924Srr27l
New member
There's been 2 other related posts, one in regard to a Rattle, and another re fitting a Decat pipe.
Both with further questions, opinions and solutions and what may occur in the case of removing a Catalyst & fitting
either a straight pipe or Resonator .
Here's some interesting notes I have on file which explains the job of exhaust silencers, resonators and the Myths of Back pressure and what effects flow (Volume) and velocity (Speed) have on an Engine's power characteristics, and bore sizes.... Which may help.
The job of Exhaust Silencer / boxes is to lessen the sounds of the engine to an appropriate and acoustically pleasing level by reducing the sound pressure emitted. OE exhausts are restricted by efficiency concerns, ease and cost of manufacturing, and of course sound level laws.
Exhaust Silencers are engineered with multiple chambers that expand exhaust gases as they pass through them, these chambers feature perforated tubes or baffles or both.
Exhaust gases pass through these perforated holes and baffles, resulting in expansion. As the gas expands, its pressure lessens, and consequently so does the sound level. Furthermore, and OE silencers are often packed or lined with materials (such as fibreglass) as a soundproofing measure to further absorb the sound. The baffling also increases engine back pressure by decreasing how fast the exhaust gases leave the system but this pressure can hamper performance.
The purpose of a Resonator is to cancel out a certain range of sound frequencies.
Sound is simply a pressure wave emitted at a certain frequency. Like waves in the ocean, sound waves have certain amplitudes (comparable to overall size), a crest and a trough. At the beach, when the crest of a wave meets the trough of wave of the same size, the two waves actually cancel each other out and there will no longer be any wave. The exact same principle applies to sounds waves. If you have two sound waves of the same size and frequency meet crest-to-trough, they too will cancel.
Typically an OE Resonator sound engineer will choose a range that is not pleasant to hear and build the resonator to eliminate that frequency. Noises that are cancelled are harsh noises or ranges where the exhaust note produced would be a loud drone or irritating buzz.
Resonators are not quite a straight-pipe, but no too far off. They have less baffling and don't reroute exhaust gases as much as a muffler which reduces back pressure, potentially freeing up a few horsepower.
[FONT=helvetica, arial, sans-serif"]Back pressure:![[FONT=verdana,geneva"]](/forum/styles/default/pcgb/space.gif ". [FONT=verdana,geneva\"]")
[FONT=helvetica, arial, sans-serif"]One of the most misunderstood concepts in exhaust theory is back pressure. People talk about it with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need back pressure" when discussing exhaust upgrades.
[FONT=helvetica, arial, sans-serif"]1. Some basic exhaust theory
[FONT=helvetica, arial, sans-serif"]Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pulses and so on. The more pulses that are produced, the more continuous the exhaust flow. Back pressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
[FONT=helvetica, arial, sans-serif"]2. Back pressure and velocity
[FONT=helvetica, arial, sans-serif"]Many people mistakenly believe that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this idea would be appealing - as wider pipes have the capability to flow more than narrower pipes. However, this omits the concept of exhaust VELOCITY. Here is an analogy...a garden hose without a spray nozzle on it. If you let the water just run unrestricted out of the hose it flows out limply at a rather slow rate. However, if you take your finger and cover part of the opening, the water will spray out at a much much faster rate.
[FONT=helvetica, arial, sans-serif"]The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be travelling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you also want make sure the pipe is wide enough so that there is as little back pressure as possible while maintaining suitable exhaust gas velocity.[FONT=helvetica, arial, sans-serif"]
[FONT=helvetica, arial, sans-serif"]Back pressure at it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust will flow backwards, which is...er... is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero back pressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range (remember the pulses!). A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of back pressure at high rpm. Thus if your power band is located 2000-3000 RPM you'd want a narrower pipe than if your power band is located at 8000-9000 RPM.
[FONT=helvetica, arial, sans-serif"]Exhaust Manifolds
[FONT=helvetica, arial, sans-serif"]The manifold is an assembly designed to collect the exhaust gas from the engine cylinders into exhaust pipes, (either one or two with Morgans). Manifolds are often made of cast iron in stock cars. Many have material-saving design features to use the least metal, to occupy the least space necessary, and/or have the lowest production cost. These restrictions often result in a design that is cost effective but that does not do the most efficient job of venting the gases from the engine.
[FONT=helvetica, arial, sans-serif"]Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates back pressure and restriction in the engine's exhaust system and limits the engine's true performance possibilities.
