Background and history

Welcome to Stock Touring LLC. Here, we will discuss the engineering goals in the USA and Europe in the 1960’s, the reasons they were different, some of the technical issues over there, here, and in general, back then. If you are looking for the process we have to turn your car into a Stock Rocket that is under “process” in the website menu.  

When the 1964 Pontiac GTO was envisaged and planned, a decision was made for the goal of the design. This probably happened early in the planning stage around 1961 or ‘62. That decision was to gear the GTO for maximum quarter mile acceleration, which entailed using all available gears including top gear. This decision focused the GTO as a car which paralleled the “Hot Rod” post WW2 car culture in California, a powerful influence at that time.

Top speed was not considered as a goal, but reached at maximum motor rpm in top gear. It would be reached or nearly reached in a quarter mile of acceleration from a stop. This kept top speed relatively low. Maximum miles per hour was around 110 or 115 and occasionally up over 120, depending on which gearing of the various differential choices available was chosen. But, the choices generally stopped well short of those “taller” (lower numerically) which would allow the GTO’s maximum top speed potential to be achieved. 

There were several reasons for GM’s choice. The GTO’s tires, as available from major USA tire manufacturers such as Goodyear, were bias ply and not up to more than an “S” speed rating in today’s terms, 113 mph. Also, only drum brakes were available on the GTO, completely inadequate for stopping from high speed. They overheat and stop working instantly. Finally, there was an issue with front end lift on many American body designs and suspension set ups. 

Aerodynamics was not considered very much if at all, and drag was also significantly higher than was good for fuel economy, wind noise, or top speed. Around 110 mph front end lift was starting to make many of these cars a bit scary, and no one in their right mind would want take them much faster.

The Europeans developed a completely different philosophy for their sports cars and fast sedans. The American high power cars were leveraging the CA hot rodding post war focus on quarter mile acceleration and drag racing, partly for necessity and partly for convenience. The Europeans focused on high speed cruising on their superhighways, pieces of which were already in place in many countries including Italy, France, and Germany. We didn’t have these until 20 or 30 years later. 

They started to design their sports cars and high end luxury cars for maximum top speed potential, so “taller” final drive gearing than Muscle Cars had was normal. The run of the mill European sedan had a tiny engine and little power so the top speed was modest, but the philosophy was the same, top speed at redline in top gear. Whatever the theoretical top speed was the Europeans wanted their cars to achieve it.

European brakes were mostly disc type by the early sixties, and Michelin XAS tires were designed for up to 150 mph, although ratings were not like today. A 1963 Jaguar XKE was geared for 150 mph, and due to its slippery shape, small frontal area, and reasonable 220 HP (gross), it achieved that at maximum rpm in top gear, forth. The quarter mile was reached in third gear. It was no quarter mile screamer, but no slouch either.

Standard European practice became to match gearing so the power peak rpm, and the amount of power generated at that peak matched the drag supported by that power amount in top gear. This point was the top speed achieved. This was not necessarily “overdrive”, which usually refers to a transmission gear beyond the 1:1 top gear in the transmission, no, overdrive was typical much later in American transmissions. A less than 1:1 gear was considered “overdriven”, e.g., 0.7:1. Today, overdrive gears typically have such a “tall” ratio that top speed is in the gear before top gear, or maybe two gears before. A new Corvette with an eight speed transmission makes top speed in sixth.  

Up to a point, better fuel economy is available if even taller (lower numerically) gearing is used, gearing beyond that needed for maximum speed. But, Europeans were not using a taller final drive ratio aiming for maximum fuel economy back then. They were aiming for maximum top speed. It was both a philosophy and a policy. Decent fuel economy was a by product of this policy, as it will be for a Stock Rocket. 

Powerful European sports cars had slippery shapes which required a lower numerical final drive ratio to allow acceleration to the aero drag limit to achieve the potential top speed. Final drive ratio is the combined top gear ratio through the transmission and differential, A new Corvette has a 1.83:1 final drive in eighth gear, top gear. This is the result of a .33:1 overdrive top transmission gear and a 5.49:1 differential gear. 

In Europe back in the day, the more powerful sports cars’ fuel efficiency was not the priority, though it might have been of greater importance than in the USA due to how expensive fuel was. European sports cars had better fuel economy as a consequence of the ”taller” by ~30% final drive ratio necessary to achieve top speed. Today, we have transmissions with many more gears and the top gear(s) are overdriven to obtain maximum fuel economy. 

Otto Cycle non turbocharged gasoline engines run most efficiently at maximum intake manifold vacuum somewhere around the start of the the engine’s torque “band”, meaning where the engine starts to make about 70% of its maximum torque. Many engines have a broad rpm band where they make 70%- 80% of peak torque. Engines with a broad torque band, a “flatter” torque curve, have have better drivability than engines with narrower, “peaky er” torque production. 

Also, an engine which has 70%- 80% of max torque at a fairly low rpm is an engine that has good fuel economy potential because it can be geared down more and still have decent drivability. Low rpm reduces friction and pumping losses, which helps a lot. Even moderate valve overlap engines should have a torque curve with ~ 70% of their max torque available by about 2200 rpm (peak around 3600). And this is the rpm ST LLC will shoot for at a little over 60 mph, depending on the cd and the engine specifics. 

Think of the maximum point of fuel economy this way: you are driving a car on a flat, straight road and have to push the throttle maybe halfway down to maintain speed. This would make you want to shift down to a lower gear as the throttle response would be nil, almost no acceleration if you pushed it further down. That is gearing for maximum fuel economy. 

