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Old 05-01-2010, 02:47 PM
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twin turbo wont stay running

just finished twin turbo build. car wont stay running at idle only when given throttle. any ideas?

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Old 05-01-2010, 03:38 PM
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Quote:
Originally Posted by Pupsvette76
just finished twin turbo build. car wont stay running at idle only when given throttle. any ideas?
Need a lot more info on the motor. Fuel injected or carbureted? From everything I ever read you never break in a motor on boost. Probably need to adjust idle screw or tps setting.
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Old 05-01-2010, 07:39 PM
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its not being broken in on boost the bonnet is not on the carb the turbos scream like they are not getting oil but i pulled the feed line they are being oiled could the down pipe be making the loud screaming there is not gasket on it now with the curb idle set really high it will stay running however i dont wanna have the car sit and idle at 200o rpm... i know thats how its supposed to be broken in but we cant even get it to the point where we can call it the break in period

the motor is a sbc 355
the turbos arent huge not sure on size
forged crank forged h beams
forged pistons
stock gm 76cc head
edelbrock performer int manifold
holley 650cfm dp
488 lift cam installed straight up

thanks for your help in advance
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Old 05-01-2010, 07:42 PM
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I'm not too sure you want to freewheel the turbos- they can easily reach some VERY high revs...
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Old 05-01-2010, 07:54 PM
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AFA the idle quality, you need to be sure you have enough initial timing to allow the engine to pull good vacuum at lower speed- but w/o idling it, obviously, until the cam's broken in. The timing at 2000 to 2500 RPM for break in could be around 34-36 (and will tolerate more, being under no load) w/o problems.

The carb needs to have the transition slot covered enough that the transition circuit isn't activated at idle, but enough to allow a good idle- again all this w/o going below the 2-2.5K RPM. It would be safe to say that the primary side transition exposure is less important than keeping the engine speed high enough for the cam to break in, so I'd worry less about the trans. slot than the RPM of the engine.

If you set the timing to 36-38 @ 2500 RPM, the carb should be into the transition stage anyway. You could use the vacuum advance if it's equipped w/one for break in. I'd set the idle screws a couple turns out from lightly bottomed for now, and break in the cam like this. After the initial break in, adjust the carb correctly for idle, etc.

Keep an eye on engine temperature.
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Old 05-01-2010, 08:44 PM
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Until you get the cam broken in you needn't even care if it will idle. You have to run it for 20+ minutes ABOVE 2200-ish rpm anyway. You idle it now and you really risk killing the cam. Get the cam broke in and then worry if it will idle.

I would remove the turbos, letting them run free is a bad deal.

You would have been a lot smarter to fab up a engine run-in stand and do this out of the car.
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Old 05-01-2010, 09:24 PM
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a lot of good info guys thanks i will break it in first with more timing and vacuum adv hooked up for now but i do have wastegates installed so the turbos should be ok
only problem i have been having is that i cant get the car to stay running so its hard to set timing
i will turn the idle mixture screw out a few turns and keep the curb idle screw in a lower position and break it in first

if you guys think of anything else let me know thanks
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Old 05-01-2010, 09:28 PM
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also should it be real hard to start the car with the bonnet on the carb... just for future reference i just feel like with the bonnet on it would be like the engine is sipping through a straw
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Old 05-01-2010, 10:02 PM
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pulling a charge pipe off and letting the compressor side of the turbo vent to atmosphere will cause the compressor WHEEL to "vent to atmosphere" also ... so dont go doing that

if you DO need to run the engine with the turbos disconnected, prop the waste gates wide open and place a restricter of some sort just after the compressor. last time i did this i used a spray can cap with a decent sized hole drilled in it.
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Old 05-02-2010, 12:56 AM
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Similar prob on my ZZ4 turbo'd

I have a 355 ZZ4 specs turned roller, etc and I installed just a single turbo 9psi. I had same issue. Turns out no matter gow much I played and tuned carb there was just no solution. Turns out I had to make my carb into a blow threw carb. It's alot of work but not hard work, just time consuming. I found the info on the net. It described how to turn a Holley into a blow threw carb.
I did this and readjusted carb and all is fine.

My neighbor even had similar problem on his C10 pickup. He cut choke housing off just like mine and ran it a little rich on idle and he's satisfied.

One more thing what kind of intake you running?
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Old 05-02-2010, 05:39 AM
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the intake is an edlebrock performer dual plane yeah i did the conversion to blow through on my holley
drilled out the pvcr
solid floats
bigger needle and seat
is there anything else i need to do? after i did these things i thought to myself it couldnt be that easy? those are real easy changes and only took 30 min is there any more changes to be made?

and as for the spray can idea can you elaborate a little where do i put the spray can i understand opening up the wastegates but im not sure on the spray can idea
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Old 05-02-2010, 10:12 AM
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any variation of this:



it keeps some backpressure on the turbo so it cant overspin, but still vents air so that you dont have compressor surge problems while you fiddle with the car

doesnt have to be a paint cap either, if you have some scrap tubing around you can tack weld a piece of flat steel over the end (off center) to acheive the same effect. or if you have 2 long enough pieces of pipe, hammer one end almost flat on each to form some restictors

this is how we tested for MAF sensor turbulence on my friends car, because it was a 5 min workaround instead of a 4 hour turbo removal.
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Old 05-02-2010, 10:12 AM
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How about a pic of your setup.
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Old 05-02-2010, 10:14 AM
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You want to somehow cap the pressure outlet side of the turbo( the side going to the bonnet) with a restrictor hole in the cap. The turbo compressor side has to push against something or the turbine wheel's speed will skyrocket freewheeling beyond the design limits of the turbine and bearings. This is true even with the waste gate propped open, that is why you have to have the compressor side pushing against some restriction.

It would be smarter to just take the turbos out right now and fab up a replacement section of pipe to fit in their place.

EDIT: lowROLLERchevy has the smart less time consuming idea.
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Old 05-02-2010, 04:45 PM
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yeah i will do that right now the charge pipes are routed to the intercooler then up to the engine but i will take the charge pipe off and cap it with the paint cap trick i dont have the have a good enough welder or the time to fab up a pipe the project is due this week
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