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GM Coil Modification

LongDistanceRider

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The stock Honda coil was rated at 15kV when brand new 35 plus years ago and with age has surely degraded. The answer to getting good spark voltage lies in starting with a good coil. The CDI system is spec'd to use .55 ohm primary resistance coil and although it seems to be OK to use one that's 1 ohm or less it's best to stick to spec. Original spec in 1978 was .3 ohm.
GM D555/576 coils from the mid '80's thru 2006 fit the bill nicely.
The GM engines that seem to require the best coil output are the 2000-2005 Bonneville SSEi versions. Here's a 20011 dated post comparing versions http://www.pontiacbonnevilleclub.com/forum/2000-2005-other-than-gxp/topic29036.html I'm sure that there have been more version created but not all are the same. Lately you can buy 3 of this type coil for @$20 on eBay, probably need all 3 to find a half way decent one.

First issue is the coil does not fit under the tank like stock. If you're willing to sacrifice looks for function this is the way to go.
201_3954.jpgYou can use the original front coil mount hole for one end of the coil, longer bolt needed.
Then make a bracket to extend from the original coil rear mounting hole to the other coil mount hole. This prevents the coil from vibrating too much and cracking.
These are what I used to mount mine
201_3961.jpg
The white spacer is to give the primary wire clearance from the frame, black spacer for the bracket to the frame so the bracket is flat. Long bolt for the coil and spacer to frame. 1 short fot coil to bracket and the other for bracket thru spacer to frame.

Second issue is the primary wires. You can cut the wires off an old coil so you have the connector or Vintage Connections can sell you the pieces needed. Now for attaching the wires to the coil. It's a small female spade connector inside. I don't have a part number to look for there. That's the plug in version. Next are 2 solder versions.
I chose to remove the white plastic cap to get access to the terminals to solder the wires on. The cap pries off and the tabs will break so plan on gluing back in place. Drill the terminal holes big enough for the wires to pass thru. Put the wires thru and solder the wires to the terminals. It doesn't matter which wire goes where. Glue the cap in place and seal it with liquid electrical tape. You'll want to set the spacer in place beforehand.
Other choice is mark the coil for an exact location of where the terminals are on the top side of the coil. VERY carefully drill in just far enough to get to the terminals. TOO fast or far and the terminals are damaged or torn loose. The solder the wires to the terminals and seal the holes with liquid electrical tape.

Plug Wires: The NGK plug caps will just barely accept 8mm plug wires. You can use a standard graphite core GM plug wire, expect them to last a year or so since they're exposed to the elements and prone to braking internally. Ideally you want wire core plug wires, you're unlikely to find them in the local auto parts store. Some of the speed shops or custom truck shops have them. Originally I used some car wires but they failed after the first year. I ordered a pair of Taylor wires but it appears they only sell set now. An internet search for solid core wire will give results for you. http://www.taylorvertex.com/product-search/?id=220444&sID=61

Pros: Good spark voltage, wider plug gap (I use .032"), smoother idle because the mixture is burning better with the hotter spark
Cons: It's not good looking

Some pictures that may answer your questions
201_3959.jpg 201_3960.jpg 201_3962.jpg 201_3964.jpg
201_3966.jpg 201_3967.jpg 201_3968.jpg 201_3969.jpg
201_3971.jpg 201_3973.jpg
 
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Is this GM-coil better than the replacements you can buy today for our Honda Twins? I like the idea of upgrading , but I don't like how it looks on the bike and I don't like the hassle to make your own spark plug cords (it's really not that simple to score loose cords with the right specs and caps).

What about this ignition coil: https://www.rexs-speedshop.com/product/htc1-suzuki-twin-lead-cdi-coil-gt-250-500/
It's meant for a Suzuki GT250 / GT500 but it has the right specs ("Strong sparking twin lead ignition coil manufactured for the Suzuki PEi system (CDi ignition). 90mm between mounting hole centres. Resistance: primary 0.6 – 0.8 Ohm, 14K ohm secondary. HT leads are removable.").

Do you think this will be a good alternative for my CB400T, or will the GM still be a better option? Any thoughts?


