The "perfect" 1.75 rod ratio
09-20-2007, 10:00 AM
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The "perfect" 1.75 rod ratio
Why is a 1.75 to 1 rod to stroke ratio the "perfect" ratio? I find this difficult to stomach. If it were the perfect ratio then all manufacturers would use it. Rod ratio is all about targeting a specific range of power with consideration to stress and loading, right? What does this magic 1.75 have that no other ratio has? It is just more equally balanced, pro and con?
09-20-2007, 02:14 PM
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Re: The "perfect" 1.75 rod ratio (RStoR)
I've done lots of searches on rod ratio. I already know a lot about rod to stroke ratio and the benefits / drawbacks. What hasn't been explained is why 1.75 is an ideal ratio. 
09-20-2007, 02:30 PM
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Re: The "perfect" 1.75 rod ratio (Insert Marijuana Here)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Insert Marijuana Here »</TD></TR><TR><TD CLASS="quote">I've done lots of searches on rod ratio. I already know a lot about rod to stroke ratio and the benefits / drawbacks. What hasn't been explained is why 1.75 is an ideal ratio. </TD></TR></TABLE>Obviously not enough, or you haven't read it anywhere that will explain it in lamens terms for you. Study a little math and/or physics and it should come together for you. You'll understand why not all manufacturers make there engines with this exact number in mind.
</TD></TR></TABLE>Obviously not enough, or you haven't read it anywhere that will explain it in lamens terms for you. Study a little math and/or physics and it should come together for you. You'll understand why not all manufacturers make there engines with this exact number in mind.
09-20-2007, 02:37 PM
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Re: The "perfect" 1.75 rod ratio (hatchling37)
To better understand my question: I understand why engines have different rod to stroke ratios in different applications. What I don't understand is why everyone is touting 1.75 as some magic number that fits every application. I can't count the number of sites that have claimed this; even on this site, with no proof of context. It's just "1.75 is the ideal ratio." For what reason? I've never seen this specific ratio - 1.75 to 1 - explained.
Modified by Insert Marijuana Here at 3:49 PM 9/20/2007
Modified by Insert Marijuana Here at 3:49 PM 9/20/2007
09-20-2007, 02:40 PM
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Re: The "perfect" 1.75 rod ratio (Insert Marijuana Here)
I'm beginning to wonder if anyone knows or if it's just repetition... Sure, maybe I could map the piston acceleration curves to RPM of all the rod to stroke ratios on a graph and 1.75 would be an average ratio. I don't know. What I wonder is why nobody else knows, but they still tout it.
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09-20-2007, 03:26 PM
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Re: The "perfect" 1.75 rod ratio (Insert Marijuana Here)
I honestly think rod ratios have been debated here for so many years everyone just doesn't want to go there again.I know I don't want to go there again.
09-20-2007, 04:04 PM
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Re: The "perfect" 1.75 rod ratio (Insert Marijuana Here)
1.75 is a good rod ratio.
I've read that anything between 1.6 and 1.8 is good.
Take my F20B for example. It has a 145.5mm rod and a 88mm stroke
my r/s is 1.65
This is a good r/s in that its more stable at high rpms. However to make best usage of this you need to have a matching close ratio transmission. Which fortunately for me I have built my trans to do so.
In order for one to make the best usage of a higher r/s ratio the following is required
1. Very High rpms. High rpms require strong rod bolts, balanced crankshaft, a high flowing oil pump, and a higher static compression. With the increase of compression, 93 octane is required, i'd go 12.5:1cr on the street.
2. A close ratio transmission. This is a lot harder to come across. Gear design is not cheap. Thus close ratio gears are expensive. Most people are too scared to modify a trans, so they're not going to be interested in changing them (read: less demand for them, more expensive) Also with close ratio trans, your cruising rpms are sacrificed. I'm at 4950rpm @ 80mph in 5th gear.
I/H/E all have to be optimized for high rpms as well, which sacrifices low end torque.
Or one can switch out the long rod/ short crank combo for a long crank, shorter rod combo. Example being the H23/F22 crank with a 95mm stroke with a 141.5mm rod This yields a 1.49ish r/s ratio. Stock gearing works better with a stroked engine also. Low end torque isn't sacrificed either. And the best part, get this. The external dimensions are the same. The engine weight remains the same.
The best part is that externally my f20b would be the same size as a 2.2 liter f22 stroked f20b. I wouldn't rev as high, but stock gears wouldn't feel so gappy because I'd have more low end power.
The moral of the story, stroking is cheaper and easier to make power with.
destroking for high rpms costs more money effects daily drivability. Destroking is for when you have a max displacement of like 2000cc for some race. Likewise, if the country that you plan on selling your engine in puts higher taxes on larger engines, and you need an engine that makes 220hp, then you might want to look into making a higher rpm engine.
Motorcycle engines and F1 engines are completely different. Those engines and drive trains were built from scratch where the initial designs were for high rpm power. The ports are huge, the bores are larger, the gears are close. On our stock cars, like a prelude for example. We would have to do heavy modifications to change the characteristics of our engine (increasing compression, close ratio gears, head package, port/polish, i/h/e, etc etc etc)
I've read that anything between 1.6 and 1.8 is good.
Take my F20B for example. It has a 145.5mm rod and a 88mm stroke
my r/s is 1.65
This is a good r/s in that its more stable at high rpms. However to make best usage of this you need to have a matching close ratio transmission. Which fortunately for me I have built my trans to do so.
In order for one to make the best usage of a higher r/s ratio the following is required
1. Very High rpms. High rpms require strong rod bolts, balanced crankshaft, a high flowing oil pump, and a higher static compression. With the increase of compression, 93 octane is required, i'd go 12.5:1cr on the street.
2. A close ratio transmission. This is a lot harder to come across. Gear design is not cheap. Thus close ratio gears are expensive. Most people are too scared to modify a trans, so they're not going to be interested in changing them (read: less demand for them, more expensive) Also with close ratio trans, your cruising rpms are sacrificed. I'm at 4950rpm @ 80mph in 5th gear.
I/H/E all have to be optimized for high rpms as well, which sacrifices low end torque.
Or one can switch out the long rod/ short crank combo for a long crank, shorter rod combo. Example being the H23/F22 crank with a 95mm stroke with a 141.5mm rod This yields a 1.49ish r/s ratio. Stock gearing works better with a stroked engine also. Low end torque isn't sacrificed either. And the best part, get this. The external dimensions are the same. The engine weight remains the same.
The best part is that externally my f20b would be the same size as a 2.2 liter f22 stroked f20b. I wouldn't rev as high, but stock gears wouldn't feel so gappy because I'd have more low end power.
The moral of the story, stroking is cheaper and easier to make power with.
destroking for high rpms costs more money effects daily drivability. Destroking is for when you have a max displacement of like 2000cc for some race. Likewise, if the country that you plan on selling your engine in puts higher taxes on larger engines, and you need an engine that makes 220hp, then you might want to look into making a higher rpm engine.
Motorcycle engines and F1 engines are completely different. Those engines and drive trains were built from scratch where the initial designs were for high rpm power. The ports are huge, the bores are larger, the gears are close. On our stock cars, like a prelude for example. We would have to do heavy modifications to change the characteristics of our engine (increasing compression, close ratio gears, head package, port/polish, i/h/e, etc etc etc)
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