wr1970

Oldsmobile Dave made a suggestion. Here goes i wonder why back in the day why a 7 inch rod wasn't installed in a 455 engine From the factory! I know it wouldn't be a fuel problem if they did because of compression . Does anyone know of any used say in the early 1969/72 of such a thing being done for a race combo?Can anyone post evidence of such work being done back then?

wr1970

The factory was never going to change bore and stroke which was my original thought that they could have..Cost involved restraints. So i did a edit and changed it back to what Oldsmobiledave and i discussed as a suggested thread.Maybe Eric will change thread title for me i went advanced but it didn't work.

captjim

OK, Neely, once again I have NO idea what you are saying. Stock rod is 6.735", right? So what is the point of a 7" rod. The rod length changes neither the bore, stroke, or displacement. If you change the rod length then you must chance the compression height of the piston to equal the deck height.

wr1970

My thinking was originally the factory could have used a 455 block with a different bore stroke and a 7 inch rod for a bigger ci motor.That thinking was flawed because never would have happened budget constraints. The second ideal was what Oldsmobiledave and i discussed which would have been a zero deck motor. I am using this in one of my motors 425 7 " rod and a 455 n crank of course this would have to have a different piston to work JMO. Sorry for confusion. I hope this is more clear.

captjim

No, it does not make it more clear. In order to add cubic inches, you need to have either a larger bore, longer stroke, or both. If you add some stroke as you suggested, you would need a SHORTER rod, not a longer one, (or a smaller journal, which would mean a completely different rod), or, a different piston. The 455 is already under square, with a 4.125" bore and a 4.250 stroke. You are talking about completely redesigning the engine.

wr1970

Jim you are reading the title i tried to delete.See the edit.This site didn't respond to my delete. I changed the thread to read 7 inch rod motor. Not for cubic in no bore change. The question at hand is this why was a 7' rod was not used when it very well could have. There is a advantage to doing this.

captjim

What would that advantage be, in your opinion?

oldcutlass

Is that the title you were looking for?

captjim

It might be, but it is physically impossible without changing the stroke. 455 rod length is 6.735" and pin height is 1.735" If you use a 7" rod, that would be .265 longer than stock. Since the p/h needs a minimum of 1.00", that still puts the piston out of the bore by .130 +/-

THey were able to use a 7" rod in the 425 with the same journal size because it had waaaay less stroke (3.975 vs 4.250) and a 1.615 p/h

wr1970

Yes Eric many thanks.

wr1970

Jim to use a 7" rod on a 455 stock crank no stroke change all that would be required is a different piston which i did mention. This will give you a zero deck. I do think this could have been done it just wasn't back then.

captjim

You are right, if you add .265 to the rod, you need to subtract that from the piston, making the p/h 1.500. But again, what is the actual benefit? Same bore, same stroke, same cubes, lighter piston but heavier rod, I see no gain??

cutlassefi

Neeley
I've done a couple 455 builds with the longer rod and consequently a shorter piston. It doesn't give you a "zero" deck automatically unless you were to measure the deck and order a custom piston accordingly, which you could also do with a stock rod 455 as well.
The longer Rod to stroke ratio raises the tq and hp peak, all else being equal. The only upsides are better wall stability due to less angularity as well as being a band aid for crappy heads. A longer rod normally enhances breathing due to the longer dwell time at the top. But most Olds aren't spun high enough to make good use of that.
My guess is Olds wanted more torque, earlier in the rpm range with the 455. The shorter rod would have helped that, along with keeping the pin heights from getting ridiculous.

wr1970

Less side load for one.

Octania

Aye, if the rod were infinitely long then it would be at all times parallel to the bore, for all practical purposes. Practical and infinite, yeah, that's the ticket.

I once composed an Excel sheet into which one could enter various stroke, rod length, whatever numbers and it calculated the position of each point of interest. From positions and time [time from RPM] we get velocity of the point. From velocities and time we get acceleration, and from F=ma one could then calculate Forces if the mass were also thrown in. I stopped at Acc. Plotted vs. crank position.