[FONT=helvetica, arial, sans-serif"]A header (aka branch manifolds or extractors) is a manifold specifically designed for performance. Engineers create a manifold without regard to weight or cost but instead for optimal flow of the exhaust gases. These designs can result in more efficient scavenging of the exhaust from the cylinders. Headers are generally circular steel or stainless tubing with bends and folds calculated to make the paths from each cylinder's exhaust port to the common outlet all equal length, and joined at narrow angles to encourage pressure waves to flow through the outlet, and not backwards towards the other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow in a particular engine revolutions per minute range and married to the firing sequence.
[FONT=helvetica, arial, sans-serif"]How did the myth about back pressure and big pipes come to be?
[FONT=helvetica, arial, sans-serif"]I believe it is/was a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a Honda Civic decides he's going to upgrade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the power band. He makes the connections in the following manner: "My wider exhaust eliminated all back pressure but I lost power, therefore the motor must need some back pressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero back pressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
[FONT=helvetica, arial, sans-serif"]Why is exhaust velocity so important?
[FONT=helvetica, arial, sans-serif"]The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this article but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle travelling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
[FONT=helvetica, arial, sans-serif"]Many designers will increase the length of the exhaust, trying to achieve a faster flow and a larger area of low pressure. Short pipes create a smaller low pressure area..[FONT=helvetica, arial, sans-serif"]
[FONT=helvetica, arial, sans-serif"]CONCLUSION Engines function best with the exhaust system they have been designed with, which has been tuned to its needs and characteristics with a goal of producing the most efficient scavenging.
[FONT=helvetica, arial, sans-serif"] If changes are made to a system's bore size, the amount of silencers are reduced & removed (Including Catalysts) the engine ECU will need to be retuned to address the changes in velocity and flow otherwise performance losses will occur.
[FONT=helvetica, arial, sans-serif"]However, some owners are more interested in the noise of their exhaust than the performance of their vehicle. If it is noise and exhaust tone you really want, that is a much simpler, non-invaisive goal and much easier to achieve.
The 968 3.0 Exhaust system has sections of twin pipes with smaller bores than the single 3.0 S2 system, this speeds up the velocity which works well in tandem with it's variable valve timing cylinder head.
Exhaust Bore size v Engine Power
why is back pressure bad? The engine need flesh air to burn fuel. The more air you have, the more completely you can burn the fuel. The more fuel you can burn per cycle, the more power you make. It is that simple.
Back pressure impedes the flow of exhaust out of the cylinder. If exhaust gasses are left in the cylinder there is less room for the fresh air that we need to burn fuel. Thus, with back pressure you can "choke” the motor. But why is the backpressure idea a half-truth? This is because velocity has to be considered, too. If you can keep velocity up, you can actually pull more exhaust out of the cylinder and more air in through the scavenging effect. So there is a bit of truth in the idea that you don’t want too big of an exhaust, but the reason isn’t for backpressure, it is because you want to keep velocity up. Tube size is what limits flow as in CFM or a volume over time. Velocity is just speed apart from volume. Take a garden hose for example. If you use a nozzle on a garden hose to choke the flow, you will increase velocity but decrease flow. As I mentioned before, you do need to have enough velocity to ensure that exhaust gasses are not pulled back into the combustion chamber in the brief moment when the exhaust valve hasn't completely closed, the intake valve is beginning to open, and the intake stroke is beginning. But velocity alone isn't enough. You have be able to flow enough volume to ensure that you are completely able to empty the exhaust gasses from the chamber. You can calculate the CFM (flow) required to support a motor of given horsepower. A commonly used estimate is 2.2 CFM per flywheel horsepower. Another common estimate needed is the CFM per sq inch of tubing which is 115 CFM per Area in cubic inches.
So here is an example: Let us assume we have a motor making 400 flywheel horsepower and we'd like a single pipe exhaust.A 3" single exhaust is good for about 340 crank horsepower. Our motor is 400 hp at the flywheel, we need an exhaust size larger than 3".Now I'm not saying that 4" would work fine, either. 4" tubing supports much more power than we are planning to make. While the flow would be adequate, the velocity would be too low because the exhaust pressure would be diminished.Thus, the rule of thumb with the calculations is to use the smallest tube diameter that supports the horsepower you are planning on making.
My 924S design / Application
I used all this info to decide what system I wanted to design for my 2.7L 8v Motor as I wanted not just the lightest system I could find
but also one that would give me the best characteristics for Road use (Low RPM) and Not (full throttle - High RPM) Track / Racing use.
Martelius in Finland sell, and make lightweight Steel thin wall tubing (1.2mm) and custom silencers with aluminium cores.
