APPLICATION OF POWER-TO-WEIGHT THEORY

By 460-BBF-Turbo-In-CC (from the legendary Car Craft turbo thread**)

"If you don't know where you're going, you'll probably end up somewhere else."

Let's say our project car is a typical V8 compact or intermediate that weighs ~3,200 lbs. Add in the following:

120 lbs. for a tank of fuel (Gasoline 6.2 lbs./gal.)
200 lbs. for extra safety equipment required by track rules
200 lbs. for "Driving ballast" (you)
50 lbs. for the various jetsam that accumulates in a real street car (tools, maps, lawn chairs, etc.) or maybe some actual ballast to improve weight distribution.

Now the car's tipping the scales at nearly 3,800 lbs. Now let's say that you've figured out that to humiliate some arrogant jerk in a Viper ACR or a Corvette ZR1 that you need at least 140 m.p.h. in the quarter mile to be safe.

Recall that 140 m.p.h. requires a minimum of one horsepower for every 4.67 lbs. So 3,800 divided by 4.67 = 814 h.p. Factor in an extra 15% "reserve" and the horsepower target is 936 h.p. Note that this assumes that the car goes through the timing traps at the horsepower peak. If it doesn't, you'll need a higher peak to make sure that the car is making the minimum required power at the point on the torque curve that the car's gearing causes it to reach the timing traps.

If we up the target to 150 m.p.h. (3.79 lbs per pony), then we need a minimum of 1003 h.p. Add in the 15% "fudge factor" and the PLAN target is 1153 h.p. It should be clear now that quarter mile speeds over 120 m.p.h. in a typical heavy street machine are going to require in excess of 500 h.p.

And the power requirement escalates dramatically as speeds inch upward (i.e. raising the trap target from 140 to 150 requires around 200 more h.p. in our example).

FIRST PRINCIPLES: FORCE EQUALS MASS TIMES ACCELERATION

By 460-BBF-Turbo-In-CC (from the legendary Car Craft turbo thread**)

"Force Equals Mass Times Acceleration (F=MxA)"

The first step in developing a PLAN for where you want to end up is determining how much power you're going to need to get there. For most Car Crafters, the "holy grail" of high-performance is power-to-weight ratio. It's the most fundamental relationship.

And for an overwhelming number of Car Crafters. power-to-weight is best benchmarked by standing-start acceleration over a quarter mile. Decades ago, Chrysler engineers worked out the math for how much power it takes to hit a target trap speed in a production-based vehicle at the end of a quarter mile.

Based on their formula:

M.P.H. Lbs Per Horsepower

100 m.p.h 12.82 lbs/hp

110 m.p.h 9.61 lbs/hp

120 m.p.h. 7.40 lbs/hp

130 m.p.h. 5.38 lbs/hp

140 m.p.h. 4.67 lbs/hp

150 m.p.h. 3.79 lbs/hp

Now to avoid disappointment and account for differences in measuring horsepower, mechanical efficiencies, and other factors, I'd add in 10-15 percent more horsepower, just to be safe.

Notice that these results don't mention engine size or overall weight. A big, heavy car with ~ 5 lbs/h.p. should have roughly the same trap speed as a small, light car with ~ 5 lbs/h.p. (excluding aerodynamic effects). It just takes more "engine" (and budget) to make a heavy car as fast as a light one.

The results don't mention [elapsed time] (E.T.) because "quickness" tends to be influenced by more factors than effective h.p. More often than not, E.T. is a measure of how effective a car is at relatively low speeds in the first third of the quarter mile. But unless you're doing something really wrong (i.e. running FWD, horrible weight distribution, "no traction" tires, "peg-leg" differential . . . .) you should be able to obtain at least the following under proper conditions on the street:

Lbs/Hp E.T. Zone

12.82 14s

9.61 13s

7.40 12s

5.38 11s

4.67 10s

Obviously, a well-sorted drag-oriented supsension and better tires will produce quicker E.T.s, but for conservative PLAN purposes, these numbers will get you "in the ballpark" on how much power to build for.

