Warren G
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Registered: 14th May 06
Location: Kent
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basicaly if a turbo has an exit point on the turbo of 2 1/4" is it still worth fitting a 2.5 downpipe/system or will that one ridge on the turbo cause a restriction so its not worth it?
yes you will get better flow over a 2.5" system over a 2 1/4" system thus giving better power but will the smaller outlet give power problem
to much science involved with boost pressures/ exhaust flow and exhaust bores for me on this one 
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craig_s
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Registered: 5th Mar 06
Location: Birmingham Drives: mk4 astra gsi
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im sure the exit on a k16 is 2 1/4" and im running a 2 1/2" system with no problems.you want a certain amount of resistance in the exhaust anyway.
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Nic Barnes
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Registered: 5th Apr 04
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Don't want any resistance in system tbh on a turbo
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craig_s
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Location: Birmingham Drives: mk4 astra gsi
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so you could have no system after the turbo at all??? i had a conversation about this not long ago with the company that built my system,and they said if theres not enough back pressure the turbo wont last long.
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Nic Barnes
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Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
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Warren G
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Location: Kent
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your missing the point of the question craig, i could run a 3" system no problems, BUT .....
you will see better torque with a smaller diameter system, but more power with a larger bore system
you need x amount back pressure on a N/A, but the less the better unless you go to the extreme.
a turbo is in its self a restriction, as all the exhaust gases and being forced throw one hole into the turbo (or via the waste gate) but once the exhaust gases exits the turbo proller with is say 1.75" and meets with the waste gate and gets bottled necked (the shape) through 2.25" bore then to a 2.5" downpipe- how much real differnce would a 2.5" outlet be?
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Warren G
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Location: Kent
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quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
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Nic Barnes
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3" is probably a bit too much on say 300 powers. Force turbo to work hard to make 300 on 2.25.
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craig_s
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Registered: 5th Mar 06
Location: Birmingham Drives: mk4 astra gsi
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yeh i see what you mean,my point is wouldnt the exhaust gas travel faster having a route to travel with a minimal resistance,rather than into a system with nearly no resistance which could cause the gas to exit slower due to not being able to pressurize the system?
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connollygt30
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Registered: 16th Nov 08
Location: West Lothian
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quote:
Originally posted by Warren G
your missing the point of the question craig, i could run a 3" system no problems, BUT .....
you will see better torque with a smaller diameter system, but more power with a larger bore system
you need x amount back pressure on a N/A, but the less the better unless you go to the extreme.
a turbo is in its self a restriction, as all the exhaust gases and being forced throw one hole into the turbo (or via the waste gate) but once the exhaust gases exits the turbo proller with is say 1.75" and meets with the waste gate and gets bottled necked (the shape) through 2.25" bore then to a 2.5" downpipe- how much real differnce would a 2.5" outlet be?
Too much science involved for you? sounds like you know alot about it to me...
Unless that jargon was copied straight from a book? 
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Nic Barnes
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Location: nowhere near ginger people
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quote:
Originally posted by Warren G
quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
To warrant a 5 inch system the power levels you would be running would be massive. 3" system do you 600+ happy
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Warren G
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Registered: 14th May 06
Location: Kent
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i guess its something you would only realy find the true answers if you done back to back dyno tests
might aswell still getting 2.5" downpipe and system then....
only running 8-9psi, but high comp
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Nic Barnes
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Registered: 5th Apr 04
Location: nowhere near ginger people
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quote:
Originally posted by craig_s
yeh i see what you mean,my point is wouldnt the exhaust gas travel faster having a route to travel with a minimal resistance,rather than into a system with nearly no resistance which could cause the gas to exit slower due to not being able to pressurize the system?
Turbo is the pressure. System restricts it.
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craig_s
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Registered: 5th Mar 06
Location: Birmingham Drives: mk4 astra gsi
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quote:
Originally posted by Warren G
quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
i totally agree with you,you cant just say have as little back pressure as possible for every engine.
a friend had an exhaust made for his clio 172 a couple of weeks ago,they told him he needed a 3" system,
i said i thought it was too big but he had it anyway,surely my engine will be flowing more gas than his so why did he need such a big system?
