignition, which will eventually cause damage.

[Way2go]

I would say that it's the timing that needs adjustment too rather than higher octane petrol.

[Kitch]

My head's been skimmed at some point, and like Matt I have to use high octane jungle juice or it'll pink itself mad all day long at lower revs.
Only difference is that valvers were only ever designed to run 4-star (apart from the weiny catalysed one), so 95 isn't really high enough to use anyway, although many do with mixed results.

With regards to the car, the 14 isn't as far off a 16 as some peple would like to believe, so I doubt there would be enough difference in performance to discuss! Bigger difference would be tax costs, fuel consumption and ease of maintainance, all of which the 14 will have the 16 licked all day long. Only thing the 16 has over a 14 is refinement IMO.

[Matt H]

Well thanks for the advise, I'll get the timing checked. It really seems fine running on 99 for now, but I want to be sure.

As for the skimmed head not making any difference to compression, I might just be misinformed... but my car really is quicker than it was, as was my dad's 16v when the head was done on that. Maybe my engine is just running a lot more efficiently now? The emissions are certainly a lot lower than they were a year ago.

[DavidRutherford]

As for the skimmed head not making any difference to compression, I might just be misinformed... but my car really is quicker than it was, as was my dad's 16v when the head was done on that. Maybe my engine is just running a lot more efficiently now? The emissions are certainly a lot lower than they were a year ago.

The emissions and the performance are definitely nothing to do with any change in compression ratio, and everything to do with the fact that the upper cylinder is now clean, decarbonised and working well. No doubt you've also had a new set of spark plugs, and maybe a new air filter? That will be the difference.

Think about this... if you take 10thou off a head (which is a good skim) that's 0.25mm removed from the valve area only. The face of the head is still in the same place, just cleaned up now. If you have 35mm valves (quite big) then you've lost only 0.5cc from the cylinder. Given that the cylinders are 340cc each, that's a reduction of just 0.14%.

Look at the difference between a high atmospheric pressure day and a low one, that's the difference between 980mBar and 1020mBar (ish), which is a difference of 4%. Makes the difference in compression ratio seem a tad insignificant doesn't it?

[Kitch]

As for the skimmed head not making any difference to compression, I might just be misinformed... but my car really is quicker than it was, as was my dad's 16v when the head was done on that. Maybe my engine is just running a lot more efficiently now? The emissions are certainly a lot lower than they were a year ago.

The emissions and the performance are definitely nothing to do with any change in compression ratio, and everything to do with the fact that the upper cylinder is now clean, decarbonised and working well. No doubt you've also had a new set of spark plugs, and maybe a new air filter? That will be the difference.

Think about this... if you take 10thou off a head (which is a good skim) that's 0.25mm removed from the valve area only. The face of the head is still in the same place, just cleaned up now. If you have 35mm valves (quite big) then you've lost only 0.5cc from the cylinder. Given that the cylinders are 340cc each, that's a reduction of just 0.14%.

Look at the difference between a high atmospheric pressure day and a low one, that's the difference between 980mBar and 1020mBar (ish), which is a difference of 4%. Makes the difference in compression ratio seem a tad insignificant doesn't it?

Agree with what you're saying, but wouldn't the design of the head have a bigger variation on the result of the skimming? If the car had domed pistons (which a normal TU3 doesn't) and a Hemi-spherical combustion chamber (which again I don't think it does, but bear with me!) surely it could make some noticable difference to the performance?
Obviously if the head has a flat valve area it will make precious little difference, but surely different designs of head that can flow air alot quicker could benefit?

[docchevron]

Very little, unless you skimmed ALOT off, even then the gains would be barely negligable.

Removing carbon will be the biggest benefit in this case.
Although if the valve seats have been recut then that will help too, ever since the demise of good old leaded 4 star I've not had a head off without needing to cut and lap the valves, the exhausts take a right battering, even with hardened seats.
But I dont know if Matt's head has been done?
Although anyone with any sense would at least lap the valves whilst the heads off..

In this case it sounds absolutley like the timings out, I've skimmed LOTS of TU engines, well, lots of engines in general actually, it it has no effect whatsoever on how it runs.

[DavidRutherford]

wouldn't the design of the head have a bigger variation on the result of the skimming? If the car had domed pistons (which a normal TU3 doesn't) and a Hemi-spherical combustion chamber (which again I don't think it does, but bear with me!) surely it could make some noticable difference to the performance?
Obviously if the head has a flat valve area it will make precious little difference, but surely different designs of head that can flow air alot quicker could benefit?

Yes, you're quite right of course. My reference in this case was entirely to the TU engine. Take for instance a jag straight-6 engine which does have quite massive combustion chambers in the head. When you're trying to recover a 40-year-old corroded-to-buggery head, you often have to take a fair swipe off the face. Best to date was something like 70thou, which is the better part of 2mm. That starts making some difference to the compression ratio, and you have to start doing clever things like measuring the cc of the combustion chamber and calculating the CR quite accurately, relating in valve timing and crankshaft geometry to be absolutely sure. (and then machining a load off the face of the piston when it's all wrong)

However, when you start modifying an engine that much the CR is not the worst concern. removing 2mm from the overall height of the engine means the cam timing is all shot to hell, so you have to start fitting vernier pullies, or altering the cam chain path to get the cam timing correct. You've also got numerous other issues with alignment of bits and pieces and it's all a right pain in the posterior.

...none of which matters when you take 10thou off a flat head of course (thankfully, otherwise a blown head gasket on a BX would be a lot more expensive than it is)

[Matt H]

Makes very interesting reading all this... Just out of interest, how much would you have to lower compression before Turbocharging becomes safe? Don't worry, I'm not planning anything, I just wondered... I know you have to lower it quite a lot on an A-series.

The valves were done, as the Doc says it would be quite daft not to, and the valve stem seals were changed... Just a general 85,000 mile overhaul.

[DavidRutherford]

Just out of interest, how much would you have to lower compression before Turbocharging becomes safe?

That depends on a number of factors. What pressure you want to run from the turbo, what the cam timing is, the crank geometry is, the cylinder head temperature, the octane of the fuel you want to use, , (and a few other less important factors)

The issue of fuel octane is an interesting one. I'm currently involved with the possibility of running spark ignition engines on bio-methane, (essentially similar to natural gas, but in this case carbon negative) which has an octane equivalence of 135(ish). This means the possibility of either running very high compression ratio engines (like 15:1 or more) or simply slapping a turbocharger on to a normal 9:1 engine. Either way, you can get much better thermal efficiency than with 85/95/100/whatever octane petrol, and it's much cleaner burning too.

Generally, most turbocharged petrol engines have CR's of between 5:1 and 7:1 or thereabouts.