Engine wants dwell above 4.0 ms max. duration

[Don]

Hello,

I've been having problems with firing at upper RPM ranges and found that the engine wants >4.2 ms of Maximum dwell duration. At 4.2, it runs well and feel it would like more but haven't tried it yet. The coil is a MSD blaster #8226.

I have the 8 pin stock Ignition module and should need to be set at 2.5ms, not 4.2ms.

Is this OK ?

Has anyone successfully run at this high of duration ? Maybe something somewhere else in my tuning is messed up making the need for high duration. The voltage change table for dwell is set at default.

Any help would be appreciated.

Don

[Matt Cramer]

As long as the module isn't getting hot and your spark output is set to going high / inverted, I wuldn't worry too much.

[WASyL]

Matt will it be similar for SBC engine?? i also have problem with running over 4k RPM, EGT goes over 950C deg. and car loses power, may it be that dwell is to short ??

best regards

[Don]

Hi Matt,

Thanks for the reply!

Although 4.2ms may be safe to keep using, I would like to understand why this engine will not run well at 2.5 ms which is per the MFG specs and as stated in many places in the megasquirt manuals for a GM small cap, external coil set up.

The Megasquirt references tell me that I may have other things that will affect it but does not say what and I cannot seem to find what it is - see below.

"Do not use misfiring and high engine speeds and loads to set the dwell - misfires under those conditions will likely be caused by other factors, and will lead to you setting very long and damaging dwell periods. Recall that the dwell is shortened with higher rpm in any cases, so increasing the maximum dwell setting doesn't help at higher rpms. For example, with a 3.5 millisecond dwell time, and a 2.0 maximum spark duration, the rpm at which the dwell begins to be reduced is just 2727 rpm on a V8."

I truly believe that this engine should run very well with 2.5ms and would like to correct whatever is making the >4.2ms duration requirement.

Any help to find what could cause this longer duration would be greatly appreciated.

Don

[Don]

One more thing. The engine revs nicely with low load at 2.5ms and a slightly opened throttle. Under load and more throttle opening it misses or bogs at higher RPM's and wants the longer duration dwell, which fixes the issue. So I am targeting troubleshooting whatever the TPS controls as the possble problem area to look into.

Any help is appreciated.

[Matt Cramer]

The TPS doesn't really affect fuel or spark directly, though the MAP sensor reading will change the amount of advance.

Do you have an oscilloscope available?

[Don]

Matt,

I do not have an oscilliscope handy, I wish I did. I did do some reversal of the things I did and found that the Pulstar plugs that I put on were the cause. I actually have the "Direct Hits" made by Pulstar. Since they are added to a stock vehicle and required no ignition adjustment, I did not think they were the cause. As soon as I removed them, the truck ran great at 2.5ms.

Below is the web address for those plugs if you are interested.

http://www.pulstar.com/

Matt - Thanks for the continued support with my issues now and in the past. I am almost finished with tuning and will be going thru emissions in the next week or two.

The truck is running excellent ! I will be increasing the boost from ~5 - 6 PSI to 10 PSI once thru emissions, then, once that pressure is tuned, will be going to 14 PSI - the same as the Syclones ran with.

Happy Holidays !

Don

[Matt Cramer]

Matt,

I do not have an oscilliscope handy, I wish I did. I did do some reversal of the things I did and found that the Pulstar plugs that I put on were the cause. I actually have the "Direct Hits" made by Pulstar. Since they are added to a stock vehicle and required no ignition adjustment, I did not think they were the cause. As soon as I removed them, the truck ran great at 2.5ms.

Very interesting. Sometimes those plugs work like normal ones and sometimes they seem to cause a lot of issues.

[Colt]

What ignition driver are you using?