Yes yes, i know, another rear vs. conventional thread, but please, read first.

So me and a friend have been having this argument for a good 4 months or so now, and i keep giving him facts after facts about how you lose energy in the exhaust with having the turbo so far from the turbo. His argument is that the exhaust is colder thus making it more dense and spinning the turbine more and that the charged air cools off because of the distance that it has to travel. But he is extremely thick headed and will not beleive a thing unless he is shown actual real world numbers. So what i am asking is this: I am looking for any info where this has been tested on the exact same car, same tune, same turbo, nothing on the car changed, except the placement of turbo.

Thanks, Jon

 

How is dense air suppose to spin a turbine alone? You want the air to be as hot as possible so the gases are spread out and are able to get good velocity going to the turbo. On 1 part your friend is right, it cools down the air making it denser BUT you only want this to occur from the turbo on the way to the TB.

Rear mounts arent bad but they take more planning and, IMO, are easier to pull off. Most people tend to wrap all the exhaust going off the headers to the turbo to hold as much heat in as possible. If done well, a rear mount will produce similar results, just at different parts of the curve but its not a huge difference. Id rather not deal with having to use oil pumps to and from the turbo to keep it from drying out or pushing oil out from the seals.

 

hot air expands, meaning there is more of it and that spools the turbo not cold dense air, otherwise we would have a water cooled exhaust housings

why do you think rearmount guys loose spool with longtube headers and why most keep their stock cast exhaust manifolds and wrap the **** out of all the hot side piping.
to keep the exhaust gases cool ? i do not think so

also why a rear mount turbo car has a much smaller A/R exhaust housing then a front mount turbo car of the same caliber ? because there is less air to move the exhaust wheel. if you install one of those turbo's that comes with the sts kit up front it will choke the same motor at 3500-4000rpm if not even lower due to the small a/r exhaust housing, but that same turbo will work just fine in the back, so you can see that efficiency will be lost.

i used to have a t63 precision turbo(up front) with a .63 a/r exhaust housing and that used to choke my motor just under 4000rpm, but spooled at a little over 1500rpm and this was on a 8.6:1 cr forged 347, even with that low CR it spooled that early. the turbo that comes in the sts kit is pretty similar to the one i used to have and on their website it makes power all the way up on the same cube motor with much higher CR, so you can see how much energy is wasted

 

 

Sorry to say but you will never see this comparison. For the simple fact that it is time consuming, difficult to get all the variables the same and their are different ways of doing both. A rearmount isn't going to be setup like a front mount because you have to do different things to get them to work so that in itself is a huge variable.

 

First off I would like to say thanks to all the great responses. Alot of great information being given. I understand the expansion of gasses and how you want to keep all the heat in as possible. I just wish someone had some actual numbers to compare. I myself do not like the rear mount setups for many of the same reason posted previously. I never did think it was a good idea, especially because of the loss of thermo-efficiency. The argument we were having is weather the exact same turbo will work as well or better in the rear as in the front, given that you changed stuff to make it all work like oil supply and ect. I have told my friend alot of stuff on thermo-efficiency but he is still "brainwashed" as i say by a instructor he had in school who is a sts sack rider.

If he has any questions when i show him this thread, i will post them so he can get some actual info from people who have alot of experience with this, like yourselves.

Thanks, Jon

 

The same turbo will generally not give the same results on a front mount as the rear. The if you take a turbo thats .63 A/R and throw it on as a front mount, it will spool too fast. Just like if you take a .96 and put it in the rear, it will take forever to spool up. I dont think its a matter of which will make more power, since 6 psi at the engine is 6 psi no matter where its coming from, its a matter of when it will be spooled.

