42 replies to this topic

[Andy@PWR]

This is just my take on the subject and is by no means some sort of industry standard.
 
The main things to consider when building an engine for use with forced induction are the following
    1. Daily driver or not
    2. How many miles per year
    3. What type of power adder is being used
    4. What type of racing is involved
 
 
When we are building an engine, we are assuming that the engine will be installed into a daily driver or at least something that will see 5,000-15,000 miles a year. With that being said, a lot of engine selection is going to be based durability and strength since we offer a warranty on 95% of our engines. 
 
We are going to base this discussion off of the 6.1L block since its the most common. 
 
The deck height of the 6.1L block is roughly 9.240". What this means is that no matter what crank, rods, pistons you use, it must be no taller than 9.240" roughly. Any taller, and the piston will stick out the top of the block. Depending on the cylinder head we use, we might purposely stick the piston out as much as .015" above the deck of the block. Keep in mind that the head gasket is roughly .040" thick also. The 6.1L cylinder head has a perfectly round combustion chamber and is counter sunk a good amount which makes it easier to run a positive deck height with the piston. With the Apaches, Eagles, or Thitek heads, you better be real careful to maintain at least .035" clearance between that piston and that cylinder head otherwise they will hit at higher rpms as the rods stretch. This is assuming you are using a steel rod. An aluminum rod needs even more clearance. 
 
Ok so we now know that 9.240" is our measurement to work with , lets see what that means for crank, rod, piston selection. 
 
371ci: 3.58" crank, 6.240" rod, 1.210" Compression height piston. 
Take the stroke and divide it by 2 (3.58/2=1.79), add the rod length and then the compression height on the piston and you 9.240". In this case, the piston will be perfectly flush with the deck surface. 
 
 
Now, if we want to increase the stroke build a 393 (Keep in mind that all 6.1 blocks have basically the same bore and that all the displacement differences are from crankshaft stroke), we will need to use the 3.795" crank. 
 
Since we know the 393 uses a 3.795" crank and we know the deck height is 9.240", all we have to do is solve for the rest. 3.795/2=1.8975". 9.240" - 1.8975" = 7.3425". So we have 7.3425" to work with in selecting our rod and piston compression height. If we use a 6.125" rod, we can run a piston with a 1.2175" compression height. If we use a 6.200" rod, the piston will have to be shorter at 1.1425". This is where you have to decide whether you want a longer rod and shorter piston or a shorter rod and taller piston. In this case, Im going to use the shorter rod because the 1.2175" piston will allow the wrist pin to sit below the oil rail of the piston where the 1.1425" piston will have the wrist pin going directly through the oil ring and will require an oil rail support ring for the oil rings to sit on. 
 
 
Now we can look at the 426. The 426 typically uses either a 4.05" crank or a 4.08" crank. They are basically the same crank, but I will use the 4.05" for this discussion. 
 
4.05"/2=2.025"
9.240"-2.025"=7.215"
Lets use a 6.125" rod
7.215"-6.125"=1.09" compression height piston
 
The reason Im using a 6.125" rod is because any shorter and the bottom skirt of the piston will slam into the counterweights of the crank if we go any shorter on the rod. We could shortent he skirt of the piston, but the piston is already hindered by having a short compression height so we dont want to make it even more unstable by shortening the skirt. 
 
So as you can see above, by increasing displacement, you have to shorten rods and compression heights on the pistons, both of which comprimise integrity of the motor. Not to mention the longer strokes will also force the piston to hang out of the bottom of the cylinder bore due to the extended travel of the piston. This forces the piston to rock back and forth increasing wear and decreasing engine life. Add boost on top of that and you have an even bigger problem. 
 
 
Based on all the info above, I prefer to use the 393 for our boosted applications or possibly the 371 but that depends on what power adder is used also. More on that later. The 426 will work, but I dont see a long engine life out of a boosted 426, at least not compared to the others. 
 
 
One more thing to look at from a structual standpoint. Crankshaft overlap. Crankshaft "overlap" can be visualized by looking along the axis of the crank. A simple sketch helps.

Draw a 2.560 diameter (1.25 radius) circle to represent the main journal.

Draw a line from the center of that circle 2.025" long. using the end of that line, draw a circle that is 2.00" in diameter. The 2.025" long line represents a 4.05" stroke. The 2.00" diameter circle represents the rod journal. The area where the circles (journals) overlap is what most folks mean by crankshaft overlap.

You could do the math to get an overlap distance:

(main dia/2) + (rod dia/2) -(stroke/2) =Overlap

In this case: (2.56/2) + (2.0/2) - (4.05/2) = .255" Overlap (For a 426)

The metal in the crank journals doesn't really "overlap" because of the main and rod journals are offset, but intuitively, more overlap should equal a stronger crank.

