The Only 3300 lb. Buick V6 in the 8s using...

[Fryguy]

FWIW, I have had a T4 T70, T4 T76, and T6 91mm on this car. I would never consider going back to a T70. Maybe 70mm turbos are different today. I'd have to take a good look at the compressor map. Backpressure would also be a consideration.

I never said you should go back to a T70. I will say you could run the same times you run now with a smaller turbo, and you wouldn't need the nitrous to spool it up. That's just my opinion though, and I'm no pro.

 

[Alky V6]

There is more than just back pressure to consider. If you look at a compressor map, you can see at what pressure ratio a given turbo will work best. For the many (most) of the smaller units, a smaller and less efficient engine will actually make more power with a given turbocharger. The OP's engine, which is small in displacement and has mild heads on it, would be a great engine to get the most out of a small turbo with because you could run a very high pressure ratio (boost level), which is where many small turbos are happy.

That's why the compressor map and the boost number is actually important, and not irrelevant. At one time I had a very mild 6200 rpm 347 engine and a GT42-76. That car went 155mph @ 3300# at around 21psi. We took that turbo off and put it on a only slightly heavier F-body which had a very nice 370cid LS engine with good heads, etc. That car at the same boost level wouldn't fall out of a tree. It was a total turd. The answer was in the compressor map. The turbo was outside of it's happy place at 21psi on that engine but it was just fine on the mild street engine. That's why the pressure ratio matters.

Do me a favor. You obviously know how to read compressor maps. Go on the internet and find a compessor map for a Borg Warner S510 turbo. That is basically the turbo I am running except for a slighly smaller compressor side. Use 1200 hp to calculate your airflow number, and use 30 psi boost to calculate the pressure ratio. Get back to us with what you find.

 

[Alky V6]

To be specific, the B/W S510 comes standard with a 95mm compressor wheel. My wheel is a 91mm, so you can adjust the compressor map in your mind a little for a smaller compressor wheel.

 

[Alky V6]

I never said you should go back to a T70. I will say you could run the same times you run now with a smaller turbo, and you wouldn't need the nitrous to spool it up. That's just my opinion though, and I'm no pro.

You're assuming that I was looking to spool a turbo without nitrous.
From the very beginning, nitrous was one of the first items on the list to be used to spool whatever turbo I would eventually choose. There was never a time while I was putting this project together where nitrous was not one of the most important specifications for the project. Knowing that now, how would that change your selection of a turbo for my project? Have you ever used nitrous to spool a turbocharger on an engine?

 

[Alky V6]

I find it fascinating that people would just fly off the handle and criticise me for certain things about my project without first taking the time to find out what my original plans and goals were for the project, and why. And please don't ask me to explain all that because I'd have to then link you to a thread that will really bore the heck out of you. You think this thread is rediculously long.

 

[RICHIE HIGH PSI]

Where do you plan on racing this car?
Bracket or heads up? {class}
Thanks

 

[KL Mallender]

Can you guys expand on this a little more, as perhaps I missed something. I was under the impression that the only difference between two identical engines running 30 pounds of boost pressure, with two different sized turbo's (or single vs twins) would be the denser air charge of the larger (or extra) turbo(s). I would agree with Donnie in that boost pressure is an irrelevant reading, it's just telling you how hard that particular turbo(s) is working to achieve that amount of pressure. How is backpressure used in the equation as being the sole factor in making more power? If you compare two turbo's with one equally matched turbo (exhaust housing, flow ability, MAP, etc.), run two 2 1/2" downpipes on the twins, and a 5" downpipe on the single, how exactly do the twins make more peak power at a given RPM (note that twins will obviously make more power under the curve)? Please explain, as I am very curious....

Good question Lethal, and Fryguy brings most of the answer: efficency, one large compressor wheel/housing will leak"backflow" much less than two smaller units causing less temperature rise of the intake charge after compression, this is how compressor maps are graphed.

