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Message started by Armen on 02/04/18 at 10:29:18

Title: Twin plug ignition?
Post by Armen on 02/04/18 at 10:29:18

While staring at a DR650 motor, one of the obvious differences is the second spark plug. Has anyone tried twin-plugging their Savage? I'v twin plugged more than a few Airhead BMWs, and the results are good. Of course, on an Airhead, the second plug is on the other side of the combustion chamber, and on a DR, its right next to the original. But, it must be doing some good, I figure.
One of the things that need to be done is to lessen the full advance timing (not idle). Usually into the 28-30 degree BTDC range.
Wondering if the DR ignition box would work with the Savage trigger? Or is there an aftermarket ignition box that allows one to adjust the timing curve? I think there is an aftermarket box for the DR.
I've got the second head off to be ported and fondled, so poking a second plug hole is no big deal.
Must be winter. Too cold to be in the garage...

Title: Re: Twin plug ignition?
Post by batman on 02/04/18 at 12:57:38

Armen ,I'm not sure the double plug in the DR was there to increase performance as much it was to make ignition more reliable, in off road duty. Our bikes have relatively fast moving flame fronts due to the four valves and twin swirl and center plug firing, I don't believe a second plug would show any noticeable improvement. However , if you used the DR coil and CDI ,I believe the Savage pickup coil would still trigger it.

Title: Re: Twin plug ignition?
Post by Dave on 02/04/18 at 13:47:02

A long time ago Porsche put twin plug ignitions into their racing cars to allow a duplication of ignition.....just like airplanes have......to allow the cars to continue racing if one ignition system failed. They also discovered the timing could be reduced and they got additional power as a result. With 2 big valves per cylinder....the sparkplugs had to be set off to the side of the head pretty far, and the flame front had a long way to travel to get all the way across the piston.

With the 4 valve design, the spark plug is pretty much in the center of the piston.....I don't know that a second plug would do much for the Savage. (It does make you wonder why Suzuki did it in the DR).

Title: Re: Twin plug ignition?
Post by Gary_in_NJ on 02/04/18 at 16:47:18

The second plug is there as a backup.

Title: Re: Twin plug ignition?
Post by Armen on 02/04/18 at 17:03:09

On airplanes and older dirt bikes, maybe, but twin plugs on street bikes are for more complete combustion. I kinda doubt the big twin cruiser like the XL1200S and the Kawi Vulcans, and the big BMW singles are worried that the primary plug will foul.
Even with a 4 valve head and a central spark plug, it is still a pretty big combustion chamber.
Prob the only way to know for sure is to find a good used ignition box and see if it sparks with the Savage trigger.
Doubt it will make any more HP, but it might make a 97mm, high compression combustion chamber a little happier.

Title: Re: Twin plug ignition?
Post by justin_o_guy2 on 02/04/18 at 20:45:42

Seems like knowing how the flame front propagates before you decide where the second plug goes would be helpful.
Ahh, screwitt, I'm tired,, that's a messed up sentence, but I don't want to fix it..

Title: Re: Twin plug ignition?
Post by batman on 02/04/18 at 20:59:05

Armen the 97 mm combustion camber is already happier, it's smaller due to the crowned top piston,and the temperature of the fuel mix is hotter due to increased compression.

Title: Re: Twin plug ignition?
Post by Armen on 02/05/18 at 02:40:16

FWIW, the Gen II SV650 went to two plugs and makes about 10 more ponies than the Gen I single plug motor. That wasn't the only change made, but it helped.

Title: Re: Twin plug ignition?
Post by Dave on 02/05/18 at 03:37:56

I have read that the BMW RT1150 had surging issues at cruise speeds, and when they fixed it the bike got 2 plugs per cylinder.......I am not sure what else they may have done.

Twin plugs allow you to run less advance - and that can help provide more power. It also allows 2 flame fronts to burn, and it can help resolve detonation issues as the temperature of the flame front is reduced (hard to believe that stuff makes any difference at 50 times a second....or whatever tiny time period the combustion cycle occurs).

I have looked at the Savage cylinder head.....and I really don't see where the second plug could fit (I have not looked at a DR head to see where the second plug is). If the second plug is within an inch of the first one....I am not sure it would be all that helpful.

Back in the Bultaco/Ossa/Greeves late 60's/early 70's days the bikes were single ignition - but often had a second spark plug hole where you could add a backup plug - if one plug fouled you could move the wire to the other plug. I remember hearing stories of guys who would not have a proper heat range plug to put in the second hole - and they would stick a plug in that hole that had too high of a heat range for the bike...they wrongly figured that since the plug wasn't hooked up to thec coil wire it wouldn't make any difference. However - the second plug is still exposed to the combustion process, and the insulator of the spark plug still gets hot and holds heat ....and the second plug that had too high of a heat range could get so hot it would burn a hole in the piston. (If I remember correctly the CZ had 2 sets of points, 2 coils and 2 spark plugs per cylinder).

