I have listened to alot of talk about turbo's, turbo lag and where the power comes on. Who can explain the A/R Ratio. What ratio is best for which power range and what A/R ratio is you 996tt or GT2 currently running.

This question ought to separate the men from the boys. It should also bring the level of understanding on this board way up.

 

I am assuming your talking AFR's (air to fuel ratio) Chad.

Stoichiometry is 14.7 for most fuels (i.e. 14.7 lbs of air for 1 lb of fuel). This would be ideal. In an intake chamber, because of numerous constraints (imperfect evaporation, etc.) most people accept 13.8 as being ideal.

In order to protect the engine, most people want to run richer than this. Lean burns hotter and can preignite. Richer just wastes a little fuel and costs minimal power as long as you don't go crazy.

For turbos, it is common to run 12-13. 11 is rich, but you should never run under 11.

Is this what your after?

 

Here to learn a little.

 

I am enlightened. Thanks! Thought it was a typo.

 

No Mercy,

Brute size is not all that matters. The turbine's A/R (area/radius) ratio basically determines where the turbo starts to accelerate. A turbine housing looks kinda like a big snail shell. Unwrap the shell and it resembles a cone. Cutting off the tip of the cone leaves a hole - the cross section area of this hole is the A in the A/R. The hole size is important since it determines the velocity at which the exhaust gases exit the turbine scroll and enters the turbine brades. For a given flow rate, the smaller the hole the higher the velocity-but the greater restriction to the exhaust-gas flow.

The R in A/R is the distance from the center of the cone's cross-section to the center of the turbine shaft. A smaller R imparts a higher rotating speed to the turbine shaft; a larger R gives the turbine shaft greater torque to drive the compressor wheel (because the lever arm R is larger).

Why is A/R ratio important? Consider two extremes: Bonneville land speed racing (LSR)versus 1/4 mile drag racing. In a LSR application, the turbo rate of acceleration is not critical, the setup can be lazy off the line, but the overall acceleration rate, once it begins, should be smooth an linear-this application generally calls for a high A/R ratio. At the drags (and on a street car), you need more aggressive, instant response, which tends to lean toward a lower A/R ratio.

Unfortunately there is no easy scientific method for selecting the proper A/R ratio. Seat-of-the-pants is important: if boost raise is sluggish, the ratio is too large. In extreme cases, the ratio gets so big the turbo's can't turn fast enough to produce the required boost. But if the turbo is too small, the turbo gets into boost so quickly that the vehicle becomes almost undrivable-and on top, it will feel like a choked up normally aspirated engine that's under-carburated. Also, what equates to a low or high A/R ratio varies by trbine series and engine displacement.

A turbo like the TO4 style turbo for low end range uses an A/R up to 0.58, for mid range 0.69 to 0.81 and for high rpm 0.96

Given an equilivent turbine trim and A/R ratio, as engine displacement increases, the operating rpm range characteristics of the turbine decrease. Then there is also the heat the unit will see from the engine and exhaust gases, which change the units efficiency curve. Wastegate location and design also affects the turbine's performance. (credit Marlan Davis)

So back to the question. What is our A/R ratio or do we even know.

 

Chad, this is remniscent of the old days! I'm tempted to get out the bible "Forced Induction and Performance Tuning" by A. Graham Bell. by the way, thanks again for reccomending to by thiat book. It's very informative.

 

Zippy,

All I really wanted to do was bring up the level of awareness. Many 996tt owners simply go to a tuner and ask for a K24, Garretts or a certain hybrid. Or they buy a certain product, stage etc. What was this product designed for. Better yet, what is the A/R ratio of the turbo? The trend has been rather obvious. Just look at the learning curve of many of the tuners. These turbo's have been out their for a long time. What we have been slowly learning has been common knowledge to a painfully few people.

A recent article in Excellence touted some 996tt's top speed. Physics dictate that most of us would not be happy with those cars. Like my above response says, there are two extremes. Maybe some of us would like the drag racer extreme. It will feel like a choked-up normally aspirated engine that is undercarbureted. I doubt most of use would really like this. Turbo's have an alot to do with the spirit of the car. I just happen to believe we all should understand what we are getting. No one turbo or specific A/R ratio will be everything to everyone.

I find it strange few have challenged and no one has asked one very important question. Whether you are seeking to reach a desired power level (for racing) or a specific boost level (on the street), you must first determine how much air flow is needed to reach your goal at a given engine displacement and engine rpm.

Anyone even interested in going further with the formulas required to do did?

 

 

Yes Chad, lead on!

 

Bring it on Captain!

 

Hey Chad,
You must be aware there is a six page thread about A/R and other turbo tricks on the rennboard? Very interesting thread.

Cheers,

R

 

Turbo size selection should begin by choosing the compressor housing (the air into-engine side of turbo). Racers operating with high-octane fuel usually base this on how much horsepower is required to be competitive in their particular racing venue. Street driven cars operating on available pump gas are boost limited, so their primary selection criterion is based on how much turbo their engine combination can accept at a specified boost level.

Whether you're seeking to reach a desired power level (racing) or a specific boost level (street), you must first determine how much airflow is needed to reach your goal at a given displacement and engine rpm.

Cfm=CID/1,728 x rpm/2 x VE

CID=cubic inch displacement
VE=1.0 for a turbo charged engine

Next you need to add boost into the equation. Turbo engineers use pressure ratio instead of an outright expression of boost pressure.

Pressure ratio=14.7 + boost pressure/14.7

The cfm required under boost would be:

Cfm boosted = Cfm unboosted x pressure ratio

In the turbo world, engine airflow is measured in pounds/minute (lb/min). To convert cfm to lb/min, a good rule of thumb for 80 degrees F at sea level is to multiply cfm by 0.07:

Lb/min=cfm boosted x 0.07

Generally on a high performance EFI engine, every 1 lb/min of airflow is worth about 10 hp, so to find the required lb/min for a race-only application, start with horsepower requirement, then divide by ten:

Lb/min=hp/10

Every compressor has a definite combination of airflow and boost pressure at which it is most efficient. When choosing a compressor, you want to position the point of maximum efficiency in the most useful part of the engine's operating range. As efficiency drops off, heat transfered to air induction side of the turbo's goes up. That hurts both power and durability. (credit Marlan Davis)

Now, what is the most useful part of the 996tt's engine's range?

 

Sorry R, I wasn't. What is the thread's title?

 

I'm interested in the formulas too. For some reason I thought the A/R ratio took into consideration the size of the compressor wheel(?). Do the various K16 hybrids have the stock A/R ratios?
Has anyone measured the exhaust back pressure - upstream of the turbos? I remember reading that the stock Buick GN's had over 50 psi of exhaust back pressure. It seems like the K16 hybrids with small stock hot side housings and big compressor wheels would have a LOT of backpressure. You are forcing 600 +/-hp of exhaust flow through a hot side designed for 420 HP.
When I comes to sizing can you skip all the formulas and just compare the flow maps (boost pressure vs flow graphs)? If you want a 25% horsepower increase can't you just pick out turbo with a map that flows 25% more than the stock K16's. Are maps available for the newer hybrids ?