CoilGT500.jpg
 
The stock coil when new produced @15K volts and over time that will decrease.
The GM coil depending on which one is used produces 30K-45K volts.
I don't know what the Rex's coil produces but I would guess it's in the 15K range.
Yes, the GM coil is ugly.
The benefit of higher spark voltage is a cleaner burn and that produces a smoother idle and idle speed can actually be set lower than 1200. Sparkplug gap is also set wider which contributes to the cleaner burn.
I'd offer to send the parts but last time I sent to Belgium the pieces sat for 6 weeks in Customs and Max had to pay import duty on used parts. Plus shipping has gotten insane.
 
Thank you for your input, LDR!

15K volts as opposed to 30-45K indeed smothers all the doubts against this upgrade, lol. I couldn't find the exact specs of the GM coil (Bremi 20425 for Europe), but now I understand the benefits of it better. I will certainly keep this in mind. I am now in the middle of figuring out why my new (old) CB400T is starting so hard when cold. If it turns out to be the coil, I 'll go for the Bremi.

The hardest part seems to be finding the spark plug wires. Thanks for the offer, but ordering from the US is insane, when in Europe. Customs and shipping are indeed sky high. It's hard to find loose spark plug cords, and if you find them, they are not copper cored. Do you maybe have a partnumber of the ones that you use? Can I use a loose 90°/90° automotive plug wire, with correct cap (to spark plug) and boot (to coil), or do I need to chance the caps anyway?
 
The spark plug end caps are for EMI or RFI control. With the modern radio and TV electronics it's really not an issue, anyone with old stuff will notice you in the close vicinity when the static hits.
NGK plugs come with the automotive end cap on them so using auto plug wires will work fine, not as solid a connection as the stock rajah clip but good enough.
Solid core wires are going to be hard to locate but you might have good results by checking with race car/truck/bike shops since that's the preferred wire type. Copper or Stainless Steel. f they don't have them they should be able to tell you where to go. I did find these https://www.ngk.com/search/all?q=racing+cable
 
Thank you for your suggestions. I found some specialised shops online like choppershop.com and westlandcustoms.com, which also seem to sell the Taylor wires you have mentioned before, but those are very expensive here. I also have found lose 7mm copper cored wire that is sold by the meter, but I 'm not sure if that option will require special tools for crimping and fastening the correct caps and boots (EDIT: seems to be easy with those Rajah-clips, no tools needed!) I can try with some readymade 90-90 GM spark plug wires and can always change them afterwards if and when I find a better option. Thanks for helping me out! (y)
 
Histoparts-bougiekabel-set-universeel-6-cilinder-motor.jpg

Will those work? They are universal 7mm copper cored wires, with a 90° spark plug boot. Will the depicted 90° boots also fit on the SAE-plugs on the coil? (we use mainly the DIN-type coils here in Europe, I'm not familiar with the SAE type..). On the other end of the wire I will fit a NGK-resistor plug cap.
 
Thanks for these instructions! Mod fits my bike for sure, and it made a noticeable improvement. I might look at moving the coil somewhere else when I get some better plug wires later on
 
Out of curiosity, what is the difference between this and the following?
For your CM450 you want the first one, 1ohm, if your going stock replacement. The difference with the GM coil is output…

The stock coil when new produced @15K volts and over time that will decrease.
The GM coil depending on which one is used produces 30K-45K volts.

Tom - 1982 CM450E / Midwest USA
 
The stock CDI unit doesn't like higher primary resistance coils, latest spec for the primary resistance was .55 ohms in the 1986 CB450SC FSM. Under 1 ohm is best, 1 ohm max. Excess resistance will affect the life span of the CDI.
Neither one of these coils is great in the resistance area but the Magna is the worst.
 
Thanks Tom and Jim - That makes sense. Good to know there is a reason for why other methods exist (such as this one!).
 
Under 1 ohm is best, 1 ohm max. Excess resistance will affect the life span of the CDI.
Neither one of these coils is great in the resistance area but the Magna is the worst.

I guess I better check mine; I have the 1 ohm 4-1 unit. (Another before I joined VHT replacement.)


Tom - 1982 CM450E / Midwest USA
 
The stock CDI unit doesn't like higher primary resistance coils, latest spec for the primary resistance was .55 ohms in the 1986 CB450SC FSM. Under 1 ohm is best, 1 ohm max. Excess resistance will affect the life span of the CDI.
Neither one of these coils is great in the resistance area but the Magna is the worst.

