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MG MGB Technical - Head porting and Carb questions

First post on here so here it goes, I've done a quick search as well as to hopefully not go over an old topic!

I'm considering porting, polishing and gas flowing the head on my B.

First of all how far is it possible to go with gas flowing the head? I have a Bell stainless exhaust fitted with a tubular manifold, would it be possible to match the exhaust ports to the size of the manifold?

Also is there anything that can be done to improve the standard inlet manifold? To my un-trained eye it looks perfectly fine but I'm sure there are always improvements to be made!

In terms of carbs I may be able to get hold of some 1 3/4 SUs from a Dolomite sprint. Will these fit? Will I need to modify the inlet manifold at all? And of course what do you do about the linkages between the carbs and the throttle + choke cables?

I think that should do for now!

Kieren
K Goldup

Ah, my favourite subject! And as to how far you can go with porting, well you can break into water ways, oil galleries, and in my case pushrod tubes! But seriously though, it is fun and satisfying, would you not agree Peter? Get youself Peter Burgesses' books 'How to power tune the MGB engine' and his head book. Look at the past threads, quite recent stuff. You are on the right line, get the ports matched up to the exhaust and inlets. The inlet manifold is really very good, perhaps just take away any casting marks for good air flow, and incidentally, big carbs may slow air flow on standard capacity, keep flow up for good cylinder filling, and low down torque. All in Peters' book. Mike
J.M. Doust

Kieren-
Inside of the early examples of the Original Equipment SU intake manifold, when viewed from the carburetor side, a distinct edge can be seen at the top of the exit side. This abrupt ridge is formed at the intersection of the crossover balance tube and presents an obstruction to the flow of the fuel / air mixture. However, when later examples of the SU intake manifold are viewed from the carburetor side, a more radiused profile can be seen in the same area of the intersection. This more radiused profile is better for airflow than that of the sharp edge of the earlier SU intake manifolds. If an Original Equipment SU intake manifold is to be employed without any modifications having been performed upon it, then this later type is the preferred type. The first procedure for improving these intake manifolds is to smooth the passage as though the goal was to transform it into a round tube. The second procedure for improving these intake manifolds is to flatten off the top of the intake manifold where the crossover balance tube adjoins the port. The purpose of this blending is to distribute the inherent design inefficiency over a larger area, allowing more flow through the intake manifold with less overall disruption. With these procedures having been accomplished, the intersection of the crossover balance tube should be carefully given a .250” (6.35 mm) radius along its leading and trailing edges. With the exit side having been roughed out, proceed to the intake side in order to perform a similar sequence of procedures. Because its runner section is shorter and needs to be a good transition to the open area, the intake side need not be worked as heavily as the exit side. Be careful and work slowly, bearing in mind that excessive removal of material in this area will cause the flow to break away even earlier, resulting in the creation of eddies that will produce increased fuel condensation as well as an increase in the vortex effect which will reduce airflow capacity. Afterwards, polish the interior surfaces with # 60-grit sandpaper. Smoothing the cast surface of the inside of the intake manifold and blending the change of cross section, as well as making a .250” (6.35mm) radius at both the leading and trailing edges of both ends of the crossover balance tube can reduce the vortex effect.
Stephen Strange

Kieren-
Sorry, but I neglected to answer your question about the 1 3/4" carburetors. No, they won't fit your intake manifold, and unless you're installing a Fast Road camshaft, they would actually decrease performance.
Stephen Strange

Talk to Peter Burgess about a porting and carb job. It's very rare to have access to knowledge and to the work of a tuning expert and even more rare to get it at very reasonable cost! The HS6's or HIF 6's can be bolted to a larger manifold and the spares to linkthem are available from Burlen.co.uk the SU manufacturer. Using second hand non matching carbs can be a false economy though as they will need rebuilding and re jetting most likely. Work out you budget and talk to Peter. I would if I lived over there. He also may be able to help you with advice regarding manifolds and efficient use of the carbs and exhaust you have.
A J Ogilvie

Kieren-
I have to second AJ's recommendation of Peter Burgess. Nobody does headwork for an MGB engine as well as he does. I won't let anybody else touch mine. Best of all, you don't have to disable your engine for weeks by sending the cylinder head to him and waiting for the work to be done. He has finished ones that he sells outright or on an exchange basis.
Stephen Strange

If you are going to do gas flow work build a flow bench like the DIY one in Peter's general book about heads.

