i was looking at parts today trying to find a reasonable but good rotating assembly. and i was looking at je pistons that wiegh about 386 grams along with some childs and albert 6" rods that wiegh 575 grams for a piston rod wieght thats 961 grams. for a crank i was looking at the 3.75" stroke "44lb superlight" crank from scat it says longer strokes wiegh more but the 3.5 stroke version is 44 lbs. i dont wish to discuss the top end of the engine. but the car will be for the street. i hope i'm headed in the right direction here. when its done it will be an m6 with 4.10's and a 9" rear. i just wondered what the effects of those "light wieght" parts will be on my power curve. so you guys who know feel free to tell me about it, my dad said i'd lose tq from it. i understand theres more power stored in a heavy rotating assembly a 60lb crank turning at 3500rpm is a lot more power stored and a 44lb one at 3500. car will be COMPLETELY n/a when done. so help me to understand, maybe my parts are all wrong.

 

Really for the street I wouldn't worry bout it too much. Unless you plan on being at the strip every weekend. I'm no expert but i know most lighter parts are found in engines that turn more rpms. Someone else may chime in and give more info from a horsepower standput, I've never really built one from all lightweight stuff.

 

Lightweight parts allow the engine to accelerate faster, because they have less inertia, or the resistance to accelerating. In a fast drag car where the engine may accelerate from 5000-8500 or so in a very few seconds, there is a noticeable difference, but at a very high cost penalty.

Heavy parts (high inertia) store more energy, and may help launch a heavy car. Yep, free revving to 3500 with a heavy-internal engine and a heavy flywheel and then dumping the clutch will help your launch with slicks. With street tires, you'll probably just see more smoke. However, with the ultra-light internals and a lightweight flywheel, you might find it difficult to launch the car.

For a street car, buy rotating parts for strength (not weight) without overspending, and spend your money on heads; you'll be faster. In other words, superlight rotating parts without good heads won't be much better than stock.

My $.02

 

I don't see how less rotating mass will decrease torque. Since torque is measured while during acceleration and a lighter rotating mass will accelerate more quickly, that should make for a higher torque reading.
I would think that as long as your lightweight parts are have adequate strength to stand up to the power you are making, there can be only benefits from using lighter parts.

 

Yes... reduced weight of rotating parts reduces the energy/HP consumed in bringing them up to speed. But the issue is "cost-benefit" ratio. It may cost you a lot of $$$ to gain a few HP with lightweight rotating parts, and that same $$$ spent on the heads, for example will gain you more in the long run.

Its sort of like looking at a CF driveshaft.... big $$$, little gain... or going nuts with reduced weight on the rear axle assembly... gun-drilled axle, light weight spool, light weight gears, drilled hub flanges.... they all reduce HP losses while accelerating, but the gain isn't really worth the cost, unless you have exhausted all the less expensive items available, and have fairly deep pockets.

 

Interal parts could easily become 2-3x the cost, for what gain? If you have the money and road race, go for it.

 

all those parts come to like $2100 if i go with the 50lb crank they're 1900. i'm not skimping on the heads either they'll be a set of very well ported afr 210's. the idea here is to build the badest motor i can afford i just landed a pretty good summer job and will probably dump from 7-10k into the car most of that being the engine since i've been setting the car up to take the power already. and the cost above isnt price shopping. we own a parts store and have accounts with many many high performance companies so i can get all of these things at cost instead of retail the price above is retail. so it'll probably cost me about 1300 for all those parts when done. the rest of the combo will be around 12:1 comp a good set of ported afr 210's.

for cam timing it would be nice to get a little help from you guys. what i was thinking cam wise was: 3111/3317 112icl 112lsa thats 230/242 .598/.576. generally i dont like to use a cam that big but after reading about the ltx style intakes having poor flow i decided that maybe a 224/236 with high lift wouldnt work so good so i extended the time the valve was open and cut some of the lift down which will save springs. thats just what i'm thinking you guys tell me. i'm sure it will have a pretty rough idle but thats why i cranked up the CR to around 12:1 hoping to make it behave a little better. so Oldsstroker or bret come on and give me some cam help.

the reasoning behind this post is i'm trying to get all my parts nailed down so i can work up a reasonable budget.

 

Solid roller, period.

thats my input on the cam

hehe

 

I was just reading the new hot rod magazine I got in the mail yesterday and they had a small article on the difference in lightweigth versus standard parts and the horsepower difference was only 2 HP. So it really becomes a personal preference in how far you want to take this.

 

Going as light as you can within your budget on the rods and pistons makes some sense, especially if the engine sees high revs. Assuming that you stay out of detonation, the greatest forces placed on the rotating assembly are inertial in nature, and of course lighter parts will help this problem. Just find that "knee in the curve" in the price-vs.-weight curve, and get the most out of your money. Per what the Doc said, you can quickly double or triple your parts bill with very little benefit.

A lightweight crank makes a lot less sense, especially if you do what I did and put a 40+ lb flywheel/clutch assembly behind it, and then drop the whole thing into a 4200+ lb car A few pounds saved in the crank quickly disappears.

 

when i said power, i guess that was the wrong word in hindsight. i guess whati really should have said was engine characteristics. like how it would act as compaired to an engine built with a standard wieght crank and a rod piston combo that wieghs 1500 grams. obviously it will rev up a hell of a lot faster. about the hard launching thing, anybody think i'll have trouble in a 3500lb car (that includes me). besides setting the car up to get into the higher rpms thats kinda the idea behind the 4.10 rear gears is to give the car a mechanical advantage. another part of it is this car will be mostly street and i'm thinking on a street tire less inertial energy would be good, i wont have problems with traction so much... although at the power level i expect there may not be such a thing as traction on the street. anybody think i need more cam than 230/242 112lsa 112icl? or could somebody recommend something for me here, no jordon SR is out, sorry.

as for the whole cost thing, it really isnt going to matter when it comes down to it this time. i've got a high paying job this summer and no bills (living at home) i've also got the 85 truck to drive while the t/a is down. i'm having my dad check up on what the parts will run me at cost. if its cheap enough i'll do it. dont worry bret's dad i'd eat glass before i skimp on heads. so let me sum this up everyone agrees that theres nothing but gain for me here with light parts assuming i can afford it? i've got the next 3 months to come up with exactly what i want to do, i have longer of course, but i dont want to wait purposely.

 

Conventional wisdom… Vizard just had an interesting test/article published in MM&FF a couple of months ago testing this with a stock and light weight flywheel, and found that even if traction is not an issue, with slicks that stick with either combination, he ran better times with the lighter combination.

 

How were the 60 foot times?

The lighter flywheel should help, especially in the lower gears, as soon as you're hooked up.

FWIW, isn't MM&FF a double oxymoron?

 

They tried a 10.5# vs a 23.5# flywheel on 2 different cars (one was 2 seconds faster then the other). They found that they ran the best times launching at 500rpm more with the lighter flywheel and ran better #’s EVERYWHERE except the 1/8 mile times, and they were at a loss to explain that. On the dyno they found that there was slightly more power available on engagement and right after the shifts with the heavier setup, but the lighter assembly caught up and passed it just about instantly and showed a 10-20hp advantage through most of the curves

 

And I second that. Only way to go.