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MG MGB Technical - Exhaust Header Porting...

I purchased a "peco" style big bore exhaust header. Just curious to see what people have done in regards to porting the header to match up with the exhaust port on the header. As I have not been able to find anything in the Archive.
JRB Mr

sorry that was supposed to be "in regards to porting the header to match up with the exhaust port on the head" where is my head, errrr
James

The hole in the header is almost certainly already bigger than the port in the head. Leave it that way, and do not try to match the head to the header. That is old school and never did anything good. The step helps to prevent reversion, and is desirable.
The big bore header is not of any use except for drastically modified engines run at high speed; you'd be far better off with a standard bore item for any street driven car.

FRM
FR Millmore

I must chime in agreement here with FRM. The stock MGB exhaust on the SC Moss engine is producing nearly 125RWHP. With the Han's SC engine with his 2.5 inch LCB header, about 107RWHP is the best to date. Vic
vem myers

Smaller exhaust bores means higher gas speed and greater scavenging effect pulling fresh mixture in during the overlap period.

You have to go a long way to beat the original cast manifold - the only benefit in changing to a tube header is weight saving.
Chris at Octarine Services

Sorry for being a little dense here. Are you saying that the standard exhaust manifold works better on a road car than the tube LCB...

Was thinking of making a change to an LCB but will think again if that is the case.

regards

Andrew
A I McGee

Hey Guys actually I am talking about the aftermarket quality of these headers, getting them to line up etc... Making sure there is no lip of the exhaust header covering the edge of the exhaust port on the head.

Oh and its a 79 B with dual su's going on it as well. Not an earlier model car.
James

James-
There certainly should not be any bits of header poking out in the port - if there are it's inexcusable, especially on a "big bore" piece.

To check, put a thin layer of modeling clay on the head, grease the header flange, and bolt it up. The fit will be clear when you undo it.

Ideally, you want head to be a bit smaller than the flange plate all around.

If you need to grind, be careful - tubular fabbed headers don't have much material to grind - you may have to do remedial welding.

If you need to grind, it's a good excuse to send it back and install a stock manifold and downpipes, with a good freeflow exhaust system.

As a matter of interest, what are the tube sizes on this thing? Branches and collector?

Andrew- Yep! The stock system IS an LCB once you include the downpipes. The tube sizes are good for even fairly "warm" engines. On hot cam ones you might loose a bit on the very top end, but you'll more than get it back in drivability in the low/middle. And it's quieter and more durable than tubular ones. The sole penalty is a couple of pounds weight - against a bunch of pounds cash!

FRM
FR Millmore

Yes I agree. When I tried lining up the gasket and the exhaust header, off of the car, seeing if things lined up, it was off. Ill have to have a look tommorow, could be the gasket or the manifold, or im just crazy. Are there any other ways to check besides modeling clay ?
James

James,
Re: Are there any other ways to check besides modeling clay ?
Yes, engineer's marking blue - should be available at parts stores.

David
David Overington

Depending on how flat the surfaces - especially the corners - are, marking blue may not show the fit very well, which is why the clay is good. The gaskets generally have a significantly bigger hole than will match either port or header, but not always. Any somewhat sticky coherent substance can be used; grease one surface so it sticks on that side.

FRM
FR Millmore

James-
If the alignment is off, send it back. As Chris rightly pointed out, a Big Bore header on your type of engine will actually decrease performance due to inadequate scavenging effect. If you want good exhaust flow on the type of engine that you're building, you best bet is an electropolished Original Equipment exhaust manifold.

Electropolishing is an electrochemical process used to smooth metal, usually prior to plating. It is commonly performed on a precision casting (such as a window winder handle) or on prepolished sheet metal after it has been formed to shape (such as a bumper) prior to plating it. The item to be electropolished is thoroughly cleaned, then immersed in a chemical bath. A current is then run through and the highest points on the surface of the metal are removed. In a sense, it is the reverse of plating in that metal is removed instead of deposited. The advantage of electropolishing a cast iron exhaust manifold is that because the item is completely immersed into the electropolishing bath, the process can get inside the manifold, reaching into remote areas and otherwise inaccessible curves so that it will polish the interior of the exhaust manifold quite nicely where human hands and mechanical tools cannot reach, the smoother surface making for reduced turbulence in the exhaust gas flow just like the smooth walls of an exhaust manifold constructed of tubular steel. I sincerely believe that an LCB (Long Center Branch) 1 ¾” diameter tubular steel exhaust manifold will not flow any better than an electropolished Original Equipment cast iron exhaust manifold if it has the same basic design.

