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Discussion Starter · #1 ·
Thanks guys! I understand.
What I'm wondering now is, at what point during exhaust mods would you need to modify the wastegate / O2 housing in order to eliminate boost creep? Would you have to do it even with just a catback system, 2.5" or 3"? And finally, does all this mean a 2.5" system (cat-back, preferably) would be a safer way to go to avoid boost creep?
 

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Discussion Starter · #2 ·
...can a big 3" exhaust system cause boost creep? If anything, I would think that a smaller system would contribute to boost creep because it wouldn't let as much air pass through as fast. Enlighten me <i>please.</i>
 

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wastegate path and exhaust path is 2 diff paths..bigger exhaust without external wastegate or wastegate flapper moding allows more air flow for more turbo spool up..but same flow of wastegate causes boost creep becuz the wastegate cant keep up with the exhaust higher flow of large exhuast. im new to all this too..but i think this is correct
 

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Let's see if I can explain this:

When you upgrade to a much larger exhaust than stock (3"), you have effectively decreased the back pressure on the system. That's why you can get more horses by upgrading the exhaust. Now that there is less of a restriction, the gases flow down the downpipe, cat(upgraded or non-existant), and cat back MUCH easier. When you mash the go pedal, you generate a lot of exhuast. The turbo spools reaches it max seting and the wastegate opens. Only problem is that if the O2 housing (that's where the wastegate flapper lives) isn't ported it becomes a restriction in the exhaust plumbing. The gases then go to the least resistance (upgraded downpipe, ect..), and mostly bypass the flapper whole. This continues to spin the turbine blade faster increasing your boost pressure. Hense boost creep.

Sound alright?

Zach
'92 Laser RS Turbo AWD
"RSX"
 

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ATKChemist is right.


Exhaust gases are lazy. When presented with a fork in the road they will take the easiest path.

So when the come to the point where they have to choose to go through the damn turbo and out the exhaust or just go through the wastegate hole they will choose the wastegate. But once you start modding the exhaust and reducing the back pressure you area making it an easier path. So at some point of modding the turbo will start looking like the easier path and some of the air that was suppose to go thru the wastegate decides to thru the turbo instead. So the boost is no longer maxed and it will start to creep up. The more exhaust that 'takes the wrong path' there is the more creep you have.

So to prevent this just make the wastegate path the easiest again. By enlarging the wastegate hole you are reducing it's backpressure and so it will stay the easier path longer :).

Is this clear?

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Quadcylla
92 Laser RS-T FWD

[This message has been edited by Quadcylla (edited October 06, 2000).]
 

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Oh and use [] for your html tags for them to work.

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Quadcylla
92 Laser RS-T FWD
 

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I also think that the hotter the pipe can get the easier it flows exhaust because the fumes are less dense. I wonder if header wrap would do anything positive.

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92 Talon AWD
Mods:
2.5" BR D.P. 2.5" Dynomax cat-back, 95 Manifold, BR upper I/C pipe, K&N, Extreme [email protected], Walbro pump, Apex AFC, ACT 2600.
 

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header wraps on what? The downpipe, that wouldn't help eliminate boost creep. On the O2 housing, you would be able to get in on there and even if you did, it wouldn't do much good. The restriction is on the inside and the housing is already damn hot. Plus it would be cheaper to port the housing yourself and eliminate creep.

Just my $0.02
Zach
'92 Laser RS Turbo AWD
"RSX"
 

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I had the same question a while ago. You shouldn't see any boost creep with a cat back, 2.5 or 3". The downpipe and cat is the biggest restriction, you replace those and your going to see creep. Big time if you went a full 3". Not sure on the 2.5", but you might see some. I am looking at exhaust systems (turbo back) to put on my car so this problem will become very clear soon. I'm probably going with 2.5" so I'll let everyone know when I get it.

Zach
'92 Laser RS Turbo AWD
"RSX"
 

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I have a full 3" exhaust...turbo back(minus the 2.5 inch thing that connects to the o2 housing...after that its all 3") including downpipe and cat. For a while I have been trying to set my manual boost controller. I have no external wastegate, stock turbo(2nd gen) and when I set MBC to about 15 punds, it boosts to like 18, but I don't wanna wait for it to go down to 15, in case my car runs lean and detonates. If I set it at 12 or 13, my car creeps to 15 or 16, but drops quicker than when I had a higher setting. Creep sucks.
 

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At the track I run a 2.5" mandrel bent DP to the open air... no creep at 20 psi. Every car seems a little different.

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Quadcylla
92 Laser RS-T FWD
 

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Yeah header wrap wouldn't do much,should have thought before I spoke, hehe thats always the problem. I have 2.5" turbo back with no boost creep at all. Hey ATKchemist, were did you go to school? My friend just his chemical engineering degree from R.P.I. in upstate NY, ever hear of it?

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92 Talon AWD
Mods:
2.5" BR D.P. 2.5" Dynomax cat-back, 95 Manifold, BR upper I/C pipe, K&N, Extreme [email protected], Walbro pump, Apex AFC, ACT 2600.

