Why the /6
From a social science point of view, this thread is quite interesting....
From a drag racing point of view, no so much. We really don't formally drag much here (even thought I knew 2 guys that dragged their 60's minis and MGA) and the fines for stunting ( a broad category of running your car over the speed limit) are quite harsh.
I am curious because i lived around small displacement, high reving and power producing engines. There is a definate trade off or cost. How are these high revving, muscle bound /6 for lugging around town or city driving and maintenance ?
You need to separate the two varieties of high-performance slant sixes into two different and distinct groups for purposes of analyzing their street manners.
The two groups have different methods of achieving their goals, though both do it by increasing (working) cylinder pressure.
Group one, and by far the most popular, consists of modifying naturally-aspirated motors. These depend on higher compression, increased carburetion (or, fuel-injection,) a better-flowing cylinder head, ignition mods that will allow for efficient spark advance curves and suitable high-rpm spark performance, headers, increased- lift and duration cams, and improved cam profiles and valve-train dynamics that will allow reliable high rpm for increased horsepower.
This all works to increase power, but mainly in the high- rpm areas, and usually comes at a cost to mid-range torque, which affects driveability, and results in poor idle quality. Most 225 slant sixes modified in this way have difficulty in making much over one horsepower-per-cubic inch, and staying very streetable, because of breathing-limitations presented by the small ports and valves in the only head available, (one that was originally-designed to work well on the 170 cubic inch 1960 Valiant/Dart.) Bigger valves can be fitted, but the ports are problematic; they're small, and can't be enlarged a whole lot. The 225's longish-4.125" stroke in not a plus in trying to use this naturally-aspirated approach technology to make high rpm power. The 170 motor is a better candidate here, (MUCH better specific output!,) but suffers from the "size matters on the street" syndrome; it's just at too much of a displacement-disadvantage to haul much weight very fast...
Which leads us to the alternative... forced-induction and the 225.
The slant six was originally intended to be an aluminum block engine, and as such, was designed with a lot of extra beef in the stress-bearing structures, such as main bearing webs, cylinder block side walls and such. It also employed a forged steel crankshaft, with main bearings the same size as those in the 426 hemi(!)...
The aluminum-block engine had a variety of unforeseen problems, so it was abandoned after 60,000 were made, but the cast iron version still used all of the heavy-construction parameters of the aluminum engine. This resulted in an unusually strong, stiff, block (not unlike a Diesel,)with a really thick deck, and still used the same 84-pound head, which has itself, a really thick deck surface.
All this resulted in a basic infrastructure that invites boost levels that would blow most other engines into a pile of scrap iron. It's not unusual to see 225 slant sixes that are turbocharged, with as much as 25 pounds of boost (and more.)
Such engines can, on race gas. make upwards of 500 horsepower, with the rather strange quirk that they don't make much power over 5,500 rpm. Some sort of breathing problem, but, there it is...
Two of these engines that I know of, have no fuel injection (just one Holley 4-bbl carb,) a flat tappet cam, and since they are red-lined at just 5,500 rom, no special ignition beyond an MSD box, or valve train heroics are called for.
They are NOT high-tech engines... basic stuff, mostly.
Milder examples employing less boost... using only ten pounds, can usually make somewhere between 250 and 270 horsepower on pump gas...
These milder turbo engines have a stock "demeanor," with gobs of mid-range torque and a smooth idle. Turbo /6s don't run well with camshafts that employ much duration because those cams invariably have a fair amount of overlap, and during that overlap period, the boost goes right out the exhaust valve... so duration is kept to around 220-degrees at .050" lift, and ground with wide lobe separation (such as 115-degrees,) to minimize overlap.
I would guess that it might be possible to have a 300hp turbo slant six with a pretty stock idle and excellent street driveability, that gets gas mileage comparable to a stock engine.
This might give you a better idea of what is available to those of us who still like to play with this old stuff....
In summation; turbos are better in some respects, but tuning the mixture is not easy... you need a LOT of patience!
You pays your money and you takes your choice! :)
