Engine Speed: Will the Larger, More Powerful GW Engine Lower My RPMs?

Q: Will my new, larger and more powerful GoWesty engine spin more slowly, especially on the freeway?

We get this question all the time (hence, the article). The short answer is NO, and here's why: The relationship between vehicle speed and engine speed has nothing to do with the engine. It is the transaxle and the size of your tires that establishes this relationship. It is true that our GoWesty engines produce more power and torque at a lower engine speed, and that's where the confusion starts. The engine has the capability to move the vehicle at any given speed at a lower engine speed, compared to the original engine. But just changing the engine does not affect the relationship between engine and vehicle speed. To change this relationship—and take advantage of the higher performance our engines offer—it is necessary to change the tire size, transaxle gear ratios, or both. 

One way to lower the engine speed at any given vehicle speed is by increasing the tire size. A larger tire will spin more slowly at a given vehicle speed than a smaller tire. Simply increasing the tire size will lower the engine speed by the same percentage as the tire size difference. For example, if you start off with a tire that is 20" in diameter and increase the tire diameter to 22", that's a 10% increase. This new, larger tire will spin 10% slower, and so will your engine. So, if at 75 mph in 4th gear your engine was spinning at 4000 rpm, with the larger tire the new engine speed will be 10% slower, or 3600 rpm. Make sense? The only other way to change the relationship between engine speed and vehicle speed is by changing the gear ratios in the transaxle.

Changing the final drive (ring and pinion set) has the exact same effect on engine speed as changing the tire size. That is, raising the final drive ratio by 10% will lower engine speed in all gears by the same percentage. So, if you were to note your engine speed in 1st, 2nd, 3rd, and 4th gears at 20 mph—and then change either the tire size or the final drive ratio—you would notice the exact same percentage change in every gear. 

Changing individual gears within the transaxle will affect the vehicle/engine speed relationship in the same direct way. However, the change will occur only in that gear. If, for example, you are happy with the engine/vehicle speed in all gears except 4th (at highway cruising speed), you could just change the 4th gear. What you have to be careful about is not creating too large a "gap" between the gears. That is, making the ratio of the 4th gear so much higher than the 3rd gear that, when shifting into 4th, the engine speed drops too much and you don't have enough power to maintain your speed.

Typically, a careful selection of tire size, final drive ratio, and individual gear ratios is the best approach. Think about what you are experiencing with your current gearing and where you feel like it could be better. A knowledgeable Vanagon shop will know all the gear ratios available for each gear and final drive and can help you make the choice that is right for you, your Vanagon and the type of driving you do together.

Just like that.

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