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Looking for some advice.
I recently had my car dyno tuned and the tuner thinks the turbo is too small and choking out the motor. I am unsure if the issue is turbo size or maybe just an issue with the turbo itself. It made 540hp/610tq at the wheels on pump gas at a peak of around 12 psi. Boost would drop off over the course of the pull. The car would pull hard to peak boost but then just hit a wall. See dyno graph below.
I am planning on some exhaust work over the winter (bigger downpipe and cutout) but I am not sure if I should be getting another 7875 with a 1.25 housing (assuming the turbo is the issue) or if I should be looking at a larger turbo with a t6 flange (assuming the turbo is too small).
The turbo isn't the problem with that dyno graph. A 5.3 with a 7875 at 12 psi won't nose over at less than 4500 rpms. That thing falls off like a rock. Do you have a measured boost curve graph? I would be more inclined to think the cam was installed wrong. What's the exhaust setup?
So I checked the cam twice and everything appeared ok. The ICL is 109 on that cam and I was within a degree, not perfect but not enough to make it nose over like that. I would think that cam should be pulling to 7k.
The exhaust is 3" from the turbo than about 4 feet from the turbo I have it split to dual 3" out to the back of the car. There is a bend right off the turbo due to the angle I put it at. (this is part of some of the rework I am planning).
Yes the springs are brand new and were checked before TEA installed them. The pushrods were checked and the correct length was installed. I don't remember the length off the top of my head but can check my notes if needed. I do plan on triple checking the cam ICL to make sure it is good.
The plugs are NGK BR6ES gapped at .025". During the dyno pulls I didn't hear any misfiring. So I don't think it is an ignition issue.
Some more background on this situation is I had the car tuned last year and it pretty much did the same thing only made about 430whp and didn't want to rev out. The tuner at that time thought there could be a valve float issue or an issue with the valves not seating. Since it was a set of junkyard 317 heads that I had put the new springs on. I figured there could be a chance of that so over last winter installed new heads and intake. Prior to pulling the motor apart I had checked the cam and also did a compression test. All the cylinders were right around 170psi.
This is why I am considering trying a new turbo as I think the motor is solid.
It's not the turbo, especially if it didn't rev out NA last year. Have you logged fuel pressure and confirmed that all 8 cylinders are firing and you dont have weak spark?
Sorry last year was with the turbo as well. Fuel pressure is good and increases with boost. When I was still on the p59 ecm (switched to Holley this summer) I never had any misfire codes or anything. I even used HPtuners to shut off injectors at idle to make sure I didn't have a dead cylinder. Like I said I dont hear any misses at RPM but I suppose there still could be something going on there. What else would I need to do to check for weak spark?
the couple times ive dealt with spark blowout (on a zx10) it didnt really sound different it just leveled off on the dyno.
do you have a flex bellows in the exhaust?
you are sure the cam is degreed right? pushrods are right?
what does your ignition hardware look like? wires, coils, coil grounds?
I do have a flex bellow in the cross over from the drivers side to passenger side. I will pull it off to make sure there isn't something weird there but it was a lined bellow.
I am going to head to the garage this afternoon and check the cam degree is good. While I have the rockers off I will double check the pushrods as well.
Ignition stuff looks to be ok but they are old coils. Plugs and plug wires are new, wiring to the coil packs is new. Grounds are good and tight at the back of the heads.
Tap the housing and install a gauge or transducer to measure back pressure. No reason to guess at this stuff its cheap and pretty important data. Will help you size another turbo too depending on the numbers. Personally I’m not a fan of a t4 on anything. But there are some nasty T4 turbos out there if you have the $ too.
I think people are missing the fact that the motor flows a ton more with those heads and is higher compression as well. Has it been bored over too?
Because a stock 5.3 or 6.0 has make more boost pressure, doesn’t mean a motor with a much better flowing top end won’t be out of turbo. If you are sure the gate isn’t blowing open and boost is falling with RPM, you are out of turbo. Pretty simple stuff… But a back pressure rig will tell you what’s going on.
It could also be the WG blowing open. You can try more WG spring and revisit you boost control and plumbing. Gap plugs to .015 to test and eliminate spark blow out as an issue too.
