Exhaust Advice

Road-race engines and ancillaries - general discussion
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beardybloke
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Exhaust Advice

Post by beardybloke » Sun Oct 25, 2015 7:43 am

I have a Tvr with a 1600t/c from a supermirofiori , unfortunately I had an accident recently when someone rear ended me . The insurance company have committed to the repair but part of that will involve replacing the exhaust system , this wasn`t great anyway as it needs to be made to fir and was part Fiat and part made up .
The insurers are going to take it to their specialist who will make it to fit the car , what I was looking for was some general advice about what would be that best system to aim for . I have rear that 4-2-1 systems are best and I am assuming that a straight through pipe , as it is at the moment , would be also best . The existing manifold is not too good as I have had to have it welded up before .
The plan would be for a stainless system made to fit to the car , I am not sure about bore size ?, the engine is not tuned at present but it is something I may look at in the future .
Any advice or opinion would be great , as I would like to have some more information before I have to discuss it with the insurers specialist , many thanks Andy
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Will01
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Re: Exhaust Advice

Post by Will01 » Sun Oct 25, 2015 8:39 am

If you are in position to get a proper exhaust made up then you are very fortunate.
You really need to get you primary and secondary pipes made equal length. These lengths should be calculated depending on setup of you engine.
Maybe here it would be best to research what you may end up doing E. G. Carbs and intake manifold and cylinder head development. Type of use also has an affect.
The ports at head face should not be port matched but should have a little step so make slightly larger ID than the exhaust port.
The collectors should be properly designed. Not the poor you get from alot of exhaust manufacturers.
I would read up on some pulse tuning and exhaust design. It can make for interesting reading.
I am sure GC will give you alot more detail.
Good luck with the exhaust build
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beardybloke
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Re: Exhaust Advice

Post by beardybloke » Fri Oct 30, 2015 7:26 pm

Thanks for the advice , you say that the ports should not be matched but that the exhaust port should be slightly smaller than the head port ?. I am assuming that I have misread this and the bore of the port on the exhaust should be slightly larger than the bore of the port on the head .
I had a look at the AR manifold shown on another post and although this would probably not fit I assume that this is the type of fit I need to end up with.
Having not had a lot of experience with this how do I know that they will not fit poor collectors , as I said I am a bit stuck with using them and I have reasonable confidence that they will do a good job I would like to be able to at least know what a good collector looks like .
I am not sure where I will end up with engine development as the car will come back in far better condition than it was before so I may have to review my plans . How much difference do the length of the primary/secondary need to be say from a standard 100bhp to a mid range 120bhp ?.
Also I had a hill climb charity run in the car earlier this year and although I made a mess of it I was only about 10% off a semi competitive time , given the TVR`s use a handicap system for their events more power is not essential just making the most of what you have got .
I am not sure if looking at what I might do in the future is the best way or just get it right for what I have now ? , I assume that I could always change the lengths later .
As I said I am just trying to collect a little knowledge so I can get the best system I can for what will always be a compromise situation .
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TomLouwrier
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Re: Exhaust Advice

Post by TomLouwrier » Sun Nov 01, 2015 12:07 pm

I think you could do worse than having a long good look at the 4-2-1 that Abarth developed for the 124. There are many pictures out there on the web and you can still get an original as well as decent looking copies from all sorts of places.
I'm sure Guy can help you out with some numbers on inner diameters and pipe lenghts for the headers and downpipes. It's been discussed here several times in the past, not all threads have been saved (but might be restored?)

My notes say (this is a direct quote from one of Guy's previous posts here)
4-2-1 - here is my best advice:

1. The collectors must not be considered part of the lengths - they are merely the open space that generate the reflections.
So you can make them as long as you like and internal taper optimum is 14 deg included angle or less to minimise turbulence.

2. Overall sum of primary and secondary pipe chord length (measured at pipe axis & not incl collectors) should be typically 34-36". The min acceptable py chord length is 27" (no less!) thus with 27" py the sy chord should be 7-9". You could equally as well run 28" & 8" or 29" & 7". Etc etc. You can go longer overall, say 37 or 38" but don't go below 27" py and don't lengthen beyond 34-36" unless available space dictates that's needed. The py pipe is measured from the head face and the pipe chords must be of equal length to +/- a few mm. Py pipes are paired 1& 4 and 2 & 3 and no other way!

3. Cannot predict the effect of various lengths suffice to say the torque won't suffer (nor power) but the position of the spread of torque will alter up/down the rev band. And no more than that!

