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Building an exhaust


geordie40

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Hi

I am thinking about building my own exhaust well at least from the 4 into 1 collector so just the cat {or decat for after MOT's) and the silencer now bearing in mind I can't weld so is it just a case of buying the relevant parts and clamping them together.

Looking at all stainless parts and using lap or butt joint type clamps.

Has anyone made their own this way and if so can I have a pic please 😀

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Thanks for replies so far, looks like every exhaust part does what I need except the cat.

Is there any reason to up the bore size slightly? currently my pipe is 2" but was thinking of upping to 2.5" (get gas out quicker)

I don't think I would get it welded as I want to remove cat when not needed (MOT time) so the lap type clamps seem best suited, I currently have one fitted between collector and straight pipe.

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I made mine, measured and planned the system out, ordered the various sections from https://www.304stainlessexhaustparts.com who cut and swaged everything to order (very helpful). I handled the kink in the body at the scuttle by just cutting and welding the section between the cat and the silencer at an angle. What I would say is, just get someone to weld it for you - my welding isn't up to the standard I'd want for an exhaust (and I don't have any stainless wire either!) so I tacked it up with mild on the car, then took the entire thing to a mate who did it for me and spotted him some beer tokens. I'm sure you can find somewhere that would do that for you.

No-one would make me an exhaust off design, they all wanted the car (even if I said I absolve them of blame if it doesn't fit!) and on top of that they wanted stupid £££ for the task so in the end I did it myself and am very happy with the result.

ExhaustDesign_v2.01.jpg

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2 inch may be better than 2.5, larger bore can some times slow the gas flow so much it removes the scavenging effect and stops efficient cylinder emptying. It has to be matched (tuned) to the CC of engine, RPM window and flow rate of the head. It will likely move the power curve closer to the red line regardless. Not all pub bragging is a good thing here. Unless the engine is force induced that is, go as big as hell then, all of the other crap is just irrelevant once you go that route.
 

Saying that in these cars it’s often nice to rev the buts off them and low down torque isn’t high on most people’s want lists in a 500kg chassis. So as long as you know what it will do to your torque curve....  easiest thing is obviously see if anyone with the same engine has maps pre and post ‘upgrade’. A dyno tweak of the ecu’s map would also be wise for best results. 

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On 4/11/2020 at 9:19 AM, geordie40 said:

Is there any reason to up the bore size slightly? currently my pipe is 2" but was thinking of upping to 2.5" (get gas out quicker)

I've discussed this on the forum before.  My Superspec has the standard twin outlet from the silencer.  On the advice of a Club member I actually blanked off one of the pipes with a bung to see if it helped the emissions (a long standing problem with the Superspec).   To my surprise the car ran appreciably better, much sharper, better throttle reaction.....   Discussions at Car Shows ("why have you blocked off the exhaust ?") seemed to show the the better performance came from increasing the back pressure in the system, which the engine seemed to like.   Obviously the exhaust system was never designed or tuned for the engine it was just made of parts that Robin Hood Engineering to get hold of, so the explanation made sense to me. 

So I would think that increasing the bore size would actually reduce the back pressure and have a negative effect.  And the same effect would be seen by removing the cat, as that would also reduce the back pressure.   So I will be very interested in the result, but it may not be what you expect.

 

1.jpg

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2 hours ago, Mrbarry said:

 Unless the engine is force induced that is, go as big as hell then, all of the other crap is just irrelevant once you go that route

Only true for turbo charged engines, supercharged engines still respond to exhaust tuning in the same way an NA engine does.  

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Hmm food for thought, it seems that to much back pressure is bad and to little is bad, but also in the equation is exhaust velocity and scavenging.

Ideally you have an exhaust tuned to your car but that's never happening (wayyy to expensive) so may keep the 2" pipe or step up slightly to 2 1/4" and will see how it goes with & without sports cat.

Keep you posted once I get bits ordered and fitted 😉

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  • 1 month later...

Well it's all fitted and looks quite smart, took it for a run today as eventually got Lambda sensor working ( see other post for that storey!!)

Sound seems a bit deeper tone withe the larger bore, as for performance it's to soon to tell.

Doesn't take long for discoloration to occur ☹️

IMG_20200504_111917.jpg

IMG_20200517_160754.jpg

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On 4/15/2020 at 11:32 AM, alanrichey said:

I've discussed this on the forum before.  My Superspec has the standard twin outlet from the silencer.  On the advice of a Club member I actually blanked off one of the pipes with a bung to see if it helped the emissions (a long standing problem with the Superspec).   To my surprise the car ran appreciably better, much sharper, better throttle reaction.....   Discussions at Car Shows ("why have you blocked off the exhaust ?") seemed to show the the better performance came from increasing the back pressure in the system, which the engine seemed to like.   Obviously the exhaust system was never designed or tuned for the engine it was just made of parts that Robin Hood Engineering to get hold of, so the explanation made sense to me. 

So I would think that increasing the bore size would actually reduce the back pressure and have a negative effect.  And the same effect would be seen by removing the cat, as that would also reduce the back pressure.   So I will be very interested in the result, but it may not be what you expect.

 

1.jpg

Once again the back pressure issue comes up, engines don’t want back any pressure You confuse this with scavenging caused by tuned sound wave pulses which is a dark art I have tried to dabble with. The calculations involved are complex and specific to your engine.

exhaust diameter at a basic level is about port velocity and includes primary, secondary and collector lengths and diameters.

in the most simplistic terms a bog standard Pinto will be just about ok with a 2” pipe from collector to tail, I would prefer 2 ¼” again on a bog standard engine but any tweeking such as an air filter or long 4 branch manifold I would consider 2 ½” as about right.

back pressure backs up to the exhaust valve, when the piston rises to push the exhaust gas out it meets the back pressure and has to work harder pushing the gasses out... you loose power pushing it out that could be used to turn the rear wheels.

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1 hour ago, Snapperpaul said:

Cheers for this Paul, I did watch and read a few articles about back pressure and scavenging and as you say it's quite complex and ideally you have a exhaust built and tuned to your engine.

In the end I went for a 2 1/4 inch pipe for my 2L Rover T Series engine, it seems ok but it's blowing slightly as only clamped atm with no exhaust paste, I will eventually get bits welded but want to make cat removable.

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