Lets talk rear mounted turbos
08-11-2016, 10:06 AM
#1
Lets talk rear mounted turbos
Yeah, the red headed step child of boost. After all of my experience in other cars, motor swaps and boost, I cant help but wonder why there aren't more examples in the VQ platform. For a group where $500 gets you an intake and 10hp, it only makes sense to look further into this. Before you reach for that reply button, I don't want to start any debates about manifold mounted vs rear mounted, lag, STS, road debri yada yada, I don't care at all and most of it is just hearsay anyway... I just want this to be an all inclusive info thread to show others a very possible option that want to go FI without breaking the bank. Please check your "what I've heard" knowledge at the door and keep your opinions to yourself. If you don't own a rear setup, have dyno sheets, helped install a kit yourself, links to knowledgeable threads or just plain want to stir the pot, please DO NOT hit the reply button..
Lets get our learn on and share what we DO know, help me, help you go fast
Lets get our learn on and share what we DO know, help me, help you go fast
08-11-2016, 01:21 PM
#3
My buddy has a STS rear kit with twin snails on his C6. At low boost the lag is not that much present but at high boost thing lags. Once boost kicks in his tires would not grip but other than that, its one of the fastest cars I've been in. Only thing I didn't like was how long the piping was and placement.
08-11-2016, 01:31 PM
#4
Because when you search them they are all several years old and the turbo community and technology has come so far since then. Not to mention they are riddled with a ton of misinformation and arguments so its better to start fresh and not let people read the old out dated stuff.
My buddy has a STS rear kit with twin snails on his C6. At low boost the lag is not that much present but at high boost thing lags. Once boost kicks in his tires would not grip but other than that, its one of the fastest cars I've been in. Only thing I didn't like was how long the piping was and placement.
Does your buddy have a dyno graph you can post to backup all this lag? Lag is not a bigger problem in rear mounted turbos, its the same as all the others. In fact, the air is 100's degrees cooler by the time it reaches the turbo and in turn has become more dense and some could argue it's even more efficient in the rear, allowing bigger turbos with the same spool time or smaller twins with practically no lag at all.
Last edited by Kris9884; 08-11-2016 at 02:14 PM.
08-11-2016, 02:37 PM
#5
Lets also squash the super exaggerated piping argument. The 12-15 feet of charge pipe from the rear turbo to the throttle bodies is hardly any different than running the piping from a turbo to and through a restrictive intercooler and back up to the throttle bodies.. I bet if you straightened out all the piping from both a front and rear setup and laid it next to each other, they would be within 1-2 feet of each other. Not to mention the huge intercoolers that a front mounted setup has to pressurize before it gets there too. Think about it.
08-11-2016, 04:41 PM
#7
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08-11-2016, 05:33 PM
#8
Compressor side is probably a wash length-wise if the rear-mount doesn't use a heat exchanger. I'm curious has to heat loss from the compressor to the throttle bodies.
08-11-2016, 06:47 PM
#9
You're missing some 411 on equilibrium and fluid dynamics. While cooler is denser, volume and thus velocity is down. There there's lots of drag removing even more energy from the exhaust stream. Total available energy to the turbine is less than a front-mount.
Compressor side is probably a wash length-wise if the rear-mount doesn't use a heat exchanger. I'm curious has to heat loss from the compressor to the throttle bodies.
Compressor side is probably a wash length-wise if the rear-mount doesn't use a heat exchanger. I'm curious has to heat loss from the compressor to the throttle bodies.
I just found this link today actually while trying to find IAT graphs, there's a TONNNN if good info!
Rear Mount Turbos... - BoostCruising
Directly from the mouth of STS back in the day:
"By mounting the turbo further downstream, the gasses do lose heat energy and velocity, however, there is just as much mass (the amount of air) coming out of the tailpipe as there is coming out of the heads. So you are driving the turbine with a "denser" gas charge. The same number of molecules per second are striking the turbine and flowing across the turbine at 1200F as there is at 1700F"
If you're super worried you can get the pipes ceramic coated and wrap them with reflective tape too.
Last edited by Kris9884; 08-11-2016 at 06:56 PM.
08-11-2016, 10:34 PM
#10
So...heat is energy and it's a very large component on how quickly a turbo "spools". If you were to compare the potential energy of two, identical masses of air, one being 400 degrees hotter than the other, the hotter mass of air will have far more potential energy.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Last edited by Ape Factory; 08-11-2016 at 10:40 PM.
08-11-2016, 11:45 PM
#11
While I'm being a stickler about the physics, there's no reason it won't work. It's just that the efficiency is lower than a front-mount. You'll have to resize the turbo as was noted on the website.
Last edited by slartibartfast; 08-11-2016 at 11:55 PM.
08-12-2016, 09:45 AM
#12
So...heat is energy and it's a very large component on how quickly a turbo "spools". If you were to compare the potential energy of two, identical masses of air, one being 400 degrees hotter than the other, the hotter mass of air will have far more potential energy.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Negative. This is the equilibrium part you're missing. The volume goes down as the exhaust gas cools, the velocity goes down. Molecules per second goes down. Total energy goes down.
