ROD RATIO - how low is too low?

[phoenix]

Jon Kasse's success in the Engine Masters challenge and Darin Morgans comments have made me rethink what I know (It isnt much) about Rod Ratios. Kasse's RR are in the 30's on his IHRA stuff. His Engine Master BB was in the low 50's if I'm not mistaken. Darin made a comment on this forum that if his Pro Stock RR were 1.65:1 he would not care. At what point does it become a concern. I'm not talking about the limitations within the block. would piston speed become a concern? piston rock?cylinder wear? Seems these low RR engines only see low RPM. And lastly,could IHRA Pro Stock RR theory carry over to a small block? Say a 400 cu in ford 302. The reason I picked this motor is because of the low deck height advantages. ie.. like a NHRA Pro Stock motor, short push rods, better manifolding? small bearings etc. Just a curios. thanks Guys. Shane

[maxracesoftware]

from experience Dyno testing various
Engine sizes and Rod Ratios

the trend so far shows
increased BlowBy CFM (6 to 8 CFM) as you go less than 1.50 Rod Ratio
if you can't run a Vac-Pump, i'd keep Rod Ratio above 1.50

and with 1.43 Rod Ratio with same Hone finish
8 to 10+ CFM BlowBy, but thats not much of a problem
with a Vac-Pump ...the higher piston speeds and shorter rod ratios,
are showing more HP/TQ per 1 CFM of Cylinder Head Flow up to a point.

usually 1.60 to 1.65+ you'll have no major ring BlowBy problems
and 1.6 to 2.0 can show 1.5 to 4 cfm BlowBy range

if you have a very good Block , (Correct Hardness and Thick Walls),
the BlowBy CFM numbers will be reduced from the above numbers

[Darin Morgan]

Jon Kasse's success in the Engine Masters challenge and Darin Morgans comments have made me rethink what I know (It isnt much) about Rod Ratios. Kasse's RR are in the 30's on his IHRA stuff. His Engine Master BB was in the low 50's if I'm not mistaken. Darin made a comment on this forum that if his Pro Stock RR were 1.65:1 he would not care. At what point does it become a concern. I'm not talking about the limitations within the block. would piston speed become a concern? piston rock?cylinder wear? Seems these low RR engines only see low RPM. And lastly,could IHRA Pro Stock RR theory carry over to a small block? Say a 400 cu in ford 302. The reason I picked this motor is because of the low deck height advantages. ie.. like a NHRA Pro Stock motor, short push rods, better manifolding? small bearings etc. Just a curios. thanks Guys. Shane

Once again, what maxracesoftware said is right on the money. The piston rock and side loading become a big factor below 1.50 with piston speeds over 4500ft/min. If its a low rpm street machine that never sees the high side of 6000rpm or 4000ft/min, then the problems with blowby are much less but none the less still present. When I discuss Rod Ratio I always dismiss people who think there is an " ideal" rod ratio and are willing to compromise the overall engine design in order to achieve this " ideal" Rod ratio. I have never said that there are not both high and low limits one must avoid. A Rod ratio of 2.2:1 or a 1.4:1 is not going to help matters any. Anywhere between 1.65 and .1.85 is fine. any less and you run into high skirt loading, blow by and frictional HP losses. Anything above 2:1 and you run into pressure lag and have to run the cross sectional area of the ports very small and maintain a higher mean velocity in order to help make up for it.The little 265 and 302 Comp eliminator engines are usually a case study in pressure lag unless they run the Aurora block with a 8.5 deck and get there ratio into the 1.8 range.
.

[phoenix]

Thanks guys. glad to see i'm not the only one bored on a saturday.

[Trev]

I found this post wery interesting
But what is Rod ratio and how do you calculate it

[Rick360]

But what is Rod ratio and how do you calculate it.

Rod Ratio is the ratio of the connecting rod length to the stroke.

Formula is Rod Length divided by stroke.

Example:

6.00" rod
divided by 3.48 stroke

=1.724 rod/stroke ratio

Rick

[Trev]

So the 400 im building now will be


3.750 stroke
6 inch rod
6/3.750 = 1.6

So is this good or bad

[Guest]

Anything above 2:1 and you run into pressure lag and have to run the cross sectional area of the ports very small and maintain a higher mean velocity in order to help make up for it.The little 265 and 302 Comp eliminator engines are usually a case study in pressure lag unless they run the Aurora block with a 8.5 deck and get there ratio into the 1.8 range.
.
Darin ( or any of the other guys) could you expand on the "pressure lag" concept. I don't understand what it is, what effect does it have, beside smaller ports (and lowering the rod ratio) is there anything else can be done about it. What sort of effect on power, etc? Thanks

[Floyd]

Jon Kasse's success in the Engine Masters challenge and Darin Morgans comments have made me rethink what I know (It isnt much) about Rod Ratios. Kasse's RR are in the 30's on his IHRA stuff. His Engine Master BB was in the low 50's if I'm not mistaken. Darin made a comment on this forum that if his Pro Stock RR were 1.65:1 he would not care. At what point does it become a concern. I'm not talking about the limitations within the block. would piston speed become a concern? piston rock?cylinder wear? Seems these low RR engines only see low RPM. And lastly,could IHRA Pro Stock RR theory carry over to a small block? Say a 400 cu in ford 302. The reason I picked this motor is because of the low deck height advantages. ie.. like a NHRA Pro Stock motor, short push rods, better manifolding? small bearings etc. Just a curios. thanks Guys. Shane

Once again, what maxracesoftware said is right on the money. The piston rock and side loading become a big factor below 1.50 with piston speeds over 4500ft/min. If its a low rpm street machine that never sees the high side of 6000rpm or 4000ft/min, then the problems with blowby are much less but none the less still present. When I discuss Rod Ratio I always dismiss people who think there is an " ideal" rod ratio and are willing to compromise the overall engine design in order to achieve this " ideal" Rod ratio. I have never said that there are not both high and low limits one must avoid. A Rod ratio of 2.2:1 or a 1.4:1 is not going to help matters any. Anywhere between 1.65 and .1.85 is fine. any less and you run into high skirt loading, blow by and frictional HP losses. Anything above 2:1 and you run into pressure lag and have to run the cross sectional area of the ports very small and maintain a higher mean velocity in order to help make up for it.The little 265 and 302 Comp eliminator engines are usually a case study in pressure lag unless they run the Aurora block with a 8.5 deck and get there ratio into the 1.8 range.
.

What about with a motor that you're not able to get the 1.65 to 1.85:1 ratio with due to the phsyical design? Is it a huge deal when you're trying to build a race engine? When does piston speed and lag become bigger factors?

For example 382 Stroker LS1 Motor 3.900 X 4.00

4.00 * 1.65 = 6.6 inch rod which won't fit

For my own purposes I wanted to run a ~ 3.73 Stroker and 6.3 inch rod, which is right at 1.689:1


Thanks for the help/advice

Floyd.

[Racer7088]

Run whatever rods will fit and start working on your heads and intake and cams. Rod Ratio is worthless and won't gain you hardly anything at all especially with really great heads like the LS1 has and it's probably a street engine anyway.

[Floyd]

Run whatever rods will fit and start working on your heads and intake and cams. Rod Ratio is worthless and won't gain you hardly anything at all especially with really great heads like the LS1 has and it's probably a street engine anyway.

The 382 Stroker I was talking about, Yes.


Mine, No. I'll eventually get it all together and try to win the Tech Series Super Stock class.