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< Performance issues ~ Engine Plans

Alberto

Posted: Mon Oct 17, 2005 12:22 pm

Junior member

Joined: 13 Oct 2005 Posts: 10

Folks

I intend to built, within next couple of years, a CR replica, with a few modifications:
1-instead of the 750A crank, I intend to use the 750C (62mm) and longer rods (maybe 750 DOHC);
2-Arias 10.2:1 pistons (62mm), that combined with F2 intake valves would do 10.1:1 comp ratio;
3-GL1000 primary gear (thanks Satanic Mechanic for this tip!)
4-CR replica cabs;
5-Yoshimura or Eagle cam (stage 1 or 2)

But I'd appreciate your comments on this preliminary spec, please.
I'm rookie, you know . . .
Any concerns, recommendations will be VERY welcome.
I'm counting on your expertise.

Do you have tips on suppliers for CR parts (original/repro)?

Thanks!

Ron

Posted: Mon Oct 17, 2005 9:23 pm

Member

Joined: 15 Sep 2005 Posts: 30 Location: Indianapolis

Alberto:
I'm not sure why as a beginner you would mess with the crank. Its a lot of work for not much result. If its for the Hy-Vo chain just keep in mind that we dragraced and roadraced that model for years with a lot more horsepower than you are expecting to make with the stock chains. Just a thought.

FYI: I have a set of the CB900DOHC rods which I would sell. They have the appearance of the RC Rods we've spoken of in that they are beefier. But the bolt diameter is stock. A good crank/rod shop could install the heavier bolts if desired.

If I were you, I'd save the time and money on the crank swap and buy the Carillo/Falicon rods in the stock dimensions. They are lighter by several oz, stronger and drop right in.

Are you using 62mm pistons and the shorter crank to stay in 750cc range?

If you want the best set-up for a streetracer, I would reconsider an 836cc Kit, falicon Supercrank and Falicon Knife Rods http://www.faliconcranks.com/ lightened alternator rotor. Ported head, larger intake valves (however I think the actual F2 valve only fits an F2 head?) and carbs. While no longer made, 29mm Mikuni Smoothbores are out there.

But if you're working with Satanic Mechanic, I can't see him steering you wrong. This site and his expertise is an amazing resource!

My current project is a rebuild of a motor I built in 1980 and put 50,000 miles on. 900cc, (resleeved) forged rods, lightened crank, ported head, RB Pipe, Mikuni carbs. Revved REALLY fast, very strong. I'm going to change over to the Falicon Crank/Rod setup. Ron

Satanic Mechanic

Posted: Tue Oct 18, 2005 7:31 pm

Moderator

Joined: 04 Sep 2005 Posts: 309 Location: Munich, Germany

Alberto,

I can only second what Ron said. Unless there are specific resaons, I would go for an engine aling the lines that Ron had laid out, i.e. 836cc, a decent cam from Megacycle (e.g. 125-65) or Web Cam (e.g. 41a grind). Both camshafts are still available. Valve springs from the'78 K7 model (or a complete head) allow for a redline of +9500 with these cams. The F2 valves will only fit the F2 head - the stock head cannot take the 2mm larger intake valves. Have the head ported, valves polished.

If you want to add some more goodies, drop in stronger con rods (altough) they are not mandatory for such a setup, have the crank done by Falicon or another good crank shop.

Old 31mm Keihin CR's would be the classic outfit for a CR replica but Smoothbores might be easier to work on (and both are hard to find, unfortunately).

Especially if this the first 750 engine you are building, I would stay away from the crank swap you suggested - even if the crank drops right in, there is still a lot of small things to consider, as ignition / dyno which are reversed on the DOHC, a primary chain tensioner which needs to be made, same for the cam chain which will is a Hi-vlo as the primary chain. It's not so much the big things which you can get sorted out relatively quickly, but there are lots of small odds and ends which will give you more headache than you might want to have with your first project.

Speaking of ignitions - a Dyna S and Dyna coils are well worth the money. The Dyna S is a drop-in-and-forget unit - once installed and set up it will stay that way.

The CR bodywork is available from Airtech: http://www.airtech-streamlining.com/hondaz/cr750.htm. CR exhausts are made by Swarbrick Racing: http://www.swarbrick-racing.co.uk/

P.S. I would loke to move that thread to the Performance forum so that others will find it more easily. I will move it in the next few days, ok?

Alberto

Posted: Wed Oct 19, 2005 11:24 am

Junior member

Joined: 13 Oct 2005 Posts: 10

Folks,

You're right! But I'm thinking a little over a few theoretical issues before freezing the lay out and start buying parts:

In the original CB750, rod to crank ratio is 97/31.5=3.08.
This proposed set up increases it in 3%.
For a given crank angle of rotation, the bigger is this ratio, the longer the piston dwells next to the top dead center. More of the combustion occurs at the beginning of the expansion. Or, in other words, more low-rev torque. In the other hand, the peak power is weaker.
In a racing (close) final ratio, more torque would be good for getting kicker responses out of corners, faster acceleration, but also lower top speeds. And low-rev means longer lasting engine.
I just rebuilt my first original CB750, and it have plenty of torque. But people claim that the racing megaphones (open headers) leads to low back pressure and low end torque. CRs heads usualy had small exhaust valves to compensate this efect. Today, some people put restrictors inserts on the pipes instead.

Well, this is just a thought . . . but perhaps a 3% ratio increase (estimate +8% in torque) would not do a good trade off for all this trouble (ignition/generator/chains/etc)? Maybe going all the way around and built a more hi-rev top end power engine would end up better overall results.

I also heard a lot over broken primary chain, but never experienced it myself.

