Scrigroup - Documente si articole

Username / Parola inexistente      

Home Documente Upload Resurse Alte limbi doc  

BulgaraCeha slovacaCroataEnglezaEstonaFinlandezaFranceza


Acquiring a Turbocharged Vehicle


+ Font mai mare | - Font mai mic

Acquiring a Turbocharged Vehicle

The essence of this book, if such exists, is to provide the performance car en­thusiast interested in turbocharging with a body of information that can be used to evaluate system designs, whether of a Factory turbo system or an after­market kit. This book is also intended as a design guide for the hobbyist who wants to build his own turbocharger system. Three viable methods exist to ac­quire a turbocharged vehicle;

buy an OEM-turbocharged automobile

buy an aftermarket kit, if available, for your specific application • build your own turbo system

The rationale behind the decision that suits your needs and requirements best is no more than a logical summary of the following:

What is the intended use of the vehicle?

What is the legality with respect to state and federal law and the year of the car?

How much power is required?

Is fear of a failure such that a factory warranty is required?

Can you make a reasonable judgment with respect to the engineering of

an aftermarket kit?

Do you have the skills, time, patience, and equipment to build your own?

Fig. 2-1. The Mitsubishi 3000GT turbocharged 24-valve V-6, Two turbos, two intercoolers, four-wheel drive, and 183 cid give the 3000GT extraordinary potential.

OEM-Turbocharged Automobile

Automobile manufacturers have build a variety of turbo cars in the last decade. One can easily wonder how some decisions are made. On one hand we have the Ford EXP Turbo, most Chryslers, and the Nissan NX Turbo. The other hand holds something like the Porsche 944, Buick GNX, and Lotus Esprit turbo, Members of the radical middle are large in number, relatively nondescript, and not entirely without merit. In most circumstances, the factory turbo engine is conservative in power output—easily understandable in view of warranties, li­abilities, and emissions requirements. Generally speaking, OEMs will not equip a turbocharger system with optimum-configuration parts. Virtually all OEM designs will have some shortcoming, whether in turbo size, intercooler capability, or restrictive exhausts. Occasionally the shortcoming is just a dif­ferent design, based on the OEM's perception of its buyers' requirements. Finding and fixing these weak links then becomes the focus of attention in ef­forts at greater performance.

** RULE: OEMs will generally provide you with a vehicle that functions nicely but is blessed with enough shortcomings that performance is far from optimum.

The first step in pursuing more performance is a complete analysis of the system design. Chapter 14, Testing the System, is your starting point. With those data accumulated and analyzed and the weak links identified, you can set out to find the necessary components to improve the system. Keep in mind that the issue here is to improve efficiency, thereby opening up the potential for huge gains in power. Increasing boost pressure is also a consideration, but without efficiency improvements, this path to power is fraught with mechani­cal risk. Once the system has been tested and the merit of each feature has been determined, start the improvement process with the weakest link. Here is where foresight becomes important. For example, an intercooler that loses only 2 psi at the factory-rated boost can be judged okay. It is okay, but only for the factory-rated boost. Likely it will lose 3 or 4 psi at any significantly in­creased airflow. That kind of loss is not acceptable.

Aftermarket Turbo Kit

The purchase of an aftermarket turbocharger system is an ideal occasion to employ this book as the guide it is intended to be. An investigation is necessary to determine the system that will meet your needs. Before a reasonable deci­sion can be made, answers to a variety of questions must be both sought and understood. The following samples will get you on the right track:

Does the system provide a correct air/fuel ratio at all operational condi­tions ?

The air/fuel ratio is a basic building block of a turbo system. It needs to be maintained over the boost range that the manufacturer claims for the kit. It is not to be expected that the air/fuel ratio will stay correct if the system's design limits are exceeded. In all circumstances, it is necessary to avoid discussing 'fuel enrichment' Either an air/fuel ratio is correct or it isn't—no 'enrich­ment' required.

Does the system provide a margin of safety on detonation ?

The attempt here is to determine whether the system installed and operat­ed per Instructions will yield useful boost and not be subject to detonation problems.

Does the system provide the necessary thermal controls to operate at the stat­ed boost pressures?

Ask for a description and explanation of these controls.

What efforts are extended toward quality control?

Fit and finish are obvious. Material selections, methods of welding, surface finishes, and other fabrication procedures should also be checked out.

Do the components carry a reasonable warranty?

Although warranties on performance-oriented components are frequently subject to severe limitations, the buyer cannot be hung out to dry. It is useful to discuss with the kit maker the warranty limitations and procedures necessary to establish the best warranty terms.

