Rear Mount Turbo Opinions

[DrifterProdigy85]

Im thinking of doing a rear mount turbo setup on my 240sx just for the hell of it. Probably use a T3/T4 50 trim. Fully wrap the exhaust in header wrap to keep heat in. Turbo will be placed where the muffler currently is. All the work would be done by myself. All intercooler pipes will be welded with maybe 3-4 couplers at the most. Wastegate and downpipe will exit out the back. Also will use a Turbowerx Scavange pump for oil return. Oil return is the biggest issue with a rear mount but that pump works excellent from what ive read. The setup might cost a little higher than the normal ebay junk setups but everything will be new and pretty much utilize all the stock stuff without taking up more room in the engine bay.

Everyones a critic and has there own opinion so lets hear em.

[RustspecS13]

If you really do it, you'll want a tiny a/r on the exhaust side because that exhaust will be much colder by the time it gets to the turbo no matter what.

RMT aren't really a bad thing for the cars that first got them. They were vettes and trucks and stuff that had big motors and cramped engine bays.

On a KA, I wouldn't bother. There's plenty of room in the engine bay for anything, I don't get why you wouldn't want to use it.

Good luck with getting good boost response. IE on/off the throttle.

But if you do go ahead with it, good luck. I'm always interested in something different, but there are better ways to be different.

~Alex

[Doya]

I totally agree with ^^ Doing a rear-mount just complicates things even more.

[DrifterProdigy85]

Having the exhaust painted and wrapped helps a ton with them. Im familier with rear mount kits on F bodies. Guys are running large 76+mm turbos with large .9x a/r. Making well into 900whp. The boost problems come with those who run long tube headers and unwrapped exhausts which loose tons of heat. Ill be using a .63 T3 housing with a 38mm gate.

STS is doing this on the 350z and Scion TC now with good results as well.

[WDRacing]

.48AR exhaust housing is a much better idea. With the 38mm wastegate you'll have enough exhaust flow to support over 400 easy. Why add lag?

Use aluminum for all of the charge air pipe to keep weight down. Hell you could use flexible PVC from the turbo to the under hood area if you wanted. It would be light, easy to install and it's cheap.

Are you going to run a decent amount of boost? What are the power goals. I'd be leery about wrapping the pipe that goes under the car. The wrap will make those pipes prone to corrosion problems. You don't lose near as much heat/flow as people assume you do. Once those metal pipes heat up, they stay hot for a long time. Ever try to work on your turbo after you started the car for a short 5 minutes? You get burned every time.

Free flow cat is also a must.

Will be a cool project, better keep us updated

WD

[PuNcHdRuNk]

not to be a negative nancy or anything but my only question would be why bother? when you factor in all the variables i.e. turbo placement, charge piping, oil lines ect it just seems like a lot of extra weight, time, money, and materials for nothing. i cannot see a single upside to a project like this, JMHO.

[DrifterProdigy85]

I don't really have a goal. Just to do something new that people don't approve of for no reason at all. The weight added would be at the rear axle and besides that, it's notmuch more than 30lbs. Won't make a difference there. It's not complex at all. Remove the muffler and weld a flange on and make pipes and oil lines. Very simple. The heat is alot lower than the turbo would be in the engine bay but that's not a big deal because that's where the turbine housing size comes into play. The lack of heat makes for less exhaust velocity which can cause lag with a larger turbine housing. But with a small housing it will raise the velocity making spool similar to a conventional setup and won't have the backpressure issues.

[PuNcHdRuNk]

so basically more money and more weight for less performance. yes it would be unique, i just think performance mods should be made with just that in mind, performance. This type of thing ranks among DONKS and the guy with the ice cream truck with hydros and twenty two's. it just makes me think, why?anyway just my .02 good luck with your project and i would love to see pics of the setup when you get it all done.

also the more piping you have to fill with psi the longer it will take to achieve boost, so not only do you restrict your options for turbos, any turbo that you use will take longer to spool no matter how you look at it.

[DrifterProdigy85]

Weights not any different than putting a turbo in the engine bay. Just a different place where the weight is added. When STS came out with the first kits everyone said the same thing about lag and response and it was all BS. The guys that run them love them and the guys that don't run them just make excuses why they don't like them. You might think the response would be bad but that comes from the size of the turbine housing not the length of the intake pipes. Guys used to say that same thing about subarus going front mount and now they all do it.

