Boost vs. back pressure

postal

Peoples champ runner up
Joined
May 29, 2001
I read a book on turbocharging several years ago. I read that book front to back multiple times. I believe the aurthurs name was Hugh McInnes. He was an engineer who had worked with turbochargers for many years. In his book he stated that a properlly designed turbo system will have 2 to 3 times as much pressure in the exhaust manifolds as it will have boost. Does any body have any experience measuring the back pressure on thier motors or have read up on this topic?

Thanks: Jason
 
I did a little researching on the net. It seams that if a turbo and the a/r ratio of the turbine housing are sized correctly to your motor for the boost level you're running at then the back pressure will be lower than the boost. Once you raise the boost to the point that your turbo and/or your turbine housing aren't properly sized the back pressure will become higher than your boost.

Hmmmm...

Maybe it isn't big cams that dont work with turbo motors. Maybe its that big cams don't work with the little .63 a/r 3 bolt housings that most GN guys run.

BTW: The reason I'm looking into this stuff is because I'm having a custom cam ground for my stage motor, and I'm picking ALL the specs on it. I have to decide on:

Intake and exhaust duration
Intake lobe centerline
lobe centers (lobe separation angle)
valve lift
valve overlap

TIA: Jason
 
No way. Exaust back pressure is always higher than boost level. If they are the same, then the turbo is 100% efficient (impossible). I also read turbochargers by Hugh MacInnes. Great book. I needed to read it over and over for one year to digest all the info, then I spilled coffee all over it and ruined it. Most turbos fall into the 50%-70% efficiency range. At 50% there will be 2x more pressure in the exaust. Re-read the section about matching turbos to various applications. There are two forms of pressure to measure. One is absolute, the other is static. (maybe I'm wrong on the names, my book is ruined) A cars tire will read 32psi, but no air is flowing. Static and absolute pressure are the same in that case. In a moving fluid, like exaust, you can measure pressure with a pipe facing the flow of fluid (air) and the velocity energy will raise the pressure reading, even though the static pressure is lower. If you face a pipe at a 90degree angle to the flow, you will get a lower number. I forgot all the details, but I know that these two different readings are crucial for measuring what is really going on.
But both numbers of pressure will be higher than charge air pressure.
 
I have seen this one posted over and over, I don't believe it. "If you put a banana in your tail pipe, you can build as much boost as you want." Sure you will build a lot of exaust pressure, but without velocity, and volume of gas to spin the turbine, there will be no boost! Just a lot of exaust pressure, like the tire pressure example. If I put a turbo in a pressurized chamber at 100psi, there will be no spinning of the turbine. Turbos need velocity/volume of fluid to spin the turbine, not just pressure. ;)
 
While turbine backpressure can be reduced by proper selection of wheel sizes, turbine housing design, etc. There can never be less turbine backpressure than boost created. Or at least not that I've ever heard of.

The usual rule of thumb is that the turbine backpressure (that in the pre turbo exhaust manifolds) will, for the most part be about double what the boost is, in a fairly good design. Sometimes a little less, many times, much more (as much as 3 times more).

But when my Son-In-Law gets back off his road trip this weekend, I'll ask for more specifics. He, after all, was a design engineer at Garrett for several years.
 
Originally posted by postal
Does any body have any experience measuring the back pressure on thier motors or have read up on this topic?

Thanks: Jason

Although I dont have a Buick (I have a 2.2L 16V Dodge) I am using what is basically the equivalent of the PT/GT61 on my car.

Making backpressure measurements is quite easy. Assuiming you already have an EGT port in your manifold, all it takes is a cheap pressure gauge (0-100 psi range is about right), some soft aluminum/copper tubing, some vacuum hose, and a compression fitting.

Basically, all I did was removed the EGT probe (which uses a 1/8" NPT thread) and inserted the compression fitting which has 1/8" NPT male threads on one side and a compression type fitting on the other. The compression fitting side goes to the tubing. Thhen, after a few feet of the tubing, I connected some standard rubber vacuum line to the tubing and ran that the rest of the way to the pressure gauge in the car. (Basically, you just need a few feet of the aluminum/copper tubing to handle the temperature close to the manifold).

Now, take your car out and flog it and compare the boost and backpressure gauges. This simple test will tell HUGE amounts of info.

I have used this test to determine when the turbo (especially the turbine side, but the compressor will play into it as well....thats another topic though) is too small. I have been shooting for something less than a 2:1 ratio on my car. The lower the ratio the better of course (i/e/ the more power youll make), but as with anything there are tradeoffs. If one sizes the turbo such that the ratio is very very small, the turbo may never even reach the desired boost level. The 2:1 ratio seems to be a good compromise for a street car to give reasonable spool and great top end power.