Title: Re: Twin plug ignition?
Post by Dave on 02/05/18 at 04:45:51

OK....here is a DR550 head on eBay that shows what is going on.

The second plug is way over on the side.......and I could see how this would start 2 flame fronts.

https://www.ebay.com/itm/suzuki-dr-650-se-96-2017-cylinder-head/222711202878?hash=item33da9f443e:g:FTYAAOSwjyhaAqR-&vxp=mtr

The DR650 coil most likely could be used.....then you would have to figure out about the timing and ignition box.

Title: Re: Twin plug ignition?
Post by Armen on 02/05/18 at 05:40:14

Dave,
up, as to the Oilheads. In fact San Jose BMW does a twin plug conversion on the early models.
I'm going to pick up a used DR ignition box and coils on Ebay and see if I can get it to work with the Savage.
If all else fails, I'll use the second plug hole for one of those compression release thingies the guys use on big Harleys.

Title: Re: Twin plug ignition?
Post by Dave on 02/05/18 at 06:17:28

Does the DR use a single coil with 2 plug wires - or does it use 2 coils (which would allow the timing to each one could be a bit different)?

Title: Re: Twin plug ignition?
Post by batman on 02/05/18 at 06:21:59

Armen, It would be interesting to see the results, it might improve fuel mileage,and power.

Title: Re: Twin plug ignition?
Post by batman on 02/05/18 at 06:23:31

Dave ,yes it uses a single coil with two plug wires. It makes me wonder if your going to two plugs if it might be possible to use just the coil ,and simplify any timing issues.

Title: Re: Twin plug ignition?
Post by Fast 650 on 02/05/18 at 09:38:02

One thing to keep in mind is that if it improves the flame travel, you will probably have to retard the timing to compensate for that. Otherwise, to the engine it will appear that the timing is over advanced. Especially if you have a bigger bore or higher compression piston installed.

Title: Re: Twin plug ignition?
Post by Armen on 02/05/18 at 18:19:09

Right, as I said in the initial post, the timing would have to be retarded.
FWIW, it seems that some DRs used a single, two lead coil, and some used two, single lead coils.
I'll look for a good used DR ignition box and a single, two lead coil. And try to make it work with the Savage trigger.
No matter what coil I use, both will fire at the same time. Although there have been experiments with slightly different timing between two plugs on one head, I believe all of the mass produced systems fire both plugs at once.
The idea is that the flame travel distance is reduced. Part of what causes detonation is the pressure building up in a large, wide combustion chamber ahead of the flame front. By reducing the flame travel distance, the time required to burn the mixture is reduced, hence the reduced timing.
Honda did a lot of experiments in the early 60's as their 6 cylinder 250s revved to previously unheard of RPMs. At first they thought that timing would have to be advanced throughout the RPM range. What they found was that the increased turbulence of the faster incoming charge moved the flame front around faster, so there was a limit to the amount of timing needed. In those days, many motors used a steep included valve angle to allow large valves, owing to sub-optimal ports. Therefor, timing was quite advanced by modern standards because the flame had to travel over the hump of a high compression piston in a combustion chamber that looked like a TeePee.
When Cosworth in England come up with better ports and a flatter 4 valve combustion chamber, timing on those motors was in the high 20's. Even the twin-plug BMW Superbikes in the mid-80's used only 28-30 degrees of timing on their 2 valve heads.
It wasn't until Yamaha tried their 5 valve heads in Superbike, and ended up with a crazy wide, thin, flat combustion chamber that ignition timing headed toward the 40+degree territory to get the mixture at the peripheral areas to burn.
Part of what speeds up combustion is the turbulence in the head. I've made some mention of tightening the squish band. I've done this on other motors, and it makes a noticeable difference in power. The mixture in the perimeter of the combustion chamber is 'squished' in toward the center.
Because I have to start somewhere, I'll start with 1mm/.040".
Not sure anyone asked.... ::)

Title: Re: Twin plug ignition?
Post by batman on 02/05/18 at 19:57:39

Armen ,Fast 650, your both right ,I did a bit of research you will have to retard the timing . I read a figure of about 10 degrees but that may be covered by using the DR's CDI. Suzuki did a lot of changes to the DR650 , n/p and r/s models in the same year used the double coil or two singles,and differed with cdi 's and rotors and stator/pick up coils, good luck with that! I think I'll just brave it out with one plug.

Title: Re: Twin plug ignition?
Post by Dave on 02/06/18 at 04:02:29

Maybe you will need to use less timing advance! :-?

(Sorry......couldn't resist). ;)

Title: Re: Twin plug ignition?
Post by norm92de on 02/06/18 at 10:06:44

Many years ago I was flying along and noticed one of the EGT gages reading 25* higher than the other. Thinking I had nudged one of the mixture controls I richened the mixture slightly and on we flew. After landing when doing an ignition check I realized one of the magnetos had failed.

I have always figured that un-burned gas was causing the increase in EGT. Clearly an engine designed to run on double ignition doesn't like single ignition.