When looking at the combustion chambers on an OHV head, why are the combustion chambers so deep? Wouldn't this lower the compression significantly? Giving you less power?
In a Model T engine the tops of the pistons protrude nearly a half an inch above the block deck, so the chambers in the head are at least that much deeper than those seen on another engine where the pistons do not go above the block.
The engine is just a pump, with an OHV there is much more going in and then back out than a standard T block because the flow is in the head and not the block. Other factors apply such as increased compression.
Let's not forget the diameter of the original spark plugs protruding in from one side or both (.500) and the added increase in valve lift when using the 1:1.5 ratio rocker arms. With a even a stock grind cam lobe that's a solid 3/8 inch (.375).
Consider that "bent noodle " crank shaft!! I own a "Laurel model 40" (also known as a Roof 8). Implies that 40 hp can be achieved. Certainly ALL the crank is good for !!!
The concept of "quench " in the combustion chamber was not yet being considered. This came in the early '30's with the advent of "tetra ethyl lead" providing consistent octane ratings and making the HORSE POWER "race " achievable.
I'm sure you've looked at the "wedge " combustion chambers of later engines and the ability to achieve quench and to control spark knock.
Not considered with T OHV heads
Great info guys! Thanks!
Les, do you know what the ratio of the rocker arms is on the Roof model 40?
Alex also, not all overheads were meant to increase the compression rate by much of over stock if any. The were used for better breathing and more power at lower end. I thing some of the RAJO's were made for trucks.
Another thing to consider is that dynamic compression ratio will increase even if static remains the same, simply by flowing more air in to a chamber of the same size, naturally it will squeeze it more.
The rockers are 1.15-1. So minimal increase in valve opening. The head runs well. It came with a original adapter to mount a stock T carb. Or remove this and add 1-1/4" throat carb. The intake "manifold " is part of the head casting.
I believe the target market was probably trucks and similar