Anybody ever swapped out a heavy( stock) rotating assy for lighter aftermarket?

-

B3422w5

Well-Known Member
Joined
Feb 9, 2017
Messages
3,696
Reaction score
4,569
Location
Portage michigan
Just curious if anybody has ever swapped out heavy factory pistons and rods for lighter aftermarket units, and went back to the track?
Reason i ask, is i have a completely stock 360 bottom end in the motor i currently am using.
I also have a 418 shortblock laying around.
Having been there, done that before, instead of at this upcoming seasons end using the 418, thinking about just upgrading the 360. I suspect the 418( it would be 12.8 ish to 1 abd maje some low 10’s steam, would be enough to break my 8.75 rear.
Think the 360 wouldnt. Be slower and good bit less squeeeze and torque.
Gotta believe a light assembly would accelerate and shift recover and notable amount quicker

lol.. edit.. Getting in the racing mood because i recently had full hip replacement and didn't know how soon i could literally get in and out of my Dart( it sits low). Well, yesterday i gave it a shot.... passed the test. So racing season is a reality
 
Wouldn't it just be easier to upgrade the rear end? Just get the lighter rotating assembly balanced but why mess with it at all if your worried about the rear. What transmission are you using?
 
Wouldn't it just be easier to upgrade the rear end? Just get the lighter rotating assembly balanced but why mess with it at all if your worried about the rear. What transmission are you using?

904
i can upgrade the 360 for good bit cheaper than buying a new Dana. Would probably sell the 418 at that point
I am getting old and been in the 9’s for years and years. Want to not have to work on things anymore.
Light 360 with heads i currently have and the extra compression new slugs( would go flattops zero deck) would easily get the car into the 10’s and sate me
8.75 live a long life that way footbreaking
 
180F5921-1C18-4F9A-80C4-F49EACFFCE24.jpeg
7FF57BEE-A318-4FA0-98AD-7754305EF868.jpeg
A926F690-CE15-4F31-81A0-DF9378336164.jpeg
What’s the matter with 8 3/4’s. My sons been 10.0’s with his at 3200 pounds and transbrake. Should you change out the gears if you race weekly every three years or so. Sure why not.
 
Sounds to me like a piston swap for the stroker is a better option.
Get it down into pump gas territory, put in a cam that isn’t set on “kill”.
Makes for a nice low maintenance mid-range racer...... that can be used on the street with no worries.

I don’t see a lighter bobweight, with no other changes, doing much to lower the ET.
 
Sounds to me like a piston swap for the stroker is a better option.
Get it down into pump gas territory, put in a cam that isn’t set on “kill”.
Makes for a nice low maintenance mid-range racer...... that can be used on the street with no worries.

I don’t see a lighter bobweight, with no other changes, doing much to lower the ET.

current 360 “ might” be 10 to 1. Its even that because i skinned the heads to 58cc and run an 028 gasket. All factory internals.
The 418 is zero deck flattops. Be roughly 12.7 to 1 with 58cc head.
The other factor is me. I know me... lol
If i swapped in that 418 i would go to a roller and likely Trick flows as well. Block is tubed already for a roller. It gets to “ where do you stop” been there, done that.
My current convertor is a little loose, cam is a couple sizes too big.
If i take this 360 out at the end of the season( will have raced it 4 years at that point) i can swap in an aftermarket flattop zero deck deal. Now i al closer to an actual 11.5/ 12 to 1. deal. Now the cam works way better, and so does the convertor.
I am positive those changes would easily get the car in the 10’s.... plenty enough to tickle me.
I have been fast in the past.... lol. Life changes.
Can sell the 418 and more than pay for the freshen/ upgrade and keep the heads i have( which still could be a good bit better)
Make any sense now?

bTW.... last two season, couple valve lashes and oil change. Zero other engine maintenance needed. Love it that way
 
Last edited:
View attachment 1715488547 View attachment 1715488548 View attachment 1715488549 What’s the matter with 8 3/4’s. My sons been 10.0’s with his at 3200 pounds and transbrake. Should you change out the gears if you race weekly every three years or so. Sure why not.

I went 10.30’s at 3400 few years ago( swapped eddie topend off my flat tappet 416 eddie headed shortblock), went to W5 heads, heads and intake. Zero other changes. Broke the ring and pinion 4th pass footbreaking it.
You start making enough power to run low 10’s in a heavy car, 8.75 rears arent going to last
Opinions vary, that my experience
 
I’d still go with the 4” stroke and a 20-ish cc piston.
Mid-10’s cr, even with a 58cc head.

I like cubes.

And the bobweight is even lighter than a stock stroke combo because of the shorter piston.

The bobweight of my (at the time) tricked out 340 was 1650g.
The trw pistoned 440 was almost double..... close to 3000g.

After the 440 was put into the car, 120lbs of roll bar and floor plates were added to the car, along with how ever much more a 440 weighs than a 340.

The 440 was 1 second and 10mph faster.

I’ll take the cubes over bobweight.
 
