Safest way to make power on stock block.

[turbows6]

I want to know what some of you guys that have put some faster cars together think about the best way is to make power safely with a stock block and main caps. That's what mine is, but has ARP studded mains, Wavelock 2000 rod bolts and polished stock rods. Diamond pistons, and balanced rotated assembly.

Given equal horsepower:

Do you think that it is safer to make horsepower by trying to keep the boost levels a little lower and make the power with more rpms and less torque.

Or do you think you are better putting more boost to it and keeping the rpms down.

Or do you think it doesn't matter and it's just down to the power you make and the rpm you make the power doesn't matter much.

 

[bison]

Keep the rpm down! I keep mine under 5800 thanks to the PTC converter. The car will run 131-132mph the way it is right now at 26psi. You really need a good converter to keep the revs down on a high hp application. Last year it was over 6000 rpm in 3rd. Now its at 5500 rpm and i have more mph. The converter works. Ive been to 5500 rpm in 3rd at least 30 times on the factory stock bottom end at about 4.5%slip, 28" tire, 3.42 rear. You can do the math from there. This engine will be retired soon. Im pressing my luck with it now.

 

[turbows6]

I know these typically aren't rev happy motors, and I don't plan on trying 6000+ rpm's, but they are similar to other turbo motors in the aspect that you can play with the powerband to an extent. For example, if you were making an arbitrary 625 horsepower would the bottom end be more reliable with a 220* camshaft/ 20#s boost @ 6000 RPM, or with a 206* camshaft/ 25#s boost @ 5400 if you had a capable cylinder head to make the power.

 

[Pablo]

Excellent question, I've been pondering the same thing myself.

I think that in our case, keeping rpm down is going to be the most beneficial thing.

In aviation the "mean piston speed" of a piston engine is a pretty important factor in the longevity of an engine. There is a rule of thumb about it that 3000 mps is the upper limit for a reliable aircraft engine. The LC2 hits 3000 mps at just a hair under 5300 rpm. If it's a rule of thumb in aviation, there must be some sound reasoning behind it.

When you consider that the V6 is inherently unbalanced to begin with, i think those unbalanced forces start to grow at an exponential rate as the rpm rises. Those are forces that are pushing laterally on the crank and thus the main caps. Getting back to the aviation thing, it should come as no surprise then that a lot of general aviation aircraft engines are flat 6s since they have a better inherent balance than a v6.

I'm still researching the subject so I'm not quite sure if there is a practical and safe limit to BMEP which is essentially the average pressure in the cyl... obviously it would have to be higher to compensate for a lower rpm limit. I think most engines are octane limited in this regard more than anything but what I do note is that where engine longevity really counts (aviation) they are always looking to improve bmep as opposed to increasing rpm.

 

["Turbo-T"]

Interesting subject. As I have read Buick V6's don't like to be wound up. Which i think is a good thing. But if I may ask is it because the stroke is too long for the bore? What exactly dictates it as being a rev happy or a down low grunter?

 

[bison]

If your going to make any serious power and test the stock bottom end then you are going to extend the rpm at least 800 over stock and more likely 1000 rpm. Theres really no way around it. Thats why you need a good converter. A 212 cam is plenty. Anymore is too much imo. The 206 cam has been well over 130mph also.

 

[bison]

Excellent question, I've been pondering the same thing myself.

I think that in our case, keeping rpm down is going to be the most beneficial thing.

In aviation the "mean piston speed" of a piston engine is a pretty important factor in the longevity of an engine. There is a rule of thumb about it that 3000 mps is the upper limit for a reliable aircraft engine. The LC2 hits 3000 mps at just a hair under 5300 rpm. If it's a rule of thumb in aviation, there must be some sound reasoning behind it.

When you consider that the V6 is inherently unbalanced to begin with, i think those unbalanced forces start to grow at an exponential rate as the rpm rises. Those are forces that are pushing laterally on the crank and thus the main caps. Getting back to the aviation thing, it should come as no surprise then that a lot of general aviation aircraft engines are flat 6s since they have a better inherent balance than a v6.

I'm still researching the subject so I'm not quite sure if there is a practical and safe limit to BMEP which is essentially the average pressure in the cyl... obviously it would have to be higher to compensate for a lower rpm limit. I think most engines are octane limited in this regard more than anything but what I do note is that where engine longevity really counts (aviation) they are always looking to improve bmep as opposed to increasing rpm.

Piston speed is cntrolled by stroke and rod length. Its not the reason we dont rev more than needed. The stock crank is most of the problem because of the flexing and subsequent cap failure.

 

[Pablo]

I know that bison How do you think i did the calculation?
Reciprocating weight is going to cause things to flex, the crank included. Inherent unbalance multiplies with rpm. Ive seen charts showing how much lateral force a crank has to resist as rpms rise and they get into the hundreds of pounds. MPS is not just for the sake of the pistons and rods. Even "balanced" engines like the flat six are not really balanced because of the non sinusoidal movement of the pistons. This all gets transferred right through the crankshaft and main caps.

 

[bison]

I know that bison How do you think i did the calculation?
Reciprocating weight is going to cause things to flex, the crank included. Inherent unbalance multiplies with rpm. Ive seen charts showing how much lateral force a crank has to resist as rpms rise and they get into the hundreds of pounds. MPS is not just for the sake of the pistons and rods. Even "balanced" engines like the flat six are not really balanced because of the non sinusoidal movement of the pistons. This all gets transferred right through the crankshaft and main caps.

This is why you should consider a forged crank for applications that have lot of flex like our v-6's.