Top Gears

I'm getting all pumped up for my vacation in a week when all of the parts that have been specified have been delivered. I'll have mostly clear decks to go into the garage and work as hard and as fast as I can on this project that has gone through several retrenchments. Someone pointed out a nasty side effect of switch to Subaru power eight months ago and I've been looking at it since. It's called gearing.

While any number of adapter plates can put all sorts of engines into your VW, that doesn't mean that your VW transmission is going to appreciate 2-3 times the horsepower than it was designed for. Not only that, but you might not much like the experience either.

The VW transmissions were built with a basic idea about their power plant that goes all the way back to the Nazi era KdF-wagen: Top speed is cruising speed. In the KdF-wagen 100kph was as fast as the vehicle was expected to go, so everything about the engine's design was made to meet that goal and not a bit more. That means that the RPMs required are quite high by comparison, not for acceleration, or passing, but just for cruise: maintaining a mostly constant speed. So the engine is designed to produce its peak power and torque at the RPM needed to maintain cruise. At any RPM faster than max power and torque, both fall off rapidly: You can turn the engine faster, but not more productively.

So let's use an example to wrap our heads around this: The 1977 VW Bus has a designed top speed of 75MPH. With the right size tires on (185R14C which are a type of truck tire) the tire stands 25.7 inches tall. Every rotation carries you a certain number of feet forward. The taller the tire, the further forward you travel per rotation. So, pass the diameter of the tire, the engine RPM and the various gears in play in the transmission, and you can get a nice chart of what speed you'd be going in 4th gear for any given engine RPM. Confused? Relax. Here's what we've got, given a completely stock engine and transmission setup from the factory. (A 1 to .82 gear ratio in 4th, e.g. one rotation of the tire for 0.82 rotations of the engine.)

Speed (MPH) per RPM
2000	2500	3000	3200	3400	3600	3800	4000	4200	5000	5400	6000	6500	7000	7500	8000
38	47	56	60	64	68	71	75	79	94	101	113	122	131	141	150

So the top speed of 75MPH is achieved by turning the engine at 4000RPM in 4th gear. Coincidentally, maximum torque for the stock 2 litre Type4 engine is achieved at 3000RPM, and maximum power at 4200RPM. So 75MPH is a good spot for that engine: it can work at that RPM effectively and efficiently for a long time. It's designed that way. The maximum torque starts falling off after you've reached freeway speed at 3200RPM (60MPH) and then it's all about horsepower to keep things turning that fast or a little better.

As vehicles got heavier over the years, different gearing was installed in transmissions to match it to the peak efficiency of the engine, trying to stay in the sweet spot. The engines and their output kept changing too, as manufacturers struggled to comply with emissions regulations, usually resulting in engines that were less powerful: A constant dance between the weight of the vehicle, the power of the engine, and the transmission in between trying to mediate the constantly flexing relationships between the two.

So why the lesson in ratios? Because the EJ22 SOHC Subaru has a very different power and torque profile. It produces so much more power and torque than the poor old Type4 many people just assumed that it's a win to just swap the engine. What they don't count on is that the whole rest of the drivetrain, from the clutch all the way out to the tires, is expecting 4000RPM for cruising speed. This means that the EJ22 is capable of wailing away under the rear deck at 4000RPM and still have more power available. But it's LOUD. The RPM required to make the engine happy (where the EJ22 makes the best balance of torque and power) is much lower.

My EJ22 is from a 1997 Impreza Outback Sport which has an unladen weight of 2915 pounds, 127 pounds lighter than the 3042 pounds of a 1977 Deluxe Bus. So what RPM would the EJ22 cruise at if it was at home, coupled to its favorite Subaru 5MT transmission in a similarly weighted vehicle?

Speed (MPH) at RPM
2000	2500	2850	3000	3200	3500	3800	4000	4200	5000	5400	6000	6500	7000	7500	8000
43	54	62	65	69	76	82	86	91	108	117	130	140	151	162	173

Aiee! 4000RPM, while well below redline for the engine, is still flailing away making a lot of useless sound and fury for no good reason. To produce good cruise behavior, this engine only needs to be doing 3450RPM. Something has to give: either the engine needs to be completely redesigned or at least get new valve cams at a minimum, or the transmission in the Bus needs to be a closer match for the engine. Messing with the engine's behavior is expensive and a little ridiculous. Putting the Impreza 5MT transmission in the Bus is do-able, but very expensive. Having the 091 transmission in the Bus rebuilt with different gearing is not cheap, but it's less expensive than either of the other options. But there's a third option that has real merit...if you're willing to embrace some 'lifestyle changes.'

Remember I said that the tire diameter is part of the equation, moving forward a certain distance for each rotation? If you increase the diameter of the tire, you go further for the same number of rotations. That means, working backward from the tire, through the drivetrain and gearing to the engine...the engine turns more slowly. So it's only a matter of playing with the tire diameter until you find the (pun intended) golden ratio. Make the tire too big and it will have rubbing, grabbing and interference issues with the suspension and the body. So there's an upper limit. Thankfully, there's a tire that will fit, and produce an improved change of total ratio: instead of the 185R14C at 25.7 inches tall, we replace it with an Offroad/Onroad multi-purpose 27X8.5R14LT. (Yes, the sizing format changes to an older truck tire standard. It's roughly the same as a 215/70R14, a nick bigger at 26.8 inches tall.) 

So what do we get for running a big, scary off-road/on-road truck tire?

Speed (MPH) per RPM
2000	2500	3000	3200	3300	3600	3800	4000	4200	5000	5400	6000	6500	7000	7500	8000
39	49	59	63	65	71	75	79	82	98	106	118	128	137	147	157

Now we're getting somewhere! It isn't perfect, and 3800RPM is still much higher than it needs to be relative to the 3450RPM the Subaru 5MT gearbox would need for 75MPH. If you want to do better, you're going to need to regear: either a different transmission, or the same transmission worked over with different ratios of gears.

Fortunately for me, I don't have any desire to go 75MPH in my Bus! It is a 40 year old vehicle now, and has a reputation for far too much 'float' in the front end at high speed. I'm not in such a hurry to meet my Maker that I want to spend most of my time at that speed. So a John-Law approved 65MPH will put me at 3300RPM with gobs of headroom in torque and power if I need it. That's about the best I'm going to get without the $1700 for a rebuilt and custom re-geared 091 Bus transmission, or a rebuilt 5MT specially modified to join a VW Bus: $4150.)

I have to buy new tires anyway: the last time the tires on my Bus were healthy was when Clinton inagurated. They hold air...barely. So it's a bargain at $500 for four tires (General Tire - Grabber AT2 27X8.5R14LT / Load Range C) that will provide me many years of roadworthy travel and may lead me into adventuring in places that street tires might not take me. Everybody wins.