Xu Nerds Required

[Kitch 4]

Hi all,

 

Looking for someone with geekier tendencies towards PSA engines than myself! I've got a 1985 BX 1.9 (carb/159A engine) here but on stripping it down have found that some clown has fitted an injection head. It's massively down on power (though this might not be head related of course!)

I've checked the block is indeed a 159A, and measured the valve head diameters at 39.4mm IN and 33mm EXH. Apparently this is the correct size for the 159A, but looking through the BX specs I can't find an injection version of the XU5 or XU9 which uses a valve that size. The injection versions seem to be the same as the 205 GTi, with inlets at 40.6mm or 41.something depending on spec. I wondered if someone has some literature or even knows off the top of the head what other injection XU heads might use identical sized valves to the carb-fed versions.

I did think maybe someone's fitted an injection head, but used the 159A valves (not realising they're very slightly smaller) meaning they won't shut properly. Would explain the loss of power (it's pushing 70bhp!) but the compression test results were decent enough. HAve checked the cam lift and it's measuring 10.2mm, which is about right.

 

I'm on the lookout for a XU9 carb-fed anyway, but wanted to get to the bottom of the current head mystery too!

 

Any help appreciated.

 

Rich

[Anthony 1,000]

Sounds like it's possibly the head from a XU9J1 / DFZ engine (105hp CAT equipped engine from a Gentry / GTi Auto etc)

 

Certainly that uses 39.5 and 33mm valves for the inlet and exhaust respectively.

[unariciflocos 49 1 Cars]

Based on the engine it came from, it may have a larger combustion chamber, which in turn will cause lower static compression, so lower power. You can calculate the compression ratio using some plexiglass and a seringe.

[DamirGTI 327]

Indeed , could be a XU9 DFZ head .. that one uses small valves and has huge combustion chambers , the biggest of the all XU heads .

 

Haven't tried measuring the chamber volume yet on these one , but they're over 40cc that's for sure (XU5 has 32cc and XU9 (D6B/DKZ) 34cc chambers) ..

 

I have a few pics of the two XU9 D6B and XU9 DFZ for comparison :

 

D6B on top , DFZ bottom

 

D6B

 

DFZ

 

D6B chamber

 

DFZ chamber

 

 

 

 

... even without measuring you can clearly see the difference between the two with regards to the chamber size and they're shape - DFZ head has "in line" cut chambers (which is unique for just DFZ heads) while D6B/DKZ has "ofset" cut chambers (as well as all XU5 heads) .

 

D

[Kitch 4]

Ah, that's interesting! I knew you guys would probably have it sussed. I'll go check it out, but either way it looks like I need a carb XU9 head as fitted to a 405 1.9 or BX19.

Guess the original head must have died and somebody threw this on it. Combustion chambers would go some way to explaining why it's down on power so much - the compression ratio has probably dropped.

 

Rich

[Kitch 4]

One more question.....I've sourced a carb head from a 1600 XU52C model. It's got bigger valves than the XU9, but the compression ratio of the 1600 engine is 8:95:1, not 9:3:1.

Does anyone have experience of fitting an XU5 head to an XU9 block, and would I get round any issues by skimming the head down to raise the compression back up etc? I like the idea of this head as the valves are as big as a 1.9 GTi, but I'm not sure what effect I'll have elsewhere. Obviously the bores are the same, so that's ok, but will I screw the compression up?

Thanks

[unariciflocos 49 1 Cars]

There's no way to tell how it will affect compression without measuring the volumes and doing the math. Get a small piece of plexiglass and fit it over the combustion chamger with a very thin layer of sealant around the bore mark, drill two small holes and fill with water using a seringe while keeping track of how much you're injecting until it's full. You'll need an extra hole for the air to escape. Take into account the volume inside the compressed head gasket and measure how much the piston is down the bore at TDC and also calculate that volume. Add combustion chamber + gasket + unswept cylinder volume and then divide the swept volume (1905/4 i believe) by this and that should give you your static compression.

 

This website maybe explains it better: https://www.rbracing-rsr.com/compstaticcalc.html After doing the math you can calculate how much you would need to skim off the head.

 

I skimmed 2 mm from a DFZ head and there was no valve clearance issue. This was to raise compression from 8.5 to about 11.

 

Compatibility wise it should fit without issue, even though the cam profile might be slightly different. Might as well fit a GTI head, they should be quite cheap. I binned a perfectly good one a few weeks ago.

[Kitch 4]

There's no way to tell how it will affect compression without measuring the volumes and doing the math. Get a small piece of plexiglass and fit it over the combustion chamger with a very thin layer of sealant around the bore mark, drill two small holes and fill with water using a seringe while keeping track of how much you're injecting until it's full. You'll need an extra hole for the air to escape. Take into account the volume inside the compressed head gasket and measure how much the piston is down the bore at TDC and also calculate that volume. Add combustion chamber + gasket + unswept cylinder volume and then divide the swept volume (1905/4 i believe) by this and that should give you your static compression.

 

This website maybe explains it better: https://www.rbracing-rsr.com/compstaticcalc.html After doing the math you can calculate how much you would need to skim off the head.

 

I skimmed 2 mm from a DFZ head and there was no valve clearance issue. This was to raise compression from 8.5 to about 11.

 

Compatibility wise it should fit without issue, even though the cam profile might be slightly different. Might as well fit a GTI head, they should be quite cheap. I binned a perfectly good one a few weeks ago.

 

Thanks, I was really just hoping that somebody would have already done it and learned the results!

 

Don't want a GTi head as it's no injected, so the ports don't match properly. That's the issue I have with the current one. The one I've found is from a slightly lower compression engine, and I just wanted to know if anybody knew what effect it would have on the engine I have....hoping someone had, for some reason, already tried!

[welshpug 1,626]

Depends if the compression is lowered due to a different head or the pistons.

 

Have a look on servicebox to see if the heads are different or not, i dont think they are.