Only Stainless can be used this thin as it's stronger than mild steel, and of course this has an added benefit of not rusting!
I chose a 2" Bore and only (1) rear back box not a 2.25" bore and 2 boxes like the original system.
However I was surprised to see when the pieces arrived from Finland they were 2" OD and hence the Internal bore was 48.4mm, quite a bit smaller
than the original ID 53mm? 924S 2.5 / 944 system but this has 2 silencers which are not straight through but restricted.
The stock 2 box system on a 924S is 19.5 Kg, the Martelius (1) rear back box system is 8Kg so the weight saving was great but the bore size was a concern as
even Martelius said it will begin to be restrictive up to 185bhp, but the engine responded well (8 hours of mapping from 179 to 205bhp & 205Ft Lbs)
So from the general characteristics and info explained above, my conclusion is the reason the motor produced the results it did
(40bhp more than the 944 2.7 production Porsche) was because the smaller bore meant the exhaust speed has (Faster velocity) and the lack of restrictive silencer (s) also gave minimal back pressure and the Manifold I fitted was a tubular 944 Turbo one plasma coated by Zircotec for better scavenging.
(Unyet guidelines for 200bhp can't be achieved with a 2" Bore pipe??)
Add on top the Head is the small valve 2.5 8v, and not the larger valved 2.7 Porsche produced... maybe the flow is faster here too?
The head's inlet ports were worked by Lindsey Racing Flow guy which increased the CFM by 28%, with a Mild Camshaft and the
Inlet Manifold Extrude hone polished (+20% CFM)
it all makes sense that the motor's inlet and exhaust systems have been optimised with the live mapping by Chip Wizards but the really surprising bit is the exhaust bore is smaller than stock!
For road use it's ideal, as this tune produces a lot of torque low down the RPM range, and peak power is @ 4500RPM, This is all on stock 2.5 8v ECU, AFM, Fuelling and Ignition.
Big is clearly not always best (in this case)... I reckon I could fit a twin cam head, throttle bodies and a larger bore exhaust and touch 250bhp but this power
would be at a higher rpm and I don't think it would be as nice and drivable on the road as it is now.
R
http://924srr27l.co.uk/wp-content/uploads/2015/05/TEch-Spec-924srr27L-.pdf
Both with further questions, opinions and solutions and what may occur in the case of removing a Catalyst & fitting
either a straight pipe or Resonator .
Here's some interesting notes I have on file which explains the job of exhaust silencers, resonators and the Myths of Back pressure and what effects flow (Volume) and velocity (Speed) have on an Engine's power characteristics, and bore sizes.... Which may help.
The job of Exhaust Silencer / boxes is to lessen the sounds of the engine to an appropriate and acoustically pleasing level by reducing the sound pressure emitted. OE exhausts are restricted by efficiency concerns, ease and cost of manufacturing, and of course sound level laws.
Exhaust Silencers are engineered with multiple chambers that expand exhaust gases as they pass through them, these chambers feature perforated tubes or baffles or both.
Exhaust gases pass through these perforated holes and baffles, resulting in expansion. As the gas expands, its pressure lessens, and consequently so does the sound level. Furthermore, and OE silencers are often packed or lined with materials (such as fibreglass) as a soundproofing measure to further absorb the sound. The baffling also increases engine back pressure by decreasing how fast the exhaust gases leave the system but this pressure can hamper performance.
The purpose of a Resonator is to cancel out a certain range of sound frequencies.
Sound is simply a pressure wave emitted at a certain frequency. Like waves in the ocean, sound waves have certain amplitudes (comparable to overall size), a crest and a trough. At the beach, when the crest of a wave meets the trough of wave of the same size, the two waves actually cancel each other out and there will no longer be any wave. The exact same principle applies to sounds waves. If you have two sound waves of the same size and frequency meet crest-to-trough, they too will cancel.
Typically an OE Resonator sound engineer will choose a range that is not pleasant to hear and build the resonator to eliminate that frequency. Noises that are cancelled are harsh noises or ranges where the exhaust note produced would be a loud drone or irritating buzz.
Resonators are not quite a straight-pipe, but no too far off. They have less baffling and don't reroute exhaust gases as much as a muffler which reduces back pressure, potentially freeing up a few horsepower.
[FONT=helvetica, arial, sans-serif"]Back pressure:
[FONT=helvetica, arial, sans-serif"]One of the most misunderstood concepts in exhaust theory is back pressure. People talk about it with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Engines need back pressure" when discussing exhaust upgrades.
