 
The first two questions most people ask me about modifications to their cars are "How much horse power will it make?" and "How fast will it go?" My standard answer is "As much as you want to spend and as fast as you want to spend it".  The title of this section, Engine - Turbo - Transmission - Rear End - Tires, is written as a single unit, which is how you must approach building a turbo charged performance vehicle. Too often thousands of dollars are spent on engines which will never realize their full potential because no consideration was given to the optimum RPM range of the engine, the transmission gearing, the rear end gearing, or the correct tire size to keep the engine in that range. Even with all of these set up correctly, incorrect suspension and/or steering geometry (which is beyond the scope of this page) will render an otherwise beautifully prepared race car non-competitive. If you have looked at the pictures of the 'small' 2.3 liter engine setup on the MPC turbo charged sand rail in the Photos Section, you looked at an engine that has been run for 8 racing seasons using the same turbo charger, block, crank, rods, cam shaft, heads, clutch, transmission, and ring and pinion. It had over 500 races on it, each one starting with a launch RPM of between 8,000 and 10,000 RPM and turbo boost levels between 10 and 20 pounds. We never broke a major part! As part of normal maintenance, we changed cylinder jugs, pistons and rings, valves, valve springs, retainers and keepers, main bearings, rod bearings, and cam bearings every two seasons. Engine oil and filter were changed after every weekend outing while transaxel lube was changed at the end of every racing year. This is to illustrate that when parts are properly assembled, matched to their intended purpose, and maintained, they will last.
This page is made up of the following sections: Engine
- Base Theory
I prefer to consider an engine a heat and pressure generator and apply the same rule, the more heat and pressure generated the more power it will make. Using this concept tends to bring to mind that if the engine generates too much heat something is going to melt (like pistons, heads, and valves), likewise, too much pressure something is going to blow (like bearings, rods, piston tops and head gaskets) regardless of the RPM. Can I install a turbo on a totally stock engine?
Will it give me tons of horse power?
If I have the heads redone is this all I need to do?
OK, OK, I will have my block and heads machined for
bolt girdles and additional bolts to make sure there is enough clamping
force to stop parts from moving and gaskets from blowing, then I can create
tons of horse power right?
What I am trying to point out is that you should not think of a turbo charger as a bolt on piece of equipment, think of it as a system. A system that includes the turbo, a special intake and exhaust system, modifications to your fuel system, carb or injectors, heads, crank, rods, pistons, transmission gears, rear end gears, and tires. If you do not want to make all of the modifications, you will end up with something you will not be pleased with. Turbo
Charger - Base Theory
When considering a turbo charger most folks tend to look at the maximum CFM rating of the compressor and ignore everything else under the assumption that the compressor and the exhaust turbine are perfectly matched out of the box. I will grant you that in stock factory applications that is probably close to the truth but, in all out performance applications, nothing could be further from the truth because of the extremes of operation in a performance application. The goal in a performance application is to get the exhaust turbine up to speed as quickly as possible however, it must be mated to a compressor wheel that will generate as much pressure as it can as soon as possible. This is a contradiction because the exhaust turbine generates the drive power and the compressor consumes that power. The larger the compressor and the higher the pressure (boost) we want, the quicker the power from the exhaust turbine is used up. Put in a larger exhaust turbine and it will take the engine longer to develop enough hot expanding exhaust gas to spin it, slowing down the compressor and causing turbo lag. At this point I am going to repeat something stated earlier, do not think of a turbo charger as a bolt on piece of equipment, think of it as a system. The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better. I am sure many of you have seen pictures of turbo charged engines with cherry red hot exhaust systems and turbo housings. The captions under most of these types of pictures proclaim outstanding horse power numbers. What most of the articles related to these pictures do not tell you is that the engine was under an extreme load. A load so heavy that the engine was almost at its stall point for a prolonged period of time. A condition that most turbo charged engines will never see. The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load. There is where the selection of transmission gear ratios and the ring and pinion ratio play a critical part. The fact that the engine must be under a load is the reason why, no matter how high you rev a turbo charged engine with no load on it, you will not see the boost gauge move. This is also where the term 'turbo lag' came from. Turbo lag is basically the amount of time it takes from the time you place a load on the engine (stomp the gas peddle to the floor and dump the clutch or, get full converter lock up with your automatic trans) until the time the engine develops enough hot expanding exhaust gas to spin the turbine fast enough for the compressor to do its job. Effectively, a turbo charged engine is a normally aspirated engine until the turbine and compressor spin up. To minimize turbo lag, it is imperative that the turbine and the compressor are properly matched to the engine as well as the engine being properly matched to the transmission gears, the ring and pinion gears, and the tires. Transmission
- Base Theory
So why do we have 5 and 6 speed transmissions today? Todays drivers not only want top speed, but they want to get to it as quickly as possible. This is called acceleration, the word that all of us performance junkies are addicted to. Maybe you are old enough to remember the stir that was created when the 'close ratio Muncie Rock Crusher' transmissions came out in the 70s. Wow, what a difference those first close ratio gears made on performance! That increase was mainly because the engine was kept in its optimum performance range throughout the gear changes. With the advent of pollution controls, automobile manufacturers realized that the longer they could keep an engine in its optimum RPM range the better they could manage the pollutants created by their engines. The end effect is that most factory designed engines today operate in a low and narrow RPM range. To get the performance needed out of the narrow RPM range, they added additional gears. This has been the practice on race cars for many years, the main difference being that race car engines operate in a much higher RPM range. Clutch
