Learn me "small" liter, high HP engines.

Everyone knows the beast below.  If you don't know it, you're wrong.
The heart of it is a Honda 600cc motorcycle engine and transmission.

One of the key things I saw from the Challenge this year was a reinforcement of what I learned in my FSAE days, and is a key tenet of motorsports.  Lightness makes rightness.  There were several tube frame chassis this year, and as everyone knows tube frames = light.  However in general people seem to ignore the engine aspect of the equation.  Iron engine blocks in tube frame cars, big ole honkin multi-liter aluminum engines and transmissions that one man could not lift with the strength inherent in his own body.

So, lets fix that.  What "small" engines pump out big HP numbers for low weight? Motorcycle engines are the easy button, of course, but what else is out there?  Jet ski engines seem to roughly be in the 1HP/lb range, but you'd need a transmission that could mate up.  UTV engines come with their own transmissions which would be useful; fabricate a makeshift driveshaft and connect it directly to a Miata IRS and shove it into god-knows-what.  It feels like slapping a Jetski engine into a $1000 X1/9, Fiero, or 914 would be a lot easier and lower CoG than a V8 or even a turbo I4.  Make an AWD, twin engine Corvair.  Maybe even make an angry Smart Four-Two, or a kei car

What other options are there?

Snowmobile is another logical option.

I think the power transmission part of the equation is what usually makes these options harder than just putting another car powerplant in any of those vehicles.

Snowmobile engines. Come with a compact CVT already attached.

ProDarwin said:

I think the power transmission part of the equation is what usually makes these options harder than just putting another car powerplant in any of those vehicles.

100% agreement, which is why I mentioned the UTV/Side-by-Side options.  I'd bet that some genius here could figure out how to mate a jetski engine to a T5.

So one thing to consider- do you want a car that can only be raced at the challenge?  If that's ok, then carry on.

If it's not, and you want to run some local events that use the SCCA rule book, be aware that car body classes require an engine that comes from a car, not a motorcycle or snowmobile.  

In reply to Mr_Asa :

Why would you want to go to the expense of a PWC engine and then mate it up to a big, clunky T5 that would probably suck half the power just to turn?

I really like the CVT idea.  If the car is fast enough, you won't have time to shift, anyway.

In reply to Pete. (l33t FS) :

Was just the first cheap manual transmission I could think of.  I'm sure there are others out there that would work.

alfadriver said:

So one thing to consider- do you want a car that can only be raced at the challenge?  If that's ok, then carry on.

If it's not, and you want to run some local events that use the SCCA rule book, be aware that car body classes require an engine that comes from a car, not a motorcycle or snowmobile.  

I'm not anywhere near a good enough driver to be competitive enough in any serious competition, so while I might do local auto-x with it, it would be in whatever class they deign shove me in.
Might be a fun weekend driver, though if it could be made reliable enough

I don't think you're going to beat sportbike for power/lb/$. The chain drive and lack of reverse isn't optimal for a car.

The thing to remember is people wreck sportbikes at an alarming rate. Snowmobiles and watercraft and UTVs aren't hitting the "well I guess I'd better part out to try and pay this thing off" threshold as often.

The big dog Yamaha snowmobile is apparently an R1 engine but with a separate-able transmission. But it's not cheap like an R1 engine is.

Robbie (Forum Supporter) said:

The chain drive and lack of reverse isn't optimal for a car.

I've been thinking about that, specifically for F-Dat. 
In general though, with a light enough vehicle your typical small block big 3 starter motor and the right gear setup you'd be able to whip something up.  Maybe include a cable operated tensioner that would pull the starter into engagement with the chain to prevent it from constantly spinning and wearing out the bearings?

I always wondered if you absolutely had to have reverse, why you couldn't use a geared starter motor to an appropriate sized ring gear on an axle, or intermediate sprocket to reverse at the push of a button. Zero drag, and only a small penalty in inertia from the ring gear.

As someone racing a car with no reverse it's not that bog of a deal. 

As for snowmobile engines the 850cc two stroke twins are putting out between 140-165hp. 

buzzboy said:

Snowmobile engines. Come with a compact CVT already attached.