Fig. 2-2. This compre­hensive and complete aftermarket system for Honda CRX cars easily shows HKS's custom­ary attention la detail. Although non- inter­cooled, for reasons of cost, the system enjoyed many excellent features, a superb exhaust mani­fold design, fuel con­trols, and compressor bypass valving.

Fig. 2-3. The idea of a complete system takes on significance with the HKS Supra turbo. Note the oil cooler, flywheel, clutch, fuel injectors, spark plugs, and the entire exhaust system.

Are proper instructions offered with the system?

Instructions should provide all the necessary information to install, check out, and subsequently operate and service the turbo vehicle.

Will consulting be provided after the sale?

This is where the maturity of a turbo system manufacturer will truly show.

If the system is to he used on a public highway, is it designed with all emis­sions-related equipment in proper order, and/or is the system on EPA- or CARB exemption-order status?

In all states, the emission question will be the most important one.

When the answers to the above questions are satisfactory, it is time to get down to the fun details, such as compressor efficiency with respect to the sys­tem flow rates and boost pressures.

** RULE: All kit makers will try to represent their systems as the most power­ful. Absolute power is the last reason to make a decision.

Building Your Own Turbo System

Any reasonably able fabricator should have no serious difficulty designing and building; his own turbocharger system. Forethought, planning, calculating, sketching, and measuring, all done in considerable detail, will be the keys to the success of the project. Perhaps the single greatest problem facing the do-it­yaurselfer is avoiding getting stuck. Getting stuck is the phenomenon of 'You can't get there from here.' For example, you can't ever hope to intercool your turbo system if you build a draw-through carb type. Creating a high-perfor­mance piece for a 454 cid V-8 with a single turbo where a twin is clearly dictat­ed will decidedly put you in a position where you are stuck. Avoid going down these paths leading to 'stuck.' The first requirement is to determine the pow­er level desired. Translate that figure into a boost pressure necessary to get the job done. That, in itself, will determine the equipment needed. The remainder of the project is the sum of the experience contained in this book.

Fig, 2-4. The Callaway twin-turbo Corvette featured a thoroughly prepared engine, two Roto-Master Compact turbos, and inter­cooling. Note the low oil drains, collector sump, and belt-driven scavenge pump located at the lower right corner of the engine


Why is a correct air/fuel ratio necessary?

Basically, a correct afr means the engine is getting all the fuel it can effi­ciently burn, but not an excess. If you err on the rich side (the safer side), per­formance declines, because a rich condition louses up combustion temperatures. Lean mixtures lead to higher charge (in-cylinder) tempera­tures, promoting detonation.

What does 'fuel enrichment' mean?

'Fuel enrichment' means, in every aftermarket sense ever expressed, an indiscriminate dump of fuel into the system. It is indiscriminate because it does not care what the actual airflow is. Any kit maker who uses the phrase will usually supply the indiscriminate dump device. Don't ever ask a kit maker what he uses for fuel enrichment; rather, ask, 'How have you managed to maintain a correct afr, to how high a boost level, and can you prove it to me?' Every kit maker will respond that the necessary equipment to maintain a cor­rect afr is in the kit. Not necessarily so. Be sure the answers are correct, as this facet of turbocharging is of the greatest importance.

What are some of the devices for maintaining a correct air/fuel ratio ?

The worst device is none. It is perhaps the most popular. It is also the easiest to install. Another equally bad device is the boost-pressure-sensitive switch that sends a false water-temperature signal to the EFI brain. This is a wholly unworkable gizmo. It attempts to add fuel when under boost by lengthening injector pulse duration. While it can double fuel flow at mid-range rpm, it can add only about 10% more fuel at the redline. The nature of timed injection (like EFI) results in a situation where the length of an injector pulse for a maximum torque cycle remains essentially constant, regardless of rpm. That fixed injec­tor pulse length becomes a greater percentage of engine cycle time as rpm in­creases.

Fig. 2 5. The BMW 2002 is a superb street rod when equipped with a water-based inter­cooled turbo and two blow-through Mikuni 44b. Ten psi boost creat­ed 210 bhp and stock vehicle driveability.

The point, is finally reached where engine cycle time is the same as the  maximum-torque injector pulse time, and then the injector is open continual­ly. This is why an injector duration increase, by any device whatsoever, cannot supply enough fuel for a turbo engine at any upper-range rpm.

Further, all additions or subtractions of fuel are instantaneous incremental changes as the switch is activated, and nothing with a large instantaneous change in the afr can be correct. The result of the 'fuel enrichment switch' is at best a poorly running, detonation-prone engine, The EFI fuel enrichment, switch is the source of perhaps 75% of turbo-related horror stories. Avoid it.