4. Py pipe ID should be such that the final py pipe internal area is equal to the valve throat area (x 2 if a 16v head) or the area of the port exit region - whichever is the larger. Invariably on a 16v the port area will be bigger than the combined valve throat area (which after all is the controlling section) and on in fact an 8V unit I tend to make the py pipe ID same as valve throat diameter plus 5mm which works well in practice and has been proven in simulation to be effective. You can go bigger - but it may not work so in general terms avoid! There is little point making ex ports huge in relation to that valve throat (s).

5. Make the sy pipe diameter 25% bigger than the py.

6. The tailpipe - which is the bit from the sy collector to the silencer (muffler) make same as sy internal diameter. As a rule of thumb no 8v TC tailpipe should be bigger than 2" bore and frankly 1.75" will suit any 1600 & 1800cc unit.
Hope this helps.

regards
Tom
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beardybloke
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Re: Exhaust Advice

Post by beardybloke » Mon Nov 02, 2015 7:21 pm

Hi Tom , that is really useful and answered virtually all my questions . I think the only confusion from this is that the primary and secondary pipes are described as for example 28"& 8"giving an overall length of 36" , I thought I understood that primary and secondary should be the same length ?.
The other advice is perfect , there has been a few articles in my TVR magazine but obviously they know nothing about a Fiat tc , firing order was one of the next questions , so thanks very much .
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TomLouwrier
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Re: Exhaust Advice

Post by TomLouwrier » Tue Nov 03, 2015 2:20 pm

Hi Beardy,

My pleasure. As discussed elsewhere in this forum the equal length py/sy or short py / long sy appear to have been proven old skool (or just plain wrong). Mind you it's not my personal experience, I'm quoting our host here. Straight from the horse's mouth if you pardon the expression :-)

regards
Tom
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Guy Croft
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Re: Exhaust Advice

Post by Guy Croft » Tue Nov 03, 2015 4:04 pm

all good Tom, well done and thanks for your email just now.

I shall assert fact as I know it to be here! Equal length py and sy will not give anywhere like the spread of torque that long py and short sy will. The optimum design of ex systems was never understood until Prof Blair's extraordinary 'Virtual 4 Stroke' software was launched (early 2000's, not quite sure). It made almost everything ever done before almost 'wrong' overnight.

One could opt for a slightly less costly setup (compared with 4-2-1) by using a 4-1 where the overall pipe length is the sum of the 4-2-1 py & sy.

The only other thing I would emphasise (beyond what Tom has said) is the merge angle of the collectors which is critical and MUST be not greater than 15 deg included angle. According to pipe diameters that can make the collector quite long. Some folk get this wildly wrong. You can get terrible power-robbing reflected wave interference if the collector is too short - and thus wider - than that.

The firing order is 1342 and on a 4-2-1 you link 1&4 py pipes and 2&3..

G
Guy Croft, owner

beardybloke
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Re: Exhaust Advice

Post by beardybloke » Wed Nov 04, 2015 7:07 pm

Thanks Guy , the advice is very helpful . At least now I know what I should have , just got to persuade them to build it for me .
The insurers are chipping in with a far bit of the cost as the system was damaged in the accident , therefore I thought it was a good opportunity to get the whole think redone at a reduced cost . Unfortunately I have been applying this logic to most of the external finishing on the car so will end up with a fair size bill myself in the end !.
Thanks for the help , Andrew.
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Guy Croft
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Re: Exhaust Advice

Post by Guy Croft » Wed Nov 04, 2015 11:31 pm

the best guy by far - BY FAR - owns Zero Exhausts in Kent,

mention my name and get Kevin to call me if it goes forward,

none of the others in the UK are remotely interested in my guidance but he is.

Most of the folk doing exhaust 'bespoke' systems for race cars are no more than welders or pipe-fitters, with little understanding of the mechanisms involved,

with no disrespect to those honourable trades....

G
Guy Croft, owner

beardybloke
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Re: Exhaust Advice

Post by beardybloke » Sun Jan 03, 2016 12:31 pm

Hi Guy , I thought I should let you know that the manifold is not going to get done at this time as the insurer`s repairer is still completing the repairs and although their exhaust supplier was willing to build a system matching your specifications he would not agree to do it as part of the repair due to the repair`s usually letting them down in terms of time scales . So when it eventually gets done the main pipe and silencer is to be replaced , as that was damaged in the accident , the manifold will have to be left to a latter date .
Not sure if you or anyone else has used them but their exhaust people are Infinity exhausts near Bristol , they seemed more than willing to build to your specification`s so I may still use them as they very local to me , many thanks for your advice , Andrew Webb.
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