Twin Spark Plug Cylinder Head

Fabian

Well-Known Member
Local time
3:38 PM
Joined
Sep 8, 2009
Messages
4,487
Location
Australia - Melbourne
Twin independent power supplies and twin Jaguar CDI's and twin spark plugs enables both spark plugs to fire simultaneously, just like any aero engine, and just like it's been done since World War One and World War Two and on every piston engine aeroplane after that to the current day.

A twin spark plug engine will not add "any" power to an engine with a small bore size, however it is very useful for redundancy purposes.
Redundancy capability isn't an issue for someone doing short trips, but you want the trouble free concept of redundancy on your side if you're 150 miles from the car or from home and a coil or CDI fails.

I like the idea of redundancy, hence my bike will now be equipped with a dual spark plug cylinder head and dual CDI's and a dual power supply.
 

crmachineman

Member
Local time
9:38 PM
Joined
Oct 21, 2012
Messages
30
Location
United States
Hi.
Well, what Fabian said was somewhat true, however I have some interesting test results that add a little more to the story.
When I was testing the first twin plug prototype on my test bike it was working fine, but I didn't really notice a power increase from the previous testing, using the single spark plug head. As time went on, I made improvements on port timing, the intake tract and the exhaust, and power started to become really awesome. This was when my motorized bicycle started to be capable of exceeding 50 MPH! Soon after all these upgrades were completed, I thought it would be a good idea to go back to the stock ignition system, with a single spark plug cylinder head, for testing purposes. Both of these heads had the same combustion chamber shape and size. It was then, when my suspicions of the twin ignition were confirmed: when I put the single spark ignition, I started getting partial detonation problems when the engine reached normal operating temperature. This was when I realized I had to make a stage 2, single plug cylinder head with lower compression. I did, and the performance for the single spark ignition engine improved, even though I had lowered the compression.
The understanding I gained from these tests is that a twin spark ignition system will allow you to safely run higher compression. Detonation can sometimes be hard to detect when it is only a portion of your fuel mixture which is detonating. Whatever amount of detonation you have in your engine, it will not contribute to power gain, it will be a power loss. Instead it will punish your engine with high stresses and heat production, which could then lead to very noticeable detonation, if it is allowed to continue.
Since twin ignition initiates the flame front from two places, it has the ability to complete the burn more quickly. This is the key to preventing detonation from starting.
Thanks
-Fred
 

Fabian

Well-Known Member
Local time
3:38 PM
Joined
Sep 8, 2009
Messages
4,487
Location
Australia - Melbourne
It's interesting that you make mention of detonation with high compression cylinder heads, because i have experienced exactly what you've described: not just detonation issues but dieseling problems on two of the high compression cylinder heads that i've tried. I had to layer up four head gaskets to drop the compression ratio below a point where it would stop detonating and this was with the Jaguar CDI set on the lowest possible ignition advance curve.

Since four head gaskets screws up the squish effect, i reverted back to the low compression cylinder head, with the Jaguar CDI set at it's maximum advance curve, which proved to be the best setting for the low comp cylinder head.
In both cases, the jetting was adjusted for the high compression and low compression cylinder heads, eliminating air/fuel ratio as a potential source of detonation.
Twin spark plugs should reduce the flame front travel distance and potentially reduce detonation problems with a high compression cylinder head.

I can't wait to try out your twin spark plug cylinder head, not just for redundancy purposes, but with the other included benefits.

In my opinion, some of the high comp heads out there are set up with excessively high compression ratios, forcing you to screw up the squish band when adding extra head gaskets to drop the compression ratio.
 

crmachineman

Member
Local time
9:38 PM
Joined
Oct 21, 2012
Messages
30
Location
United States
Absolutely!

It's interesting that you make mention of detonation with high compression cylinder heads, because i have experienced exactly what you've described: not just detonation issues but dieseling problems on two of the high compression cylinder heads that i've tried. I had to layer up four head gaskets to drop the compression ratio below a point where it would stop detonating and this was with the Jaguar CDI set on the lowest possible ignition advance curve.

Since four head gaskets screws up the squish effect, i reverted back to the low compression cylinder head, with the Jaguar CDI set at it's maximum advance curve, which proved to be the best setting for the low comp cylinder head.
In both cases, the jetting was adjusted for the high compression and low compression cylinder heads, eliminating air/fuel ratio as a potential source of detonation.
Twin spark plugs should reduce the flame front travel distance and potentially reduce detonation problems with a high compression cylinder head.

I can't wait to try out your twin spark plug cylinder head, not just for redundancy purposes, but with the other included benefits.

In my opinion, some of the high comp heads out there are set up with excessively high compression ratios, forcing you to screw up the squish band when adding extra head gaskets to drop the compression ratio.


Yes! That's right!
It is likely that those high compression heads you have been using were optimized for clean, carbon deposit free engines using race fuel. All my testing with these cylinder head designs was using 93 octane fuel you can easily get here in the U.S. I wonder how your fuel in Australia compares to ours, but many people here consider our fuels to be pretty inferior to fuels common twenty years ago. And you are right to worry about what all those extra gaskets would do for your squish band; not good.

Several people have requested that I make special high compression versions of the heads. Since it is not too much trouble for me to do so, I decided that I would make some of those as well. Those heads could work well for two scenarios; The special high compression versions could be used on advanced engines using race fuel, or could be used with engines using stock exhaust and pump fuel.

-Fred
 

Fabian

Well-Known Member
Local time
3:38 PM
Joined
Sep 8, 2009
Messages
4,487
Location
Australia - Melbourne
I completely agree with your assessment of the situation i.e. race fuel vs pump grade fuel.