**Reposted with permission of the author.

LIMITATIONS ON HOME-BUILT TURBO PROJECTS, PART III

“Genius has limitations, stupidity is boundless” – Anonymous
By 460-BBF-Turbo-In-CC (from the legendary Car Craft turbo thread**)
LIMITATIONS (Part 3)

7. “The Man” – Car Crafters in many states have to watch out for the long-arm of the law. In fact, CC’s failure to do a big-cube turbo project is probably in part because CARB hates retrofit turbocharging. And even in the states where there’s still a little liberty left, Car Crafters face restrictions on noise, how much engine can be hanging out, and other legal limits.

8. Sanctioning body rules – If you’re going to run in any organized competition, then sanctioning body rules are going to be a limitation. For turbo Car Crafters, it may mean limiting the number of “power adders” and adding extra safety equipment. It probably will require adherence to specifications on fuel, fuel additives, cooling strategies, wiring, and plumbing.

9. Reliability and maintenance – This one is the fusion of time, money, parts quality, and practicality. At least until recently, Austin Coil could rebuild John Force’s AA/FC after every quarter mile pass. But most turbo Car Crafters expect their creations to last for thousands, if not hundreds of thousands of miles. And while some elevated maintenance may be tolerable, constant fussing and repairs get old fast. So the PLAN for a realistic street/strip turbo car has to take into account reliability and reasonable service intervals.

If you don’t know your limitations going in – or ignore them – then the outcomes of your turbo build will be waste and frustration, if not expensive disaster.

**Reposted with permission of the author.

LIMITATIONS ON HOME-BUILT TURBO PROJECTS, PART II

By 460-BBF-Turbo-In-CC (from the legendary Car Craft turbo thread**)

"The man with insight enough to admit his limitations comes nearest to perfection.” – Goethe

4. Fuel quality – The trickest engine in the World is useless if you can’t obtain or afford its fuel. The most obvious limit on the amount of power you can jam out of a street/strip engine is going to be fuel quality. A few lucky Car Crafters have access to E85. An even smaller number will put up with the hassle of using racing fuel. But everywhere else, practical street/strip cars have to burn weak pump gasoline most of the time.

5. Parts quality – It does little good to PLAN a 1000 h.p. turbo engine if you’re using a production block and reciprocating assembly that self-destructs at ~ 600 h.p. So we’ve got to be realistic about how much abuse we can expect the parts we’re forced to work with will absorb.

6. Loyalty and style – Most turbo Car Crafters are going to build a make and model of car that appeals to them for reasons other than pure objective performance. That usually means the turbo project car is going to be heavier, less aerodynamic, and less weight-balanced than optimum.

And our powerplant choices are also often made based on personal history, brand loyalty, or whatever we’ve already got. They’re not usually based on what is the best engine design available to meet our chosen performance parameters. But a great thing about turbocharging is that it can be a huge equalizer for those committed to “obsolete” engine designs.

Moreover, many of the choices we make in crafting a project car are for reasons other than hitting a performance target. As David Freiburger once observed, this sport involves a big fashion component. It also nurtures some fairly strong traditions and peer pressure. Thus, one of our limits will be our self-imposed sense of what looks and feels “right” without regard to objective performance.

The GN/Type-T Regal is a perfect example of a turbo project car that is often picked for reasons other than optimum performance potential. Objectively, they're boxy. They're relatively heavy. They have poor weight distribution. They're saddled with an obsolete two-bolt main six and a cast crank. The cylinder heads are fairly restrictive. The factory turbo system was hardly optimized. And in stock form, they're down about 80 h.p. or more to many modern DOHC V6s.

So from a purely objective assessment of performance potential, there are plenty of better choices for a turbo project car. But over the past 25 years, Turbo Buick Car Crafters have often built these cars to be brutally fast and quick! And we're all glad that they have! The point here is that if you're carrying a torch for something that's off-beat or that has less than optimal hop-up potential, you should be honest about it going in and adjust your PLAN, budget, and performance goals accordingly.

**Reposted with permission of the author.

LIMITATIONS TO BIG-CUBE HOMEBUILT TURBO PROJECTS, PART I

BY 460-BBF-Turbo-In-CC (from the legendary Car Craft Big-Cube Turbo Thread)

A man’s got to know his limitations.” – Harry Callahan, “Magnum Force” (1973).

The Jetfire V8 and recent Indy V8s can only provide limited guideposts for Car Crafters on turbocharging.

