What are the pro's and con's of using a 5.7" rod length vs 6.0" in a 383 stroker? What is the actual affect on performance?

 

the longer the rod, the less rod angle there is- easier on the piston skirt

 

Any 383's we have built have 6 inch rods in them as we are able to internaly balance the crank liter pistons and less friction and more dwell time at TDC

 

I am going to order parts within the next two weeks. Thanks for the input but my question still is what performance difference does these rod lengths make? ie: torque, hp, rpm, etc.

 

We build a lot of circle track engines that are suppose run 5.7 rods but we have done some with 6 inch rods and the same componets we see that the 6 inch rods has a flatter torque curve and seem to carry the horse power longer compared to the 5.7 roded engines.

We have built some 358's with 6.250 rods and those engine really carry the HP to a higher RPM and do require differant cams on these engines.

 

if any and i mean if any probally less than 5hp at peak rpm but is easier on other internal components

i allways say put in the longest rod you can afford that being said in a 383 base street engine you will see no power gain and a little more extra wear on the piston skirts and bores with the 5.7 to the 6 inch rods

 

Sounds like BLOCKMAN is recommending the longest ones you can fit for some added power?

I have also been told the added dwell time he mentions helps burn the mix better?

Some long rods make for the pins to go into the oil ring area! Not a problem generally.

Harry P.Hunter

 

From what we have seen on the dyno the torque curve is flater and the horse power is better and it seems to carry longer another words it doesn't fall off as fast like the 5.7 or shorter rod engines.

And with the longer rod and more dwell time it requires less timing as well.

 

We have built 383's with SRP and wiseco and Diamond pistons and 6 inch rods which which require a spacer ring but if we also use the Maule pistons which use a metric ring package and no spacer ring is required.

 

The test I want is to see is this.
Two engines"5.7 & 6.0" built as identical as possible w/very high DCR that can't take full adv.
Then test to see which engine can take most ign adv and make most power. This is where I think the extra hp/tq will be made.
I have TFS heads, 6" rod, FT pistons w/2 valve reliefs,.040 quench,
8.3 DCR.
Engine runs best on 32*. Have never had any issues w/timing
280 rwhp 315 rwtq w/stk rams horns.

 

My 427 SB with 3.85" stroke has 6" rods. Std height block, not a raised cam block. I believe the DCR is about 8.6.

The wrist pin is in the oil groove, no big deal.

Heads are Dart Pro 1 CNC and the JE pistons are dished to give me about a 10.8:1 static CR. Thus, there isn't a lot of quench area with the ~1/8" deep dish in the piston top.

now regarding timing, engine analyzer suggests the following at WOT

1500 rpm 15 degrees
2200 19
2900 21
3600 22
4300 24
5000 29
5700 35

Does that seem about right? I don't want to get into a detonation issue, and the motor runs fine with that initial timing curve on 91 octane, but I don't want to leave any power on the table, either.

Motor cruises fine with 48 degrees total advance at a 40% MAP.

I can easily change the timing with the MEFI box.

Thanks,
Doug

 

Jon Kasse already did this test when he engineered and built his Engine Masters winning 385 Series BBF a few years ago.... The short rod won out. The reason's he gave make good sense and coincide with what I have learned in engine building. I'm not going to repete something you can research for yourself.

IMO high velocity piston short rod motors have their advantages and disadvantages just as the long rod motors do. In my experience it takes very different cyl heads, and camshaft combinations to take full advantage of each.

However I cannot stress how different the competing theories are in this area of engine building. Because in addition to those (2) there are others who think it makes absolutely no difference, ie. "the rods simply connect the pistons to the crankshaft and nothing more theroy".
Will

 

Engine Analyzer is a great program and I use it.

However it cannot predict the minute differences that either enhance or degrade Combustion Chamber Effeciency (actual combustion chamber configuration, fuel burn rate, plug gap and heat range, quench, piston desgin, intake port swirl, fuel atomization, ect, ect) which is one of the biggest factors that determine how much timing you can actually run in the real world.

For example just change your value for the combustion chamber from "Wedge" to "Compact Wedge". You will see your projected timing values/spark advance go down (ie less required timming), less spark knock counts and your TQ/HP increase.

The values it uses in this area are very rough guesstimates.
Will

 

Thanks for the comments! What I have concluded with the input is that the difference between the two is minimal, unless you are constantly pushing the engine in the peak power range. The reduced wear by using the 6.00" is not a major concern because of the friction reducing alloys and suface coatings available now. And this will not be a high mileage engine. Any additional input will be appreciated.

 

I have a chassis dyno appointment set up Thursday, that will get the timing sorted out. Plus fuel pulse widths, as right now everything is set by theory, which runs fine, but no doubt could be better.

Thanks,
Doug

 

with Corkvette. I remember reading years ago in a magazine article, Joe Sherman built a 383 SBC with stock 400 rods and then with 5.7. The shorter rod made like 3-5 more HP until around 55-6000 and then the 5.7 took over by about the same amount. The increased wear makes the 6" the better choise IMHO.

 

Have read everthing I can find on this. Difference seems to be minimal for dual purpose car. Further complicated when you add forced induction into equation?

 

Also note the actual spark advance numbers underneath the chart.

This is thru 1 5/8" primary tube Hedman headers on a chassis dyno, mufflers and full length 2.5" exhaust, the small headers are choking the motor over 5K RPM, but good for lower speed torque, and for a cruiser/stop light bandit, which is the usage is, probably fine.



200 ft-lb of torque at idle, BTW.

Note the anomoly that starts about 2200 RPM, where power/torque suddenly drops, then takes a while to recover.

This was noticed on the 327 old motor also, though on the 327 motor, it started about 400 RPM higher, there is obviously something either in the intake, or exhaust that is causing a reversion, or something, in that RPM range. I would suspect exhaust, as higher cubes = more air flow, thus a lower RPM at which it started.

The final WOT spark curve was

800 RPM 16 degrees
1200 19
2000 21
2400 22.5
2600 23.2

everything above 2600 was 23.2.

More advanced timing lost torque, dunno if it was from 91 octane gas and incipient detonation, or of the Dart Pro 1 heads are fast enough burning that any more timing isn't required.

Engine Analyzer suggested up to 36 degrees adavhnce over 4000 RPM. Maybe the exhaust restriction had something to do with it, or maybe EA itself was just making wild guestimates.

The EA Torque curve was pretty close to actual, factoring in EA gives you flywheel HP, and I was measuring HP at the ground, with a 15% or so drivetrain loss from theory.

Only bummer was I didn't have too much in the way of brakes after the dyno pulls, the 25 year old rubber hose at the rear was too old and to close to the exhaust pipe and started leaking. At least with the dual MC, I was able to limp 4 miles home with minimal brakes.

Doug