[FONT=helvetica, arial, sans-serif"]1. Some basic exhaust theory
[FONT=helvetica, arial, sans-serif"]Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pulses and so on. The more pulses that are produced, the more continuous the exhaust flow. Back pressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
[FONT=helvetica, arial, sans-serif"]2. Back pressure and velocity
[FONT=helvetica, arial, sans-serif"]Many people mistakenly believe that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this idea would be appealing - as wider pipes have the capability to flow more than narrower pipes. However, this omits the concept of exhaust VELOCITY. Here is an analogy...a garden hose without a spray nozzle on it. If you let the water just run unrestricted out of the hose it flows out limply at a rather slow rate. However, if you take your finger and cover part of the opening, the water will spray out at a much much faster rate.
[FONT=helvetica, arial, sans-serif"]The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be travelling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you also want make sure the pipe is wide enough so that there is as little back pressure as possible while maintaining suitable exhaust gas velocity.
[FONT=helvetica, arial, sans-serif"] [FONT=helvetica, arial, sans-serif"]Back pressure at it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust will flow backwards, which is...er... is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero back pressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range (remember the pulses!). A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of back pressure at high rpm. Thus if your power band is located 2000-3000 RPM you'd want a narrower pipe than if your power band is located at 8000-9000 RPM.
[FONT=helvetica, arial, sans-serif"]Exhaust Manifolds
[FONT=helvetica, arial, sans-serif"]The manifold is an assembly designed to collect the exhaust gas from the engine cylinders into exhaust pipes, (either one or two with Morgans). Manifolds are often made of cast iron in stock cars. Many have material-saving design features to use the least metal, to occupy the least space necessary, and/or have the lowest production cost. These restrictions often result in a design that is cost effective but that does not do the most efficient job of venting the gases from the engine.
[FONT=helvetica, arial, sans-serif"]Inefficiencies generally occur due to the nature of the combustion engine and its cylinders. Since cylinders fire at different times, exhaust leaves them at different times, and pressure waves from gas emerging from one cylinder might not be completely vacated through the exhaust system when another comes. This creates back pressure and restriction in the engine's exhaust system and limits the engine's true performance possibilities.
[FONT=helvetica, arial, sans-serif"]A header (aka branch manifolds or extractors) is a manifold specifically designed for performance. Engineers create a manifold without regard to weight or cost but instead for optimal flow of the exhaust gases. These designs can result in more efficient scavenging of the exhaust from the cylinders. Headers are generally circular steel or stainless tubing with bends and folds calculated to make the paths from each cylinder's exhaust port to the common outlet all equal length, and joined at narrow angles to encourage pressure waves to flow through the outlet, and not backwards towards the other cylinders. In a set of tuned headers the pipe lengths are carefully calculated to enhance exhaust flow in a particular engine revolutions per minute range and married to the firing sequence.
[FONT=helvetica, arial, sans-serif"]How did the myth about back pressure and big pipes come to be?
[FONT=helvetica, arial, sans-serif"]I believe it is/was a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a Honda Civic decides he's going to upgrade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the power band. He makes the connections in the following manner: "My wider exhaust eliminated all back pressure but I lost power, therefore the motor must need some back pressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero back pressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
[FONT=helvetica, arial, sans-serif"]Why is exhaust velocity so important?
[FONT=helvetica, arial, sans-serif"]The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this article but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle travelling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
[FONT=helvetica, arial, sans-serif"]Many designers will increase the length of the exhaust, trying to achieve a faster flow and a larger area of low pressure. Short pipes create a smaller low pressure area..
[FONT=helvetica, arial, sans-serif"] [FONT=helvetica, arial, sans-serif"]CONCLUSION Engines function best with the exhaust system they have been designed with, which has been tuned to its needs and characteristics with a goal of producing the most efficient scavenging.
[FONT=helvetica, arial, sans-serif"] If changes are made to a system's bore size, the amount of silencers are reduced & removed (Including Catalysts) the engine ECU will need to be retuned to address the changes in velocity and flow otherwise performance losses will occur.
[FONT=helvetica, arial, sans-serif"]However, some owners are more interested in the noise of their exhaust than the performance of their vehicle. If it is noise and exhaust tone you really want, that is a much simpler, non-invaisive goal and much easier to achieve.
The 968 3.0 Exhaust system has sections of twin pipes with smaller bores than the single 3.0 S2 system, this speeds up the velocity which works well in tandem with it's variable valve timing cylinder head.
Exhaust Bore size v Engine Power
why is back pressure bad? The engine need flesh air to burn fuel. The more air you have, the more completely you can burn the fuel. The more fuel you can burn per cycle, the more power you make. It is that simple.
