head mods

[helsbymgtf]

I have a vvc 2004 yr and wondered if any one has had a head mod done on one or any info regarding same

[steve]

John you need to contact Dave Andrews - he is without doubt THE K series cylinder head mod guru. Check out the following link to his website.

www.dvapower.com/

[sniperpenguin]

.... and the first thing he will do (if your serious) is strip away the VVC mech, surprisingly

[Debs]

Another alternative is Roger Fabry at Sabre Heads.

www.sabre-heads.co.uk/1.html

Roger was the guy responsible for the 'Motorsport Head' (now marketed by PTP as the 'Vulcan Head'). He was also responsible for the 1400cc turbo MGF Bonneville racer.

You don't have to strip away the VVC for gains in power. However for serious gains you go to solid cams because the VVC inlet cam(s) is compromised in the degree of valve lift. With the stock 80mm bore the valves are shrouded until they are lifted well off their seats. TBH the same hapens with the 82.5mm bore Judd and Scholar engines which is why I am going to an 84mm bore.

That said, going to solid cams is expensive because you need blanking plates as well as the fact that the VVC inlet cam has a larger journal diameter than the exhaust cam.

[helsbymgtf]

many thanks gang, I would keep the vvc mech, to my way of thinking ,it is a good system for a road car.
on looking at cost it is not just a head as I would have to replace my pistons(why u have to replace windscreen washer I don't know) as they are only good for 185 bhp (a friend with a lotus found this out to his cost)

[Debs]

It's not so much that the stock pistons are only good for 185bhp (as is often quoted), but that the pistons can't take excessive rpms and break up at around 7500rpm. For eg, my motor puts out 240 bhp on NOS quite happily but I don't rev above 7000.

In this respect the TF 160 pistons are better than the rest of the range because they are stronger.

However, even with forged pistons, failures are still common at around 8000rpm.

The reason for this is that the K Series motor was never originally designed to be a 1796cc unit (it was originally an 1100cc unit). In order to stretch the engine to this capacity it was initially over-bored (1600cc), but more importantly stroked. That is to say a longer stroke crankshaft was fitted.

Whenever you stroke an engine, while you inevitably produce more torque (because you have a longer effective lever arm), you run into problems with higher rpm and it is rpm you need for power (in normally aspirated engines) because bhp = torque x rpm.

Firstly, the longer you stroke a motor, the more under-square (bore : stroke ratio) it becomes (the 1796 K is 80mm bore x 89.6mm stroke), and the less willing to rev.

Secondly (and more importantly) if you use a longer stroke crank in an otherwise unmodified block then you have to shorten the con rod (else the piston will hit the cylinder head). This reduces the con rod : piston deck ratio and causes 2 major problems:

1. A short con rod, long-stroke, motor such as the 1796cc K Series develops rather extreme rod angles as the pistons move up and down the bores. This produces excessive sideways thrust on the pistons as well as a rotatory torque action applied to the piston. This in itself can lead to the development of bore ovality and (in extreme cases) piston failure.

2. A reduced con rod : piston deck ratio (and believe me it's well short on the K, in fact it borders on the limit of acceptibility in a tuned motor) leads to excessive piston accelerations away from TDC and BDC. These acceleration cycles can weaken the piston skirt (especially around the ring lands). Obviously the shock loadings increase (by a factor of a square) as the rpms increase and it is this that causes piston failure at high rpm (classic High Cycle Fatigue failure).

It is for these reasons that the motor I am developing will have a destroked custom steel crank of 88mm stroke, larger bore 84mm forged pistons, and longer custom steel con rods (with a block to head spacer plate). That way I can increase capacity to 1951cc, have a motor that is more willing to rev but, most importantly, have a motor that will produce reliable rpm.

[helsbymgtf]

thank you once again debs I did think 185 was a bit close giving rover had made the vvc 160 with only 25bhp tolarance.(now some one will say the 160 only makes 150bhp)
as for rpm 7200 with lots of torque will suit me as a road car needs torque not bhp because you can't drive around at 6-7thou rpm IF my engine started out with 155bhp it should now be around 180

[jupilerman]

nice explanation Debs. How do you manage 84mm bores? Is this within the current tolerance of the liners? surely not?
Problem for some of us (at least for me) is the quite big figure (£) for conversions like that. Steel crank, one off pistons and rods. Although I believe that in i.e. wisceco´s catalogue you could find some forged pistons for other cars with that bore and appropriate gudgeon pin/top o/piston sizes. Better price that a one off IMO.