SGX 91RS

[Chris Wood]

allen you are not alone.. i have talked to alot of people that have smoke holes in the pistons.. i believe the new versions comming out will not have a dish in them and that should help

chris

 

[dwilfong]

Yes Ray .010" is fine on a 91. Tom, I knew his head volume must have been a communication error.

Yes the pipe needs to be .470" all the way David. As I recall the 80 stinger is s only about .420" I would stick with the 80 pipe. The 90 pipe will only give you more torque that you don't need.

6.3 cup is too much for a 90 SGX unless your looking to go 100 mph. 5.8" cup on that H-50 will put you in the low 90's and the boat will handle better. Just shorten the pipe and let it breath (stinger) and you will go fast with little effort.

Thanks Andy. Will be fun trying to open up the inner stinger. Your memory is real good .420 it is. Got to scratch my head a little on this one.

Ya I did bend it a little to much at the pond. But it sure did go. Think I will hold off on propping it up till I get the hang of it.

The fancy button and all the whittling on the P/S it sure seams to like the cup. the stock 1667 made it jump out of the water at launch.

Just want to keep up with the new twins. :lol:

David

It worked!!!

The RPM's were over the top. Way over the top blew the guts out of the plug seal and all. Could not catch up with the needle and burn a pin hole in the piston. :angry:

Time to start over.

Just did the same thing today!!! Recently Andy Brown modded running 1667 @ 6.5" Macs 15cc at 11-1/2" with 65%. I knew it was lean from the last time out and I had to go to the manual adjustment on the needle. I backed it out 1/2-3/4 turn but I guess with the cool weather and the added pitch on the prop it wasn't enough. Yes, I am frustrated but I guess it happens. I will tell you this, I have never had to be so "gentle" with a motor when it comes to rich/lean as I do with the CMB 101. I remember my 2 AB modded Mac 45's as well as a 67 and I could run on the edge every run (take a plug every time even) and never put a hole in the piston or chipped up the edge of the piston and head button like I have seen on the CMB!!!! This is my second piston/sleeve in this motor. First, stuck. Second, hole in piston!!! On a side note, boy was it fast!!!!!!

Allen

This is my second P/S on this eng. The first one came with the eng used. Put a hole in that one with 50%. modified the original button to get the chamber size and shape where I want it. Juggled the cc , timing and squash and came up with a winner. Thanks to Marty Davis and his new eng Program software. The MSV was the trick to running high nitro.

Just need a good prop to put the power to the water.

David

 

[dwilfong]

allen you are not alone.. i have talked to alot of people that have smoke holes in the pistons.. i believe the new versions comming out will not have a dish in them and that should help

chris

Chris what this about the new P/S not having a dish. Any Idea when this will happen.

David

 

[Buckshot]

I feel your pain David.

I had to go with billet pistons in my twin to keep them together.

Chris,

I belive that they are using Grade S**T Aluminum for their pistons in the 91's & 101's.

B

 

[Tom Foley]

I feel your pain David.

I had to go with billet pistons in my twin to keep them together.

Chris,

I belive that they are using Grade S**T Aluminum for their pistons in the 91's & 101's.

B

Who is doing Billet Pistons that have a good fit ? I have spoken to Paul Stirton, Aus. but have not sent the sleeves over ...probably would have been done by now . Also some enlighten me as to the " WHY " behind the dished piston if we are having to increase combustion chamber size anyway ? I burned one as did Mike Costanzo now you , David , Allen , Chris , whats going on ?

 

[Buckshot]

I feel your pain David.

I had to go with billet pistons in my twin to keep them together.

Chris,

I belive that they are using Grade S**T Aluminum for their pistons in the 91's & 101's.

B

Who is doing Billet Pistons that have a good fit ? I have spoken to Paul Stirton, Aus. but have not sent the sleeves over ...probably would have been done by now . Also some enlighten me as to the " WHY " behind the dished piston if we are having to increase combustion chamber size anyway ? I burned one as did Mike Costanzo now you , David , Allen , Chris , whats going on ?

Tom,

All the old Evo and EvoII 100's I have also have a dish in the piston but have never blew a hole in them. I have an RS 101 that I bought from Tidewater that has never had an issue with the piston like the latest models. I blew thru two pair of the latest RS 101's in my twin before I knew what was wrong. I'm almost certain that CMB made a change to their casting material in the latest 101's and this is why everyone is having an issue. If they are just going to make them flat tops then the problem will not go away.