Back pressure impedes the flow of exhaust out of the cylinder. If exhaust gasses are left in the cylinder there is less room for the fresh air that we need to burn fuel. Thus, with back pressure you can "choke” the motor. But why is the backpressure idea a half-truth? This is because velocity has to be considered, too. If you can keep velocity up, you can actually pull more exhaust out of the cylinder and more air in through the scavenging effect. So there is a bit of truth in the idea that you don’t want too big of an exhaust, but the reason isn’t for backpressure, it is because you want to keep velocity up. Tube size is what limits flow as in CFM or a volume over time. Velocity is just speed apart from volume. Take a garden hose for example. If you use a nozzle on a garden hose to choke the flow, you will increase velocity but decrease flow. As I mentioned before, you do need to have enough velocity to ensure that exhaust gasses are not pulled back into the combustion chamber in the brief moment when the exhaust valve hasn't completely closed, the intake valve is beginning to open, and the intake stroke is beginning. But velocity alone isn't enough. You have be able to flow enough volume to ensure that you are completely able to empty the exhaust gasses from the chamber. You can calculate the CFM (flow) required to support a motor of given horsepower. A commonly used estimate is 2.2 CFM per flywheel horsepower. Another common estimate needed is the CFM per sq inch of tubing which is 115 CFM per Area in cubic inches.
So here is an example: Let us assume we have a motor making 400 flywheel horsepower and we'd like a single pipe exhaust.A 3" single exhaust is good for about 340 crank horsepower. Our motor is 400 hp at the flywheel, we need an exhaust size larger than 3".Now I'm not saying that 4" would work fine, either. 4" tubing supports much more power than we are planning to make. While the flow would be adequate, the velocity would be too low because the exhaust pressure would be diminished.Thus, the rule of thumb with the calculations is to use the smallest tube diameter that supports the horsepower you are planning on making.
My 924S design / Application
I used all this info to decide what system I wanted to design for my 2.7L 8v Motor as I wanted not just the lightest system I could find
but also one that would give me the best characteristics for Road use (Low RPM) and Not (full throttle - High RPM) Track / Racing use.
Martelius in Finland sell, and make lightweight Steel thin wall tubing (1.2mm) and custom silencers with aluminium cores.
Only Stainless can be used this thin as it's stronger than mild steel, and of course this has an added benefit of not rusting!
I chose a 2" Bore and only (1) rear back box not a 2.25" bore and 2 boxes like the original system.
However I was surprised to see when the pieces arrived from Finland they were 2" OD and hence the Internal bore was 48.4mm, quite a bit smaller
than the original ID 53mm? 924S 2.5 / 944 system but this has 2 silencers which are not straight through but restricted.
The stock 2 box system on a 924S is 19.5 Kg, the Martelius (1) rear back box system is 8Kg so the weight saving was great but the bore size was a concern as
even Martelius said it will begin to be restrictive up to 185bhp, but the engine responded well (8 hours of mapping from 179 to 205bhp & 205Ft Lbs)
So from the general characteristics and info explained above, my conclusion is the reason the motor produced the results it did
(40bhp more than the 944 2.7 production Porsche) was because the smaller bore meant the exhaust speed has (Faster velocity) and the lack of restrictive silencer (s) also gave minimal back pressure and the Manifold I fitted was a tubular 944 Turbo one plasma coated by Zircotec for better scavenging.
(Unyet guidelines for 200bhp can't be achieved with a 2" Bore pipe??)
Add on top the Head is the small valve 2.5 8v, and not the larger valved 2.7 Porsche produced... maybe the flow is faster here too?
The head's inlet ports were worked by Lindsey Racing Flow guy which increased the CFM by 28%, with a Mild Camshaft and the
Inlet Manifold Extrude hone polished (+20% CFM)
it all makes sense that the motor's inlet and exhaust systems have been optimised with the live mapping by Chip Wizards but the really surprising bit is the exhaust bore is smaller than stock!
For road use it's ideal, as this tune produces a lot of torque low down the RPM range, and peak power is @ 4500RPM, This is all on stock 2.5 8v ECU, AFM, Fuelling and Ignition.
Big is clearly not always best (in this case)... I reckon I could fit a twin cam head, throttle bodies and a larger bore exhaust and touch 250bhp but this power
would be at a higher rpm and I don't think it would be as nice and drivable on the road as it is now.
R
http://924srr27l.co.uk/wp-content/uploads/2015/05/TEch-Spec-924srr27L-.pdf