Oh that is cool!!!!  Bet it is ear bleeding loud and so great all wrapped up together. 

The Rudezon's 2 stroke V8 seems to be good and light but maybe not small with the expansion chambers...

Appleseed said:

I always wondered if you absolutely had to have reverse, why you couldn't use a geared starter motor to an appropriate sized ring gear on an axle, or intermediate sprocket to reverse at the push of a button. Zero drag, and only a small penalty in inertia from the ring gear.

Ring gear and starter motor reverse systems are a thing in the offroad mini buggy world. Here's an example, looks like the ring gear even doubles as the brake rotors in this design.

https://theedgeproducts.com/collections/the-pircv-plans/products/cv2-quaife-conversion-plans-and-laser-files-digital-download

In reply to newrider3 :

That's f'ing awesome.  I love it

It's hard to beat a rotary for power density.

In reply to APEowner :

Except in reliability. 

In reply to Mr_Asa :

After seeing This SxS/Beetle mashup I'm really liking the idea of vehicles like this. 

In reply to Pete Gossett (Forum Supporter) :

That is gorgeous. 

Mr_Asa said:

In reply to APEowner :

Except in reliability. 

High performance rotaries are extremely reliable.  It's the anvil part of my rallycross RX-7.  It just keeps going and going and going.  (As long as you keep dust out of it, and have a decent cooling system)  I haven't had the refreshed engine on the dyno, but I'm running a pair of 1100cc injectors out to 85% duty cycle, so, it's "sufficient"   Peak torque is at about 4500 but "peak" is kind of a misnomer, as that is simply the highest point of the table.

Light, however, they are not.   They're very dense.  A Duratec four is about the same weight, and has more power capability.  At the RXNC this year, there was a cammed 2.5 swapped NC that had the same WHP as my bridge ported 13B used to make before the refresh, except his engine was tame enough that you could put a cat on it.  However, you still have to worry about throwing rods or valvetrain problems, and it takes some tricks to keep the timing chain happy in a motorsports environment.  (Basically you modify the auto tensioner to become a manual tensioner, and you have to keep up on its adjustment)

The key advantages rotaries have is the low engine inertia and low amount of engine braking.  This makes it VERY easy to regain traction when the tires spin.  I think my entire rotating assembly, with flywheel, weighs what most 4 cylinder cranks weigh.  And there is nothing to break, especially if you have an '86-up rotating assembly, which can handle in excess of 10k RPM with minor work.  Very minor.

APEowner said:

It's hard to beat a rotary for noise density.

FTFY

Pete. (l33t FS) said:

Light, however, they are not.

I think rotaries got their reputation for being lightweight in the 1970s, when most piston engines were iron block, iron head, etc.  Lots of development work on making piston engines lighter, not so much on rotaries.

boooooosssst

Prior to working on the T50 Gordon Murray was convinced small light weight was the future.  There is something to be said for the performance that is unlocked by low weight. 

My personal attraction to it is the low operating costs.  The MG burns through consumables at a hilariously Low rate.  I've done 8 trackdays on 1 set of brake pads, the same used take off set of Tires, and despite being a thirsty rotary go through 3.5 gallons during a 20 min session.  That's at 1800lbs with driver.  

I'd love to build something lighter just to see what it's like, but for a full W2W cage with automotive drivetrains I seem to end up at 1450-1500 lbs for the build.  

When I build another challenge car if will likely only meet challenge rules and be optimized so I would be shooting for <700lbs with a bike engine probably.  

That said I really want to find a 2stroke outboard.  Apparently the V6 200+ HP Mercury engines only weigh around 120 lbs.  Most of the outboard weight is in the drive unit and transom bracket/lift mechanism.  The V8 2 stroke linked in videos is an old Evinrude apparently (or one of the ones on YouTube is).  The challenge is they are usually very expensive. 

Also I looked at Jetski engines and apparently they are heavier and lower reving then their snowmobile counterparts.  Despite being basically the same designs they are built slightly different for the applications and the snowmobile ones are the light performance ones.