Performance improvement from reducing rod journal size

[BradH]

I know it's common to reduce the rod journal diameter as a mod for improving engine performance. What I haven't come across is any documented (published) testing that quantified the improvement for any particular build. So, I'm curious what people here can -- or are willing to -- discuss from their own experiences.

An interview I read years ago with a circle track engine builder mentioned only in passing that whatever gains he saw in his testing of reduced journal size weren't significant until 8K+, but nothing in terms of the actual improvements was included. Thanks - Brad

[Walter R. Malik]

I know it's common to reduce the rod journal diameter as a mod for improving engine performance. What I haven't come across is any documented (published) testing that quantified the improvement for any particular build. So, I'm curious what people here can -- or are willing to -- discuss from their own experiences.

An interview I read years ago with a circle track engine builder mentioned only in passing that whatever gains he saw in his testing of reduced journal size weren't significant until 8K+, but nothing in terms of the actual improvements was included. Thanks - Brad

From what I can tell, the biggest improvement for power is that there will be less oil "throw off" from a smaller journal so, the amount of improvement will depend upon how efficient the oil control in the pan was working in the first place.

[rebelrouser]

I was under the impression that reducing bearing journal size was to reduce bearing speed, and friction, so the improvements were mainly in longevity and the ability to run higher RPM's. On my Chrysler engine reduction of bearing journal size is mainly to make more room so you can get in a bigger stroke. Plus big block Chevy rods are cheaper to buy, than a Mopar rod dimension.

[1972ho]

It’s one of those monkeys see monkeys do.

[Stan Weiss]

Let say you are talking about a V-8 and 4 rod journals. If you remove .1 pound (45.36 grams) from each journal that is a total of .4 pounds. Next we next need to look at the stroke. Let use 4". We can look at different RPM but must also decide on the engines acceleration rate. I am going to use 3000 RPM per second.

Stan

Code: Select all

__Weight_(or_Req._Inertia)_=_0.4#____Radius_=_2.0_Inches____Rear_Gear_Ratio_=_1.0

____________________________Rear_____Aero_____Rolling____________Rear_W__Accele___Time___Rate
__RPM______MPH___Velocity___Wheel___dynamic___Resist.__Elapsed____Horse__ration__Differ_Rev_Per
__________________ft/sec___Torque__Drag_-_HP____HP______Time______Power__in_G's__ential___Sec

____0.0____.000_____.000____0.000______.000____.000____.0000_______0.00__0.0000__0.0000_____0.0
__500.0___5.950____8.727____0.108______.000____.000____.1667______0.010__1.6274__0.1667__3000.0
_1000.0__11.900___17.453____0.108______.000____.000____.3333______0.021__1.6274__0.1667__3000.0
_1500.0__17.850___26.180____0.108______.000____.000____.5000______0.031__1.6274__0.1667__3000.0
_2000.0__23.800___34.907____0.108______.000____.000____.6667______0.041__1.6274__0.1667__3000.0
_2500.0__29.750___43.633____0.108______.000____.000____.8333______0.052__1.6274__0.1667__3000.0
_3000.0__35.700___52.360____0.108______.000____.000___1.0000______0.062__1.6274__0.1667__3000.0
_3500.0__41.650___61.087____0.108______.000____.000___1.1667______0.072__1.6274__0.1667__3000.0
_4000.0__47.600___69.813____0.108______.000____.000___1.3333______0.083__1.6274__0.1667__3000.0
_4500.0__53.550___78.540____0.108______.000____.000___1.5000______0.093__1.6274__0.1667__3000.0
_5000.0__59.500___87.266____0.108______.000____.000___1.6667______0.103__1.6274__0.1667__3000.0
_5500.0__65.450___95.993____0.108______.000____.000___1.8333______0.114__1.6274__0.1667__3000.0
_6000.0__71.400__104.720____0.108______.000____.000___2.0000______0.124__1.6274__0.1667__3000.0
_6500.0__77.350__113.446____0.108______.000____.000___2.1667______0.134__1.6274__0.1667__3000.0
_7000.0__83.300__122.173____0.108______.000____.000___2.3333______0.145__1.6274__0.1667__3000.0
_7500.0__89.250__130.900____0.108______.000____.000___2.5000______0.155__1.6274__0.1667__3000.0
_8000.0__95.200__139.626____0.108______.000____.000___2.6667______0.165__1.6274__0.1667__3000.0
_8500.0_101.150__148.353____0.108______.000____.000___2.8333______0.176__1.6274__0.1667__3000.0
_9000.0_107.100__157.080____0.108______.000____.000___3.0000______0.186__1.6274__0.1667__3000.0
_9500.0_113.050__165.806____0.108______.000____.000___3.1667______0.196__1.6274__0.1667__3000.0
10000.0_119.000__174.533____0.108______.000____.000___3.3333______0.207__1.6274__0.1667__3000.0
10500.0_124.950__183.260____0.108______.000____.000___3.5000______0.217__1.6274__0.1667__3000.0
11000.0_130.900__191.986____0.108______.000____.000___3.6667______0.227__1.6274__0.1667__3000.0
11500.0_136.850__200.713____0.108______.000____.000___3.8333______0.238__1.6274__0.1667__3000.0
12000.0_142.800__209.440____0.108______.000____.000___4.0000______0.248__1.6274__0.1667__3000.0

_Averages____________________.108__________________________________.129__________0.1667_3000.0

[ClassAct]

It reduces bearing speed and it reduces the Bob weight quite a bit.

