optimum rod/stroke ratio, opinions?

[Chris Horoho]

stroke = 63mm
bore = 81mm

stock rod length = 119.5mm

the ratio for a stock stroke busa is

stock ratio = 1.896
2.5 mm shorter rod ratio = 1.857
5 mm stroke w/ 2.5 mm shorter rods ratio = 1.720

[Chris Horoho]

graph from another link

Rod Angle “n” Ratio Examples Comments
13½° 2.142-1   High speed motor with small ports. Best breathing with small ports
14°   2.067-1     
14½° 1.997-1   Long rods for good breathing with small ports
15°   1.932-1   Long rods to help breathing with small ports. Responds well to stroke increases (“n” value too large for intake port size)
15½° 1.871-1   Responds well to stroke increases (“n” value too large for intake port size)
16°   1.814-1 Mopar 383/400 Approximate “ideal” compromise between stress & breathing (1.81-1)
16½° 1.760-1 Chevy 327 Good choice for motors with good breathing
17°   1.710-1 Mopar 360
Ford 302, 351W, 460 ”Safe” limit for thrust angle. Approaching practical limit for street motors
17½° 1.663-1   Approaching practical limit for street motors
18°   1.618-1 Chevy BB 396/427 Approaching practical limit for street motors. Good power due to large intake port
18½° 1.576-1   Limited street use
19°   1.536-1 Chevy BB 454 Good power due to large intake port
19½° 1.498-1   Not practical for street use due to short pistons
20°   1.462-1 Chevy SB 400 Poor peak power. Longer rods are used in any serious application


good link to have a look at
http://www.victorylibrary.com/mopar/rod-tech-c.htm

[Chris Horoho]

another graph from the link above

Best Combinations of “n” Values & Intake Characteristics
“n” = 1.45 - 1.75 more compatible with: “n” = 1.75 - 2.1 more compatible with:
Large intake port volume vs. motor size
(”J” head on 273) Small intake port volume vs. motor size
(stock 452 head on 498” RB stroker)
Single-plane or 360° intake manifolds
(Edelbrock Victor, Torker & Torker II, TM7. Holley Strip Dominator. Offenhauser Equa-Flow, Port-O-Sonic. Weiand X-Celerator, Team G) Dual-plane 180° intake manifolds
(Edelbrock: LD340, CH4B, DP4B, Performer & Performer RPM, Streetmaster, SP2P. Holley Street Dominator. Weiand Stealth, Action Plus)
Large carburetor vs. engine size
(273 with 750cfm) Small carburetor vs. engine size
(440 with 600cfm)
Moderate engine speed
(pick-up, RV, towing) High engine speed
(peak power more important)
Tall axle ratio
(2.76, 2.93, 3.23, 3.55 and/or with tall tires) Short axle ratio
(3.91, 4.10, etc. and/or with 25 or 26” tires)

[Chris Horoho]

here is another great link to read

http://www.stahlheaders.com/Lit_Rod%20Length.htm

[sportbikeryder]

Most of the high horsepower turbo V8 guys (2000+ hp) over at turbo mustangs say a good piston design (piston crown thickness and ring packaging) ALWAYS takes priority over rod ratio. Many use 5.7" rods with 3.75" stroke cranks (1.52).

I have zero experience in rod selection, just being an internet forum tuner...

John

[Chris Horoho]

ya im tryin to debate about shorter rods with turbo pistons to remove the fact of needing a spacer
plus itll drop the compression just a hair more

still deciding what to do
and thought id get some opinions

[sportbikeryder]

If you want shorter rods, you can probably find some watercooled 1100 rods. I am not 100%, but I am pretty sure they are 2mm shorter and have the same big end. Should be able to find some pretty cheap.
John

[Chris Horoho]

If you want shorter rods, you can probably find some watercooled 1100 rods. I am not 100%, but I am pretty sure they are 2mm shorter and have the same big end. Should be able to find some pretty cheap.
John

im looking to make 400-450 hp for a while before going up a little more
crower shortened rods (for 5mm stroker) is what im thinking of going to

[Chris Horoho]

they would be 2.5 mm shorter
which would be the 1.857 ratio
in my opinion I don't see where that would be enough of a difference in a street bike to hurt anything
does anybody agree or disagree?

[sportbikeryder]

I would think you'd be fine. Especialy since you are probably not going to ned to rev the snot out of it with a turbo. I mentioned the watercooled rods thinking that you may be able to find some Carrillos or something that someone had new in the box on the shelf, covered in dust and would be cheap because the bikes are old and not in high demand anymore...

[Chris Horoho]

ya thought about that but would they handle the power?
just wouldnt want to lose the rods out the front



they are 2.5 mm shorter by the way
i used to have oil cooled 1100's
they are still one of my favorite bikes

[1BADASSBUSA]

Pinky the ratio you should be shooting for is 1.7

That's on all motors.  It's what all Pro-Stock Cars & bikes run....

Mopar 360 was a wicked motor for just that reason.....

[Dave.C]

Have not done a lot with bike motors in relation to changeing strokes and rod length, but on my car motors, for example on my small block chevys 350/383/400 and all my stuff had a supercharger and I found the longer rod you can stuff in the motor the better it keeps the piston ay TDC longer and the longer the rod the less stress there is on pistons and walls, this probably dont meen anything to you guys, with the Busa motors, I think because if you go to a 5mm stroke you are probably limited as to you piston and rod combinations, another thing I found is with a blown motor you don't need a lot of cubic inches to make big power in fact I also have found that a little smaller motor even with a lot of boost 30-35 psi will stay together longer than a motor with a long stroke and short rods, don'tmeen to bother you guys but its to cold out to ride today   

[Chris Horoho]

my idea is to get away from the spacer all together and install shorter rods with the turbo pistons
id be within .01 of a spacer in difference of height

its not even within a 2% change to see a difference
but i thought id get some of your guys opinions

[Chris Horoho]

angle at stock length rod is 14.76716
angle at shortened length is 15.06849

[BLACK_DEATH]