8 Valve Xu Head Limitiations, And Other Engine Bits

[welshpug 1,657]

A little bored given the crappy weather, and mindful of the mileage my 205's original engine is racking up, I was thinking of building up a higher compression 8 valve lump for my 205, 1.9 with 1.6 pistons, and am wondering what the limitations of the standard top end is, and would standard bottom end parts work as a budget build without grenading given the 8 valve tends to produce its power at lower rpm?

 

the questions I have are as follows:

 

at what point does the head need work, would I benefit significantly from doing so at a fairly low level of tune? IIRC I do have a lightly tickled 8 valve head in the shed already, the exhaust ports have been smoothed up a bit

 

I am thinking as with most pug engines and going from what I have seen of Sandy's engine builds is that the valves are plenty big enough (though I'm off to read this topic again shortly... http://forum.205gtidrivers.com/index.php?showtopic=31290&st=0 )

 

a quick working from memory of dish and chamber volumes indicate that I'll be around 10.8-1 with an unskimmed head, no doubt the head I have has been skimmed in the past.

 

what is the max lift for std valve gear? I take it that RPM is more a bottom end concern given the tappets are solid as standard.

 

for the bottom end, I am thinking as mentioned of some XU5JA pistons as they have a smaller dish and bump the compression, some standard 1.9 rods checked for weight discrepancies.

 

I do have an XUD9TE crank to hand which is reputed to be forged steel.

 

I have an XU7 block to hand, and an XU7 oil pump, as well as XU9j4 10J4RS and XU10J4 oil pumps.

 

Camshafts, not sure what I could use, may depend on limitations of standard valve train, IIRC I do still have a PTS max Torque cam if it has not rusted away in the garage

[welshpug 1,657]

hmm, read that big valve thread, and none the wiser, I'm not building a top spec engine so stock it is

[petert 586]

Why not use Mi16 rods and pistons? Lighter, full floating. The standard valve seats are great for reliability but poor for performance. As a minimum, I'd do a 3 angle grind and blend into the ports. Smoothing exhaust ports is just therapy for you. Time is better spent blending the inlet seat into the port. Max lift is around 0.460-0.465", if you want 0.007" between each coil at full lift. If you don't have 11:1, I wouldn't use the Grp A cam.

[welshpug 1,657]

Purely because a member on here has a set of new xu5 kit of liners and pistons.

 

What would the xu9j4 piston give under an 8v head (d6c, dfw almost non-existant here)

 

Whats the benefit of being fully floating? I guess that its not a contributing factor to the mi issues given that an mi running 8v rods still knocked out a shell.

Edited October 11, 2012 by welshpug

[petert 586]

A D6C Mi16 piston gives approx 11+:1. The dish in the XU5 piston is approx 9cc, compared with approx. 4cc for the valve pockets in the D6C piston. The benefit of full floating is mainly easy of assembly.

[welshpug 1,657]

Fair enough, thanks for the figures, just remembered I have those in the garage...

 

Will dig that head out and get some pics.

[pug_ham 244 3 Cars]

A little bored given the crappy weather, and mindful of the mileage my 205's original engine is racking up, I was thinking of building up a higher compression 8 valve lump for my 205, 1.9 with 1.6 pistons, and am wondering what the limitations of the standard top end is, and would standard bottom end parts work as a budget build without grenading given the 8 valve tends to produce its power at lower rpm?

This is pretty much the spec of my budget build engine from this topic bar with a Piper 272' cam & a slight head skim from the previous engine.

 

The few miles I did with it fitted showed a nice torque spread & decent power but if it was an improvement over before I wouldn't like to say, although it did drive very nicely just too long between driving old spec & new to honestly compare.

 

g

[welshpug 1,657]

Had a look at the MI pistons I was donated by DCC, they're buggered I think, quite heavy wear on the thrust sides.

 

I do have some flat top SRI pistons as it happens, even more compression

[Baz 421]

IMO headwork is a dark art, ones that look good often never make the numbers and all the Longman heads i've seen in depth don't ever look that wild, yet make very good power.