Any of us playing with "Boost" has seen the results(or lack thereof) of cranking up boost and to a point it's all fun and games, then the wall of diminishing returns is found, when dealing with turbos that wall might be the compressor side on some and the exhaust on others.

The easiest way to diagnoiss which side is helping or hurting you is to measure temperature output of the air charge out of the compressor(before intercooler) you can then "connect the dots" on a compressor map to "See" where you are playing.
Measuring the exhaust side is much simpler with pressure measured before the turbine and after(normally atmospheric unless a restriction exists)

For any compressor wheel there will a necessary shaft rpm to achieve "Boost", this rpm is supplied by the exhaust wheel (still with me?)
If the compressor requires more shaft torque (speed) to aquire compression than the turbine can supply/transmit to shaft, exhaust pressure(pre-turbine) will need to increase in order to overcome the resistance of the compressor trying to do it's job, this is expressed in backpressure(pre-turbine)
Anytime exhaust pressure is higher than intake pressure there will be a natural but unwanted "EGR" condition.

Many accept their EGR penalty by changing valve timing (overlap) to accomodate this contamination, while it lessons the contamination it severely hurts the engines natural breathing throughout it's operating range.

In my experience, most turbocharger combinations (compressor side vs turbine side) are out of wack, way too much compressor for the turbine driving it.

I'll wager that many here do not understand how this contamination seriously affects tuning in reguards to fuel and timing requirements,( A conversation for another day)
If someone here has a running car and a temperature probe leaving the turbo and a pressure probe before turbine and the paitence to understand this, I would be more than willing to lay out the data in a seperate comprehensive thread.

Good luck,

Kevin.

 

[Alky V6]

Where do you plan on racing this car?
Bracket or heads up? {class}
Thanks

I have been racing the car at Barona Drag Strip for the last 6 years or so. I live in the TnT lanes. After I'm done with playing around with it, I may settle down and pick an index class to race it in. Doubt it though. That may bore me too much. I like tinkering too much. So much that the car is never really consistent, performance wise, for too long. I'm always playing with something on the car. Lately, it's the chassis. Instant centers and shock settings. Very boring stuff to share. Although, once you get into it, it is interesting when you start to see changes to the car from different settings.
A lot of the people up at the track that like watching my car have come up to me and told me that the reason they like watching the car is because you can never count on what it's going to do. I may make a change to something and it turns out to be a flop. Then the next run turns out to be a new best for the car.

The car is basically a personal lab experiment of mine. And fun to drive, too.

 

[Dusty Bradford]

Can you guys expand on this a little more, as perhaps I missed something. I was under the impression that the only difference between two identical engines running 30 pounds of boost pressure, with two different sized turbo's (or single vs twins) would be the denser air charge of the larger (or extra) turbo(s). I would agree with Donnie in that boost pressure is an irrelevant reading, it's just telling you how hard that particular turbo(s) is working to achieve that amount of pressure. How is backpressure used in the equation as being the sole factor in making more power? If you compare two turbo's with one equally matched turbo (exhaust housing, flow ability, MAP, etc.), run two 2 1/2" downpipes on the twins, and a 5" downpipe on the single, how exactly do the twins make more peak power at a given RPM (note that twins will obviously make more power under the curve)? Please explain, as I am very curious....

The compressor side running more efficiently does help the power somewhat. In the real world with a liquid intercooler I can still take the single turbo putting out hotter air and get it back to under 80 degrees. So considering that an engine it taking in 80 degree air at 30# of boost from either a single or twins, you have to look at the exhaust side. Most people never bother to look at the back pressure. Like in the example given earlier. I bet if you were to look at the back pressure the 370ci engine is running under alot more back pressure. It is true the smaller engines will make better use out of a turbo, all things considered.