I am using a completely standard exhaust/muffler and the both of the high compression cylinder heads (using a single head gasket and Australian south eastern seaboard 98 octane fuel) dieseled at low vehicle speed with high cylinder head temperatures and under heavy load.

I am not sure how Australian south eastern seaboard 98 octane fuel compares with American fuel, because there seems to be differences in the way fuels are tested and given octane numbers, depending on global location.
 

crmachineman

Member
Local time
9:38 PM
Joined
Oct 21, 2012
Messages
30
Location
United States
I completely agree with your assessment of the situation i.e. race fuel vs pump grade fuel.

I am using a completely standard exhaust/muffler and the both of the high compression cylinder heads (using a single head gasket and Australian south eastern seaboard 98 octane fuel) dieseled at low vehicle speed with high cylinder head temperatures and under heavy load.

I am not sure how Australian south eastern seaboard 98 octane fuel compares with American fuel, because there seems to be differences in the way fuels are tested and given octane numbers, depending on global location.

Hmm,
Okay, well I guess it's mystery for now. So you are using stock exhaust on your bike? I hope you will get good performance from what you ordered. I'm thinking you might want a higher compression head. Well, since they are already on the way, you might have to get an expansion exhaust and get ready for a pretty big boost.
-Fred
 

Fabian

Well-Known Member
Local time
3:38 PM
Joined
Sep 8, 2009
Messages
4,487
Location
Australia - Melbourne
Hi Fred,

I am perfectly happy with the cylinder heads you have sent me - if they have a lower compression ratio than the high compression cylinder heads i've previously used, i'll be a very happy camper.

An expansion chamber exhaust is desirable but in Victoria (Australia), new legislation has been passed (after a police pursuit with a high powered electric bicycle), giving police extraordinary levels of subjective assessment in deciding if a motorized bicycle isn't law compliant with the 200 watt maximum power output, allowing the police to hit you up with a bunch of tickets/fines, simply on their "belief" that your bike is over specification, of which you then have to prove to police that your bike is law compliant if they confiscate the bike.

An expansion chamber is such an obvious visual giveaway that you're trying to squeeze more power out of the engine, plus it's visible from the rear of the bicycle. At least with the standard exhaust and 2-stroke engine, it is not visible from the rear of the bicycle.
 
Last edited:

crmachineman

Member
Local time
9:38 PM
Joined
Oct 21, 2012
Messages
30
Location
United States
Boy that makes me mad!

Hi Fred,

I am perfectly happy with the cylinder heads you have sent me - if they have a lower compression ratio than the high compression cylinder heads i've previously used, i'll be a very happy camper.

An expansion chamber exhaust is desirable but in Victoria (Australia), new legislation has been passed (after a police pursuit with a high powered electric bicycle), giving police extraordinary levels of subjective assessment in deciding if a motorized bicycle isn't law compliant with the 200 watt maximum power output, allowing the police to hit you up with a bunch of tickets/fines, simply on their "belief" that your bike is over specification, of which you then have to prove to police that your bike is law compliant if they confiscate the bike.

An expansion chamber is such an obvious visual giveaway that you're trying to squeeze more power out of the engine, plus it's visible from the rear of the bicycle. At least with the standard exhaust and 2-stroke engine, it is not visible from the rear of the bicycle.

Hi Fabian

Oh, man! That is so wrong! I would be so unhappy if some police officer pulled me over and gave me a ticket because of what kind of equipment I had on my bike. That just hurts!
Okay, well keep us informed of how it goes.
-Fred
 

KCvale

In memory of KCvale 1959 - 2019
Local time
9:38 PM
Joined
Apr 18, 2010
Messages
3,508
Location
Phoenix, AZ
You can spin the cylinder head around 90 degrees; having twin plugs poking out the left and right hand side of the bike.
The head yes, but not the cylinder itself ;-}

I can't really see 2 plugs on one of these 48 or 66cc motors being better, actually worse it would seem to me other than the higher compression because 2 plugs are in the head.

Unless you boost the voltage from the Mag somehow you are going to get less spark from each plug when the whole point is to get everything that little mag makes to the plug.

I have only built 50 or so of these 48-66cc 2-stroke bikes but one thing I can you for sure is the one best investment you make is an $8 NGK BPR#HIX Iridium spark plug.
The # is a number between 5-8. Higher numbers like 7 are best for really hot climates where lower numbers like 6 are best for very cold places. I run 7's here in the desert.

Unless you are running a way off gas mix or tiny NT like carb on a 66cc I can pretty much guarantee you will feel the difference and they go in most every customer bike I build but the bottom basement budget bikes which I don't build many of.

Just a tip, and here's another, your biggest performance gain is going to be from a tuned Expansion Chamber Exhaust no matter what carb, head, ignition system, boost bottle, bla bla bla you try.
Next to the cheap and easy plug change, it is straight to an X-pipe next on a performance build for me.

Again, just my 2 cents but I make my living building high end MB's and that's how I do it.
 

Fabian

Well-Known Member
Local time
3:38 PM
Joined
Sep 8, 2009
Messages
4,487
Location
Australia - Melbourne
@ crmachineman

It makes me seriously mad, and it is utterly wrong that a police officer (in Victoria) can give you, not just one ticket, but multiple tickets totaling around $2,000 even if you have a valid car and motorbike licence and without any convictions or outstanding infringements, just because of the equipment you ride.
If your bike is confiscated, you then have to pay the $600 it costs to get it out of the impound centre, but you can only do so if you can prove to the police that your bike makes no more than 200 watts maximum power.

Every day, i ride my motorized bicycle, knowing that at some point (it's going to cost me $2,000 and) i'm going to come across a police officer who will throw the book at me, even if i'm riding the bike in a lawful manner; obeying the road rules.
 
Top