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DIY Turbo Discussion greddy on a 1.8? homebrew kit?

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Old 07-23-2008, 09:03 PM   #81
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Originally Posted by AbeFM View Post
Lastly, be sure your wastegate rod has a little pre-tension on it. Sometimes, if they are only barely shut, they can blow open slightly, and will leak exhaust even at low boost, slowing spool.

What's your exhaust like? TIll I went to a 3" catless, I never got full boost till ~3800. Now I get it maybe 3k, maybe 2650 uphill in 5th.
My exhaust is 2.25". Regardless, I am going to do a simple test with a mityvac pump to see when the wastegate starts opening. If that isn't enough, I want to hook up a string pot or a modified TPS to my data acq. and get a relationship of boost vs. actuator rod movement. That should help narrow down the cause of the slow spool: exhaust restriction or improperly set wastegate.


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Originally Posted by AbeFM View Post
My guess is, looking at your piping, that you WILL see the boost falling off, depending where you take your boost signal (right ahead of the throttle plates is ideal for reasons I won't get into here).

Which piping? Charge or exhaust? I am taking boost signal off of the compressor right now. Can you point me to more info about this?


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Originally Posted by devin mac View Post
looking awesome, man. glad to see you're at the stage you are!
Thanks, wouldn't have been possible without your help.


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Originally Posted by 18psi View Post
Also the dropoff is a bit weird to me. At the low boost level you are pushing, it shouldnt be dropping off that low.
There is no dropoff? In fact there is creep I would imagine.


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Originally Posted by rb26dett View Post
PF, how high are you planning to rev it to? If you add PW and AFR to your logs it will be more easy to see what torque is doing as the revs climb.

I'm not sure what you are supposed to rev those B6 engines to, but give it a blast and log those two parameters along side to see what's really going on.
Revving to 7000. I am working on getting a constant AFR through the MLV VE analyzer. Will post a 4th gear WOT log and then it will be easy to see this.

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Originally Posted by rb26dett View Post
Boost creep is where you get more than you want at higher rpm, what is the gate supposed to give stock? If it's 5.5psi, then yes, that's exactly what it is, if its 8psi, you probably have some leakage or something causing down low to not make target.
Not sure since I clocked the turbo and disturbed the wastegate mounting, so I will hook up the mityvac pump and test this. The leakage angle is intriguing. Can you explain a little bit please.


Appreciate everyone's help in ironing all the bugs out.
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Old 07-23-2008, 09:30 PM   #82
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http://abefm.smugmug.com/gallery/116...54351231_aiCXV pics, a little explination. But, at low flow, the intercooler doesn't present much restriction. At high flow, there is a restriction in all those bends and the intercooler itself.

Take the signal at the turbo, it's constant boost there, minus a varying number across the IC set up. Take it at the manifold, and the wastegate stays shut even when the piping is at 22 psi, overspinning the turbo, overheating the air. Take it right ahead of the throttle plates, and the manifold will always see the same pressure anytime the turbo is capable of supplying it, and you vary what the motor sees with the throttel.
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Old 07-24-2008, 05:25 AM   #83
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Nicely said Abe. I have a partly finished write up if you want. PM me for a link if interested.

Thinking about it again, leakage probably wouldn't do what you have in your logs. It would just push the knee point to the right, whereas creep does exactly what you show.

2.25" for the exhaust can't be much of an issue down low where there is stuff all flow, but it could hurt up top. If it's really bad I guess you could still see some improvement in bottom end spool by fixing it.

Fred.
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Old 07-24-2008, 01:45 PM   #84
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Well, if you want to stop messing around and find out what's going on - esp since I had the 2.25 ex and when I went to 3 it was night and day... Try this:

Pick a day you hate all your neighbors (a turbo quiets it down some, so it won't be that bad) and break the exhaust at the bottom of the downpipe - either leave the ex hanging there or remove it, but let the gas leak out. Drive around the block (I suggest doing this where it's not crowded, or simply wait till the car is warmed up to make the break.