[Edited on 29-09-2010 by craig_s]
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Warren G
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Registered: 14th May 06
Location: Kent
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quote:
Originally posted by Nic Barnes
quote:
Originally posted by Warren G
quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
To warrant a 5 inch system the power levels you would be running would be massive. 3" system do you 600+ happy
to look at something like that, i always think of the extreme
i know there is no need for something that big unless if was on a 9.0l v50 lol
you said basicaly the turbo would be fine with no exhaust, but surely you would loose power?
i know some people still run 2.5" system on 350-400bhp cars, but the see more torque than the people running 3" systems
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Nic Barnes
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Registered: 5th Apr 04
Location: nowhere near ginger people
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Egt
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Warren G
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Registered: 14th May 06
Location: Kent
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quote:
Originally posted by craig_s
quote:
Originally posted by Warren G
quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
i totally agree with you,you cant just say have as little back pressure as possible for every engine.
a friend had an exhaust made for his clio 172 a couple of weeks ago,they told him he needed a 3" system,
i said i thought it was too big but he had it anyway,surely my engine will be flowing more gas than his so why did he need such a big system?
[Edited on 29-09-2010 by craig_s]
n/a is different to turbo in repects of exhaust because of how the turbo unit runs
unless he was reving to 9-10k with some very larry cams putting 270-300bhp down, then he will loose torque over a 2.5" and power would most likely be the same if not less due to like you said before, due to a type of back pressure
you only need big bore exhaust like that if you was reving high with high gas flows, but giving a bigger bore you will loose power down low
the primers have alot to do with power due to back pressure pulsing, hence why equal lenght manifolds etc should realy be suited to the cam speck
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Warren G
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Registered: 14th May 06
Location: Kent
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quote:
Originally posted by Nic Barnes
Egt
i see, that the main reason? less restriction, less strain on turbo, giving lower exhaust temps
so if a turbo's working to hard the temps will raise? that correct
is that the techical way to see how hard the turbo is working so to speak?
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neil h
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Registered: 28th Sep 06
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quote:
Originally posted by Nic Barnes
3" is probably a bit too much on say 300 powers. Force turbo to work hard to make 300 on 2.25.
So am i reading this right, for something like an Astra VXR a 3'' system is a bit much and something more like 2.5'' would be better?
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Nic Barnes
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Registered: 5th Apr 04
Location: nowhere near ginger people
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If 300 is only aim for power then should make it ok on 2.5 if want more go bigger.
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SAL
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Registered: 19th Dec 05
Location: Radlett, Hertfordshire
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talktoburt.co.uk
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Warren G
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Registered: 14th May 06
Location: Kent
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burt's to low comp for me
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csweatherston
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Registered: 16th Jan 06
Location: Devon
User status: Offline
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quote:
Originally posted by Warren G
quote:
Originally posted by Nic Barnes
Will be fine. Turbo works off exhaust to spin it so restricting the flow from it causes it to work harder to suck more air in.
if there was no waste gate then yes you would have a problem, but all it will do is spool up quicker to the required boost, to which then the waste gate controls
But taking things to the extreme, why dont people, forgetting costs/room etc 5" exhausts? is it because tyicaly you dont see any gain over 3"
if a N/A car had too big primers it would loose power, theres a perfect size for the cc/ exhaust flow at x revs
With a 5" system, you would have a larger space to fill with the spent exhaust gas, before it leaves the system.
Gas velocity will still play a part in expelling the waste quicker, not so much as N/A though.
Letting the gas's leave quicker keeps load down on the turbo, and keeps temps down.
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alan-g-w
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Registered: 9th Nov 07
Location: Glasgow
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I'm not sure on the turbo side of things but I know the basics of how an engine works.
I was always under the impression that back pressure didn't come into the equation with turbo'd motors. The way I think about it is this - for an N/A enigne (let's say XE) you've got the headers going straight onto the head of the engine with a pretty much hollow pipe going straight out the back. This system has back pressure because the diameter of the pipe (exhaust) has a direct relation to the power of the engine for the fact that the combustion chambers go straight into the exhaust. So smaller headers/ exhaust diameter = less power, too big a dia. and it's going to be like a fart in a gym hall and not have the back pressure to 'control' it.
With a turbo (let's say a LET) instead of combustion chamber > manifold > exhaust you've got combustion chamber > manifold > turbo > exhaust, to keep it simple. Because there's a turbo (and manifold) in between the chambers and exhaust does that not mean that the only back pressure from the exhaust is against the turbo? Because as far as I know the turbo can't 'run backwards' as in put pressure towards the head (correct me if I'm wrong) so in turn can't have any effect on how it's running.
The whole thinking behind that comes down to the fact that since the turbo is a compressor with its own exhaust, for it to run as efficient as possible it needs to exhaust the gases in it as quick as possible. Therefore there's not any need for a system to be a maximum size like you would need on an XE, since the back pressure would be in part 'controlling' how the engine's running.
As said, I'm not dead sure about all that but that's how I think about it from what I've been told.
[Edited on 30-09-2010 by alan-g-w]
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