I don't know of anyone other than Brad that is making pistons for customers. I made my own and have no desire to build them for anyone else because it is a TON of work.

B

p.s. I have NEVER had to increase the combustion chamber on the big motors but then again I don't feel it is necessary to run "BIG" nitro in them either.

 

[Piston1]

I feel your pain David.

I had to go with billet pistons in my twin to keep them together.

Chris,

I belive that they are using Grade S**T Aluminum for their pistons in the 91's & 101's.

B

Who is doing Billet Pistons that have a good fit ? I have spoken to Paul Stirton, Aus. but have not sent the sleeves over ...probably would have been done by now . Also some enlighten me as to the " WHY " behind the dished piston if we are having to increase combustion chamber size anyway ? I burned one as did Mike Costanzo now you , David , Allen , Chris , whats going on ?

Tom,

All the old Evo and EvoII 100's I have also have a dish in the piston but have never blew a hole in them. I have an RS 101 that I bought from Tidewater that has never had an issue with the piston like the latest models. I blew thru two pair of the latest RS 101's in my twin before I knew what was wrong. I'm almost certain that CMB made a change to their casting material in the latest 101's and this is why everyone is having an issue. If they are just going to make them flat tops then the problem will not go away.

I don't know of anyone other than Brad that is making pistons for customers. I made my own and have no desire to build them for anyone else because it is a TON of work.

B

p.s. I have NEVER had to increase the combustion chamber on the big motors but then again I don't feel it is necessary to run "BIG" nitro in them either.

To eliminate detonation piston burns open the volume to .9 - 1cc. Try it, you will like it. It can be done using primitive tools or send your head button to Steve Woods and he will take care of you..

 

[Joe Warren]

A few of us are currently Running RS101 & Vac 91`s on 60-65 perent. The head volume With the piston volume needs to be approx 1.0cc Squish .010.012. Do not mod to intake timing excessive. clearence the intake valve to .003 .005. One of the bidggest problems the RS91-101 have is that they over cool. If you have a third channel you will contuine to needle down to increase heat so it will hit the pipe. Most all of the rs101`s will benifit to restrict the water. Plug one hole in the divider plate run one open... 2 holes open is Way too much cooling....... The success will increase if you let the engine make heat & not needle down to make heat....

 

[steve wood]

new 1.01 pistons do NOT have the dish,and new engines and piston/liner sets are available now!

 

[Chris Wood]

from my understanding guys the thickness in the middle of the dish is thin VERY thin. and that is part of the reason they are burning holes in them. not enough material thickness in the dish area of the piston.

as dad said the new ones no longer have the dish

chris

 

[Marty Davis]

from my understanding guys the thickness in the middle of the dish is thin VERY thin. and that is part of the reason they are burning holes in them. not enough material thickness in the dish area of the piston.

as dad said the new ones no longer have the dish

chris

Chris:

That will make a change in the head button necessary because of the area that is in the dished piston is no longer there.

 

[Piston1]

from my understanding guys the thickness in the middle of the dish is thin VERY thin. and that is part of the reason they are burning holes in them. not enough material thickness in the dish area of the piston.

as dad said the new ones no longer have the dish

chris

Chris:

That will make a change in the head button necessary because of the area that is in the dished piston is no longer there.

That would be approximately .1cc volume addition.

 

[Chris Wood]

from my understanding guys the thickness in the middle of the dish is thin VERY thin. and that is part of the reason they are burning holes in them. not enough material thickness in the dish area of the piston.

as dad said the new ones no longer have the dish

chris

Chris:

That will make a change in the head button necessary because of the area that is in the dished piston is no longer there.

Yes it will marty.. not sure if cmb is going to change anything or not on the head

chris

 

[dwilfong]

On my current head in my .91 I am running .95cc with the dish and .014 clearance. The button is designed different than the stock shape. It is a modification of the design Marty did on my 45RS and MAC 21.

With the clearance at .010 I blew the hole in the piston with this head. I made a timing change and set the MSV in the range Marty specified and It is golden now.

I think there is some thing to be said about chamber shape in keeping these eng alive on high nitro.

Also the modification I use on the P/S reduce friction area as well as open up the intake track and shorten the runner to the boost and transfers,

Have blocked the lower chamber on the cooling head. But am going to make a new cooling jacket to only cool the top of the button leaving the bolts on the out side.

