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3/30/12 4:48 p.m.

Um, we did it with a snowmobile motor. Check it out: http://blog.caranddriver.com/lemons-goodbad-idea-of-the-week-two-stroke-miata/

It freakin' works!

MG Bryan
MG Bryan Dork
3/30/12 4:52 p.m.

That's berkeleying awesome!

Ian F
Ian F UltraDork
3/30/12 4:56 p.m.
Evil_Genius wrote: Um, we did it with a snowmobile motor. Check it out: http://blog.caranddriver.com/lemons-goodbad-idea-of-the-week-two-stroke-miata/ It freakin' works!

That's awesome! I bet it's loud as hell.

Now get back to work on the Volvo 1800/Miata rack conversion!

CLNSC3 Reader
3/30/12 5:02 p.m.

Personally, I would never rely on a jet ski engine to be reliable. They are such finicky, temperamental engines. I have worked in powersports sales for a long time and it never surprises me anymore to see even a low hour, well maintained machine blow up for no apparent reason!

CLNSC3 Reader
3/30/12 5:03 p.m.
Appleseed wrote: Don't they have a total loss cooling system? Seems like that might be a problem.

The Rotax engines in Sea Doos have a closed loop cooling system. Its a pretty cool system, the hull basically acts as a radiator with coolant lines running through it.

dean1484 SuperDork
3/31/12 9:20 a.m.
http://www.thejetworks.co.uk/rotax4tec.htm said: The Rotax 1503 engine is an engineering masterpiece, the only 4 stroke engine specifically designed for watercraft use, the only watercraft engine with a closed loop cooling system. It is now available in several variations from the new 135hp found in the 2006 GTI range right up to the wopping 215hp supercharged intercooled monster found in the Seadoo RXP/RXT/GTX LTD and is also available in various Sea doo jet boats. It has 1503cc 3 cylinder’s single overhead cam pushing 12 valves. The engine is a dry sump variety running with twin oil pumps and a tip over protection system to stop the oil from flowing to unwanted areas in the case of the watercraft being inverted. Valve timing is designed in such a way that every time you turn off your 4 tec equipped Sea doo the engine will stop with all 12 valves firmly closed to make water ingression almost impossible. The exploded diagram shows the supercharged intercooled engine.

A better look at the Rotax motor

Amoung other things I noted that there is a waterpump.

dean1484 SuperDork
3/31/12 9:23 a.m.

This has me thinking really hard about things. It is almost 50 HP more than my stock 924s motor.

I wonder what the torque numbers are.

OHHHHH I know put one in an FC!!!!. With a 9K red line it would mimic the 13b quite nicely

dean1484 SuperDork
3/31/12 9:28 a.m.

I think I know what I am trolling CL for. The bad is that summer is close by so the prices for anything watercraft will go up fast at the moment. This may have to be a fall winter purchase.

Imagine one of these in a go cart?

motomoron Dork
3/31/12 10:01 a.m.
fasted58 wrote: Exactly. I've got an uncompetitive DSR - a 1999 Radical - in I'm swapping the Suzuki GSXR1000 for a Suzuki 1300 Hayabusa to get in a more favorable weight class, CSR. The 1000 motor has a stock Suzuki oil pan with the bottom chopped off and a piece of 1/8" AL plate tigged on. It seems to oil fine and the car pulls about 2Gs. For the 'Busa I bought a Rilltech pan which has 3 oil chambers w/ flapper doors, it's what the guys with the Stohr cars run. In SCCA racing bike engines are used in C and D sports racers and formula B, which are formula continental cars converted from inline 4 Ford motors to liter bike motors. At a regional level bike engines are relatively cheap to run. Blow it up? Some kid just totaled his bike and made a new motor for you in 10, 9, 8, There - new motor - that'll be 2 grand. And they happen to come with 6 speed transmissions w/ dog gears which are designed for clutchless upshifts, and electro/pneumatic paddle actuated shifters w/ auto-blip downshifting are readily available. At Summit Point where you shift (up and down combined) about 14 times/ lap it's an easy second. Quaife makes a chain-drive specific diff, and reverse isn't a problem on a 900# car.
DaewooOfDeath wrote:
They also like to explode given any side loading, right? I thought about a bike engine but they seem to be a) small displacement b) chain drive c) oil control challenged One of the biggest things that had me looking at water craft motors was the factory dry sumps. I also need something that's thin so I can fit more of those aforementioned ground effects in the car.