When purchasing an ignition coil, would the D576 coil work? The black one that's $70ish on ebay? I looked for the silver one that you were using and couldn't find one so I figured buying the nicest model would suffice. Is this correct? Is the D576 good for the GM mod?
 
The D576 is the highest output spark voltage of that type so that is the best one to get.
The grey one I have was for an early Chevy Cavalier and lower output. Need to replace it one of these days. At the time I did it 8-9 years ago this mod was fairly new and no one knew about the higher output coils.

Auto parts stores usually carry them in stock
 
Okay! Thankfully, that's the one I purchased from ebay. I got it for 63$ new. I bought a nice set of copper core plug wires to match. Would you reccommend iridium plugs with this mod? And if so what should I gap them to? And what should I set the idle to? When I get the D576 and check the resistance on the primary side, what should I expect to read? .33ohms? Also LDR, I sincerely appreciate your continued support with these bikes!! You're more helpful than you know.
 
I haven't used the Iridium plugs to date, just the regular D8EA's, so it's hard to say on the gap. Probably in the .030-.035 range is my guess.
Idle speed should still be set at 1200 although you may be able to drop it some.
Primary resistance needs to be 1 ohm or less but unless there's a published spec for that coil I have no idea. Doesn't matter as long as you note the beginning number for reference if you have ignition issues in the future.
 
I haven't used the Iridium plugs to date, just the regular D8EA's, so it's hard to say on the gap. Probably in the .030-.035 range is my guess.
Idle speed should still be set at 1200 although you may be able to drop it some.
Primary resistance needs to be 1 ohm or less but unless there's a published spec for that coil I have no idea. Doesn't matter as long as you note the beginning number for reference if you have ignition issues in the future.

When reading the resistance of the primary side on the acdelco coil D576 it's 2.1ohms. Does it need to be plugged in for the reading? Or unplugged? When unplugged that's what it reads and it's brand new. And tips?
 
When reading the resistance of the primary side on the acdelco coil D576 it's 2.1ohms. Does it need to be plugged in for the reading? Or unplugged? When unplugged that's what it reads and it's brand new. And tips?
That's way too high for resistance, it'll have a negative effect on the CDI. 2.1 sounds like it's defective right out of the box.
I can't find any actual specs on the coil but the D555 version is usually in the .8-.9 range
 
When reading the resistance of the primary side on the acdelco coil D576 it's 2.1ohms. Does it need to be plugged in for the reading? Or unplugged? When unplugged that's what it reads and it's brand new. And tips?

So it turns out that I had my multimeter lead plugged into the wrong hole, when I would test the leads it would read 1.8ohms. Not .1 so I swapped the negative lead into the other port and it read .1. I then tested the D576 and it read .5. Installed it, gapped the plugs to 40 and lowered the idle to 1100. She runs like a dream.
 
So it turns out that I had my multimeter lead plugged into the wrong hole, when I would test the leads it would read 1.8ohms. Not .1 so I swapped the negative lead into the other port and it read .1. I then tested the D576 and it read .5. Installed it, gapped the plugs to 40 and lowered the idle to 1100. She runs like a dream.
Now that sound much better.
 
It's kinda like when I brought my '74 Chevy into the new age with a junkyard HEI distributor, because as a poor college kid there was no way I could ever buy a new distributor, but let's please forget about the new OEM dash pad I paid $350 for from Perry Chevrolet maybe in 1987 ("Are you sure? These are just barely still available, we don't know what color it will be, and no refunds ...") and then let sit in storage for the next 20+ years before it got TOSSED.

Is this a fair summary?

This mod applies to "any" vintage Honda Twin. Maybe even something as archaic as a 2016 Rebel.

The D576 is the best version of this coil. Searching for "GM D576" reveals Rockauto (for example) currently sells the AC Delco copy for about $50; local auto parts stores should have equivalents. Find a way to mount it and provide it a signal.

Get a pair of wires that snap onto the coil, and that make either a snap or rajah connection for your spark plugs, now enjoying wider acceptable gap (assuming the orig spec plugs can safely be opened up). Wire cores are best, but graphite cores should work (my 40-yr old OEM graphite wires work fine?) Use quality wire caps with low resistance so that you don't freak out your CDI (if you have a CDI.)
 
