What changed 4.5" stroke?
 

I've seen a number 4.5" stroke stock block combos lately. In addition, one 4.65" stroke stock block combo.

Not long ago this was -

• Keept it under 5500 rpm
• Side loading is exponential with rpm
• Piston skirt exposure
• Etc, etc.

What changed?

Well, it sure seems common sense changed.
Not worth the risk.

Stronger rods so the big end is not as wide and rods with a different center to center to ease the loading of the cylinder walls.

And I would guess that the 4.65” stroke can only safely be used in a stock block if it’s hard blocked.

But in general I would not stroke a stock block to 4.500” even with 3” mains.

Nothing has changed. It takes a lot of work to make the longer strokes make competitive power in a traditional Pontiac engine. Seldom would I use a 4.5 arm in a factory block build. Very narrow spectrum that one will shine when used in a stock block. Just because folks build them, it doesn't mean they make good power.

I have always wanted to take a mellow approach to a 4.5 build, ... Stock 3in main block, out of the box 85cc E"heads adjusted to 10-1 compression, Obviously great rods and pistons, Crower 60919 cam with 1.65 rockers, decent intake and exhaust tracts, stock stall converter with a 2.56 gear out back and drive the pants off it....... Seems like the 4.5 arm is less suited for high RPM and longevity this case would dimmish both inherent shortcomings and still be enjoyable to operate... .. I already have the crank and block.

I like your thinking. You still have an issue with lack of support for the piston skirt.

Totally understand that Paul.. That's why i would use modest head flow with a modest cam and a rear gear to keep things low RPM so as not to exacerbate that issue.. Question tho.. Is the aftermarket blocks lower cylinder wall placement any closer to crank centerline for more piston to cylinder engagement bottom stroke?

Be nice if they extended the lower cylinder in the shape of the typical piston skirt to avoid crank weights and give more skirt engagement.. Then the pin section / cylinder would perhaps be next issue

4.5 may be a short fuse but these things are toys now and were never meant to be extensively modified and live forever

I found these tidbits years ago that might provide some interest...

First, increasing the stroke greatly changes the angular relationship of the rod and cylinder. The rod is at a much sharper angle in relationship to the angle of the cylinder, generating enormous amounts of friction between the piston rings and cylinder walls, causing both friction and ring sealing problems. At lower RPM ranges this friction is easily overcome, but as RPMs increase, the friction often reaches intolerable levels. Most strokers have horrible fuel economy, and it's not just because of the extra performance equipment.

The second factor is directly related to the first. The increased rod.cylinder angularity also increases the piston velocity. That is, the piston will move up or down within the cylinder bore at a far greater rate of speed than a shorter stroke engine. Also, the piston "dwell time", that is the amount of time that the piston remains at TDC, is decreased. These factors have the effect of making the engine extremely sensitive to detonation, as well as making the engine sensitive to under-flowing heads. To illustrate, grab a bicycle tire pump sometime, and first try to operate it as quickly as you can. Then, slow down the pumping action. You will notice that it is far easier to move the pump handle slowly. Your engine is no different- slow down the piston, and the engine has less work to do. Again this condition is aggravated by high RPM conditions. The important thing to remember is that no matter how fast or slow the piston goes, the engine moves exactly the same amount of gasses during the same number of crank degrees. This situation could be improved by using the longest rods possible, which would increase the rod ratio and somewhat alleviate the angularity problem.

Here's an interesting one from Car Craft.....

"Think about rpm for a minute. That's revolutions per minute, referring to an engine's crankshaft speed. At 6,000 rpm, that's 100 revolutions per second, or one revolution every hundredth of a second. If you've got a 4.250-inch stroke, each piston is traveling 70,8333 feet per second at 6,000 rpm, which is about 48 mph, and it has to change direction twice every hundredth of a second. And you wonder why stuff breaks."

With a 4.500-inch stroke a piston will be traveling 75 feet per second at 6000 rpm.