Also depends how stingy that dyno is. The gen1 stuff is usually done around 600-650. But you may be hitting that level on a different dyno. And others may hit over that on an inflated dyno. So you can’t really go by other peoples dyno numbers or boost levels IMO.
. But you may be hitting that level on a different dyno. And others may hit over that on an inflated dyno. So you can’t really go by other peoples dyno numbers or boost levels IMO.
you dont even need numbers on the graph to see something is terribly wrong
recently had a friend install a 'fresh' 6.0 in his turbo fairmont and it was a total turd, wouldnt make boost, wouldnt rev. turned out the genius that installed the cam used an adjustable gear, put the top gear lined up with zero mark and the keyway in the -2 slot. im amazed it didnt bend valves, and that it even ran. runs great now that its fixed.
not saying thats the case here, but its an easy mistake for someone thats new to engine building to make.
I was planning on measuring backpressure and have the stuff to do it. Unfortunately I didn't get it done prior to the dyno tuning. I will be getting that installed by the weekend. The motor is stock bore with the summit forged pistons. I am also with you on the top end flow, that is part of the reason for wanting to redo some of the hot side stuff. I have been going at it in phases. The big thing for me at this point is if I am redoing the exhaust side should I be looking at a different turbo anyway and build around that or stick with something similar. I guess that the backpressure readings will let me know for sure.
Currently I am running a 3 port mac (plumbed like a 4 port) on a 6lbs spring. I had it set up reading MAP for dome pressure but it was spiking so instead of paying the tuner to mess with PID's we just went to open loop. After about 60% duty cycle we couldn't get any more than 12ish lbs out of it. So maybe there is something going on with the wastegate. I will add more spring and go back to the MAP/dome control and see if I can dial it in to hold the boost
The Dyno was a Mustang and the tuner said don't be upset if the numbers are low so I figured the dyno is stingy. I also think the dana 60 is eating up some power too.
The timing chain and gears were GM replacement pieces and I lined them dot to dot. I also degreed the cam last year. I could see how something like that could happen on the adjustable sets but that isn't the case here. I am still going to recheck the cam degree because it doesn't take long and will rule it out. Also just for some background I have been working on cars/motors for 35+ years but we all make mistakes from time to time. This is my first turbo motor though. I would say after verifying cam timing and pushrods it has to be either the ignition or exhaust/turbo related. I may just buy some new coil packs and put them in to see if there is a difference as well to rule that out.
Sorry last year was with the turbo as well. Fuel pressure is good and increases with boost. When I was still on the p59 ecm (switched to Holley this summer) I never had any misfire codes or anything. I even used HPtuners to shut off injectors at idle to make sure I didn't have a dead cylinder. Like I said I dont hear any misses at RPM but I suppose there still could be something going on there. What else would I need to do to check for weak spark?
Thanks
Have you read the plugs yet? Should tell you if you have signs of weak spark or a bad injector. Have you put an IR temp gun on the primaries to see all 8 are firing? It's easy for a cammed V8 to be down a cylinder and it might not be obvious.
I had 8lb springs in and had 2.8:1 back pressure on my 317 headed 5.3 @ 26lbs on the small 1.10 T4 s475. So even at stupid high back pressure amounts it still made boost and power.
Your pressures do seem on the low side to me. But we don't know how well your top end flows either.
Plumbed like this? Have you confirmed at 100% DC the top port sees 100% of the manifold pressure and the bottom port has zero?
The plugs all look about the same maybe a little on the rich side but no one plug stood out as being more sooty than the rest and no signs of detonation.
Like I said before I did test the motor at idle to make sure all cylinders were firing I turned each injector off and could hear/see the change in rpm with each cylinder. So at least at idle I know all cylinders are firing.
Correct it is plumbed like the picture. I believe the tuner went from 50 to 70% duty cycle and saw no change in boost pressure. I want to say he tried 100% and got a spike of over 20 and let off and settled at just leaving it at 70%. I will have to confirm the actual pressures once I have a sensor wired in I will make a few runs to see what happens.
Thanks
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