 

much like a front mount, r/m, supercharger or nitrous.. , you have to built, modify and tune tune tune to make each system work to it's full potential. but i'm guessing the question was aimed at those who probably just slap a rear mount kit on and thinks they will break records with it..lol

 

i think people read a little too much into these things. yes front mounted turbos will be more efficent. but you are not working with a 2.0ltr 4 pot engine here! also lots of people are making big power with rearmounts these days. i think LMR has made over 1100rwhp out of one of their cars (could be wrong)!

i say go with whatever is the best solution for the car you have. if that menas a rearmount then so what? make it the best you can and be done with it. there are always bigger and better options out there but you have to do what is wright for you, not what someone says on the internet! lol

Just my opinion.

Chris.

 

I've gone back and forth on this changing my opinion and I think I have finally settled. When you look at some of the best kits from LMR, TTI, Ky Turbo and others, both can make huge power, LMR has proven a rearmount can run as fast as you want to run.

I would chose a front mount if you don't mind removing A/C most likely standing the radiator up, etc. Rear mount if you want to keep the engine compartment intact.

 

"His argument is that the exhaust is colder thus making it more dense and spinning the turbine more and that the charged air cools off because of the distance that it has to travel"

If I remember correctly from my thermodynamics class. For turbines the efficiency is based on the temperature difference between before and after the turbine. So efficiency is based on the heat difference between hot exhaust going in and the cool atmosphere on the opposite side. This temp difference makes it spool.

Your friend is right that the colder exhaust is more dense. However the cooler exhaust is closer in temperature to the atmospheric temp (so there is less of a temperature difference between the hot and cold side). Therefore the efficiency is less. Cooler exhaust will spin the turbine less.

You would not want to put a turbo that was sized for front mount onto a rear mount setup or vice versa because they won't be sized properly

 

These arguments are like a lot of "Engineer" arguments. There is theory and there is reality. In the real world the sts turbos work pretty damn good, they tend to spool up a bit later depending on what turbo is used,etc than a similar sized setup up front mount. But they can still make huge power in singe or twin form and can run very quick and have run very quick both et and mph. Just look on this board and you can see that.

Pros.sts is pretty easy install, leaves the engine bay nice and stock looking. Is great if you might be selling the car later on don't hack and slash anything for it.
Cons ,the older kits had more issues, ground clearance, scavenger pump and pcv issues think as well The newer kits seem to have addressed most mabye all of these issues.I have personal experience meaning have either had rides in or driven two sts lt1 cars and one sts ls1 car. They were fast, fun,sounded great.
I went front mount in my build as wanted twin and no rear mount twin kits were out for my car.
My install was long ,not overly hard but still had to mod quite a bit on the car here and there. It kept air and you didn't really have to move alt or battery to back or any of that stuff. I am pretty happy with my front mount kit. I can still get to my spark plugs and engine access is pretty good. No real ground clearance issues with my kit.

Anyway off track. Theory is just that theory. Rear mounts work, front mounts work. All good.

 

No engineer says "it won't work", just that "it won't work as well." I see turbo and cam selection as a myriad of trade-offs. Putting the turbo that far away isn't a trade-off because you don't actually gain anywhere.

The turbine side makes however much power is required to spin the compressor to the desired boost level. Turbine power is a function of mass flow rate and the change in enthalpy of the gas. Enthalpy is a function of temperature only. The lower the inlet temperature, the greater the inlet pressure has to be to force a greater mass flow rate through. More turbine inlet pressure means more exhaust backpressure the engine has to work against.

But. . . you didn't ask for theory - you asked for test results. Several years ago, forcedinductions did a back-to-back test of a rear vs front mount. The front mount made 20 - 50 more rwhp, if I remember right. Also, I might add, that you rarely find a rear-mount car running over 140 mph in the quarter, whereas front-mount 140+ cars are everywhere.