For a 393 or 371, you will have more overlap for 2 reasons. They have a shorter stroke and a larger rod journal. (Some 3.795 cranks also use a 2.0" journal)


The main reason I brought up crankshaft overlap is because the crankshaft is what turns the transmission and everything behind the transmission. Its the workhorse of the engine and if it isnt strong, it isnt going to last. Add a supercharger to the nose of a crankshaft and now we have even more stress on that crank.



With all that being said, I have one final point to consider. Turbo vs Supercharger. Superchargers are belt driven and displacement isnt going to affect how they build boost, rpm will. With a turbo, it is driven by exhaust gases. Larger engines have more exhaust gas and result in faster spooling turbos. They also can be a hinderance if your turbo piping isnt ideal and it can create too much back-pressure from too much exhaust gas.


To summarize, we typically like to use a little longer strokes on the turbo charged engines as they will spool the turbos a little better.

Supercharged engine: Stock crank 3.58" - 3.795" crank
Turbocharged engine: 3.795" - 4.00" crank


Again this is all just my opinion. There are several examples of me going against all of what I just said and I will continue to contradict myself in the future, but this is a rough guideline to start with.

[legmaker]

nice write up.  let the discussion begin.....

[StevoHemi]

I can read stuff like this all day long. Always enjoy learning about the HEMI.

 

Thanks for posting Andy!

 

Stevo

[Shaggy SRT]

Great info, Thanks!!!

[70Barracuda]

Nice write up, thanks for posting it.

[!FST4DR]

This is just my take on the subject and is by no means some sort of industry standard.
 
The main things to consider when building an engine for use with forced induction are the following
    1. Daily driver or not
    2. How many miles per year
    3. What type of power adder is being used
    4. What type of racing is involved
 
 
 

 

Andy,

Would "how much" of any certain power adder matter as well?

[Andy@PWR]

Available strokes.
3.579
3.593
3.795
3.800
3.900
4.000
4.050
4.080
4.200
4.250

Andy,
Would "how much" of any certain power adder matter as well?

Absolutely. A 10psi application would be fine on any of the available engines. This is more for guys trying to get the most out of their application.

[Adanac Charger]

Great info, Andy.... very cool that you're willing to share this.

 

I'd love to see your (easy-to-read) take on piston speed and rod ratio for G3 Hemi's.

[Chally]

This was really cool to read. thanks for sharing!

[toofart]

Knowledge is PWR ... thanks Andy.

[Drp6982]

No love for the nitrous motors ?   

[Andy@PWR]

No love for the nitrous motors ?

Nitrous motors are more like extremely powerful NA motors. They dont need the crank to turn them and they dont need exhaust gas to turn them. 400 shot on a 6.1 is never gonna be as strong as a 400 shot on a 419. Nitrous motors fall into the NA motor category for me. Still need extremely stong parts though. You already broke a crank way back when and who knows what broke this weekend. Im still betting on a piston or bearing. I have yet to see someone break a Manley crank and I seriously doubt you hurt one of those 300m rods. Will be interesting to see.

Great info, Andy.... very cool that you're willing to share this.
 
I'd love to see your (easy-to-read) take on piston speed and rod ratio for G3 Hemi's.

The more I read and the more I experience, I dont think rod-ratio is as critical as most once thought. I would rather have a shorter rod if it meant keeping the wrist pin out of the oil ring groove. I am playing around with some cool new combinations right now. Mani has a pretty cool motor being built for his car

[Drp6982]

Well, I did turn the motor over in the pits and it seemed to spin somewhat feely so I doubt I snapped a crank like before.

 

So if Nitrous is as an extreme NA, How big would you go CI wise for a race motor only (No street  miles) on a 500 shot ?  Is what we did as big as you safely feel ?

[Andy@PWR]

Without switching to aluminum, I wouldnt go any bigger. Another .005 to .010 on the bore wont hurt but I definitely wouldnt increase stroke. You can go up to 444ci with the aluminum block and be fine.

Well, I did turn the motor over in the pits and it seemed to spin somewhat feely so I doubt I snapped a crank like before.
 
So if Nitrous is as an extreme NA, How big would you go CI wise for a race motor only (No street  miles) on a 500 shot ?  Is what we did as big as you safely feel ?

Its probably piston, wrist pin, or bearing.

[legmaker]

^{i see some AL in drp's future......}

[Drp6982]

^{i see some AL in drp's future......}

 

 

I really dont want to, only being that the motor that goes in the car I know will get abused and will need replacing at some point. I would do alum for a strip strip car though

[dodgetime]

atsa really cool write up, you come up with the good facts on these motors

[Andy@PWR]

As long as you dont break a rod, aluminum blocks are easier to repair than iron blocks. Aluminum blocks have $50 sleeves that can be pulled out and replaced as needed. With an iron block you can only rebuild so many times.

[Corrupted]

Good right up.

[TA340]

Cool info. Glad a 393 stroker is high on your list of supercharged set ups. Thanks as well for sharing.