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Garrett GT35-R surge questions.


big-phil

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I got a problem from my last trip to the dyno, video here:http://www.youtube.com/watch?v=kvc00Qy2VLU

We (Bryce..owner of Alamo, and Chris) Thought it was the WG spring being to small. I upped the spring from a 7 lb to a 15 lb. With no change in the flutter. I also ported the WG hole, smoothing it out, and opening it up. Creep is MUCH better. I then put a stiffer spring in the BOV, and tightened it all the way with no change. So I then blocked off the BOV (no BOV) also with no change in the flutter (two runs then put the bov back on)

 

So I now read about compressor surge. I hit this wall of fluttering at 20-21 lbs of boost and above. Is my stock head, intake, and cam causing this? Is my .63 exhaust housing on my turbo too small for the 25lbs of boost i'm trying to run?

 

I really don't think its the turbo. Everything is spot on up untill I try to run above 20lbs boost.

 

I'm THIS close to putting on my ported head, Isky cam. But want to know what I'm dealing with.

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Yes, it sounds like the flow point through the stock components is causing a minimum flow surge situation.

JeffP has a GT35R with a .63 and the turbo is out of air before 25psi with his ported head. Going to a larger A/R on the turbine will only increase boost threshold point. By 'out of air' I mean you can see the HP level off and it's obvious the engine is nowhere near it's power potential...but the turbo won't make any more power or pressure no matter what you do. STONEWALL is what the compressor terminology is for that flow point.

 

It IS possible to surge the compressor at lower pressures than you would think, this is a minimum flow surge you are experiencing. Unfortunately it's the same sound as a natural surge (maximum flow, maximum pressure).

 

In essence they are the same thing...for the pressure you are at, there is not enough flow for stable compressor operation.

 

Have you tried manually lifting the BOV open slightly while under boost near the surge point to see if it supplies more air and doesn't surge? When a compressor surges you have to either lower the pressure, or give it more ability to flow...usually this occurs simultaneously.

 

BTW, the power and HP at 9psi is roughly very close to JeffP's testing. He was making 380Tq at 4500rpms, at 8.39psi of boost. It's more now, with higher pressures... GT35R. Jeff had some BOV or low wastegate spring related issues with his setup as well, you might get some info from him directly if you ask. He changed the spring to a heavier unit a couple of times to make it better. Different issue I suspect, though...

 

and after hearing it... yeah, low flow surge...

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Thanks Tony. While on the dyno I think the flutter cause the bov to go off slightly then it seemed to pull through, like you were saying. It started at about 20lbs, gauge bounced, you hear fluttering, then it pushed to 25lbs holding steady. When you say"increase boost threshold point" with a larger AR turbine. Do you mean I would be raising my stonewall point? I.E. with the .63 stonewall is at about 20lbs, but if I had a .82 AR turbine stonewall would be higher?

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Boost Threshold Point is the rpm under load whereby the engine turbocharger combination is capable of making full boost. I'm figuring you make 2-3 psi at WOT at 1500-2000rpms, and as you pull under load, like clockwork at 3400rpms the boost gauge shoots to 17 or 25psi. Meaning your boost threshold is 3400 rpms.

 

If you put a .82 housing on there, the point where you make that transition from 2-3 psi to full boost will likely be higher---say around 4000rpms instead.

 

Stonewall is the point where the turbocompressor is flowing as much air as it can, and can't flow any more. Usually this is at a point below maximum pressure or design pressure. The hot side A/R will not affect this at all. The compressor sizing and operational speed, as well as the flow map will determine where flow stonewalls. Stonewall can stress the blades, but you won't hear anything. JeffP gets to 25psi or thereabouts and the horsepower graph just flattens out. It's a very distinctive curve on the dyno when you are out of air, as opposed to surging, or AFR issues. This doesn't taper off, it just flat plateaus (er...around the same HP that Garett says is maximum for the frame capabilities). You aren't hitting Stonewall at 20psi, you are hitting the SURGE LINE due to MINIMUM flow. Stonewall deals with MAXIMUM flow. Surge deals with MINIMUM flow.

 

jeff's GT makes more pressure than 25psi, but won't make appreciably more power. On a stock head he would likely run higher boosts than the worked head. Moves where the compressor is operating at all around the pressure/flow map. By upping the boost at that point (once stonewalled) you run closer to surge (natural surge) than stonewall, and that is bad. You can run in stonewall, you will just not make any more power. If you up the pressure from stonewall, not only do you not make more power, you run the chance of natural surging the compressor at some point and it's stressful on the blades.

 

in industrial compressors that are known to have to run in stonewall before being ordered, usually the compressor wheels will get extra treatments like shotpeening and stress relief so they can stand the continual stress of flowing everything it can at any given pressure.

 

I digress...

