[BMFH]

Hello All

Can anyone give me any input on the benefits or problems of using the 6.80 rod in a stock 455 with stock crank and D port iron heads?

I'm working on building one for the street and occasional strip fun.

I know that a longer rod is supposed to help with octane intollerance problems, I am planning on running approx 11.0 Compression ratio on the engine.

Any information will be appriciated.

Thank You all for your inputs, in ADVANCE.

Take Care

Dusty

[tom s]

none unless you want to turn your rod size down to BBC size to save a little money on the rods.tom

[Tom Vaught]

The 6.8" Connecting Rod will require cutting your 455 crankshaft Rod Journals (Stock= 2.250") about .050" in diameter order to use the Chevrolet (Stock= 2.20") rod bearing designed for that 6.8" rod. Not a big deal for a competent crankshaft shop.

Then you will need to buy some custom pistons with the right compression height for your 455 crankshaft/block so that the pistons will work with the rods larger pin (.990") diameter. You will also have to decide what the bore needs to be for your block and order the right rings, etc for the set-up.

Good Luck.

Tom V.

For a stocker 455, you might be better off (less money) with some stock length aftermarket rods for your deal.

[77 TRASHCAN]

Tom V.,
I've read other places about the longer rod being more octane friendly, as the OP'er stated....
With the piston at TDC a fuzz longer, and cyl pressure builder higher during this small amount of time, wouldn't a shorter rod be more octane friendly, building slightly less cyl pressure w/ a shorter rod??? Seems a shorter rod w/ less TDC dwell time could help an octane sensitive engine, especially on the street.

I remember reading about Smokey Yunick talking about one of the benefits of longer rods, was the ability TO build more cyl pressure because of the increased dwell time @ TDC...
Thanks
Jeff

[t money]

As others have just said if you turn down a 455 crank from 2.250 to 2.200 to use chevy 6.8 rods you will need custom Pistons. But are you taking about offset grind your crank the .50 making it the 4.25 stroke and use the 6.8 rods and most pontiac vender have the piston in stock and at a good price???

[chiphead]

My 400 is a long rod deal. It revvs really freely and pulls hard past 5500 with a 230 duration cam and 240cfm heads. I did notice that it had low idle vacuum, mostly due to 9.3 compression. But the long rod makes the piston leave TDC a little slower which hurts idle vacuum too. With the long rod, a little more dwell at TDC helps the heads appear to flow a few more CFM, due to increased scavange during overlap. In theory the smaller rod journal results in a slower bearing speed which allows a bit more RPM. The piston side loading is supposed to be better as well.

For a street/strip deal, I really don't see the point. Stock dimension parts can make plenty power and turn 6200 RPM with forged parts. If this was a road race engine that had to turn high RPM for several laps, then it may be worth it. Same for a drag engine turning 7000+. My issue with the long rod deal, if you ever want to change anything in the rotating assy, you're stuck with custom pistons and a custom ground crank. I wish I could swap a set of 400 shelf pistons with a smaller dish to bump compression, but I'm stuck. This engine ran well but I don't think I'd build another.

11:1 compression with pump gas and iron heads? Yeah, don't do that.

[tom s]

I have used as short as 6.oo and as long as 7.04 rods and for my money I would just use stock length rods,pistons etc.The juice aint worth the squeeze IMHO.Tom

[Skip Fix]

There are some piston speed calculators out there and a 6.625 vs a 6.8 on most of our stuff is minimal. Using a lighter piston(higher pin placement) will probably be offset by a slightly heavier rod.

Cost of "off the shelf" stroker pistons vs a "custom" is also not a big deal and gets you right where your actual deck height is.

Like Tom S said not much gain-unless you already have 6.8 rods.

[Tim Corcoran]

I agree with all that has been said on the rod length deal. I have a 473 ci 455 motor, 4.21 bore X 4.21 stroke and custom J&E pistons. It started out as a low 12 second street machine with mildly ported 6X heads, a mild hydraulic flat tappet cam and 9.5:1 compression. It's now a strip only low 10 second car with a solid roller, E-heads and 12.3:1 compression (race gas). I have been very happy with the combo. I believe the biggest advantage of running a long rod on a street car is it changes the angle of the arm and has less side loading of the piston on the cylinder walls on the thrust side. This may give the bore longer life.

Regardless of which rod you run I don't think you want to run 11.0:1 compression with iron heads and pump gas. I would either run different heads or dish the pistons and get your compression to 9.5:1 for a street car. With the right cam you can get away with a little more maybe 10.0:1 but I wouldn't do it. The best way to make power for a street car is to port the heads and use a small cam and don't push it on compression or you may end up with a disaster.

[GOAT WHORE]

Originally Posted by 77 TRASHCAN

Tom V.,
I've read other places about the longer rod being more octane friendly, as the OP'er stated....
With the piston at TDC a fuzz longer, and cyl pressure builder higher during this small amount of time,
Thanks
Jeff

How would the cyl pressure build just because the piston dwells at tdc longer?

