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I guess the real question I should be asking is what have some of you done, or have heard of being done, to make the entire assembly spin faster?
If I were trying to build a slant that would spool up faster, I would run a 170 or run a 1980s era cast crank motor. The cast crank engines introduced during he 1976 model year had lighter internal components and head. The cranks were lighter and the timing gears were lighter.
If you are trying to keep your original forged crank 225 and make it spin up faster, just get your rotating assembly balanced, run different pistons with thinner rings for reduced drag, run lightweight synthetic oil, and mill the block and head to achieve the dynamic compression ratio you want. There aren't many tricks you can do with a slant six crank beyond balancing, offset grinding the journals, or cutting and rewelding the crank for increased stroke.
The modifications to the crank will be limited to what the engine builder knows how to do once we get to that point, though... I have a few choices of builders currently, but unfortunately none of them are with-in a 1 hour drive in any direction and their work-loads vary so I've had limited interaction with any of them. All I know is that they are good enough for professionals in the NHRA, so they should be good enough for my Slant.
Quote: I shifted my 198 rod 2.2 turbo piston motor at 6500 normally, 6750 sometimes and had it over 7000 a couple times just to see if it would. No balancing, no grinding, just a Car Quest crank kit.
To spin one up you need a good head and valvesprings, and a decent camshaft.
The last thing I would do is build a 170. When you already have no cubes why handicap yourself further? High rpm don't mean crap if you don't have the nuts to get it up there to start with.
The displacement thing is kind of what I thought, too. Displacement builds torque, torque at higher RPM builds horsepower. If you don't have a lot of torque you have to spin the engine that much exponentially faster to build the power. As a rule of thumb, a stroker engine is going to produce more power and torque in the entire power band, while the non-stroker can spin faster and may be able to produce slightly more peak horsepower with the right supporting mods (head). I was originally thinking of stroking the 225, so definitely not taking any steps backwards on that one, especially since I don't really know where to start at getting a 170 around here... don't have all day to scout junk yards (even if I would love to).
With the 225 being as torque-y as it is, finding a way around the limitations of drag/resistance of the pistons and head flow rates to get the RPM's up means it could theoretically build a lot more power... I've read that the most powerful NA motor put down around 370 horses on an engine dyno, but then again I am confident that is on race fuel, really high SCR and DCR, and a bunch of parts that cost a lot more than I'm willing to spend, which is why my goal is really 250+hp at the crank... ideally it would be closer to 300hp at the crank so that I could get to that magical 250+ at the wheels... may be more fantasy than fact though.
Quote: Not sure i am following this obsession with making the motor spin faster, ie 7K.
It's not for the RPM's I'm obsessed with, it's the power. 
I was only trying to answer the question of how to spin it faster to get to that peak power, the engine running out of steam at high RPM being the limiting factor, which is typically air and fuel based on what has been stated in this thread.
My general ignorance is in the limitations of the motors, but then again, I learned a lot of new and at least newish stuff from veterans on here that may not post their every mod or every experience every time they do something new and 'unconventional'... that's the type of gold I was looking for, and it seems that I got all of that and a bag of chips. The last few bits may be on me to figure-out with the engine builder.
As an example of this general goal of trying things to get more power, I had an unofficial power record for a stock engine internal and stock turbo 2008 Mitsubishi Evo X for the State of Texas (if not USA) that had every type of bolt-on with a slightly uncommon (custom) turbo-pipping that was not offered by the major kit producers (but purchased for less from a US-local shop that specialized in these), e85 conversion with injectors, fuel rail, pump, and a nice tune by Cobb Tuning here in town. Stock internals, stock turbo, stock just about everything else except everything on both sides of the turbo. That thing put down 410 awhp and 450-ish awft/lbs at all 4 wheels on an all-wheel Mustang dyno. That was the 'race' tune. The daily tune was 371 awhp/411 awft/lbs.
At the time and based on all of the dyno data available to Cobb Tuning in Plano at the time, there were people running $1,200 FP Red and Black turbo upgrades with less power at the wheels. 
I never got to take it to the track as it was cruelly (but very ironically) taken from me by a $6K Kia Rio running the middle of a red light and undoing the tens of hours I spent epoxying custom carbon fiber fenders, bolting-on parts, etc. in the garage on weekends. But hey, fun was fun while I had it. The experience of driving that thing on the street was a blast, even though it was built for autocross (that it never got to see)... and it gave a few Porsche's, AMG's, M's, and Corvette's a run for their money. 
Yes, I am indeed a "special" kind of special. 
... But back on topic, the reason I want to spin it faster is because of this, specifically:
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... The only reason to wind it that high is if the gear ratio spread combined with the rear axle ratio and tire size allows such a pronounced drop in rpm during launch or while up shifting that the revs are needed so the rpm drop puts the mill right in the powerband for the build....
The tire diameter will be a static 27" with 255's for street and strip. Not currently planning on multiple sets of wheels.
3.55 gears are going in the back for daily-driving, and may consider going up to a 4.10 if I want more out of it and have enough top-end RPM to sacrifice... I'm still planning to run the lower-drag A904 as well (to conserve as much power as possible from the engine to the ground), so due to my gearing limitations I will have to spin the motor a certain speed to A.) be able to get to 80mph on the highway sometimes, B.) run the 1/4 with some RPM to spare... gas mileage will suffer over 60mph no matter what, but anything above 10mpg is acceptable to me at 80+mph... my daily commute does not have me traveling above 50mph, 65mph is fine for most place I have to go to ever, 80mph needs to be there if I'm in a hurry and the roads permits it, and 80+mph needs to be attainable when racing based on how fast my car can get going in the 1/4... don't want to be pegged at red line for the second half of a 1/4 mile, and not really looking for anything running shorter distances.
So now you know there is a little method to my madness... outside the other times when it's just pure madness. 
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