Exhaust manifold tuning

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dal2.0litrefrogeye
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#1 Exhaust manifold tuning

Post by dal2.0litrefrogeye » Fri Apr 03, 2020 2:57 pm

Hi all , thinking about making a pair of over frame exhaust manifolds , I've got the space as I don't have brake servo ,
Now in the past I remember talking to an engine builder and being told if you can build exhaust manifold build it with the opposed cylinders connected, so along the lines of 1&6 5&2 etc which obviously isn't how a normal pair of headers tubular or cast are together , the reasoning for this it encouraged gas flow and made the engine a bit free-er reving ,
Anyone heard this or know this for definite??
Cheers for any ideas. Darren
Its a way of life not a hobby
Darren . 64 4.2 modded 69 4.2

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Tom W
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#2 Re: Exhaust manifold tuning

Post by Tom W » Fri Apr 03, 2020 3:39 pm

Very interesting.

Doing it that way pairs the cylinders so there’s the least chance of the positive pressure from one cylinder affecting the one it’s paired with. 1 & 6, 2 & 5 and 3 & 4 exhaust at 360 degree intervals. Even the race camshafts only have a duration of 310degrees or so, so no chance of interference.

Having said that, the standard grouping of 1,2,3 and 4,5,6 is pretty good. The cylinders in each group are separated by 240 degrees, so not much interference, particularly with standard cams (256 degrees duration). Given that tubular headers are typically reasonably long (compared to a standard manifold), there’s little chance of the flow itself diluting an adjacent cylinder. The problem is more pronounced on an inline 4 where the positive pressure from one cylinder in the middle of the exhaust stroke can find its way into the adjacent cylinder that’s on the end of the exhaust stroke. Rather than the exhaust drawing in fresh mixture for the cylinder that’s in the valve overlap period, the flow is reversed and the charge gets diluted.

What’s critical, and will be very difficult to get right without a lot of trial and error, or some fancy measuring kit, is the correct length of header. As well as considering flow, there’s the positive and negative pressures in the system due to the sound waves in the exhaust gas. The length of the pipe, and it’s interaction with its paired pipe could have either a very beneficial or negative effect on the performance. Where you place the junctions, and where the effective end of the pipe reflects the waves all has an influence. Some of this will be constrained by packaging as well as mathematical ideals. You’ll also need to decide whether you want 3 independent systems, or whether you’ll group all 3 into one. Again, it’s probably heavily packaging dependent.

I’ve not seen a system like this on a Jag before, but some systems exist for sporting 6 cylinder triumphs.
Tom
1970 S2 FHC

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Gfhug
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#3 Re: Exhaust manifold tuning

Post by Gfhug » Fri Apr 03, 2020 5:48 pm

Darren, there is a fair amount of info out there on tuned exhausts for aero engines. A simple search will show you some thoughts (search Lycoming tuned exhaust). I think the advantage for them is being a horizontally opposed engine means you can balance which exhausts to twin together a bit easier. It can lead to quite long pipes, which need to be all the same length before they can be twinned together.

Hope that might help

Geoff
S2 FHC Light Blue
S2 OTS LHD - RHD full restoration

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PeterCrespin
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#4 Re: Exhaust manifold tuning

Post by PeterCrespin » Fri Apr 03, 2020 5:55 pm

"Now in the past I remember talking to an engine builder and being told if you can build exhaust manifold build it with the opposed cylinders connected, so along the lines of 1&6 5&2 etc which obviously isn't how a normal pair of headers tubular or cast are together."

He was probably a 4cyl or v8 guy. The six is two three-cylinder 120 degree triples and plumbed as such via two systems siamesed 3 into 1. The V12 is two sixes, plumbed as four triples. As with inlet plumbing, longer/narrower pipes tend to enhance things at lower revs but restrict higher up, whilst shorter/wider tracts are good at high revs but less so lower down.

