Posted by PapaDean on Friday, March 23, 2001 at 10:23PM :
In Reply to: Engine rebuild posted by BOB on Tuesday, March 20, 2001 at 11:30PM :
When I suggest "camming up", of course I mean to install a camshaft with longer duration and/or higher lift than what was stock. When a cam of greater duration is installed, bottom end power is lost because of poorer cylinder filling! This longer duration allows greater exhaust gas contamination of the intake charge, as the intake valve is opening earlier and the exhaust is closing later. The resistance to the intake flow then cuts the cylinder filling and power drops off. As the rpms go up, the intake charge builds greater inertia (even just air has mass, after all) and overcomes the resistance of the residual exhaust. This situation is exaccerbated as port volume goes up (from porting), as that larger volume cuts port velocity, thereby also hurting cylinder filling at lower rpms. Advancing or retarding the cam changes the intake opening point, which increases or decreases the exposure point of the fuel air charge to the residual exhaust.
When the static compression ratio is raised, power is gained. You can quibble about how much, but it is seat-of-the-pants felt! This gained power helps overcome the loss of power from installing a cam of longer duration. That's why most cam grinders, in their catalogs, suggest increased compression for their cams, especially as they get longer in duration.
Piston domes really don't get that much in the way of flame front propagation, even in as high a compression motor as, say 12:1. (This has been a debated item for years.) Incidentally, Pro Stock drag engines have for years run ratios from 16:1 up to 20:1 !!! Yup, shades of a diesel. For a flathead to have high compression ratios, one has to work at it. These things came with low ratios in the first place (except maybe the Twin H Hudson of the early '50's - the engine intended for NASCAR and called the "high altitude option"). Because of the valve pockets needed in the head, it is very difficult to get the chamber volume down, especially when those pockets have to be larger and higher to accomodate bigger valves and higher lift cams. By the way, don't, I say, don't, relieve a flattie block. All it does is hurt compression ratio and does nothing to promote air and fuel flow into the cylinder. For you youngsters out there, that means grinding down the block area between the bore and the valves. Just slighhtly round off the sharp edge of the cylinder to promote easier turning of the fuel down into the bore. Don't go too near the highest point of the top ring travel, though.
As always, when you modify a stock engine, you are changing OEM compromises to a set of your own compromises. The trick is to make it all work together and to be able to afford the mods you want to make. I'm a great fan of installing as large of intake valve as can be had, but finding those rascals for something other than the typical small block V-8 Chevy/Ford/Mopar gets expensive, and in the case of what I'll call unusual engines, darned near impossible - without spending some $$$ or getting the help of a friendly machinist who would be willing to pour over parts books to find a valve out of something else that can be used. I just now remembered that you can special order any size and shape valve you want from Manley. They'll custom make you 1 or 6 or 8. For a street motor, get their street stainless one-piece valves. They are slightly softer than an OEM valve, but will bend and not break like an OEM valve is you kiss a piston (or head in a flattie) from valve float. That softer valve should be easier on a non-seat inserted block, too.
OK, off the soap box for Papa. Actually, I didn't know this place existed, but my #1 son emailed me a hot link to it and I thought I'd drop a little of the knowledge I'd learned since getting that first copy of Hot Rod Mag back in 1958. (The guy who gave me that mag ports heads and runs a dyno for a speed shop in NW Ohio and is my life-long best friend.)
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