dj4monie

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.

#Rotor

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...

MetricMuscle

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.

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.

#Rotor

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.

Robb235

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.

MetricMuscle

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.

Slitherbeast

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.

#Rotor

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...

Slitherbeast

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?

#Rotor

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.