Only my first para was OT. I asked if my summary to this thread was accurate. Now it makes no sense where it is.

Valid point. I copied your post back to this thread. Just trying to keep things in order, and I would do it for your threads the same as I would do it for anyone else's here.
 
I've been doing some research as I'm not interested in the funny mounting of the GM coils.

Dynatek has a "race coil" that is 0.7ohm and 11.8K secondary. Model number is DC9-1: https://www.dynaonline.com/dyna-igntion-coils-set-dual-output-0.7ohm-race-only-high-energy-dc9-1/. There are vendors on eBay that sell just that coil and not a set for ~$120. Dimensions look to be similar in mounting, and is actually a bit smaller at 40mm width compared to 45mm for the original. Mount spacing is about the same at 102mm.

The original secondary is 7-8K. What is it on the GM coils? Assuming 5K caps (or even 10K in Canada, which apparently works fine) that's ~7 + 5 + 5 = ~17K total resistance on secondary for OEM OR ~7 + 10 + 10 = ~27K for Canadian OEM. With XD01F caps you get ~12 + 1 + 1 = ~14K total resistance on secondary with Dyna, OR XD05 ~12 + 5 + 5 = ~22K. You might even be able to just run them without the resistor in the caps.

They also make a 0.5ohm that is "mini" coil that is 16K secondary. With XD01F's that's about ~18K total secondary which would be practically the same output as the US models and it's even closer to original spec. I don't know the dimensions of that one, but it's also cheaper: https://www.dynaonline.com/dyna-ign...al-output-0.5ohm-dual-plug-conversion-dc11-2/. The downside to this one is you'll need to crimp one end with a boot and solder, just like the GM coil mod but shouldn't be a huge deal.

Has anyone tried this? I don't see why it couldn't work, especially on a new CDI unit.

I'm willing to guinea pig one of them, but may take me a few weeks until I'm able to do so. It's a lot more expensive than the GM coil, but should last just as long and easily hide-able for those like me that want stock appearance.
 
Disregard part of what I said. I called Dynatek, they suggest that mini coil that's 0.5ohm. He said at 16K secondary it shouldn't matter as much, even with CDI and that's the one they use on a lot of their CDI conversion kits.

Dimensions:

4.2in long x 2.3in tall at widest point x 1.6in width.
2.9in to 3.5in distance from mounting points.

Converted to mm.

107mm L x 59mm H x 40mm W
Mount spacing: ~74-88mm

So, I believe this would work, it's just very tiny and you will have to make a small bracket for it to work because original is about ~102mm for mounting distance. The rep suggested this one to use based on 0.3-0.5 primary and 7-8K secondary for the CM400. I made sure to clarify that this is a true CDI bike and not a transistorized kettering ignition.

Took a look on eBay. The kit does come with the crimps and boots. So really all you need is the 7mm solid core wire. This can be found at treatland or the other vendors listed above. https://www.ebay.com/itm/380974784948

These specs are very similar to the Suzuki GT500 coil Rubicon posted a while back. He never said if it worked well or not. If it did then this Dyna coil seems like a good option.

What do you guys think?
 
I see no reason that this isn't a viable alternative.
The secondary resistance will have no effect on the CDI since it's not connected to it. IIRC the higher the secondary resistance the higher the spark voltage.
I would still use the 8mm wire core wires, more insulation for less EMI, and the XD05F resisted caps.
 
Sounds good. 0.5ohm is the lowest they make. I'll give it a try in a few weeks and report back, but I'm sure it will be just fine.

I sent treats an email asking about sourcing 8mm. You never know. Their 7mm is pretty good stuff.

EDIT:

Treats said they checked with their supplier and they don't have 8mm any more. In fact, their 8mm line was actually marketed for Dynatek. The manufacturer has no plans to bring it back or supersede it with something else.
 
Yeah buddy. My original coil was failing (I think, was getting worse and worse misfires and loss of power) so I went ahead and did this. The info in this thread made it so easy. Now I’m running better than ever. Thanks!
 