 

All the .86 housing will do is up the point where you make full boost, and maybe breathe better out the back end of the car---if you haven't taken exhaust backpressure readings you don't know if you need it or not. Likely not.

 

My life is revolving around surge at this point, see what you can take away from this article someone sent me a couple of weeks ago... I got to read so much of this crap it gets numbing and I wonder if I'm relating it even halfway cogently sometimes without getting to geekspeakish...

http://www.documentation.emersonprocess.com/groups/public/documents/articles_articlesreprints/d350790x012.pdf

I thought the part about 'basic surge concepts' was pretty good. I think I will link this in the other post as wel...

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Phil, I highly recommend you not go to a larger turbine. The .63 is a perfect match for the L28. You might get more peak power from a .82 but the spool would noticeably suffer. Like you, I used to have a highly custom internal gate. I could never get it to stay closed no matter how big my spring was. All porting the wastegate offers is to provide a larger surface area for exhaust pressure to push on the flapper door. Remember, the higher the boost the less wastegate size you need since most of the exhaust is used to move the turbine. As a fix, I made a custom divorced wastegate port that allowed a 40mm Tial external gate. All my problems were solved. The fabrication was a pain but I can almost guarantee that your boost issues will be gone. You may still have a surge problem with stock components but the boost flutter would be gone. Let me know if you want more detail. Your car should easily be over 400rwhp.

 

I'm curious on one other point. Are you really running a 2mm head gasket with stock pistons and a ported head? If the answer is yes and your combustion chambers are worked over you are well under the stock 7.4:1 static compression. That's too low in my opinion.

 

P.S. On an external gate, the rule of thumb is boost should not exceed 2.5x the spring pressure. 2x is preferred.

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I'll take any help. It seems like when I put on the ported head, cam, intake. I'll make more power at lower boost, but i'll still stonewall around the same 20ish lbs of boost?? Still reading and learning.....

 

Now you're getting it! Your FLOW will be MORE than you would be at a lower flow and higher pressure (think Pounds Per Hour)...

 

What you will get is the inability to make any more pressure, but you will not be ANYWHERE NEAR the surge point.

 

I figure right now, you hit a minimum flow point to surge the machine---your BOV pops as a result and that flow stabilizes through the turbo and humps you to another spot on the map (eventually) where your engine is capable of taking enough flowfrom the compressor that you aren't in surge.

 

That one or two little farts keeps the turbo pumping.

 

At a higher flow rate through the opened up head and with hte bigger cam, you will be 'lower' on the compressor map, and 'to the right' in relation to flow.

 

PSI does not make power, FLOW makes power.

 

Understanding this is the first step in figuring out how to make REAL power with a turbo car. It's not as simple as Corky says to just throw boost at it. 8PSI on a GT35R is like 17PSI+ on the same engine, with the stock turbocharger. You have to find the PSI/FLOW relationship that matches the engines capabilites or your power goals. Garrett gives horsepower ratings, and they are based on PPH of airflow. You have to calculate the flow capabilities on the engine, and match that to power levels. If your engine can flow well, then you choose a turbo with the matching PPH to support hte power, at the boost you calculate it will take to move that air into the secondary compressor (the engine). This is where plenum sizing comes in as well, because a small plenum can exacerbate surge issues because the centrifugal compressor can overfill the plenum quicker than the engine can digest it--it can spike pressures, and cause a surge.

 

I digress...

 

As JGK says, stay away from that .82, the compressor and turbine setup you have should be able to support (oh, let me grab a number out of my hiney...say.....) 658RWHP at around 17-25 lbs of boost.

 

If you are running stock bottom end, limited to under say 6500rpms, then lop say 100hp off that, maybe 150. JeffP has run his head on a stock L28ET block and posted results on his web page or here I think. He was a little frustrated when he made more than 450HP to the rear wheels on his stock bottom end. Mainly because a large hairy inciter was there grinning and saying "I told ya!" LOL Nothing like finding out you spent $$$ on a bulletproof bottom end that will still break the pistons when you detonate, and finding you could have tuned the maps to that point on your old stock block engine!

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Phil, I highly recommend you not go to a larger turbine. The .63 is a perfect match for the L28. You might get more peak power from a .82 but the spool would noticeably suffer. Like you, I used to have a highly custom internal gate. I could never get it to stay closed no matter how big my spring was. All porting the wastegate offers is to provide a larger surface area for exhaust pressure to push on the flapper door. Remember, the higher the boost the less wastegate size you need since most of the exhaust is used to move the turbine. As a fix, I made a custom divorced wastegate port that allowed a 40mm Tial external gate. All my problems were solved. The fabrication was a pain but I can almost guarantee that your boost issues will be gone. You may still have a surge problem with stock components but the boost flutter would be gone. Let me know if you want more detail. Your car should easily be over 400rwhp.

