What /6 is Best For Forced Induction 170, 198, or 225?

[JGC403]

I was reading in another thread that the /6 head was originally designed for the 170c.i. version. Also that its difficult to put decent sized valves in the head because of the small bore size of these engines.
So keeping in mind we are stock with a factory type head that was designed for 170c.i. and are limited to valve size. Lets port the head, put the biggest valves in we can, and put a turbo on the engine.
So for maximum horsepower, what size engine would be the best? Would the smaller 170 be the best since the head was originally designed for it, or does the old saying still hold true even with a turbo and small head "no replacement for displacement" and go with the 225? Or the 198 that falls in the middle?

 

[Killer6]

I was reading in another thread that the /6 head was originally designed for the 170c.i. version. Also that its difficult to put decent sized valves in the head because of the small bore size of these engines.
So keeping in mind we are stock with a factory type head that was designed for 170c.i. and are limited to valve size. Lets port the head, put the biggest valves in we can, and put a turbo on the engine.
So for maximum horsepower, what size engine would be the best? Would the smaller 170 be the best since the head was originally designed for it, or does the old saying still hold true even with a turbo and small head "no replacement for displacement" and go with the 225? Or the 198 that falls in the middle?

No matter what, maximum torque & power production will come from the 225, but the RPM range will be NARROW compared to the smaller mills. It depends on what Your
plans are, street romping/drag racing, or road racing/country road switchbacks and long stretches wound out? Theoretically headflow & boost should = same output
potential at different RPM's, but reality says not really, frictional factors and stresses go up by the square of the increase in velocity....there are losses.

 

[JGC403]

Is there a max rpm that is a general guideline for these engines, that the bottom end really doesn't like to go above?

Around what RPM does the 225, 198, 170 run out of airflow with a ported head?


I just pulled up allpar site on the /6 its interesting to see how the rpm drops for peak horsepower going down the list.

Engine......Years.............C/R...............Horsepower...........Torque
170........1959-1963.......8.2 to 1.........101 @ 4400...........155@ 2400
170........1964-1966.......8.5 to 1.........101 @ 4400...........155 @ 2400
170........1967-1969.......8.5 to 1.........115 @ 4400...........155 @ 2400
198........1970-1971.......8.4 to 1.........125 @ 4400...........180 @ 2000
198........1972...............8.4 to 1.........100 @ 4400...........160 @ 2400
198........1973...............8.4 to 1...........95 @ 4000...........150 @ 1600
198........1974...............8.4 to 1...........95 @ 4000...........145 @ 2000
225........1960-1962.......8.5 to 1.........145 @ 4000...........215 @ 2800
225........1963...............8.2 to 1.........145 @ 4000...........215 @ 2400
225........1964-1971.......8.4 to 1.........145 @ 4000...........215 @ 2400
225........1972...............8.4 to 1.........110 @ 4000...........185 @ 2000
225........1973...............8.4 to 1.........105 @ 4000...........185 @ 1600
225........1974...............8.4 to 1..........105 @ 3600...........180 @ 1600
225........1975...............8.4 to 1............95 @ 3600...........170 @ 1600
225........1976-78..........8.4 to 1...........100 @ 3600...........170 @ 1600
225-2V...1977-78..........8.4 to 1...........110.......................180

 

[rumblefish360]

Speaking from what i see on V8’s, a /6 isn’t going to act to differently than any other engine that is pressurized. So in a way, your correct on all accounts. A lot depends on how you want to play it and for what purpose.

The cylinder head in a N/A engine will flow only so much air. A larger engine ingest more are quicker than a smaller engine. If the head can only feed a 225 to 5000 rpm, the smaller engine will be able to rev higher because of the volume of air it ingests is equal to the volume of the larger engine at a lower rpm.

All of this kind of goes out the window when your pressurized.

Bore and stroke come into play more so at this point. I think bigger is better on engine and turbo.

 

[Killer6]

Speaking from what i see on V8’s, a /6 isn’t going to act to differently than any other engine that is pressurized.

