I've seen these rear mounted turbo systems (at back bumper) for cars and wondered if anyone had seen them on street trucks? They seem poorly located for off-roading but for a street truck seems fesible.

I figured there would be mad turbo lag but testing (on tv's hotrod I think) seemed to not have much lag at all. Also the long tubing running back to the engine compartment negated the need for intercooling as the ambient air cools the pipes.

Anyone done or thought about a home-made system like this? Maybe a couple Thunderchicken turbos. Wonder if it would effect the O2 sensors.

Just curious

 

There's no lag if you keep it wide open throttle all the time.

 

The higher the velocity and temperature of the exhaust, the more energy it contains to rotate the turbo.

Since exhaust does come out the tailpipe and is at least warm if not hot, there is energy coming out the tailpipe that could spin a turbo. This is why those rear-mounted turbo kits work.

However, there are huge energy losses (heat and velocity) thanks to the exhaust system absorbing heat from the exhaust (the exhaust system gets hot to touch, right?) and of course the act of muffling is done through splitting or redirecting airflow in the muffler's body.

So a more optimal solution is to put the turbo as close to the exhaust ports as possible, to maximize the potential heat and exhaust velocity availability to spin the turbo. This is more efficient. Also, by having the turbo that close to the engine, allows the intake side of things to be very short also. This cuts down on perceived turbo lag because the more volume something has the longer it takes to "fill up".

If you were to take a 5 gallon contractor style air compressor (the pancake style), and turn it on, it will take a minute to fill the tank to full pressure. If you were to attach a 60 gallon tank, it will now run for 10 minutes to fill both tanks to full pressure.

Having the exhaust, and intake tubes running down the length of the vehicle does the same thing. ONe can side the turbo so that these effects are lessoned, but it's still not an optimal situation.

But it does work. I know two people who have installed kits like these on camaros, and they're very pleased with them. I could feel the kick of the rear-mounted turbos when I drove their cars. I just know that if they put the turbos underhood, there would be even more kick and they could have used a larger turbo. But niether guy wanted the underhood temps nor did they want to get into tuning which is pretty much required when you're achieving more than 5psi of boost pressure in the intake manifold.

I guess I'm saying I don't find 5psi worth the effort.

 

when you say side the turbo, do you mean smaller exhuast side or larger compressor side?...or neither?

 

Aaah, a typo! I meant "size the turbo".

Please excuse my clumsy fingers.

 

lol, ok....but did you mean the same thing as I was saying? Makes sense to me that a smaller turbine on the exhuast side and/or larger compressor would be needed to achieve similar results as an under-hood turbo.

 

You can increase the size of the intake compressor a little as compared to the exhaust side, but only to a point. If the mismatch is too large, then the smaller exhaust turbine cannot provide enough power to spin the compressor side enough to generate consistant useful boost. Aside from not providing enough boost, it also creates a situation whereas the smaller exhaust turbine acts as a cork, as the intake side will resist rotation so much that you won't get an optimal 80,000-100,000 RPM range on the crossshaft.

Nothing is free, there are always tradeoffs. THis applies regardless to where the turbo is mounted.

In the case of the rear mounted turbo...

In order for the turbo to spin, there needs to be a minimum amount of energy pointed at it. If you were to hold a turbo to your lips, and blow with all your might, it's unlikely that you'll get it to spin. Aim a small nozzle attached to an air compressor and it should spin fairly quickly. Probably no higher than 2000 rpm though.

Turbos, when sized correctly, will spin at 80,000-100,000 rpm. Some turbos are designed to spin faster, and they are more expensive.

With a rear mounted turbo, that minimum pressure has to be established. The longer the exhaust system, the more volume it can hold before that pressure is realized, and the turbo starts spinning at a reasonable speed. So that is where the lag comes in on the exhaust side.

Once the turbo is spinning, the compressor side then has to pressurize the intake tract which if it's the length of the vehicle, that also is a large volume and takes a moment.

.5 second lag on the exhaust side, and .5 second lag on the intake side, adds up to a second between when you romp the throttle and the turbo does something other than make noise.

I'm deliberately exhaggerating to illustrate the point.

Also keep in mind that power is not created, otherwise we'd have perpetual motion engines and never have to refuel them.

Since that does not exist, whatever power is necessary to produce high velocity compressed air on the intake side, is required on the exhaust side to turn the crossshaft.

In fact, turbos are typically 50-60% efficient, nowhere near 100%.

So a mismatch in intake/exhaust sizing is very noticable, actually. Like I said, a small amount is okay, but typically within say, 15% or so is usually a good rule of thumb.

Remember, you do have to tune this, after it's assembled

 

Solution to having decent perfomance and cooler under hood temps...long tubes and turbos mounted on the header flange, provided theres enough clearance.
I dont think I would want to mount my turbo(s) under a street vehicle anyways, seems like they would get beat on pretty hard with debris, etc...

 

The solution is to coat the inside and outside of the headers, keep the turbos coolish by making sure there is airflow through the engine compartment.

My 75 twin turbo dodge pickup didn't get that hot under the hood, but at the back of the hood by the cowling I had installed 90 zillion little vents.

 

well that would be the best solution yeah lol...or get two little hoodscoops like the ones you always see on trucks, and actually cut a hole in the hood so air flows down onto the turbo.

 

Well, this time around based on how things seem to measure out, I'm going to have to bubble my F350's hood to clear the turbos, OR have them peek through.

 

Doesn't matter much, as of the last time I checked, the company doesn't offer one for Fords.

-Kerry

 

just cuz noone makes the kit for you, doesnt mean it cant be done....not that I would.

 

They are relatively simple to install becuase you don't have the underhood packaging problems. I've seen one installed on an F-150 with STS's universal kit.

I've also seem many Chevies make good power with them. If they offered a smog-legal kit I'd have considered one over a blower.

 

Yeah, that's the end of the world now isn't it. No kit so it's impossible to do!



The above is a picture of the mustang kit. What do you see? I see a turbo, wastegate, tubing, clamps, oil line hoses, fittings, bolts, wire ties, and stainless intake tubes that fit together.

I guess STS are the only vender who sells such parts and they aren't available anywhere else in the world, so if a kit isn't available you simply can't do it, ever.

My obnoxious sarcasm aside... this isn't difficult to figure out. You buy a kit that's sized properly for the engine in your vehicle, and use their parts to bring the intake tract as far up to the engine as possible, and fangle the last few feet and it's done and still, almost a bolt-on.

We're talking thin wall stainless tubing here - easy to buy, easy to weld, easy to bend.