If you consider it, the big end of the rod can be smaller, therefore lighter. The bearing is now smaller, and lighter. The rod pin is now smaller...and lighter.

That means rather than Swiss cheese the counterweights you can reduce the OD to get the mass off of it and maybe drill a couple of small holes.

Bearing speed and lighter components are the biggest reasons to do it. I do it on everything. I don’t run Chrysler bearing sizes unless I can’t get them to change the rods.

[dannobee]

We tested this way back when, before piston guided rods and even smaller rod journals became common. SBC, small journal crank was worth 6 hp over large journal. Honda size rod bearings were worth an additional 4-5 hp.

[RDY4WAR]

We tested this way back when, before piston guided rods and even smaller rod journals became common. SBC, small journal crank was worth 6 hp over large journal. Honda size rod bearings were worth an additional 4-5 hp.

What was the build? Are we talking 6 hp on a 700+hp NA small block turning 8000+ rpm or your typical 450 hp 383ci? Was the power found only at the very top of the rpm range or were improvements seen everywhere? Do you remember other variables to the testing such as bearing clearance, ring seal, oil viscosity and temperature, windage control, etc...

[Stan Weiss]

It reduces bearing speed and it reduces the Bob weight quite a bit.

If you consider it, the big end of the rod can be smaller, therefore lighter. The bearing is now smaller, and lighter. The rod pin is now smaller...and lighter.

That means rather than Swiss cheese the counterweights you can reduce the OD to get the mass off of it and maybe drill a couple of small holes.

Bearing speed and lighter components are the biggest reasons to do it. I do it on everything. I don’t run Chrysler bearing sizes unless I can’t get them to change the rods.

Do you happen to remember how many grams per journal?

Stan

[RDY4WAR]

2.100" journal
- Circumference = 6.597"
- Square Area = 3.464"^2

1.889" journal
- Circumference = 5.938" (-9.9%)
- Square Area = 2.806"^2 (-19.0%)

I imagine near 20% of the journal's mass is a pretty significant weight reduction (relatively speaking). It may be more when accounting for oil passages.

[dannobee]

What was the build? Are we talking 6 hp on a 700+hp NA small block turning 8000+ rpm or your typical 450 hp 383ci? Was the power found only at the very top of the rpm range or were improvements seen everywhere? Do you remember other variables to the testing such as bearing clearance, ring seal, oil viscosity and temperature, windage control, etc...

Sorry, nascar 358" dry sump small block, 8000 rpm circa late '90's. The other variables were held as close as possible. The test was done to quantify the big vs small journal difference. Pistons were the same, "only" cranks and rods were changed. Cranks were balanced to reflect using the same piston/ring package. Don't remember the difference at lower speeds as we were obviously focused on the high rpm performance.

[BradH]

Interesting stuff, although I have to admit I'm not exactly sure what Stan's spreadsheet is telling me.

In my case, it's the new / next build for my Mopar. Stock rod journal size is 2.375 and, as mentioned above, it's pretty common to switch to BB Chevy rods with 2.20" journals. However, most people do this as part of changing to a longer-stroke crank, whereas for me it would be keeping the existing stroke and swapping rods to go with having the journal size reduced.

My junk can go about as fast as the car will ever be legal for with the standard 3.75" stroke; cost-benefit is probably poor for the expense of having the new unbalanced crank cut down and getting matching rods, etc. It's a 451 ci engine turning maybe 7200 max, so it ain't exactly leading edge technology here.

[nhrastocker]

What was the build? Are we talking 6 hp on a 700+hp NA small block turning 8000+ rpm or your typical 450 hp 383ci? Was the power found only at the very top of the rpm range or were improvements seen everywhere? Do you remember other variables to the testing such as bearing clearance, ring seal, oil viscosity and temperature, windage control, etc...

Sorry, nascar 358" dry sump small block, 8000 rpm circa late '90's. The other variables were held as close as possible. The test was done to quantify the big vs small journal difference. Pistons were the same, "only" cranks and rods were changed. Cranks were balanced to reflect using the same piston/ring package. Don't remember the difference at lower speeds as we were obviously focused on the high rpm performance.

It is the same story I heard from a friend of mine that managed several NASCAR engine shops. He even showed me some connecting rods with a 1.77 rod journal size that were were used in many of their engines.

[SchmidtMotorWorks]

Oil temperature reduction.

[engineguyBill]

The amount of increase due to small journal crank pins depends on many variables in the engine combination/design. The torque/horsepower attributed to small diameter rod journals does not just show up via magic. The small rod journal has a significantly smaller surface area, which will reduce internal friction. Reduction in friction results in more power, but there are several factors and they each contribute to a very minimal friction reduction (i.e. bearing width/length; bearing diameter; piston ring thickness; piston ring radial tension; etc.; etc.