Then you'll know what you're missing. If you really like it, get a dump pipe. More likely, build a 3" exhaust. And just live with the late spool, I did for over a year.
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Old 07-24-2008, 02:34 PM   #85
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Thanks, wouldn't have been possible without your help.
bah, all i did was send you something that you never got to put on your car ;-)
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Old 07-24-2008, 02:37 PM   #86
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A 3" exhaust is overkill on even a 500+ hp engine. 2" is good for 150+ 2.25 more than 200 which is probably where he is on a b6t with 8psi. I've gotta ask Abe, did your 2.25 use some bodgy baffled-to-hell mufflers and a LOT of particularly sharp edged crush bends?

I know a guy making 400whp on a 2.5" system from a 2.5 litre v6 on 14psi, you just can't tell me that the size matters that much without backing it up with some context and physics.
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Old 07-24-2008, 02:45 PM   #87
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Here in a country where we don't make the rest of the world choke on our polution (ha!) the OEM exhaust has 2.25 diameter, with muffeler, resonator, catalytic converter, etc etc.

Presumably he didn't make a 2.25" system custom, he's referring to the OEM exhaust. I like 2.5", and it's likely what I would have made (2.75 without batting an eye)....

But... 3" is cheap. That's a big plus. More to the point, the volume of the muffler you use determines how quiet it is. And I'd read a lot of testing, and they said for the best reduction in noise, a bigger muffler is better. Since I wanted a straight through system, I had to go to 3".

Taking out the bends... will be where your gains are. Did I need 3" for performance? No. I went the 3" route because it was the best way to keep from waking up the whole neighborhood when I drive, and from getting eyed by cops, and to have the girlfriend actually WANT to ride in my car when we go somewhere....
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Old 07-24-2008, 03:02 PM   #88
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My mates B6 NA mx5 had an oem 2" on it. it should still be suitable for spool purposes, if not top end, with the exception of the rear muffler, then again, US spec cars could be wildly different to his jap or NZ one.

http://i238.photobucket.com/albums/f...remounted2.jpg
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Old 07-24-2008, 05:08 PM   #89
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This is definitely a point for discussion: i have read here about people who went to a 2.5" or a 3" from an OEM exhaust (mine isnt OEM though, its levnubhin's old ebay exhaust, plus I don't have a cat) and noticed improved spool.

This is counter-intuitive because the smaller exhaust shouldn't hurt in the low end. Yet it seems to. Why is this?

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Old 07-24-2008, 05:36 PM   #90
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The OEM rear muffler is probably pretty bad, that could easily be why, combined with even more hard core cat converters in some parts of the states it could make a big difference. The exhaust on my mates early car would be fine up to some reasonable power (enough to take it from slow to acceptable) because it is all straight through.
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Old 07-24-2008, 06:12 PM   #91
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Quote:
Originally Posted by The_Pipefather View Post
This is definitely a point for discussion: i have read here about people who went to a 2.5" or a 3" from an OEM exhaust
I saw this when I went from the stock exhaust to a custom 2.5" with a metallic cat and straight through resonator and muffler. Changing nothing else, it made an immediately noticeable difference on a BEGI-S with a 2560 only running at 6-7 psi. I was smiling as I drove away from the exhaust shop with the new system in place.
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Old 07-24-2008, 06:24 PM   #92
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In some kits, with the 99-00 2 cat cars, there's actually a cat between the motor and the turbo - sapping all that energy!

But more likely, the further along cat is the problem - turbos would on differential pressure, a cat after the turbo ensures that the effective 'barometric' pressure is quite high. In a steady state things aren't that bad, but you need a high force to overcome the rotational inertia of the turbo.

I bet a 1.5" STRAIGHT through pipe would be better than a 2.5" pipe or even a 3" (if you could find one) that meets the worlds' strictest noise and pollution laws. And remember, a car that's NA needs a lot more muffler and resonator on it than a turbo one, so an NA exhaust is much more restrictive.

Anyway, trumping all this: Go for a ride with the exhaust off. If it doesn't make a difference, we're barking up the wrong tree. :-)
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