Dual spark plugs or squish?

[Truckedup]

A sort of spin off of the peak combustion pressure thread.
The original poster of that thread and me are friends and race bikes in the same land speed racing class. I used to have the record but be bettered it by a few mph, the dirty rat
Both our engines are the same model Triumph two cylinder, 1930's hemi technology with a big piston dome for high compression. The heads done by the same guy . Same exhaust and similar carbs.
But there are differences...his engine has around 11.75 compression and dual spark plugs per cylinder to get fast combustion with the high piston dome..
My engines use a lower compression ,10.5 flatter dome piston designed to get increased squish by a narrow band around the bore circumstance.
There are some differences in cams and the rpm ranges of the engines but I'm trying to focus on the combustion process.
Comparing dual plugs to increased squish, can you say that one is more efficient than the other? Better use of fuel , that sort of thing? Thanks

[ptuomov]

I think (but don’t know) that hemi and semi-hemi two valve heads make more power with twin spark plugs. The squish can be engineered with twin sparks, too, I believe.

[Dan Timberlake]

When converting a BMW boxer MC to dual plugs the timing can and should be retarded a couple of degrees.
The conversion is sometimes done specifically to reduce tendency to part thottle detonation on the smaller displacement street bikes with stone age centrifugal-only advance curve.

R60 piston with shallow dome and unknown squish clearance --
https://cdn11.bigcommerce.com/s-y6x5xb1 ... 48.jpg?c=2

Very modest dome on R90 pistons.
https://i.ebayimg.com/images/g/d5kAAOSw ... -l1600.jpg

[panic]

A common adjustment for hemi with dual plugs is take off 5 degrees, but that's a 3-1/2" piston. The bad part: if one plug fouls, that cylinder will run very hot.

[PackardV8]

The original poster of that thread and me are friends and race bikes in the same land speed racing class. I used to have the record but be bettered it by a few mph, the dirty rat
...his engine has around 11.75 compression and dual spark plugs per cylinder to get fast combustion with the high piston dome..
My engines use a lower compression,10.5 flatter dome piston designed to get increased squish by a narrow band around the bore circumstance. . . Comparing dual plugs to increased squish, can you say that one is more efficient than the other? Better use of fuel , that sort of thing? Thanks

You sort of answered your own question. Knowing how difficult those few MPH are at your level of refinement, at present, his twin plugs and higher compression are just that much more efficient than your squish.

[falcongeorge]

I think (but don’t know) that hemi and semi-hemi two valve heads make more power with twin spark plugs. The squish can be engineered with twin sparks, too, I believe.

This did not prove to be the case in the late hemi (which has a MUCH bigger bore than the triumph) in the 1970's when Chrysler released the twin plug D-5 head, most Pro-Stock teams that initially used two plugs quickly plugged the other hole and reverted back to the single plug configuration. Most teams didn't see any gain. My guess is, its down to the extra point of compression, and the dome shape necessary in the triumph dictates the extra plug. So its not the extra plug that is making more power, its the extra compression the use of the extra plug makes possible.

[dannobee]

Going way back here, but wasn't the Pro Stock D5 relegated to single plug use by NHRA rule? I do know that the second plug hole did offer more engine setback because of rules. In any case, the D5 head isn't a true hemi but more a spherical cap, with a much shallower depth than the regular Chrysler Hemi, so its flame propagation is different than a "true" hemi.

Anyway, if you look at classes with more liberal rules, twin plug hemi's outperform a single plug squish chamber. Porsche proved this over the years by running twin plugs in every class where legal. When they were called out by the sanctioning bodies in various classes because it didn't come from the factory with twin plugs, they developed a single plug head with a squish pad on the side opposite of the plug (air cooled 962 engines). Suboptimal, but better than a single plug hemi. Obviously later stuff had 4 valves and the ubiquitous "pent roof chamber" that is common today. The pattern here is the distance that the flame has to travel before pushing the piston down. Shorter is better. Two plugs = faster burn = higher compression ratio or more boost before detonation. And less ignition timing needed.

If you look at a "modern" Dodge hemi chamber, it's a twin plug 2 valve head with a squish pad on each side of the chamber. Sort of the best of all worlds if you're limited to a 2 valve engine.

And if you've ever piloted a piston engine airplane, what do you look for on your magneto check? An rpm drop when you cut out each magneto.