The P/S is new from Stu only about 8 weeks ago and it has the dish. That is why I was asking when this will happen.

Where can you get the right alloy for making pistons. Want to start making my own.

David

 

[Rick Doane]

On my current head in my .91 I am running .95cc with the dish and .014 clearance. The button is designed different than the stock shape. It is a modification of the design Marty did on my 45RS and MAC 21.

With the clearance at .010 I blew the hole in the piston with this head. I made a timing change and set the MSV in the range Marty specified and It is golden now.

I think there is some thing to be said about chamber shape in keeping these eng alive on high nitro.

Also the modification I use on the P/S reduce friction area as well as open up the intake track and shorten the runner to the boost and transfers,

Have blocked the lower chamber on the cooling head. But am going to make a new cooling jacket to only cool the top of the button leaving the bolts on the out side.

The P/S is new from Stu only about 8 weeks ago and it has the dish. That is why I was asking when this will happen.

Where can you get the right alloy for making pistons. Want to start making my own.

David

I am having a hard time trying to figure out why you guys are running such low head clearance on these motors. Can someone explain what advantage this has?

Rick

 

[Joe Warren]

On my current head in my .91 I am running .95cc with the dish and .014 clearance. The button is designed different than the stock shape. It is a modification of the design Marty did on my 45RS and MAC 21.

With the clearance at .010 I blew the hole in the piston with this head. I made a timing change and set the MSV in the range Marty specified and It is golden now.

I think there is some thing to be said about chamber shape in keeping these eng alive on high nitro.

Also the modification I use on the P/S reduce friction area as well as open up the intake track and shorten the runner to the boost and transfers,

Have blocked the lower chamber on the cooling head. But am going to make a new cooling jacket to only cool the top of the button leaving the bolts on the out side.

The P/S is new from Stu only about 8 weeks ago and it has the dish. That is why I was asking when this will happen.

Where can you get the right alloy for making pistons. Want to start making my own.

David

David I received 2 New RS101 P/L today they are Both FLAT TOP pistons......

 

[dwilfong]

On my current head in my .91 I am running .95cc with the dish and .014 clearance. The button is designed different than the stock shape. It is a modification of the design Marty did on my 45RS and MAC 21.

With the clearance at .010 I blew the hole in the piston with this head. I made a timing change and set the MSV in the range Marty specified and It is golden now.

I think there is some thing to be said about chamber shape in keeping these eng alive on high nitro.

Also the modification I use on the P/S reduce friction area as well as open up the intake track and shorten the runner to the boost and transfers,

Have blocked the lower chamber on the cooling head. But am going to make a new cooling jacket to only cool the top of the button leaving the bolts on the out side.

The P/S is new from Stu only about 8 weeks ago and it has the dish. That is why I was asking when this will happen.

Where can you get the right alloy for making pistons. Want to start making my own.

David

David I received 2 New RS101 P/L today they are Both FLAT TOP pistons......

Looks like I just missed the boat.

Great for the next project though.

Did the price go up. :lol:

 

[Joe Warren]

On my current head in my .91 I am running .95cc with the dish and .014 clearance. The button is designed different than the stock shape. It is a modification of the design Marty did on my 45RS and MAC 21.

With the clearance at .010 I blew the hole in the piston with this head. I made a timing change and set the MSV in the range Marty specified and It is golden now.

I think there is some thing to be said about chamber shape in keeping these eng alive on high nitro.

Also the modification I use on the P/S reduce friction area as well as open up the intake track and shorten the runner to the boost and transfers,

Have blocked the lower chamber on the cooling head. But am going to make a new cooling jacket to only cool the top of the button leaving the bolts on the out side.

The P/S is new from Stu only about 8 weeks ago and it has the dish. That is why I was asking when this will happen.

Where can you get the right alloy for making pistons. Want to start making my own.

David

David I received 2 New RS101 P/L today they are Both FLAT TOP pistons......

Looks like I just missed the boat.

Great for the next project though.

Did the price go up. :lol:

About the same.

 

[dwilfong]

Testing today went real good. Andy hooked me up with a prop.

55% at 11" and the throttle is as sharp as a tack.

Now its time to mess with the strut and loosen it up just a tad.

Thanks Andy for the killer prop.

David

 

[allenrichards1]

This is with a bum piston that I "stuck" a few months ago. Its running real rich but at least its running again without having to buy a new piston and sleeve.