Rod length even with in practical ranges does indeed matter. And the accelerations do strange things at BDC/TDC...

wr1970

Mark back then they could have had a piston that would have worked. We now just order a custom piston. The other thing i would point out is higher compression by going zero deck. I don't know if i would call C or a E crappy heads.If you think you can do better with a 455 stock motor and then compare it to a long rod motor at zero deck. I will disagree.Same bore same stroke all things being equal.JMO

cutlassefi

What's this Zero deck thing?
Both combinations had "zero" deck and very very similar specs. The build with the longer rod made comparable power but at a peak that was about 200-300 rpm higher than the shorter rod combination, and it took less timing. Plain and simple.

wr1970

The stock 455 never came out factory Zero deck! If you don't understand this i can't explain it. When i said all things being equal that didn't mean the stock 455 would get a different piston to bring it to zero deck. I was saying 7 in rod different piston allows the same cubic engines to have this example to not be down in the hole and the other would be. The factory 455 piston is down in the hole which is a lower compression.

captjim

Are you forgetting about piston to head clearance? The Olds engines had the piston down .020 +/- and a shim gasket, equaling .040 +/- piston to head. Now, if you zero deck you have to use a thicker head gasket to get your clearance. You can't run zero deck and a shim, once parts start wearing you are bound to smack the head or a valve. IMHO

wr1970

Don't tell me what i can and can not do Captjim.I have a engine that has the 455 block,425 7" rod,stock 455 stroke crank at zero deck.Go do your spin doctor crap somewhere else.

captjim

What head gasket are you using? Are you using a shim?

wr1970

Captjim your drivel and thread derailing is getting old.The other thread went three pages with your crap.I don't care what you think plain and simple.

captjim

You aren't going to answer because you are running the piston at zero deck and an aftermarket .040 head gasket, which actually LOWERS the compression ratio over an engine with the piston down .020 and a .020 shim. You can mock me and call me names, but it does not make you correct.

Back to the original question, ask yourself this; Oldsmobile was already building a 7" rod, and could have easily used it along with a 455 piston with a 1.500 p/h. But, they chose instead to manufacture the engine with the 1.765 p/h and 6.735 rod. Why? I don't know, but I know they are smarter than we are and went to the trouble of building it that way.

cutlassefi

How about this, please tell us the difference between a block, any block with the pistons .029 in the hole and using a .011 shim gasket vs having a "zero deck" and using a .040 compressed gasket? All else being equal.

Thanks.

wr1970

Mark how about this the piston is at the top of the deck it is a aftermarket piston.Now am i or am i not at a higher compression than a stock 455. What i was getting at they could have done this years ago with a 7" rod in my opinion is a advantage.That is all i was saying.You can answer this why beat around the bush with your question to me.If you say no then i say this is the best answer.

captjim

No, the compression ratio is NOT higher. You still need to maintain a minimum of .035 piston to head. You can do that in any combination of gasket thickness and deck. If you zero deck you actually LOWER the compression ratio since the gasket bore is larger than the cylinder bore. There is NO way the factory is going to mass produce an engine with a piston-to-head clearance of .015 just to gain a tiny bump in compression ratio.

wr1970

Well i guess i don't understand ! With no dish in the piston i thought at zero deck it must be at a higher compression.Maybe my thinking is wrong.

captjim

That would be correct. I have explained it twice in this thread. Perhaps someone else can try and do it more clearly.

cutlassefi

You told me you could take constructive criticism, so here goes.
You're at the very least, confused.
There aren't any engines built with 0 deck, not even today with much better machining practices. Years ago the machining was even worse so they built in tolerances. You won't find any older engine with any where near a 0 deck for that reason, no matter what piston and rod are in it.

So to answer your question, I think, whether you achieve the 0 deck with a taller piston, rod or whatever, it'll be no different than having a piston in the hole and using a thinner gasket plain and simple.

With all due respect you need to be done here Neeley, Jim was just trying to get to the crux of your question. You didn't make that very easy, sorry.

wr1970

Accepted.

80 Rocket

100% spot on

To expound a tad bit further, there is no need for the factory to go zero deck (even though with stack up of tolerances it would be almost impossible in an assembly line setting) and the length of the rod plays a very VERY minor role in the output of the engine (sometimes none).

The way I look at the length of a rod is that you have the 1) engine stroke and 2) the ring pack placement......which will determine the rod length, that's it. As mentioned, the length of a rod cannot change stroke, period.