[biggie]

To play devil's advocate, MOST guys who have them seem to be much larger displacement V8 guys. Possible big difference in lag/response when on a I4. I've never seen one on anything other than V6/V8, but I'm sure would be an intersting project.

[WDRacing]

If I were doing a remote setup I'd use a T28. Or I'd simply accept the lag issue.

Maybe one of those CT26's or something along those lines. Something normally to small for the KA. The external wastegate will assist in the flow once you're moving more air around so you won't have a restriction.

This isn't the most efficient or best means to boost a car, but no one is claiming that. This is just what someone else wants to do...which is awesome. Because I don't know anyone that has done this on the 240. I know a guy who used a T25 on his Cavalier, 1.8 IIRC. Full boost by 3k and he even used the stock exhaust system right up to the T2 flange where the muffler was.

This will work, it's just different.

[DrifterProdigy85]

What I might do is start with something cheap and small just to see how it will workout and keep costs low if it fails. If it all works out then ill go with a small t3 turbo with a small exhaust housing.

[WDRacing]

I'd suggest tossing a stock SR T25 on there. They are cheap as hell and internally gated. So the initial setup to see how everything works will be ultra cheap. Maybe a RB20 or RB25 would work...they still spool crazy fast but will support a touch more power.

[DrifterProdigy85]

Yeah i was thinking a T25 just to get started with too. That way i could get used SR injectors, MAF, and a Nistune program for my own tuning. The T25 will be a good testing turbo. Small turbine to keep exhaust velocity high and a small compressor for good response. I had one on a SR so i know exactly how they spool on a 2.0L. It was instant on a 2.0 let alone on a 2.4L. The RB25 T28 would be good too but i hate that 6 bolt outlet. Depending how i mount the turbo i could use the RB Elbow facing toward the rear of the car.

[C-Kwik]

You don't lose near as much heat/flow as people assume you do. Once those metal pipes heat up, they stay hot for a long time. Ever try to work on your turbo after you started the car for a short 5 minutes? You get burned every time.

But your exhaust pipes will be much more managable than your turbo 5 minutes after shutting down. The pipes have a great deal more surface area than your turbo does. As such, it sheds a great deal more heat.

The lack of heat makes for less exhaust velocity which can cause lag with a larger turbine housing. But with a small housing it will raise the velocity making spool similar to a conventional setup and won't have the backpressure issues.

Backpressure will be higher with a smaller turbine as airflow through the engine increases. The limitations of smaller turbines doesn't change with location.

[WDRacing]

Backpressure will be higher with a smaller turbine as airflow through the engine increases. The limitations of smaller turbines doesn't change with location.

An external wastegate will help with the flow. Not to mention people use T25's for low boost on their KA's already for 8 psi. An additional 38mm's will be more then enough to reduce any restriction.

[DrifterProdigy85]

The exhaust velocity changes over distance, pipe size, bends, and heat. Placing the turbo at the rear will see alot less velocity from the distance and heat. Backpressure is caused when the exhaust velocity reaches so high that it builds up creating pressure which becomes the restriction. Sizing the turbine housing is what balances the distance and loss of heat. The larger the engine, the larger the turbine housing will have to be because the larger engine has the ability to put out alot more exhaust velocity. Thats why a .60 turbine housing might work on a 2.4L but when you go to a 3.0L, you will need to step up to a larger housing because of the increase in velocity. There is no exact number for what turbine housing you need. Rear mount setups need a smaller turbine housing period. The size needed is gonna be based off the engine size and setup.

[C-Kwik]

An external wastegate will help with the flow. Not to mention people use T25's for low boost on their KA's already for 8 psi. An additional 38mm's will be more then enough to reduce any restriction.

A wastegate only diverts flow when enough energy is available to have reached the desired boost level. Before then, the flow is still restricted. Still, smaller turbines are less efficient at higher flow rates requiring a lot more pressure differential. To achieve the higher pressure differential, the wastegate stays closed placing more backpressure on the motor.

The exhaust velocity changes over distance, pipe size, bends, and heat.

