1979 MGB with Rover 4.2L V8 (owner: Mike Cook)
Major Design Considerations
Weight BalanceFRONT TO REAR: The ideal weight distribution for a front engined, rear drive car is 50/50 front-to-rear. Most small cars have a slight-to-medium weight bias toward the front of the car. Unless the engine you're installing is lighter than the one it replaces, the forward bias may well be increased. Consideration should therefore be given to moving the engine as far rearward as practical. Firewall and legroom considerations may have a limiting effect on location.
CENTER OF GRAVITY: If all else is equal, the lower the center of gravity, the better your car will handle, so the engine should be as low as practical. The limiting factors may be ground clearance or clearance to the steering rack (and/or front suspension crossmember). If the oil pan is too low, a guard should be fabricated to protect it from road hazards.
BrakingContrary to what may seem obvious, and what you will be told, increasing the power of your car does not necessarily create a need for better brakes! For street cars, unless you significantly increase the weight of the car by the installation of a heavier engine, braking requirements are not increased. Why not? Your road speed is more determined by legal limits and surrounding traffic - you simply reach terminal speed quicker. Simply because your car can now exceed (say) 140 MPH doesn't mean you'll ever drive it that speed. By the same token, common sense and safety concern will keep you from exceeding the speed limits much more then you already do. If your brakes will safely stop your car from a speed of 70 MPH now, they will still do so regardless of the available horsepower.
For "performance driving" (e.g. "track days"), because you can reach a higher speed quicker, your average speed will be higher. In that case, you will be using your brakes harder than before. As a result, it may well be appropriate to make brake system improvements to reduce "fading" (i.e. increased stopping distance due to increased brake pad temperature.)
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While it's true that braking power requirements on most re-engined British sports cars won't be increased, it's also likely your current brakes could be improved!
If you modify your brakes, it's absolutely VITAL you maintain (or create) proper front-to-rear brake bias. The last thing you want is for the rear brakes to lock up before the front brakes in panic stop situations. The braking power of a sliding tire is only a fraction of the braking power of a slowly rolling tire. If you car is at even a slight angle to the direction of travel - as it often will be in panic stops - and the rear wheels lock-up, the loss of braking power will cause the rear end of the car to come around. It's so much easier to control a car when it's pointing in the direction of travel!
If you plan to improve your brake system, it's highly recommended you go to the library and read a few good books on the subject. You have no business endangering the lives of other road users. Replacing the rear drums with discs without improving the front brakes, for example, may seem like a good idea, but it could prove fatal!
HandlingJust as with the brakes, the simple act of increasing horsepower does not necessarily mean an increase in handling capability is needed. However, just as with the braking, it's almost certain the car could use some handling improvement. If you're comfortable with the way the car handles now, and you haven't disturbed the weight and weight-distribution too much, there is no reason not to leave the suspension alone. On the other hand, it would seem a shame to have to slow down un-necessarily for corners. As we're talking sports cars here, it just makes good sense to improve the handling as much as your budget and need for comfort will allow. (Bone-jarring suspension setups can become tiresome if the car is to be used for "regular transportation".)
Drive Train PhasingOften overlooked, improper drive train phasing can be a source of severe vibration. There are two aspects of drive train phasing: u-joint orientation and engine/pinion angle.
U-JOINT ORIENTATION: Any time a shaft is driven at an angle through a u-joint by another shaft, there will be a speed variation in the driven shaft as the u-joint rotates. By inserting an intermediate u-joint into the driven shaft, the changes in speed can be cancelled. To do this, the yoke at the input of the shaft must be in the same plane as the yoke on the output.
ENGINE/PINION ANGLE: Speed variations in the drive train will only cancel out if they are equal. In order to be equal, the driveshaft angle as it leaves the transmission must be equal but opposite to the driveshaft angle as it enters the differential. In order to accomplish this, the engine/differential must be installed such that a line drawn through the center of the crankshaft is parallel to a line drawn through the center of the pinion gear. The crank centerline can be above, below, or to one side of the pinion centerline, but the two lines MUST be parallel.
The two lines must be parallel, but they must NOT be concentric, as the u-joints must be allowed to work to prevent bearing failure. For street use, the maximum angle should be limited to three degrees or less, but more than zero.
Tire Size / Rear Axle Ratio / Transmission Ratios / Speed CalculationsGreat care should be given to making sure that the wheels, tires, rear axle, and transmission gear ratios are all designed to work together. If you want a high speed race car, you will be very disappointed if the engine runs out of rpm at 70 mph because your overall gearing is too low. It doesn't matter how much power you have, when you reach redline, you also reach your top speed. By the same token, if you are building a dragster, you don't want to bog coming off the line because your gearing is too high. Horsepower won't do you any good if you leave the line in the equivalent of second gear because you used the wrong gear ratios or tire size. To make the decision easier, you may want to download the following Microsoft Excel spreadsheet. (The spreadsheet contains no macros.) To get a copy, click on the sample spreadsheet printout below.
Disclaimer: This page was researched and written by Dan Masters, and has been edited by Curtis Jacobson. It formerly had a "Cooling" section, but that has been removed because a new seperate article on cooling system design considerations has been added. Views are provided without warrantee or guarantee. Apply at your own risk.