Well... I think that some good porting with a grinder on the stockers would increase flow while retaining heat and not adding additional piping area to the cooling equation.

 

Want proof...read Post #1 and about Jon's car. What the engineers said makes total sense, at least to me. Then again, I'll be at FAU for ME come September.

 

I have a set of custom rear twins....I have Macs...mids... They are ceramic coated... I run 2.5" strait pipes over the axel directly into the turbos...there is no X or H pipe.
The strait portions of the pipe are mild steel.. I ran them "unwrapped" and then "wrapped" and there was very minimal gain in spool time...like 100-150 rpm. It all helps though I guess.
The next begging question..is what would happen if I took those same 2.5" pipes and made them 2.25 or 2" .. Thats another aspect to this whole story...Velocity. Hmmmm

 

Thats the next thing I'm wondering.

 

one car is not proof, they must have a leak or something for no boost, read my sig i make 5.5-6 psi on LT and 3 inch exhaust all the way back. my car makes good boost and it doesnt take forever to come in, all im saying is everyone is saying an engineer said this and that, well my car must be a freak of nature then, because it goes against all engineering talk???? im curious to see someone take a stock exhaust STS set up car, install LT and exhaust and get some track times to compare. and what does FAU for ME come september mean???

 

Jammer... good poop! I love this thread... That is funny that we are discussing the same challanges that a turbo housing itself faces. We just magnify that arguement when its rear mounted.

1:What is the most efficent way to maintain heat (effecting volume and airspeed)? Wh discuss the total surface area of manifolds vs. headers (headers having much more) and ceramic coated vs wrapped vs unprotected...

2: A major consideration with a turbo is the AR size.... what we have not discussed is that the entire exhaust track on a rear mount has similar effects... 2"-2 1/2"-3" ehaust piping size. Hell... I am running the stock Y pipe and it is on my list to replace as I think its restricting... it may be an advantage by increasing the exhaust velocity by size restriction....

No wouldnt the exhaust... 2"-3" have the same velocity regardless based upon the AR on the turbo flow limitations?

 

This has really got me to thinking and looking at my system. I am pretty much a FI noob, but I am no stranger to science. I like the STS system for what it is and only want to make it as good as it can possibly be and perhaps max it out one day with a low comp motor. That being said, here are some of my thoughts:

A rear mounted turbo can be made to spool as quick as a front mounted turbo of the same size or it can make the same peak power, but not both. You have to reduce A/R to get spool time down on a rear mounted system, but it chokes off sooner, hence lower power. Or you can go the other way, like I did and go for more power and get a little more lag.

If you want to talk energy equations, you have to do it right. Exhaust contains two types of energy; kinetic and thermal. What makes it hard is that the two are related and are generally governed by the equation pressure*volume = mass * gas constant * Temp (PV=mRT). If you re-arange the equation you get mR=PV/T. Since mass and R are constants, as temperature goes down you have to reduce either pressure, volume or both (in this case both).

So, as the gas comes out of the cylinder it has its max temperature and velocity. It loses a slight amount of speed due to the exhaust gas becoming more dense (taking up less space due to cooling) and frictional type effects, but these are probably small enough to be negligible. As it makes its way to the rear of the vehicle it loses heat which causes a loss in temperature (touch exhaust and see if you want to see heat transfer work). As the gas loses heat it lowers its temp along with its pressure and volume to conserve mass. It is important to note that the massflow is essentially constant in this closed system. What drives a turbine to accelerate is pressure and effective area upon which it acts. The gas at the rear of the car has less pressure and therefore causes the turbine to spin up slower. Thus the necessity for smaller A/R to make up the difference. If you go with smaller exhaust pipe, the exhaust flow will accelerate faster and spool will be quicker, but the the system will choke off at higher flow rates, producing less peak power.

Once you have spin-up accomplished, power to run the compressor side is essentially from the inertia of the gas flowing through the turbine and power levels will approach those of a front mount.

Other critical things that affect spool and peak power are exhaust leaks, pipe bends and discontinuities in the flow cross section. For instance, I have an old exhaust cutout that is capped off in the I-pipe, which can't be helping my situation. I also have several 3 bolt flanges that have small leaks that I need to get fixed. I will address all these things and try to measure the difference.

BTW I am a rocket scientist by trade and no I didn't stay at a Holiday Inn last night.

 

joe... fricking outstanding. I am only a Technical systems engineer and planner so the thermodynamics portion of this eludes me some. I think however you explained it in a fashion that even I can understand.

I believe that you have concurred with the statement that I made that restricting the size of the exhaust piping in effect is the same as reducing the size of the AR on the turbo housing?

If that assumption is correct then a relatively simple methodology to test the effect and balance of Air velocity vs: Volume (I think I have that right) would be to take a 12" portion of the exhaust piping close to the turbo, cut it, flange it, and create a variety of reducers that change in.... oh say 1/4 or 1/2 inch measurements. The mandrel type reduction would minimize airflow disturbance while still causing the desired backpressure resulting in velocity increase… am I off or on here?

The results could then be measured on a dyno with a base tune and large AR turbo (as the AR would be controlled more by the pipe reduction then anything).

