Looking for some DOHC horsepower

[cdaiscool]

To be honest, I wouldn't do it. Only because I already have a fast/turbo car, and don't want to have a super-fast bike. Especially 140+ on a 1981 bike, with 1981 tech.

I know myself waayyyy too well. I would end up racing and quite possibly getting killed. I'm good. :)

OP, if you do want more info on turbos, let me know. I've got more'n enough for F.I., but it can give you some info with regards to overall putting a turbo in.

[Half-Caf]

cdaiscool wrote

I've got more'n enough for F.I.

What?
Thanks, I think  However, I don't think I'm ready to go down the turbo road.

shinyribs wrote

I wonder what the the bore and stroke of a factory built 900 is vs a bored out 750.

Shiny, ask and you shall receive -I found this on the deuces wild forum:



Any help crunching numbers and interpreting stuff like this is more than welcome.
Keep in mind that this is my first bike ever, so no one should give me too much credit! I'm probably just too dumb to not know when to quit. But who cares, right?

[shinyribs]

Looking at the valve timing-the open and closing measurements-the 750 has earlier opening  valves. That will produce an engine with more bottom end power compared to one with later valve timing. Basically,the 900 should be a little more highstrung than a 750,which should be a little bit more torquey.They oddly close at the same time???

BUT,the 900 has a longer stroke which will always produce more torque since the longer crank throw gives the piston/connecting rod a greater mechanical leverage over the crankshaft.Greater leverage,easier to push hence increased torque.

So the 900 has more stroke to help with torque down low,so it could afford the later opening valve timing without giving up bottom end.But that same valve timing helps with upper rpm breathing.

The reduced compression helps prevents detonation at higher rpms that the 900 should see,but will also give up some bottom end power.Again,it has a stroke to afford it.

The larger bore will bring the cylinder walls away from the valves providing the 900 with better breathing by unshrouding the valves.

Basically,bigger is always better.

[Half-Caf]

Thank you much for the interpretation!
I'm guessing that these are more points for the 900 top end swap vs boring the 750?

[shinyribs]

If you can bore a 750 that far than it would do the same,provided a 750 head would flow would a 900 head already flows.It's that long stroke of the 900 crankshaft that is the babydoll of the whole operation. Stroke is always a good thing. If you compare it to a V8, a Chevrolet 427 and a 454 have the exact same bore size.4.250'' The 427 has a 3.75'' stroke,whereas the 454 has a 4.00'' bore. 427's are great to zing around in a lightweight Vette,but if you plop one in a heavy Chevelle a 454 would wear it out. Torque is king. You'll never go fast if you can't get moving in the first place and torque is what initially gets things moving.

[Half-Caf]

Hmm, sounds like I will make that crank fit even if it doesn't want to!

reminds me of Tools' sig:
"It couldn't be done, but the darn fool didn't know it, and did it anyway."

[shinyribs]

Exactly! Have you checked to see if anyone makes a stroker crank for a DOHC? That would make your life easy.

Here are the bore sizes as related to the cc for a 750. These kits are for SOHC's but it should relate.


A 65mm bore with a 750 crank gives 836. Shows how much extra capacity is afforded by that 900 crank since the stock 900 bore is 64.5mm


*notice the part about breaking rods...thats how you know you're on the right pathYeeeeeee-haw!

[shinyribs]

APE advertise a stroker for a SOHC.Maybe they would do a DOHC???




The fee is $1,100-1,200 depending on how much you want it strokedBetter get that 900 crank stuffed in there!

http://aperaceparts.com/crankwork.html

[Half-Caf]

shinyribs wrote

The fee is $1,100-1,200 depending on how much you want it strokedBetter get that 900 crank stuffed in there!

Pshh, pocket change  

But for real, I'm gonna try my darndest with that crank!

[cdaiscool]

Half-Caf wrote

cdaiscool wrote

I've got more'n enough for F.I.

What?
Thanks, I think  However, I don't think I'm ready to go down the turbo road.

Awww, turbos are fun!

And F.I. means Fuel Injected. I've done my (un)fair share of research on fuel injected turbo set-ups.

[Half-Caf]

Gotcha
Well, if you ever do decide to turbo yours, let me know and maybe I'll come give you a hand.
You're only a few hours from Chicago, right?

[cdaiscool]

Holland MI.

You can get to Chicago in around 2.5, 3 hours from my place. I haven't completely decided yet. Maybe I'll look at the price of creating my own manifold and everything versus boring out the motor. Leave it a 750, but let 'er breathe a little.

Though I should go to the 823 kit as this will allow me to get forged pistons, a much needed assistance to reliability.

