Rear Mounted vs. Front: pros and cons
 

I haven't found a lot of profound information on rear mounted turbos, their pros and cons (keep the bs for yourself please). Most I found is biased (STS promotion, direct or indirect).

What DIY experience is available?
What to watch out?

I am soon starting my first "homemadeturbo" project and front or rear mounted is one of the first strategic decisions to be made.

Volker

I don't have any hands on experance with rear mounted, but I have been told there could be oiling issues and boost issues with the long lines to the front.

i have seen a rear mounted turbo on a rx8 here in fl that was pretty nice. it does need a external oil pump to transfer oil so far back, but with the long charge pipes going under the car an intercooler is not necessary. A nice medium sized front mount would help alot with really low ait's. lag isnt much a big deal either as long as you can do the math to get everything sized up right. i think its a pretty cool thing if a conventional setup isnt available or impossible.

i say go for it it cant be that bad

I ran across some info on a '89 z24 cavalier build. RMT Z24 I also found a thread on the build on a gm f-body site Another Remote Mount Turbo System - Third Generation F-Body Message Boards There was a lot of info between the two. Finding a pump to return the oil to the pan seems to be main issue. I found some bio-diesel people modifying small block Chevy pumps to electric inline pumps here DIY, How to make a SBC veg-oil pump for cheap - Forums I was planning to use a pump like this for my car, but it will be awhile till i can say how it works. still gathering parts and planning build.

OP, what car are you trying to do a rear mounted turbo setup on?

Also, the oil issues can be remedied as long as the system is properly thought out. Boost should not be an issue, the only thing you should have to watch for is the clearances for the piping, making sure it does not impact anything... as well as using proper high pressure oil lines that are covered from the elements...

some disadvantages are:

turbo LAG due to the colder exaustgases.
Turbolag caused by the very long intake pipes.
oil drain needs a extra pump.
airfilter is under the car.
the turbo can get wet and crack when you drive throug water on the road.


advantages.
no turbo manifold.
no downpipe, just a short pipe as exaust.
Cool spinning sound from the turbo. (Jet-sound)
easyer to fab to your exaustsystem.
no need for drilling a oilreturn in your oilpan

I think the low ground clearances would also affect the "fab" ability of the install...

Turbolag caused by the very long intake pipes.

There is actually no lag caused by a long chargepipe.. its all about the exhaust setup screwing you.

The real issue with STS's is the oiling... you have to make sure its perfect or you will have issues.

do it...so youll be different..other than that i dont know much bout rear turbo's

Honestly I don't see any point in going remote mount if you have the ROOM to do a front mount. It's a LOT of extra piping, your turbo/piping is exposed under the vehicle, and you have to worry about designing the oil feed/return.

^ agree if there's room up front, mount up front.

With room I'd probably say mount up front as well. I am a little bias toward rear mounts tho having done it myself.

Rear mount will give you lower engine bay temps and cooler intake air temps. Lag is not an issue provided you pick a smaller turbine A/R and you keep the exhaust temp as high as possible when it reaches the turbo. When the turbo is cool boost sucks but once the turbo/exhaust warms up there is not much difference with front or rear mount. Just have to design it properly.

Oil system was pretty simple and has worked perfectly. Many turbo vehicles use scavenge oil pumps where the turbo is mounted fairly low, the Porsche 911 turbo for example, only difference is the oil lines are a little longer. Make clean runs and keep the lines tied up neatly and you'll have no problems.

Even with the long compressor plumbing and oil lines I believe my install was easier than many front mount installs I've seen. Either front or rear mount can be great or crap, it all depends on the install and if you do your homework when designing it. I covered many of the details in my build thread, check it out.

That's my thoughts exactly. More material, but easier to do.

I certainly have a bunch more questions, but first would like to go through your write up (your Q45 is probably as close as it gets to my Pacifica 4.0; I consider installing 2 small KKK K26 or K27 making use of the dual exhaust setup my 'signature' series car currently has; hoping for low rpm torque from the quick spoolup small turbos).

Could you send the link to you build thread?

"I covered many of the details in my build thread, check it out. "

rear sleeper haha

I agree, I want to do a setup!!

One thing is the fact that you must have an oil accumulator mounted to drain the oil into a air free sump that feeds a special oil pump that will return the oil to the engine oil pan. Turbo piping from the rear of the car will entail the proper turbo a/r selection and piping choices to ensure a velocity that will provide good throttle response and charge air to the engine. The only advantage I can see is that you almost have to buy a "kit" that has the bugs worked out of it. Underhood installation will provide you with less
piping choice selection miscalculations and in my opinion more help form knowledgeable
people from support forums to help you out. Good luck!

