SX/SXI Timing curves and advance plate

[brker1]

Hello,

I am hoping you can clear things up regarding the timing curves of the sx/sxi and their years. This is all in regards to running the R&D advance plate and if it is safe to do so on the sx, sxi, sxi pro.

Do all the big pin dual carb sxi's have the same timing curve. From what i've been able to read is that the 95sxi's differ from the 96 and later years and that all the sx's are the same... is this true

I guess this all boils down to is if the advance plate is worth the potential trouble of toasting an engine if the tune is not spot on or you get some bad gas.

I have a 95 sxi and am now questioning running the advance plate or not

Any insight would be much appreciated

[Group K]

Re: SX/SXI Timing curves and advance plate

I'll start with some basics. Every "ideal" modern ignition has an "advance-retard curve". This curve is managed by the CDI box. At low starting speeds, the ignition timing is somewhat retarded for easy starting. As rpms increase towards 6000rpm, the CDI box advances the timing very quickly to a "maximum advance" at about 5000-6000rpm. As rpms increase beyond 6000, the cdi progressivly retards the ignition timing. By offering this variable timing, the CDI offers strong mid-range acceleration, along the the desireable retarded timing values needed to stave off detonation at high rpms.

Based on the parts program I have, all the Kaw CDI boxes (which create the advance retard curve) are the same from 1995 to the last SXR. I know that all these boxes also have an 8000rpm limiter (higher than anyone needs to turn).

When you install an advancer plate, you increase the ignition advance at "ALL" rpms ... not just mid-range. This added advance can work okay for stock-class and freestyle engines that are not held at rpms for a very long. But for closed course engines that run at high rpms for a long time, the added advance of the plate (at high rpm) is a very bad thing that can result in rapid overheating and/or piston scoring.

For this reason, aftermarketers have made racing CDI boxes that offer even more advance at 0-6000rpm only (to help low end acceleration), and then retard sharply after 6000 to the same high rpm timing that a stock CDI box offers. You would think that such a CDI box is ideal for all applications.... but not so.

Race boats usually run on race gas, and are very seldom "cruised" at 5000-6000rpm. If you install a racing CDI box on a 91-octane setup, it will generally work very good as long as you hold it at peak rpm most of the time. However if you "cruise" your 91-octane boat (with the racing CDI) at 5000-6000 rpm, there is a very high chance that the heavy mid-range advance will cause detonation ... and swiftly thereafter a siezed piston. This is a very common occurance on well modified 91-octane SuperJets that use the MSD box. The MSD box is great for racing .... but very risky in a 91-octane boat that might occationaly get cruised..... we see it all the time.

Hope that helps. Respectfully, Harry Klemm

Hello,

I am hoping you can clear things up regarding the timing curves of the sx/sxi and their years. This is all in regards to running the R&D advance plate and if it is safe to do so on the sx, sxi, sxi pro.

Do all the big pin dual carb sxi's have the same timing curve. From what i've been able to read is that the 95sxi's differ from the 96 and later years and that all the sx's are the same... is this true

I guess this all boils down to is if the advance plate is worth the potential trouble of toasting an engine if the tune is not spot on or you get some bad gas.

I have a 95 sxi and am now questioning running the advance plate or not

Any insight would be much appreciated

[JonnyX2]

Re: SX/SXI Timing curves and advance plate

Awesome words of wisdom!

[honda_killer]

Re: SX/SXI Timing curves and advance plate

Ok now what about the 94 750 sx do the cdi box have no ign curve at all? How would a r&d 6° plate work with this? Good? Bad? I bought one a while back and been a little worried about installing it. as far as the ski goes it a sp with factory wet pipe and milled head about 175 180 compression always ran on 91 thinking about e85 in the future.

[wielgot]

Re: SX/SXI Timing curves and advance plate

Ttt

[kweasel]

Re: SX/SXI Timing curves and advance plate

Ttt

You need to mark the flywheel check the timing with a light. I have two early 1992 cdis marked 3729 that advance 5 and never retards, this is bad with aditional static timing. The latest 750 cdi 3738, advances above idle and retards 5 degrees around 5000rpm. There are a couple of boxes that are common but I have not checked, 3733, 3734, 3735. Just make sure the high rpm timing is down below 17 degrees for pump gas.

[JonnyX2]

Re: SX/SXI Timing curves and advance plate

Ok now what about the 94 750 sx do the cdi box have no ign curve at all? Yes it has a curve. How would a r&d 6° plate work with this? Good? Bad? Please read Harry's reply he addresses installing the advance plate. I bought one a while back and been a little worried about installing it. as far as the ski goes it a sp with factory wet pipe and milled head about 175 180 compression always ran on 91 thinking about e85 in the future. Raising compression increases chances of detonation. Lowered octane increases chances for detonation. Advancing the timing increases the chance for detonation.

I am curious though what is the advertised octane rating of e85

[88kawi5fiddy]

Re: SX/SXI Timing curves and advance plate

the octane rating is fetched-up carbs if you ever let the ethanol sit in the tank, carbs, or engine. e85 makes power, but ethanol attracts water like nobodies business, and boats, they go in water. It would solidify diaphrams in, I imagine, a couple tanks or less if carbs aren't drained...

Per wikismedia - "If a retailer chooses to post octane, they should be aware that the often cited 105 octane is incorrect. This number was derived by using ethanol’s blending octane value in gasoline. This is not the proper way to calculate the octane of E85. Ethanol’s true octane value should be used to calculate E85’s octane value. This results in an octane range of 94-96 (R+M)/2. These calculations have been confirmed by actual-octane engine tests."

so 95avg +/- 3.

[honda_killer]

Re: SX/SXI Timing curves and advance plate

Yeah The fact that alcohol attracts water has been keeping me from using it in the ski. I use it in my turbo drift car and it loves it. Here in az they advertise e85 as 108 RON but i do understand it changes depending on the blend

Alos this is the statment that had me thinking that the pre 95 did not have an ign curve. "Based on the parts program I have, all the Kaw CDI boxes (which create the advance retard curve) are the same from 1995 to the last SXR" so do the 94 just have a different curve

[wielgot]

Re: SX/SXI Timing curves and advance plate

You need to mark the flywheel check the timing with a light. I have two early 1992 cdis marked 3729 that advance 5 and never retards, this is bad with aditional static timing. The latest 750 cdi 3738, advances above idle and retards 5 degrees around 5000rpm. There are a couple of boxes that are common but I have not checked, 3733, 3734, 3735. Just make sure the high rpm timing is down below 17 degrees for pump gas.

is that all it has is 5* on the 3729 cdi. Im running that cdi on my 750 SC with the advance plate and it pulls hard!!

[kweasel]

Re: SX/SXI Timing curves and advance plate

is that all it has is 5* on the 3729 cdi. Im running that cdi on my 750 SC with the advance plate and it pulls hard!!

Im talking about the timing curve. The old box advances 5 and stays there just like a yamaha superjet cdi. Kawasakis cant use as much high rpm advance so it is important to know exactly where the timing is. A newer cdi will respond as well with lower octane requirement.