Dual spark plugs or squish?

[ptuomov]

Angled squish on both the piston and head might improve things a little bit, but is much more time consuming to make work. The other thing to take into account is that the hemi head burn speed is largely determine by the amount of tumble. The greater the tumble one can generate and sustain, the faster the burn. This also puts special requirements on the “dish” on the piston crown and is one reason why in normally aspirated applications semi hemi and tighter valve angle work better than hemi and wide valve angle. Two plugs, angled squish, and a head and piston crown combination that really spin the tumble wheel on the flow bench would be my pick. I think Porsche probably optimized this the furthest.

This all based on just thinking and reading about old race engines, these are not personally observed facts.

[ptuomov]

I’m under the impression that both of these two kinds of twin-spark designs worked well in air cooled two-valve Porsches:

#1:

0BB352FA-EF62-439F-9534-C26D622C0A8C.jpeg

AEAA8292-E0A6-438B-AD34-80C71A37F828.jpeg

#2:

6FAD960D-325A-49B7-9FDB-12FB2A3A05FB.jpeg

70905979-549C-4357-AA65-09AF2AD041C2.jpeg

[Truckedup]

There are 4 valve pentroof heads available for these engines made by aftermarket and even Triumph during their last years of production in the early 80's...They can be found, they will bolt on with piston changes, they are expensive and often need work to correct defects like valve seats falling out. The aftermarket heads may not be legal in the modified production class.
Changing valves angles would require drastic changes that are beyond our capability. Some using these engines in the. 750 version for road racing do extreme modifications to the intake ports ,a lot of welding and moving valve spring seats to raise the ports.They claim 80 hp from 45 cubic inches at 8000 rpm.. Getting details about it are difficult, no one wants to talk about it.
Ignition timing, Dorset with the dual plug experimented a lot and settled on 30 degrees. Mine was 37 degrees when on the dyno 5 years ago.I never tried different timing .
Currently he is still racing, my bike is retired for now ...

Both Dorset and me have 750 Triumph street bikes modified to run without detonation on lower grades of pump gas with static compression ratios of 9.2 for mine and 9.5 for his..No surprise his has a dual plug head and mine his modified for a tight squish band..
I suppose the fun ,if you can call it fun ,is getting the most from the basic design ...

[ptuomov]

Just thinking out loud, without having any actual model specific knowledge of information:

Can you increase stroke within the rules? My theory here is that by increasing the stroke you could make the piston crown smaller (for the same compression) the piston lighter, rod shorter and lighter, and get a better combustion space. The lighter components in turn might make the longer stroke tenable from the durability perspective. Just an idea?

Can you try your tight-squish small-crown design with a second spark plug just to see whether it burns better and makes more power even without increasing the compression ratio by increasing the piston dome volume?

Have you measured how much tumble your head generates? I think that as far domed pistons go, yours probably sustains tumble pretty well.

Looking at the old Porsche two valve heads and pistons, it seems that they tried to generate a tumble channel that is somewhat wider than the intake valve and then fill in the rest of the space with aluminum in the head and the piston. Can’t read dead people’s minds”, though.

Porsche tried for years to get the air-cooled four valve head to work but they would always melt in tests.

[Dan Timberlake]

Is that detonation damage on the squish over by the exhaust valve ?[
attachment=0]70905979 porsche hed contrast .jpg[/attachment]

[ptuomov]

Is that detonation damage on the squish over by the exhaust valve ?[
attachment=0]70905979 porsche hed contrast .jpg[/attachment]

I don’t know but I don’t think so. These may be newly cast heads. It could be just casting inclusion revealed by machining.

[Truckedup]

Just thinking out loud, without having any actual model specific knowledge of information:

Can you increase stroke within the rules? My theory here is that by increasing the stroke you could make the piston crown smaller (for the same compression) the piston lighter, rod shorter and lighter, and get a better combustion space. The lighter components in turn might make the longer stroke tenable from the durability perspective. Just an idea?

Can you try your tight-squish small-crown design with a second spark plug just to see whether it burns better and makes more power even without increasing the compression ratio by increasing the piston dome volume?

Have you measured how much tumble your head generates? I think that as far domed pistons go, yours probably sustains tumble pretty well.


Looking at the old Porsche two valve heads and pistons, it seems that they tried to generate a tumble channel that is somewhat wider than the intake valve and then fill in the rest of the space with aluminum in the head and the piston. Can’t read dead people’s minds”, though.