As for actual measurements, on my car at 25 psi boost, I measured 45 psi in the exhaust manifold with the PT/GT61 turbo. Thats a 1.8:1 ratio and is in my opinion pretty stinking sweet for a street car.

Its taken me a few turbos to get to this point though. Some past results were as follows (Which I also posted in the PT61 thread---

T3 55 trim compressor with 0.42 a/r housing paired to standard T3 turbine in 0.48 a/r housing

13 psi boost = 43 psi backpressure

T04E 50 trim compressor with 0.50 a/r housing paired to Stage 3 T3 turbine in 0.63 a/r housing

20 psi boost = 42 psi backpressure

Basically......power has shot through the roof with decreases in exhaust backpressure.
 
Over the years a few people with different combinations have posted back pressure measurements on TR's and other cars, and I guess my opinion is that on any kind of drag car or street car the backpressure will be anywhere from maybe as low as 1.25 or so times the intake pressure or higher. Any less and you just can't spool the turbo before the race is over :). In a more steady-state situation you can get the back pressure down to the boost pressure or below (remmeber, it's the heat energy in the exhaust much more than the pressure that is driving the turbine) - I think someone posted that back when Formula 1 ran turbos (or maybe it was on an Indy car?) that they were able to do this. I've always wondered what the ratio is on big diesel trucks where the turbo sits in boost for hours and hours at a time.

As for numbers that I can remember, in the gnttype archives someone measured about 45 psi back pressure on a stock turbo at about 20 psi boost, and Todd King measured in the mid 30's at 20-25 psi boost with a 72 turbo and GN1 heads.
 
Usually exhaust back pressure is greater then boost pressure. On some oems, it can be over 2x as much, I've never read of it approaching 3x. From what I've read the Mopar turbo 4s were the record holders at 2.5x.

There is a point called *Crossover* where boost pressure can be higher then exhaust back pressure. For the sizing of turbo to do this, means usually having a very high RPM operating range. Some of the *ricer* car mags have had several interesting articles on it. Your 1,200 HP Supra's are all typically members of the having reached crossover club.

Frank Parker some years ago posted some interesting data off of his Syclone at DIY-EFI/GMECM some years ago about stock vs something similiar to a 60 turbo. Been a few years ago, so I'm rusty on the details, but there was a shocking difference.
 
I don't have any backpressure data but a experence I can share. When I got my GN it had the stock turbo that was in bad shape, and a crappy cat back exhaust that was rusted out.I put a combo of mods. (TA-49,bluetops,etc. with a 3"DP,3"highflowcat.,& Hooker Catback exhaust). I got everything installed, set TPS,FP, etc. mixed up some xylene 100 oct. took it out to my "testing grounds" gave it a rolling start blast & it was "yeehaa" till about 15/16 psi then it really fell flat & struggled. I went back to the shop gutted the cat back on the hwy. & WOW! 20 /21 psi & pulling like a mutha! That was with no change other than to gut a brand new 3" highflow cat!
 
Carl, the only time boost pressure is going to be higher than exaust, is when you take your foot off the gas, and the inertia of the spinning wheels will make high boost pressure while nothing is comming out of the exaust. Other than that, what you are describing is close to a perpetual motion machine. impossible. There is no such thing as a turbo with an efficiency range near 100%, much less over 100%! Heat does not make a turbo spin. Heat just keeps the exaust gas from shrinking and loosing velocity, and volume. Heat is good. An example of this is like putting a turbo in a hot oven....is it spinning? No.


When you guys are measuring exaust back pressure, are use measuring absolute or static pressure? Very different, as described in the book mentioned "Turbochargers", in the section for matching turbos.

There will always be less volume of fluid leaving a combustion chamber than entering it. Thats why intake valves are bigger than exaust.
 
Originally posted by psikuout
I don't have any backpressure data but a experence I can share. When I got my GN it had the stock turbo that was in bad shape, and a crappy cat back exhaust that was rusted out.I put a combo of mods. (TA-49,bluetops,etc. with a 3"DP,3"highflowcat.,& Hooker Catback exhaust). I got everything installed, set TPS,FP, etc. mixed up some xylene 100 oct. took it out to my "testing grounds" gave it a rolling start blast & it was "yeehaa" till about 15/16 psi then it really fell flat & struggled. I went back to the shop gutted the cat back on the hwy. & WOW! 20 /21 psi & pulling like a mutha! That was with no change other than to gut a brand new 3" highflow cat!


What does that have to do with anything? Nice little story though.

