Do you have a general or tech question for Corky Bell? Send an e-mail to Corky at comments@bellengineering.net. We will sort through the e-mail and pick the one's we feel will be most relevant to turbo and supercharger applications. Corky will post those questions and answers on this page. Corky will also post some comments here too. 


Remember that unsolicited suggestions, ideas or other submissions to BEGi will be treated as non confidential and non-proprietary. Corky does reserve the right to use names if questions are extreme in nature. We will not post questions or answers to every e-mail received. See our Terms of Use and Privacy pages for more information.

Topic:
Boost Creep

The problem:
Over the years we have dealt with boost creep, or even out of control boost, many times.  Nine of ten times it has involved the integral wastegates.  As the T25/28/30 Garrett series turbochargers have come into wider use, we seen the growth of compressor flow rates nearly triple yet the integral gate remains the same size.  I’ve no idea what goes through the design guy’s heads.

These creep/overboost problems were originally encountered with the IHI turbos as applied to the old Z car and RX7.  We solved the problems then and the same techniques have proven successful today.

In my view, the creepy problem stems from a limited flow capability of the gate valve, and the geometry of the turbine inlet in the immediate vicinity.

What we’ve done from the start of our current designs:
With some historical fear of integral gate creep problems we felt some concern in avoiding the problems at the outset of the new designs.  With some past judgments applied, it appeared the streaming of the valve would accomplish what we needed and was only a two minute operation.

History:
While it is often difficult to tell if you have solved a problem or not affected anything, at least we had no reports of over boost for two years.

As with the best laid plans...... we’ve had two reported incidences of creep in the last month.

What we have now incorporated into every integral gate turbo passing through our office:
Clearly the need was to flow more exhaust gas through the gate's valve, when open, and less through the turbine.
Here the discussion must become a bit vague for fear of telling competitors how to fix their similar problems. 

Streamlining, re-routing, directing and expanding requires a 45 minute milling machine operation.  Perhaps not the best deal for the company bottom line, but ................

On asking garrett to fix the problem in production with some casting and machining changes, they very clearly stated that old and tired cliché;  “we’ve never seen such a problem, you must have done something wrong.”

I bet you've heard that a few times................
Turbo System Updates

As often stated in these pages, I find system improvements irresistible, regardless of large or small.  With the entire fabrication done in house, I am free to incorporate any change I wish at any point in the production process.

The intercooler cap configuration has been changed to a much more streamlined shape.  The transition from a tube shape to the cap interior is always a tough spot and a source of considerable drag.  The new shape enjoys a very gradual transition of section areas thus producing a far more streamlined flow.  In my view, the new shapes are superior to the classic AC inlets/outlets. 

A second change to the inlet/outlet caps is at the charge air channels at both top and bottom of the IC.  Our older design required a sudden change of direction to both enter and exit the channel.  With a far more gradual change of direction, the flow throw these two channels increases slightly.  With a bit of luck, the improved flow on the end channels will also find it easier to shed another BTU or two. 

So, what does it all mean?  Two things in my view.  My best guess is the flow improves between 1 and 2%.  Not exactly a big deal, but it is better and onesies/twosies continue to add up.   Secondly, it represents my willingness and intent to keep the Miata systems, while clearly the best engineered and crafted systems ever offered, even better yet.


Minor mods to the 1.8 exhaust manifold design.
Corky's Area Rule

The gas velocity distribution through the Turbine Outlet pipe must see a steady decrease.  Exhaust gas velocity is controlled by the cross section area, hence, a steady decrease in velocity dictates a steady increase in flow area. 

Our design starts with a 2.25" diameter exiting the turbine, with 1.5" diameter tube exiting the wastegate. These diameters match the turbine and wastegate outlets from the turbine housing.  Bigger or smaller tubes at this point will cause severe section area misfits.  This offers the turbine a path of 3.63 square inches, and the gate a path of 1.54 square inches. When operating at less than gate controlled boost, the flow area is the 3.63 square inch figure.  When the gate opens, the flow area becomes 5.17 square  inches.  Part way down the Turbine Outlet pipe, the primary diameter increases to 2.50" for an area of 4.6 square inches.  When the main pipe and vent tube combine, approximately 30" downstream, the diameter changes again to 2.75" for a flow area of 5.52 square inches. This use of Corky’s area rule does not yet achieve the perfect taper, but it obviously avoids the technical/functional discrepancy of exiting the turbo with 5.17 sq. inches only to dump down 11% to a smaller 4.6 sq. inch tube.  Requiring the turbo to pump uphill by the need to accelerate the gasses through a smaller section area is not the path to power.