The comparison of these engines does show that success in turbocharging (huge torque & massive horsepower) depends on more than just sticking on any old turbo laying around.

But neither the Jetfire nor the Indy V8 would be good street/strip mills. Back in the early ‘60s, Oldsmobile dealers were often busy replacing blown head gaskets or removing Jetfire turbo systems and replacing them with four-barrel carbs. Bore corrosion was epidemic. So the Jetfire stands as a cautionary tale of what not to do.

And an Indy V8 on the street would produce horrible fuel mileage and unsatisfactory power below 5,000 r.p.m., even if you could afford the huge “buy-in,” packaging, and servicing costs. So Indy engines merely suggest general areas for improvement of street/strip turbo powerplants.

Thus, a practical PLAN for turbocharging a OEM production street/strip engine means building within the walls of a vastly different “box” of needs and performance expectations.

But before we identify our performance parameters and goals, we’ve got to take stock of the outer limits. We’ve got to know our limitations.

These limitations should be regularly consulted as we PLAN and build. They are a “reality check” on our turbocharged enthusiasm and creativity.

LIMITATIONS (Part 1):

1. Money – The big limit is always going to be money. While it’s not the same sum for every Car Crafter, everyone has a maximum budget. The only way a home turbo project becomes feasible for most Car Crafters is by avoiding the expense of custom-built parts and costly “experts” whenever possible. Thus, turbo Car Crafters are mostly stuck with mass-produced OEM and aftermarket parts and self-help labor. What is cheap and readily available often trumps what would be best.

2. Time – A close cousin to money is time. Most street/strip turbo Car Crafters are not engineers. Most are not professional fabricators. Most cannot spend every waking hour ironing out build problems or contemplating turbo tuning theory. So something that’s quick, relatively simple, and easy-to-build is almost always better than something so time-consumingly “perfect” that’s never completed.

3. Practicality – If it’s not practical, we won’t drive it very much. It’s that simple. A street/strip machine that guzzles race gas, or idles at 1500 r.p.m., or won’t start when it’s cold, or has to stop for refueling every 75 miles, or takes three hands and three feet to operate, or hides small children behind its giant hood scoop won’t stay attractive for long in the “real world, ” regardless of how “wicked” and “gnarly” it is in our imagination.

PLAN back from where you want to end up.

BY 460-BBF-Turbo-In-CC (from the legendary Car Craft Big-Cube Turbo Thread)

To some Car Crafters it's self-evident why turbocharged Indy V8s "processed" more than four times more air mass than the 1960s Oldsmobile Jetfire V8. Others may just throw up their hands and mutter something about "racing engine" and "unobtainium."

But we can learn important things about planning PLAN from the Indy V8s.

While the Indy V8 engineers may have had huge budgets and the liberty to design purpose-parts (something most Car Crafters will never have), they were forced to design in a fairly restrictive rules box.

THE GOOD: The "N Factor" (number of combustion events per minute) of PLAN was rules-limited to eight cylinders and 12,000 r.p.m (although development had once taken r.p.m. levels as high as 15,000). That's 48,000 cylinder firings per minute at redline. That sounds like a lot until you realize the cubic-inch displacement factors (L & A) were limited to 2,650 cubic centimetres (161.7 cu in; 2.65 liters) That means each cylinder was only a little over 20 cubic inches.

The engines needed to last only 700 miles or less between major rebuilds. The designers were free to use many premium materials, dual overhead camshafts, multiple valves per cylinder, multi-point fuel injection and other strategies to optimize high r.p.m. power.

Fuel was detonation resistant methanol. Thus Factors A (area) and L (stroke length) were not as dependant on avoiding detonation (more on this later). And relatively high compression could be employed to extract more work from the "working fluid" (fuel + air).

THE BAD: The Indy V8s were limited to one turbocharger and very low intake manifold pressure. They could not use charge cooling (although it was hardly necessary considering the cooling effect of alcohol fuel and the low "boost" levels).

They needed to be responsive over a moderately wide r.p.m. range. They also needed to be a structural member of the chassis. But weight and packaging were restricted by chassis aerodynamics and handling considerations.

