Super Jet 760 cylinder exhaust port sizes ...are they the same as a 701?

[lamar311]

Hi all,

I have an odd set up that I bought my ski with. Its a 61x bottom (light port work done) with a 64U top end. I just recently had it bored to 1mm over and it runs good but I am wondering if I can port it to get a little more?

I have access to the templates for 701 porting and want to know if this porting will work? I am aware that the ports are 2mm higher than 61x or t but I plan to run without base gasket and already have my head and domes adjusted to fit the proper squish band.

Seems to be so many conflicting posts regarding this topic. I have heard just ditch it and go 61x, but considering I have only half a season on the bore and the new top end, I'd like to do some porting myself to try and squeeze more bottom end out of her.

Looking for advice/experience.

Thanks in advance.
B

 

[yamanube]

I ran the same setup, I took a sport port template and enlarged it a bit to accommodate the extra bore size. I didn't do anything to change the port timing and didn't touch anything but the exhaust ports and very slight tunnel porting on the cases. It's a decent little motor, I have had very little to compare it to back to back as far as other similar displacement, ported motors but I am happy with it. Add some compression and it helps.

 

[TheBuzzard]

just making the exhaust port wider is going to help but the transfer ports are also a big part of getting power. thats why limited class racers always wanted a double zero cylinder stamp from the factory. thats the best sleeve port alignment with the casting and it gives you the best flow without porting.

 

[oside-tanner]

Isn't the 64u the stamping on a 701 cylinder head, what does the stamping on the cylinder say

 

[TheBuzzard]

all 760 cylinders are cast 62T, just like 62T 701 cylinders. the 760 sleeves are a bit larger than 701 sleeves and thats why you cant just put a 760 cylinder on any other cases, other than 760, without a bit of machine work/grinding.

 

[lamar311]

I ran the same setup, I took a sport port template and enlarged it a bit to accommodate the extra bore size. I didn't do anything to change the port timing and didn't touch anything but the exhaust ports and very slight tunnel porting on the cases. It's a decent little motor, I have had very little to compare it to back to back as far as other similar displacement, ported motors but I am happy with it. Add some compression and it helps.

Did you see any performance gain? or did you not run the motor previous to the porting? I have it running at 190 compression right now (yes I know that is scary high). It pulls well and might not be worth pulling apart and porting but I might be to drop the port timing. I figured if I am dropping the port timing I might as well port it. But wanted some advice on the sport porting for the 701 cylinder. That way I could just buy the template and go from there.

 

[js/sj]

Focus less on exhaust port timing and focus on transfer angles. The reason 760 cylinders and 62t cylinders don't "hit as hard" mostly has to do with transfer port angles. The angle of theses cylinders decreases "bottom end" because of the scavenging levels. Have a real builder that actually knows these things build you something. I'm not saying exhaust port timing has nothing to do with a different power band but there is way more going on then most builders even know.

Sent from my DROID RAZR HD using Tapatalk 2

 

[lamar311]

This motor is kinda my transition motor before I have something built. So perhaps I leave it be for another season as it was hitting pretty hard but my ski was waterlogged like crazy.

 

[yamanube]

Did you see any performance gain? or did you not run the motor previous to the porting? I have it running at 190 compression right now (yes I know that is scary high). It pulls well and might not be worth pulling apart and porting but I might be to drop the port timing. I figured if I am dropping the port timing I might as well port it. But wanted some advice on the sport porting for the 701 cylinder. That way I could just buy the template and go from there.

I pieced it together from parts so I never ran it without porting, it had more power than the previous 61x/61x unported motor I had, not as snappy down low but linear. It wasn't a monster and I wouldn't recommend it to anybody unless they had a bunch of cheap parts and time to kill, if you are paying for the parts/work, there are other places to put your money. I wanted to try porting my own motor, these was the least expensive combination of parts I had at the time, I will likely do some work on the transfers and a bit more to the cases and put it back together this winter. I lost a crank seal a while back and this motor got swapped in favor of a 62t 701 that went with the ski when I sold it.