Wastegate Tube Separation
The wastegate vent must be segregated from the turbine outlet as far as possible.  Avoiding a colliding blast of wastegate gasses into the back of the turbine is necessary for maintaining low back pressures and smooth flow through the turbine.  The best situation is offering the wastegate gasses their own complete tailpipe.  While impractical, the longer the two pipes are separate, the greater the power.  We have chosen to separate the wastegate gasses as far as the flex joint and the standard catalytic convertor will permit.  This is about 30". Treating the vent tube in this manner is generally believed to be worth 3 to 5% in total power output. Competitors have all chosen either zero, or two inches.  I don't mind suggesting again that this is neither cheap nor easy.

While not perfect yet, this is clearly the highest science downpipe in all 'Miatadom'. 

Welding Method
It is not for reasons of rapid production that race cars are welded together exclusively with the tungsten inert gas process called TIG, or more commonly, heli-arc.  The heli-arc process is used because it is the best.  It penetrates the best, produces the strongest weld, the prettiest weld (with a little practice) and leaves no splatter.  No one has ever looked at a wire feeder weld and said: “Wow, look at the nice welds.”  That expression is reserved exclusively for the heli-arc. Some acetylene guys could do it, but they've all retired.  The wire feeder (MIG, wire welder...) is best suited for lawn chairs and pole barns.  We use the heli-arc exclusively and will NEVER cease to do so.  No other Miata performance equipment maker does so.  
 
Craftsmanship: Flange to Tube Weld
While cracks almost always occur at the edges of a weld, where the material has had its basic heat treat removed by the welding process, the techniques of fabrication can offer some relief to the loss of heat treat.  For example, the weld of the flange to a tube must have the tube extend through the flange and weld 360 degrees on the inside, then welded 50% on the outside at about 1.0" increments.  This permits half of the unwelded, thus undamaged, tube material to go through the flange and attach inside. This technique eliminates a ring of un-heat treated material from circling the tube.  It is estimated that the basic strength of a welded joint using this technique is three fold stronger than simply laying one bead around the outside.   Guess which is cheaper and easier?

Craftsmanship suggests tube joints must be concentric, bends and straights must merge smoothly, flanges must be deburred, holes chamfered, O2 threads re-tapped, and the flanges ground flat.

The surface finish must be presentable.  We prefer to power buff the stainless, and paint the mild steel.

The intersections should all match with no diameter changes. Smooth flow of the exhaust gas requires no sudden changes of cross section area.  Never/ever/never hook a 3.0" tube up to the 2.0" exit of the turbine.  If a section change is required or desired, try to keep a gradual taper in between the two sizes.

We were offered a very kind compliment recently when a kit recipient asked if he could purchase an additional downpipe to mount above the mantel on his fireplace.

Material
Stainless. The choice of material with a good high temperature strength is necessary.  Stainless steel satisfies this need most easily.  Further, its corrosion resistance is virtually the best there is.  

Intercooling & Interesting Twists

The point of this argument is: The heat that finds its way through the walls of the tubes to and from the intercooler greatly contributes to the overall effectiveness of the intercooler system.  Taking full advantage of that benefit is a must for any designer. 

Premise: Never insulate a compressor outlet tube.

Testing my own turbo has produced an interesting set of charge temperature numbers. This was particularly evident with respect to the temperature changes in the tube from the turbo to the intercooler. Rather than produce a bunch of equations, I’ll offer the data and suggest a couple conclusions.  The conclusions are rather obvious and have design quality and experience implications way beyond the data presented here.

The basic layout of the system: The turbo is positioned in the same place we originated 15 years ago,  just 3.5 inches out from the head and an inch above the port centerline, right between cylinders two and three. One major change is the discharge of the compressor is now downward and out into the wheel well area. The tube goes forward to the intercooler, through the long tube IC core with baffled inlet, then out the opposite side, turns aft and up into the throttle. Pretty straight forward, but clearly not well understood, as evidenced by other designs offered up in the market. This layout was chosen specifically for the purpose of shedding heat wherever possible.  For this test, all tubes were made from mild steel.

While seemingly obvious, but obviously not well understood, is the need to let the compressor outlet tube discharge some of the heat from the compressed air exiting the turbo.  There is a substantial amount of heat that can be removed from this tube before it gets to the intercooler.  EVEN IF THE TUBE STAYS UNDER THE HOOD IT WILL LOSE HEAT, AS THE UNDERHOOD TEMPERATURE IS NOT AS HIGH AS THE TEMPERATURE INSIDE THE TUBE. 

We have taken the compressor outlet tube into the ambient air stream as quickly as possible. Hence, downward from the turbo and out into the wheel well.