[Half-Caf]

In doing some more readin, I've found some people recommending throwing the 900 crank in the DOHC 750 -I'm hoping this is good news...
Also, I may not be alone. Someone posted this on Deuces Wild:

deuceswild guy wrote

I just replaced the Cams. I've never really got a good answer if the 900 head is a good upgrade to the 750. Some have said that it would create dead spots, which makes sense. But if you bore your 750 to accept cb900 pistons, that should be a non issue. that would give you 810cc's. I've just been gathering up the parts for a full 900 conversion. I'll bore the 750 crankcase to fit the 900 cylinder sleeves, than install the 900 crank, cylinders, head, cam chains, tensioners etc.
I'm only hoping that my 750 exhaust will still work due to the small change in height.

There is more discussion of the cam swap on that page as well

Also, I have some turbo thoughts for those more adventurous than I. Using the 900 jugs, pistons, head and rods with the shorter 750 crank would lower your compression. Now I don't know much about turbos, but isn't lower compression desirable when using one?

[cdaiscool]

lower compression allows you to more safer tuning - if you make a mistake,  you are more likely to be safe with lower compression. However, the less compression you have, the less power you make. It's a trade-off.

[shinyribs]

I was reading "that page" and came across this:

"The massive stroke on 900 explains why they blow up so easily. Stroke is WAY too long for a performance motor, Honda still was stuck in the '60s."

"Both 750 and 900 rod virtually the same on pin end, you are correct, the narrow spot right under pin boss is the problem. 900 breaks faster because of the huge stroke which is way out of line for a motor that runs that kind of rpm. Rod angle increases which makes it easier to break at high speeds. 750 has less load on it, with less stroke the piston speed in foot per minute drops off, piston speed is one of the first things one looks at when thinking about making big power. Once 4000 fpm is crossed you are asking for motor blowup. Long stroke really shoves that number up. That's why you always see high perf motors that are oversquare, meaning bore much bigger than stroke. Short stroke lowers piston speed and big bore allows for bigger valves and better breathing into cylinder at higher speeds. 750 is exactly square at 62 x 62 mm, 3897 fpm. 900 is undersquare at 64.5 x 69 mm., 4293 fpm (?!!), not the hot ticket for a revving motor that LASTS. Fine as a stocker, but press on it and it will crack."

Now I'm no bike mechanic,but concerning the motors we used to build for our drag cars...it sounds in line with the same reasoning we used to use. Anytime we ever built a stroker motor it was because we were trying to shove a heavy car down the track. Build a lot of torque and gear it high so it won't spit parts at the topend...and DO NOT miss a gear!A longer stroke causes the connecting rods to whip around like mad...after awhile they end up shooting out of the block and on the track if you over do it.LOL! I saw a 383 sbc(350 chevy with a 400 crank in it) puke its guts out in the waterbox one day. It is an odd sight to see a car still doing a burnout,still hearing the motor run, but watch rods,pins,pistons and coolant go flying out of the block all at the same time.Wish I woulda had a camera.

[Half-Caf]

Shiny, I also read those posts. Basically what I took away is that a 900 is almost just a 750 that is tuned to the max -which is kind of what I'm looking for
However, it seems that running performance mods on a 900 is kind of asking for trouble. Although, I guess boring one out would be relatively safe...
I guess this backs up my point:
"If you hotrod 900 you'd best limit rpm to 10,000 max unless you like shrapnel. Honda RSC used to limit Freddy Spencer to 9500 rpm on the factory racers, "They blow up at ten" was the word of the day."

But my question is this: I will basically be running a stock 900 if I do the swap, aren't stock 900's all over the road? Are they blowing up left and right? OR.. Are they all being driven by grandma's who never take them past 5k?
Could Honda's engineering be that bad?

If the answer to that last question is "yes", then perhaps I would be better off to keep the 900 head, bore out my 750 jugs to accept the 900 pistons (64.5mm), and use my 750 rods and crank.

Now keep in mind that this bike will be driven on the road, not at the strip. The only time I see myself going anywhere close to 10k is if I accidentally shift into neutral while I'm really pushing it...

[shinyribs]

I agree. I don't really see Honda building a 900 that tosses rods whenever pushed a little too hard. I think you are probably right on that.But on the "i'll never see 10k"comment...yeah right!

[Half-Caf]

I guess time will tell, eh?
Word on the street has it that the 900's looove to rev. I think it wouldn't hurt to change the gearing a bit to make it a little easier on the motors at highway speeds. I think I wouldn't sacrifice much low end due to the amount of torque I should be getting..

[Half-Caf]

shinyribs wrote

If you get it and decide to part it out I might be interested in the tank. Especially if it fit a '78 frame,which I highly doubt.Never hurts to dream...

Shiny, got that bike today and I think you might change your mind on that tank.

[Piute]

                    Lil glass custom locking cap better then new.