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Triumph TR6 - Single vs multiple exhaust pipes

My 71 TR-6 was stock with a single exhaust from the manifold to the mufflerand I plan to replace it with a stainless steel system throughout. Since I'll already be in this deep, would there be advantage in going to a multiple pipe exhaust system? I understand that I'd need a different manifold or headers to do so, but since I'm investing substantially in the exhaust system, going a little futher and exploiting a design change seems to be a marginal increase in overall cost, especially if there's increaed performance/reliability/etc to be had. Comments?
db
D R Baker

You'll loose about 8hp thru dual exhausts. I'm thinking of spending money to go from dual back to a single exhausts.

Perhaps we could work a deal. I have a set of Falcom Stainless steel pipes and mufflers. Loud as hell. Looks good too.

Out...

John Parfitt
73 5 Speed.
John Parfitt

I would think that loss of power would be relative to the the large diameter single pipe system. For the stock type single pipe, the cross sectional area of the exhaust is smaller than the total for the stock twin pipe system used on the TR5/TR6PI and later cerb TR6s. Remember that for the carb TR6s such as in this market, power was little changed over the entire production run of the TR6.

The early NA spec cars had higher compression ratios, but also had milder cams, the close inlet port spacing heads and single pipe exhaust systems. Later NA spec TR6s had lower compression ratios, but the same cam as the later TR6PI cars, the wide inlet prot spacing of the PI models and the same twin pipe exhaust. Net result, about the same power in either engine with some slight shifting of the power curves.

In my mind, it comes down to what you want out of the car. If you want what amounts to a stock system, but in stainless, just get the stainless replacement system for the single pipe cars. You might pick up some slight difference by going to the later twin stock type twin pipe set up, but it would be slight at best and would involve finding a twin pipe exhaust manifold and all the mounting hardware for an early type twin pipe exhaust system. This will raise the price of admission, but not by leaps and bounds.

On the other hand, if you are talking going the header route, then go ahead and do it right with a header/large single pipe system such as the Rimmer Bros "Sports System D," part number RR1400SS at 369.00 quid less VAT. The large single pipe type of exhaust system that will help make the best power, but it works best if you are putting more stuff into the pump called an engine. If you are running stock everything else, not too much will be gained. At a $1.75US to 1.00, that works out to $645.75 US. Plus you have to throw in shipping, so it would involve parting with a fair amount of brass, say $750 or so by the time it gets to you in Rocket City. At least you wouldn't have to pay the 17.5% VAT since it would be shipped over here and out of the reaches of Inland Revenue.

Now for a little soap box "opinionating" on headers and street cars. They just aren't worth the effort, especially given the issues with the TR6. They can get in the way of starter removal and replacement, they are extremely close to the frame on a TR6 and with the cheesy TR6 motor mounts are almost guaranteed to wind up hitting the frame and be damaged at some point. They are never truly "bolt up and go," there is always some fiddling to be done. I could go on, but it is late. If it is a street car, my take is to do a good port matching job with a stock manifold and run it. It will make for less work time (and correspondingly, more fun time) in the long run. As always, your mileage may vary and well, we know about opinions and comparisons to the more undignified, nether regions of the body. Main things to remember are don't be afraid to ask questions and that it is your car. In the end you should do it like you want and not worry too much with everybody else and their opinions.
SteveP

John, Steve,
Thanks for chiming in. John, I'm surprised that you'd lose HP with a larger cross section which ought to offer less back pressure, thus less work for the engine. An uneducated opinion.

