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Discussion Starter #1
Wierd boost issue. FIXED!

Here's my set up.
357 Windsor, 8.5:1, custom cammotion turbo cam, Canfield avenger stg3 heads, victor Jr. Intake, Holley 750 built by Brent Davis (crutch). Twin t-70 turbos with .68 ex housings. Fully built 4r70w trans 3.27 gears, 275/69/15 MT dr's.

Same me setup I've had for a long time except I upgraded the old 57mm turbos to twin t-70s. Also swapped my old race c4 for the 4r70w. I've only had it out for a few weeks. Have NOT went WOT yet, my wide band died and I haven't installed my new one. Anyway when I get to 2700-3000 rpm the turbos start bleeding off, sounds like a bov but it's coming from the turbos and it alternates from the drivers side to the pass side. Very wierd.. My old turbos never did this. I do have small filters on them I wonder if they are choking the turbos? It's almost like they are to the point of making boost but not quite and they start popping off... And one else have this happen?? Did a burnout in 1st gear and it didn't do this just when I shifted into 2nd gear it started chirping.. It's got plenty of power.. Feels like it just wants to go WOT,,, At 55 mph in 3rd gear half to 3/4 throttle its hauling ass but still "chirping" the boost gauge bounces from 5-10 psi..


Turbos too big?
 

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Discussion Starter #2
Sounds like the turbos are surging.... Wonder if anything can be done to fix this?
 

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Yeah it sounds like you're going into compressor surge, which means you're not getting enough airflow for the pressure ratio you're at. It isn't a huge problem per say at part throttle but at WOT compressor surge can break a wheel. At part throttle it can reduce bearing longevity and increase wear. What size turbine are you coming from and what size did you go to? As far as I know if all parts are in good working order (mainly blow-off valve), the only way to fix compressor surge is to go with a better matched turbo(s). I don't think there's much you can do from a tuning standpoint because it's a physics issue.
 

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Discussion Starter #4
I went from 57mm turbos with .69 housings to twin t70 with .69 housings.. You think bigger exhaust housings (.81 or even .96) will fix my problem ?
 

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As far as I know, an A/R ratio change won't cure surge outright. It's a wheel mismatch problem for the pressure ratio and airflow quantity that the engine is operating at. What size turbine wheel was in the 57mm turbos? I assume the T70 is a 62 to 66mm turbine. The best way to determine the fix is to calculate airflow for your engine, divide by two(you're running twins) and then plot it on the compressor map for the turbo model you have. If no map is available then you can find a map from a turbo that's similar. I don't know your cam specs, but twin T70s is alot of turbo to spool up, my guess is you're not getting enough shaft RPM which is why you're going into surge. What's the .050 timing specs?
 

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Yeah it sounds like you're going into compressor surge, which means you're not getting enough airflow for the pressure ratio you're at. It isn't a huge problem per say at part throttle but at WOT compressor surge can break a wheel. At part throttle it can reduce bearing longevity and increase wear. What size turbine are you coming from and what size did you go to? As far as I know if all parts are in good working order (mainly blow-off valve), the only way to fix compressor surge is to go with a better matched turbo(s). I don't think there's much you can do from a tuning standpoint because it's a physics issue.
I had that exact problem with a Buick GN for the same exact reason. I paid the price for another swap but then it was only one.
 

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Discussion Starter #7
Yea I'm not sure bigger exhousings will do the trick. Might just sell these and get a pair of 61's
 

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What is your BOV setup?
 

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I'd go over everything first just to make sure things are working as they should before swapping out the turbos. Verify the the BOV is working properly, boost reference signal to the gates and BOV, run these turbos without the filters. Do you still have the 57s? If you still have the issue after verifying everything is working, you could put those back on and if it goes away then you know the issue is the T70. If it is, then you could spring for a pair of the 61s. And I need to look into what's different on that turbo vs the traditional Garrett legacy turbos because the simulation says that turbo pair should be okay on your setup as far as surge is concerned.
 

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Discussion Starter #10
I have a Chinese eBay 50mm bov mounted by the carb and a dsm bov mounted next to the inter cooler. I first just had the dsm bov on there. But didn't think it was big enough. Two things I noticed with these. The vaccum lines are long I will try to shorten them up for better response. And the 50mm bov was hardly working, it has a super stiff spring on it. Shows no signs of wear from the piston moving. Can I just cut a coil off of this stiff ass spring or do I need to order a new one?
 

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Discussion Starter #11
I dont have the 57's. Here are the specs on the 70's
Cold side
.70 ar
4" inlet and 2.5" outlet
Inducer 70mm
Exducer 99mm

Hot side
.68 housing
3" vband
Inducer 74.5mm
Exducer 65mm

The 61s are identical except for the compressor wheel (61mm/99mm)
 

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It's not the turbos. I think at the 1/2 throttle you are creating enough vacuum to open the bov's.
Have you put compressed air to the vacuum lines going to the bov's to make sure they aren't leaking?
 

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Yeah, I agree. There is an issue with one of those BOVs that's causing the issue you're seeing. The springs are cheap for the Ebay BOV. Did you run the car with the 57s and the Ebay BOV? Or was that new to the system as well as the T70 units?
 

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Discussion Starter #14
I ran the 57's with both bovs, never had this issue. If the bovs were leaking wouldn't it help my issue, letting air escape instead of going back through the turbos? Would I need a weak spring on the 50mm unit? I might just take the old dsm one off and make a block off plate.
 

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Discussion Starter #15
Was doing some reading would I want the bov to be slightly open at idle? Kinda like a supercharger set up..
 

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My bov's are closed at idle. I'm trying to have one slightly open at idle. My turbos will alternate blowing out of each like you describe only while idling.
 

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Discussion Starter #17
I took out the bov spring and cut two coils off of it so it would just hang open at idle. That helped my problem but didn't fix it. I'm surprised how much air these 70's move at idle. Then took off my air filters and went for a drive. All I could hear is sweet turbo spool no more surging.... looks like these filters were choking my turbos. They are small but I don't have a lot of room, with the compressor housings being in the inner fenders. Never thought that would be the fix, lol. My bov is MUCH more responsive now, I could barley hear it before.


Jeremy
 

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That's great you got it figured out.
 

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This happened to A friends car a while back and it was strange.Cruising lightly down the street it would make all kinds of sounds.

Wooop pssssshhh pssssshhh pshhhhhh pshhhh side to side.This was the ponydown twin turbo kit.I think it was a vacuum/spring issue.
 
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