Last edited:
current 360 “ might” be 10 to 1. Its even that because i skinned the heads to 58cc and run an 028 gasket. All factory internals.
The 418 is zero deck flattops. Be roughly 12.7 to 1 with 58cc head.
The other factor is me. I know me... lol
If i swapped in that 418 i would go to a roller and likely Trick flows as well. Block is tubed already for a roller. It gets to “ where do you stop” been there, done that.
My current convertor is a little loose, cam is a couple sizes too big.
If i take this 360 out at the end of the season( will have raced it 4 years at that point) i can swap in an aftermarket flattop zero deck deal. Now i al closer to an actual 11.5/ 12 to 1. deal. Now the cam works way better, and so does the convertor.
I am positive those changes would easily get the car in the 10’s.... plenty enough to tickle me.
I have been fast in the past.... lol. Life changes.
Can sell the 418 and more than pay for the freshen/ upgrade and keep the heads i have( which still could be a good bit better)
Make any sense now?

bTW.... last two season, couple valve lashes and oil change. Zero other engine maintenance needed. Love it that way

Don, this is the fourth year for my 434, 11.2-1, .700” solid roller and all I do is change the oil and run the valves in each spring. It has been 6.40 @ 109 on pump 93, 3350 pounds.

I understand about breaking stuff, on the second pass my 408 hooked up it broke a fresh set of 4.30 years in my 8 3/4. I invested in a S-60 then rebuilt and sold the 8 3/4.
 
It’s not the power but the weight that breaks the 8 3/4. Their pretty reliable until you get over 3300# and high 10 sec. et’s. Then you need good caps and bolts. A lot of racers are going to the S60 from Strange. For about what it cost to brace, put axles, spool and all the good stuff in the 8 3/4, you can have a much stronger rear. Just bolt it in and go.
 
Here's a comparison of two very similar sbc engines that I installed in my street/strip manual 4spd car, only significant difference between them was bobweights...
...Engine #1 was 4.04" x 3.48" w/ 5.7" i-beam rods, hypers with gas ported spacers and 1.2mm rings (12lb oil), 49lb crank and heavy 8" balancer, 1863g bobweight.
...Engine #2 is 4.03" x 3.48" w/ 6" aluminum rods, forged pistons with lateral gas ports and 1.5mm rings (14lb oil), 42lb crank with pendulum style counterweights, drilled rod journals, 6" balancer, 1492g bobweight .
Both engines had flat tops with nearly identical quench and compression. Exact same intake and carb, same carb calibration. Exact same cam installed on the exact same intake centerline. Exact same flywheel and pressure plate installed in exactly the same car, same weight, with exactly the same gearing and tires. Even though these tests were a couple weeks shy of 2 years apart, both tests are on the same location with zero tire spin and conditions were very close to the same. The car itself was basically a time capsule between the two tests...I lost engine #1 a few weeks after the test, and i had other irons in the fire so the car sat until engine #2 was ready to install...just picking up where i had left off with regard to developing the car.
Here's the average rates that each engine gained rpm WOT thru the gears according to the RacePak files...
1st gear 2000 to 4000 rpm- engine #1 1634 rpm/sec........engine #2 1910 rpm/sec (276 rpm/sec difference) = 18.8% gain
1st gear 4000 to 6000 rpm- engine #1 1975 rpm/sec........engine #2 2217 rpm/sec (242 rpm/sec difference) = 12.2% gain
2nd gear 4000 to 6000 rpm- engine #1 1070 rpm/sec.......engine #2 1116 rpm/sec (46 rpm/sec difference) = 4.2% gain
3rd gear 4000 to 6000 rpm- engine #1 535 rpm/sec.........engine #2 541 rpm/sec (6 rpm/sec difference) = 1.1% gain
No 4th gear data available for comparison.
The quicker an engine sweeps thru a gear, the more acceleration rate you will gain from lightweight components. These two engines might both make around 500ft/lbs each running steady state NA, making engine #2's 12.2% gain in 1st gear from 4-6k roughly equal to around a 60hp advantage over engine #1.

Sometimes it helps to think about what happens in opposite extremes...
A given engine has a maximum acceleration rate that it can gain rpm without any external load at all...like a neutral free-rev. At that point all it's power is being used to accelerate itself, and no power is left over to do external work. The lighter an engine's rotating assembly, the easier/quicker it is to accelerate. Sweeping thru the heart of it's torque curve, engine #1 in my example above could gain rpm without a load at the average rate of 8500 rpm per second. Engine #2 could gain rpm without a load at the average rate of 11,515 rpm per second.

On the other end of the spectrum as a car accelerates and works it's way thru the gears, it eventually reaches a point where the engine can no longer accelerate the car. At that point all the engine's power is being used to overcome friction/drag, and there is no power left over for acceleration. This is also the point where the weight of the rotating assy no longer has any effect at all on the power output of the engine. All the torque the engine is making is reaching the transmission's input shaft, no power is being absorbed by the rotating assy as inertia. Operating WOT against maximum load, engine #1 and engine #2 both make the same power.

