Porsche Enthusiasts Club Forum

I spoke to H&S yesterday and they told me that someone had agreed a group buy price with them and was going to post... Anyone?

I did

The link is further down the main page.

Count me in for the Fabspeed full monty please Jon, including the 3" Turbo elbow

Doh, got it, thanks...

Advice from JMG...?

Jon,

The chap at H&S advised his '3in reducing to 2.5in' product over a straight 3in exhaust (as per Fabspeed) on the basis that the 3in item would harm spool. He reckoned his 3in into 2.5in would bring the torque on earlier in the rpm range. Any thoughts on this? If you were aiming for a 400bhp car, is his recommendation good? Which one would you fit?

Hmmm.. Well my problem is that anyone going for an exhaust, will already have chips, will probably have a MAF kit, and will probably have an aftermarket turbo, as that is when the cost of an exhaust will be outweighed by the performance limitations of the standard 2.5" exhaust systems.

As an example, putting a 3.0 inch system, or any performance system on a car with a standard turbo, a DPW, EBC and MAF kit will see no peak gains, but will see an improvement in torque and spool up.

However, with a hybrid turbo rated at above 400 BHP a standard system will leave you stuck at about 350BHP peak and 380 ftlbs of torque at about 3500 to 3800 rpm, with spool up to 1 bar being at about the same point as peak torque. Once you then fit a 3.0 inch system, we have found you should be getting about 400 to 420 BHP peak with about 390 to 400 ft-lbs and spool up occurring about 500 rpm sooner. At which point the cost of the fabspeed or any 3.0 inch system has taken 500 rpm off the lag, increased peak horsepower by about 70 BHP and peak torque about 500 rpm sooner and rising by 20 ftlbs.

I have never fitted a 2.5 rear section to a 3.0 inch front section as that is contrary to the thermodynamics and physics of the system as I understand them.

A 2.5 inch pipe has a cross sectional volume that is 28% less than the cross section of a 3.0 inch pipe (just over 7 square inches for a 3.0 inch pipe, versus just under 5 square inches on a 2.5 inch pipe - forgive me if my math is off, but it is late!).. which means that the exhaust gasses to have contracted by 28% to be at the correct volume at the tailpipe due to cooling to maintain equal back pressure along the whole system, which I guess H&S are aiming for.

In particular we have combined gas law, or ideal combined gas law which is used to predict how a gas will expand and contract and what pressure it will be at with any set temperatures and volume. In particular Charles law will be as easy fine law to dictate how the gas will contract as the volume decreases or temperature decreases... We use similar laws when working on intercooler design as well as exhaust design.

This roughly means that if the EGT exiting the turbo is for example 600 degrees C in a 3.0 inch system, by the time it reaches the exhaust tailpipe (moving at the speed of sound, so in a very short time period) it will have cooled by 200 degrees to still be at the same pressure in the 2.5 inch section as it was at the 3.0 inch section using charles law to work out the exhaust gas pressure and temperature.

And even if it would have cooled by this amount in this very short time period, the change in pipe diameter would have to be increase along the entire exhaust length to make it worthwhile, otherwise the vortexing gasses caused by the abrupt change would cause losses, as well as the sudden increase or decrease in volume would again cause a step up in pressure and temperature, under the same gas laws.

However, if you maintain a 3.0 inch diameter along the entire exhaust, you will find that there will be a reduction of exhaust temperature (I cant recite exact amounts) but you will also get a pressure drop (again predicted by gas law and more specifically Charles Law), which will in fact help extract the gas behind it.

Back in the 70's there was a fad for extractor exhausts, especially in Porsche and VW tuning circles, and even by the Porsche teams, which used the exact opposite to what H&S are doing, where the exhaust diameter increases along the entire length of the exhaust, with the sole purpose of forcing the exhaust gasses to cool and contract along the exhaust, so that an exponential drop in pressure occurs by the time the tailpipe is reached, literally causing negative pressure behind each exhaust pulse in a normally aspirated car.