Exhaust gas velocity will vary throughout the exhaust system and will do so primarily with the conditions present at each section of pipe. When a passage narrows, the gas speeds up. If it gets wider, it slows down. But instantaneous flow through the entire system will be somewhat constant for the engine's airflow. But the velocity is not constant.

Placing the turbo at the rear will see alot less velocity from the distance and heat.

Exhaust gas velocity will not be affected by distance in that of itself. For any given volume of gas flowing through a pipe, the velocity will be dependent on the rate at which the gas flows. The heat loss of course will affect velocity. Bear in mind though that none of the velocity in the pipes are relevant to the operation of the turbo (I'll explain below).

Backpressure is caused when the exhaust velocity reaches so high that it builds up creating pressure which becomes the restriction.

No. Velocity is the effect of higher flow (for a given pipe size). And flow is the result of a pressure differential. Backpressure is merely the pressure measurement of the flow restriction.

Sizing the turbine housing is what balances the distance and loss of heat. The larger the engine, the larger the turbine housing will have to be because the larger engine has the ability to put out alot more exhaust velocity.

I can get just as much exhaust gas velocity out of a small motor as I can a large motor. All I have to do is put in the appropriate pipe size. What a smaller motor can't do is have as much flow (assuming same VE and peak RPM) as a larger motor. That said, larger motors have an advantage as the higher flow rate of gasses allows both high velocity AND more mass. Since Kinteic Energy is equal to (Mass x Velocity^2)/2, if both motors are generating the same amount of velocity for their respective systems, the engine with higher flow will still be outputting more exhaust energy as there is more mass of the gas.

Thats why a .60 turbine housing might work on a 2.4L but when you go to a 3.0L, you will need to step up to a larger housing because of the increase in velocity.

Assuming you are talking about the A/R ratio, its not a larger housing. The only thing the A/R changes in a given turbine family and trim is the oriface that the gas passes through just before it hits the turbine wheel. Changing the size of this orafice (or nozzle) is what controls the velocity of the gas for a given amount of flow. Simply put, you have it backwards. You don't run a larger A/R to deal with velocity. You run an A/R appropriate to the engine (with consideration of the turbine wheel size itself) to achieve the ideal gas velocity for that turbo.

Rear mount setups need a smaller turbine housing period. The size needed is gonna be based off the engine size and setup.

To achieve similar spool rates, yes, a lower A/R is necessary. But lower A/R's flow less which makes them bigger restrictions. Smaller turbines are also generally less efficient at higher flow rates. Larger turbines, once in their operating range of flow can work with smaller pressure differentials across the turbine. This ultimately means less backpressure, which means the motor is diverting enrgy that would be used to accelerate the car into pushing the exhaust gas out of the cyulinders. Furthermore required engine airflow rates aren't going to change so matching a compressor to the motor isn't going to change. The smaller the turbine, the more limitations you have on available compressors. You can't exactly get efficient operation by trying to couple a T66 compressor with a T25 turbine. Hell, even something as common as a T04E would likely be too big with anything less than a T3. Basicvally the compromise between fast spool and ability to make more power becomes more of an issue with a rear mounted turbo.

[DrifterProdigy85]

Well I know nothing. But I'm still gonna do it to see what happens. The worst that will happen is it doesn't work that well but I still have the parts to use elsewhere.

[WDRacing]

Any turbo that spools early will be a restriction at higher rpms and load. It doesn't matter where the turbo is, Chano is only regurgitating fact. Smaller holes will obviously be a restriction at some time...duh.

When the turbo is at full song, the wastegate is open and it DOES relieve back pressure inside the manifold...or else the turbo wouldn't stay at one particular boost.

People have added larger external wastegates and made power because it increases exhaust flow. Can't argue fact. Just like a smaller turbo will be restrictive, a larger wastegate will flow more.

Do it. It will work fine.

[DrifterProdigy85]

Once spring rolls around and my dad doesnt need the tractor in the garage then ill start on this project. Im gathering funds up right now. Got about $1k so far. I doubt i will use near that but i got it just incase.

So far the 350z guys love the STS kit. There spooling a 60-1 8-10psi @ 3500rpm. Im not looking to make any big power with a rear mount setup, just an attempt to be creative and make a tad more power. You never know, it could suprise some people. Everyone ive talked to with a remote mount setup has nothing but good things to say about them. Ive only seen one instance where a guy had trouble making over 15psi with his LS1 and swapped to stock manifolds and wrapped the exhaust and that was all fixed. I think its worth a try for something new. Its kinda like back in the day when the SR20 started to get big and few people started talking about turboing the KA. So many said it was a junk truck motor and couldnt handle it. Now look what it started. Lol.