The final drive ratio, or “top gear” ratio, determines how fast your engine spins on the Highway at any speed. Now, for max efficiency some cars use a lower gear for 50 to 65 mph, and a top gear for 65+. Some throttle response is always preserved to help “drivability”. Drivability might actually help efficiency if it is not taken too far. Muscle Cars take it way, way too far. 

Many cars with 8 or 10 speed transmissions approach the maximum efficiency and fuel economy point in each gear at a certain speed, but none actually get really close to maximum efficiency because some degree of throttle response is more comfortable to have. Otherwise, your car would be downshifting on barely perceptible 2% grades. Some do. This would be annoying at a minimum, and not at all efficient overall for a three or four speed transmission car. This is because a three gear car that downshifts from 3rd (top gear) to 2nd has a large change in engine speed that would make the engine spin much faster than a gear change from 8th to 7th in a ZF eight speed automatic. Running in second is not efficient in a three speed car in any circumstance except at such a low speed that 3rd stalls the motor, theoretically. In the real world it won’t stall due to the torque converter stalling first, basically disconnecting the engine as a fluid clutch.

Engine speed changes as speed increases or decreases in any gear, so as an engine slows down to below its efficient operating rpm, the transmission will downshift to make the rpm’s higher. Surely you have experienced this. Of course, in a manual you have to downshift yourself. Downshifts by an automatic trans are designed or programmed to occur before the maximum efficiency point is reached, because that would be close to “lugging”, which is the point perceived by a driver as a warning message that the motor is about to stall. Actual gear choices and downshift rpm’s are always compromises for drivability.

Stock Rockets won’t get anywhere near a maximum efficiency gear, just a lot closer than the ridiculous gearing your car has now. The Europeans never got maximum efficiency in top gear because they were not aiming for it, but they got a lot closer than GTO’s, SS396 Chevys, Cobra Jet Mustangs or Torinos, or Dodge Hemi Cuda’s or Chargers did. Muscle Cars, were ridiculously under driven gear wise. It was almost a joke, except in a drag race. 

Let’s be specific, these cars “geared out” at 100 to 120 mph. They went screaming up to and past the motor’s power peak at ~5000 rpm to their ~ 5400 rpm (or whatever) redline, where spinning the motor faster meant possible damage. Whatever speed this equaled to was top speed. GM’s 3.23 differential gear came standard with automatic transmissions and air conditioning. This gearing was considered relatively “tall”. That 3.23 gearing put the engine rpm at 2485 at 60 mph with the average tire diameter on Muscle Cars of the day. And that gearing delivered a top speed of 121 mph at 5000 rpm, usually the maximum power rpm, and 126 to 130 mph if running to redline. 

Normally, a large 400 cubic inch displacement, high compression motor’s maximum efficiency should probably be at around 1200 rpm at 60 mph, but aggressive valve timing keeps muscle motors inefficient below around 2000 rpm, because no variable valve timing was available and they prioritized high rpm HP. 

Motors can have their valve timing mechanically optimized for any rpm (within maximum internal mechanical loads). Valve timing, including “overlap”, was a way to optimize high rpm power. Valve timing “overlap” means the intake valves open before the exhaust valves are fully closed. Muscle motor’s intake valves open much earlier than if the motor priority was max power at 2800 or 3600 rpm. Motors from the 1920’s and 30’s did that.

Overlap helps the engine breathe at high rpm, but it’s bad for low rpm efficiency. Plus, since motors have the characteristics of a pump, the faster it turns the more power it puts out, until it hits some rpm limit such as maximum rotating mass loads, valve spring response limits, or gases’ flow inefficiency. Hear that lumpy idle? That’s why variable valve timing was such a big deal when it was developed around 1990. Modern motors change the valve timing to be efficient at any rpm. Goodby lump, lump. 

Most GM Muscle Cars with manual transmissions in the late ‘60’s had standard final drive gearing of 3.55. This gearing had the motor spinning at 2700 rpm at 60 mph with the tire diameter of that day, and top speed was around 120 at redline. Now, even 122 cubic inch displacement 2 liter engines turn about 1400 rpm at 60 mph if turbocharged, maybe 1700 or 1800 if not. Today, variable valve timing and even variable valve lift are common. 

A 2017 Audi A5 AWD two liter four (122 cu in) runs at 1100 to 1300 rpm from 20 to 50 mph, using several gears, and speeds up to maybe 1400 at 60. Yet, it makes as much peak power as most base Muscle Car engines at 6000 rpm, and the car would do over 150 mph if it was not governed to 130 mph so as not to exceed the speed rating of its “H” rated tires. It’s aerodynamic, .25 coefficient of drag (cd) for the coupe, or .34 cd for the convertible. And since it has eight gears it still uses five in the quarter mile doing about 14 seconds flat at 101 mph with 253 net hp, better than most Muscle Cars. 

Optional axle ratios for Muscle Cars were normally higher numerically, not lower. This made it even harder to put down power, since the cars were always front heavy and light at the back. That’s not ideal, and spinning the rear tires was ridiculously easy. Fuel economy was terrible because the motors were rotating more than twice as fast with a 3.90 differential/ final drive (3000 rpm at 60) than is normal by today’s standards. Plus, the differential ratio was also the final drive ratio, since the top gear in the transmissions of the day was always 1:1. 

This setup was wasteful, limiting, and hurt the all around usefulness, expense to operate, durability, and day to day “livability” of Muscle Cars.