Rickman CR900 "GS" engine choice poll - big bore GS550-741/789cc vs GS750-894cc

[Chuck78]

Alright, help me decide here... I got a Rickman CR900 ex-racer frame with additional gusseting & GS1100E engine mounts. I think I will hold off on the monstrously beastly GS1150 idea (too much power for a guy who can't resist the throttle twist!) in favor of the 1st gen 8V engines.

I'm building this thing to be a very serious corner burning canyon carver, and brakes/suspension/handling is the whole reason I snagged a Rickman road racer frame. Here are my latest options:


#1 894cc 11:1 GS750 Megacycle or Web cams Keihin CR29 smoothbore carbs maybe some head work
*uses 850 cylinders bored out +2mm, and custom JE Pistons that I can order duplicates of from a friend's past order ($700 pistons)
*I need 2 of these pistons anyway to build a 475cc GS425 road racer, and have 3 spare 750 engines
++big cc's = big smiles, big power potential
++I have plenty of spare parts
+It is a CR900 frame, 894cc's just makes sense if you're nostalgic!
--750 engines are quite wide, I come close to scraping the stator cover on extreme corners on my GS750-920cc. racers have cut and welded an angled piece on there for same reason)
--weight: 750's have a massive crank, GS1000 engines are lighter by a good 12lbs with lighter crank, no kickstarter. (I do battery delete/downsize& starter delete kickstart only to save weight)
--4 of 5 crank main bearings are obsolete from Suzuki, but they are roller bearings and hardly ever wear out, bearings are available from suppliers but the races I assume are proprietary. I have not checked the rod bearings or the thrust washers for availability. I do have 3 spares though!



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**#2-#4 have these advantages/disadvantages:
++much lighter than a GS750 engine, better cornering, frees up some hp, + starter delete+battery downsize makes it an ultralight 4cyl vintage rocket!
++much narrower engine than a 750, more cornering clearance
++crank bearings are common to all other 2 valve per cylinder Suzuki's except 4 of the 5 GS750 crank bearings, all still available
+I have 1 spare 550 engine with a shifting issue, but 77-82 parts are common
+I have a GS650 MAC 4:1 header I could use on this
-may need to add even more additional engine mounts to the frame, already need to bronze weld different lowers for the 750 and touch up some previous repairs.
-can the 6 speed GS550 transmission handle this much power? Can a GS650E 5 speed fit in place of it, it's gears are 1/5th wider per gear so I assume)
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#2 741cc GS550 (*IF* my hunch that 550 and 750 combustion chambers are the same, see other thread photos, will tear down engine next month to measure)
*around 9.25:1 compression using GS650 cylinders bored to the max of 65mm with stock GS750 pistons
*base model MegaCycle or Web cams are the max lift I can run, machining deeper valve pockets would only decrease compression ratio)
*80-82 GS550 BS32 CV carb head for larger intake ports, Keihin CR smoothbore carbs
-not blisteringly fast, but sufficiently fast


#3 789cc GS550 (*IF* I can resleeve a GS650 block with 750 sized sleeves but with smaller steps at the top flange, and the above #2 comb chamber criteria)
*10.9:1 789cc using some NOS 798cc 67mm GS750 11:1 pistons I have acquired, 80-82 GS550 BS32 CV carb head
*requires resleeving a tightly spaced GS650 block even tighter, custom sleeves or the top step flange on 750 sleeves machined down
++789cc 10.9:1 with good cams will make a very fast "little" 4 cylinder!
-may run a bit hotter due to less metal in the cylinder block (+ could possibly run WIseco 844cc GS750 pistons as well, -but even less cylinder wall thickness)


#4 741cc 10.25:1 GS550/650
**Need to contact Wiseco and see if they still have 30 year old CAD drawings of their 65mm GS650 big bore pistons, & could make a limited run of them
*would run a GS650 cylinders bored to the max of +3mm and a GS650 closed chambered head, Wiseco pistons or else make custom JE or CP pistons for $$$
+++GS650 heads are a closed chamber raised port design for more power and efficiency due to more complete combstion and more direct flow into the chamber, a stock 673cc GS650 makes same hp as a stock hemi head 77-79 8v GS750, less torque due to lower displacement though. Pistons are specific to the head design.
++would be every bit as fast as the 789cc hemi head 550/650/750 hybrid, slightly less torque but not by much
-more costly, should I spend the $700 custom piston money on the 894cc for more power instead?
-have to do more legwork to see if Wiseco can re-make these out of production pistons, but many GS550/650 owners would kill for more sets, group buy???

[Chuck78]

I may just build up both a 550 big bore and the 894cc, but still looking for guidance/opinions.

From my other thread, the bike will basically look like this but Suzuki powered and VTR1000F 41mm cartridge forks with 6 piston gsxr1000 brakes & cbr900rr rotors


GREAT frames! Reynolds 531 lightweight high strength 1.25" tubing, beautifully bronze welded and nickel plated

[Chuck78]

Should added a 798cc gs750 build, $500 cheaper for me! Wasn't highly considering it but may if others chime in on 798cc vs 894cc for $500 additional expense

[Nessism]

I vote for a stock GS1000 engine. It's lighter than a big bore GS750 engine, and MUCH cheaper. The driveability will be awesome and it will make good torque, something that can't be said for a 550/650 based engine.

[Chuck78]

I'm very attached to the kickstart engines, however... Yes, a GS1000 would be more than fast enough stock with k&n & 4:1, but where's the adventure in dropping in a freshened up bone stock engine?!? The 894cc would surpass a stock gs1000 but have a bit less low end response.

