It usually has more to do with the rod to stroke ratio. The higher ratio is equivalent to less friction and less side loading. However the 6.2 rod is out of the question with the 4.125 stroke. My pins are right on the oil control rings with a 6.125 rod and 4.075 stroke. The bigger bores allow for use of bigger valves, and moves the valves away from the cylinder side walls where airflow is slowed. Try to utilize the longest rod for the best ratio.

 

Everyone agrees that it's better to build a race engine with a certain displacement limit by using more bore than stroke. But if you've got a given block and want the most power from it, you'll always be better off maximizing your bore and also then maximizing your stroke (except from a cost perspective). You will never be better off with the same bore and less stroke unless perhaps you're hoping for greater engine longevity (ie. endurance racing). As I stated earlier, this assumes that your intake manifold limits your rpms anyway...Under certain conditions, your short stroke engine could rev to 9K while your long stroke engine could only go to 7.5K and under these conditions the lower displacement engine could make more max HP. But again, this is an extreme race situation and I think the question is being asked in the sense of a street-driven LS1.

 

I don't think that is a problem as much as its a fact that a bigger bore size (i.e. 4.125 vs. 4.060) always helps unshrouds the valves, which then allows the heads to breath better (more air flow) and make bigger power throughout the entire rpm band!!

I can tell you that my new 427 C5R based motor with 4.125 bore size makes 100rwhp more at 4000 and 4500 rpms vs my old iron 427 motor that had a 4.060 bore and both engines use the same heads and cam and other than the different blocks the only other difference is that my iron 427 utilized a 4.125 stroke where my current alum. 427 C5R based motor uses a 4.00 stroke, the same way that GM RACING achieves 427cid in their C5R Vette Lemans RaceCar.

 

I did alot of research on this before deciding on building a 408 to go with my Procharger. When you run that much stroke (4.125) then reliability and longetivity starts to become an issue, especially with forced induction and nitrous. Going .060 on the bore just makes the problem worse. With power adders it's usually better to go conservative on the motor in order to get more out of the power adder itself. Think of it this way, what's going to give you more power, 19 cubic inches or an additional 50-75 shot.

 

Which would give more power: that 75 shot with a 408 or that 75 shot with a 427? The only reason it's best to go with the 408 are:
1. cost
2. the amount of power adder is too much for the extra displacement (although extra stroke won't make the engine weaker as much as extra bore could with thinner cylinder walls).
3. the planned operating range is too high for the stroke but this is generally not going to be the case with the power adder.

So is it cheaper to add displacement through stroking or boring? Boring if you go to an iron block and stock rotating assembly. If you're upgrading the rotating assembly it's going to be cheaper to stroke. A stroker will make more torque down below 4k rpms where it's valuable on the street. As long as you don't want higher than stock rpms it is still strong enough to endure any power adder. The short stroke engines only reign supreme in race motors...and then again, wide bore AND long stroke will still be better than just wide bore and short stroke unless the racing class limits displacement.

 

The cost shouldn't be any different between a 408 and a 427. Unless a 4.125" stroker crank costs more than a 4.000 crank. Also most machine shops charge a standard rate for boring and honing whether you go .030 or .060". What I meant with a 75 shot is that a 408 would more than likely be more reliable when it comes to juice or boost so you might be able to push a little more juice through it then you would on the thinner walled 427.

 

Wait, were you talking about a 408 derived by stroking or boring? I thought you were talking about boring which would give it the same thin walls as the 427 achieved through boring and stroking. So both would handle nitrous just as well.

 

I think he is talking about a 4.030 bore x 4.00 crank to get a 408ci setup. I think the max you can bore the iron block is 4.060 x 4.00 to get a safe 414ci. Now that would make a kick azz NA setup, but any type of power adder would decrease the life of the motor. Yes you can go with a bigger crank to achieve 427ci, but I think you would sacrifice rpm for more torque. People are fascinated with revving motors high. I guess it comes from watching too much NASCAR. I personally think if you can accomplish 500rwhp at 5600rpm instead of 6300rpm you would win most races on the street if both setups are geared the same, but we love to hear that motor hummm. I am going with a 408ci because my engine builder recommends this setup for me. My setup is going to see an occassional nitrous, but the more I get NA the less I will have to spray..

 

I believe he is talking about boring and stroking and for iron 408 you only have to bore 4.030 and use a 4.00 crank and for 427 you must bore ironblock 4.060 and use a 4.125 crank!!!

 

MTI 427 at what rpm does your engine peak?

 

Exactly. I was talking about going 4.030 with a 4.000 stroke compared to 4.060 with a 4.125 stroke. I meant that with using a power adder it would be better to go with the 408 for reliability reasons. I think the 408 is the better bet especially with a blower because you can take advantage of the high rpm's where you make your power. If I was going n/a though I would probably go with the 427 for the extra cubes.

 

Can you explain why the 408ci would rev higher than the 427ci? I would think that would depend on the cam..

 

You're right, and it would depend on alot of things. But generally it's easier for a motor with a shorter stroke to spin faster at higher rpm's because the weight is closer to the crankshaft centerline. example-IRL cars. Short stroke, small displacement motors spinning 18,000rpm. This doesn't mean you can't spin a 427 to the moon, just a rule of thumb.

 

For example.. If I chose a 260/260 lsa 114-116 with my 408ci I could spin her to 8000rpm

This would be a hydr cam.. I wonder how much tq I would lose going with a cam that big?