Repeat:
The design principle: First; The air charge temperature inside the compressor discharge tube is higher than the underhood temperature,  and Second, the discharge tube must get quickly out into ambient air temp where it can shed even more heat.

Here is how we tested it:
Data: We measured temps from right at the compressor discharge and at the IC entry.  This data was consistent over several days of runs.  Forward speed was held to 60 mph in 4th gear by dragging the brake. Boost was 6 psi. Time under boost was held until temps stabilized.... about 20/25 seconds.  Tough on brakes, but we needed good data.  The tally is an average of 12 measurements, none of which varied more than 3 or 4 degrees.

NOTE: This is a measurement of the temperature drop through the turbo discharge tube only! 

Ambient F   95/96 F 
Turbo Exit F   189/191 F 
Temp Rise F   94/96F 
Intercooler Inlet F   169/172 F
Temp Drop F  19/22 F

This data clearly states that approximately 20F is removed from the system through the walls of the tube between the turbo and the intercooler.   Cool, eh? 

Keep in mind that this data represents only 6 psi boost.  At 12 psi, the compressor discharge temps will be approximately 90F higher yet.  If so, while assuming the “efficiency” of the tube will remain the same, then the simple tube from the compressor outlet to the intercooler will discharge approximately 40/45F.  Downright newsworthy.

General underhood area temps at the compressor discharge were 112/118 F.  Temps in the wheel well area were 97/101 F.

This data states three things absolutely clearly:
1. The temperature inside the compressor outlet when under virtually any boost is far higher than the surrounding areas, thus heat exits the tube through the walls. 
2. Behavior of the tube: The temperature removed from the system by the compressor outlet tube alone was 19 to 22 degrees F. Approximately equal to the temperature gain accompanying 1.5 psi boost. 
3. Temperature out of the Intercooler, is within 5 degrees F of ambient.   

Conclusions #1: 
Anyone insulating the Compressor Outlet tube is blowing in the wind. That means their concept of the heat flow is backwards.  If done so, their “Quality of Design” is subject to serious question. 

Conclusion #2:
The compressor outlet tube should be made from a material with a high heat transfer capability, such as aluminum.  This is precisely why we introduced the “multi-material tube set” moons ago with just that, an aluminum compressor outlet tube.  Perhaps a bit in excess of reason, and not of world shaking proportions, but a maximum effort system should have a compressor discharge tube made from silver.  Now, wouldn't that be a hoot?  Maybe expensive too...

Conclusion #3: Porsche will one day build either aluminum, copper or silver compressor outlet tubes, and when they do, we will once again be able to claim we plagiarized their design.

Summary: Regardless of where the turbo is located,
NEVER, NEVER (NEVER SQUARED??) INSULATE THE COMPRESSOR OUTLET TUBE. 

What is considered insulating a tube?
Wrapping in in fabric or using a heat retaining tube like silicone turbo hose

One more reason why the BEGi “quality of design” is superior to ALL others.

Should any reader like to verify our data, I’ll lend our digital thermometer to anyone for a reasonable deposit.  You must use our Intercooler, tube system and duplicate our test conditions.

'02 Dyno Run

While Lanning's car remains a hoot to drive (hence, the speeding ticket), the fuel system works marvelously well, and the dyno numbers are satisfying. I will still maintain my position that the design quality of the individual components are the key element to performance rather than the boost pressure one is willing to run. The factor that determines the real value of a system sums up as the power per PSI of boost. That number reflects the merits of the manifold, turbine outlet pipe, cool air supply, intercooling, intake flow losses, fuel system accuracy....everything the designer is supposed to do well. In my view, the system producing 205 rwhp at 8 psi is a superior design to any system producing 250 rwhp at 18 psi. I reckon we ought to find some better tailpipes one day.

The Water Cooled Bearing Section of the Turbo

The Water Cooled Bearing (WCB) was not a device that came along and magically made turbos reliable.  The turbo has always been reliable, when offered proper care.  The crux of the matter; information regarding proper care of the turbo was not offered by anyone.  OEM’s were either ignorant of the care required or didn’t want to scare prospective turbo vehicle buyers away by listing increased maintenance needs.   This idiotic situation lead to a zillion early turbo failures and the lingering B.S. of today that turbos are generally unreliable.  An interesting fact regarding 18 wheeler freighters; the time between overhaul of their turbos is approaching 1,000,000 miles.  Another: the last three centuries of the Le Mans 24 Hour Sports Car Race have been won by turbocharged vehicles.  The equivalent road mileage wear estimate of a race car is 1000 to 1 (Porsche, among others). The Le Mans winner usually covers about 3,300 miles in the 24 hours.  Does that actually suggest the equivalent road wear of three million miles?  You can bet that they cross the finish line still operating under boost.
<>
What info was needed?

