Easy question: What really limits the rpm on the Gen II Lightning motor? Why can't we twist 'em to 6000 or more without seeing the rods sticking out the side of the block?

 

weak rods.

 

Piston speed.

 

I have had mine up to 6,300 on a Dyno. My shift points are around 5,500. At 6,000 you may start seeing problems.

 

The post on horsepower and torque has inspired me to try and get at least 6200 out of the engine, but I want to maintain reliabilty. The answers of "Weak rods" and "piston speed" both point to the reciprocating assembly being too heavy. With the overhead cams, the valve train looks like it would be happy at 7000. If the rods are made of butter and the pistons are heavy, the lower end is doomed to fail if the engine is run much higher than the design spec allows. A blown motor needs good pistons, but they don't have to be heavy. Sounds like a set of H-beam rods and something like JE pisrons is required here.

 

Don't forget the rotors in the blower which turn faster than the motor and are supposed to whip soon after the designated engine RPM limit.

 

There are other factors to consider besides the rotating assembly if you want more RPM. The SOHC heads really don't flow much air, you'll need an incredible port job or a set of 4V heads. The standard 1mm oversize valves aren't light enough to use at high RPM, so you'll need a titanium valve or one of the ultra alloys. Also, the comp, modmax, and crower valvesprings are only slightly better than stock.

For 6500+ RPMs you'll want a dual spring. You can have a custom valve made with the stem designed to use a chevy LS1 spring and machine the spring perch a little, or you can look to the motorcyle world. There's some cylcle springs that work that are KILLER. Don't forget a cam profile that gives up low RPM power in favor of high RPM power, and a deeper gear and looser torque converter to make up for low speed slugishness.

The best bet is a Cobra stlye 4.6 with a 3.70 bore. Engine size becomes almost irrelevant with a blower, the smaller 4V engine will rev much higher, have less parasitic losses, and be lighter. It'll make more power too. There's 4 cylinder turbo cars running 6.60's now, so a 4.6 with a lot of boost should scream. If you insist on a 5.4, offset grind the crank for slightly less stroke and machine down the rod journals to 2.0 and use a chevy rod with a big pin. Slap on a set of 4V heads and the intake pictured below with a procharger or turbo, then hang on.

 

You guys have to give the piston speed thing a rest, it aint the problem, rod length, stroke length, piston weight.....not the problem.

rscoleman, I have new favorite Mod motor spring....check out Endura-Tech .com
Dale

 

I am womderigg, Bad as L, what are the limiting componants to more rpm capability?
The posts about the increased efficiency of the Magnum Powers blower at high rpm`s as well as the torque vs rpm vs power thread has me revisiting ideas for low gearing and increased rpm.
Your thoughts are appreciated and respected.
(btw, I have few posts, but have read every thread and every post since this forum started. A lot has changed as we have learned and wiyh all the new items available it looks like we have many choices for building power thru varying methods).
Jim

 

Yo, Bad as L:

I'm looking for the solution here. Better springs are a good thing, but what's the key? You say it isn't rods, piston weight, stroke, etc., but you don't tell us what it is!

What is it ??????????

 

i'ved had no problem spinning my motor to 6000 rpm's evershift. its not the rods ect. its the heat and overspinning of the blowers that kill them. why do you think nitrous is used by the big guns to overcome that issue.

 

Dale, care to explain???????? I'm not suggesting it is any of the things you listed either. Please expand.

 

Well I mentioned valve springs but it goes alot deeper than that, its valve control in general. The fact that we have hydraulic adjuster is a big pain in the rear.

On a supercharged motor, the boost acts directly on the valve train. On a 2 valve mod motor the valve springs are good for about 6200 rpm or so. On the lightning motor with 8 lb of boost the engineers lowered the rpm limit to 5000 or so for a shift point.

One of the reasons they did this was because if you calculate the area for the backside of the intake valve and muliply that by 8lbs of boost acting directly on it then you can take that value and subtract it from the stock spring rate. So in round numbers lets say the intake valve has 1.75 square inches of area X 8 lbs = 14lbs. The stock L has about 74 lbs of seat pressure so now at full boost your valve spring has and effective 60 lbs of seat pressure.

Now at 5000 rpm the valves are opening and closing around 41 times per second......60 lbs of seat pressure is not very much to try and keep things under control at this speed. Here's were the hydraulic adjusters come in....as soon as the valve starts to seperate from the camshaft motion the hydraulic adjuster will take up the slack, and literaly hold the valve off of its seat a small amount.

There is a bunch of other things that come into play here like the frequency of a valve spring and what happens when you reach that frequency, cam lift rates, and cam lift period. The reason I run Comp cams is because I think Crower is out of the mind with some the cam lifts they have......they don't have a spring that will cover that in my opinion. The springs we have are not even 1" diameter and about 1.7" installed and you want to run near or at .600 lift with a spring like that?? Not in my motor.

Anyway you guys can take this info and run with it, but I can tell you its not a secret and I can't believe it hasn't caught on here....lord knows I have tried to mention it in the past. Call any cam manufacterer and tell them you have and engine...any engine, make it a small block chevy with a hydraulic cam and ask them to spec out a spring for you.....then tell him you have 17 lbs of boost and see what he says.

Here's another one for you...Robert Yates has a mod motor program going right now and you can ask him about any part in his motor and he will tell you.....just don't ask about valve springs .
Hope this helps
Dale

 

Thanks Dale!

FWIW, I agree completely.

Joel......

 

Let's assume that the valve train fails to keep up at high rpm and the valves wind up hanging open. Usually, valve float results in a lack of further acceleration rather than engine destruction. I currently have a Lightning engine on a stand with holes in both sides of the block. I'm pretty sure the heads of the valves are still attached to the stems. My concern here is what fails and why. I've long believed that engines fail in tension; that is, when the force on the rod, as it it trying to pull the piston back down, becomes so great that the big end goes out of round and the bearing material touches the crank journals. This occurs because the rod pushes the piston up and the clearance between the rod and the crank is virtually nil on the top of the journal. But when the piston gets to the top it hesitates, not moving, for a nanosecond while the crank continues to turn and the clearance is taken up on the far side of the rod journal. At that point the acceleration on the piston becomes close to infinite. It goes from stopped to moving so quickly that the interia pulls the big end of the rod out of round or breaks the piston or breaks the small end of the rod or whatever. It simply comes apart. Top fuel can fail in compression, where the net force on the crank is great enough to force the crank right out of the block. The rest of us suffer tension failures. I'm more than happy to address the valve spring issue, but I suspect I better look at the rotating assembly as well.