I do not make exhausts, it is not my business, but I do tune cars and select components to use, or manufacture our own whenever there isnt the right thing out there... However Gas law and its basic components, including Gay's, Boyles and charles, dictate gas pressure, temperature and volume, which no one can get away from, even someone with a good trading name.. physics laws are laws until proven otherwise, and until we start talking about these as the old laws, and a new Haywards law, or Scotts law, I think the physics I have mentioned hold true.

You can make a 944 turbo exhaust cool by 200 degrees along the length of the exhaust while on boost and at reasonable RPM, but that requires an increasing diameter of exhaust pipe along the length, rather than the opposite, at which point you throw out the whole reason for the calculation, to see if there is a benefit to a reduction in exhaust diameter.

If you are going to get a H&S then get one which is a 3.0 inch along the length, from the turbo all the way to the tailpipe... If you get a fabspeed one, I know what results you will achieve from previous experience, however with H&S I can not say.

I am always fair in my review of a product, so if I ever get hold of a H&S I am happy to put it up against a fabspeed, a competition I would surely like H&S to win, as their product is British, and buying British is the way we can get the UK economy improving (forget quantitative easing and daft political policies) and if the H&S ones beat the Fabspeeds, I would back them in a second... But the explanation I have seen for how the 3.0 down to 2.5 inch would make more power, falls short of impressing me, I am here to be converted though.

There is already a H&S group buy setup and running by Scott, so it would be daft to split the camp into two and have half the people looking at a fabspeed group buy and half buying a H&S, so my advice might be to buy the H&S on that group buy, I am sure it will be a good system and the gains will be similar to what I have seen on a fabspeed system, just DO NOT BUY THE ONE THAT REDUCES TO 2.5 inches. BUY THE 3.0 SYSTEM!!!

I wonder if H&S would knock a few quid off the extractor without the polished finish ?.

While I appreciate the logic, my experience with a lot of bike exhaust systems including Akrapovic ,which are as good as anything probably, is that very few gain something without losing it somewhere else, ie 15 bhp at the top end is often at the expense of 5 bhp in the midrange . I tried an almost open system on my turbo and as well sounding like a tractor my backside dyno felt it spooled up 1k later though it did come on stronger over 5k , fitting some slightly restrictive baffles restored the response which didn't fly well on rennlist amongst the bigger is better crowd. Exhaust efficiency starts at the camshaft/turbo/compression ratio stage I think.

Some people do not entirely understand how a turbo works, many people think of the exhaust turbine as being a windmill being blown round by gas flow, however the gas flow only contributes about 20% of the turbine energy. 80% of its energy comes from a pressure differential between the turbo inlet and the turbo outlet.

Increase the inlet pressure and you get an increase in turbine shaft energy.

Decrease turbo outlet pressure and you also get an increase in turbine shaft energy.

Increase turbo inlet temperature, or even decrease the heat loss from the turbine inlet and you increase turbine efficiency through an increase in turbine inlet pressure..

Decrease turbo outlet temperature, or even increase the rate of thermal loss from the outlet of the turbine and you get an increase in turbine efficiency through a turbine outlet pressure drop..

The second two above experience a directly proportional change in pressure differential through thermal changes, which a lot of people miss. Especially when they start using the wrong kinds of thermal management on the wrong components, such as thermally wrapping or coating a turbine housing, which is another important area where pressure is designed to drop through expansion, which also causes cooling.

Changing the thermal and pressure dynamics of your pre and post turbine exhaust plumbing will always increase turbine energy at any rpm and therefore spool up as well as turbo efficiency... However, choke the inlet too much and you will loose energy at higher rpm due to a reduction in flow and therefor a ceiling to the amount of available energy to the turbo as well as causing rising exhaust gas temperatures which can damage your engine.