[ptuomov]

I can think of two theoretical benefits from twin spark hemi or semi hemi chamber. First, there’s the direct effect of two flame fronts burning the charge faster than a single flame front. Second, as mentioned earlier, two plugs allow one to use a high compression piston dome that sections the combustion chamber into two parts for part of the cycle without screwing up the combustion. But this is just me thinking it thru.

[Truckedup]

The original poster of that thread and me are friends and race bikes in the same land speed racing class. I used to have the record but be bettered it by a few mph, the dirty rat
...his engine has around 11.75 compression and dual spark plugs per cylinder to get fast combustion with the high piston dome..
My engines use a lower compression,10.5 flatter dome piston designed to get increased squish by a narrow band around the bore circumstance. . . Comparing dual plugs to increased squish, can you say that one is more efficient than the other? Better use of fuel , that sort of thing? Thanks

You sort of answered your own question. Knowing how difficult those few MPH are at your level of refinement, at present, his twin plugs and higher compression are just that much more efficient than your squish.

Jack, unfortunately the engines are not equal.. Let's just say for this story, if all was more equal,Is the turbulence from the squish better at making power from given amount of fuel than two plugs with no squish factor?

[PackardV8]

Is the turbulence from the squish better at making power from given amount of fuel than two plugs with no squish factor?

Are we asking about maximum power production or max BMEP or max fuel economy?

An extreme example were the turbocharged Offenhausers at the Indianapolis 500. Given unlimited boost and unlimited fuel, they were competitive with modern V8 designs. Once the strangulations limited boost and fuel, they were goners.

So yes, no, maybe, it depends. The GM LS head and piston design does a wonderful work of managing squish to complement DI. Here are a couple of race designs



The Gen III Mopar hemi has both squish and two spark plugs. Here's a Diamond version



Maybe, the inherent limitations of the Triumph hemi have a different answer.

[dorset]

i've got the dual-plug triumph 650 head. the domes are really problematic:



there's more or less no squish area on this motor, and when the piston is rocking over TDC the dome is more or less a giant wall in front of the intake valve. i run C12 leaded gasoline, MON+RON/2 = 112. with a near-stock 38 degrees advance, it was borderline on detonation with single plugs. with dual plugs and the same fuel, it's fastest at 30 BTDC with no trace of detonation.

there's not a lot of room on top between the cylinders, so the center plugs are 10mm.

[Truckedup]

In comparison to Dorset, I use this type piston..The squish is .032 on a 2.8 inch bore. Since the photo was taken the head combustion chamber was lightly modified to an increasing squish clearance near the top of the piston dome. This may seem a bit trivial compared to modern engines but crowding 66-68 HP from 40 cubic inches on gas puts us at the top of the heap in our class...

[englertracing]

Is the answer as simple as..... whichever one takes less timing is more efficient?
Or is it which ever one makes more power.....
Hmmm
I'm thinking
Can you alter the valve angles?

[dannobee]

Here are the Gen 3 hemi chambers. The latest one is on the right. If you're stuck on the valve angles, you could still weld up some quench pads on each side of the chamber, then cut down the piston dome to the correct CR. The quench pad on the left head is similar to what Porsche did with theirs (except on only one side, being a single plug head). If you're able to change the valve angles, you'd be able to weld up the bottom of the chamber and possibly use a flat top piston. No doubt a lot of work, but at least it would bring the chamber up into this century.

[hoodeng]

Many years back i saw some Triumph chambers welded on the sides to fill them in and pistons cut to suit, as we know the quest for comp is a dog chasing its tail, we fill in here and then cut off there for clearance. Before NC we did this with mills and lathes we effectively only cut in two axis' as getting a compound shape would have required the action of the third axis to achieve, hand fitting and finishing was how it was done for the compound shapes.
Bear in mind manufacturers were always first in generating compound shapes, the home machinist could only dream of emulating their products, although now a small NC machine, or printer for castings is not unknown in privateer hands.

As pointed out your friends comp is considerably higher than yours at 10.5:1, which for your performance works pretty well, the big question is do you have a budget to get into a slug out? Have you squeezed other avenues like barrel decking and head decking to see if there is more there to be had? Even half a point will help.

Can i ask what ignition you are using?



Cheers.