Another popular scheme is to proportion 'fuel enrichment' according to boost pressure. While this sounds better and is better, it is still technical non­sense. The situation is created wherein the same amount of fuel would be add­ed at 3000 rpm and 5 psi boost as at 6000 rpm and 5 psi boost Obviously, fuel requirements would double at twice the rpm, but the boost-proportioned fuel­er would deliver the same quantity of fuel regardless of rpm. Not a workable mechanism,

The change to larger injectors is a valid approach to adding fuel. This gener­ally requires other changes to reduce the larger injectors' flow at tow speeds, so off-boost operation will not be too rich. This can be done by reprogramming the ecu or altering flowmeter signals. With boost pressures greater than 8 to 10 psi, the larger-injector approach is a necessity.

Another popular device is to send the lambda (tailpipe oxygen sensor) sys­tem to full rich when under boost. Lambda systems have control of approxi­mately 8% of the fuel delivery Combine that with 50% more air [7 psi boost) and the engine becomes intolerably lean. This method is unfortunate, at best.

Fig. 2-6. A straight­forward, low-cost design from Perfor­mance Techniques for the Mazda Miata. The absence of an inter-cooler and compressor bypass valve keep boost down and the cost more affordable.

What is compressor surge, and how can it be countered?

Compressor surge is the rapid fluctuation of turbine speed caused by the throttle's being closed under boost. Rapidly spinning air compressors (turbos) can go unstable briefly when this occurs. The fluctuating speed can be damag­ing to the turbo and the accompanying noise is obnoxious. The condition can be alleviated with a compressor bypass valve that opens as the throttle closes and allows air exiting the turbo to vent back to the front This keeps the flow up. Many modern turbo cars are equipped with such valving, but seldom are they big enough to handle high-flow, high-boost systems, A useful fringe bene­fit to these valves is that they reduce lag and perceptibly increase fuel economy.

What is a reasonable price to pay for a turbocharger system?

The lowest-priced system that offers

a correctly sized turbo

a correct air/fuel ratio under boost

boost control by controlling turbine speed

proper ignition timing

proper thermal controls

a margin of safety on detonation

quality components

Such a system can put together a good argument for being the best value. It is popular to believe that you get what you pay for; but there are turbo kits costing nearly $4500 that do not have a correct air/fuel ratio or even an iron exhaust manifold. Conversely, kits are available that have alt the above at a price less than $2500. A reasonable price? This must remain the prospective buyer's decision, based on a thorough knowledge of what he gets for his money.

What paperwork should he included with a turbo kit?

Instructions and warranty are self-explanatory. Cautions and operating procedures must be well detailed and conservative.

What are the warranty implications of installing a turbo in a new automo­bile?

All factory warranty on drivetrain components will be voided. There are, however, several circumstances to consider. You can purchase an aftermarket warranty to cover your vehicle for all non-turbo-induced or -related problems: It is currently in vogue to sell these policies with turbo systems under the intended misconception that your drivetrain is warranted against 'turbo-in­duced' failures. Not so.

Fig. 2-7. A simple, effective, low-boost system for the small-block Chevy. Note the additional fuel injectors, lack of intercooling, and warm-air pickup for the filter.

If one breaks his turbo engine, it is not going to be paid for by anyone's war­ranty—exactly the same situation as waiting until the factory warranty ex­pires and then adding the turbo. Which means that waiting out the factory warranty before installing a turbo accomplishes nothing except insuring that the mechanism is one-third used up pre-turbo. Furthermore, it eliminates the fun of ever owning a nice new automobile with enhanced power. It is rare for a modern automobile to have an engine/drivetrain problem within the warranty duration. Those problems that do appear are generally minor and will likely cost under a hundred dollars to repair. To preserve the warranty for many thousands of mites to avoid a possible hundred-dollar component failure rath­er than enjoying the extra performance seems to me the poorer choice. To as­suage your concerns, call the car maker's regional office and discuss with a service rep the areas of the drivetrain that have been a warranty problem.

Fig. 2-8. Turbo Engineering produced this low-mounted turbo specifically for the Chevy Camaro, With intercooling, a large series TO4 turbocharg­er and generous flow paths, this system could offer power levels in excess of 500 bhp.

Politica de confidentialitate



Vizualizari: 966
Importanta: rank

Comenteaza documentul:

Te rugam sa te autentifici sau sa iti faci cont pentru a putea comenta

Creaza cont nou

Termeni si conditii de utilizare | Contact
© SCRIGROUP 2023 . All rights reserved

Distribuie URL

Adauga cod HTML in site