While I'm being a stickler about the physics, there's no reason it won't work. It's just that the efficiency is lower than a front-mount. You'll have to resize the turbo as was noted on the website.
While I'm being a stickler about the physics, there's no reason it won't work. It's just that the efficiency is lower than a front-mount. You'll have to resize the turbo as was noted on the website.
All great points, but were getting stuck on the lag issue like it makes having a rear mount not possible. Were talking a couple 100 RPM's OTHERWISE, its still a turbo'd G37... For arguments sake sure, lets all agree a rear mounted setup will not spool as quickly as a front setup, completely agree, no arguments there but lets move on. Even if a ridiculous 1000 RPM was added to the spool up time so be it, you'll run an 11.6 instead of an 11.2, lets please move on and get back to point.
As for cost, I have to disagree. The bolt on kits start around $7k while leaving several things out, most importantly the labor of the install. We could piece together our own setup for half that while still using a BorgWarner turbo, Turbosmart BOV/WG, boost controller of your choice etc. The only custom work you couldn't do with just basic tools is the welding for the exhaust piping, even then its not intricate at all, just welding an elbow onto a T4 flange and making it all straight and tucked. Any competent exhaust shop that makes custom exhaust could do it. Most guys that have done it said they were charged $4-500 including the piping. There are tons of threads on Corvettes, Mustangs, Trans Am's, Goat's running twin rear turbos and adding up all the costs. They are always much cheaper.
08-12-2016, 09:58 AM
#13
So...heat is energy and it's a very large component on how quickly a turbo "spools". If you were to compare the potential energy of two, identical masses of air, one being 400 degrees hotter than the other, the hotter mass of air will have far more potential energy.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Rotary engines are great examples of how this works. Their EGT's can be hot enough to melt aluminum. They can typically spool a much larger turbine, more quickly, than their piston counterparts. And it's not due to volume.
Edit: There's also the header/collector design which plays a major roll in tuning the exhaust pulses to be more effective at spinning the turbine. You loose that with a rear mount.
But getting back to the original question, I'd think the biggest disadvantage would be drivability. It wouldn't be an easy car to handle as the boost came on and wouldn't have the flexibility of a typical turbo system. I think it'd still cost a pretty penny to do it right and it'd be pretty close in cost to a properly-designed turbo system.
Explain what you mean, I cant think of any situation where a rear turbo would make the car less drivable than a front turbo setup? Also fill me in on the flexibility of a front install as oppose to a rear install, what does that even mean?
08-12-2016, 11:16 AM
#14
Boost response. You're convinced lag won't be an issue (I'm pretty sure it will be on a 3.7L v-6 with a rear mounted turbo) but I've driven high hp cars with big turbos and tons of lag and it's not fun when you have to turn the wheel left or right. If I had a penny for every time I crushed an 800hp Supra with my measly 400hp RX7, I'd be a rich man. It's not fun to try and handle a car mid turn at 4 or 5k rpm when the boost hits. I prefer lots of area under the torque curve as the car will scoot out of corners and have that swell of low end power. Just easier to control than going from 125 to 300hp in over a 300rpm range.
Honestly it sounds like you've already made up your mind so I say go for it. I'll be rooting from the sidelines. I like the idea of removing the hot turbo from the engine bay honestly. I'd look at adding a water/meth kit for intercooling.
Honestly it sounds like you've already made up your mind so I say go for it. I'll be rooting from the sidelines. I like the idea of removing the hot turbo from the engine bay honestly. I'd look at adding a water/meth kit for intercooling.
08-12-2016, 12:21 PM
#15
Boost response. You're convinced lag won't be an issue (I'm pretty sure it will be on a 3.7L v-6 with a rear mounted turbo) but I've driven high hp cars with big turbos and tons of lag and it's not fun when you have to turn the wheel left or right. If I had a penny for every time I crushed an 800hp Supra with my measly 400hp RX7, I'd be a rich man. It's not fun to try and handle a car mid turn at 4 or 5k rpm when the boost hits. I prefer lots of area under the torque curve as the car will scoot out of corners and have that swell of low end power. Just easier to control than going from 125 to 300hp in over a 300rpm range.
Honestly it sounds like you've already made up your mind so I say go for it. I'll be rooting from the sidelines. I like the idea of removing the hot turbo from the engine bay honestly. I'd look at adding a water/meth kit for intercooling.
Honestly it sounds like you've already made up your mind so I say go for it. I'll be rooting from the sidelines. I like the idea of removing the hot turbo from the engine bay honestly. I'd look at adding a water/meth kit for intercooling.
.. Not to mention, why would you be hitting full boost in a corner unless you meant to? I had a .91AR bolted onto the stock (3.0) 2J in my Lexus SC and the lag was obvious but it didn't make the car worthless, I cant imagine a 3.7 with a smaller turbo mysteriously being worse? In comparison, I'd probably go with something more in the 70mm range for the G. I haven't decided whether or not I personally will boost my car but just wanted to publicly discuss the options for everyone and hope to learn more about it myself as well.
Last edited by Kris9884; 08-12-2016 at 12:26 PM.