Vitesse Cylinder Porting

[Wilts289]

Having been inspired by Darkspeed's porting adventures on /forum/viewtopic.php?t=9887&highlight=rover+cylinder+head,
at Christmas I duly asked for (and received) head stand, porting kit, burette and a pair of Rover heads.(still got the hankies and socks though)
As it turned out, the heads were from a Vitesse so some of the good stuff had been done already, i.e. valve throats a bit bigger, waisted stem valves etc. The main thing that concerns me however is that as the valves are recessed into the head by approx 120 thou compared with normal SD1 heads, how does this affect the dreaded tappet pre-load problem, i.e as valves are recessed 120 thou, they protrude out of the top of valve guide an extra 120 thou. which must effect the preload clearances substantially!!
The second question also relates to the recessed valves, should I chamfer the edges of of the recesses?

p.s engine in question is a 4.2L with Crower 50230 and 500cfm Edelbrock

[kiwicar]

I think your biggest issue with recessed valves is that they will restrict flow, if that is 120 thou recession from standard there is something seriously wrong. Think about it 120 thou is effectivly removed from your valve lift, that is a lot! I would strip the heads, investigate and probably renew the seats and valves, because that much recession is far too much.
Best regrads
Mike

[sidecar]

I think your biggest issue with recessed valves is that they will restrict flow, if that is 120 thou recession from standard there is something seriously wrong. Think about it 120 thou is effectivly removed from your valve lift, that is a lot! I would strip the heads, investigate and probably renew the seats and valves, because that much recession is far too much.
Best regrads
Mike

Hi Mike,

It's not valve recession that has pulled the valves up 120 thou, Rover built some of the heads like that for some reason! (Which escapes me at the moment).

I don't know how Rover got round the fact that this would have an affect on the pre-load, I suspect that they just did not worry about it!

The best way of getting the pre-load spot on is adjustable pushrods (Other than adjustable rockers), even then the three engines that I've had a hand in building with them have had to have the pushrods shortened before the ball ends were fitted.

All part of the fun!


Cheers,

Pete

[davemgb]

It's not valve recession that has pulled the valves up 120 thou, Rover built some of the heads like that for some reason! (Which escapes me at the moment).

Maybe Rover could then use those heads as a basis for competition work with larger valve lifts but not pockets in the pistons or shortened guides?

Dave

[stevieturbo]

If the valves are recessed that much, fit new valve seats.

Otherwise the heads would be considered scrap IMO.

[badger]

Guys,
1. It isn't 120 thou, it's 1.2mm!! 120 thou is 3mm, they ain't recessed that much.
2. 1.2mm at the valve is 0.75mm (30 thou) at the follower with a 1.6:1 rocker arm ratio.
3. To the best of my knowledge, Rover never altered any other parts, they must have reckoned that the hydraulic followers had sufficient operating range to cope with the change..?

I seem to remember a reason given in Hardcastle's book, but it escapes my mind at this point tin time, as does the whereabouts of my copy!

[Wilts289]

to quote Hardcastle's (I'm keeping the apostrophe!) Tuning Rover V8 Engines book, (page 20 in my copy)

"..Vitesse heads are superior to SD1 type but are from same casting. The valve diameter is the same, but the valves have waisted stems and are machined behind the valve head to increase flow, which they do successfully, the valve throat has been machined to increase the diameter some 40 thou, but more interestingly the the valves have been recessed approx 120 thou into the combustion chamber, restricting low valve lift inlet flow but assisting overall exhaust flow"

answers on a postcard!!!!

[stevieturbo]

Sounds like a mistake. There is no possible way it could assist flow at low or high lift if they are recessed anywhere near that amount.

And bigger ports are not always a good thing in terms of making power.

Some good chat recently over on PH.

This thread led to a couple of other good ones

http://www.pistonheads.com/gassing/topi ... ve+jobs%3F

[kiwicar]

Hi
To repeat myself, put new seats in and get the valves back to the face of the combustion chamber, rover may have chosen to reduce the flow on the inlet side by cutting the seats 120thou deep to in some way ballance the flow when compared to the exhaust (probably to reduce unburnt hydrocarbons in the exhaust and a site cheeper than a new cam profile or improved head casting) but despite how hard Hardcastle may try and dress it up in "Woo Woo Magic you mere mortles will not understand unlike megga clever me. . . . " it is still reducing flow which reduces power, not what you are after.
Best regards
Mike
PS sorry I find the man an arogent condersending TW@

[Wilts289]

Stevie,
went to that link and spent over an hour following it. Immediately cancelled my order for valve seat cuts at the local engine shop! Have to say it makes slightly depressing reading for base level people like me that often just pay up in the belief that we are dealing with so called experts.

[sidecar]

Hi
PS sorry I find the man an arogent condersending TW@

I also think that his book is a load of crap!

[kiwicar]

Hi
There are some very simple "rules" that govern flow in a port,
Firstly a port will only flow as much as it's greatest restriction will allow it
At high fluid speed laminer (smooth) flow offers the least resistance.
At low very speed turbulant flow offers least resistance.
The transition from one sort of flow to the other is often highly chaotic and it is often better to get the port to operate in one ragime for longer if you can just so that it behaves more consistantly, and that what you do to help laminar works for longer as it may spoil turbulent flow.
Even if the port is flowing at near supersonic speeds in the centre the layer of fluid (air and fuel droplets) near the surface or the port wall is pretty much stationary (this is directly relevent to surface finish).
The narrower the port the higher the velocity through it for a given volume of flow per minute.
Resistance to linear flow goes up at the square of velocity
Resistance to turbulent flow goes up at the rate of the third power of velocity.

From the above, at high flow speeds the greatest restriction to flow can and often is an area of turbulant flow, not necessarily a phisically narrow part of the port.
Sudden chances in cross sectional area or changes in direction of flow can cause the flow in the port to become turbulent leading to a restriction in flow.
From the above you can see that the anything that delays turbulent flow forming will help flow overall and especially high speed flow. . . therefor the valve shape and proximity (on the port side) can have a great effect on the overall flow of the port by keeping the flow into the chamber smooth and non turbulent, however if you prevent the port opening rpoperly by having the valve stuck half open it will cause too much restriction and choke off flow.
It is not magic, all of this stuff is simple but you do need to need to look beyond the "black art mythology that grows up around it all.
There are further things to consider here but thay are just simple principles of design that use the above rules.
Bst regards
Mike