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Can't seem to get a definite answer, I don't want to damage anything, so short story car was tuned open downpipe on 15lbs boost, I added full 3in to the back into dual 2.5 at the axle and lost 5lbs boost. Logic says the added back pressure made the turbo less efficient and now the waste gate dumps at lower boost, so is it okay to add duty cycle until I'm back at 15lbs of boost?
Yeah, you might be ok. The other thing is to make sure you've got plenty of crankcase venting. The additional backpressure will increase crankcase pressure and need to be vented. Worst case is the turbo will have too much backpressure and damage the journal/bearing. A lot of guys have bought a electric cutout to open under boost to fix this issue.
Yeah, you might be ok. The other thing is to make sure you've got plenty of crankcase venting. The additional backpressure will increase crankcase pressure and need to be vented. Worst case is the turbo will have too much backpressure and damage the journal/bearing. A lot of guys have bought a electric cutout to open under boost to fix this issue.
I dont really.want a cutout, i want the same power all times not opening it to gain and closing to lose, it is straight 3in to 3inle axle and then 2.5 duals no muffler so it's already free and loud, I have an oil cap breather and passenger valve cover into a catch can with drivers plugged hole.
I dont really.want a cutout, i want the same power all times not opening it to gain and closing to lose, it is straight 3in to 3inle axle and then 2.5 duals no muffler so it's already free and loud, I have an oil cap breather and passenger valve cover into a catch can with drivers plugged hole.
I assume your wastegate dumps to atmosphere and the cutout would be the same concept. People wire the control so that it automatically opens when the engine sees boost and then closes when it doesn't.
Your venting sounds like it might not be enough aswell. I run two 10AN hoses to a large single catch can from Radium. I was running a single 10AN hose and it wasn't enough to stop the rear main from leaking.
I assume your wastegate dumps to atmosphere and the cutout would be the same concept. People wire the control so that it automatically opens when the engine sees boost and then closes when it doesn't.
Your venting sounds like it might not be enough aswell. I run two 10AN hoses to a large single catch can from Radium. I was running a single 10AN hose and it wasn't enough to stop the rear main from leaking.
I jist think its pointless to spend the time and money on a cutout for 3in straight pipe exhaust when it wokt be quiet closed lol, and you dont think thr 2 lines from the valve cover and the oil cap is enough? I figured just the oil cap breather would be alot of venting
The cutout isn't for the exhaust system to be quiet. It's to drop the back pressure on the turbos exhaust turbine. If you wanted to add a muffler you certainly could but the cutout is just there to relieve back pressure.
I dont really.want a cutout, i want the same power all times not opening it to gain and closing to lose, it is straight 3in to 3inle axle and then 2.5 duals no muffler so it's already free and loud, I have an oil cap breather and passenger valve cover into a catch can with drivers plugged hole.
A single 3" is good for a turbo 4cyl. Not so good for a V8. Unless that 3" is dumped a couple feet from the turbo. I went from 3" full exhaust to 3.5" and it woke it up. The unfortunate fact is you just killed your power doing that without a cutout.
A single 3" is good for a turbo 4cyl. Not so good for a V8. Unless that 3" is dumped a couple feet from the turbo. I went from 3" full exhaust to 3.5" and it woke it up. The unfortunate fact is you just killed your power doing that without a cutout.
On his combo (a 5.3L with a 78/75 vs gen 2 cast turbo) I would wager that the only real change is a slight increase in lag and probably no power loss, with boost kept at 15 psi of course.
On his combo (a 5.3L with a 78/75 vs gen 2 cast turbo) I would wager that the only real change is a slight increase in lag and probably no power loss, with boost kept at 15 psi of course.
when I had a single 3” on my 7875, is see 14-15psi with the cutout closed and 17-19psi with it open on a back to back run.
Yes with it open vs closed your gonna see more psi, but, with a boost controller you can get back to 15psi I'm just not sure in the effects of 15psi with the added back pressure
when I had a single 3” on my 7875, is see 14-15psi with the cutout closed and 17-19psi with it open on a back to back run.
Yeah, re-read what I wrote. Also boost going up and down when you remove/add a restriction in the exhaust is pretty typical especially if the gate doesn't have the best priority.
Yeah, re-read what I wrote. Also boost going up and down when you remove/add a restriction in the exhaust is pretty typical especially if the gate doesn't have the best priority.
