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Thread: Tuning the twin turbo

  1. #1
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    Tuning the twin turbo

    Well I finally got everything lined up to tune the car. Dave flew in from Ohio to work his magic. We street tuned it Wednesday morning. Everything functioned like it should and idle and street manors are excellent. So we loaded it up and headed to Lincoln. Strapped it to the dyno and proceeded to make a few base line pulls. Waste gates have 14lb springs in them. It was boosting to around 10 to 11lbs of boost. Hp was in the 300 rwhp area, not really impressive I felt. Dave added some timing and we started dialing in boost. We quit at around 20lbs and 450 rwhp which I felt was low for my expectations. We could have dialed in some more boost and maybe a degree or so of timing and hit 475 to 480 but I called it quits for the time being as the tune is not super agressive and We felt there was something holding it back from making expected numbers. The really bad news is the full boost isnt there untill almost 5000 rpms makeing it a dog on the street or off the line in a quarter mile race. This is not what we expected and everyone was at a loss as to why it wont spool with small turbos. The water to air IC worked great and keep ACTs low. Hottest day of the year. 100 degrees in the dyno shop. Dave and I brained stormed a lot to try and figure out what is limiting the cars performance. Dave felt I needed to talk to the maker of the universal kit I used to fab up my system. I talked to STS tech guy yesterday and layed out to him the entire story and list of mods for the car. I sent him detailed pictures of how the system is set up and give him detailed information so he could evaluate the problem. He told the problem was very obvious. The cam in the car now, while great for a supercharger is totally wrong for a rear mount turbo. He spent about 20 minutes schooling me on the why and where withs of cams. Now before a lot of you guys with turbo cars jump in and say use this or use that. A rear mount turbo cam has to be a certain spec to work the best with a rear mount turbo setup. Cam specs are not the same as a front mounted turbo. We all know that to make the best HP possible we need a combination of modifacations that make sense and work together. My cam is speced out with lots of exhaust duration and lift, more so that the intake side. It also has a lobe seperation of 112. The recommendation given to me by STS was a lobe seperation of around 116 and they have installed one cam with 118. Second recomendation was to reduce the exhaust duration and lift dramaticly. Those two things alone would solve the poor spooling and its low power. He told me I would have made sick power if I had a closer to stock cam than the big stick in it now. The tech guy said he worked for a major cam company specing out cams for a living and seemed to have a good working knowledge of why the recommendations work. To bad I can remember everything he said. I plan to do some more looking in to this and talking to Dave about a cam to replace my bump stick. I hope to have a better recommendation today from STS as to what are good numbers for a new cam and discuss this with Dave. So unless I figured out something else wrong with the combination it looks like a cam change in the near future.

    I would like to say that Dave was a wealth of information and worth bring him in to tune the car. He even made my dog fall in love with him and Benny one of the local SC guys has help me through fixing a lot of things and getting the car ready. Even solved a electrical problem keeping the alternator from charging the battery. Thanks to both of you


    Ken

    93 SC Whipple powered 11.648 @ 119.6
    Twin Turbo, air intercooled..yet to see

  2. #2
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    Ken, can you post your current cam specs.
    Making woosh sounds since 2010!

  3. #3
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    Quote Originally Posted by neverfastenough View Post
    Ken, can you post your current cam specs.
    Intake lift .602 Exhaust .610 Duration 224/228 Lobe seperation 112
    Vale timing @ .050 Open Closed
    Intake 4 BTDC 40 ABDC
    Exaust 50 BBDC 2- ATDC

    Numbers off the cam card

    Ken

    93 SC Whipple powered 11.648 @ 119.6
    Twin Turbo, air intercooled..yet to see

  4. #4
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    Ken you will figure it out!!! You have a beautiful car and a monster lies beneath!! Good luck w/ finding the right cam for your setup! look forward to the end result.

  5. #5
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    Guess a picture of the card would cover everything.

    KenName:  CAM CARD.JPG
Views: 564
Size:  164.2 KB

    93 SC Whipple powered 11.648 @ 119.6
    Twin Turbo, air intercooled..yet to see

  6. #6
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    Ken and Ben did an excellent job making sure everything was ready and the overall install/prep was excellent. Ken was constantly expecting something he did would go wrong but it didn't and everything went really smooth. I just wish we'd had a clue how sensitive these turbo's would be to back pressure. Live and learn!
    Quote Originally Posted by Miller View Post
    Ya thats why i tape mine down. People think its bc i dont have a moonroof seal (which is true) but its really to keep my roof from ripping off .
    Email me here.

  7. #7
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    Minor set back. Pretty good results for having a wrong cam shaft. Congrats on not blowing up on the dyno.

  8. #8
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    Ken,

    Looks like the same cam I've got in my 91 SC or really close to it. So they are thinking the higher than usual exhaust back pressure from the small turbines and intake/exhaust overlap are the reasons for slow spooling and less than expected power ?

    Is there any way to block off your wastegates to confirm they were staying closed until you reached the desired boost level ?

