Boost vs. back pressure

[TurboTR]

Well IME the bp is generally an upward sloping line function of rpm. So to be complete, it needs to be specified at the same rpm, under the same conditions. I didn't see any rpm conditions stated.

Postal I did some experimenting with advancing the cam like that and it does indeed help low end power and spooling. It generally traps more air per stroke at low speed advanced like that, but then of course the tradeoff is less air at higher speeds. Since the engine is now trapping more air per stroke at low speed (ie the VE has gone up there) it is effectively slightly "larger" at that rpm range, and we all know what effect more cubes generally has on spoolup.

TurboTR

 

[SloGN]

Well IME the bp is generally an upward sloping line function of rpm. So to be complete, it needs to be specified at the same rpm, under the same conditions. I didn't see any rpm conditions stated.

Postal I did some experimenting with advancing the cam like that and it does indeed help low end power and spooling. It generally traps more air per stroke at low speed advanced like that, but then of course the tradeoff is less air at higher speeds. Since the engine is now trapping more air per stroke at low speed (ie the VE has gone up there) it is effectively slightly "larger" at that rpm range, and we all know what effect more cubes generally has on spoolup.

TurboTR

why does it have to do with rpm? i'm looking at the backpressure as a whole amount thru the rpm band @ a certain boost level.

@ diffrent rpm's yes there can be a difference in BP but how do u scanvenge a turbo motor you don't unless u use boost to help blow the exhaust out and make a cleaner charge.

one thing the buicks need is to keep the exhaust valve closed as long as possible to amke the most out of the cylinder charge. but there again we have exhaust port problems that can't get he exhaust out really fast so we have to open the exhaust valve under some pressure to get he charge out faster thus wasting some of that cylinder presure to do work. in the end make more power.

 

[SloGN]

Well IME the bp is generally an upward sloping line function of rpm. So to be complete, it needs to be specified at the same rpm, under the same conditions. I didn't see any rpm conditions stated.



TurboTR

i can see that happening in a NA engine. but a turbo motor is diffrent. you can make max boost @ say 3k rpm and you have a sudden upward rise in cylinder pressure that in turns makes a sudden rise of back pressure.


just my thoughts

i'm just a young pup tring to learn all i can

 

[TurboTR]

Well, never mind then.

TurboTR

 

[SloGN]

Well, never mind then.

TurboTR

if i'm wrong please correct me no harm done


i hope i didn't step on any toes.

 

[XxDarkSidexX]

WOW GREAT READING!! just found this link, and am reading barly 1/4 way thru and im quite interested. A lot of experience was posted on this thread it would be wise to read alllll of it.

thanks everyone who posted their results and opinions, its good stuff!

 

[postal]

Hey SloGN thanks for posting your results. The opening of the dump tube making the BP go up has me a bit stumped. My guess would fall inline with yours in that some how that has to be turbulence related. There has to be added turbulence where the dump splits off from the main exhaust or at the outlet of the dump itself. I would really be interested in any other test results you came up with.
Is there anything you can do to the compressor inlet track? Allowing the compressor to breath better should help lower the BP.

TurboTR
Thanks for your input on advancing the cam. Some of the stuff that you and the other guys have pointed out in this thread really makes me scratch my head at some of the other guys out there. I have read responses in other threads where guys state that the cams arent very important in these motors. MY feeling, which is backed up buy alot of the posts in this thread, is that a turbo motor has some very unique senerios that the camshaft must perform under. With all the different setups we have on these cars I cant buy into the three sizes fits all approach that we are offered. What I mean buy that is for the longest time we basically where told to buy a 206/206, 212/212, or 218/218. All symetrical cams from both a duration and centerline or valve timing standpoint. With the two completely different senerios the intake and exhaust systems have to deal with on our cars how can having the intake and exhaust valve "mirror image" each other be the optimal choice?

Mike Licht is offering some cams that vary from what we used to be offered, but Mike was probably the only vendor who was trying to get us to run more aggressive cams back when he was running Modern Muscle Car, or atleast look at different options. I have ordered two custom rollers through Mike recently. I got on compcams site, went through there master lobe catalog, and picked the intake and exhaust lobes I wanted, picked the lobe separation angle, and picked the intake centerline. Both cams I picked have longer duration exhaust than intake and have earlier (advanced) exhaust lobe centerlines. If anyone is interested I'll post the exact details.

Thanks for the input guys: Jason

 

[LUCKY1313]

Could it be that after removing the DP it allowed better flow which inturn let the compressor pump more & easier which then created more exhaust flow to cause more BP.



Jess

 

[lrayzer]

From my experience, the open dump allowed more or quicker boost spool, which as expected would yield more back pressure.

I really like the comments on cam profiles, we have had the old ****ty choices for too long, almost like the old days of 3/4 cams for the small block.
Keep your minds open, these are still engine just with a huge intake pressure, which in turn requires some help in the exhaust tract. Mike is a great guy, not a follow the lead dog guy.