Steve, a good discussion. Thanks for your comments. You've helped a decision to stay with the single pipe unless something changes before I actually get to the point of making that purchase, still some months away. After I discovered PAECO Engines in Birmingham, I've redirected my thinking on this restoration somewhat. The engine has about 75K miles and for all I can see and learn from the previous owner, ought to be in fairly good shape so I had not planned a complete engine overall...just peripheral and relatively minor stuff, at least for awhile. Now, though, with the prospect of getting my engine redone for about $2K (short block) and upping the performance to 168BHP, I've begun to consider reallocating more of my budget toward the engine and not knowing about how the exhaust might affect performance and more particularly reliability it seemed like a logical area to explore. Probably not a lot of return, but then since I had the expense of a new system planned anyway, if I could gain some by going to the twin pipes, maybe it'd be worth it. I guess from what you've described, it comes down to whether I can find a twin pipe manifold reasonably. If not, then I'm probably best served by keeping my single pipe manifold and just replacing the current system.

My objective in the end is to produce a daily drivable car that I can enjoy driving to work or, if I'm so inclined, off on a jaunt across country without fear of failing along the way. I'm observant of staying as close to original as reasonable, but incorporating improvements offered by technology advances as I can. For example, I intend to install an electronic ignition system and do the oil filter change with a cooler...those sorts of things so staying faithful to everything original is not an objective.

Appreciate your thoughts and ideas. Stay tuned. Maybe since we're relatively close geographically (certainly relative to many on this BBS), we'll have the opportunity to compare notes in person sometime.
D R Baker

I have a '71 that I just rebuilt. I raised the C/R to 9.5to1 and installed a stage two cam. I am going to convert to a double outlet exhaust. I bought a used header off a '76 and ordered a stainless system. I wanted to help it breathe a bit better without spending an arm and leg. I'll report in with results after the conversion. BTW I also installed 1.6to1 roller-rockers and upgraded all the valve hardware,seats,guides,etc.
Mike Crane

Mike,
What you've done is kind of what I'm planning. I'll be very interested in your comments after you've had a chance to test the results. What did you do to change the CR and why 9.5:1 as opposed to say 10 or 11:1? What do you estimate the change in perforamance will be?
db
D R Baker

db--I raised my CR to about 9.5:1 by shaving 0.120" off my stock (4.550" thick, 7.75:1 CR) head. The shave amount was estimated using info on Richard Goods' website. This CR seems to be a consensus figure tempered by machine shop cost, pump gas use, reliability, and not wanting to strengthen the drivetrain to handle additional power. Also, I believe this CR is what the PI TR's ran at. I also installed a warm cam (Isky Z-19), new valve springs, tubular pushrods shortened by 0.120" from stock; the dynamic timing is set at 8 degrees BTDC (950 rpm). I could probably advance some more. I find that 89 octane gas works fine (no pinging) except for engine run-on at shutdown. The run-on disappears with 91 octane gas.

These mods have completely transformed my TR6 and I highly recommend them. No dyno results, but my butt says the effects are real!

Rick O.
72 TR6
Rick Orthen

Rick,
Wow!!:) Sounds like you have a bundle of energy with a seat behind! Used to know a guy who had TWO (2) COBRAs, one a 289 cu in and one a 427! Never could understand why you'd want to sit behind about 450HP in a 2 seater, but something greater that the stock 105HP in the TR6 seems to be a worthwhile goal. The TR5 was stock at 150BHP but the PI was responsible for a good apart of that.

Take a look at http://www.paeco.com/ and you can see how they beef up the engine with titanium push rods and Molychrome or titanium piston rods, enlarged combustion chamber with a longer stroke and milling the head etc. Looks like you're already well toward the 150 mark, maybe more. PAECO will build a TR 6 engine with 256 BHP, but advises that it'll be short lived requiring frequent tear down and rebuild, like once a year or so if the inherrent performance is exercised. Typical racing engine.

Thanks for the info.
db
D R Baker

Going back to the single vs. double pipe set up, remember that you have to think in terms of stock single pipe and the large bore single pipe. The power gain relative to the dual pipe system is for the big bore single pipe set up and not the stock one.