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat...............707HP/650TQ

    My Garage

  9. #9
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    Quote Originally Posted by David Neibert View Post
    Ken,

    Looks like the same cam I've got in my 91 SC or really close to it. So they are thinking the higher than usual exhaust back pressure from the small turbines and intake/exhaust overlap are the reasons for slow spooling and less than expected power ?

    Is there any way to block off your wastegates to confirm they were staying closed until you reached the desired boost level ?

    David
    The first thing the STS guy wanted to know was the lobe seperation, when I told him 112 I could almost hear him shaking his head over the phone. Seemed to feel that was a big big problem. Than asked about exhaust lift, once again said that was not good. He even went as far as to suggest I install some 1.6 rockers to lower the lift some to help. Duration way to much for the intended use. I am waiting to hear recommendations. Sometimes going big is not better. Waste gates are plumbed into the rear exhaust so no way to really block them off. I beleive your lift on the exhaust side was a little lower than mine but you had more duration. It makes sense if you start to think it through that rear mounted turbos have diffrent problems due to their location and need a diffrent consideration. Corey has suggest I install the extra 5lb springs in with the 14lb springs to equal 19lbers. All in an attempt to have power come on quicker. The only problem with that is I wouldnt be able to run pump gas. The 10 to 11 lbs on the lowest setting is pump friendly at the moment. Other suggestions were shots of nitrous to spool the turbos. In the long run I would rather solve the problem rather than bandaid it. If a correctly speced cam gives me what I am looking for than I will tear the engine down again. Only have done it a half dozen times already so whats one more time. We are always telling new guys that if you mod something to move more air than you need to upgrade injectors. Everything works together to get a desired result. Running a cam speced for a rear mount turbo just seems to make good sense over a pretty agressive supercharger cam. Dave pointed out that coreys almost stock cam made over 500 rwhp, if bigger is better, 600+ should have been a cake walk with my bigger cam

    Ken


    Ken
    Last edited by kenewagner; 05-09-2014 at 12:37 PM.

    93 SC Whipple powered 11.648 @ 119.6
    Twin Turbo, air intercooled..yet to see

  10. #10
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    I don’t remember what my stock cam was lsa wise but it still had more exhaust lift and duration than intake, just a fyi. It was either 112 or 114.

    To the comment on back pressure pre turbo as David asked. It hasn’t been measured yet according to ken. Normally the high backpressure would be a quick spool and poor top end performance.

    Ken how did the boost curve look, it will tell a lot. Especially if the wg is starting to creep open around 10psi and then fully open at your desired 20 psi set point. I’d want that wg closed to the last possible second.

    Btw I ran 20psi on 19 degrees on pump gas. My motor showed signs of detonation though. I’d ask Dalke if you can lower the timing some so you can experiment with the heaver springs on the street.

    BTW again, if youre interested in my cam it may be available. I may be a bit off but i beleive its 206/212 553/562. I have the card somewhere.

    As far as the sts guy goes, I think he likes high LSA because raising lsa would lower overlap if nothing else changed. I think...... no expert on that. And some people think overlap is the end of the world on a turbo car.


    Maybe this will stir up some interesting conversation, got this from the GN forums

    stock gn cam

    Advertised Duration : 258/262
    Duration at .050" lift : 192/196
    Lift .384/.408
    Lobe Separation 107 degrees
    Overlap 38 degrees

    Hard to find GMC cyclone stock specs but the off the shelf aftermarkets seem to be 110-112lsa with around 200/200 500/500
    Last edited by neverfastenough; 05-09-2014 at 01:39 PM.
    Making woosh sounds since 2010!

  11. #11
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    We can easily do a cam that will eliminate overlap. It's much harder to actually put it in. Lift is pretty much irrelevant, but most people look at lift and assume high lift means long duration so you can't seem to have a conversation about cams without lift coming into the discussion. I already know what we need to do, it's just a bummer that Ken has to take his stuff apart again.
    Quote Originally Posted by Miller View Post
    Ya thats why i tape mine down. People think its bc i dont have a moonroof seal (which is true) but its really to keep my roof from ripping off .
    Email me here.

  12. #12
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    Dig out them wrenches ken! Bright side, that engine has to be easier to work on with all that stuff out of the way. Might even be easier to just yank the motor.
    Last edited by neverfastenough; 05-09-2014 at 02:33 PM.
    Making woosh sounds since 2010!

  13. #13
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    I know what their theory is. In a long, rear-mounted hot side, there are multiple pressure zones that achieve their peak pressure at different times. First, the pressure will be highest in the headers because of the collector. Once that happens, you must maintain that pressure until the downstream pressures are high enough to spool the turbo. In order to keep those pressures high, the valves cannot stay open in overlap too long or else that pressure will be lost from reversion back into the cylinder. Every overlap event, the exhaust system can potentially lower the pressure in the hot side. However, I do not agree with the less exhaust duration with the cam that Ken has. The piston speed and volume of gas that is exiting the exhaust port will help spool the turbo. The pressure in the hot side will not overcome those pressures, to a certain extent, but definitely not in the magnitude of Ken's cam specs. The rear mount system differs from the front mount turbo system because of the variable exhaust pressure building, but in the end result it's still exhaust pressure and airflow.

