bell-auto-services wrote:

Dont go to large a diameter as the back pressure lost does have side effects.


Pete

they're 63mm diameter so same as standard

K9F wrote:

Harry,
I am surprised at James for being in the exhaust making business

James is quite an accomplished Equine Dental Technician (Horse dentist), and only sells exhausts as a part time hobby. His systems used to be made by Magnex (to his recipe), until they went pop a few years ago, so not sure who makes them now?

I badgered him for years on the downsides of 3" bore exhausts, as at the time that was his USP. As Pete says, too big is not a good thing! The larger the bore, the slower the gas flow, and the deeper the resonance. This also causes excessive heat soak, and with the reduction in back pressure, can overspeed the turbocharger resulting in a rapid failure. I guess you can only stomach so many come backs, and so I assume he's now gone to 2 1/2" bore as I suggested he should have done many years ago.

Not sure about his other USP which was his movable exhaust hangars. Not a single flange joint on these either, as they're all slip joints. That's not a good thing if you consider the shape of a Defender exhaust, and the way it naturally falls when on the floor. You can multiply this effect several times when exposed to the vibrations from the diesel engine, and so if the U clamps are not tightened up enough, the sections of exhaust can slip.

Another issues with slip joints is they have a tenancy to blow/leak exhaust gasses, and so need a sealing compound to help them do their job.

ALIVE exhaust systems have always had flange joints (Our USP). When we launched our exhausts many years ago, nobody else in the LR aftermarket made their exhausts in this way, but now several have adopted this method, mostly due to the reasons above. It does make them more expensive to make (around £20 for every pair of flanges), but they bolt together only one way, and will not move, or leak, once put together.

Our exhausts are available in a number of variations too, with single or twin silencer options.

Quote:

The larger the bore, the slower the gas flow, and the deeper the resonance. This also causes excessive heat soak, and with the reduction in back pressure, can overspeed the turbocharger resulting in a rapid failure. I guess you can only stomach so many come backs, and so I assume he's now gone to 2 1/2" bore as I suggested he should have done many years ago.

Quote:

Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.

Thanks for the dig, Ian.

Although you rightly point out that this particular thread is related to the puma, the principles of exhaust size and construction are much the same across all turbocharged engines, and so my points were meant to be a generalisation of benefit to all. Of course, a lower back pressure will have benefits, in allowing the turbocharger to spool up quicker, and this is indeed why we fit decat downpipes, and remove the bypass type silencers in favor of a more free flowing mid pipe design.

Regarding the bore of the exhaust, this should be sized depentent upon peak exhaust gas flow, to ensure a rapid expulsion of gas (and heat) from the engine. 2 1/2" bore is adequate for all but the most highly tuned Td5 models, where 2 3/4" is required. The size of the bore most certainly does have an effect on noise and harshness.

Quote:

Over speeding can be an issue on all turbo's, but especially FGT with a normal mechanical actuator.

However, as we are talking about Pumas' - they use a VNT and a REA (rotary electric actuator)… in this case within sections of the calibration there is open loop overspeed protection maps built in.

Not in the Puma Defender 2.4 Calibration there isn't! The only turbocharger related open loop control maps are to control the REA with calculated approx aerodynamic force on VNT, and Demand position for VNT actuator in %. All values in this map are set to null, as there is no overspeed protection, other than the turbo pressure limiters on that model.

A VNT turbo can be destroyed through overspeed as easily as a FGT, and often do. Usually as a result of a sticking actuator.

Sticky wrote:

I also use the Griffin exhaust on my 110 Puma, it made an immediate impact on driving especially those low speed gear two and three changes. I found with the new exhaust I could stay in third for longer.

In fact I am more pleased with the effect the exhaust change had than the tuning, although removing the speed limiter was fantastic.

It is always interesting to see a vendor laying into someone else's design, I would love to see Jame's response about the Alive system too.

Yes thats how I feel too