Stohr/ Suzuki DSR, fastest class in SCCA. Small displacement, chain drive, not oil control challenged, w/ ground effects.

If watercraft engines were viable in C or D Sports they would be populating the field. If a one liter engine is too small in displacement there's always the Hayabusa.

Ian F
Ian F UltraDork
3/31/12 11:29 a.m.
dean1484 wrote: Imagine one of these in a go cart?

That basically describes a typical A-Mod car. A 3 cylinder 2 stroke snowmobile engine adapted to work in a car. I doubt using a pwc engine would be much different, engine-wise. The transmission is the tricky part which is partly why a snowmobile is an easier donor.

Curmudgeon MegaDork
3/31/12 1:38 p.m.

BBR Shark. Fast as stink. 3 cyl snowmobile engine, belt type torque converter, has lots of cool/smart little details when you look closely.

I wonder if a 4 stroke SC'd PWC engine could have a belt type converter added easily...

fasted58 SuperDork
3/31/12 1:41 p.m.

... then there's the 'ol Water Buffalo

Curmudgeon MegaDork
3/31/12 1:42 p.m.
Evil_Genius wrote: Um, we did it with a snowmobile motor. Check it out: http://blog.caranddriver.com/lemons-goodbad-idea-of-the-week-two-stroke-miata/ It freakin' works!

Better not stall it on course. http://www.youtube.com/watch?v=DwbS4mVxYD4

oldopelguy Dork
3/31/12 5:28 p.m.
Curmudgeon wrote: I wonder if a 4 stroke SC'd PWC engine could have a belt type converter added easily...

Of course it could: Two pillow blocks and a chunk of shaft the size of the converter plus a BMW Guibo and a pair of flanges for it. One flange on the converter shaft, one on the PWC driveshaft. Some combination of large drill bit and/or shaft spinning in drill press and grinder cutting it down will make a flange fit well enough to weld up.

2/5/21 12:28 a.m.

*Dons flame suit*

Hello, GRM fam!

This is my very first post here and it's on a totally dead thread, but before i get hate for that, I think it's actually worth taking another look at this topic with almost a decade of aftermarket development and part depreciation from the last post until now... Also I have an unhealthy obsession with power dense lightweight engines, as well as straight 6 engines, so I can't help but get excited going down this rabbit's hole.

Ok, so here's the current fantasy that leads me to posting rather than lurking. 

I want to design (maybe even build) a high revving 2 liter straight 6 that'll make all the F20C VTEC boys jealous.

Piggybacking off the hardwork of motorcycle engineers is the obvious route to take. Kawasaki ZX12R with a first gen ZX14R crankshaft gives a 1320cc 4 banger, which, with 2 extra cylinders would be a 1980cc straight 6. 

Why stroke a 12 instead of using a 'busa? Power band. 

The relevance of this is that the Kawasaki STX12F engine and STX15F engines in the PWCs are directly based on the ZX12R and ZX14R. It's hard to know exactly how interchangeable parts will be, but if they're at least in the ballpark, then I'm in luck (and you are too, if you like the idea). 

As discussed in this thread, motorcycle engines come with a sweet six speed sequential. But what if you're not 100% about chasing every 10th of a second and like a good old H pattern? Or what if it's not going into a dedicated track car and instead of turboing your Miata, you want 300hp naturally aspirated. Except most V6s are a bit big and heavy (relative to MC engines) and then you get into the "just get an LS" debate... Anyway, I'm getting on a tangent here.

If you want to adapt to an existing car transmission, then you are stuck with mating the sequential to the H pattern and that's just silly. Enter the STX12F. It's the ZX12R without the tranny and a slightly different head to accommodate different intake and exhaust, haven't been able to find out about cam specs yet, though I think compression is the same. It's really hard to find more specs on these engines, but in an ideal world, ZX12R intake bolts on, or entire head swap is easy. ZX14R crank and rods drop in, keeping ZX12R pistons (after a lot of digging, I found that wrist pin and journal sizes are exactly the same on both engines, the ZX14R uses the same block as the 12, just bored and stroked).

I think I'd spec a custom camshaft that leaves intake valve open a little longer during compression stroke to drop the dynamic compression ratio to a more same level. This may not actually be necessary, based on what I've researched on stroker builds for these motorcycles, but I'm happy to lose a little power in the name of higher efficiency (greater expansion to compression ratio) and less stress, less prone to knock, etc.