Don't just test your head on the flow bench. Test your silencer as well. You want 2.2cfm per target bhp at 10.5 inches of mercury equivilent. (according to David Vizard)
David Witham

Thanks for all of the help and information so far. I forgot to mention I emailed Peter Burgess via his website a few days ago but have yet to get a reply. He probably gets hundreds of emails a day the same as mine!
K Goldup

Get a copy of Peter Burgess' B tuning bible, all you need to know.
Allan Reeling

Hi Kieran

I will get round to replying...not quite sure what you are asking though?

Peter
P Burgess

Hi,
I'm basically wondering how big you can go with the exhaust ports before you start getting into trouble by going through oil ways etc.
The exhaust manifold (from memory) is approx one to one and a half inches wide.

If your un-clear with anything I asked in the email please let me know. I was trying to keep it short & sweet but this may have lead to me missing out on vital information.

Thanks
Kieren
K Goldup

Kieren,

As I recall, and it's been a few years, the ex port is most restricted where the boss surrounds the ex valve guide. So, once you open that up as far as you can, I suspect that opening the rest of it up more may not help much.

However, virtually everything I know about MGB heads I read in Peter Burgess's book.

So, if you don't already own the book, back up, stand still, put the grinder down, and buy the book.

The book is way cheaper than buying another head to replace one that you screw up.

Charley
C R Huff

Hi Kieren

To decide how far to go you must first decide what sort of use you expect to put the engine to.

Unless you are doing full race there is no need to open the ex ports. Likewise, unless you are doing a Fast Road spec of at least a 285 Piper there is no need for bigger carbs. The standard manifold works fine with no messing.

Peter
P Burgess

I intend to use the car for the odd track day but it will spend 95% of its time on the road.

One of the questions in my email is which cam would you recommend for this use?

Although if bigger carbs will only have an effect when used with the 285 cam that kind of decides for me!

(I have Peters tuning MGB 4 cylinder book and it is a very helpful point of reference)

Thanks
Kieren
K Goldup

Peter,

I thought I remembered that one of the road heads described in your book recommended cutting out a good bit of the iron around the exhaust valve guide. Am I remembering wrong, or is that area not correctly called the port.

Charley
C R Huff

Hi Charley,

There is a difference between opening up the ex ports and modding the gude boss. We mod the guide boss for Fast Road and above specs and mod the ex port for SC spec heads and race spec heads.

Hi Kieren

Sounds like Fast Road spec sort of head with 285 cam and bigger carbies should suit your purposes, might be worth thinking about 1.69" inlet valves too. Re ex port matching, we like a step from port to ex manifold even on full race use.

Peter
P Burgess

Thanks for taking the time to reply Peter, You have been most helpful!

Whilst I've got you cornered, are all of the heads you do converted to run on unleaded?

If I can get the funds together (I keep spending it all!) I think I'd rather buy one of your heads than attack it myself or give it to someone else.
K Goldup

Thanks Peter.

It sounds like I understood the directions and just didn't have a handle on the terminology. That's better than the other way around.

Charley
C R Huff

Hi Kieren

Unless otherwise instructed we produce all our heads leadfree. You mentioned you might end up with some spare castings, we are always happy to do a deal to take them off your hands against the price of a head :) ps what are funds? Just having to fork out for a Main Dealer only ex system for my 'cheaply' purchased Mitsubishi Pinin 4x4 (already forked out £400 for 4 tyres!).

Hi Charley

I suffer from lack of communication, forgetfulness etc etc the older I get. Must make sure not to declare war by mistake :)

Peter
P Burgess

Peter,

As far as I'm aware 'funds' come from a credit card and you just keep using it until it wont give you any more!

I know where your coming from on the cheaply purchased side of things. The first B I bought I got for only £500! Then as the recommissioning commenced we found out why it was only £500. Has been very useful for spare parts so far though including a front cross-member, engine, diff... Even a window for my Dads V8!

Again thanks for all your help and advice so far (I'm sure I'll soon be after more). Once I have the engine in pieces I'll be on the phone to discuss which cylinder head is best for me and to try and strike up a deal!

K Goldup

This thread was discussed between 22/03/2011 and 29/03/2011

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