Fuel consumption and on-board capacity were both limited (limiting the amount of fuel enrichment for in-cylinder cooling). The engines needed to produce enough power to overcome extremely high-drag bodies at speeds of more than 230 m.p.h. and to accelerate 1500-1600 lbs from 35-150 m.p.h. harder than most doorslammers several times per lap on road courses.

So how did they make it work?

Mostly by concentrating on maximizing Factors P & N within the limits of the rules.

Optimizing Factor P (cylinder pressure) on low "boost" required all of the standard "all motor" hop-up techniques: Generous breathing, optimized by oversized bores (Factor A), large valve curtain area (the total amount of valve opening), large straight ports, pressure-wave tuned intakes manifolds and exhaust headers. They also increased efficiency through high compression and lowered crankcase backpressure by dry-sump scavenging (less air resistance to the power strokes from the back of the pistons).

The aerodynamics, bearings, and rotating assemblies of indy car turbochargers were also optimized for efficiency and improved responsiveness(more Factor P quicker, at lower r.p.m., and with less charge heating) Even the intake air bell was aerodynamically engineered to swirl the air into the turbocharger compressor to improve its efficiency.

To maximize Factor N, they built everything to withstand and enable sustained high r.p.m. They moved the torque peak closer to the rev limit. Stroke length (Factor L) was reduced to serve the needs of Factors N and P (bigger bores allow for more valve curtain area; shorter strokes reduce loading on the reciprocating assembly) They also helped get more of the work accomplished by Factors P & N through to the wheels by targeted reductions in rotating mass and reduced oil windage.

It should be clear that the success of the turbocharged Indy V8s didn't occur from just slapping a turbo on the engine and calling it good. The engineers instead focused on where they wanted to end up and the restrictions placed on getting there. Then they adjusted the PLAN factors to achieve their targeted results.

Fail to PLAN, plan to fail.

BY 460-BBF-Turbo-In-CC (from the legendary Car Craft Big-Cube Turbo Thread)

Corky Bell's celebrated turbo book Maximum Boost: Designing, Testing, and Installing Turbocharger Systems introduces readers to the acronym PLAN to explain power production:

P -- Pressure (as in cylinder pressure)

L -- Length (as in stroke length)

A -- Area (as in bore area)

N -- Number (as in number of combustion events per minute)

Bell uses PLAN to show why turbocharging is more practical and cost-effective than most other forms of Car Crafting at boosting the output of production engines.

PLAN is also useful to explain yesterday's low boost comparison. The Indy V8's greater ability to consume air (and thereby extract power from fuel) is the result of optimizing PLAN within restrictive rules. The Jetfire's weak numbers are mostly from ignoring PLAN.

First, let's look at the Jetfire:

The Jetfire V8 used an early, less-efficient, smallish turbo compressor drawing through a restrictive gasoline carburetor. It forced the hot supercharged air-fuel mix into a constrictive cast iron intake through small ports regulated by a mild camshaft.

Although the Jetfire's static compression was in excess of 10:1 (which in many driving conditions jacked up peak cylinder pressure beyond the detonation threshold), it exhausted the waste gasses into simple log manifolds that joined together before a single scroll turbine section. The turbine exhausted into a small, restrictive exhaust and muffler system. Maximum effective RPM was less than 5,000.

In terms of PLAN, average cylinder pressure (Factor P) was held down by inefficient induction. It was artificially spiked by high compression, but that did not increase the amount of "working fluid" (air and fuel) available for combustion and ultimately led to less available power than an equivalent peak cylinder pressure produced by other means. Heat and kinetic energy use was not optimized.

The number of compbustion events per minute (Factor N) was also limited by induction design, cam timing, valve size, port size, short-block design, materials and exhaust design. (Pumping up Factor N is usually the most expensive way to increase power because it requires either adding more cylinders or increasing R.P.M.)

Thus, although the Jetfire has a small advantage in displacement (Factors A & L) over the turbo Indy V8, it is far outweighed by other factors limiting mass air flow.

As Bell points out at some length, Factors A & L are generally hard to change and require huge increases to yield big results. Spending $2,500+ on boring and stroking most engines will typically produce only a faction of the power of the same investment in a turbo system.

On the other hand, starting with a big-cube V8 can reduce the need to hike the P and N factors to budget-crushing levels to obtain tire-melting power!