[Nessism]

I don't know man. Torque is good. Get a big bore kit for the 1000 engine if you really want to spice things up. Put money into a clutch hub rebuild w/heavy duty springs and backplate. It makes a huge difference on these bikes.

The smaller engine is a non starter on a project like that, doubly so if you intend to sell it at some point down the road.

[Rensdw]

I voted for the 894cc option so you don't have to change the sidepanel decals

Seriously though, In my humble opinion: "just dropping in" a 1000cc is the easy way out.
Are you going to race it? if no then the fun factor is more important right?
If resell value is the main argument, then why build a custom bike in the first place.

my 2 cents, more like 1-1/2 cents

[Chuck78]

Well I'm leaning towards the 800cc or 900cc route, and with a much lighter bike and 155lb rider+gear, less hp and torque will give the same seat-of-the-pants feel as a 550 lb gs1000 and 155lb rider... my gs750 920cc I got down to around 485lbs with fuel& oil from 532. The Rickman will be even lighter, 470lbs maybe? With a gs550-789cc engine, I'm guessing I could be at 435-450 lbs. Big difference.

I'm going to have to find a scale and weigh the engines in my shop for a better assessment. Once I finish a few more house rehab projects, I'll be pulling the 550 head, then cc the combustion chambers and test fit 750 pistons into one.

Unfortunately I don't think my hvac refrigerant scale goes over 100lbs, have to use a bathroom scale. I can't easily pick up a gs750 engine, but I can a 550 engine. Still hefty but noticeably lighter.

[Chuck78]

I can also order up 1023cc pistons from JE for the 750, but that's addING a lot of weight to a short stroke rotating assembly that likes to rev high, and the 16mm wrist pins are really put to the test on that large of a piston...

As far as torque, major boosts in midrange torque can be had from playing with the cam timing, 106 lobe centers or so, or altering lobe seperation.

I was also looking into running a slightly less aggressive orifice exhaust cam than the .380" lift 261°@.040"megacycle intake cam, this will help a lot with lower end while still letting the intake breathe well. The exhaust gets pumped out by force, the intake charge is sucked in, so bigger intake valves and more lift really help out there. Pushing the exhaust out by force requires less.

[Nessism]

Chuck,
you remind me of a good friend of mine that just can't leave any vehicle he has alone.

True story...
I bought an RX-7 with a blown (12A) engine for cheap. Found a good low mileage engine and went through it, including porting the side housings, which like porting a two stroke increased power significantly (but also drastically increased emissions). Just about this time I came across a combo deal of HP parts: header/exhaust system, intake manifold, and a small Holley carb. When done that little 12A was a RUNNER! Had it dyno tested and it was making about 40% more power than stock with perfect driveability. Problem was the exhaust was so dirty there was no way in heck it was going to pass emissions testing. This is where my friend came in. He drove the car and had to have it. He knows "people" that can get him a smog cert so no issue there. So he takes the car and proceeds to start modding it further: changed rear end ratio, close ratio transmission, different exhaust system, all new brake and suspension system, new wheels/tires (replacing those I installed), etc. He took my little price point econo project and outspent me by double, and added only marginally to the performance. Don't take me wrong, my friend built a hell of a nice car, it's just that costs went threw the roof for only marginal gain.

[steve murdoch]

Slight hi-jack, Chuck.
I don't know what front end you are you hoping to use but i have an old Rickman front fender.
Not perfect but fairly solid. 4.5" fork leg spacing. I think i is from a Kawi kit.
You can have it if you can use it.

[Chuck78]

Steve, I most certainly would want that! I'll be making custom fender mounts so anything can be accomodated on the VTR1000F Superhawk front end.

[Chuck78]

Don't take me wrong, my friend built a hell of a nice car, it's just that costs went threw the roof for only marginal gain.

Well It is a Rickman, & the cheapest good running condition Rickman I've seen was $4500, 2nd cheapest $6,000, 2 for $8,000...The rest of them, the nicest examples, always go for $12,000-14,000, some upwards of $18-25,000...

This is a labor of love, and trying to build the ultimate vintage bike but staying under a liter as I love the 8v kicker motors

[Chuck78]

Man this is a tough choice for me, but I think I may have to just build both engines as money allows (after my old house sells this spring/summer), & bronze weld on provisions to mount interchangeable motor mount plates. I wish I had a spare GS1000 & GS1100E bare crankcase here to mock up the engine mounts for all the above! Maybe GS400 Larry (theironlung1976) can save me his GS1100E crankcase that now has a rod protruding through a fist sized hole in his 82-83 gs1100e crankcase...

coupla gs1000 engines at the local salvage yard. Wouldn't mind having a rebuildable core so I could look into what it'd take to adapt the smaller gs1000 crank bearings (&all other gs400&larger 2 valve per cylinder gs roller cranks) to fit in a gs750 kickstarters crankcase, doing the Yoshi method of running a 4 link longer cam chain and modifying a 1000 head to fit a 750 cam chain idler gear on the middle of it for high rpm longevity (team yoshi blew up every gs1000 endurance racer engine until they put the 750 idler gear into the 1000 head, I'd just use the 750 head with bigger valves, no welding)

[Nessism]

Have you looked at the right hand side footpeg and brake lever to see if they are designed to fold out of the way and clear the kick lever you hold so dear? In the photo showing the Rickman Kawasaki the kick lever is removed, and it appears that the lever, if present, would whack the peg/brake lever if anyone attempted to use it.