Haven't had a chance to try that Dynatek coil yet. Might be another month or two until I can do so. But I will get around to it. If it works well enough I'll have my buddy do a CAD drawing a mounting adapter and get some made up.
 
I got around to ordering the Dynatek Coil. Should be here next week and will keep you updated on the results. I think it should work fine, then will see about getting brackets made up as well as making the harness adapter.
 
Also, Jim, I took a look at NGK's site. They only have 7mm racing cables. I went ahead and just got 7mm from treatland. I know 8mm is preferred, but 7mm should be good enough for testing. My plan is to offer a plug and play kit with the Dynatek replacement if it works OK. I.e. 8mm wires with the XD05F's trimmed to the correct length, with new plugs, the bracket installed, and adapter. All you have to do is just replace the plugs and coil. I also plan to just make the adapter itself available for those who are OK with sourcing the rest of it on their own.

I guess finding a supplier for 8mm might be hard to do apparently. But we'll see.
 
The purpose of the 8mm is the added outer cover for insulation. In reality the 7mm will be perfectly fine.

I kind of figured as much. The other thing I don't care for with the NGKs is that it's red. I know most people don't care, but I like to try and keep things as original looking as much as I can.
 
The Dynatek DC11-2 appears to work just fine. Need a spacer and a very small bracket, but was pretty painless to setup. Haven't given it a real road test yet, however. But I'm sure it will be fine.

20230322_123109.jpg

 
Yes, I just took it for 5 miles up to the gas station, filled up and came back. Seems fine. I did notice I can start it easier, feather touch of the starter which is pretty nice. Haven't tried to see if you can lower the idle like the GM coils. I'll test it more over a few days and report back. But seems good so far.

20230322_125144.jpg

FWIW, I used the Treats "Hypalon" solid core 7mm wire for this. Seems fine. It was a pretty tight fit over the weather boots. It's pretty flexible too, which is nice.
 
I mean, honestly, you could just bolt it with the one bolt as is and run it this way. But, I'd like to make it a bit cleaner. Having brass spacers and a simple straight bracket to extend it another 10-20mm or so then it'd be about as good as it's going to get imo.

The nice thing is that it is labelled on the coil for the positive side with a red dot. It shouldn't really matter since it's AC, but just for completeness sake the Yellow out of the CDI is indeed the positive.

Another benefit is the output voltage is similar to the GM coil. ~36kV under load. Should be plenty.
 
Put 20 some-odd miles on it today with that Dyna coil. Some sustained speeds at 75mph for a couple of those miles. Works great as far as I can tell.
 
Gave it a test with low RPM. The Dyna coil can do 800rpm just fine at idle. On the Hondamatic I had to do more like 850-900rpm. It doesn't stall, but if you give it a very quick 1/16" blip it almost sounds like it will. 850-900 eliminates that.
 
Been over a week and 100 miles, still running quite well on the Dyna coil. It got pretty cold out over night and was a good test for that. With the original coil it tended to be quite annoying to get it started. Only took 3 kicks, fired right up.

Will get with my buddy sometime soon to see about a proper bracket. In the meantime, if someone wants a new coil that is hidden under the tank it's a great option. You can mount it with one bolt seems OK. Not ideal, but works all the same.

If someone gets a Dyna for their bike and would like a simple adapter for the Yellow and Green from Coil to CDI PM me I can make you one.
 
Coil is still working great. I have been busy with a lot of life events lately, but my car has been out of commission for a few weeks which also gave me more time to ride the 400. No issues at all. Going to look into a small slotted bracket. My dad knows someone that could stamp out 100 of them for me cheap and I'd just have to drill them as needed. I'll be looking into that soon.
 
Cross-posted from another thread, but here is a better mount that I made up from an old computer peripheral bracket. Works well. Taken measurements of the brass spacers I found laying around and it looks like MCM has one of similar dimensions. If they fit OK then I will look into getting this simple bracket cut up at a sheet metal shop and drill them myself per order and start offering these as a kit. With the XD05F caps unfortunately being a thing of the past supplies for this are low and I will have to start making the ends for this as well. This means you'll have to switch to DR8EA plugs, but in reality you can probably run without a resistor plug or cap and it should be fine. This was mostly to satisfy RFI requirements for radio and TV interference.

20230607_151150[1].jpg
 
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