TOPIC:

How much rpm will a 750 take, and is the rods stronger than a 650???? What about the crank.

to Old Skul, I think I am coming to reality. Everyone here thinks 72mm is the max piston size. I think I can build a fairly durable motor using them. Still don't think forged pistons are the way to go, though. I think kz750 rods are the way to go also. The larger rods from a zx-9 are heavier and would therefore put more stress on the crank in the RPM environment I am contemplating. So 72 is where I am going. Now the question is JE or ARIAS. The last set of pistons I had made were by JE, I have to look up my reciept But I think they were 600 and some change. I thought that was very good. I'm thinking of 12:1. I have a call in to ARIAS now. JE was so easy to work with though and it was six weeks from the first call till the pistons were in my hands. People keep talking about the 650 head and intake tracks being better for midrange. I think a 750 head is a must for 72mm pistons. I'm still unsure about the carbs but 31,s would probably work the best. Cams will definately be from Web, they have worked great on both of my other bikes. My 920cc Z-1 is extremely fast with only the CBX faster this year in a straight line. I,m going to up my Z-1 to 1015 this year until this bike is done. Anyone want to see these bikes in action can go to AHRMA.org and click on photos of Barber motorsports park race in September. Both bikes are painted silver No. 120 with the little bike having red and blue scallops on the tank. And yes that bike behind me in heavyweight is a Moto Guzzi of Wil Harding that is supposedly 1380cc built by Stan the Guzzi man out of Florida. He won the National Championship this year. If anyone wants pistons made I can ask if there is a price break on quantities. Thanks all. There are some good minds on this site. Jeff Hinds

If it's not to much trouble, will you post the specs of your 920 motor? I'd really like to see the static (calculated) compression ratio, the cam specs, and what type/octane fuel you are using, as well as the ignition timing and advance specs.
Apparently it's been trouble free?

I'm just curious as to what it took to build a front runner 900 for road racing

Oh yeah, if you go to Lorcan's site www.750turbo.com there are some people there working on a group buy of JE pistons, and I think the quantity price breaks were listed.

Post edited by: steell, at: 2006/01/04 12:49

To Ahrma 120,

Good Luck with your project. And let us know about thos pistons!

I'd actually considered building a similiar project, but hadn't gotten off my butt to do it. I think the smaller flat plate area, combined with the better handling of the 650 should result in a very competive package. Bumped out to 880cc you shouldn't be giving up much in power.

In regards to the 650/750 head. I'd be hard pressed to make that call. The 650 head flows very well at my 80hp level. At 11,500 RPMS it still seems to be breathing fine. The smaller combustion chambers should also provide a nice compression bump. If you can do it a back to back dyno comparison with different heads would answer alot of questions posted on this site...... At the horsepower levels your going for the larger 750 valves might make a significant difference. If you can have both heads tested on a flow bench that might help answer the questions as well.

A few people asked how do you rev a 650 past 10K rpms? The answer is cams and carbs! My web cams (street grind) move the power peak up to about 11,000 rpms. The bike has low torque below 6500 rpms though...

Am curious does squish band come into play. Would a smaller combustion chamber give a better squish band with a given bore. I'm sure theoretically it would but how about practically. Would that make or break the decision wether or not to use a 650 or 750 head. A 750 head has the same 77-93-77 carb spacing as 900/1000 and older pre 93 GSXR's. Canadian 1985 750 GSXR's came with a 29mm flatslide, which I have on good authority can be bored to 32mm. Pretty rare carb. I believe most went south years ago to be run on Yank bikes in 86, when the Yanks finally got GSXR's, but with CV carbs.

Post edited by: RomSpaceKnight, at: 2006/01/05 15:17

I have a couple of 750 heads here, and they don't really have a squish band, just a hemi combustion chamber the same size as the bore. I think using a 650 head with a stock combustion chamber on a 750 would create a squish band due to the smaller diameter of the 650 combustion chamber, and I have been wondering what effect (if any) that would have.

GPz1100 has the same carb spacing as the 750 also.

I figured in stock trim would not be much of a squish band. Was wondering about a big overbore situation. I think 2 strokes may benefit more relatively from squish. I would wonder about big bore high comp pistons fitting in relatively small combustion chambers. But that would be where you would bring out the plastigauge and plan ahead with advice from your engine builder.

Oh, I know Kawi makes great motors. But 88 hp is a long way from 120 or even 100. I doubt a thirty year old design can take 13,000 rpm for half an hour flat out. And if you blow up the motor over 800cc, is it still a "650"?

in my personal experiance with bikes,cars and boats if you can go with a smaller combustion chamber rather then a higher dome on a piston its easier to make power because when the fuel burns its easier for it to travel across a flat area than it is to travel up and down a dome,is this what you mean by squish?but you have to be careful not to shroud the valves with the combustion chamber!

A squish band is located around the edges of the combustion chamber and is generally useless if it's more than .045 deep (top to bottom). What a squish band does is send a jet of high speed mixture shooting towards the center of the combustion chamber from all directions, increasing turbulance in the chamber, and generally allowing a higher compression ratio by delaying the onset of detonation. This happens as the piston approaches top dead center. Generally, the closer you can get the piston to the head in the squish area, the better off you are, some builders actually make the piston touch the head at max rpm and temp, but that's cutting things a little to fine for my taste.
AFAIK, squish is not normally used in engines with hemi combustion chambers (as in the KZ's).

You are right about the piston dome, increasing the dome height is the worst way to increase compression, but sometimes that's the only option.

Post edited by: steell, at: 2006/01/05 22:30

CB/GS/XS, you have to remember it's not a 650 anymore - it's an 810 or 880. No one would doubt a kz900 could make that much power and punch out a zx6r as big as it will go and it will easily make those numbers.

The 650 motor was overbuilt from the get go, and is the same bottom end as the 750 turbo and later model zr-7s. In the 750 turbo it made 110 hp from the factory stock. This means the bottom end can handle the power and the block with an oil-cooler can shed that much heat. These motors are very strong and can take alot of mods with very little issue.

the ol man used to tell me some of the best engines he ever had were the ones where you could just see the witness marks on top of the pistons,but im with you the last time i built one of those 0 clearance handgrenades the only thing salvagable was the 73 cases and a couple of trans parts,but it was a hard running s.o.b before the verybig BOOM!!!!!!!!!bwaha