If you can stay awake for all this, here is a comparison I wrote awhile back for a ball-park cost-analyis for going with a turbo225 and a small block of equal power.
YMMV...
Comparison of turbo Slant Six 500 HP build vs. 500 HP 318/340/360 small block build
The following is written in reference to the 225 motor. The 170 is a different kettle of fish.
It would seem that slant 6 motors (remember, this is a 225 thread) were built (though, not intentionally,) to be force-fed air and fuel.
Whether it's done with a supercharger, nitrous oxide, or a turbo, it once-and-for-all, makes an end run around the built-in breathing problem that slant sixes suffer from birth. Because of their small bores, they can never have the kind of breathing that will deliver horsepower in the 1.75 hp-per-cubic-inch range. Not with that original, cast-iron, 2-valve head, at least.
The engine is rpm limited because of the long (4-1/8") stroke, so it's never going to deliver the goods at 8,000 rpm. That's the rpm where most small-block Chevys that are really "built" seem to make big power. I'm talking the small-displacement, short stroke models.
RPM? Forget it.
Because of the fact that the original plans for the slant 6 included an aluminum block (and, the fact tht that aluminum isn't as strong as cast iron,) the basic specifications for the slant 6 block were robust, to say the least; they had to be... aluminum needs more mass and are to be as strong as it needs to be for reliability, than cast iron. Well, the engine that went on to live in Mopars of various descriptions for many years, turned out to be cast iron.
But, the changes in the cast iron version, from the aluminum parameters, were minimal.
That meant that the resulting engine was an unusually strong critter, not unlike a Diesel, in basic construction.
A forged steel crank with mains the size of a 426 Hemi’s made for an equally-strong bottom end.
Along the way, someone a whole lot smarter than I am, realized that what this all meant was, you could boost the s**t out of this little motor without hurting it. Boost = performance increases!
Tom Wolfe and Ryan Peterson built the prototypes for the engine that is in the '64 Valiant that Freddie and I plan to run. We bald-facedly copied their lead, and we are HOPEFULLY going to run within a second of their cars. There are only minimal differences in their "recipe" from ours. But, they did it first; we're just copy-cats.
In this diatribe, I am going to try to justify why anyone would want to go this route, ($$$$$$$$$$$,) and perhaps point out a few reasons why it might not be such a good idea, after all...
There are (at least) two kinds of people out there; those who just want to go fast, and those who want to go fast and prove something in the process.
Anybody can stick a big engine into a light car with easily predictable results: It's gonna be F-A-S-T!!! Those 440+ cubic inch A Bodies are hard to outrun... and, with good reason! Ma made it pretty easy to drop an RB engine into an A-Body, and beyond getting it to hook, the problems in getting it to go fast are not actually what you'd call "insurmountable." Whoever said, "There's no substitute for cubic inches," said a mouthful!
Senor' Schumacher has made the task of installing a big engine into a Dart or Duster a lot easier, with his motor mounts and custom-fit big-engine-in-a-in small car headers. The appeal is almost overwhelming, if you love "speed."
Some folks, though, look at that operation and say, "Ho Hum... It SHOULD be fast; big block in a small car.... so what?"
Some of those detractors want to produce a fun car with a smaller engine, but not TOO small.. There are the 318/340/360 guys who don't want the hassles that go with the installation of a third-member-breaking big block, but would still like to trim a few Corvettes.
To them, a small-block is the answer; they don't want to mess with a slant six, because 1. They don't like the way they sound, and 2. it's hard to build one that will outrun most Corvettes. They probably have never SEEN an 11-second slant six car. Or, a 12-second one; thirteen second slant six cars are not even that plentiful.... so, they know that they can stick a set of headers on a 340, raise the compression to 11.5:1, go with any one of a hundred different solid lifter cams, and presto," a low 12-second car that will embarrass most street driven anything, Corvettes, included.
