Xu9 8V Flat Top Pistons In Xu9J4 Engine?

[wld 3]

Hi. I'm a long time member but never posted so far.

I'm starting to rebuild a BX16v (DFW) engine with mild mods and a Peter Taylor stage 1 cam.

I found out that some XU9 engines use a flat top piston with 37.5mm compression height. The old carburetted XU9S (159A) and also the XU9J2 (D6D) 125HP from 405SRI/BX Gti use these.

Is there a reason that they should not work in a XU9J4 after machining the valve relief pockets in them? Of course, they must be used with the 8v conrods.

37.5mm compression height means the top of the piston is flush with the deck, so ideal squish height and around 10.8 static compression so It could be the cheapest way to get high comp pistons for a 1.9 mi16

 

[petert 586]

That's correct, 8V pistons don't have valve reliefs, aren't full floating and heavy. Thus expensive to make work with 16V parts. The cheapest and easiest way to increase the CR is use a 0.7mm Cometic gasket. Alternatively, skim the deck and liners.

[wld 3]

Wouldn't the 0.7mm gasket get a DFW compression up to only 10.2 or so?

Beside weight, what are the main disadvantages of the press fit pistons/rods?

A mate has an XU9S laying around and I can get the pistons and rods for free.

[petert 586]

How will you cut the reliefs?

[wld 3]

I'll have to ask the guys from the machining shop. I know they modify valve reliefs for bigger valves on some cars. The skirts may need some cutting too, for the oil squirts.

In my neck of the woods machining is still affordable, as long as it is not something too complicated. They skim/deck iron blocks all the time, but are not so keen on doing liners.

So, any technical reason this would not work providing the valve reliefs and skirt mods are done? Are the pistons and rods able to sustain the higher revs needed? Redline on a SRI engine is ~6400rpm

Of course, first I'll have to compare weights.

[welshpug 1,657]

with all the work and faffing involved, I would rather just buy some new MI16 higher compression pistons.

 

XU9 pistons as mentioned are very heavy, likely very worn these days, you'll need new piston rings (which come with the new pistons anyway)

[petert 586]

I would charge at least $200 to cut pockets from scratch. Then you've got notches for squirters. I really can't see how it's feasible. Even if you bought some 2nd hand D6C pistons and new rings, you'd be approaching the price of new pistons.

[welshpug 1,657]

that's close to what guys here charge to make standard gti6 a bit deeper.

[wld 3]

Just spoke with the guy i know from the shop. He will charge me 50-60 euros to modify the pistons. normal price ~100euro

Forged pistons will be around 600 euro.

Already got set of new Goetze liners and rings for ~150EUR

Probably my best bet will be to find someone to deck the block/liners instead of going the 8v piston route.

[petert 586]

Even decking the liners will be expensive. You need to mount them on a mandrel in a lathe and face them to length. Time consuming and time equals money.

[wld 3]

I know, my guy said he may be able to do it but has to build a fixture just for this and can't be bothered.

Peter, I'll be using a Stage 1 reground cam bought from you by a friend of mine several years ago, and never used. After decking ~1.3mm and using this cam, the valve pockets of DFW pistons must be altered or there is enough clearance? Also, what pulleys should be used? #4 (intake) and #2(exhaust) or another combination and offset keys?

Thanks.

[petert 586]

You can take 1.3mm off the deck and leave the pockets untouched with that cam. Use a #3 inlet for best results. Retard the exhaust cam if you can with an offset key, otherwise leave it std.

 

The depth of pockets is the same between DFW and D6C pistons, relative to the gudgeon pin. ie DFW piston = D6C piston - 1mm off the crown.

[wld 3]

Thank you!

[wld 3]

I took apart the XU9S engine and weighed pistons+rods+rings. The 16v ones were with pin and circlips, for a correct comparison.

110g difference: XU9S=1274g vs XU9j4Z=1164g

I forgot to note them somewhere, but I'll weigh them again in a few days and take pictures.