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DT360A Variable Timing CDI

  • 2noSmoke
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DT360A Variable Timing CDI was created by 2noSmoke

Greetings all,
I'm wanting to try a modern dynamic timing CDI on my DT360A.
The engine porting and head is completely standard, is fitted with an expansion chamber, richer jetting to suit, and much better freer flowing air filter. I'm running 98 Ron fuel.
Does anyone on this forum have experience with this mod and what was the maximum timing advance the engine would accept without detonation issues ?







Power on regardless. 
17 Oct 2021 06:39 #1

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Replied by GFrench on topic DT360A Variable Timing CDI

how much compression do you have right now?
If it ain't smokin', it's broken!
1970 RT1 360
1970 RT1 360
1971 CT1 175
1971 DT1 250
1971 DT1 250
1971 DT1 250
1971 RT1 360
1971 RT1 360 "Black Widow"
1974 DT 360
1976 IT 400 / redesigned as a 75 MX400
1978 XT 500
1980 YZ 465
1984 IT 490

17 Oct 2021 07:06 #2

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Replied by akara1 on topic DT360A Variable Timing CDI

The timing you can get away with depends on a few factors such as 
Squish clearance
compression ratio
fuel quality
spark quality
jetting
expansion chamber

I can get away with 4mm btdc at lower engine speeds and then retarding to 2.5mm btdc at 6000rpm.

I use a tight squish clearance, stock DT head volume, 93 octane, jetting is clean
The following user(s) Liked this Post: MarkT
Last edit: 17 Oct 2021 10:12 by akara1.
17 Oct 2021 07:48 #3

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Replied by MarkT on topic DT360A Variable Timing CDI

Yes, you definitely want to have timing retard as rpm increases
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
17 Oct 2021 07:59 #4

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Replied by 2noSmoke on topic DT360A Variable Timing CDI

Currently have 144 psi, throttle closed cold cranking, ( I know, forgot to open throttle)
The fully retarded static timing range of the stator plate provided by the new dynamic CDI had the timing rising to nearly 30 degrees BTDC, ( timing light but haven't converted to BTDC yet) at around 3,500 which I think it's way too much and evidenced by the severe detonation above 170 degrees head temp. (Temp sensor under head attach nut)
I am aware the head design of these Dt360's is not ideal, but want to see what can be achieved before modding the head.
Ankara, do you know what 4mm BTDC equates to in degrees ? Also when you say lower revs, approximately what rpm ?
17 Oct 2021 08:37 #5

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Replied by akara1 on topic DT360A Variable Timing CDI

my exhaust Port is higher than yours so my compression is roughly 130psi.
 

I am using timing curve 8A in this picture but slightly more advanced
so I see around 24 degrees at 3,000 and 18° at 6,000 roughly

 
Last edit: 17 Oct 2021 10:26 by akara1.
17 Oct 2021 10:26 #6

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Replied by 2noSmoke on topic DT360A Variable Timing CDI

Thank you for your reply Akara, that is the type of info I was hoping for, and very much in line with what I would expect to be about right for these motors. 
Best I can measure, standard 2.9 mm is 22 degrees, Yamaha are no fools, (even in the seventies ) so a safety margin for us lessor folks and varying fuels around the world would be wise at 22 deg.
I was just wondering what others might have been able to achieve, knowing it's very subjective to individual engines.
I did contact the manufacturer, but did not receive answers to my questions about their timing advance range for this engine. Will try again.
Did you have your head machined to alter the squish band and also restore the volume ?

Regards, 



Power on regardless
 
17 Oct 2021 18:24 #7

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Replied by MarkT on topic DT360A Variable Timing CDI

I don't believe Yamaha was conservative with their timing specs. 

Two strokes have very different timing requirements than four strokes... As rpm increases, two stroke timing requirements decrease.  At high rpm (~10,000+ rpm) the turbulence, squish, and pipe design of a good two stroke will make most power at about 15 degrees advance. 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
17 Oct 2021 18:56 #8

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Replied by 2noSmoke on topic DT360A Variable Timing CDI

I just read Turbodan's thread about the SC500 build where he mentions the CDI he has on a DT360A, 
I'm hoping he will chime in with his experience of that unit on the 360.
 
17 Oct 2021 22:52 #9

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Replied by 2noSmoke on topic DT360A Variable Timing CDI

Well I guess I screwed up, I didn't ask about the timing curve before purchasing the dynamic curve CDI.
The below is the response from the manufacturer when I enquired about the curve after not being able to adjust out severe detonation once it reached about 170 degrees head temp., despite extending the adjustment slots to further retard it.
I would be very surprised to learn that there are many, if any two strokes out there running correctly with 30 degrees ignition advance.
Currently I've modded the stator to reduce the maximum advance to 27 deg. (5.1mm ) however this is still way too far advanced and detonating badly.
Has anyone here had positive experience with reducing the squish clearance that might allow use of this CDI ?
Or back to the fixed timing I go.

Reply below:

"Hello Max

Yes the ignition does advance to these levels (22 deg.@1,100 rpm to over 30 deg.@ 3,500 rpm) before retarding off. This can be controlled by virtue of the electronics in the box at the design stage. The curve included in the kit price has been tested and works very well in the majority of engines.

The kit is sold as is with the one timing curve (as supplied), unfortunately at this time we are not able to offer a design service to produce different curves. We have available only the stock CDi units. You can alter the slots which will move the datum point of the timing curve"
21 Oct 2021 05:32 #10

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