The 150 hp in the early PI cars primarily came from the higher compression ratio, hotter cam and better breathing intake port and exhaust configuration compared to the early NA spec cars. Later PI cars had a milder cam and were rated at 125 hp (same cam, inlet port spacing, and exhaust used in both later NA spec TR6s and PI cars, compression ratio was reduced further for later NA spec cars so power did not increase). This later cam was somewhere in between the early carb cam and the early PI cam.

Given pump gas these days, 9.5:1 is probably about as high as you want to go. Any higher and you have to think in terms of octane boosters, race gas, aviation gas, etc. The Richard Good table is a nice table and is in line with values we have taken on engines, but it is a table and will not give you precise information for YOUR engine. Having said that, the best way to determine your compression ratio involves some work as part of the engine build. First you have to determine the cylinder volume and any zone volume if you did not zero deck the block and pistons. You have to check the combustion chamber volumes and tweak accordingly to even them up. This could very well be an iterative process as part of the head work. From the cylinder volume, the combustion chamber volume, the volume of the head gasket opening and other corrections for deck heights. This will give you the compression ratio for your engine as it is being built.

To get the measurements, we take physical measurements for the bore/stroke, gasket volume and deck heights, then start crunching the nubmbers. We typically do short engines first to the desired deck. We take an educated guess based upon the head thickness as part of determining the deck height we want for building the short engine. Then we turn attention to the head. To determine combustion chamber volume, we use a burette and isopropyl alchohol with a dye in it to make reading the burette graduations easier. The alchohol has a very low surface energy and will wet just about anything making for a good measurement. Once we have that information in hand, it is back off to the machine shop to have the head shaved as needed to achieve the desired compression ratio. Match those ports to smooth out the flow of gases.

On the PAECO stuff, I dare say that the high hp motor is definitely not anything you would want on the street. The compression ratio would have to be extremely high, so pump gas is out. It would also have to have a very agressive cam in it. Agressive cams means that it will not be making power low in the rpm range (where you would be running a street motor). That much power also means that it is moving a great deal of air through the motor. Since stoichiometric ratio is stoichiometric ratio regardless of the amount of air, then correspondingly huge amounts of fuel would also be going through the motor. I bet if you looked at the exhaust real close, you just might be able to see charred tiny dinosaur bits. I know one guy that has one of their Stage II motors with 40 DCOE carbs in his street TR6 and loves it, I know another that bought one of their race motors for his G prod Spitfire and had nothing good to say about the motor. My own dealings with PAECO have been mixed.
SteveP

To (my mate) db!

You may be interested in checking out the following for some general info on a range of things you will need to consider.

http://home.att.net/~jroal/perftheory.htm

As far as exhausts go, there has been a lot of experimenting done over four decades. The 6/3/1 has been found to be the best for all but the highest revving race engines and many systems which achieved the best results have been 'modified' to be commercially viable for manufacturers. The modifications reduce the performance from the 'ideal'.

I spoke to Simon (the MD) of the British company Phoenix Exhausts on this very matter. http://www.phoenixexhausts.co.uk(Phoenix is VERY highly regarded in UK for its TR6 products). They had their 6/3/1 manifold dyno tested in Germany and it gives a 'bolt on' 13bhp extra. The thing is, that their design is NOT the most efficient you can get (unequal primaries), but is commercially the best (for them and us!). They were faced with pressure to provide a system that would suit a range of Triumphs and some comprimises were necessary. Even so, their system is hailed by many enthusiasts as being one of the best around. A custom made set would give the best results, but is $$$.

Unless you are in the seriously high rev range all the time, our straight sixes need an 'interference' type header which scavenges gases from the exhaust stoke even when inlet mixture is starting to enter the cylinder. There is a magic length for the primary pipes when fabricating a header (20", with the first 3" straight). This length is determined by the wave pattern so that when exhaust pulses from cylinders (firing 180 apart) the end of one pulse co-incides with the beginning of the next and so on. The shock wave of the exhaust pulse creates a negative pressure (rarefaction/suction) behind it, which assists the following pulse along etc etc. Unfortunately limited space in the engine bay makes it tricky (but not impossible) to do.