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Wouldn't a 4.5" stroke 6.8" rod 1.2" compression height piston have the piston in about the same place ABDC as a 4.25" stroke 6.625" rod combination?

Stan

Steve,
Yes, increasing the stroke does increase rod angle. That is why if possible you should also increase rod length.

Stan

even with the 4.5 stroke, wouldn't that 2.56 gear make it somewhat anemic out of the whole?

The longer stroke is hard on everything. It's worse with a production block. 6.8 rod is a bunch better than a 6.625 rod as far as stressing the block.... I think you already know this.

We've made 850 with a factory block and 4.5 stroke. We made similar power with a 4.21, the later looked better everywhere when we freshened them. Other than a few applications I'm not seeing an upside to the longer arm other than bragging rights.


I could be wrong, I'm a simple head porter that got drafted into building engines I'd be happy to hear about others long and short stroke builds.

I should mention many years ago we were involved in three similar high end builds for the time. Two 4.25 and one 4.5 stroke engines. The 4.5 arm came up noticeably short compared to the smaller engines. Today those results would be different. My point is the odds are against you if you're going to order a "stroker kit" and go set the world on fire with a long stroke, stock block combination.

There is a board member that had a professionally built 4.75 aftermarket block motor that had a bunch of top notch guys involved. I remember the crankshaft being removed pretty quickly. Maybe some of the folks involved in that build will chime in.

Now that I think about it there's been at least two high end long stroke motors sponsored by members of this site..... Pretty sure both of them removed the long arms not long after there first dragstip outing. Hopefully they offer there input.

These were issues I considered before doing a 350/413 stroker with a 4.25" crank. My justification included three primary points:
1) Stroke on the 455 is almost equal at 4.21";
2) Using a 6.8" rod kept the rod to stroke ratio at 1.6;
3) The weight of the 350 piston being less than larger displacement bores.

Given those parameters and the use of Johnson reduced travel lifters, in discussions with techs at Butler Performance and my contacts within the TransAm Race Series crews, concensus was that valve float would be the RPM limiting factor. The engine had no trouble spinning to 6200 rpm on Reher Morrison's dyno.

The Childress 935HP TA engine has a 3.48" stroke with rev limiters set at 9200rpm. The 525HP TA/2 Chevrolet LS3 engine is a 3.622" stroke with rev limiters set at 6800rpm.

Larry Jones’s dad has a 4.75” with an aftermarket block in his 700+HP GTO. Mostly a street build, I think BES built it.

Calvin Hill might be the other 4.75 big cid drag and drive? Not sure?

4.75 in a Pontiac not common at all.

4.5 has been fairly popular in the SD455 and the 71&72 455 HO’s at one of the local shops (Willard Machine). I think Rotella’s in Omaha have at least one, mild street builds though.

I think it would move out quite well.

I was thinking of similar build. Ported 6X-8 heads, maybe around 9:1 compression ratio

There's a good tech article on SD Performance that Dave performed, title is Ram Air manifolds vs. Headers. A 4.5 stroke crank in a stock block.498 cubes, Ram Air manifolds. It's a good read.
Keep the RPM's under, say 4500.........

Side loading of the piston is the biggest issue with the 4.5” stroke, along with the short cylinder of a Pontiac block pulling the piston down out of the bottom of the bore. Short piston life and quickly diminishing ring seal I’m thinking.

To end up with the same 1.57:1 rod to stroke ratio as a stock 455 which ain’t that great to begin with (6.625” rod - 4:21” stroke) you would need a rod close 7.1” long with that 4.5” crank.

Is it possible to put a rod that long into a stock deck block or would you need to go to a tall deck block? I’m thinking there’s no way to move the pin that far up into the ring pack on a stock block.

Makes me wonder exactly how GTO Jones is getting away with that 4.75” stroke in his 571. Which block standard or tall and what rod length?

Your spot on.... 7.1 won't work

For folk's reading pleasure. See date of article (1962).

Looks like they utilized 421 SD rods.

PS: Edit - looks like the PDF is too large for Forum @ 644 kb,