What do you think?

 

joe question for you, as the exhaust gases cool going down the exhaust the gases become more dense, in theory wouldnt this make more efficient turbo spool due to the fact the turbine blade would have more dense gases pushes against the blades?? it makes sense to me , you may loose some velocity due to the cooling gases but it should be more dense, thats how it works on aircraft turbines??? i think the velocity on a front mount turbo is better just because its closer to eng exit, where the rear mount has more piping to get exhaust gases up to speed???? could a venturi be installed prior to the turbo to help reaccelerate the gases prior to entering the turbo???

 

smoke... I wont answer for joe, but IMHO... the volume (or mass dont remember which) increases with temp. The hot air actually has more volume to spin the turbine on the turbo. I am not completely familiar with the thermodynamics of a jet engine, however I know that the maximum expansion of the air is upon combustion... the more relative distance you go away from the point of combustion the slower and cooler the air goes.

The best think I can offer you is that the velocity of gases expanding at the source are parallel to a chamber of a gun... at ground zero of the combustion the rapid expansion of the gas can propel a bullet to high velocity..... however if it worked in the manor that you describe the bullet would actually be more optimally placed at the furthest place possible away from the combustion.

Take a drag tire for example... measure the air pressure when cold.... then go and smoke them up and get them hot.... you will see an increase in pressure on the tire. Gas expanding = more relative volume

Hope this clears things up some.... and I hope I am not too far off.

 

I wonder if coating the exhaust pipes with this stuff would make a difference:

http://www.swaintech.com/store.asp?pid=10969

Or would wrapping the pipes with header wrap be just as good.

 

longrange , thanks, i understand a bullet being pushed out a barrel and a tire is air that is trapped, on a turbo you want to turn the tubine, like an airplane propeller, hot air is less dense than cool air, thats why your car makes more power on cool days than hot, a jet eng turbine is just like a big turbo wheel exhaust gases push against the fins spinning the turbine. im not an engineer im just trying to understand the dynamics of this thread, just adding another point of view. thanks for the response it does make sense. i think i will wrap my exhaust from the LT back and see if it helps spool better, cant hurt anything.

 

Again...what are your headers/ypipe coated with? You can have headers/ypipe that are coated and it would retain heat within the exhaust gas flowing through them which inturn help keep the turbo hot enough to boost.
Since you can't answer this, I'm going to assume that is the case. The other 2 cars, mine and Jon's have coated headers, but not ypipe. So there is a lot of heat loss from our ypipes, which in turn the exhaust gas loses heat also. Hell, not even 5 minutes after driving my car around, the ypipe is cool enough to touch/change out widebands/etc.

FAU = FL Atlantic University
ME = Mechanical engineering.

 

Joe, basically you said the same thing we have been saying just in simpler forms. *English voice*Bloody good show ol' chap! */English voice*

 

if you read my sig you will see it says JET_HOT. of course they are coated who wouldnt have coated headers and ypipe? the beginning of this thread talked about LT and stock manifolds thats what my contention has been about!!! hell just a few responses back me and longrange were discussing density of exhaust as it travels, down exhaust , i understand what your saying, i didnt realize you were running a complete uncoated exhaust. im not trying to undermine this thread just trying to understand why one car acts one way and another acts different, i want the same things you do , max power out of my setup. thanks for the info and i look forward to keeping this thread going until we all can have what we are looking for in our STS systems.

 

hey less of the taking the **** out the english! only us english are allowed to do that!

only f*cking old farts over here say that!

Chris

 

I think everyone here has a pretty good grasp of what is going on with our systems, there are just a couple of things I wanted to clear up.

-Cars run better in the colder weather because the air is more dense and you get more air and fuel into the cylinder for each power stroke.

-Mass never increases or decreases in this type of system (as long as there are no leaks in the exhaust or turbo). The amount of air leaving the cylinder head is basically the exact same amount that exits the tail pipe in whatever units you want to use (i.e. lbs/min, Kg/sec, etc.). There is very slight deviation from this during hard acceleration, but it would be very hard to detect. What is noticed is a drop in temperature and pressure. This is what slows the spool.

-The air can be speeded up by using smaller piping, but this comes at a cost of peak power. Just narrowing a small section of the tubing will speed it up in that section, but it will return to its previous velocity upon exiting the restricted section minus frictional losses. Again this will sacrifice peak power.

This is going to be a learning process for all of us, but if we keep sharing the info, we and all of those that come after will benefit.

 

Chris, the English people I met in Israel and along my travels just crack me up, ya'll are cool in my book.

 

Ahhh ok got it... understood the cooler air portion, your right about the smaller portion of pipe, I didnt give that enough thought. Good thoughts.... hmmmm

 

Thanks mate!

yeah the english are pretty 'normal' comperad to the scottish and irish! lol

and dont even menssion the welsh! LOL

back to topic! (sorry) there is a really nice thread oncustome rear mounts onthe corvette froum i will try and find when i get home. one very good guy talks about the relationship between AR's, spool up and choke! really good post.

thanks Chris.

PS. thats not a dig at any of the above countries, i love them all!