Think of whats going on inside an engine. During the exhaust stroke the exhaust valve is open, the piston is pushing against 50psi of exhaust pressure to clear the cylinder out. On the intake side of other cylinders you have 30psi of boost in the intake helping you push down on the piston during the intake stroke. So in both cases you have 30 psi of boost helping you. Now think of the twins. They have 25psi of backpressure. The crank and piston is working against half the pressure. If you wonder how much power can this be. Remove all the spark plugs from the engine and turn it over by hand. Now pressure the cylinder with 25psi of air from your shops compressor, now try 50psi. There's your answer.....it's alot of effort.

Now if you compare a really large single to flow the same as twins you have to go very big. Like the 91mm Don has. It has a very large turbine side so he's able to get the back pressure less than the boost. So you can size a single to make the same power as twins but it's very hard to spool the single due to the size required. Nitrous would be required with the single where as twins will spoool by themselves. Which brings on another topic that doesn't look right on paper but works in the real world.

Say you have a 274ci engine with a twin 6765's. That's just 137ci per turbo. On paper a 137ci V6 Buick would never be able to spool a 6765 without nitrous assist. Yet the twin 67's actually spool faster than a single 88mm and make as much or more power.

 

[Street Lethal]

There is more than just back pressure to consider. If you look at a compressor map, you can see at what pressure ratio a given turbo will work best. For the many (most) of the smaller units, a smaller and less efficient engine will actually make more power with a given turbocharger. The OP's engine, which is small in displacement and has mild heads on it, would be a great engine to get the most out of a small turbo with because you could run a very high pressure ratio (boost level), which is where many small turbos are happy....

I would definitely agree with you there regarding a smaller turbo with nothing being touched, especially in a street application. But when you velocity port the smaller cylinder heads, and remove the valve guides, the 1.84" isn't really a restriction at all anymore, especially with a cam with the right amount of duration, which gives us our RPM ability with smaller valves, so it will take whetever you can give it. Not to mention, there are ways of keeping a much larger turbo in it's efficiency range to satisfy a smaller displacement engine; nitrous, stall speed, gears (tranny included), and tire size, just to name a few. Boost pressure alone is really an irrelevant reading though, because it can be controlled, and changed simply by opening up the intake tract without even touching the cam and heads, so velocity is definitely the key, not a pressure reading. Boost pressure tells us how hard the turbo is working to achieve a particular amount of forced air with the particular setup being used. Think of it this way, your running 30 pound injectors at 15-psi on a 300 horsepower engine, and the injectors are almost going static with that particular setup. Now, you open up the intake tract after the turbo, throughout the intake manifold, but not touching the cam and the cylinder heads (larger charge tubes, larger throttle body, ported intake), and at the very same 15-psi of boost pressure, are you running any leaner? If so, why...?

 

[Alky V6]

Good stuff, guys.
This is exactly why I usually stick with pushing on with a thread I start, even with some of the type of criticisms that come up. There are great moments where good information is shared. Most times, it can be just basic stuff to the more experienced, but I tend to hope that the inexperienced are also here, and hopefully learning something out of all this.

 

[Mike E]

... Which brings on another topic that doesn't look right on paper but works in the real world.

Say you have a 274ci engine with a twin 6765's. That's just 137ci per turbo. On paper a 137ci V6 Buick would never be able to spool a 6765 without nitrous assist. Yet the twin 67's actually spool faster than a single 88mm and make as much or more power.

The combined moment of intertia is probably much smaller for the 6765's compressor/ turbine wheel vs. the 88. The same concept that bringing a light flywheel up to speed takes less power than a heavier one. It not just the mass though, moment of intertia for a round object also takes into account the square of the radius or diamter.

 

[Street Lethal]

The same concept that bringing a light flywheel up to speed takes less power than a heavier one. It not just the mass though, moment of intertia for a round object also takes into account the square of the radius or diameter....

... not to mention where the weight on the flywheel is located, as a heavier flywheel with most of that additional weight being located in it's center mass, and not it's outer diameter, would make it just as easy for the motor to spin as it does the lighter flywheel.

