Picture this: A .040 over, stroked 350 displaces 385 cubic inches:

My new pistons & rod assemblies are 128 grams lighter than the old ones. I'm going internally balanced, with a smaller diameter harmonic balancer, and a ten-inch converter versus the original stock 12-inch.

How will this change the behavior of the engine? (Assume the converters are equal in efficiency and stall speed)

 

basicly what you should notice will be alot smoother at all rpm and quicker revving.

 

I'm going to go from what I've read. Not from experience (only a few engine swaps in my background).
A lighter engine (small balancer, light flywheel/flexplate, smaller converter) = a faster reving engine which is good for stuff like autocross and stick cars. If you have an auto I've been told that more weight is going to make the car faster. It's more torque, that easy. With a heavier setup there is more rotational weight which is more torque and so when going up hills the thing will chug right up and won't be as inclined to slow down and downshift. The idea of having more torque is good for the 1/4 mile. With a heavier setup you'll notice the shifts are smoother and the engine doesn't drop rpms as much as a super light setup would. It's been proven that an 8" balancer will make you faster in the 1/4.
I'm not all that sure on the smaller torque converters because they are more complicated than just rotating mass. The smaller the converter usually the higher stall speed which will let the engine come off the line with more torque. Does that make sence? Feel free to correct me because I'm not SURE about all this, this is just what I've learned and read about.


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, Jon (350 TBI!)
91 Red My website

 

I'm not sure I can agree with the 'more torque' argument: Torque is force as applied to a moment arm. I don't think that having a heavier mass will necessarily increase the force placed on the moment arm. The force comes from the expanding gases pressing on the piston. However, I thought about it last night: The reciprocating mass has inertia. Inertia is a physics term that represents "resistance to change". A body in motion will stay in motion until some force is applied in the opposite direction to cause it to stop. Same thing in reverse! Therefore, a reciprocating assembly that is heavier has more inertia, and more resistance to change. Thus, more force is required to speed it up, and to slow it down.

A lighter reciprocating mass will be quicker revving, but will also be easier to slow down. How would this affect the driveaility? Well, I suspect that I will see that the engine may be more susceptable to hunting, and surging caused by a big cam. Any misfires will be easier to notice as the engine momentarily slows. (Less inertia, more easily slowed down by frictional losses) It will stutter more at idle. However, when the throttle is cracked open, it will rev faster.

At least, that's my prediction, anyway. Does anyone else concur?

 

One thing less mass does is make the engine live longer. It is easier on bottom end parts. less likely to put things into low earth orbit.

 

Well put! However, I imagine that there is a practical limit. For instance, my Lunati pistons are 195 grams lighter than the Speedpro forged pistons I removed. I suppose the Speedpros were better suited to Nitrous because of the thicker dome. Since I'm planning on ceramic coating the piston tops anyway, I think we'll have a good chance of surviving the 6800 RPM blasts & the occasional application of nitrous.

It's a forged 4340 crank with forged 4340 rods, Lunati pistons, SFI approved balancer, SFI approved flexplate, and everythings being balanced to +-1 gram. The 4-bolt block is line bored and running ARP studs, with windage tray, crank scraper, and oil drainback standpipes. It should be okay for most stuff under 7500 RPM, I imagine.

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Daniel Burk
http://www.isthq.com/~dan/fcar.html
'84 Trans Am WS6/L69
KB SFC, Moser axles, Torsen Diff., Spohn Adj. torque arm,
Ported 305 heads w/1.94"intake valves, Comp Cams XE262H, Griffen alum. radiator,
Turbine Technologies 2500 stall converter, underdrive pulleys, Crane Hi-6 & more.
1.05g skidpad verified.
Best of 14.039 at 100.82 MPH in Stanton, MI
New! 275-40-ZR17 tires on ROH ZS 17 x 9.5's

 

there is a limit. NOS and blowers are one of the main factors. Most generally available pistons (not counting the some of the latest two ring and feather weight pistons), will take 200 shot of NOS no problem. When you combine that with a good high quality rod (no featherweight 530 gram stuff here either), it will live fine. In general, all 430 gram 350 type pistons and 480 gram 400 pistons will take just about anything you throw at them in my opinion. Especially when combined with a good after market rod. I don't even think H beams are necessary. I like I beam rods better. I got set of the cat 4340 I beams that weigh 550 grams and use capscrews.

I know someone that has a stock bottom end 400 with hyperutecic pistons. He spins it 6500 everyday and has a 250 shot. I don't agree with the concept, but it lives.