Generation IV Internal Engine - Thinking about forging a 4.8L just for fun.

I have been doing some research on the gen4 4.8L. So far I have found that the 4.8L and 5.3L share a block and pistons. I like the short stroke of the stock crank. Wiseco makes a .020 over bore piston for the 5.3 at 1.30 compression height. With the 3.268 stroke and a 6.300 rod at standard deck height 9.240, the compression height calculates to 1.306. Does anyone think that if i get a set of 9.300 rods and a set of wiseco 3.80 pistons that it would work out?

Edit: I would be in the hole .006 right?

I'd get a custom piston instead of a longer rod. The longer rod will increase your piston dwell time at tdc and bdc, ultimately driving your piston speed up.

Custom pistons are normally only 50 bucks over the price of 8 shelf pistons. I'd imagine that rod will be more than 50 over the cost of a normally stocked 6.1 or 6.125 rod. Also with the custom piston you can more accurately place the top of the piston in relation to the deck for better compression ratio and quench area.

I'd get a custom piston instead of a longer rod. The longer rod will increase your piston dwell time at tdc and bdc, ultimately driving your piston speed up.

Custom pistons are normally only 50 bucks over the price of 8 shelf pistons. I'd imagine that rod will be more than 50 over the cost of a normally stocked 6.1 or 6.125 rod. Also with the custom piston you can more accurately place the top of the piston in relation to the deck for better compression ratio and quench area.

Mean piston speed across the entire revolution of the crank is the same no matter what the rod length is. Speed is simply a function of distance divided by time. Distance is the length of the stroke and then multiplied by RPM to give you total distance traveled over a segment of time (in this case one minute). The dwell time at TDC and BDC will increase with a longer rod, thus *piston acceleration* will be different. Here is a great illustration (http://www.pistonheads.com/gassing/topic.asp?h=0&f=66&t=1103886&mid=0&i=40&nmt=Ideal+Camshaft+Lobe+Centreline+Angle&mid=0) about halfway down the page that shows acceleration (not distance traveled by the piston) with varying rod lengths and the same stroke. Think of acceleration as the distance traveled relative to the last position measured. So if you think about that in units of a degree, faster piston acceleration occurs with a shorter rod, not a longer one.


Now back to the original discussion at hand. I built a 6.300 long rod motor using off the shelf parts. 3.625 Eagle Stock stroke Crank, 6.300 Eagle Rods, and Diamond 4" Stroker Pistons for a 6.125" rod, put my combo .010 in the hole before any decking. Mine is a blower motor, and I felt the reduced rod angularity was a nice bonus, as their was no cost difference. If I was NA I would trade that stock stroke crank for a 4" arm any day. I went 6.300 because I was originally intending to use a stock crank, bought the rods, and then found out my stock crank was no good. To avoid buying new crank and rods, I picked up an Eagle crank at a good price and ran with it.

Thanks for the input guys. I forgot to mention that this setup would possibly see boost. What is your take on the longer rods versus the custom pistons in a boost application? As I understand it the longer rod would have less of an angle as the crank rotates, thus giving it more strength?

A longer rod will have increased dwell time, plus will side load the cylinder wall less than a shorter rod. Stronger? No.

Anymore input on this subject guys? I'm really thinking about trying this out. The 4.8L has a fairly long rod anyhow. I think they are 6.275? Correct me if I am wrong. .025 more isn't very much longer than the stockers. I did some calculating with the pistons .006 in the hole and .053 head gaskets with 64cc chambers, my compression would be around 8.70:1. That is a little lower than I want, but it would do for a boost application.

It really depends on what the goal is, if I was running a drag racing class that was limited to certain turbo size (like x275) I would consider it, otherwise, I would rather have the extra stroke. I would never sacrifice cubic inches for rod length, given an otherwise equal playing field as any advantages to be gained with the long rod combo would be easily beaten by the cubes.

I would also say that my motor is around 8.8-8.9 to 1, running a D1SC Procharger. If I could do it all over again, I would up the compression ratio a little bit, probably closer to 10 -1. Off boost it's pretty lazy.

I have been doing more research and it sounds like it would benefit what I plan on doing with this motor. The rod to stroke ratio is 1.93, which happens to be good for longevity and endurance. The rod will be at less of an angle at 90 degrees ATDC, thus making it less prone to bend and has reduced sidewall load as recently stated.

Has anyone ever used a set of small block chevy rods on an ls crank and pistons? The only company that makes an ls specific 6.300 inch rod is lunati. Im looking for something a little cheaper.

I use eagles, which have the gen 1 offset, without issue.

Thanks for the input. I took my mic and measured the big end width of some 4.8 rods, then compared them to some sbc rods. I came up with a few thousandths difference. I think everything will work fine. I'm going to go talk with my machinist in the morning to see if we can get the ball rolling.

Alright guys. I'm going to build this motor. Now my question is if a forged h-beam rod will support over a 1000 horse even with a 700 horse rating? The reason I ask is because the stockers aren't rated for what they have stood up to.

I don't think the stock rods have a "rating". It's just what people have pushed them to and gotten away with.

I wouldn't try to push any rod far past it's rating esp with boost.