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How much of compression would I lose if I would take a 5.4" rod in stead of the 5.426" rod ?
I would also use a set of ZE heads.

I think 0 on the static CR, but dynamically, where the piston achieves max velocity changes, so it would be like changing your cam profile....

But a change that small is insignificant.... but insignificantly decreasing it.

I doubt it will not change the CR.

Isn't CR depending on the volume of the heads and the position / shape of the piston.

If the rod can't push it 0.026" further up; it means the area where the mixture is compressed is larger.

WhiteFinish wrote:How much of compression would I lose if I would take a 5.4" rod in stead of the 5.426" rod ?
I would also use a set of ZE heads.

Your resulting static CR with ZE heads would be 9.822:1, give or take.

So.... basically 0.

Ze heads are lower CC than DE heads too. Thats probably a bigger difference.

0.56CC difference at TDC with 0.026" shorter rods. <- wrong 3.7CC

ZE heads are 2.5CC smaller.

Plane down the heads a little to make up the difference.

Ryan wrote:So.... basically 0.

Ze heads are lower CC than DE heads too. Thats probably a bigger difference.

0.56CC difference at TDC with 0.026" shorter rods.

ZE heads are 2.5CC smaller.

Ryan, I think you should check your numbers. The difference of 0.2:1 CR I pointed out is WITH ZE HEADS IN BOTH CASES

And, considering the total capacity of each cylinder is 416.166 cc, the difference with the shorter rods is 7.408 cc, not 0.56 cc.

You're right. Hasty and totally wrong.

volume swept by an 8.45cm bore and .06604 cm of stroke is h*pi*r^2 = 3.7035 CC...

DISPLACEMENT of each cyl is 416CC, not the capacity. (you did bore x stroke)


But without knowing full volume at either TDC or BDC, I can't comment on C/R...

since changing from 1:10 to 4:13 is NOT the same increase as from 10:100 to 13:103 even though they both go up 3.

But it will be small.

Ryan wrote: volume swept by an 8.45cm bore and .06604 cm of stroke is h*pi*r^2 = 3.7035 CC...

You're right.
I made a mistake. I doubled the volume, because I confused a shorter rod with a shorter stroke. Actually, the difference in CR would be lower than I stated before, because, even when the piston is stopping .26" earlier at TDC, it's also going down .26" further at BDC, so, without doing the math, I'd guess the final CR would be a little over 9.9:1.
Sorry...

Ryan wrote:DISPLACEMENT of each cyl is 416CC, not the capacity. (you did bore x stroke)

Nope. I actually did 2497/6...
Either way, the math is the same.
And yeah, it's the displacement. Sorry for the misnomer.

Ryan wrote:But without knowing full volume at either TDC or BDC, I can't comment on C/R...

The volumes could be calculated easily, but I don't think it's worth it.

Bottom line, Whitefinish, using the shorter rods might make you lose 1 or 2 hp at the most.
But, because of the shorter rods, the stroke/rod ratio would go down, which would affect (negatively) your RPM range. At the same time, using the shorter (lighter) rods would call for balancing the rotating assembly. IMO, too much work (and money) to get less hp and a shorter redline than stock...

EDIT: You'd lose 0.53 bhp.

Go for longer, lighter rods, shorter pistons(from the pin to the block deck, with a tapered skirt to help ease side loading), and rev it to 10K with Colt 218 cams.

Oke, I understand I loose a bit of CR.
Just wondering how much I should raise the piston in order to compensate.

MrMazda92 wrote:Go for longer, lighter rods, shorter pistons(from the pin to the block deck, with a tapered skirt to help ease side loading), and rev it to 10K with Colt 218 cams.

Why do you come up with this combination ?

The decreased weight allows for significant reduction in Crank weight(below even the KLG4 crank weight), which will do more good than anything for high RPM applications. As it is, it would certainly make the engine rev faster, and if the parts are quality, it could increase the lifespan.

You decrease a LOT of weight by making a smaller piston, and a longer rod. The problem this typically creates is an increase in side loading force against the cylinder wall, for which you'll want a "longer" piston skirt to help distribute that force more evenly. If you taper the skirt so it's longer at 90 degrees and 270 degrees, and shorter at 180 and 360, then you can find a balance between strength and light weight.
Those positions are only an example, my main point was a 4 corner approach to the piston shape, but the angles make it sound more confusing than it is.

I'm not actually sure where the 218 Colt cams top out in the Revs, so that part was kind of a "wouldn't it be nice!" addition. I haven't seen too many people mess with that, none actually, surprisingly.

WhiteFinish wrote:Oke, I understand I loose a bit of CR.
Just wondering how much I should raise the piston in order to compensate.

MrMazda92 wrote:Go for longer, lighter rods, shorter pistons(from the pin to the block deck, with a tapered skirt to help ease side loading), and rev it to 10K with Colt 218 cams.

Why do you come up with this combination ?

0.026" would make it exactly the same.

If you're going with custom pistons, wiseco makes 11.5:1 pistons for us, thats your only real upgrade option as far as I know.

WhiteFinish wrote:How much of compression would I lose if I would take a 5.4" rod in stead of the 5.426" rod ?
I would also use a set of ZE heads.

Do you have these rods sitting around? Just wondering why you asked about these in particular.

He's probably got access to the Ford small block rods that are readily available, and beefier than stock. IIRC, those were 5.4" exactly, and fit with zero/minimal modification.