 

I'm curious on one other point. Are you really running a 2mm head gasket with stock pistons and a ported head? If the answer is yes and your combustion chambers are worked over you are well under the stock 7.4:1 static compression. That's too low in my opinion.

 

P.S. On an external gate, the rule of thumb is boost should not exceed 2.5x the spring pressure. 2x is preferred.

I have a 38mm turbonetics external wastegate now. I have not put the ported head on yet. But yes I have 2mm head gasket with stock p90 head. Still that has my compression very low. I've blown 3 motors so I wanted to be safe.
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  • 3 weeks later...

If you are going to a larger cam and ported head you need the larger housing. That small housing at 25psi is choking the hell out of that turbo. People do this same crap in the 350z community. Tiny turbine housing on a gt35 and can't figure out why they can never make power past 5500-6000 rpms. Or they use a turbonetics kit with a 2" down pipe and wonder why no more power after 4500 rpms. I say larger housing and use the 38mm gate. Or if you use the smaller housing you need a bigger wastegate, probably an external gate.

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If you are going to a larger cam and ported head you need the larger housing. That small housing at 25psi is choking the hell out of that turbo. People do this same crap in the 350z community. Tiny turbine housing on a gt35 and can't figure out why they can never make power past 5500-6000 rpms. Or they use a turbonetics kit with a 2" down pipe and wonder why no more power after 4500 rpms. I say larger housing and use the 38mm gate. Or if you use the smaller housing you need a bigger wastegate, probably an external gate.

 

A GT35R on a 3.5 Liter and a GT35R on a 3.0 Liter are two different things. JeffP's making 658+ to the wheels on a GT35R at 7500rpms...

 

"Choking the hell out of it" might be an opinion, but I wouldn't state it as fact...

 

Most of the time cars fall flat on their face at 5500-6000rpms is because they are using some permutation of stock cam timing. If you are throwing boost at it, you will make hellacious numbers with hellacious boost, but not above that point.

 

All you do with a larger A/R on the exhaust housing in narrow the powerband. A .63 A/R on an L28 will kick in at a boost threshold of 3400 like clockwork and make full boost. And pull to 7500+ if you have the cam and headwork to accompany it (till you stonewall the compressor section).

 

But run that same car with a stock cam, and you still come on at 3400, and run to 5500-6000.

 

Now, put a .82 on there. First example now starts with a boost threshold of 4500 rpms, and will pull till the compressor stonewalls at the same point, around 600-700 HP at 7500-8000 rpms. Result: Narrower Powerband.

 

Second example, boost threshold is now 4500, and pulls to a peak at 5500-6000. Result: Narrower Powerband.

 

With the numbers being reached, airflow through the compressor is nearing maximum. It's not a bigger A/R on the turbine side he needs for more power, its more flow from the compressor side of the equation. Though admittedly the flow at only around 380 HP should not be anywhere near stonewall. Nearer to 700hp, you're stonewalling, the compressor will physically not flow any more air thorough it, you need a bigger compressor.

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So tony, if I'm reading your comments right...if the boost pressure stops rising before the point at which the WG should keep it at, the issue is that the head and cam are out flowing the turbo at that point? And to fix it the only thing you can really do is to get a bigger compressor side?

 

And just how destructive is it to the turbo to run at that point?

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This is a very informative thread, especially since my setup is very similar to Phil's: GT35R, 3.0L, high lift cam, ported head, etc. The main difference is that mine has flat top pistons, giving a higher static CR @ 8.5:1. It will be interesting to see how mine performs relative to Phil's...if I ever get mine running :(.

 

Tony - Thanks for sharing your knowledge on the subject. People like you make this site a valuable resource.

 

Cheers,

Ross

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Stonewall is not as bad as surge. Surge is much worse.

 

I would not make the generalization you made about boost pressure stopping. Stonewall would be more likely to be seen as pressure coming to WG pressure, and as the engine speeds up and sucks more air, you see the boost start dropping off no matter what you do, or you can't make more boost when turning up the WG dialaboost thingamajig.

 

If you are using less flow than the turbo produces, you will be able to boost more, until you surge.

If you are flowing well, the turbo should technically be able to flow more as it speeds up, as it has a non-linear production rate...but if it can't keep up then you are into stonewall.

 

Most manufacturers give a PPH or Horsepower rating for their compressors. When you are near the HP maximum, or beyond it...chances are you are nearing a stonewall point. On a GT35R with a restrictive head, you might start surging at 25psi, and see the dyno do one thing, well below 600 HP (Minimum Flow Surging for the pressure being attempted.)

With a good flowing head/cam like JeffP's, you will see the engine make say 658hp at 21psi, but nothing more at 25... Matter of fact, the power peaks at a slightly lower rpm, then plateaus almost flat as the RPMS cross 7000 onwards (whereas at 21psi the plateau didn't start till 7400 rpms)... This is the compressor not being able to flow any more. Not a flow restriction issue, a compressor delivery issue.

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