 

[lamar311]

Thats where I am at with this motor. It came with the ski...I had sold the hull only while back I have read so much about port timing, transfers, etc, etc. Some say they can easily make this motor rip...others tell me its a slug and get rid of it.

For now...I 'll see if my other head will allow for the proper squish sans base gasket and if it does...I will pull it apart and port it. If not...I will leave it be.

 

[lamar311]

Anyone have any more information on this?

Thanks,
Brad

 

[lamar311]

top

 

[lamar311]

anyone else?

 

[motobehrens]

Your time and money is probably better spent tuning your pump to your existing engine, until you save up for a complete motor/pump package.

In any case, if you want to compare differences between the 3 different cylinders, I have some of each you can measure.

 

[lamar311]

I know you are probably correct. This motor was actually pulling pretty hard but it just doesn't have the snap a ported 701 with a bb sleeve has. Was wondering if there was any way to manufacture that as I have a pretty good set up. I did get the 10/16 hooker in there and it seems to pull very hard off the bottom. If I could get a little more out of the motor that pitch should be perfect. I might try running the milled 760 head that I have and run without a base gasket to drop the port timing a bit. Most I have spoken with tell me this is what I am missing.

I certainly would love to put a mic on the 701 61x cylinder to the 760 62t cylinder. I don't really want to sink any additional money into this motor but it is a brand new top end, only a season old. I am chopping the hell out of my hull so I was hoping to add a little additional pop. Due to the amount of water in the tray area I am sure I 'll already feel a difference but while its apart...why not?

Your time and money is probably better spent tuning your pump to your existing engine, until you save up for a complete motor/pump package.

In any case, if you want to compare differences between the 3 different cylinders, I have some of each you can measure.

 

[motobehrens]

If I could get a little more out of the motor that pitch should be perfect.

That's a very expensive mentality!

My 10/16 actually measures out closer to 11/17. Yours may be the same. My FX1 setup is similar to yours, and I run a much shallower pitch. There was a very large acceleration difference between the 8/14 and the 10/16(11/17)

Finish your hull and we can throw on a shallow pitch for a couple hrs to see how it responds.

 

[lamar311]

That's a very expensive mentality!

My 10/16 actually measures out closer to 11/17. Yours may be the same. My FX1 setup is similar to yours, and I run a much shallower pitch. There was a very large acceleration difference between the 8/14 and the 10/16(11/17)

Finish your hull and we can throw on a shallow pitch for a couple hrs to see how it responds.

Chris..I think you are right..that is an expensive thought process. I could very well be over propped but Imrpros thought I would be right on. Fortunately I can repitch my prop 2 degrees up or down (do you ever do your own pitch work?...I never have but have a buddy that does it).

Chris at Epic thought 9/15 would be better for my application but I would be very interested to try a shallower pitch. I really appreciate your help. I would really prefer to leave my motor be as it is running great and has a fresh top end.

 

[lamar311]

Well Chris the hull is complete. I am about to install trim set up with a larger bore nozzle and want to take you up on swapping out into a more shallow prop. I think I should run it one last time with the 10/16 and see how she reacts to the larger nozzle. It only 2mm more than what I currently have on it but the power is definitely there ..just not on immediate pull. I yank the gas and get some response but it takes about 1/4 of a second before I get the snap I am looking for.

The ski is really nose high but really is a blast to ride.

Here is the link on pwc to the build.
http://www.pwctoday.com/showthread.php?t=417485

I still have some cosmetics to clean up but she's pretty much done.

 

[motobehrens]

You finished that thing pretty quick! Looks good. Sure, let's try a different prop. Just let me know what your schedule is, via pm or fb.

 

[mxr181]

what did you figure out with this setup? Considering building a similar setup (lowered & ported 64X w/ special domes). What prop did you end up going with?