    I do agree with the theory, for the most part, but I do not agree with their opinion on the magnitude of your cam specs. How do they actually quantify that? It's hard to determine unless you test it. Your cam is rather dainty compared to the cam that I used. You have 2 degrees of overlap on that cam and I had 27 degrees. Rear mounted turbo systems do not vary that greatly in overlap. My exhaust duration was also 260 with .600" lift. With this cam, my heavy, 74mm, 1.1 A/R turbo hit peak boost by 5000rpm. If I had to do another cam, I would take the LSA out to 118.

    I would suggest to raise the LSA to 116-118 (depending on the duration you choose), raise the intake/exhaust duration, and run a straight-up duration split or regular split with a few more degrees higher than the intake duration.

    Just a little food for thought!
    1994 SC
    4.5L Twin Turbo SVO
    Project GTFD

  14. #14
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    Quote Originally Posted by CMac89 View Post
    I know what their theory is. In a long, rear-mounted hot side, there are multiple pressure zones that achieve their peak pressure at different times. First, the pressure will be highest in the headers because of the collector. Once that happens, you must maintain that pressure until the downstream pressures are high enough to spool the turbo. In order to keep those pressures high, the valves cannot stay open in overlap too long or else that pressure will be lost from reversion back into the cylinder. Every overlap event, the exhaust system can potentially lower the pressure in the hot side. However, I do not agree with the less exhaust duration with the cam that Ken has. The piston speed and volume of gas that is exiting the exhaust port will help spool the turbo. The pressure in the hot side will not overcome those pressures, to a certain extent, but definitely not in the magnitude of Ken's cam specs. The rear mount system differs from the front mount turbo system because of the variable exhaust pressure building, but in the end result it's still exhaust pressure and airflow.

    I do agree with the theory, for the most part, but I do not agree with their opinion on the magnitude of your cam specs. How do they actually quantify that? It's hard to determine unless you test it. Your cam is rather dainty compared to the cam that I used. You have 2 degrees of overlap on that cam and I had 27 degrees. Rear mounted turbo systems do not vary that greatly in overlap. My exhaust duration was also 260 with .600" lift. With this cam, my heavy, 74mm, 1.1 A/R turbo hit peak boost by 5000rpm. If I had to do another cam, I would take the LSA out to 118.

    I would suggest to raise the LSA to 116-118 (depending on the duration you choose), raise the intake/exhaust duration, and run a straight-up duration split or regular split with a few more degrees higher than the intake duration.

    Just a little food for thought!

    Not running headers but heavy ported cast manifolds. Can you add a little more insite on your last statement? sounds like you advocate more duration but I dont understand "
    and run a straight-up duration split or regular split with a few more degrees higher than the intake duration." Sounds like you agree with the LSA of 116-118

    STS sent me an E mail that offered no numbers other than they said a asymmetrical grind is recommended with less lift and duration on the exhaust side. Duration to be determined by who ever specs my cam and based on RPM acheivement requirements. Still trying to talk to a live body again

    Ken

    93 SC Whipple powered 11.648 @ 119.6
    Twin Turbo, air intercooled..yet to see

  15. #15
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    Quote Originally Posted by kenewagner View Post
    The first thing the STS guy wanted to know was the lobe seperation, when I told him 112 I could almost hear him shaking his head over the phone. Seemed to feel that was a big big problem. Than asked about exhaust lift, once again said that was not good. He even went as far as to suggest I install some 1.6 rockers to lower the lift some to help. Duration way to much for the intended use. I am waiting to hear recommendations. Sometimes going big is not better. Waste gates are plumbed into the rear exhaust so no way to really block them off. I beleive your lift on the exhaust side was a little lower than mine but you had more duration. It makes sense if you start to think it through that rear mounted turbos have diffrent problems due to their location and need a diffrent consideration. Corey has suggest I install the extra 5lb springs in with the 14lb springs to equal 19lbers. All in an attempt to have power come on quicker. The only problem with that is I wouldnt be able to run pump gas. The 10 to 11 lbs on the lowest setting is pump friendly at the moment. Other suggestions were shots of nitrous to spool the turbos. In the long run I would rather solve the problem rather than bandaid it. If a correctly speced cam gives me what I am looking for than I will tear the engine down again. Only have done it a half dozen times already so whats one more time. We are always telling new guys that if you mod something to move more air than you need to upgrade injectors. Everything works together to get a desired result. Running a cam speced for a rear mount turbo just seems to make good sense over a pretty agressive supercharger cam. Dave pointed out that coreys almost stock cam made over 500 rwhp, if bigger is better, 600+ should have been a cake walk with my bigger cam

    Ken


    Ken
    If you can't mechanically block the wastegate, applying air pressure to the top of the waste gate from your manifold, the turbo outlet or an independant source is the easiest way to test spooling with gate blocked. Doing this will provide max boost because the air hose is trying to keep the gate closed instead of working against the spring to open the gate, so you have to be careful not to over boost the motor. Do you have a diagram showing how you plumbed the boost controller and wastegates ?

    Just think it's worth testing the wastegates to make sure they are staying closed until reaching peak boost before swapping the cam.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat...............707HP/650TQ

    My Garage

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