Anyway, to make my point more clear, the goal is to obtain a bunch of Kawasaki parts, hope they Lego together to build a badass 1.3 liter I4 that can adapt to say an NA Miata 5speed. Whether or not everything Legos together, the next step would be to take very precise measurements of everything, spend a bunch of time in the CAD software that I use. Making a perfect 3D model that corrects for where certain parts inevitably don't Lego together, copy and paste 2 more cylinders in the middle, rotate the crank from 180 to 120°, plus all the other tweaks to make it into a straight 6 and also gives the engine the same bolt pattern as *dons a second flame suit* an LFX V6, because TR3160s are cheap, abundant and can handle much more torque than the ND's skyactiv-MT. (Because, oh yeah, didn't mention, one hypothetical day after a few hypothetical years, we're totally going to supercharge with hypothetical engine.)

Then I'd take this beautiful, wonderful design that took 1000s of dollars in research parts and countless hours of measuring, then drawing to the precision investment casting plant 30 minutes from my house and send the crank and cam specs to reputable manufacturers for quotes only to find out I don't make nearly enough money to afford all those custom  parts and regret ever writing this post.

But that's all in the future, in the meantime I'm gonna go grab my wallet and spend some time on eBay.

Besitos, Ed




bgkast (Forum Supporter)
bgkast (Forum Supporter) PowerDork
2/5/21 1:40 a.m.

In reply to N52B30 :

Don't forget a build thread wink

DaewooOfDeath SuperDork
2/5/21 5:37 a.m.

In reply to N52B30 :

I'd love to see this!


(Btw, not a lot of flaming on this site.)

In reply to N52B30 :

Welcome to the asylum! Stay for cookies. We like your thinking here.

nocones UberDork
2/5/21 7:35 a.m.

It appears that this is $1500 for both Ski's..


The 1997 1050 polaris makes 120 HP.  I wonder how hard it would be to use both of them in a car..  6 cyl, DI, 2 stroke, 240 HP, ~280 lbs.  That would be quite the site.  Not sure how much you could recoup, but 2 complete hulls with jetdrives and a titled trailer have to worth something..

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
2/5/21 8:35 a.m.

In reply to nocones :

Hell I'd probably buy the titled trailer from you. 

nocones UberDork
2/5/21 8:37 a.m.

In reply to Robbie (Forum Supporter) :


TurnerX19 SuperDork
2/5/21 8:56 a.m.

Why stop at a six? With two 4 cylinders you could much more easily make an inline eight. Take the power out of the middle to eliminate the torsional problem old straight eights suffered from. Phase the crankshafts by 90 degrees for smoothness.

nocones UberDork
2/5/21 9:12 a.m.

In reply to TurnerX19 :

Most of the 4 cyl and other 150+ HP skis are substantially more expensive.  It seems like the sweet spot for value is the 1997-2002 performance market which was mostly 120-130 hp tripples.    

But for a non Challenge car project you could definitely start at a 4 cyl.  Or 3 tripples..  or 4 trips in a V..  .   I need a moment.

I'm wondering how hard it is to make one rotate backwards.  I know some 2 stroke quads just start the engine backwards for reverse.   If that could be done you could have one engine oriented output forward, one output rear which would put the intakes in between the cylinders (or backwards with a. Hot valley like the outboards use).  Then run a jackshaft down the middle with a reduction sprocket at each end.  1 motor runs forward the other reverse and the jackshaft drives a standard car transmission in the middle in whichever direction you want.  

N52B30 New Reader
2/5/21 11:51 a.m.

In reply to nocones :

This spurs the idea of those bi-motor builds that pop up from time to time. A lot of these PWC engines are designed to produce more torque lower down compared to MC engines. It'd be a challenge to adapt a clutch in between, but if you had one front-mid engine and one rear-mid engine connected each to a front and rear differential...

Off the line acceleration would suck, but once you got going, it would totally rip.

N52B30 New Reader
2/5/21 11:57 a.m.

In reply to TurnerX19 :

Wasn't there a straight 8 that Ford designed like 60 years ago that was supposed to be transversely mounted and pull power off the center? I think their design used similar note and stroke to their small blocks at the time and was prohibitively large and got axed well before making it into production. 

In our case, if we have two compact 4s mated together it would fit much better. 

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