[WDRacing]

The key to the entire setup is the return pump. Everything else will work fine. Are you going to use an intercooler?

[DrifterProdigy85]

I think they carry Scavange pumps at the hardware store near me. If not, i may spend the extra money on a Turbowerx Pump for the safety. I will be using an Inncercooler. A small one around a 20x6x3 core. Gotta grab some SR 370cc injectors and a Z32 MAF. Debating on how i wanna tune it. Nistune or somekind of piggyback. I have a laptop and consult cable so i might just do Nistune and get it over with.

[C-Kwik]

People have added larger external wastegates and made power because it increases exhaust flow. Can't argue fact. Just like a smaller turbo will be restrictive, a larger wastegate will flow more.

While a larger wastegate can flow more, if nothing else changes, the turbo will still require the same pressures to operate. The wastegate will still only divert enough of the exhaust gas to maintain the pressures needed to maintain the desired boost level. If the wastegate diverts more of the gas, the turbo won't be able to boost as much. I can't think of any way that this would not be true assuming both wastegates are able to flow enough air to prevent boost creep.

[WDRacing]

If the turbo reaches 8 psi at 3000 rpm (generic numbers), then the wastegate opens so that the increased exhaust flow created with more rpm has somewhere else to go. There by keeping a semi consistent and constant back pressure. Assuming the compressor is efficient enough to produce plenty of volume. Wouldn't the larger wastegate be able to reduce the added back pressure created by using a smaller turbine flange? That way you can use the smallest flange possible to reduce as much lag as possible.

Noting that this method is NOT best for a max power type build. But for a lower power goal, say 300whp, if you used a GT2876R or whatever T2 flange turbo that comes with a T4 compressor, you'd have more then enough flow from the compressor side and way less lag from the T2 flanged exhaust housing.

Just my thoughts on this type of thing. Why add lag?

[C-Kwik]

If the turbo reaches 8 psi at 3000 rpm (generic numbers), then the wastegate opens so that the increased exhaust flow created with more rpm has somewhere else to go. There by keeping a semi consistent and constant back pressure. Assuming the compressor is efficient enough to produce plenty of volume. Wouldn't the larger wastegate be able to reduce the added back pressure created by using a smaller turbine flange? That way you can use the smallest flange possible to reduce as much lag as possible.

No, because for a given engine airflow (which implies a specific engine RPM at WOT), the turbo will need a certain pressure differential in order to maintain the desired amount of boost. This is independent of the wastegate. So a wastegate will bleed off enough air to hold the pressure differential. To do this, regardless of wastegate size it needs to bleed off the same volume of air as the volume of the exhaust system before the turbo remains constant.

Additionally, as engine RPM increases, so does airflow. This means the compressor has to spin faster to keep up with the airflow demands. Just look at any compressor map. If you drew a horizontal line at any pressure ratio to represent a constant boost curve, as airflow increases, so will the indicated compressor wheel speed. The end result is that as airflow demands increase, so does the demand for exhaust energy from the turbine. So no wastegate will hold its position once the boost threshold is reached. It constantly adjusts to the input signal from the intake.

Noting that this method is NOT best for a max power type build. But for a lower power goal, say 300whp, if you used a GT2876R or whatever T2 flange turbo that comes with a T4 compressor, you'd have more then enough flow from the compressor side and way less lag from the T2 flanged exhaust housing.

Just my thoughts on this type of thing. Why add lag?

This is part of the limitation I was referring to. Big mismatches in compressor and turbine wheel sizes will create a slip loss. When this occurs you actually end up with a laggier turbo:

http://books.google.com/books?...false

Regardless, a larger wastegate does not make up for the lower flow rate of a smaller turbine.

[WDRacing]

At the point where the turbo becomes a restriction to total flow, having a larger wastegate will move more exhaust. The turbo already has all the exhaust energy it needs to spin the compressor fast enough to achieve the desired boost. So it's up to the wastegate to bleed off the excess back pressure. A smaller hole won't move as much as a larger one.

How does this not make sense?

