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MG MGB GT V8 Factory Originals Technical - Porting, Why not shorten the valve guides?

Does anyone have a clue why the valve guides areŽnt shortened, while porting, flush with the roof in the exhaust port. The BOP Rover guides are so much longer than the guides in a small block Chevrolet and those are grinded flush while porting the exhaust bowl.
It seems to me there would be alot of flow potential in that area of the Rover head.

Cheers
Erik
Erik

Erik,
The valve guide is there to keep the valve in place and resist the side loading forces imposed upon it by the action of the rocker as it moves up and down.

The other main function of the guide is to provide a heat path for combustion temperatures going into the valves.
The exhaust valve is of course hotter but both valves are exposed to the combustion process.

Shortening the valve guide will lessen the area available for heat flow and also decrease the surface area resisting side loading.

It may be possible to add a small aerofoil shaped piece ahead of the valve guide where it extends into the flow and improve the flow that way. I have not tried it but others may have done.

Cheers , Pete.
Peter Thomas

Peter and all,
Thanks for your comment and I understand the the need for the cooling and guidance tasks of the valve guides. But I still wonder if one uses stainless steel exhaust valves which are less heat sensitive. What do you think? I also reckon, since the small block Chevy has short valve guides and often ground away in the bowl on ported heads, why wouldŽnt it work on a BOP/Rover head?

Cheers
Erik
Erik

Erik,
If it is a common modification on the small block Chevy I see no reason why it would not work on the BOP/Rover engine.
Good luck with it all.

Cheers , Pete
Peter Thomas

Erik,

Firstly are we talking about road modified or full race as the requirements and therefore modifications are totally different.

If it's for a fast road engine you do not need to remove the lower part of the guide or the cast area in the port as it is there for a good reason.

The usual mods are to install bulleted guides or modify the existing guides to the same spec, if fitting a figh lift cam you may need to machine up to 100thou. 2.5mm, off the top of the guide to give clearance for the additional lift and fit appropriate valve springs matched to the cam.

The biggest increase in flow for a road head comes from opening up the valve seat to actual seat size and then blending in the port bowl area below the valve heads. and preferably fitting waisted stem valves if you do not already have them.

This relatively simple mod will yield an additional 30+ BHP on a 3.5 engine without even a change of cam.

If you are looking at more extreme mods I recommend getting a copy of Des Hamill's How To Power Tune Rover V8 Engines.

Kevin.

Kevin Jackson

Kevin,
Thanks for your reply. I recieved the heads from the machine shop yesterday. They have had a full race treatment. The tuner builds heads for the drag racing comunity and had the full treatment of the valve seat as you suggested. He said they were radiused,guess he meant hte wery smooth transtion from the seat to the seat bowl. And yes the guides were shortened 2.5mm and blended in the port as per hes modifications of the chevy small block modifications. Too late now for second thouhts. Valve springs are one coil less and valves are waisted stems on both in and ex. Exhaust is stainless.

We will see how it works.
I could send pictures if you like.

Erik
Erik

Erik

The head mods sound about right, do post pictures if you can.

Did you retain standard size valves.

Kevin.
Kevin Jackson

Kevin,
Yes I kept the standard valve sizes, maily because I have a 3.5 block and was worried about shrowding of the valves. I donŽt know if that was wise or not. The initial plan from me wasŽnt a full race port job but the guy doing the ports got carried away. He has got a good reputation, as mentioned earlier, in the rally cross and 1/4 mile circles. IŽll post post pictures during the weekend if you can tell me how to upload them.

Erik
Erik

Erik,

Just store the pictures on your computer in the normal way in MY Pictures.

At the bottom of the panel for adding a post is the upload a picture facility, you can only do one at a time, then once loaded click submit as usual.

Kevin.
Kevin Jackson

Kevin,
The link for uploading is only visible, when logged in as a member. Pictures will come during the weekend.

Cheers
Erik
Erik Kallstrom

Many years ago, I cut back the valve guides on my MGA head while doing a bit of port enlarging and improving.

Don't do it.

It seemed like a logical thing to do to get the flow up, and that it probably did, but they just don't support the moving valve very well when cut back to the head. The valve rattles around and soon wallows out the guide seal area and then you get a well lubed upper cylinder area. Not to mention the loss of heat removal from this area...I was down to a quart of oil every couple hundred miles before I re-pulled the head and pressed in new guides..Bronze types too.I was younger then and thought the factory too conservative too, but they are correct in saying don't mess with the guides on the "B" block heads. Get a cam with monster overlap insted.
Wayne Hardy

Kevin, Here comes a row of four postings whit imagaes,

Erik

Erik Kallstrom

Second picture.
Erik

Erik Kallstrom

Third

Erik

Erik Kallstrom

Fourth,

Erik

Erik Kallstrom

Erik,

Looks like he's done a nice job, difficult to see the transition from the valve seats to bowl in the picture with the valves in an open position, if you haven't assembled them yet another pic with the valves out and plenty of light would be nice.

Also he does not appear to have over done it on the prot runners which is a common mistake on a road engine as it robs power at low RPM's

Kevin.
Kevin Jackson

Kevin,
The engine guy seemed to know hes way around with the american constructions. I will take pictures tonight.

If youŽre in to Yank iron, have a look at this.

http://www.bigmeet.com/

Erik
Erik Kallstrom

Kevin, at your request.

Erik

Erik Kallstrom

Erik,

Thats exactly how they should be done, you'll like the way the engine runs.

Kevin.
Kevin Jackson

This thread was discussed between 20/02/2009 and 12/03/2009

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