[This message has been edited by TomsTSI (edited October 06, 2000).]

[This message has been edited by TomsTSI (edited October 06, 2000).]
 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Hey ATKchemist, were did you go to school? My friend just his chemical engineering degree from R.P.I. in upstate NY, ever hear of it?

<HR></BLOCKQUOTE>

Sorry never heard of RPI.

Virginia Tech, (Blacksburg, VA) Chemical Engineering. Working as a chemist at the Radford Army Ammunition Plant. Hey, I get to play with explosives, and rocket propellants (Hellfire missles, ect...)! It's not engineering, but it supports my DSM habit ;)

Zach
'92 Laser RS Turbo AWD
"RSX"
 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by AWDMIKE858:
I have no external wastegate, stock turbo(2nd gen) and when I set MBC to about 15 punds, it boosts to like 18, but I don't wanna wait for it to go down to 15, in case my car runs lean and detonates. If I set it at 12 or 13, my car creeps to 15 or 16, but drops quicker than when I had a higher setting. Creep sucks.<HR></BLOCKQUOTE>

I have never heard of a T25 creeping. The only reason that the larger turbos creep is because they flow so much more air (450cfm for a T25 vs. 650cfm for a 20G). That air has to exit somehow and when it does it takes the "path of least resistance"--as my Physics prof. used to say.

More than likely your car is simply experiencing a spike to 18psi and then falling back down to 15psi. Even at 18psi, you don't have too much to worry about. The fuel system will be maxxed out, but it can sustain 18psi. It's the turbo that can't sustain that kinda boost. The T25 will simply fall on it's face above 6Krpms at that kinda boost. In fact, modifying the boost with a T25 is pretty pointless since it can't even sustain the factory boost levels.

You might also have a boost controller that is not calibrated correctly. If your boost holds strong until about 5500-6000rpms, then I would have to say that this is the case, but if it hits 18psi for only a split second then I would say that it is just a spike.

You also never mention if you have an aftermarket BOV. If not, then this is a huge part of the problem since the stocker can barely hold 14psi.

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'95 Mitsu Eclipse GS-T

16G Killer, 2.75" RS-R exhaust (turbo back), HKS intake, Apexi BOV, 10lb. flywheel, Centerforce DF, Greddy FMIC, 8.5mm wires, NGK 7's, Apexi Super AFC, Greddy Profec, Energy Suspension motor mounts
 

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The reigning standard "mini" 16G title belongs to Mark Hessler with a [email protected] The big, killer, super acronyms must not have helped much as far as I know, no one has gone faster than 11.90s with these turbos.

As for getting a FWD deep into the 12s, it can be done but few have done it on less than a 20G. The best "big" 16G time is Eric Cavalieri with a [email protected]

In my opinion, the 16G is a waste of money. The 20G represents a better mid-range turbo, and trust me, once you're used to the 20G you won't think its all that large. The streetability of the turbo isn't that bad as long as the exhaust system is large enough to allow the turbo to spool and it just keeps on making power until redline unlike the 16G.
 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>If not, then this is a huge part of the problem since the stocker can barely hold 14psi.
<HR></BLOCKQUOTE>

I never saw nuke state he has a 2G, since he didn't you cannot make this statement. If properly plumbed, a 1G BOV can withstand over 25 psi without leaking. Should he be using the 2G BOV then you're absolutely correct, and nuke should toss that thing in the trash.
 

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Hey yeah I got a greddy type-s... and for a second i was using spike and creep interchangibly...sorry. It spikes to 18 then drops to about 15.5 or so. I adjusted the MBC a little today, and I didnt see a lot of the spiking. Isnt it bad even for a second if the stock fuel system( i did the rewire for the pump tho) sees 18 lbs of boost... even for a second??? of course it drops back down to safer levels but doesnt the 2 or 3 seconds matter a lot??? Thanx
 

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Discussion Starter · #19 ·
Sorry for not clarifying, guys. I have a '91 GSX with stock turbo & BOV. Mods include a boost gauge and manual controller, removed airbox, K&N filter. I want to upgrade the exhaust someday when my budget can sustain it (poor college student mode ON). My goal is not to create a 9 sec. monster (haha, yeah right), but I still would like to have a "unique" vehicle. Basically, I want to avoid the expense involved in the extensive mods that come after the basic ones are done. All this exhaust talk is great help to figure out which direction I should go. I think I will go with a 2.5" cat-back exhaust, but I'm interested in Thermal R&D's system, and I don't think they make a 2.5" .... only 3". Anybody know otherwise?
 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Donnie:
(450cfm for a T25 vs. 650cfm for a 20G<HR></BLOCKQUOTE>

Just curious but I thought a t-25 flowed 305cfm or maybe 350cfm and a 14b was the turbo that flowed ~450cfm? I could be wrong, just wondering.



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Jerry
11.98 @ 113
best mph of 115.99
My Ride
 
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