dhirocz

iTrader: (2)

OK, here's my situation. I've got a GM HT383 crate motor that I've done alot of upgrading to...bearings, comp roller lifters and cc306 cam, AFR heads, moly metric rings, etc...now before I button up the motor, I considered replacin the stock PM rods for something stronger. It seems wrong to me that the crank be a 4340 forging but the rods not. That and I'm planning on getting pretty close to the 500 HP mark and increasing my compression ratio.

pistons at TDC are .029 in the hole
pistons are custom hyperutectic, 12cc dished, std. bore
crank is 4340 Cola crank 3.8" stroke (yes, 3.8")
rods are stroker PM rods, studded, 5.7"

one piston collapsed a skirt and that cylinder was burning oil. I got the engine for free under warranty from my buddy at the dealership and replaced all parts that were questionable. However, I'd really rather have 4340 rods as well. I have one that I consider questionable since it has marks on it even though I had the assembly OK'ed after replacing the bad parts. GM says that the rods will not be in stock for some time, up to a year, something about the manufacturer going out of business. So getting to the question:

Since the pistons are .029" in the hole at TDC, could I just lengthen the rod to 5.85" to make up for it? I'd rather have around 10.5 or 11:1 compression. Or am I screwing up the math somewhere? I've seen these rods before...

If I keep the stock rod length, could I just use a set of 4340 I beam floating rods and weight match them? I would loose the increase in compression, but I'm around 10.2:1 with 64 cc heads, the stock hyperutectic 12cc dish pistons and .015" head gaskets. I have the weight of the piston, but I'd rather not spend any more right now than I have to as I've spent alot of money in the last couple months and dont need to spend more right now on custom weight matched forgings. Thanks for your time!

EvilCartman

iTrader: (1)

Eagle 4340 H-beam rods would be my choice. You can probably find a good deal at www.cnc-motorsports.com for them. They were less money than Summit or Jegs for the 6" rods I bought.

ede

if you have 5.7 rods pistons .029 down and go to a 5.85 rod without any other changes the way i figure it you'll have your pistons .129 above the deck. you could use 5.85 rods if you changed to a piston with a differant wrist pin location. PM rods are among the and strongest factory rods you can buy and as strong as or stronger than the factory pink rods.

rleprechaun

Eagle rods are good and priced friendly for a forged 4340 rod. Stick to the 5.7 rod and deck the block .019.

formularpm

Scats got a set of 4130 5.7/6.0s that they say are good to 550hp and 7500rpm. $195 at cnc-performance.

DTL504

iTrader: (1)

1. If you change to a longer rod you will have to get different pistons because the ring will be closer to the oil ring lands.

2. If you do deciede to go with the longer rod get the longest rod possible (open for debate) because it will give a better angle and less wear on cylinder walls. 5.7, 5.85, 6.0, rods are identical in price so it just a matter of preference.

3. I would zero deck the block, get a good Quench area .035-.045 (using the gasket to get this quench) and have the heads cc to get your desired compression.

4. Eagle H-Beam Rods get my vote, they will give you a better Cam to rod clearance and rod to block clearance.

5. To figure out compression there are a lot of variable you would have to input your data into the calculator and it will give you a good reference point on the Dynamic and Static compression.

Quench Area is VERY important to prevent detonation which will destroy your engine. So it's best to have a higher compression than to have a large Quench (.055-.080). Thats a time Bomb waiting to happen and a under performance engine.

Here Read This: Download the DCR Calculator at the bottom of the page to help get the correct quench.

http://members.uia.net/pkelley2/DynamicCR.html

dhirocz

iTrader: (2)

Well the way I see it, with .029 below the deck and a .015 compressed head gasket thickness, I should be at 0.44 quench. Not optimal, but close. That still leaves me with a compression ratio of around 10.2:1. Not high enough, I want at least 10.7:1. I dont think it would be worth it to cut down a set of AFR's that much, but I would be best off with a 58cc chamber from the looks of it. It looks like I might be stuck with a custom flattop piston...

dhirocz

iTrader: (2)

Hey, I got a dumb idea....if I upgrade to a 5.85 rod and the pistons pop .129 above the deck, then if I use a std. bore 396 stroker piston I should only be .004 above the deck, so I'd be running a slight negative deck. Then I could use a 0.045 head gasket and then I'll have a .041 quench, right? That's probably more work than it's worth though...probably better off just getting pistons...

BILLY SMITH

i agree with DTL504 , got with the eagle rods, deck your block your to far in the hole if performance is what your after leave piston .005 in the hole or zero deck, run a fel-pro .045 gasket........here is something you can go figure on ....for ever .006"-inch you mill from a closed chamber small block head you reduce chamber volume by 1cc

RWB____s

Just a question? If you are building a new engine and went through the expense of a Cola crankshaft, and possibly aftermarket rods , Why are you running Hyper. pistons instead of stepping up to a good aftermarket forged piston? That would be
a good thing to do for future power addons.

ede

i was with RWB when i read this the first time, but never said anything because ther are advantages to running hypers

dhirocz

iTrader: (2)

Well, from my understanding of it, Hyperutectics are great pistons for the street on N/A motors. They have a low expansion rate unlike cast or forged (especially forged)pistons. I understand that the main downfall of hyperutectics are that they crack under detonation or high shock loads making them ideal for only N/A motors. The reason I have them is simply this...they were free. Yes, the whole engine was free, the only costs I have in it are the upgrade to moly rings, better bearings, oil pump and driveshaft, and the cam/timing chain change. The rest is the same as the standard GM HT383 crate motor. I think it's retarded that GM would put in a 4340 forged crank and only hyperutectic pistons and PM rods, but they never planned for these things to make more than the 340HP they originally were rated for. but for the price, I couldn't pass it up . I got it from a friend of mine at a local dealership, who wanted to get rid of it...it collapsed a skirt on cylinder 8, and twisted a rod. Dunno how, but I dont trust pink rods too much...my experience is that they suck over 380-400 HP. I've seen 'em come apart on stock motors at the rod cap... bad bolts I guess. No way I'll trust 'em over 400, not close.

My guess is I should do both...upgrade to a 5.7" 4340 floating rod and have a piston weighed, and have a set of leightweight forgings made as flycut flattops to raise the compression ratio, 0.10 in the hole (so I have room to deck the block), double my head gaskets (using felpro 1094's, 0.15" compressed thickness, 4.100" bore), so I not only have an indestructable rotating assembly, but have room for nitrous, etc. I guess 11.25:1 would be a good goal with pump gas. Suggestions on custom piston manufacturers would be great...never had to make a custom set. Anybody?

I guess this'll mean I'll have the only completely forged HT383 shortblock in existence But even after all the expenses, I'll still be under the $1000 mark for the shortblock including the cam and chain. So I guess if I think about it I shouldn't skimp here...anybody have any input?

DTL504

iTrader: (1)

Lets look at the three different pistons and you can deciede which one fit your budget and performance goals:

Cast pistons are inexpensive and are much more reliable than they're given credit for. Good for moderate normally aspirated street performance.

Hypereutectic pistons are an aluminum alloy with a higher silicon content than typical cast pistons. Good for higher end normally aspirated street performance. They have some sort of heat reflecting properties and don't expand as much as cast or forged slugs, so you can run tighter piston-to-wall clearances than with either of the others.

Forged pistons are the most "heavy duty" of the three. They can take more punishment, so they're the choice for higher horsepower engines running high compression, nitrous oxide, or forced induction.

Most hot rodders tend to go forged because they want "the best of everything". And there's nothing wrong with that.

DTL504