[Schurkey]

Originally Posted by GOAT WHORE

How would the cyl pressure build just because the piston dwells at tdc longer?

IF (big IF) we accept that the piston dwells at TDC longer, that gives additional time for the flame front to burn a larger amount of mixture. More burn = more pressure.

Realistically the difference in dwell time is so minimal as to be a non-issue.

[GOAT WHORE]

Gotcha, for some reason I was thinking compression

[Cliff R]

We've built a good number of these engines in both configurations, and there is NOTHING there in power difference, octane tolerance, or anything else that is measurable on a dyno, shows up at the track, or felt by the "seat of your pants" testing them on the street.

I suppose this happens since the Pontiac rod is plenty long enough for the stroke right to start with, at least for the RPM range the engine will be used in.

Some will argue the smaller bearing is an advantage, but we've never had the first issue one with the Pontiac set-up.

All of my own engines have been Pontiac dimensions, zero issues anyplace when it comes to longevity of the parts, performance, octane tolerance, etc.

The shorter rods probably allow the engine to twist up a little quicker as they are lighter and yank the piston around with a tad more authority.

The key to getting good results with these parts is to use the lightest parts available, and pistons with modern ring packs, etc.

The long rod deal was started WAY back with the introduction of the first "stroker" kits, they were BBC rod journal diameters, slightly longer 4.25" stroke vs 4.21", with 6.8" long rods. Probably done to simplify things and use more readily available and slightly less expensive parts more than anything else.

For your street 455 engine build I would use Pontiac parts if using a Pontiac crank rather than trying to grind it down to BBC size and/or increase the stroke slightly. If you have purchased a crank already set up for BBC rods and 4.25" stroke, absolutely nothing wrong with that deal, but from what I've seen here it brings nothing to the table in terms of improved performance other than what would be found from the slightly increased CID.....FWIW......Cliff

[77 TRASHCAN]

Originally Posted by Schurkey

IF (big IF) we accept that the piston dwells at TDC longer, that gives additional time for the flame front to burn a larger amount of mixture. More burn = more pressure.

Realistically the difference in dwell time is so minimal as to be a non-issue.

Smokey Yunick always advised using the longest rod possible, in anything he built. The cyl wall loading was one reason, and building more cyl pressure, due to increased dwell time @ TDC.

Another thing he did was off set the wrist pins .060" toward the non thrust side. Doing this allows the con. rod to be on it's downward motion when the piston is @ TDC.
The O.E. folks do the opposite to make the pistons quieter. I asked my engine builder friend once about off setting the wrist pins for power (on the street), his comment was, "You won't like it, the noise"....

Keeping ALL this in mind, Smokey built whoop ass endurance engines, seldom anything for the street. Engines that got thoroughly tested in races, a whole different world than winging it down the quarter mile somewhere between 10-15 seconds, depending on your combo...

Cliff's explanation says it all.

When Jon Kaase built his engine masters Pontiac (507 or 508 cubes?) He first selected a bore size that had a great selection of piston ring types, then the rod length, not going for any "long" rod deal. All his decisions on this build pertained to the RPM's, fuel, and hopeful power outpu the engine might make. It's a great read, it's floating aruond the internet...somewhere...

[tom s]

Daren Morgan has always said the rod hooks the piston to the crank.Pick the piston with the best ring pack for your app and what ever rod hooks them together is what you need.Tom

[Lee]

Longer rods are MORE prone to detonation. Look at the Engine Masters motors. I've seen some of the builders state that they used SHORTER rods to minimize detonation (such as when using 11.5:1 with 93 octane).

If octane is not a limiting factor, then I'd use a longer rod. Use the shorter rod if you are approaching the limit of the octane you are using.

[tpssonic]

Originally Posted by Lee

Longer rods are MORE prone to detonation. Look at the Engine Masters motors. I've seen some of the builders state that they used SHORTER rods to minimize detonation (such as when using 11.5:1 with 93 octane).

If octane is not a limiting factor, then I'd use a longer rod. Use the shorter rod if you are approaching the limit of the octane you are using.

I agree with Lee. If the piston spends more time at TDC (by using a longer rod), then it will have more time to produce/pack free radicals into the combustion chamber and allow more time for deto to occur. You can sometimes stave off deto running the same cylinder pressures at higher rpm as the cycles are shorter in terms of time. Call me crazy, but I think that running wider lobe seps and longer duration cams help keep deto at bay due to 1) lower dynamic compression, filling the cylinders less efficiently at lower rpms (due to inefficient filling as a result of increased overlap due to longer duration and intake closing events), AND by running at a higher rpm when making peak torque, as it gives deto less of a chance to develop, even though you more than likely are filling the combustion chamber more efficiently (and making more power if the rest of the system -carb/heads/exhaust, etc. is up to the task). JMHO

Tony