Porsche tried for years to get the air-cooled four valve head to work but they would always melt in tests.

The class is engine displacement, but any bore and stroke. The dry sump crankcase is very tight around the crank limiting stroke ....
I have never done any measurements on the engine other than clearance checks....I have no idea how to measure or guesstimate tumble...

[ptuomov]

I have never done any measurements on the engine other than clearance checks....I have no idea how to measure or guesstimate tumble...

I don’t really, either!

I think they measure it with computer simulations these days.

I think that, for old-school flow benches, they sell an accessory “tumble wheel” that’s just a paddle fan wheel that is placed with the axle front and back and the wheel between intake and exhaust sides. The faster it spins with the intake valve opened, the more tumble the engine generates. I don’t know if there’s a way to test this with the piston in the bore in a flow bench, there always a way I guess but I don’t know it.

[dannobee]

Is that detonation damage on the squish over by the exhaust valve ?[
attachment=0]70905979 porsche hed contrast .jpg[/attachment]

No. Porsche 911 engines are obviously horizontally opposed and the exhaust is the lowest part of the cylinder when in the car. Any moisture/fuel/gunk ends up by the exhaust valve when it sits. The electrolysis/corrosion is fairly common. That one could have been where there was a pinhole after welding up something before. And if you notice, the quench pads weren't machined as one would with a CNC.

[Truckedup]

I'm more of a bike builder than an engine guy. I like fabricating from a pile of mis matched parts. Order the new stuff, do all the measurements and tell the machine shop details on this and that. Do the cam timing , etc, but the head work is farmed out to a trusted guy....I don't tell him what do do other than what type of racing...Then assemble it and so on
These are naked frame land speed racers, no aerodynamic devices allowed. The rider's ass is on thinny padded or bare metal, body stretched out with helmet flush on the fuel tank, hands 16 inches apart at 130 plus mph...with limited power the rider's hand out of position can cause extra wind drag and a few mph..The vibrating engine stressed to the limit inches under the rider and could seize unexpectedly ...Real basic racing....

[dannobee]

Well, now that you've put it in THAT context! If it were me, I'd be more inclined to talk to a couple of trusted aeronautical engineers and do some wind tunnel testing instead of trying to wring more power out of the engine. Frontal area and Cd are horribly inefficient on motorcycles. And those guys might turn you on to some tricks that offer solid gains without anyone else suspecting anything.

Since the basic equation for speed is largely dependent on Cd and frontal area, I'd try to learn as much as possible about those and minimize as best I could.

[dorset]

over 100mph on these turds and its all fighting the wind. i ride with my head down to the side of the tank looking forward with one eye on each side of tbe left fork tube. then i try to stick my ass up in the air until my back feels cold.

theres limited aero allowed in the class-- nothing ahead of the rider or behind the rear wheel, a front fender is okay.

what is "tumble?"

[englertracing]

I like the quench porsche chambers putmov posted, but I think to make that work you need a shallower valve angles

[ptuomov]

I like the quench porsche chambers putmov posted, but I think to make that work you need a shallower valve angles

I don’t think it’s either/or between those two designs, unless one is going for extreme compression ratios. I think that even with the wide valve angles, one could “move” some of the metal from the piston into the cylinder head and create an engine that can be run at higher rpm safely. It is true to get the most out of the design by being able to run a flat-too piston, a tighter valve angle is probably required.

[Truckedup]

what is "tumble?"

K, as I understand it, the incoming air rolls over like a curl as it enters the chamber . It's created by port and or chamber design from what I'm

I like the quench porsche chambers putmov posted, but I think to make that work you need a shallower valve angles

I don’t think it’s either/or between those two designs, unless one is going for extreme compression ratios. I think that even with the wide valve angles, one could “move” some of the metal from the piston into the cylinder head and create an engine that can be run at higher rpm safely. It is true to get the most out of the design by being able to run a flat-too piston, a tighter valve angle is probably required.

Some run these engines up near 8500 rpm as I mentioned with the 750cc road racers...But I think they may be using aftermarket engines cases and for sure after market cranks and rods.
My engine has R&R rods but a stock but nitrided crankshaft. So it's tuned to make power at at 7000 rpm . Above 7500 rpm the stock crank life is just a few hours. I can't afford custom billet cranks like my competitor in this thread, lol