I read what you wrote. I don’t agree. If you have 15psi on a choked up setup vs a free flowing setup, you’re gonna make more heat, you can’t run as much timing and it’s gonna make less power. My boost goes up when I open my exhaust and I’m pretty sure my waste gate is in a pretty ideal location.
I read what you wrote. I don’t agree. If you have 15psi on a choked up setup vs a free flowing setup, you’re gonna make more heat, you can’t run as much timing and it’s gonna make less power. My boost goes up when I open my exhaust and I’m pretty sure my waste gate is in a pretty ideal location.
Wait, that's what you call an ideal location? The ******* wastegate is damn near 180 deg backwards from the actual exhaust flow, then makes two or three more 90 deg turns before dumping into the exhaust. And you wonder why opening the cutout makes your boost go up?
You can disagree with me all you want but I would put money on it making damn near the same power on his setup if boost is held constant whether it has full exhaust on it or not... and it's totally normal to have to increase wastegate duty cycle when you introduce an exhaust restriction, and no that doesn't mean that you have lost a bunch of power if any.
Wait, that's what you call an ideal location? The ******* wastegate is damn near 180 deg backwards from the actual exhaust flow, then makes two or three more 90 deg turns before dumping into the exhaust. And you wonder why opening the cutout makes your boost go up?
You can disagree with me all you want but I would put money on it making damn near the same power on his setup if boost is held constant whether it has full exhaust on it or not... and it's totally normal to have to increase wastegate duty cycle when you introduce an exhaust restriction, and no that doesn't mean that you have lost a bunch of power if any.
That is what i was thinking, it's 3in straight pipe into 2.5 dual at the axle so not super restricted, but def added back pressure with the run to the back of the car, back pressure equals pre turbo pressure working harder to spin the wheel so with out upping thr duty cycle it's going to open the waste gate at a lower boost pressure.
That is what i was thinking, it's 3in straight pipe into 2.5 dual at the axle so not super restricted, but def added back pressure with the run to the back of the car, back pressure equals pre turbo pressure working harder to spin the wheel so with out upping thr duty cycle it's going to open the waste gate at a lower boost pressure.
you think a single 3” is not restrictive for a turbo engine? 3” isn’t enough for a 400+ rwhp NA engine. Much less a turbo engine. Will it work? Sure. Is it robbing power? Definitely.
I did a pretty similar setup on my TA. It was a 408 into a 7675 precision turbo, 3 inch back to a GMMG catback. The turbo would make peak power at 4200 rpm and then boost fell off hard. The other thing I noticed was the turbo would howl under load. I guess give it a shot and if the turbo starts to howl and the impeller starts touching the compressor housing you'll know you have too much back pressure.
I went through this sane crap. I changed my exhaust several times. At first I had dual 2.5”. Then I changed to a single 3”. Lost power and boost. I didn’t want to come to terms with the. So I kept swapping mufflers. Didn’t get my power and boost back until I put a cutout right at the end of my down pipe. At that point I knew what needed to be done. I ran a 3.5” from turbo to muffler. And then 3” tail pipe. I built it all including the muffler. I’ll do 3.5” tailpipe when I get some free time but for now it runs a lot better than the 3”. What you fail to realize is exhaust gasses are hot and expanded tweeds the from. As it travels back, it cools and contracts. So something smaller in the rear isn’t as much of a power killer as it is up front. What the OP did is just the opposite of what he needed. This is mine:
That is what i was thinking, it's 3in straight pipe into 2.5 dual at the axle so not super restricted, but def added back pressure with the run to the back of the car, back pressure equals pre turbo pressure working harder to spin the wheel so with out upping thr duty cycle it's going to open the waste gate at a lower boost pressure.
Correct.
Originally Posted by Kfxguy
you think a single 3” is not restrictive for a turbo engine? 3” isn’t enough for a 400+ rwhp NA engine. Much less a turbo engine. Will it work? Sure. Is it robbing power? Definitely.
Huh? 3" works just fine for a 400 rwhp N/A engine and in fact it works just fine on an 800 rwhp turbo engine assuming the turbo isn't maxed out. It sure isn't optimal but it isn't necessarily robbing power if boost is constant and it wasn't at the limit before.
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Boost control for added back pressure on turbo 
And you wonder why opening the cutout makes your boost go up? 