I did a remote mount turbo in a 91 S10.
The turbo is mounted in the bed. The oil return setup consist of a 1/2 drain hose down to Tilton oil pump. The pump used in circle track cars to pump oil from the rear end to a cooler and back. So its made for hot oil. From the pump a 3/8 line runs along the frame and to the front and up to the valve cover. This has worked with no issues for 2 summers.

As for spool up weather it is front or remote mount I think it comes down to converter stall speed, exhaust A/R , timing , and fuel ratio.
If the converter is loose the other 3 dont effect it as much. If the converter is tight the motor will need lots of timing and a lean to slight enrichment to come up as fast as possible.

I started with a loose converter on this combo that would flash 3000 on a stockish 4.3 v6. Boost was instant. Seriously just short of N/A motor reaction. I swapped the converter for a different one that is much tighter and it has slowed spool time a bit.
Works kind good though cuz on street tires you can stab it from a stop and roll out and then it freeks out with power .

I made one for my 1990 miata. made a video on youtube. was really an inexepensive build. I was so impressed with its' ability to perform, even with such a crappy build that i am currently attending welding school with the desire to make kits for this type of layout.
lag is minimal to nonexistant. anything over 6psi and you will need an intercooler.
the only thing you really need welded is a flange for the turbo to the exhaust pipe.
oil pump was a turbowerx scavenge pump built for remotes. $299.
the trick is the turbo. getting the right sized hot side, coupled with a large enough compressor to feed the motor properly.

the rest of it is how well you have built your car. Engine management is KEY! you have to be able to control timing and fuel. PERIOD. all the boost in the world won't help you if you don't tune for it.

others were correct, mount it well, tie up the oil lines, and enjoy a much less expensive layout. oh, and the concerns over water and rain... utter hogwash. there have been low mounted turbos for DECADES... and it hasn't bothered them at all, and rear mounts are placed higher. lowest turbo i ever saw was slung on a 911. no one complained then...

pics and dyno from mine
https://www.homemadeturbo.com/attach...ine=1334313642
https://www.homemadeturbo.com/attach...ine=1334313642
https://www.homemadeturbo.com/attach...ine=1334313642
https://www.homemadeturbo.com/attach...ine=1334313642

Thread revival! Sorta.

Has anyone considered using a power steering pump to provide the turbo with oil pressure and scavenge? I am in the process of depowering the rack in my '96 Miata and it occurred to me the output pressure might be just right?
-It has to pull vacuum on the return side as well.
-The pump is small and already on the car.
-The pump pulley is actually also the tensioner for that belt so removing it altogether would be a pain.
-Inline oil cooler on the way back from the turbo to pump.

I need to confer with Darth Rohmer as to my scheme.

power steering pump runs 1000 psi or more. not a good supply for a turbo that needs 40psi or less.

I believe that is what it can make for a short period of time if dead headed, no place to pump to. The more I think about it, the pump will probably not make any pressure until there is a restriction, a load so to speak. Using a return style fuel pressure regulator should work to maintain whatever pressure you want.

i rear mounted a gtk-1000 on an 02 trans am - engine built for boost made some really sick power (1218rwhp@28.2psi) but was bad in water - cracked turbine housings - later front mounted the same car and made less power(had to add timing and fuel) much hotter iat temps car would detonate but had no boost lag

also rear mount is much more expensive than front mount because of all the extra piping and the scavenge pump on the oil system - but is much cooler on the intake air temp

sparky..... not much more expensive.. a good oil pump is the same cost as a cheap manifold. and the rest of the piping is usually straight.. fewer elbows and couplers...
an excellent stainless gasbacked/purged tig welded equal length divided manifold costs more than some turbochargers....

how did you crack a turbine housing?

i ran mine for over a year through crappy weather.... not a single issue with the turbo.... even went throught some deep puddles in an already low car and never had any cracking issues....

Thanks for the info Sparky. Might you have any pictures?

I don't see why the scavenge pump has to be so expensive or hi-flo capable. The trickle of oil coming out of a turbo with the correct restriction jet, for me a .060 or 1.5mm hole at ~35psi, should be easy work for a small electric fuel pump sized pump. I have two candidates should I go that way, a Holley Red fuel pump for carburetors, high volume, low pressure, around 7psi max.
https://www.homemadeturbo.com/attach...ine=1343739549
...and/or a motorcycle turbo scavenge pump. I can't find a picture of the one I got via eBay but it is small. I'd be more worried that a larger pump would introduce air into the system or have other issues due to too little oil flow. I may actually wire the pump in series with a fan so both run slower and constantly.
Sparky or Rohmer, did either of you ever measure how much oil flowed thru your turbos? What size restriction jet did you use or did you have to?