In addition, I would suggest having the exhaust manifold Jet-Hot coated. Jet-Hot coating is a ceramic coating that can be applied to coat both the interior of the exhaust manifold as well as the exterior. The heat will have nowhere to go except out through the exhaust system, thus it will greatly reduce underhood temperatures. This is a significant factor as exhaust manifolds often reach temperatures above 400° Fahrenheit, thus the air being inhaled into the engine being denser, more fuel can be mixed with it to result in a more powerful fuel-air charge. Another benefit is that the setting of heat-sensitive 1½” SU HIF4 carburettors can remain more consistent. One word of warning to those considering Jet-Hot coating or any other type of ceramic coating: Be sure that the entire surface of the manifold, both the interior as well as the exterior of the manifold and that of the flanges is coated so that the heat of the exhaust gases will pass on through the system instead of being absorbed and trapped in the metal of the manifold, otherwise the manifold will create the same warpage problems as in the case of wrapping the manifold with insulating wrap. Jet-Hot has a website that can be found at http://www.jet-hot.com/ .

I made these modifications to my last small-bore engine that was in my car (and now resides in a friend's car), and found the results to be excellent.
Steve S.

Steve, I am interested in this idea of electropolishing the inside of the cast manifold. I assume the process described on the following web site is what you mean.

http://www.anopol.co.uk/

Would I be correct in thinking that allowing the inside of the manifold to rust after it has been electropolished will undo the good work, at least in part?

Can you confirm that you are sugesting electropolish and then Jet-Hot? I was wondering if the Jet-Hot might have sufficent surface filling properties to make the benefit of electropolishing minimal. I have to say I know very little about Jet-Hot.

Last question, I think, what is in the chemical bath? The process description on the above site reminds me of the electrolytic removal of rust which I have done with some success.
David Witham

thanks for the info.
Im keeping the header and exhaust for now.
Anything is better than the 75-80 intake/exhaust setup.
Ive also heard of electro polishing, there is something else that does the same thing where it polishes the inside passageways of exhauts, heads etc. Was on this bbs but I do not have the link as i am at work.
James

David-
Yes, that website describes the process adequately.

Yes, allowing the electropolished interior of the exhaust manifold would, at least in part, undo the advantage of the polished surface. That's one of the reasons for having the interior as well as the exterior of the exhaust manifold coated after electropolishing.

I can't say for sure what's in the bath. I'm sure that this is something that you can't do in your bath tub.


James-
I believe that you're thinking of Forced Extrusion Honing. Extrusion Honing is great because it can port areas where it is otherwise impossible to do so, such as inside the long runners of the exhaust manifold. In this technique, a dense mixture of abrasive clay is forced through the interior of the manifold, polishing the surfaces to an even greater degree than can be achieved on a casting through electropolishing. Smaller more restrictive areas in the head act like a venturi so the mixture of abrasive clay flows faster there. Faster flow equals more cutting action and thus the Extrude Honing process by nature removes material where it's needed the most. This cutting mechanism is very good at producing runners that flow equally. I have seen a cylinder head in which both the intake and exhaust ports have been subjected to this process and it is very impressive. This service is available from Extrude Hone. Their website can be found at http://www.extrudehone.com/ .
Steve S.

Hi BigFace- A totally idle question: Would there be cost=effect in extrude hone then jet hot coating a stock cast exhaust manifold?. Assumes that even baby-buttock smooth, the hi scavenge effect is still there after jethot.I must agree that the stock unit is not surpassed by the Peco header as far as my butt can tell. Peter Burgess claims both a 3-5 hp midrange gain on the Peco, but talks of power loss when going to the same LCB header on another engine, I guess from scavenge loss, eh? Cheers, Vic
vem myers

Vic-
The Cost=Effect is all a matter of personal values. It's true that a decent tubular header might be obtained for less money than that of extrude honing and/or electropolishing, but tubular headers are much, much noisier. At certain parts of the powerband they can actually resonate into an annoying ringing sound. In addition, tubular headers are welded assemblies, and their welds have been known to crack under the repeated stress of heating and cooling. The cast iron exhaust manifold has none of these problems.