Looking at these two extremes makes it easier to understand how acceleration rate can have such a huge effect on dyno data. The two otherwise identical engines will make about the same torque when operating against maximum load at a constant rpm, but if engine #1 were dynoed at an acceleration rate of 8500 rpm per second, it would make zero torque on the dyno. Engine #2 still has power left over to move the needle.

If you are running wide open in a boat across the ocean, less crankshaft weight will probably hurt more than help you. If you are a dirt track sprint car on the pole during a re-start, less crankshaft weight is going to be a “BFD”! Most of us here will fall somewhere in between.

Grant
 
Are you an engineer my friend. Lol. Lots of info to digest but without exact weather conditions you left out a major factor.
 
No mention of the heads in the above post.
Were the exact same heads used for both examples?
No additional work to the heads between tests?
If so, what was the ET difference?
What are the ET’s when corrected to a std day?

I’m pretty sure my friends Stock eliminator Pontiac 350 has a heavier bobweight than Dons 360.
Along with a much smaller cam(.420 lift), and heads that have smaller intake valves(1.96) and probably flow 60cfm less(200cfm), and saddled with the factory dual plane intake manifold.

It’s been about 1/2 a second quicker and gone 4mph faster(10.80’s@123, @3300lbs).

IMO, there are many many things on the list of go fast goodies that will have a greater impact on the ET than a few ounces of bobweight.

Unfortunately, even if Don undertakes this approach..... we won’t get a real
answer, since other aspects of the engine package will be changed along with the bobweight(compression, ring pack, etc).

In Stock eliminator, the NHRA legal pistons are stock weight, and you can only use approved connecting rods.
So, there are NHRA legal Stocker 360’s running as quick as Dons car(and quicker), with very similar weight rotating assy’s, that don’t have the benefit of ported aluminum heads that are milled to 58cc’s and have 2.02 valves, aftermarket intakes, 950 carbs and .560 lift cams.

Just sayin......
 
Last edited:
Are you an engineer my friend. Lol. Lots of info to digest but without exact weather conditions you left out a major factor.

Just throwing out an observation, if I had weather I would add that as well. That said, there's a reason a 600hp SB is usually quicker than a 600hp BB. Rotating assy weight is going to matter less with a Powerglide that only loses 800rpm on a single gearchange, but it matters quite a bit more with a manual trans car that loses 2000rpm on each of it's 3 gear changes.

Grant
 
Just throwing out an observation, if I had weather I would add that as well. That said, there's a reason a 600hp SB is usually quicker than a 600hp BB. Rotating assy weight is going to matter less with a Powerglide that only loses 800rpm on a single gearchange, but it matters quite a bit more with a manual trans car that loses 2000rpm on each of it's 3 gear changes.

Grant
Auto Racing Weather « Air Density Online
You can get historical data for a track here.
 
Wow guys, thanks for outstanding feedback
Makes it interesting to read and learn this stuff.
You are correct Dwayne. If i just freshen up This 360 and add more compression and less bobweight, other factors all the sudden come into play.
My cam all the sudden becomes a much better fit. And my probably one step too loose convertor likely works better too.
Those above two things are part of my thought process in this most definately. Not just the compression and lighter bobweight.
I have grenaded an engine before going down the track going way faster than this Dart runs. These 40 ish year old stock rods dont instill tons of confidence in me.
Its amazing, older you get, more you think about such stuff. Not sure why.... lol
 
So, the question becomes.......
How much quicker does it need to go to justify the expense of a new rotating assy(that doesn’t include more cubes)?

I feel like you’ve already made your mind up on it...... so how much quicker does it need to go for you to say “mission accomplished”?
 
So, the question becomes.......
How much quicker does it need to go to justify the expense of a new rotating assy(that doesn’t include more cubes)?

I feel like you’ve already made your mind up on it...... so how much quicker does it need to go for you to say “mission accomplished”?

If you had read my earlier post you would have noted i would be happy if it dips into the 10’s. It will.
You seem to be ignoring why i am looking to do this.
The motor( at the end of this season) will have 4 racing seasons on it. I race a lot. I also street drive it.
Its due for a freshen up. It current has stock factory rods and factory cast replacement pistons.
To add nice forged zero deck flattops( raising the compression a couple points) and scat rods is about 1k, plus gaskets, bearings etc, that would be needed at any freshen up.
So i am adding 1000 bucks basically to a standard freshen up, plus a balance job.
I gain better compability with my current cam and convertor, better rods and rod bolts( piece of mind), and a fresh motor. And less bobweight
For the extra grand, its great bang for the buck.
 
1st problem is I don't see my local "Rockport Int'l Raceway" on the list of tracks, 2nd problem is the history function on that page only goes back to Feb 2016. Engine #1 run was 8/18/2014, Engine #2 run was 7/30/16.

Grant
Thought it might help. I'm lucky my track is on the list.
 
Don,
If you’re pulling the motor for a freshen anyway....... it does make sense to get the OE pistons and rods out of it for the way you’re using it.
 
-
Back
Top