I am currently excited about the possibility of using my power steering pump to provide oil pressure and scavenge to the turbo. The power steering rack will soon be de-powered, a popular mod on Miatas, as the PS makes the steering twitchy on the highway and is reasonable at parking lot speeds. Not only can the PS pump system provide plenty of pressure and flow, it also provides suction on the return side for a vacuum scavenge. This will allow for a separate oil supply system from the engine using OE parts that are already on the car and not known to have much failure.

I have lots of pics on my home PC n will post later - I was out and abouts one night doing some serious street racing and I bottomed out that is how housing cracked - the reason I said rear mount was more expensive - I work in an exhaust shop and used my factory manifolds with a crossover for the front mount setup - pipe I get comes in 10' sticks at cost. The pump I used for the rear mount was like $400. Is a sts pump.

Wish I would have thought of an external fuel pump for scavenge - hot oil doesn't hurt the pump? I always thought a fuel pump was cooled by the fuel going through it.

A fuel pump is not designed to pump oil but with the Holley, everything is metal except for a flat gasket piece made of rubber or Buna or Urethane of some sort. I figure if it will withstand fuel it should withstand oil but if it won't then just cut a replacement one out of Buna or Silicon.

If the pump is placed a few feet ahead of the turbo and the oil run thru a cooler or piping I don't think the heat will be an issue. The motor on the Holley is external and should not have to work very hard, I'd think it would be fine.

Not sure how its more expensive.

Kenton Hurst (The turbocharged Cavi Z24 that runs 11.8 with a Rear Mount) said he used parts lying around at first and the only real expense was the oil pump and mandrel bends. But the stock engine's fuel system had enough headroom to allow 5-6psi of boost (free). Once he sorted out his traction issues and slipping clutch, he was running 13's. With a custom chip, he ran 12's. With a new turbo larger turbo, Megasquirt and larger injectors it runs 11's now, makes 383hp/413ft of torque, dead stock GM 3400 engine (internally).

Here's some runs with the old turbo -
In fact searching around the net and here, there' a rear mount in a Nissan 200SX, less than $1,000 invested in the turbo system.

There is also a RMT 3rd Gen Maxima -
Thread here - First ever Rear Mounted Turbo maxima - Maxima Forums

I don't understand all the poo-pooing of rear mounts, I see many practical advantages one of them being cost. In most applications, you don't need much more than header wrap on the exhaust leading to the turbo to help spool and water injection if you want to run more than 6-7psi without an intercooler.

There is plenty proof of concept not even including STS because people are curious and adventurous.

I think some of the problem is being butt-hurt. Imagine you spent thousands of dollars and somebody else got the same or better results with less money spent and a smaller engine. I would congratulate not hate but that's America for you.

Just to prove it works, I want to do a rear mount. A few bucks more for piping, big deal. When you add up all the pipe length used for an intercooler, you'll find its very close to what it cost to fab up a charge pipe from the turbo to the engine inlet.

This is the first time I've heard of somebody bottoming out and damaging the housing. I would call that rare the law of averages bear that out, some people are going to have issues but the majority if you follow a few basic concepts will be successful.

Lastly, RMT improves bang for the buck if you can avoid the usual N/A trappings like cold air intakes, minor engine tuning and exhaust system upgrades.

I'm going to have to be a bit of a stick in the mud here... But!!!!!

How exactly does the exhaust gas temperature affect the way the turbo is driven, in a positive way?

As far as my version of common sense goes, the denser you can have the exhaust gasses, when it enters the turbine, the easier it would be to propel it. It is after all the gas molecules colliding with the blades of the wheel, that makes it spin, heat has no practical application with regards to getting the turbo to spin. This then obviously leads me to believe that a tailpipe turbo setup, will actually have less lag, when looking from the exhaust-temperature point of view, and also the turbo will probably last longer as well, it not being exposed to so much heat any more....

I don't know, maybe I was just sleeping in rocket-science class... :D

The first explanation I got on Miata.net went as follows.
"Yer a complete friggin' idiot, you don't understand anything, you need to go read and educate yourself and then come back."
I didn't find this helpful so I continued to pester the members there with my questions and got many good answers and explanations, eventually.

Heat is energy. The exhaust pulse coming out of the cylinder is a hot expanding gas. Some of this heat energy is used to spin the turbine.