 

[Dusty Bradford]

Yes, the rotating weight has alot to do with it. But....how about a single 94mm vs twin 88's on a 400ci engine. You would think the twin 88's would be more difficult but I can say that with a converter right on the edge of being loose enough to spool a single 94mm, the 88's come up easier. The difference though is the amount of launch rpm needed with the twins. To get the car to 60' and 330' fast with the twins takes more rpm.

 

[Alky V6]

I find it interesting to study the combinations of other people. Particularly if they bother to share detail about the combination and why they picked what part. The reason why I generally will not criticise a person for their combination is because I understand what it takes to share your combination with others. Especially if it is an odd ball combination. Besides, there is always something to learn from others. Whether all the information you might gather is useful to you or not, there still might be that little tid bit of information that you weren't aware of before that you can hide away in your tool box that may become useful to you in some future project.
OK. I'll shut up.

 

[Alky V6]

I have to inject something for argument here. Someone argued that 2 smaller turbos yielded a wider useful range than one large single. Can someone elaborate on that?

 

[Street Lethal]

I find it interesting to study the combinations of other people. Particularly if they bother to share detail about the combination and why they picked what part....

To be quite honest here, I think that that is all it really comes down to here, and that is what it always came down to, the harmony of the valvetrain. The problem with turbo applications is keeping the turbo spooled, but in the perfect world they would all spool instantaneously, and if that were the case, the bigger the better regardless of the engine. It is like comparing superchargers, an F3 Procharger is way more efficient than a D1, but then you'll have guys saying that a D1 is more suited for engines making x amount of horsepower and it's better on the street, which is nonsense. Guys now are running turbo wastegates on the intake tract of their centrifugal supercharger setups so they can control the amount of boost being applied (as opposed to relying on pulley size), then stuffing the largest supercharger that they can get on there and pullying it up. Doing that both maximizes and controls that power throughout, making the whole menu on Procharger's website of varying compressor setups being completely irrelavent, which is why you will never see them come out with a wastegate for a supercharger, because then you would only see one type of supercharger being sold, the biggest one....

 

[Mike E]

Yes, the rotating weight has alot to do with it. But....how about a single 94mm vs twin 88's on a 400ci engine. You would think the twin 88's would be more difficult but I can say that with a converter right on the edge of being loose enough to spool a single 94mm, the 88's come up easier. The difference though is the amount of launch rpm needed with the twins. To get the car to 60' and 330' fast with the twins takes more rpm.

If you know where a compressor map can be found for them, it would be much easier to put real numbers to this instead of my guesses. For the discussion I'll use this old school y2k 88 compressor map. Focus on the compressor rpm lines and ignore the effeciency island lines. It would be really nice to lay the 88 and 94 mm compressor curves over each other.

The only problem with what I'm figuring here is I'm guessing that the 94mm wheel compressor map is similar to the 88mm map, but maybe a small shift to the right. As long as the compressor speed lines drop of exponentially as they go higher in air flow on both curves and they are relatively close together this theory is the same.


Each 88 moves half the amount of air that a single 94 does for the same amount of boost (pressure ratio). If you look at how the compressor rpm curves are laid out, generally the compressor rpm curve gives low airflow at high boost and high airflow at low boost.

Both the 88's operate in the relatively low air flow at high boost range. This can be done with low compressor rpm. The 94 has to be at the same pressure ratio but higher airflow, so you end up going to a higher compressor speed. So, all things being equal (boost and airflow) the 88's each have to turn less compressor rpm than the 94. Using what I said above about moment of interia and power required to speed them up. The 88's don't take near as much power to spin them up into there operating range on the map vs. the 94.

The engine rpm it takes to get them there depends on the turbine wheel map. The turbine wheel and compressor wheel are locked together, same rpm. For the 88's you need low turbine rpm, and for the 94 you need high turbine rpm. Lets assume the turbine maps are the same for both turbos and they still look something like the compressor map. (big assumption, I've never seen a turbine map but it's still the basic fan curve) For the 88 at low turbine rpm, low airflow (half of the 94 turbo) your in the range of low pressure ratio (backpressure). For the 94 you'll need more turbine rpm and have twice the airflow going through it, so you'll be much higher up on the pressure ratio.

