When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Putting some miles on my freshly rebuilt 5.3 and trying to do some VE tuning with the VS Racing S480. Everything short of WOT is fine but during prolonged WOT(ie: winding out 2nd or 3rd) it gets to my designated 17psi (AEM tru boost) and then it oscillates between 17 and less. Is that indicative of too small of wastegates ? I have twin TIal 38mm WG. Or is it that the springs are too light? I'm running 10psi springs in each WG. I've tried both the 3 port MAC and 4 port MAC and the sensation is slightly less pronounced with the 3 port but it still occurs. I'm looking for my old TurboSmart Boost Tee but haven't located it just yet. I'm going to try that next to see if has any effect.
Gate size should be fine. What are you using to apply some pressure? Compressor housing, onboard air, CO2? How much oscillation is there? 17 psi to what? What does the hot and cold side consist of? And finally, what size is the turbine?
The compressor housing supplies the dome pressure in my configuration. I think it's going from 17psi to 15psi then back, repeat. Hot side is headers with one side of the engine feeding half of the split housing and the other side of the engine feeding the other side of the split.
Turbine is 102/92. and the compressor is 80mm.
Originally Posted by 3 window
Gate size should be fine. What are you using to apply some pressure? Compressor housing, onboard air, CO2? How much oscillation is there? 17 psi to what? What does the hot and cold side consist of? And finally, what size is the turbine?
Yes, one WG on each side, each between the header collector and the T6 flange. They are plumbed the same. But I do wonder about them 'fighting each other'. I'm getting the car up on jackstands here in the next hour to do an inspection of everything. It does seem to be something about the wastegates to me because If I don't have the gas pedal at 100%, just aggressive, It runs great and pulls hard. It just seems that once it gets to the PSI point of venting to the atmosphere that the phenomena starts. Both Wastegates are still relatively new
Originally Posted by ddnspider
So you have two 38mm WG venting the S480? Are they plumbed the same? Any chance that it's the 2 fighting each other causing the oscillation?
What are you using for a boost controller? If it's electronic, sometimes you have to reduce the amount of duty cycle the controller can give or take away to control boost so it's not oscillating like that. Most times I've solved oscillation issues it's been the controller over compensating trying to hit a boost target.
Interesting. As a test, I routed the vacuum tubes so that the waste gates are just operating off the spring. With the 10lb springs, it could only attain 8.5 psi but at least the oscillation was now gone.
My boost controller is the AEM Tru-Boost. It has been in service since 2015 and this is the first time I've encountered this. Prior incarnations were the original setup (single T4 PT7675), Twin GT35's and now this year, the single S480. I don't remember a variable of the duty cycle in the configuration but I assume you mean I need to increase the spring pressure so that the Boost controller isn't having to do so much. .
Originally Posted by LetsTurboSomething
What are you using for a boost controller? If it's electronic, sometimes you have to reduce the amount of duty cycle the controller can give or take away to control boost so it's not oscillating like that. Most times I've solved oscillation issues it's been the controller over compensating trying to hit a boost target.
Last edited by mightyquickz28; Aug 2, 2021 at 08:36 AM.
Interesting. As a test, I routed the vacuum tubes so that the waste gates are just operating off the spring. With the 10lb springs, it could only attain 8.5 psi but at least the oscillation was not gone.
My boost controller is the AEM Tru-Boost. It has been in service since 2015 and this is the first time I've encountered this. Prior incarnations were the original setup (single T4 PT7675), Twin GT35's and now this year, the single S480. I don't remember a variable of the duty cycle in the configuration but I assume you mean I need to increase the spring pressure so that the Boost controller isn't having to do so much. .
That's a good call. If manual removes the oscillation then that would point to a D/C issue on the controller.
On the tru-boost I often have to set the spring pressure or wastegate crack pressure whatever they call it a bit lower than what's actually in the gate to avoid a spike before it settles down to its higher rpm hold.
How is the controller getting its pressure sense? Is your manifold map tied in?
Reason I ask, the turbo pressure (PRE IC) is almost always higher than the manifold pressure, so it may be fighting itself. I'd tap the manifold, as that's the boost pressure you want to maintain.
Also, do you have a small bleed on your top WG ref line? LIke 30 thou or so worked well for me.
When I'm in closed loop the boost "porpoising" is caused by the boost levels overshooting the goal and the controller pulling back. Or undershooting the goal the controller trying to add to compensate. Has to do with PID control. Not familiar with the AEM tru boost, but on my ECU the PID controls are adjustable so you can remove this affect.
Hmmm... I'd forgotten about that aspect (setting the spring pressure in the program configuration) . Thanks for the reminder. I'll go over to the shop after a bit and check the setting.
Originally Posted by SLOW SEDAN
On the tru-boost I often have to set the spring pressure or wastegate crack pressure whatever they call it a bit lower than what's actually in the gate to avoid a spike before it settles down to its higher rpm hold.