We can agree that I'm right

No one does point counter point like you buddy, cept maybe 480SX. I miss that dude alot.

[C-Kwik]

At the point where the turbo becomes a restriction to total flow, having a larger wastegate will move more exhaust. The turbo already has all the exhaust energy it needs to spin the compressor fast enough to achieve the desired boost. So it's up to the wastegate to bleed off the excess back pressure. A smaller hole won't move as much as a larger one.

The energy demand of a turbo is ongoing. Its not a once its spooled, it stays spooled type of device. Energy is constantly required to continually compress air. That said, in order to extract energy from the exhaust, a certain amount of air is needed to flow through the turbine. How much depends on the amount of energy needed and is facilitated by the pressure differential across the turbine. The wastegate controls this pressure by bleeding off air. How much air it diverts will be the same for a given turbo, regardless of the wastegate size (excluding a maxed out wastegate). A larger wastegate will simply not open the plunger as far, as the wider area will allow more air through.

If the wastegate fully opened once the turbo was spooled, it would let too much air through and the boost pressure would drop.

How does this not make sense?

Because you're explaining it.

We can agree that I'm right

No one does point counter point like you buddy, cept maybe 480SX. I miss that dude alot.

We can both agree that we both miss 480sx. But he would agree with me on this as well.

[WDRacing]

Because you're explaining it.

This very well could be. But that just means I'm right, but have failed in my explanation

We can both agree that we both miss 480sx. But he would agree with me on this as well.

Actually Nate and I had discussed this very topic quit a few times. Which is why I miss him right now because he sides with me...for obvious reasons.

[RustspecS13]

At the point where the turbo becomes a restriction to total flow, having a larger wastegate will move more exhaust. The turbo already has all the exhaust energy it needs to spin the compressor fast enough to achieve the desired boost. So it's up to the wastegate to bleed off the excess back pressure. A smaller hole won't move as much as a larger one.

You really don't under stand turbos as well as you think.

Take my stockish RB20 with stock turbo. 10psi stock, 217whp on a dynapack. If I ported my WG or added a bigger flapper, I wouldn't gain any HP. I might actually loose HP right when peak boost his, because the WG flows more sooner and the WG is setup for the stock. How ever over all power will not be affected, because the WG stock or 60mm external will only flow enough to maintain boost.

Say I put a t4 sized compressor trying to run 10psi again. It would probably spool worse then stock since I'm trying to spin a very heavy compressor with a very tiny exhaust housing. Then say you still use this stupid t2/t4, and you get close to 275-300whp, the stock exhaust housing will completely limit you.

That tiny amount of housing area and wheel area (ex side) can only shove so much air through it. If you tried to flow 350hp worth of air through it, it just wouldn't do it. It would skyrocket your EGTs since exhaust just wont move, and the combustion chambers would still be partially filled with exhaust and you'd make absolute crap for power.

But rustspec what about my giant 90mm WG that can flow around that restriction? The WG will try to go for the boost its told to hold. Its a compressor pressure sensing device, not a exhaust manifold pressure sensing device. EX pressure is what drives it and it needs a lot of it, if the ex housing is a restriction to power, then that turbo wont make that power. The WG will just flow exhaust around it and you wont make any power.

As far as WG flow, on my stockish rb, at 3K when its making full boost, the WG just starts to crack open to maintain 10psi. So were making 22lb/min of flow to make 220whp, and the engine is making 23lb/min of potential air flow so the WG flows 1lb/min to level off boost at 10psi.

As the RPMs go up, and the total mass air flow increases exponentially with RPMS, the WG has to open further to maintain 10psi. So at 6K rpm engine mass flow is around 30lb/min, Turbo required power for only 10psi is only 22lb/min, WG flow is about 8lb/min to keep boost at 10psi.

Now at the same time the engine mass airflow increases, the power to drive the turbo to maintain 10psi goes up as well. But the available exhaust energy goes up faster then the needed exhaust energy. That's why some cars like DSMs have boost creep at high rpms. All a bigger WG does for them is control boost to what the user wants, it doesn't add any HP at all. The WG is a restriction yes, if you want to run low boost. It does not ever keep you from making HP.

Now I didn't use mass air flow and lb/min terms properly but crap its 1 am.....So I'm going to bed now.

~Alex