It is important to understand why the pre-1975 Original Equipment exhaust manifolds and LCB (Long Center Branch) 1 ¾” headers have nominally the same performance. The size of their internal passages are nominally of the same diameter, thus the velocities of the exiting combustion gases passing through them are the same. This high gas velocity is critical to power output at low engine speeds because the greater the velocity of the exhaust gases, the greater their inertia. Due to the high degree of directional inertia, the exiting combustion gases continue to flow exclusively out of the exhaust valve even though the intake valve is opening. In order to properly understand this phenomenon, one needs to view the cylinder as an extension of the combustion chamber. The combusting fuel-air charge exerts pressure upon the piston crown, accelerating the piston down the cylinder. It must be understood that due to the geometry of the crankpin and connecting rod, the piston is decelerated as it passes 90° After Top Dead Center, this geometry-induced deceleration becoming increasingly severe as Bottom Dead Center is approached. However, the combusting gases continue to accelerate downward, their inertia causing them to pile up on and increase pressure on the piston crown. Due to this inertia effect, at Bottom Dead Center the pressure at the roof of the combustion chamber is actually less than the pressure of the atmosphere immediately atop the piston crown. Because all forces in nature tend to equalize, at this point the pressurized atmosphere atop the piston crown expands upward, increasing its upward inertia as it approaches the roof of the combustion chamber. If the exhaust valve is open to the point that it has sufficient airflow capacity, the inertia of the exiting exhaust gases will remain sufficiently high enough to literally scavenge the atmosphere from inside the cylinder, creating a partial vacuum of as much as 7 pounds per square inch less than the ambient atmospheric pressure outside of the engine. This in turn allows the incoming fuel-air mixture to be pushed in not only earlier, but also at a higher velocity (and thus a higher quantity with better fuel atomization) by the greater atmospheric pressure outside the engine, thereby increasing power output. All other factors being equal, a larger diameter exhaust manifold would decrease this critical velocity, and with it, its benefits. The approximately a 30% greater cross section of the Big Bore system reduces exhaust gas velocity, which in turn reduces scavenging effect in the combustion chambers at low engine speeds and thus increases “Pumping Losses”.

This is because backpressure does not increase in direct proportion to gas flow out of the exhaust ports. Instead, backpressure increases in proportion to the square of this gas flow. As backpressure increases, scavenging of exhaust gases from the cylinders decreases, then stops altogether, thus increasing the demand for the engine to expend power to pump the exhaust gases out through the exhaust system. These “Pumping Losses”, as they are termed, thus rise dramatically as air flow (and power) increases. In addition, because of the fact that as backpressure increases, pressure within the combustion chamber increases, some of these pressurized combustion gases will escape out of the open intake valve, displacing the incoming fuel-air charge. These combustion gases have to reenter the combustion chamber prior to the fuel-air charge, partially filling the volume. In consequence, a smaller volume of fuel-air charge enters the cylinder with the result that power output suffers. These factors can rob the engine of as much as six to eight horsepower. For practical purposes, this is almost equivalent to the difference between the output potential of a shorter-duration camshaft lobe design such as that employed in the Piper BP270 camshaft and that of a scavenging-dependent longer-duration camshaft lobe design such as that employed in the Piper BP285 camshaft.

On Big Bore engines, you'll see the 3 Hp to 5 Hp gain that Peter mentions. On small bore engines, you'll see less power that you would get with a 1 3/4" system or a polished Original Equipment exhaust manifold. In short, Big Bore headers are for Big Bore engines, period.
Steve S.

Steve S,

Very informative. Just out of curiosity what header would be better when using a Moss Supercharger and standard pistons setup.

Pre 75 header or Peco Header?

Thanks Ray
Ray 1977mgb

Ray-
As far as I can figure, the pistons should have little influence on header design. If you want your eningine to really howl without trading away too much reliability, I'd suggest contacting Peter Burgess about the head that he developed specifically for supercharged B Series engines. He would be able to give a better suggestion for a header than I can as I have not worked with a supercharged B Series engine. Click on this- http://www.mgcars.org.uk/peterburgess/
Steve S.

This thread was discussed between 02/07/2007 and 10/07/2007

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