The information needed was simply frequency of oil changes and the quality of the oil used.   In my experience the situation boils down to two alternatives; mineral based oils and synthetic based oils.  These two perfectly able lubricants have different high temperature capabilities, thus making one more suitable for turbocharger operation.  The net result is, while both can be used safely with the turbo, the synthetic can be used substantially longer between change intervals. 
If the OEM’s had stated the oil quality and change requirements as:
  • Mineral Oil: use the best available, and change it every two thousand miles.
  • Synthetic: Use the best available and change it every five thousand miles.
.... then, the turbo would have been spared a dubious reputation.  Would fewer people have purchased turbo vehicles?  Probably not.
Why is this lubricant material and internal change so frequent?
It is well within the imagination to suspect the temperature range oils are subject to is higher around the turbo center section area than with the engine in general.  Higher temperature tolerant oil is more expensive than lower temperature tolerant oil.  Reasonably logical.  The cost is from the additives that resist charring of the basic oil molecule at high temps.  As the oil gains mileage, the high temp additives slowly get used up in process of defending against the btu’s. Eventually, enough of the additive is smoked, and the oil becomes junk.  Turbo bearing coking sets in and the oil flow to the turbo gets blocked and the turbo dies.  The additive materials are still working at 2000 miles for mineral and 5000 for synthetic, but sufficiently diminished such that the risk rises rapidly of blasting the remaining additive into oblivion becomes higher by the day.   So, its time to change the oil. 
Since temperature in the center section is the cause of the oil deterioration, it is very valid to approach a better solution to oil change intervals through reducing that temperature.  The WCB does that.  The effect of the temperatures on oil deterioration does not go away, rather it is significantly diminished.  Thus the high temp additives last longer.  Hence, the recommended oil change interval is significantly longer for both mineral and synthetic.
Suggested:
  • Mineral: Use the best available and change it every 4000 miles.
  • Synthetic: Use one of the best available and change it every 10,000 miles.
The net cost of a WCB and related plumbing is about $150.  Therefore, it appears to me that the water cooled bearing is, in actuality, nothing more than a cost savings device and a serious convenience.   It was clearly the information that was key.
By-Pass Valve

The by-pass valve, blow-off valve, anti-surge valve are all intended for the same purpose; that of suppressing a funny and obnoxious chirping noise made by turbos when lifting off the throttle.  The noise is technically called “surge.” The “blow-off” valve has slowly acquired the implication that it vents to the atmosphere, while the other two names have leaned toward implying the air is vented back into the system in front of the turbo.  The difference is that the blow-off replaces the real surge noise with an equally obnoxious whooshing blast noise made when the vented air hits the atmosphere.   The “anti-surge” valve, with the air ducted back into the system is almost completely quiet, and when heard, amounts to only a gentle “sigh.”  This is the reason our system has the anti-surge valve.  It is distinctly more civilized.
Fuel Pressure Regulator

The Rising Rate Regulator enjoys a rather interesting history in the American performance car industry.  It was invented in 1975 by a Texas Aggie graduate holding a degree in physics.  He later became one of America’s foremost race car designers.  Today he is a practicing veterinarian running his own large animal clinic in Arkansas.  If I mention his name, I’ll probably lose a friend. 

I have been credited with inventing the regulator.  Clearly, not so, but I was the first person to bring it to market and did so in 1975.  We have enjoyed brisk sales of the regulator ever since. 

The fun part of the story is the number of times it has been copied.  To date, I find eleven separate copies.  It is not unreasonable to expect a good idea to find its way around the market place.  I'm flattered that our production, promotion and use of my friends invention has been so widely copied.   Another fun part is the fact that virtually all the copies had completely interchangeable parts and could be mixed and matched with its function intact. 

Over the thirty years, I have completely redesigned the regulator three times, always adding features and improvements.  Sure enough, our latest design, introduced four years ago, has been copied four times.  All copies appear to be interchangeable once again.  It took me a while, but I finally figured out why copiers made such an exact copy; THEY DO NOT UNDERSTAND HOW IT WORKS.  A bit of evidence to that fact is that I personally know one of the copiers, and he does not have the foggiest idea of what makes it tick. 

Virtually all of the copiers have referred their troubled customers to me for assistance.

Oddly, I do not object.  It is my design and I can fix whatever problem one may have with it. In so doing, please offer me one small credit; this regulator was designed by me, the original is marketed under the name of my company BEGi, and I can offer you the best (and perhaps, only) service after the sale. 





To Order: Call 830-438-2890 or fax at 830-438-8361.


Copyright © 2006 BEGi. All rights reserved.