Stock muffler

[GT3]

It's well established that a restriction in the Polo GTI's exhaust system is the down pipe. However, some have managed to convince themselves (and others) that the remaining stock system isn't restrictive. I've taken some shots inside the stock muffler to illustrate how restrictive it is. ...and while it might give off a nice note (cheers Sam ), it's no high-performance muffler.

The first chamber is very restrictive, the only straight-through section is the centre hole you see (less than 1cm in diameter). Most of the exhaust flow goes through the most restrictive second chamber (see grey/heat marks in side shot), then has to turn again into a third chamber before being directed into twin pipes that travel the remaining length along the muffler. Fairly obvious that it's not free-flowing. ...so remember "don't believe the hype"

[Bubu]

Standard mufflers are almost always restrictive. I wonder if there is some power gain if the rear muffler was to be replaced.
I remeber reading somewhere (probably in '21st Century Performance' book) about an RX Mazda which had its tip changed, on the end of the muffler, and it acually gained some power from just chaging the tip.

[Timbo]

Mufflers are, by definition, restrictive. That's how they do their job. Unfortunately, it's hard to tell how restrictive they are by looking at them; to do that, you need to measure air flow; the challenge is to increase flow and still have a legal muffler.

[GT3]

Mufflers are, by definition, restrictive. That's how they do their job. Unfortunately, it's hard to tell how restrictive they are by looking at them; to do that, you need to measure air flow; the challenge is to increase flow and still have a legal muffler.

No, by definition they're designed to "muffle' the sound, not restrict flow. Even with three chambers and u-turns, you need further evidence of insufficient flow Agree that the challenge is to increase flow and still have a legal muffler. That usually entails a straight-through design (no corners, chambers) and sufficient packing/insulation to keep it quiet. I replaced my stock muffler with a straight through high-flow and it's quieter than the stock one.

[Grant]

mufflers reduce sound by using resonating chambers (where opposite sound waves cancel each other out). however i agree that just looking into the muffler doesn't prove its restrictive. BUT, i'm guessing that most aftermarket performance mufflers would be designed for high flow, which is always better than stock. But this doesn't mean they will be louder.

Cat converters on the other hand reduce noise by restricting flow.

[Buller_Scott]

so is dp alone enough to release a few extra ponies? or do you then have to do high flow custom made cat, or full turbo back, and then a fmic to match?

im looking at achieving around 100-105 flywheel kw, and i was thinking chip+filter+Seat cai+ dp should do the trick, without getting too carried away (im not like some of the guys here- i CAN be satisfied and i dont want numbers that are TOO large......

any info would be great as i still am struggling to really understand the facets of exhaust systems and how to take advantage of what i've got without doing too much alterations....

[VW GTI]

so is dp alone enough to release a few extra ponies? or do you then have to do high flow custom made cat, or full turbo back, and then a fmic to match?

im looking at achieving around 100-105 flywheel kw, and i was thinking chip+filter+Seat cai+ dp should do the trick, without getting too carried away (im not like some of the guys here- i CAN be satisfied and i dont want numbers that are TOO large......

any info would be great as i still am struggling to really understand the facets of exhaust systems and how to take advantage of what i've got without doing too much alterations....

From results of our Brisbane Polo dyno day, it's pretty safe to say on a GTI the downpipe gives about 7kw atw, the FMIC about 5kw atw (and sustained power) and together they give 12kw atw sustained.

[Buller_Scott]

From results of our Brisbane Polo dyno day, it's pretty safe to say on a GTI the downpipe gives about 7kw atw, the FMIC about 5kw atw (and sustained power) and together they give 12kw atw sustained.

yeah i read abit of that thread- really educational.

vw gti, when you say "sustained" how do you mean? if i intend to do my listed mods and thrash the car up the hill at buller (16km's windy stuff) is there a chance that my intended mods (seat cai, chip, bmc panel, dp) will see the car overheating WITHOUT fmic?

or do you mean sustained as in power supply is readily available right up until redline? sorry about my ignorance...



has anyone any stories/ experiences with doing mods APART FROM intercooler, and having the engine overheat? if so, what were the nature of the mods and how long before engine played up?

[VW GTI]

It's nothing to do with engine over heating, that would be the radiator not doing it's job.

The intercooler cools the air going into the engine. Cooler air is more dense which means more air gets in and that means more air coming out. So basically, the intercooler keeps intake temperatures down meaning power will be higher for longer, so the "5kw atw" gain would more be because the temperature of the air is cooler.