You’re right: this set up keeps the bike under 750 cc (actually 748cc), required for superbikes category at the time the CRs where around, and I’m a purist!
Plus, the 62mm bore, 10.2 to1 comp ratio, combined with approximately 0.5cc bigger combustion camber (F2 valves - estimated) keeps the compression ratio a little over 10.0 to 1. This could run on 90 octane gasoline, maybe. Must retard the ignition not to let it detonate at low-rev.

Yes, I’ll need a machine shop to fit the 2mm bigger diam. valves. Smoothing and boring may be a better way out. But there is very little GOOD machine shops in my area. Sending it to another country is not an option.

Regarding the carbs, the smoothbore Mikunis sound good! I saw some old pictures of non-factory CRs on the net and now I recognize them. They are probably a lot easier to get new rebuild kits too! Thank you!

Over the rods, well, Carrillo itself asked for +US$300.00/each. Considering your recommendation, and this abuse, I think I’ll take the Falicons if no cheaper Carrillos show up soon at eBay. The knife shape probably is more aerodynamic and helps 'cutting' the air and oil inside the case. Very nice.

I need the rear drum, oil cooler, gauges. I intend to build new aluminum alloy fuel tank, oil tank, seat, tail, pegs, handles and headers.
Is it really hard to find all this parts? Is it worthy? They are all available, but they are fiberglass made or too expensive (+US$1200.00 for a fiberglass kit!).

Where could I find the carbs? CRs or smothbores?
Somebody asked me for US$3200.00 for the rear CR repro drum brake. It is very look alike, but I think I could built one myself in a lower budget . . .Or wait for eBay, some day . . .Or if you know of any for sell.

Thank you for all this help and attention!

Satanic Mechanic

Posted: Fri Oct 21, 2005 1:52 pm

Moderator

Joined: 04 Sep 2005 Posts: 309 Location: Munich, Germany

Alberto,

I see you did your engine building homework Very Happy

Very good! But, like you said, you would pay a high price for an estimated 8% increase in low end torque. If you put a tenth of the time the crank conversion would take in a good head porting job, you will probably get more out of it with much less hassle.

61.5mm pistons (2nd over) should get you to exactly 749cc. There were slipper pistons made by Yoshimura with that bore, but they may be hard to find.

The Falicon rods should be good enough for your engine, no doubt. I am sure you should even get along with the stock rods, but of course the Falicon rods are much nicer Wink

.

All the CR bodywork is hard to find and/or expensive, especially if you don't want the fiberglass stuff from Airtech. A lot of people are working on CR replicas or planning to build one, and the supply on original parts isn't very good. OTOH, an experienced metal working shop should be able to build tank, seat and oil tank if you provide them with a frame and pictures.

Neither Smoothbores nor the old style CRs are manufactured anymore. The only way to get them is by looking around. They show up on Ebay every once in a while. The new CRs are still available from Sudco, @ US$ 600+

US$ 3200 for a CR rear drum sounds a bit overprized, especially for a repro. You can find them here but the price tag is Euro 2050, which is roughly US$ 2500. Add shipping and taxes, and you are gettiing close to the US$ 3200. It's still a huge amount of money... if you are really lucky, you may find one on Ebay one time, probably mislabeled so that only few people recognize it... Or build one yourself, like you said.

Ron

Posted: Tue Oct 25, 2005 11:54 pm

Member

Joined: 15 Sep 2005 Posts: 30 Location: Indianapolis

Satanic Mechanic wrote:

Alberto,

SNIP
US$ 3200 for a CR rear drum sounds a bit overprized, especially for a repro. You can find them here but the price tag is Euro 2050, which is roughly US$ 2500. Add shipping and taxes, and you are gettiing close to the US$ 3200. It's still a huge amount of money... if you are really lucky, you may find one on Ebay one time, probably mislabeled so that only few people recognize it... Or build one yourself, like you said.

I'm guessing that rear CR Rear Drum does not have a cush drive? If not, probably not practical for a street racer. Sure is pretty though. I had no idea such things were available. Once again, this site is great!

Alberto

Posted: Thu Nov 17, 2005 4:15 pm

Junior member

Joined: 13 Oct 2005 Posts: 10

Folks,

I think you all are right. This plan will take too much time and effort and cost too much trouble for the output. I'm reconsidering the overall lay out with 2nd over Wiseco pistons, Falicon rods and a good port a polishing. Now it occured to me that there are not too many shops I can trust the job in my area.
I'm still considering a 750AT cranck I saw on eBay, just because it has the stronger primary chain gear. Do you know if the fitting to the case will be just drop on?

Could you tell me what shop would you recomend in the Continental US? I intend to make contact with them, get quotation and sending parts from eBay directly, so they will send me the parts with the service already done. Please advise for head and cranck.

Regarding the CR details, that will have to wait for more budget . . .

Ron

Posted: Thu Nov 17, 2005 7:56 pm

Member

Joined: 15 Sep 2005 Posts: 30 Location: Indianapolis

Alberto wrote:

That's sounding more reasonable. This link is from SatanicMechanic's website: http://www.aperaceparts.com/ (APE) but I also know this outfit from way back in my 1980 +/- drag racing days, and still have parts from that era in my garage. I would'nt have any qualms letting them do the head and crank work and I like their prices. Besides bolt in parts, the best bang for buck is from the ported head, and a lightened crank, includong lightening the alternator, unless you have a later model which is already trimmed.

They are in Austin TX which isn't as far as California, which is probably where other top name machinists are. Save a little on the freight.

APE handles Carrillo rods, but they would probably fit the falicon's if that's what you want. Final tip, don't get too hung up on the primary chains. Unless you're making turbo type horsepower, the stock chains are fine. I think anyway. Ron

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