Bottom line is, that if you look at a commercially produced header, ensure that A) it is a 6/3/1 and B) that they are connecting 1/6, 2/5 and 3/4 ports and C) try to get the primary pipes are as equal in length as you can get.

Continuity in the rest of the system is an advantage. A 2.5"OD single pipe is ideal but limitations on muffling exist. It is possible to put two mufflers in series but space can be tight. Depending on how much noise you're prepared to put up with, you may find that splitting back to a twin system (at the diff) enables you to get two good sized 'straight through' sports mufflers side by side. Your pipe diameter to these is smaller (1.75"). A well made splitter should not result in any significant drop in efficiency.

All this said, you may be wondering if it is all worth it - quick answer - YES!

I have not yet read a book on improving the performance of the TR which does rate the improvement of the exhaust as a significant factor in the overall picture.

Cheers
Roger

PS : A word on 'twin' systems.
Many aftermarket'twin' systems are 6 into 2 at the header. They separate the exhaust pulses into two parallel streams and do not allow for any resonance which creates the desired 'suction' effect. This is the reason for a drop in power.
The TR's factory twin system was used in conjunction with a cast iron manifold which had a resonant cavity just above the flange. This cavity allowed the exhaust pulses to interact to form a more continuous stream of gas (a short 'header' of sorts). There is nothing wrong with a twin system, so long as there is a chance to combine the pulses into a continuous single flow before the gases are then split into two. (My twin system with standard cast iron manifold gave 119hp (SAE,corrected) at the rear wheels).





Roger H

Steve, Roger,
You guys are awesome. I never expected such a technical discussion. Had no clue that there was so much to consider about this issue. It's clear that y'all are serious about what you do and just as clear that I have an awful lot to learn. In my business, there a saying about being prepared for the things that you don't know that you don't know!! Thanks for the discussion. I don't know where I'll finally take this LBC, but you're really given me a lot to think about as I continue this adventure. Just this morning on the way to work, I followed a BRG TR6 into my own parking lot. No clue to whom it belongs, but he apparently works for my company so I'm off to find out. Made me jealous that he was driving his and I not mine. Lots to do before I get to that point, unfortunatley, but it'll come.

Roger, the 6/3/1 setup you described with a split at the differential and 2 mufflers...where would they fit? Parallel? and if so is there room to fit them up into the frame or would they hang below and be at risk of often dragging. Would I have to modify the frame (chassis) to accommodate the attachment or are there points to affix support for such an arrangement?

Thanks again for an intriging discussion.
Doug Baker
D R Baker

DB
Rick Orthen pretty much covered the subject. I was looking for increased performance(which I got) without losing any reliability. Basically I brought my '71 up to the 150PI specs but without the petrol injection.
Roger Williams books "How to restore TR250/TR6" and "How to improve TR5.TR250,and TR6" are good resources. See Richard Good's website for more information on compression ratios-www.goodparts.com.
He also has a informative article on roller-rockers.
I hope to get some dyno figures after the exhaust system is installed.
Mike Crane

DR

My mufflers are custom made - oval, 6.5" x 3.75" x 16". Inlet / outlet is 1.75" pipe up 1/2" above the bottom of each unit. This enables 1.5" clear between muffler and underside of car. They're located inside the chassis rails, starting at the diff mounts and finish before the tubular crossmember under the trunk floor. They're not parallel with 6.5" between them at the diff end and 14" between them at the back of the car. The pipes split at the rear of the 'tunnel'. You can use the original hanger on the crossmember, and fabricate another to match (for the second unit).

Standard mufflers with a centre outlet will hang slightly lower. Pipes rake up to follow the general contour of the underside to give ground clearance on a steep incline.

I could email you a photo or two if you want.

Cheers
Roger H

Roger,
Thanks for the offer of photos. Email is shown. I'd love to see how you achieved your setup.
db
D R Baker

This thread was discussed between 19/05/2004 and 24/05/2004

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