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PostPosted: Mon Nov 14, 2005 10:38 pm 
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Is there actually any gain to be had form replacing it for sub 350hp applications?

I'm hoping we can have some debate on this as I've just fitted the Ivana downpipe I bought from JP which is essentially the 3in Aussie copy from eastern europe. I also got a new 3in flexi pipe fabricated as my stock flexi pipe was past its best and leaking at one section.

Since fitting I have noticed all the usual comments are true, boost is higher due to the less restrictive exhaust side and the turbo comes on boost sooner and low rpm pull has definately increased.

However, after fiddling with the relief valve in the bay to get my boost back down to 1 bar, I'm getting boost spiked in 5th gear to 1.15bar.

They are very small spikes in time duration and I'm quite hopeful fitting an EBC with in gear boost mapping will cure the problem. I'm aware the boost controller has limitations in its application, panic not, I dont need a zimma debate. If a quality electronic boost controller doesnt solve the problem, I'll port the wastegate, after that, I'll add restriction.

The point of the post is, if 3in downpipe plus 3in flexi and standard exhaust can give the boost spike problems associated with large bore aftermarket exhausts, is there really any point in replacing it as a stage one modification?

It seems to me, that as a 2.75in system, it does a good enough job to remain with the money saved from an aftermarket system being used on an electronic boost controller to compliment a 3in downpipe and improved filtration/cold air feed.


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PostPosted: Mon Nov 14, 2005 11:16 pm 
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Firstly – Boost spike? Please clarify – Spike is typically a short term peak as you come on the gas, this settles to the correct boost after a short period and is not RPM dependant but a transient phenomena. A boost spike is more likely to be an EBC or relief valve incorrectly set up or a sticky actuator. Most suffer from boost creep once restriction from the exhaust is removed. This differs in that boost will rise with RPM. I myself have suffered with WAAAAAAY too much boost due to a 3” free flowing exhaust throughout (read 1.7 bar before I noticed and backed off). I have now installed a 2” restrictor and boost is now controlled to 1.35 bar ish (very changeable dependant on gear and load). (I do also have a big hybrid with poor wastegate).

The exhaust system as standard is not 2.75” more like a 2.5” ID bore with serious restrictions ( read less than 2”) in numerous tight spaces. As such it is undersized IMHO for a 300bhp vehicle. More power and economy could be released by a bigger exhaust.

Obviously on the flip-side of this we have the turbo wastegate design which causes serious problems when trying to keep boost low with a free flowing exhaust. (It has been described as the twin-entry deign is too effective at shoving the air into the turbo when compared to the poor path out of the wastegates).

The ideal solution IMHO would be to have a 3” free-flowing exhaust with a wastegate design that can still control the boost. This will reduce back pressure, providing more power and economy (remember exhaust restriction is a pumping loss as far as the engine sees it). From what I understand neither the CT26 or CT20 even when ported wastegates are adopted can control boost sufficiently. (The only exceptions I know of are DonB who had his wastegate ported by Owens and Kris Carter who was just very lucky somehow!). I have however done quite a bit of research on this as I suffer most and am hoping my new hybrid design will cure my boost creep.

As far as your question go’s – I have to totally agree that it is totally pointless purchasing a large bore – free flowing exhaust to have to insert restriction to control boost. Retaining enough of the stock exhaust to naturally supply that restriction is a very cheap route and most sensible if the turbo cannot control the boost. The money spent elsewhere would be more beneficial.

I also have a boost controller capable of different levels of boost in different gears and RPM, I have adjusted this to the best I can but still get enormous boost creep, all it allows me to do is raise the low end RPM boost levels to meet the high end boost creep level.

Another point of interest is that I suffer more boost creep post installation of a induction kit.

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PostPosted: Mon Nov 14, 2005 11:49 pm 
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Our terms are differing. With the relief valve I have always had what I called an initial surge of boost where the boost would go to about 1.1bar then settle quickly to 1 bar steady. Only time it hit more than this was on the rolling road with the extra resistance/loading that provided causing it to hit fuel cut.

In this case, I was testing my new setting on the motorway. 3rd, 1.1bar then 1 steady, 4th 1 steady, 5th 1.2 and immediate back off. Later on a section of dual carriageway, a 4th gear blat gave 1.1 then 1 then a sudden spike up to 1.1/1.15 and immediately back to 1. This is what I was meaning by spike, as if it was a spike on an otherwise steady boost pressure. The sharpness of the rise and fall would best be described as a blip as the needle on the boost gauge literally blipped.

I'm going to go warm the car up and have another examination.


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PostPosted: Mon Nov 14, 2005 11:51 pm 
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Quick addition. I was under the impression the main restriction in the 205 system lay in the 2nd cast section of the cat casing and in the flexi pipes curve over the subframe. Is there really bad area's in the loop over the rear suspension and driveshaft as well?

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PostPosted: Mon Nov 14, 2005 11:58 pm 
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It sounds like the free flowing exhaust is emphasizing the general charateristics of teh relief valve - It is not a great boost control device (but amazing considering teh cost!) It is the general nature of the relief valve to spike and then drop down - mine always did.

The best form of boost control is an upgraded actuator IMHO. Secondly an Electronic boost controller, Thirdly a Relief Valve, Finally a Bleed Valve. I woudl propose that you spend your money on a more sophisticated form of boost control rather than a free flowing exhaust system throughout which is likely to give rise to boost control problems.

The main restrictions are Cat, Flexi where it is crimped, silencers and finally the crimp over the rear axle area. In what order I am not certain TBH. It sounds like you have enough restriction in the exhaust to still control boost fine. Upgrade the Relief valve and all should be well.

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PostPosted: Tue Nov 15, 2005 7:18 am 
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2nd and 3rd gears are given the usual, although slightly more pronounced, surge to 1.1/1.15 bar then settle to approx 1. Didnt have enough road to test 4th or 5th but I will try later.

I think I'm going to swap over the relief valve set up from my CS which has a relief and bleed valve working in series and holds boost far better and doesnt have the initial surge I referred to above. Just straight to 1 usually.

Hopefully father christmas will bring an electric boost controller, or my mate will be able to source me one fairly cheap.


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