 

[Fryguy]

I would definitely agree with you there regarding a smaller turbo with nothing being touched, especially in a street application. But when you velocity port the smaller cylinder heads, and remove the valve guides, the 1.84" isn't really a restriction at all anymore, especially with a cam with the right amount of duration, which gives us our RPM ability with smaller valves, so it will take whetever you can give it. Not to mention, there are ways of keeping a much larger turbo in it's efficiency range to satisfy a smaller displacement engine; nitrous, stall speed, gears (tranny included), and tire size, just to name a few. Boost pressure alone is really an irrelevant reading though, because it can be controlled, and changed simply by opening up the intake tract without even touching the cam and heads, so velocity is definitely the key, not a pressure reading. Boost pressure tells us how hard the turbo is working to achieve a particular amount of forced air with the particular setup being used. Think of it this way, your running 30 pound injectors at 15-psi on a 300 horsepower engine, and the injectors are almost going static with that particular setup. Now, you open up the intake tract after the turbo, throughout the intake manifold, but not touching the cam and the cylinder heads (larger charge tubes, larger throttle body, ported intake), and at the very same 15-psi of boost pressure, are you running any leaner? If so, why...?

A given compressor will definitely be more efficient when there is X amount of boost rather than Y amount of boost. That's why that number on the gauge does truly mean something. At the same time time, two turbos that both make the same X amount of boost on a given engine can make much different power numbers, even if the inlet temp is similar. In my previous example, both the GT42-76 and the S88 that replaced it were making around 20psi on that good 370. The power difference was HUGE. As Dusty mentioned, some of that may well be due to back pressure, but I know for a fact some of it was due to compressor size. When I picked that turbo for the 347, it was clear we'd be riding the edge of the map.

To be quite honest here, I think that that is all it really comes down to here, and that is what it always came down to, the harmony of the valvetrain. The problem with turbo applications is keeping the turbo spooled, but in the perfect world they would all spool instantaneously, and if that were the case, the bigger the better regardless of the engine. It is like comparing superchargers, an F3 Procharger is way more efficient than a D1, but then you'll have guys saying that a D1 is more suited for engines making x amount of horsepower and it's better on the street, which is nonsense. Guys now are running turbo wastegates on the intake tract of their centrifugal supercharger setups so they can control the amount of boost being applied (as opposed to relying on pulley size), then stuffing the largest supercharger that they can get on there and pullying it up. Doing that both maximizes and controls that power throughout, making the whole menu on Procharger's website of varying compressor setups being completely irrelavent, which is why you will never see them come out with a wastegate for a supercharger, because then you would only see one type of supercharger being sold, the biggest one....

This is not exactly true. If you run a turbo outside of it's range on a compressor map (bigger or smaller) you can have issues with surge or low power.

Also there is a huge difference in rotating mass with superchargers, and especially with the big prochargers. I know of two different cars that were quicker with the F2 than the F3 at similar boost levels....some of the reason may be rotating mass, some of the reason may be the compressor itself. Both were small block cars....a large big block probably would have acted differently.

 

[turbobitt]

Yes, the rotating weight has alot to do with it. But....how about a single 94mm vs twin 88's on a 400ci engine. You would think the twin 88's would be more difficult but I can say that with a converter right on the edge of being loose enough to spool a single 94mm, the 88's come up easier. The difference though is the amount of launch rpm needed with the twins. To get the car to 60' and 330' fast with the twins takes more rpm.

I was told directly from an Honeywell engineer that came to our facility for testing about 7 years ago that you would always be better with one big turbo than two small turbos. His reasoning was that you double the ineffiencintcies(don't know the spelling) of the system. I don't have any experience to agree or dissagree but it made sense at the time. Turbo technology has changed over the years so this may be outdated.

Allan G.