You can see what heatsoak does in the graphs of Rhys's car from our dyno day. He was running at 123kw atw and then after heatsoak set in (warmed the pipes etc up) the power dropped to about 102kw atw. No other changes to the car.

Eddy's and Sharkie's cars didn't change at all over just as many runs, Eddy's probably had more.

[Buller_Scott]

It's nothing to do with engine over heating, that would be the radiator not doing it's job.

The intercooler cools the air going into the engine. Cooler air is more dense which means more air gets in and that means more air coming out. So basically, the intercooler keeps intake temperatures down meaning power will be higher for longer, so the "5kw atw" gain would more be because the temperature of the air is cooler.

You can see what heatsoak does in the graphs of Rhys's car from our dyno day. He was running at 123kw atw and then after heatsoak set in (warmed the pipes etc up) the power dropped to about 102kw atw. No other changes to the car.

Eddy's and Sharkie's cars didn't change at all over just as many runs, Eddy's probably had more.

how long (after roughly how many minutes) did this heat soak set in and cause Rhys' car to drop power (never been to a dyno day, dont know format)?

oh and thanks for the replies- they're really informative. if i can avoid an intercooler (cost), i will, but i also DO like to rip it up the bends on the way to buller, and when its a clear road that's about 16km's and 10-12 mins of thrashing its orse off....

[Spec83]

It's nothing to do with engine over heating, that would be the radiator not doing it's job.

The intercooler cools the air going into the engine. Cooler air is more dense which means more air gets in and that means more air coming out.

To expand on this point.. The cooler the intake air, the more fuel and timing advance can be added to the engine before the air/fuel mixture becomes that hot (due to block temperatures) that it ignites in the cylinder bore before the ignition stroke of the engine.. This is very bad as it sends the whole engine out of balance and results in a condition commanly known as 'knocking'...

The basic principal of a combustion engine is - (fuel+air) * spark = Killo'wasps'

I belive the 1.8T has built in knock sensors which retard the timing to prevent this from happening..

[Buller_Scott]

To expand on this point.. The cooler the intake air, the more fuel and timing advance can be added to the engine before the air/fuel mixture becomes that hot (due to block temperatures) that it ignites in the cylinder bore before the ignition stroke of the engine.. This is very bad as it sends the whole engine out of balance and results in a condition commanly known as 'knocking'...

The basic principal of a combustion engine is - (fuel+air) * spark = Killo'wasps'

I belive the 1.8T has built in knock sensors which retard the timing to prevent this from happening..

thanks for the detail- i just wanted to ask, so does that mean if i have other mods done but HAVE NOT done the fmic, that creates knocking due to excessive heat/ pre-mature fuel ignition?

if so, when would this happen? is 'knocking' something that typically is really only seen on dyno days/ track days, or do you know of people who have had daily driver setups without intercooler but with other mods, who have had this problem? cheers

[Bubu]

thanks for the detail- i just wanted to ask, so does that mean if i have other mods done but HAVE NOT done the fmic, that creates knocking due to excessive heat/ pre-mature fuel ignition?

if so, when would this happen? is 'knocking' something that typically is really only seen on dyno days/ track days, or do you know of people who have had daily driver setups without intercooler but with other mods, who have had this problem? cheers

We are so way off the topic of origial post. You should have just started a new thread. Anyway...

In short no it does not create knocking. As Spec83 said the Polo is ftted with a knock sensor which enables the engine to adjust fuel and timing as necessary.

Kocking can occour on just about any car and it can damage your engine internals very quickly. Knocking most commonly occours when the engine is under load.

I have known people to have problems with knocking on their turbocharged cars. However this is usually due to bad tuning or too much advance timing (which relates to tuning).

All this is not really a problem for you if you have a current model Polo because the factory ECU is very smart and it wont allow the engine to 'knock'. It will probably go into a limp mode at worst case scenario.

[Timbo]

There's an important detail here: IIRC, Buller_Scott's car is a diesel, so some different principles apply

[VW GTI]

how long (after roughly how many minutes) did this heat soak set in and cause Rhys' car to drop power (never been to a dyno day, dont know format)?

oh and thanks for the replies- they're really informative. if i can avoid an intercooler (cost), i will, but i also DO like to rip it up the bends on the way to buller, and when its a clear road that's about 16km's and 10-12 mins of thrashing its orse off....

Well, at the dyno day it was only a matter of minutes of running the car, but the only 'cooling' there is is a big fan at the front of the car (I think the fan was even turned off briefly to speed the process up?).

When you're driving air is rushing past you all the time and would probably be cooler air (especially up in the mountains). So the timeframe changes entirely.