....but when folks run water/alcohol injection to control detonation, the cooler denser exhaust charge helps with turbo spool up. :confused:

Corky Bell commented on my thread and dispelled many misconceptions alotta folks have about the heat energy. A traditional turbo does use some of the heat energy but he guessed maybe around 50% of it and a rear mount would be less than that by maybe 5% so no big deal there.
He had recently dyno'd a 3rd generation Miata with a make shift rear mount turbo and reported that lag was minimal and some adjusting could improve it. It made boost and good power, he was ultimately not thrilled about the added complexity and layout. This shut some up but some others continued in a funk of disbelief. "How could I ignore 20 years of turbo development and research and wanna run it away from the head"....like race cars do....and the Porsche 911....and a Subaru WRX etc. The 911 and WRX have to because of engine design but the race cars, don't know the class, use header mount turbo systems.

I will try to retain as much heat in the exhaust system thru coatings and wrap where I can. This will also help keep the temp under the car cooler.

that is where I'm having a hard time getting to the mechanism of how the turbo can convert heat energy, into rotation.

If in fact , his figure of 50% was true, would it then not stand to reason that a turbo would actually spin up somewhat, by just taking a blow-torch to the outside of the turbine housing, getting it all red hot and half transparent.


where I can see losses in such a system , indirectly as a result of heat-loss, would be the resultant "decompression" of the charge, as it now tends to take up less space, from being cooler, resulting in a lower thrust-pressure when it reaches the turbine. This pressure loss, can however be addressed by system design. for instance, having the plumbing towards the back, ever so slightly be reduces in volume, to compensate for this "shrinkage". This will also address the apparent decrease in charge velocity through a straight pipe of considerable length.

:)

The velocity of the exhaust gases are going to flow faster when its hotter. This is why you want the turbo closer to the exhaust manifold. As the gases flow downstream, it cools down and doesn't move as quickly. Hope that helps.

A Rear Mount Turbo is more like a garden hose with a sprayer on the end of it, a nozzle. This nozzle is aimed at the turbine wheel and will provide a restriction so as to speed up the exhaust flowing thru the turbine. The velocity will be dictated by this nozzle as will exhaust back pressure. This is also true of a front mount, of course, and both will have pressure before the turbine as rpm goes up.

The bottom line is it will either work or it won't. The first turbo I'm gonna try is an inexpensive eBay China-Charger which will provide me with data so as to choose a better spec'd turbo if need be.

an engine is an air pump. air in at 100*F has volume depending on displacement and volumetric efficiency . air in without combustion equals air out. same size. the only thing that happens is fuel is added and spark. With combustion,that volume of air is expanded with combustion to drive the pistons down the bore. when the exhaust valve opens, that leftover energy explodes into the exhaust manifold. This is where the largest free volume of exhaust is. the manifold becomes a plenum that gets pressurized to drive the turbo. once the exhaust gases leave the manifold , it is starting to shrink as it gives up it's heat. So ,in my opinion, the farther away the turbo is from the manifold, the less efficient it will be. the shrinking exhaust gases and the larger plenum created by rear mounting will cause lag . Some will claim no lag but i find it hard to believe.

Now that you mentioned the valves opening and that positive pulse being sent down the exhaust.... I can see yet another very significant benefit to not having the turbos in the manifold, that of Harmonic tuning.

Indeed a very nice tool with which even a normally aspirated application can be mildly charged, if done properly, and with the exhaust headers un-hindered, this can still be applied 100%... of coarse it is not just the exhaust side of things that will need to be tuned, but the inlet side as well...

But with each pulse kicking the turbo will help to drive it faster all the while building exhaust manifold pressure and increasing the amount of pulses as rpm climbs. It is the initial gas expansion velocity in the exhaust manifold that drives the turbine! I don't understand what tuning you are speaking of?

Harmonic tuning...

It basically works like this... if a positive pressure pulse is sent down a tube, the moment it reaches the end of the tube, a negative pulse of almost equal amplitude, get's sent back up the tube...at more or less the speed of sound. Heat, gas pressure, velocity, etc etc etc, plays a role in this speed, complicated, not going to go there... :)

now calculate how far the pulse will travel at more or less the speed of sound, in the time it will take the exhaust valve to close again. Devide the distance in 2 and you have the length of each exhaust runner for the engine to be tuned at that RPM.

don't worry about the positive pulses, they still carry on after going through the collector.


exactly the same for the inlet side, there it's just a negative suction pulse getting sent out from the valves, through the inlet runners, ITB's and when it gets to the mouth of the ram-tubes, a positive pulse goes back in, getting to the valves just as they are about to close... ramming that extra little bit of fuel-charge in past the valves before they seal shut....

then you can start to play with cones, like in the exhaust of a 2-stroke engine... Now you can stretch that rpm range out, giving you a much more manageable power-band, so to speak.

it is this harmonic tuning, that requires a NA exhaust manifold to be equal length, so that the pulses are evenly spaced at their tuned RPM range.