Q: How is the controller getting its pressure sense? Is your manifold map tied in?
A: There is a port off the side/end of the S480 compressor housing going to a port on the MAC valve. The manifold pressure is used to feed the gauge / controller.
Q: Also, do you have a small bleed on your top WG ref line? LIke 30 thou or so worked well for me.
A: No Bleed hole. I thought it was desirable to have it airtight. I've not heard of this technique. What does it do for you? Soften the approach to max PSI?
Originally Posted by Forcefed86
How is the controller getting its pressure sense? Is your manifold map tied in?
Reason I ask, the turbo pressure (PRE IC) is almost always higher than the manifold pressure, so it may be fighting itself. I'd tap the manifold, as that's the boost pressure you want to maintain.
Also, do you have a small bleed on your top WG ref line? LIke 30 thou or so worked well for me.
When I'm in closed loop the boost "porpoising" is caused by the boost levels overshooting the goal and the controller pulling back. Or undershooting the goal the controller trying to add to compensate. Has to do with PID control. Not familiar with the AEM tru boost, but on my ECU the PID controls are adjustable so you can remove this affect.
I don't want to over complicate as i'm not sure how your setup is plumbed and I have no exp. with a true boost controller. Prob best listening to someone that does.
So if you have your feed from the turbo and your controller info from the manifold you have 2 different pressure values. I'd *think* you would want to Cap the line at the turbo and run a manifold press line to the MAC valve. THat way your mac valve pressure and MAP pressures are the same.
On the bleed hole it depends how you have the MAC valve plumbed, and which valve you use. Generally on a 3 port to get max resolution you want it plumbed like so. In this instance you def. don't want an air tight top dome as the boost pressure can't escape when the valve goes to zero duty. This also doesn't let it control the boost properly as it will make the valve sluggish. But again. That may not be how yours is setup. The 4 port has a built in vent. 3 ports do not if plumbed this way.
Last edited by Forcefed86; Aug 4, 2021 at 01:59 PM.
Currently, that picture depicts exactly how I have mine configured. Sounds like I need to either cap off the compressor port and drive everything off the manifold source or cap off the manifold port and drive everything off the compressor housing port. I'll look for a .030 drill bit in my drill bits for the top port bleed off.
Thanks for all the feedback guys . It is appreciated.
Oh, I did go look at the spring rate/crack pressure and it was still set to 14.5 psi from my prior setup. Since I now am running 10psi springs in each gate, I set the controller to 8psi springs to put it below the actual rate a bit, as slowsedan suggested.
Originally Posted by Forcefed86
I don't want to over complicate as i'm not sure how your setup is plumbed and I have no exp. with a true boost controller. Prob best listening to someone that does.
So if you have your feed from the turbo and your controller info from the manifold you have 2 different pressure values. I'd *think* you would want to Cap the line at the turbo and run a manifold press line to the MAC valve. THat way your mac valve pressure and MAP pressures are the same.
On the bleed hole it depends how you have the MAC valve plumbed, and which valve you use. Generally on a 3 port to get mac resolution you want it plumbed like so. In this instance you def. don't want an air tight top dome as the boost pressure can't escape when the valve goes to zero duty. This also doesn't let it control the boost properly as it will make the valve sluggish. But again. That may not be how yours is setup. The 4 port has a built in vent. 3 ports do not if plumbed this way.
Well, thanks to you good people, the oscilation/porpoising (loved that term) is gone. I set the spring rate at 8 (10psi springs per Tial), and put both the gauge feed and the controller feed to the intake. With no other changes, instead of going to 15psi then dropping a few PSI then going back to 15psi, it now stayed rock solid at 12psi, which is what I guess the controller was trying to maintain before but the compressor housing sourced pressure must've shot past before the controller dropped it down. I've set my 'A' setting from 35% duty cycle to 50% and now it's running a rock solid 14.5 psi. I'll swap the 10psi springs out and put the 14.5 springs in the wastegates and experiment some more. I'd like to run 21psi at the track. Test n tune are on Wed nights here so maybe I'll get lucky and have some decent weather tomorrow or even next week.
Thanks again!
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Slideshow: From wobbling harmonic balancers to failed EBCMs, these are the issues that define long-term C5 ownership and what repairs typically involve.
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Slideshow: A modern Camaro transformed into a retro icon, this limited-run "Bandit" build blends nostalgia with brute force in a way few revivals manage.
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Slideshow: Cadillac didn't just crash the high-performance luxury vehicle party, it showed up loud, supercharged, and occasionally a little unhinged...
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Slideshow: A one-off sports car that looks like a vintage Italian exotic-but hides a C6 Corvette underneath-just sold for the price of a new mid-engine Corvette.
Boost oscillation - What is the likely culprit? 