The cylinder head in a N/A engine will flow only so much air. A larger engine ingest more are quicker than a smaller engine. If the head can only feed a 225 to 5000 rpm, the smaller engine will be able to rev higher because of the volume of air it ingests is equal to the volume of the larger engine at a lower rpm.

All of this kind of goes out the window when your pressurized.

Not quite, the same rules apply for the same amount of rise in atmosphere provided by the compressor, a smaller engine will still RPM higher with a cylinder head of a
given flow capacity. Take the little 2.2L, the IROC R/T 4 valve head raised the RPM range of that mill by nearly 1,000 RPM at the same boost, the same would've happened
if the engine had been de-stroked to the same displacement/CFM ratio with the 2 valve head...................................................

 

[rumblefish360]

The cylinder head is still the limiting factory. When I said it all goes out the window when you pressurize it is because of the wide and varied parameters introduced and variables that go with it in how it operates.

He also doesn’t have the luxgury of a 4 valve head in his slant 6 so why would you even introduce that into the conversation? It seems that it was done to make me look dumb.

P.S. My Grandfather inlaw worked on those in the beginning before Chrysler decided on turbo cars thank you very much.
I have more stories than you have and had hair.

Next....

 

[Killer6]

The cylinder head is still the limiting factory. When I said it all goes out the window when you pressurize it is because of the wide and varied parameters introduced and variables that go with it in how it operates.

He also doesn’t have the luxgury of a 4 valve head in his slant 6 so why would you even introduce that into the conversation? It seems that it was done to make me look dumb.

P.S. My Grandfather inlaw worked on those in the beginning before Chrysler decided on turbo cars thank you very much.
I have more stories than you have and had hair.

Next....

LOL, I think You should ease up there Bro', Your statements seem contradictory. But I think We agree & don't realize it, ..mostly...
1) "The head is the limiting factor"--Yes, exactly why I brought up the 4-valve head, it doesn't have to be, it could just be O/S valves & porting or whatever...it's more flow.
2) "It all goes out the window when You pressurize"--What "all" are We talking about? The example I gave shows the relationship of headflow vs displacement w/= boost.
The OP is talking an identically prepped head on two different size engines, the smaller one will rev higher at the same boost & turbo capacity, that is all I'm stating.
Depending on how well the system is designed and responsive the turbo is, the effective RPM band could be a lot wider with the smaller engine, if that is needed.
3) I never introduce anything here to make anybody "look dumb" Sir, My intention is to see that the OP gets accurate info as much as possible, I don't need the former.
4) I'm glad You have a relative that was part of Mopar's history, and I hope You have those stories written down to keep & share, peace...............

 

[pishta]

I think the 225 would be best for all blown apps. More stroke time, more torque. turbos dont need to be engine spooled to the moon if you size them right. Blowers same, do the math and have them blow to your motors limit by pulley sizing. The slant head (as with most heads) will make the same power with different displacements to a point as it only flows X amount of air, its the displacement that moved the power band high or low. Notice the peak HP@RPM of the 3 sizes. pretty close on HP, very different on RPM . I would think the larger displacement would have a broader power band? Look how peaky a motorcycle motor is, and then a 440. I could be mistaking.....273/360--same cam, different power bands. 170 will rev, thats what it took to make power, same with the 273 (weight?) but with different dynamics as it took the same crank as a lowly 318...which took the same crank (stroke) as the highly regarded 340..

 

[whitepunkonnitro]

If I was going to build an all out, take-no-prisoners, high boost or spray Slant, I would DEFINITELY go with a 170.
The engineers that designed and built these things focused on the 170 as the performance engine. They considered the 225 a compromise best suited to hailing heavy sedans and taxi cabs around. One engineer was quoted as calling the 170,."the perfect engine" and there are tales of them running for days on end at WOT on the Chrysler dynos back in the day.
One of my daily drivers has over a million miles on it without ever being opened...a 64 Dart with a 170.
I abuse and neglect that poor thing without mercy! In five years, I've changed the oil once! And yet she keeps humming along like a brand new mill.
The 170 is beyond doubt, the superior Slant