At its most basic, you can just copy a successful design and hope it matches your intake and cam setup.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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#5 Re: Exhaust manifold tuning

Post by mgcjag » Fri Apr 03, 2020 7:00 pm

Hi Darren....on a D type replica and unfortunatly can see the interesting part
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Steve
69 S2 2+2 (just sold) ..Realm C type replica, 1960 xk150fhc

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Tom W
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#6 Re: Exhaust manifold tuning

Post by Tom W » Fri Apr 03, 2020 7:34 pm

That still looks like 2 three into one manifolds to me. It doesn’t look like one and six are paired together.

You need to group the pipes like this Triumph manifold.

https://www.goodparts.com/product-category/exhaust/

No idea if it will fit though.
Tom
1970 S2 FHC

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#7 Re: Exhaust manifold tuning

Post by PeterCrespin » Sat Apr 04, 2020 12:09 am

It's a Realm replica with oodles of space to play with exhaust shapes but yes I believe it's the normal 6-2 Jag pattern. No postwar road or race Jag I-6 ever changed from two groups of three into one, right up to 1997 and the last AJ16 cars, including the supercharged version. Even if they then went into a single pipe for all six before splitting into two tailpipes again.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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dal2.0litrefrogeye
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#8 Re: Exhaust manifold tuning

Post by dal2.0litrefrogeye » Sat Apr 04, 2020 5:31 pm

So on the same tac , does anyone know if over the frame headers definatly free up some more horses ???
Its a way of life not a hobby
Darren . 64 4.2 modded 69 4.2

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#9 Re: Exhaust manifold tuning

Post by Tom W » Sat Apr 04, 2020 5:58 pm

Depends on where you want your improvement to be. I’d specify the system to best suit that, then see whether that fits better inside or outside the front frames.
Tom
1970 S2 FHC

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#10 Re: Exhaust manifold tuning

Post by PeterCrespin » Sat Apr 04, 2020 7:12 pm

dal2.0litrefrogeye wrote:
Sat Apr 04, 2020 5:31 pm
So on the same tac , does anyone know if over the frame headers definatly free up some more horses ???
What Tom said.

There's nothing magic about over the frame except that sharp bends right next to the port are not ideal. Doesn't stop plenty of cars and bikes making monster power though...

Over the frame gives you a bit more room to experiment but better the right system inside the frame than a guesswork system over the frame. Judging by the pipe size on the Realm above, it would lose out on bottom end but be quick if everything matched for top end power.
1E75339 UberLynx D-Type; 1R27190 70 FHC; 1E78478; 2001 Vanden Plas

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#11 Re: Exhaust manifold tuning

Post by steve3.8 » Sat Apr 04, 2020 8:15 pm

Have you got this book Darren , plenty of over the frame exhaust examples. The modsports guys knew how to achive the gains.

https://www.ebay.co.uk/itm/Jaguar-E-typ ... Sw~9Feh1bf
Steve3.8

64 3.8 fhc, 67 4.2 fhc

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Thor
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#12 Re: Exhaust manifold tuning

Post by Thor » Sat Apr 04, 2020 9:02 pm

loads of examples out there Darren
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#13 Re: Exhaust manifold tuning

Post by Thor » Sat Apr 04, 2020 9:07 pm

Here is the same one on the car
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#14 Re: Exhaust manifold tuning

Post by Dawnpatrol » Sun Apr 05, 2020 4:10 pm

Hi Darren,
All the works exhausts followed the same convention even on the wide angle heads as far as I can see. Which as the standard manifold 2 x 3 into one, 654 and 321.
It's all about the primary pipe length and diameter before the merge being tuned to the characteristics of your engine. George Buck designed the LWE overframe manifold. The formula is in the back of Peter Wilsons book on the XJ13 IIRC.
If I can put my hand on the book I'll try and post a picture.

Michael
Michael
1961 OTS LWE, 1965 OTS, 1966 FHC.

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peters3103
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#15 Re: Exhaust manifold tuning

Post by peters3103 » Wed Apr 22, 2020 8:01 am

My 3.8 runs over the frame tubular manifolds. Unsure of any real gains and to be honest I’d probably revert back to standard if I were given a pair of shiny new black manifolds.

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