- Base Theory
Rear
End - Base Theory
Now lets put a 30 inch long pipe on the end of the wrench and try to twist and break the bolt again. Bet you broke it without much effort. Why? Well you multiplied the amount of torque (pulling power) you could apply to the bolt by a ratio of better than 2 to 1 (no it was not 3 because the wrench was inside the pipe by at least 8 inches which made the pipe extention about 22 inches long). This same thing happens with the ring and pinion gear ratio in your rear end. The higher the ratio, the easier it is for your engine to twist the tires until they break traction. This applies to a 50 horse power engine as well as a 500 horse power engine. Obviously a 50 horse power engine will need a lot higher ratio than a 500 horse power engine to break traction with the same size tire.....that is one of the things you have to take into consideration when selecting the ring and pinion ratio. Tires
- Base Theory
Take a look at the tires on one of the salt flat high speed race cars, pretty skinny and pretty hard compared to what you see at the drag strip even though the speeds are about the same. The reason is because the salt flat cars are trying to reduce drag and rolling resistance to a minimum to achieve top speed without the primary concern of quick acceleration like the drag strip cars. Drag racers must achieve maximum acceleration at the cost of increased rolling resistance to win races and thus the contradiction. Over the years new rubber compounds have allowed gigantic strides toward the goal of maximum traction with minimum rolling resistance but because no two race cars are the same, it is still a balancing act to optimize it for each racer. Short
Block Preparation
As you probably have read in many performance magazines, trued surfaces on the block are absolutely critical if you expect other parts being installed in or attached to the block to mate up and function properly. Besides checking for correct head seating surfaces, all bores should be checked for roundness and proper clearances. Remember you are going to be building up pressure in parts of this engine that you would never approach with a normally aspirated engine. All of the bores fit and finish are crucial as too loose will cause excessive blow-by and/or scuffing and too tight will cause seizure. To minimize stress cracks, all edges on the block, crank, rods, pistons, flywheel, clutch, and timing pulley should be radiused. To cut down on windage in the oil sump area, 'knife edging' the counter weights on the crank is also beneficial. All of the mentioned moving parts should be balanced, including the valves, lifters and push rods. Valves, lifters and pushrods you say? Absolutely! Why spend a lot of time and effort putting together a set of matched valve springs only to assemble them with unmatched parts and then expect to get matching horse power from cylinder to cylinder. A common practice on VW and Porsche opposed cylinder blocks is to machine the block and install threaded inserts to allow the use of larger 10mm head studs in place of the stock 8mm head studs. This operation is needed for the additional clamping power required to keep the heads in place when running upwards of 15:1 compression ratios. On all out competition VW and Porsche blocks, two additional studs per cylinder are added for a total of six studs per cylinder instead of four. One other area you need to pay particular attention to is ventilation. The pistons and crank are generating the equivalent of hurricane force winds within the block that are changing direction in fractions of a second. To minimize the force that the crank and pistons have to work against, make sure that your blocks breathing and scavenge lines are large enough to avoid any restriction. Head Preparation
Besides checking for correct head seating surfaces, all bores and seats should be checked for roundness and proper clearances. Remember you are going to be building up pressure in these parts of this engine that you would never approach with a normally aspirated engine. All of the bores fit and finish are crucial as too loose will cause excessive blow-by and/or scuffing and too tight will cause seizure. To minimize stress cracks, all edges on the heads should be radiused. I am going to repeat that all moving parts should be balanced, including the valves, lash caps, retainers, and push rods. Valves, retainers and pushrods you say? Absolutely! Why spend a lot of time and effort putting together a set of matched valve springs only to assemble them with unmatched parts and then expect to get matching horse power from cylinder to cylinder. To do things right we need to look at: Intake and exhaust port size - while larger means more power at the high end it also means the loss power at the low end. Port matching - exhaust and intake - do you really want 'speed bumps' in those high speed tunnels? Valve spring seats - keep them absolutely perpendicular to the valve stems and watch their thickness. Valve springs - single and multiple springs, open and closed pressure, installed height, and valve stem length, coil bind. Valve spring retainers and valve locks - lock them down tight or allow them to rotate? Valve guides - what material and how much lubrication? Stainless or titanium valves - lash caps Valve seats Combustion chamber size and material left in the area Rocker arm geometry, rocker stands, shafts and shaft support Rocker arms and ratio - cheap horse power - cam bearing eater? Roller rockers and oiling - Pushrods - tapered vs straight - what does that have to do with harmonics? Intake Preparation
Exhaust
Preparation
Turbo Preparation
Initial
Timing
Nitrous
Oxide Preparation
Transmission
Gear selection
Clutch Selection
Ring
and Pinion Gear Selection
Tire
Size Selection
A rear engine car gets a lot more initial bite with the same size tire than a front or mid-engine car does. On launch the front and mid-engine car must transfer the weight back to the rear axle while in a rear engine car the weight is already there. The front and mid-engine car will always need a wider, not taller, tire (larger paddles for sand drags) for the same amount of initial bite. A taller tire can be used to fine tune the final gear ratio. Once you have determined the correct transmission gears and the correct ring and pinion ratios to keep your engine in it's power band, you may need to adjust the height, not width, for the same. To select the correct tire height, first determine the RPM that you need to run at. Next select the MPH you want at run at. Then pick the gear (1st, 2nd, 3rd, 4th, 5th or 6th). Adjust the tire height until both the RPM and MPH are where you need them. NOTE: Do not use tire height to try to make up for poor gear selection, stuff will break!
      
Last Updated 01-21-2008 If you have any comments, questions, or suggestions, email us at supportgroup@m-p-c.com Copyright © 1996-2008 Multi Path Communications All rights reserved |