Mike

 

Here is a canidat for you. last year I built a rear mount kit for my 02 WS6 with a 408. I installed a GT-88 on it. That turbo is what I need to get me to my HP goals. I am looking for 900rwhp. I have been running it now for almost a year. During this time I have down sized the turbine housing, swapped to different intake manifolds, hunted several times for leaks on the hot and cold side, I sprung and resprung my waistgate, I have wrapped, wrapped, and rewrapped my exhaust. I have played with the tune, I have swapped the cam to a turbo cam, I have redone my piping a few times, I had a 3400 stall, had it restalled to 4400, I swapped the 4L60 to a built 4L80 with a transbrake, I have installed a 2 step, all in an attempt to reduce the lag time the GT-88 has on the back. NOTHING I have done has helped the situation. I am now going to pull this stuff off and build a truck manifold front mount kit for the GT-88. I have no dought with it up front it will have little lag, and will spool up on the line with the Tbrake and 2step. The GT-88 will spool up back there, just not until I hit 5500rpm. Once it comes online, HOLY **** BATMAN! You would not believe the power! But the lag time is way, way to much.

I think a rearmount has its place, with the right (smaller) turbo, but if you go to a bigger turbo like I did and expect to make it spool fast, you better be ready to have another power added like nitrous on it to make it spool up fast. Rear mounts work good, to a point, then if you want more HP out of it, you will find a rear mount will not spool the larger turbos very fast, and you will end up with either two power adders, or moving everything up front.

I will try to remember to post up my results once get the GT-88 up front latter this year.

My .02$

 

Don't need to look any further than LMR to find rear mounts going 140+. I few years ago I would have agreed with you 100%, today the rearmounts are working very well, the engine compartments can be a compromise on todays large turbo Fbody cars. I'm not disagreeing with you just stating that todays rearmounts are a much better option than just a few years ago. Also todays systems both front and rear mount can make more power than most people are looking for.

Also the rear mount is getting alot of engineering attention to address it's defiencies, such as spool, now the spool valve is starting to get attention and seeing real results. There are alot of very smart people who like to be challenged that are tinkering with the rearmount. I remember up until 2002 a 500rwhp F body was a rarity. So again I agreee with you but I love watching people advance the rearmount feasibility.

 

If you want to see allot of data on this, search for my posts and find my 3-4 threads with millions of pages in them detailing everything we tried to make this rear mount stuff work.

 

why not try the quick spool like Zombie?

 

^has to be the funniest smiley on the site

 

Yeah for the average guy 140 mph and low 10 or 9 second 1/4 is plenty fast. For the ulitmate race guys well front mount may still be a bit better way to go. As for lag ,cmon you put a big single turbo on something even a v8 and its going to have some lag.
You can help it out off the line with two step,antilag ,nitrous but even just driving around the powerband will be pretty narrow. I have same deal on my 97 talon. Big turbo similar to gt35R . Its manual car.I use two step to build some launch boost,antilag gives me a lot more if want to use it. Could spray it have the intake set up for it but haven't yet. I can gear down when in higher gears say in third in city to second and have pretty much instant spool. Just going up thr the gears driving normally you notice its a pretty laggy thing.

Actually thats one reason some talon guys are getting almost as good a times as the big turbo guys on much smaller turbos. They spool up so much faster and while they don't make as much absolute power the much broader powerband seems to make up for it.
I am still going big one on my race talon and leaving for now my laggy big one on my 97 talon. My 99TT has very quick spool up on the standard aps turbos and will only go up a bit on the LG ones according to LG and other members running them.

For the average street guy with rearmount would stay smaller and have the right ar ratios. Not sure how much gears affect the spool up might help more on the rear mount. some guys really seem to like 3.08 type of ratio. I hate weak gears off boost personally but whatever floats your boat.

Some people used to really argue the sts couldn't /wouldn't work. They quoted theory till the cows came home. But after a few personal rides /drives in sts cars I knew they were wrong.

 

Quick spools work great, if the turbo you pick has the right available turbine housing. Quick spools do not work with turbine housings that are not divided, like mine. They do not make that kind of housing for the GT-88's.

I think once you get to the 88's and up, the selection of turbos that has the correct type of housing's offered that will work with a quick spool becaome less and less. The turbos smaller then 88's thereare allot more turbine housing options.

Quick spools are great, as long as the turbo you want has the right kind of turbine housing that will work with it.