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502 RAMJET Operating RPM Ranges
Thinking about the intended usage of the vehicle you will be installing your RAMJET engine into will allow you to make good decisions
on your total vehicle package. To get the most effective usage from your crate engine, you should attempt to design the rest of the
vehicle to complement the effective engine RPM range. Consider the weight, gearing (transmission and axle) of the vehicle, as well as
the vehicle speeds and geography (hilly or flat) that you will most likely be driving in the majority of the time.
The RAMJET 502 engine makes peak horsepower at 5100 RPM. A good rule of thumb would be a highway cruising speed that places
the engine at an RPM in the range of 60-70% lower than peak. Using this example would place optimal highway engine speed at
1500-2000 RPMs. Heavier vehicles (over 3500 lbs) would perform better using a combination at the higher end of this range and
lighter vehicles (under 3000 lbs) would be better suited near the lower end of this range.
Things that impact final gear ratio are:
• Tire Size
• Axle Gear Ratio
• Transmission Gear Ratios (with or without overdrive)
• Torque Converters (stall speed selection and/or with or without lock up feature)
Some handy formulas are included below to help make informed decisions. Insert your numbers into these formulas to help you
decide what will work best for your application.
Tire Diameter = Measured in inches
MPH = Typical highway cruising speed
Ratio = Final Gear Ratio (Transmission Gear Ratio multiplied by Axle Ratio)
Constant = 336
Final Gear Ratio = (RPM x Tire Diameter) divided
by (MPH x 336)
RPM = (MPH x Final Gear Ratio x 336) divided by
(Tire Diameter)
For example, using the first formula to estimate Final Gear Ratio:
2000 RPM x 30" Tire Diameter
IMPORTANT: Remember to include overdrive transmission ratios in your final calculation.
For example, if using a 4L80E transmission, the overdrive ratio is 0.75. In order to get to 2.74 final gear ratio, you would calculate as
follows:
2.74 (Final Gear Ratio)
0.75 (Trans O/D Ratio)
At this point, the estimated ratio you determine may not be a ratio that is readily available. You should then review ratio options higher
or lower depending on other factors listed above (personal preference and driving habits, geography, overall average vehicle speed,
vehicle final weight, ect...).
Remember that the examples given here are strictly for demonstration purposes and your results may vary. The consumer
assumes all responsibility for determining actual usage conditions.
Ramjet 502 Engine
TITLE
65 MPH x 336
=
3.65 (Estimated Axle Ratio)
MPH = (RPM x Tire Diameter) divided by (Final Gear
Ratio x 336)
Tire Diameter = (MPH x Final Gear Ratio x 336) divided
by (RPM)
= 2.74 Final Gear Ratio
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