 

[Ormand]

There is a recent thread on one of the boards- either this one, or TBS.com, with measured pressures. The guy was checking to see what the effects of running an open dump were, vs a "test pipe". He was getting about 30-35psig, for boost of about 20psig. So, depending on how you figure the "ratio", it was either a little more than 1.5 to 1, or if you convert to psia, it was more like 1.3 to 1.
The energy of the exhaust stream is a function of temperature, and of velocity, and of MASS! A race car, running on pure alky, will have an A/F ratio as low as 6 to 1. So on an engine like that, it is easier to reach "crossover", because there is a higher mass of exhaust relative to the mass of air being compressed. That means more energy to drive the turbine. It is also possible to use tuned exhaust on an indy type engine, or a Formula 1, to maximize the velocity component. Most use some insulation, too, to maximize the heat component.
Running the stock headers on a turbo Buick, without insulation, and running gasoline, 1.5 to 1, or 2 to 1, is going to be more realistic for street cars. Choosing a set-up to minimize turbo backpressure, or minimize the pressure ratio, is likely to put the max power in a very narrow band, and at high rpm, not necessarily good for street.

 

[SloGN]

There is a recent thread on one of the boards- either this one, or TBS.com, with measured pressures. The guy was checking to see what the effects of running an open dump were, vs a "test pipe". He was getting about 30-35psig, for boost of about 20psig. So, depending on how you figure the "ratio", it was either a little more than 1.5 to 1, or if you convert to psia, it was more like 1.3 to 1.
The energy of the exhaust stream is a function of temperature, and of velocity, and of MASS! A race car, running on pure alky, will have an A/F ratio as low as 6 to 1. So on an engine like that, it is easier to reach "crossover", because there is a higher mass of exhaust relative to the mass of air being compressed. That means more energy to drive the turbine. It is also possible to use tuned exhaust on an indy type engine, or a Formula 1, to maximize the velocity component. Most use some insulation, too, to maximize the heat component.
Running the stock headers on a turbo Buick, without insulation, and running gasoline, 1.5 to 1, or 2 to 1, is going to be more realistic for street cars. Choosing a set-up to minimize turbo backpressure, or minimize the pressure ratio, is likely to put the max power in a very narrow band, and at high rpm, not necessarily good for street.

it's also gonna take forever to spool-up a big turbo.

thats why the import crowd is able to use a ver large turbo in respect to engine size due to the fact that they has 9500 rpm's to do the work. if they choose a turbo that spools @ 2500 rpm then it would choke the motor @ 9 k rpm and loose power.

turbo back pressure is also a key fact into know how hard your going to turn the motor. for us 6k is about the limit or less. witht he less back pressure it's not gonna spool as good. but there is a fine line between too much and too little with these cars.

 

[Jerryl]

:biggrin:

 

[Junior Samples]

wow

Ate my dinner on this thread...old lady none too pleased but im researchin.

great thread

 

[Boost231]

its been 2 years since this thread made it to the top. i have read this thread over and over again over the last few months. i figured this is a good thread for everyone to read and maybe shed some light on or data on. Im trying to see how inefficient my turbo is on my setup. i might move up to a biger a/r from a .63 to an .85 before buying a new turbo, we will see. i guess first step will be to log back pressure. i will try that this year if i can and see what i come up with.

 

[Jerryl]

After reading this thread a while back, I started working on a simple calculator just to get things clear in my mind.
Now, I realize that the calcs are static and all theoretical, but is appears that at 23 PSI boost and a 2:1 BP ratio, the piston will need to move down the hole about 38deg before the boost pressure = chamber pressure.
(stock dish, stock chambers, stock HG, stock rods lenth/bore/stroke, etc).

What this means (in my mind ) is that the charge can not theoretically enter the cilinder untill the boost pressure equals "chamber pressure", which reduces theoretical displacement to around 205CID.
The calcs do account for the affect of the valve timing events on displacement.

Opening the valve before pressure equalizes "should" induce backflow into the intake port, wich is why cam timing retard works so well.
The calcs comfirm this.

Now, I know that loosing 25CID may not be a huge deal, but add 25+231 and we end up at 256CID and we all know that is HUGE!

It is realized that there are many other variables such as charge velocity, cam timing, etc. in the real world . . . . . which I am not smart enough to account for.

Anyway, it is an interesting excercise ............. at least to me. :tongue:

 

[bison]

After reading this thread a while back, I started working on a simple calculator just to get things clear in my mind.
Now, I realize that the calcs are static and all theoretical, but is appears that at 23 PSI boost and a 2:1 BP ratio, the piston will need to move down the hole about 38deg before the boost pressure = chamber pressure.
(stock dish, stock chambers, stock HG, stock rods lenth/bore/stroke, etc).

What this means (in my mind ) is that the charge can not theoretically enter the cilinder untill the boost pressure equals "chamber pressure", which reduces theoretical displacement to around 205CID.
The calcs do account for the affect of the valve timing events on displacement.

Opening the valve before pressure equalizes "should" induce backflow into the intake port, wich is why cam timing retard works so well.
The calcs comfirm this.

Now, I know that loosing 25CID may not be a huge deal, but add 25+231 and we end up at 256CID and we all know that is HUGE!

It is realized that there are many other variables such as charge velocity, cam timing, etc. in the real world . . . . . which I am not smart enough to account for.

