Two Valve Crossflow heads: Hemi vs. quench areas.

Hello all,

This is just for learning purposes. All photos stolen online.

This here is an 18rg combustion chamber. As you know, it the legendary twin cam 8V wide angle Toyota engine from the 70's. The other one being the 2TG (also pictured).



2TG

If you only had a pic of the combustion chamber alone, you might be hard pressed to tell them apart. In short, they share the same smooth dome, hemispherical heads.
(2tG)

Also apparent is how they have a perfectly circular squish area all around the chamber.

In order to get high compression out of these hemi heads, you need a fairly large dome on a piston. Like so.

Fast forward to the late 70's and early 80's, and Toyota comes up with the 5MGE. Still a DOHC 2valve per cylinder wide angle design (for a total of 12 valves on a six cylinder), but notice the different combustion chamber.



Up close you can see how it is not hemi, with ridges and much larger quench areas. (i'm pretty sure this is olskewltoy's photo)


Still, the wide angle of the valves means that you still need a fairly large dome on a high compression piston. However, it is visibly smaller than an 18rg or 2tg high dome piston.

These were advertised as Venolia high comp pistons.



So here's a question:

For all out NA, engine swaps not allowed, but who cares what you have inside, so long as the spirit of the car is preserved: WHICH wide angle 2 valve design is better?  What would you do up your compression ratio? Domed pistons? Lots of quench?

Why?

Pls. mention advantages and disadvantages of each design regarding valve shrouding, quench, piston dome height quench, susceptibility to detonation and whatever else you can think of.


Edit:
Welding and remachining chambers is a thing I just learned is done by racers. This guy says he'd rather do that and risk shrouding just so he doesn't have to run such big domed pistons.  https://www.facebook.com/BenAlamedaRacing/photos/a.599136520185727/743321749100536/?type=3&theater  Do you agree with him? Disagree?



(This is simply for knowledge's sake. I am not working on a project right now. Was just wondering)

To go NA, you’ll need high compression. Higher compression can ping when you have sharp edges. My recommendation, would be taking the head to a known Toyota builder to have it worked for the desired compression ratio. That way, the builder can take any sharp edges out. The sharp edges can “glow” when it gets super hot, this can Preignite fuel, causing detonation. The 18rg head is amazing. Any “G” head is Yamaha made. The 2t isn’t worth crap unless it’s got the Yamaha head on it. Good luck prying a good 2tg head out of someone’s hands. The 18rg can be pseudo built into the 152e Toyota engine. 

The hemispherical head design can have material added to it to increase compression ratio as well. The heart shape design is basically a hemi design with material already added. Of note, the heart shape design is what you typically see on modern day engines

In reply to randedge :

I know the answer and it is Jaguar!! Well sorta.  Early Jaguars had true Hemi heads. The advantage of a Hemi head is airflow!  In short the more air you can get in the more power you get out.  

Squish area is all about pollution.  The more turbulence in a Combustion chamber the fewer      Unburnt hydro carbons and lower the pollution level  is.  Jaguar used the true Hemi until 1965 with the advent of pollution control they enlarged the bore while retaining the same head.  The result is two cylinders essentially moved forward two backwards and two remained centered.  The result increased squish area while keeping exterior dimensions the same. 

Look at the horsepower of the 3.8 compared to the new 4.2 they were the same even though the bore was significantly bigger. Torque went up slightly but dropped  in the following year. 

Finally the uneven loading of the connecting rods meant the factory red line became more of an absolute than a recommendation. Going over the 5500 rpm limit meant frequent crankshaft breaking between the #5 & #6 journals. Even as massive and well forged as the crankshaft was. Please note the stroke on a Jaguar is a massive 4.17 inches!  Giving phenomenal piston speed and forcing an absolute minimum weight piston.  

The common cure was to use the 3.8 block and just sleeve the cylinder out to the 4.2 sizeplus the .040  overbore  allowed.  That reduced the water jacket size causing the engine to run hot which was cured by the better water pump impeller, moving more water faster.