Pop-up Piston and Flame Front in an L-head Engine

[3Pedals]

I have a question concerning replacement piston design and flame front in my 1924 Dodge Brothers L-head engine. The engine was designed with 4:1 compression and a massive 297 cc combustion chamber. The factory piston rises from the deck by 0.115" at TDC. The spark plug is directly over the intake valve, and the 4 cylinder engine produces 37 hp. I am designing a higher compression replacement piston and don't want to make an error or create problems. The goal is only 50-60 hp with a modern piston design and thoughtful cam re-grind.

I have thoroughly modeled the combustion chamber and have a pop-up design that fits well and increases the CR to approximately 5.8:1 by increasing the compression distance of the piston by 0.625" while leaving 0.100" plus the head gasket thickness, estimated at 0.050" min., of piston to head clearance (0.150" total). The piston would now rise to 0.740" above the deck. Knowing that the flame front is coming from the side and that the entire crown of the piston is parallel to the top of the CC, I've chosen to avoid creating a quench situation. At 0.100" of total Piston to head clearance I get 6:1 CR, which was my target.

Does anyone have insight on piston to head clearance in this type of configuration? Milling the head is not an option. Also, the CC surface is a rough cast. Perhaps polishing the chambers could aid in charge heat retention and propagation, and might be advantageous in reducing carbon buildup. Am I playing with fire? (pun intended)

 

[Lou Scannon]

I don't have experience myself but the Ford flathead V8 was extensively developed for higher performance by hot rodders and aftermarket suppliers, mainly in the 1950s. I suggest you research what solutions were generally adopted regarding increasing compression ratio while promoting combustion and breathing efficiency, and try to adapt them to your engine. I think one of the main tricks was relieving both the piston and cylinder block to provide a path for gas exchange as well as flame travel.
For sure there are Ford flathead enthusiast websites that could provide a good source of such information.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[RossABQ]

I have a Ford flathead V8. They thrive on turbulence of the fuel mix, created by a quench. The mantra with these is to have around 0.050" clearance between piston and top of combustion chamber (with fully torqued head gasket). Stock these engines had up to 1/8" clearance on the driver's side, about 0.075" on the passenger side. Milling the heads to get down to the desired clearance sometimes requires fly-cutting the contours of the CC back into the head (see attached pic, as you mill the head, you lose the CC above the circumference of the piston).

Interesting that your plugs are over the intakes; on the Fords the plug is slightly offset to the exhaust side.

 

[SwinnyGG]

I don't have a huge amount to add - but I would suggest that your line of thinking on the combustion chamber finish is a good one. Polishing out that rough finish is going to reduce the surface area of the combustion chamber by a significant amount, and I'd expect you'll see some gains as far as combustion efficiency as a result. You're also going to remove a lot of surface area for unburned fuel and combustion products to condense onto, which is a good thing.

 

[3Pedals]

Thank you for your valued input. I did speak with a fine gentleman who owns an engineering company dedicated to flathead Fords in New Mexico. Though the preferred method to increase the compression ratio in an L-head engine is by way of the cylinder head and combustion chamber, he feels the pop-up piston is workable. My concern with the quench area is that it is typically reserved for only a portion of the piston crown leaving the top of the piston at least partially "unquenched". In my case the entire top surface will uniformly be subject to the effect of quench, which concerned me when considering flame front and the lack of space for it - not knowing if this is even a factor.

Knowing what little I do about fluid dynamics, it is my understanding that while the flame front is transient (something like an expanding wind crossing the chamber), the resultant pressure, though ever changing, is uniform in all areas of the chamber at any moment in time. The example of brake fluid comes to mind were it is the transfer of pressure in the line, not specifically flow or mechanical movement that gets the job done in a hydraulic automotive braking system. If, in fact, the pressure is the same everywhere in the CC, the piston will be forced down (out) of the area just as well as if the pressure originated from above. I'll squeeze it up a bit more once I can confirm my crushed head gasket thickness.

 

[TugboatEng]

Do keep in mind that you have readily available 91+ octane fuels and PAO/Ester based oils that will allow you to run much higher compression ratios and will reduce deposit formation even without other significant changes to the engine.

 

[3Pedals]

If only I could attain higher compression without destroying the originality of the engine. The only mechanisms available to me to raise the compression are a new piston design sticking way up into the CC or by welding up the combustion chambers, which is not a reasonable choice. I don't see myself commissioning a new cylinder head and there is no aftermarket, especially regarding performance upgrades, for these engines. It's an antique Dodge Brothers, not a Model A or Flathead Ford for which there are a myriad of better options that include cylinder heads.

By my calculation, if the piston were to rise to zero piston to head clearance, I would only get to 6.6:1. At 0.050" of PH clearance, I achieve 6.32:1. If the head gasket, which measures 0.102" in hand (double sided copper) ends up thicker than 0.050" once compressed, It gets even lower by 0.12 of CR for each additional 0.010". You see I'm kind of stuck. On the bright side, I'm not greedy either. 6:1 will be a nice improvement.

In the early 1920s, they regularly dealt with detonation in these 4:1 engines running unregulated and unreliable 60 octane fuel. That's why we had manual spark advance on the steering wheel.

 

[TugboatEng]

If you want to think outside of the box, get your piston stickout with a stroker crank.

 

[gruntguru]

Good idea. The increased displacement will give you a tad more CR as well.

je suis charlie