As far as I know, there is nobody that builds an engine based on rod length. Maybe there is an application where you can tailor the rod length to your liking for certain boosted applications, but it is not the main focus of the build.

wr1970

I am thanking Captjim for trying to explain this to me.Thanks to Mark cutlassefi which cleared it up. Also to 80 rocket. My thinking was wrong about more compression. The improved rod angle is much better still a added benefit. Does it out weigh leaving the engine in stock form i don't know that it matters. Again thanks to all.

madmax442

I'm going to go with my own theory that 455 cubes was enough in Oldsmobiles eyes to haul anything they had around with ease! Cadillac already had 472 and then 500 cubes. I just don't think Olds cared if their stroke was going to get bigger. I think they would have introduced a new block if they wanted more cubes but that's just me.

Warhead

Gotta stick my nosey *** in this, not stirring shyt, but maybe I am.
2 guys who built around rod length:
Smokey Yunick
Bill Jenkins

I like what Jenkins had to say on the subject, Smokey was using his theory with a boosted application, and use the longest rods you can afford. Jenkins was applying it to naturally aspirated SBC/BBC drag racing engines, but was very specific about the relationship between rod length, lobe seperation, and port volume.

Reading their books should be required.
JMO

80 Rocket

I have read Smokeys power secrets book and also his auto-biography.

I can't say I've read any of Grumpys stuff.

Smokey says use the longest rod you can with offset pins in the pistons. The rod length is never long enough according to him.

I mentioned that rod length is determined by 1) stroke and 2) ring pack placement. If I were getting custom pistons made I would place the ring pack in the appropriate position for the application I am building for (ie: N/A, boosted) and then also consider tailoring it for the longest available aftermarket rod available.

I don't exactly split atoms when it comes to rod length in an Oldsmobile because I am not racing a heads up class where the competition is high and a few HP could be the difference between winning and losing. I am also not going to spring for a custom made rod so I can have the longest possible rod. I will look at the available aftermarket ones and go from there.

The OP seemed to start the thread thinking Oldsmobile left a whole bunch on the table in reference to rod length of 7" vs. 6.735". My contention is that it is negligible in the stock application.

Your condescension precedes you.

Humbly,
'ol 80 Rocket

captjim

Not only that, they already had the 425 7" rod and went to the trouble of making the 6.735" rod. I am going to give the engineers the benefit that they knew what they were doing and that the 7" rod was not the best for the 455.

Warhead

Smokey used his TC small block Indy program to cement his opinion on the long rod, and what he says makes sense. But when you read Jenkins book, I feel he gives a clearer picture of the "why" these things are going on. Especially how the short rod has a more effective and immediate pull on an intake port even if it is an oversize port, in an engine designed for acceleration. If they would have worked this train of thought on the SBO test program with the BBO heads on it, (I.E. used a shorter rod with that program) we may have seen an entirely different outcome. You can not dispute the results Ford (omg!) has had using very large heads with the 302 Windsor, and 302 Boss engines. Nevertheless, I agree that the 455 could have just as easily been built with the 7 inch rod, a crank and piston change was necessary. Rods not needed to be changed, they obviously felt it would do what they designed it to do better with the shorter rod.

Wasn't being condescending, just posting to a comment. Do not take it as anything more than that.
Jim

80 Rocket

You seemed to allude that people should have read certain books before they comment on a certain topic. My apologies if I took that the wrong way.

I do realize different rod lengths will have an effect on an engine......my point is, how much hp difference will you see on a stock 455 Oldsmobile? Or a stock SBO as you mentioned?

If we are going to talk rod length, we should be talking cylinder head port volumes and flow as well. On a limited induction engine, I would be using the longest possible rod.

I'm not really prepared to get into a debate about rod length because I really don't have much data (ie: none). I suppose it might be something to give some thought to on my next engine build. However, I really don't figure it to be wise going back and forth (hell, maybe we would be in agreement) talking about other peoples theories on rod length.

bobus8

I built an olds 455 about two years ago and I didn't worry about rod length so much as a concern about the stretch of the big end of the rod so I just bought standard length H beam rods and flat top SRP pistons. Problem solved. After a deck shave I had .012 deck clearance and used cometic .056 gaskets to give 10.7 CR with RPM heads. Engine runs great. No spark knock with fairly radical cam.

fleming442

Maybe piston casting/alloy technology wasn't up to speed to support the 425 rod carryover. My CP's have a 1.25" CH. I'm sure there was a reason, and only the Olds engineers know what it was. Half of them probably aren't with us anymore.