Anyway, it is an interesting excercise ............. at least to me. :tongue:

I tried calculating this out myself but came up with so many variables i gave up. Ive become backpressure limited at 26psi@ about 750-800hp with an S trim in a .85 3 bolt with an internal gate and HD spring. Ill datalog backpressure when i get it going again. Im guessing 60-65psi@26psi. I am retarding my cam 2* from where i had it before and using better springs looking for more mph. On paper it should pick up without adding more boost. We will see. Its nearly impossible to run any of these 3 bolt turbos hard without hitting a back pressure problem. Using a large frame and a/r with a big wheel spooling with N2O is the only way i see it happening if your purely looking at it from efficiency. Your info about the theoretical effects of displacement based on backpressure may explain why there is so little gain in adding cubes when running the 3 bolt ex housings with small ex wheels. Backpressure just comes in at a lower engine speed. There is definitely a lot to be gained in the cam selection/timing in heads up racing like TSM since they are so choked by an inefficient turbo design.

 

[WSLN 6]

Bison, would I be at the same backpressure limit as you even though I am using a 3.5" external gate downpipe?

 

[charlief1]

I just found this thread, and since I'm just an ignorant mechanic after alll, what I gather from it is that using a larger lobe and duration on the exhaust will allow the engine not only to breathe better but produce more power. I've been arguing this for years with my buds and one of them is just beginning to get this through his thick head. The one thing I can really tell is that on the earier designs they sized everything backwards. A large exducer and a small inducer with a small .42 AR rating. This was corrected on the 84 and up cars. This thread has pretty much confirmed what I was trying to find out innitially about compression on the inducer side. I may have to do some more experimenting with the stock 83 exducer and the 301TTA inducer. I don't think it would work because of the size of the exducer but it may lend itself to a higher RPM app if I ever get to that point.

I truely wish Bruce was still around along with some of the other "old guys" that used to be here. Their info is still here which is a blessing but to have them on hand would benefit all of us. They are missed, even if I never did get to meet them.

 

[bison]

Bison, would I be at the same backpressure limit as you even though I am using a 3.5" external gate downpipe?

No because you may have a different surface area on the wastegate valve and more or less spring in the gate. My wastegate hole is 1.1 inches. If your running an external with a big valve and the hole is large you need a lot of spring to keep it shut.

 

[Jerryl]

I tried calculating this out myself but came up with so many variables i gave up. Ive become backpressure limited at 26psi@ about 750-800hp with an S trim in a .85 3 bolt with an internal gate and HD spring. Ill datalog backpressure when i get it going again. Im guessing 60-65psi@26psi. I am retarding my cam 2* from where i had it before and using better springs looking for more mph. On paper it should pick up without adding more boost. We will see. Its nearly impossible to run any of these 3 bolt turbos hard without hitting a back pressure problem. Using a large frame and a/r with a big wheel spooling with N2O is the only way i see it happening if your purely looking at it from efficiency. Your info about the theoretical effects of displacement based on backpressure may explain why there is so little gain in adding cubes when running the 3 bolt ex housings with small ex wheels. Backpressure just comes in at a lower engine speed. There is definitely a lot to be gained in the cam selection/timing in heads up racing like TSM since they are so choked by an inefficient turbo design.

I appears that the timing of the exhaust lobe is very important during the cam installation process since the closing event is the dominent factor in residual cylinder/chamber pressure. The intake opening will fall where it is based in LSA. Of course, this all depends on the cam itself.
For a cam with a very wide LSA, ensuring the exh lobe closing event is where the cam specs is supposed to be appears to be less critical.

Compression ratio also appears to have a positive affect on the cylinder filling.
Not much, but it does help.
All else being equal, it appears that an 8.3:1 vs a 9.1:1 compression engine will gain a theoretical 3CID with a -2deg crank rotation difference in presure equalization.
Very apparent how cam timing will make or break the performance.

Interesting is the benefit of a larger bore diameter as well in reducing required crank rotation before pressure equalization.
Like I said, it is an interesting model.

I thought about a larger turbine housing to get BP down but it would only make the HP peaker. For a track car it would be acceptable, but it would not be fun to drive on the street. I will need to run some numbers with the BP you are seeing. Interesting for sure . . . . .

I just found this thread, and since I'm just an ignorant mechanic after alll, what I gather from it is that using a larger lobe and duration on the exhaust will allow the engine not only to breathe better but produce more power. I've been arguing this for years with my buds and one of them is just beginning to get this through his thick head. .........

Charlie,
You provide concistent valiable input and I don't think there is a ounce of ignorance in your body.

I think the longer exhaust event will allow earlier pressure bleed down and from my research it appears that the magic number for opening the exh is around 30-40 deg BBDC. No real revelation I know but the closing event is where the difference is, especially on the turbo motor.

The increased lift just ensures quick pressure bleed so the piston does not have to fight the pressure, but, if the velocity is low, the exhaust system pressure can backfeed into the cylinder.
I think too much lift (>35% of diameter) combined with extended duration may do that.
I know I have not yet told you anything you don't know yet . . . I am merely "thinking on the keyboard" . . .