3.8 MK 2 Resto progress.

I tried to fit the exhaust today, I have stainless silencers and tailpipes but as I bolted the first of the two downpipes I heard a ping, yes the manifold cracked at one of the studs and it wasn't even tight! If I cant find another in my pile of spares I will be considering steel downpipes. The cheapest I've found for a pair so far is £765 inc VAT from Hayward and Scott, they look pretty good considering most of the other suppliers are charging well over a grand. Does anyone know if any suppliers are doing a pair for less money?

Thanks, Ray

 

Try contacting Rob Burston on 01823400128. Based in Taunton he is a classic Jaguar breaker. Has around 120 classic Jaguars on his farm. Many Mk2s so try phoning him and ask what he has got.

 

A pair of front & rear black vitreous enameled manifolds from Barratts is only £390 incl. VAT. With slight imperfections £230.

 

Just checked Barratts £115.31 inc vat, That's for an after market which is pretty good value, Now do I just replace the one or buy a pair of the stainless 6 branch for a few more hundred quid ??

 

It depends what you are looking for. A pulse tuned exhaust manifold will push power & torque up the rev range.

Me ~ I'm not trying to build a race car & I really like the XK engine's low end torque so I would stay stock.

 

The standard Jaguar exhaust manifold is reasonably well made. To make significant gains for road driving from tuning the exhaust manifold, the primaries have to be very long and the layout becomes very difficult. It's more practical to use an arrangement like Roger Bywater's where you keep a standard manifold and join the exahust systems into a large diameter pipe near the gearbox. The reflected negative pressure pulse might not be as big, but it will coincide with the exhaust valve closing (at more typical engine speeds).

On the other hand, a nice stainless steel manifold looks good.

 

Should you decide to order a manifold from Barratts. Order by P/N from Parts Manual. They have a habit of muddling pics on their website.

Mk2/S Type manifolds:










E Type manifolds:

 

Cheers! I'm still waiting for a few more quotes for a S/Steel 6 branch to come in before I make my mind up
Ray

 

I did see some dyno tests on E Types and the conclusion was the cast iron manifolds produced more power and torque than a stainless 6 branch. Not much more, but more. Choose stainless 6 branch to look pretty, stay stock for go.

 

There are manifolds & there are manifolds when it comes to tuning & bhp/torque in a specific rev range. First you need to choose carbs & tune the inlet system. Then flowbench the head inlet & exhaust (& if fanatical cut 3 angles on valve seats). Then tackle/pulse tune exhaust manifolds in conjunction with cams & with exhausts in the equation.

I'll stay standard thanks. These are long stroke engines as it is with high piston speeds. I don't want to rev the lungs out of them.

 

That's debatable what's pretty is, I think a stainless headers get rather ugly when they turn all black with colours.
A good set of cast iron manifolds with the porcelain in good order look correct, classy and original.
I always thought stainless headers on a car that weren't mean to be there always looked like a red neck did his thing.

 

I feel the same way with aluminium radiators. Just don't look right. That said I put stainless headers or manifolds as we call them in the UK on my MGB GT V8 but only because they used to crack for a past time. The originals were cast iron but about a foot long and the engine vibration would crack them. The Jaguar manifolds on the other hand are short so there is no real necessity to change from the original.

 

The improvements in volumetric efficiency associated with single inlet runners of 20 to 80 cm are fairly well known. It’s tempting to assume that similar length primaries in the exhaust manifold (header) will have similar effect. However, the exhaust is different.

The whole idea of tuning by runner length is a matter of arranging for the peak of a reflected pressure pulse (of the right sign) to arrive back at the inlet/exhaust valve a little before it closes.

Think of a pressure pulse propagating down a pipe and reaching an open end( like the bell mouth of a racing induction manifold). In order to match the pressure outside the open end, reflection has to change the sign of the pulse: if a positive pressure pulse comes down the pipe, then a negative one is reflected back up (and vice versa). There’s also a reflection of opposite sign from a point where the pipe increases in area, though the effect is weaker.

It’s easiest to consider a single pipe with no branches running to an open end. If it’s an inlet runner, the ‘suck’ into the cylinder makes a negative pressure at the inlet valve that’s biggest at around half way down the inlet stroke of the cylinder. That pressure propagates down the inlet runner to the open end, reflects back as a positive pressure. For maximum effect, we want the positive pressure pulse to arrive back at the inlet valve around bottom dead centre; that’s 90 degrees of crankshaft rotation later. At 3000 rpm, 90 degrees takes 1/200 second. If the pulse propagates at the speed of sound of about 330 metres/second, it travels about 330/200 = 1.6 metres in that time. Since the pulse goes up and down the runner, the required length is half of that or about 80 cm. There’s an old experimental study of ram pipe lengths for a Jaguar racing engine and the best length for mid-range was 32 inches, which is close enough to 80 cm to support the argument.

Why isn’t 80 cm (32 inches) good for the exhaust? There are two reasons. One is that the exhaust gas is hot making the speed of sound more like 520 metres/second. The other is that the positive pulse (that we want to reflect as a negative) comes more from closer to just after the opening of the exhaust valve as the high pressure in the cylinder is released. So we have a pulse that’s travelling one and a half times as fast and we want it to take one and a half to twice as long to get back to the exhaust valve. The 80 cm that we calculated before becomes more like two metres or more. That’s way too long to arrange as a primary length.

Whatever you use as the manifold, in order to make gains from tuning lengths at road type engine speeds, it’s necessary to have an area increase in the exhaust at about two metres down from the exhaust valve.

What I’ve described is all a bit back of the envelope and there’s a load of other stuff going on, but I hope that it’s sufficient to be indicative.