Basicly, the name of the game in getting more performance from any given turbo charged engine (as well as the more known about ideas such as chipping, bigger turbos, better wastegates) are the following.

Pre Turbo exhaust inlet

decrease thermal loss
increase gas temperature
increase pressure

Post turbo exhaust outlet

Increase thermal loss
Decrease gas temperature
Decrease pressure

Do this and you can move your turbos efficiency around in your rpm and boost range, which can convert a badly setup aftermarket turbo into a really efficient one, or if done badly can loose loads of power.

The benefit if this, as long as you do not choke your top end flow, you really can make gains without losses.

Every turbo has an efficiency window, beyond a set turbine shaft rpm (for that turbo) the turbo will become inefficient, so the key trick is to make sure you alter the dynamics in a way which sees the turbo hitting its maximum efficient rpm close to peak power, and still is able to spool up as very low as possible, a bit of a juggling act, but certainly possible.

The thing I like about 944 turbos when they are tuned to the 420 BHP range, is you can eat your cake and have it, with a nice 400 ftlbs of torque, full boost (1.2 bar) at bellow 4000 rpm and the car feels like it was always designed to have that power and no real loss of reliability.

Some of you at this point may wonder what the point of a 2.8, 3.0 or 3.2 engine would be.. Instantly, with just the same performance parts as the 2.5 420 BHP car, but with earlier spool up and much more power off boost... But start to match bigger turbos' a better inlet, and more to this and BHP goes up, be delicate and precise about the parts you use and so will torque while keeping the low spool up.. But for a road car a 2.5 with 420 BHP and 400 ftlbs of torque is wonderful.

Anyway, back to the point.. pressure differential is where it is at with a turbo..

With a non turbo, Paul is absolutely correct, depending on how good the standard kit is, all you can do is remove unnecessary equipment (catalytic converters, EGR systems) improve efficiency and after that you are left with robbing power from one place to make power else where.

Some of this probably makes no sense, as I do tend to ramble on somewhat.

Just skim-read this thread and I'm 100% with Jon. Why would you want a turbo exhaust to get smaller towards the end after the turbo?

Wasn't it Corky Bell that said the best turbo exhaust is no exhaust? lol. (post-turbo that is)

Maybe because as the charge is cooling it's contracting. I put a bigger bore exhaust with fewer silencers from the turbo back on a Cosworth years ago and it dropped bottom end response. It's hit or miss whether some blokes in a shed can build a substantially better road system than Porsche. Sure with a bigger turbo you need a larger extractor/downpipe but I'm not convinced it's as important 6ft + downstream of the turbo.

Off-boost it wouldn't surprise me if there was some detrimental effect but should be offset somewhat by a slightly lower boost threshold.

There are better methods of improving off-boost performance though without closing the pipe and really they should be explored as all tuning has a knock-on effect.
Personally though I prefer to optimise on-boost performance and efficiency, as this is where the bigger gains are.

I'm ~500cc down on you guys and considering going larger than 3".

You would think so and I was disappointed when my big bore stainless Cosworth exhaust with only one rear box seemed to have delayed spool, the same again with a much less restrictive 944T system. Let us know how that big exhaust works out.

Interesting... US 944 GTR race cars have the exhaust coming of the front wing ie. About 2 feet long, but they don't spend much time off-boost!!

I am really interested in Pauly's experience with a cosworth dropping bottom end response after removing some restriction from the exhaust system (larger bore or less silencers).

I am no expert on Cosworths, but I do wonder if the reduction in exhaust back pressure, increased evacuation of the cylinders in the exhaust stroke and therefore lowered low RPM manifold pressure. And if the cosworth used a MAP sensor for load sensing, it might have given different timing and fueling relative to the lower manifold pressure.... Just a guess as I do not ever remember seeing a VAF or MAF in a cosworth engine bay, but do know they had a map sensor. Which might explain Pauly's experiences.

Did you try having the car re-set up with the bigger bore exhausts on the cosworth Pauly?