Enter the slant 6 turbo, the type of engine that most regular-guy Mopar enthusiasts think is an oddball, weird combination that yeah, may be pretty fast, but has to be expensive!!! Right? I mean, you don't get 2+ horsepower per cubic inch out of a slant six without a ton of costly, cutting-edge technology!
Well, that's just not true.
Let me point out what ~I~ have found out about this turbo six business that has made me wonder whether it might not be actually CHEAPER than building an equal-power small block. Especially, if you already HAVE the /6, but will have to buy a V8.
Here are a few ways that the turbo slant six can be a cheaper alternative to an equally-powerful small block.
For purposes of comparison, let's compare two 500 horsepower engines; one normally-aspirated 360, with time-honored, normal hop-up mods to produce 500 flywheel HP and a turbocharged slant six with equal power.
Lets start with acquiring a rebuildable engine "core."
People give away slant 6's all the time. The one we are attempting to build was, in fact, given to us. It was on its way to the dump, if we didn’t want it. “Free” is always good…
That scenario is also possible with small blocks, but not as easy... and virtually impossible to find a "free" rebuildable 340. But, you don't HAVE to start with a 340; it can even be a 318... but that won’t be as easy. A 500-horsepower 318 is not hard to imagine, but probably would need some pretty good heads, and 12-1 compression. It would also need to be rpm capable, to a large degree. (7,000?)
Not so hard with a 360, but they are not as much in abundance for free, I think. Could be wrong about that. More like $150 for a rebuildable "core."
Advantage, slant 6.
There is more of everything to buy for 8 cylinders compared with 6. Pistons, valves, bearings, rings, valve springs... retainers, keepers...
Advantage, slant 6
The driveability of a turbo slant 6 is not much different from a stocker, in that the main thing(s) that destroy driveability, are radical cams with a fast, ragged idle, and big ports that allow the fuel to fall out of suspension (at low rpm) in the ports, due to low velocities brought on by the size of the ports. The turbo slant six cams don't have much more duration than a stock one, and the ports, even in ported heads, aren't very big.
Advantage, slant 6.
The slant six's that have been turbocharged with high-boost (over 20 pounds,) don't seem to like rpm's and don't NEED rpm's to deliver the goods. Tom Wolfe and Ryan Peterson, the two examples I am citing here, both contend that their engines have a de-facto red line of about 5,500 rpm. With such a low red line, the reciprocating stresses, even with a 1-and-an-eithth-inch, stroke, are low enough that these engines will never fail due to bearing loads brought on by excessive piston speed, That is MY opinion; nothing more. Making 500 horsepower from a normally-aspirated small block is going to require that you spin it, probably fast enough to put engine life in jeopardy, if you do it very often.
Advantage, slant six (my opinion)
Because of the relatively low rpm operation of the turbo slant six, the valve train can remain, with stock pushrods and rocker arms, due to the low valve spring pressures required. The money you DON'T have to spend on needle bearing, rollerized rockers, special, heavy-duty pushrods and roller lifters is money saved.
Advantage, slant six.
The rear axle ratios in the two quickest A Body turbo slant six cars that I have seen evidence of, are 2.76:1 for the strip AND for the street. The turbo motors are weird, in that the car slows down with normally-"steep" rear gears, such as the 4.56:1 units often found in small-block cars. The turbo motor seems to make more power (not unlike a "fuel" motor) when it is "held back" and not allowed to increase rpm quickly. The significance if this is, the turbo motors also can use the same ratio for highway driving AND drag strip action. The small block "built" motor wouldn't think much of a 2.76:1 rear end on the drag strip, nor would it perform up to its potential, with a 4.56:1 on the highway. So, if you build a small block and it does double duty, you really need two sets of gears; one for the drag strip and one of the highway. No problem; you can change third members in a couple of hours (or, less.) But, they don't give away 8.75" A-Body housings these days, and neither is it cheap to buy and maintain two sets of third members, with different ratios; one for racing, blah, blah, blah...