Well I was waiting for Bruce to chime in. I may be wrong about the boost vs./back pressure, but I just can't see how that "crossover " could possibly happen.
 
Originally posted by turbo2nr

When you guys are measuring exaust back pressure, are use measuring absolute or static pressure? Very different, as described in the book mentioned "Turbochargers", in the section for matching turbos.

Static pressure.

Boost pressure measurements are also static pressure.
 
Originally posted by bruce
From what I've read the Mopar turbo 4s were the record holders at 2.5x.

Unfortunately, I never measured mine purely stock, but with the exhaust opened up (3" with no cat and Ultraflow muffler) and running 13 psi boost, it was 3.3:1!

With the factory cat in place, it would have been an even worse ratio.
 
Carl; It was the f1 cars that you was referring to. During the height of "The turbo era" the F1 cars were producing around 900-1300 H.P @ 60-80 pounds of boost !!!!!! Which is truly " AMAZING" for a 90 C.I.D motor with 80's technology :eek: :eek: And from my understanding the backpressure was very very low.
 
Originally posted by mec108
Carl; It was the f1 cars that you was referring to. During the height of "The turbo era" the F1 cars were producing around 900-1300 H.P @ 60-80 pounds of boost !!!!!! Which is truly " AMAZING" for a 90 C.I.D motor with 80's technology :eek: :eek: And from my understanding the backpressure was very very low.

I read an article once that stated that they were able to achieve significant crossover. I assume thats a big reason why they were able to get such monster power outputs!

The CART cars are also pretty cool. I understand they make 900 hp out of 2.65L and are only allowed about 2 psi boost!

(The rulebook states "The allowable intake manifold pressure shall be limited to a maximum of 34 inches of mercury absolute for the 2002 season.")

1 psi is about 2.036 inches of mercury, so 34 inches of mercury is only about 16.7 psi absolute. So, for a typical 14.7 psi ambient pressure, theyve only got about 2 psi of "boost" available.
 
Originally posted by mec108
HEY Larry; Is this the last season for the turbo cart cars? Mike:cool:

I am not sure as I havent followed the series all that closely. In 2003, we had an event down here (St. Petersburg, FL) and it was alot of fun to attend. Seeing and hearing those cars close up is pretty stinking wild.

But, somehow things went awry and St. Petersburg no longer has an spot on the schedule, so I havent paid much attention since :(
 
The pressures I quoted were psig, not psia - pounds per square inch gauge, not absolute.
 
I would be very interested to hear what some of the serious race V6 motors are running. My car is a 2.3 Ford (now 150 cu.in.) currently with a T72/.81 Q trim. With this combo at 27 psi boost measured at the throttle body I get a true .85 to 1 TIP ratio (23 psi at the turbine inlet) at max rpm.

With a 1.00 turbine housing it was even lower - .8 to 1, but made it a little slow to spool for the pro tree races I run.

This is mainly race with some street use but even so I can lightly powerbrake it into boost as early as 2-2500 rpm with full boost available by about 4800 or so.

Now this might be a little Supra-esque to you guys but seeing as my motor only runs from 6000 to 7800 going downtrack it works pretty well for me.

Do the race Buicks see similar numbers?
 
Originally posted by turbo2nr
Carl, the only time boost pressure is going to be higher than exaust, is when you take your foot off the gas, and the inertia of the spinning wheels will make high boost pressure while nothing is comming out of the exaust. Other than that, what you are describing is close to a perpetual motion machine. impossible. There is no such thing as a turbo with an efficiency range near 100%, much less over 100%! Heat does not make a turbo spin. Heat just keeps the exaust gas from shrinking and loosing velocity, and volume. Heat is good. An example of this is like putting a turbo in a hot oven....is it spinning? No.

I think your right and wrong both. If a turbine section was no more than a paddel wheel then you would have to have alot higher turbine pressure to build enough power with it to drive the larger compressor wheel.

These are turbines though, and therefore convert heat energy into rotational power. I think the bigger turbine housing and wheel combonations can creat the same amount of rotational power to drive the compressor with less exhaust pressure.

I'm going to pull some made up numbers out of the air to try to get my understanding of the concept across:

250 cfm of 1600 degree air @ 30 psi can create as much power in a turbine as 200 cfm of 1600 degree air @ 40 psi.

Now the bigger turbine housings allow more exhaust to go through the turbine. Because more exhaust went through the turbine more heat energy was allowed to be converted into rotational energy. Now that the HEAT is doing more of the work the pressure can drop down and still drive the same load. I think you can get to the point that the heat is doing so much of the work that the exhaust pressure can drop to the point that its lower than intake manifold pressure.

Thanks for all the inputs so far: Jason
 
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