The turbo slant six car can easily make do with a "one-ratio-fits-all" rear end. A late model, A Body 8.25" rear end from a junk yard will be lots cheaper (or, one out of an Aspen/Volare car) and will come with highway (and drag strip) gears already in it.... and is plenty strong for this application. More money saved.
Advantage, slant six
Because a high-stall converter is neither desirable nor necessary, turbo slant six converters are going to be cheaper than a 4,000-5,000-rpm unit that would be probably necessary for a wildly-cammed small block. Once again, the street driveability issue comes to light. The tighter slant six converter would not create as much heat as a high-stall, small block unit would, in daily driving.
Advantage, slant six
I BELIEVE that a turbocharged slant six motor is about 80-100 pounds
lighter than an iron small block. Can't prove that, but I'd bet on it.
Advantage slant six
The "bling" factor at shows might be of interest to some. A nice-looking small block has a LOT of competition at car shows and usually needs to have something really special, in cosmetic appeal, to win an award, just because there are so many... But, a slant six with a turbo on it is such a rarity, judges HAVE to pay attention.
Advantage, slant six
Then, there are the negative factors... and, there are some!
You can always put a turbo on a small block and go much faster than you could EVER hope to go with a slant six
Advantage, small block
No roller cams are available for slant six engine (no available roller-tappet cores) so, the ZDDP issue is always a problem.
Advantage, small block
You REALLY need both an intercooler AND a chemical intercooler (alcohol injector) for a hi-boost turbo slant six, and they don't give these away. None is needed on a normally-aspirated small block,
Advantage, small block
I don't think that a turbocharged slant six is a very good bracket car for drag racing, because of problems with turbo-spool on takeoff, and consistency. We are not building our car to run brackets; if we wanted to win bracket races, we'd build something else.
Advantage; small block
Detonation under boost will destroy a turbocharged motor on boost, quicker than you can say "turbo." So, fuel of sufficient octane is always going to be a problem. E-85 would be the perfect hi-octane fuel, but the quality of it at the pump is so iffy, you just can't trust it when it comes to boosted motors. The normally-aspirated small block, with high-compression pistons is choosy when it comes to octane, too, but the results from normally-aspirated detonation are usually not as "catastrophic" as when it happens with, say, 25 pounds of boost. So, I have to say that the turbo slant six is a problem child in that area. Bear in mind that I originally said a "500-horsepower" turbo slant six. That's what we are talking about, here, But to be realistic, the great bulk of whatever turbocharged slant six motors come to pass, MOST will never see boost levels that high, and the picture changes greatly at 7-10 pounds of boost. But, that wasn't the argument, here. Soooooo...
Advantage, small block
The sound of a well-tuned, high-revving small block at full song, is music to almost everyone's ears. Slant sixes with turbos are quiet: the turbo impeller sort of homoginizes the sound waves...
Advantage small block
Due to the very-limited rpm range (less than 5,500rpm, tops, usually) the slant six turbo motor doesn't need a high rpm ignition system like a high-winding, 500 HP, normally-aspirated small block. A stock distributor will work fine, with no worries about effective spark at 7,000 rpm... 'cause, that boosted slant 6 is never gonna see even 6,000 rpm, much less 7,000...
Advantage, slant six
There are no aftermarket (aluminum, or otherwise) cylinder heads for the slant six, so the best you can do is to port the original head, and add some cheap 1/75"/1.5" valves (some folks have used 318 valves.) And, there's only ONE head to deal with, so there's just no place to spend money (of the quantity the V8 car can absorb) on the head. A complete ported head for a slant 6, ready to run, will be cheaper than a pair of aftermarket V8 heads that will support 500 horsepower, I believe.
Advantage, slant six
I hope that after reading all this meandering post, I have made a case for it being actually cheaper to build a 500 HP turbo slant 6 than it is to make the same amount of power with a normally-aspirated, small block V8.
