Connecting Rod Length
- Thread starter propain37
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In the simplest terms, a longer connecting rod will push the piston up further, increasing compression, which usually makes more HP.
However, you can only go so high before the piston will come into contact with the valves or the head.
You can also read about piston "dwell time" if you want to go more in depth, I believe one of the ARC Racing videos (on facebook and youtube) talked about that topic at some point.
However, you can only go so high before the piston will come into contact with the valves or the head.
You can also read about piston "dwell time" if you want to go more in depth, I believe one of the ARC Racing videos (on facebook and youtube) talked about that topic at some point.
bob58o
SuckSqueezeBangBlow
For the types of engines we use, piston rod length is generally used to get the compression ratio we desire.
Rod length to stroke ratio doesn't change that much.
For example a 3.308" connecting rod with a 2.165" stroke has a 1.53 :1 rod length to stroke ratio. Increasing the rod length to 3.348" makes that ratio 1.55 :1. Only a 1.53% change.
But changing from a stock length rod (piston sitting 0.040" in the hole) on a 8.5 compression ratio engine to a 0.040" over rod (piston flush with deck) makes about 9.5 compression. ratio. That is an 11.76% change in Compression ratio.
That being said, in general the internet says that increasing 1 point in compression ratio is good for about 3% increase in max HP.
Rod length to stroke ratio doesn't change that much.
For example a 3.308" connecting rod with a 2.165" stroke has a 1.53 :1 rod length to stroke ratio. Increasing the rod length to 3.348" makes that ratio 1.55 :1. Only a 1.53% change.
But changing from a stock length rod (piston sitting 0.040" in the hole) on a 8.5 compression ratio engine to a 0.040" over rod (piston flush with deck) makes about 9.5 compression. ratio. That is an 11.76% change in Compression ratio.
That being said, in general the internet says that increasing 1 point in compression ratio is good for about 3% increase in max HP.
bob58o
SuckSqueezeBangBlow
What engine do you have?
The way to check how much of a longer rod you can use is by measuring. Take the head off, put the piston at top dead center, lay a straight edge across the cylinder deck, and use feeler gauges to see how far below the deck the piston sits. That measurement plus the head gasket thickness determines the piston the head clearance. 0.030" is a clearance to shoot for. Some do less, but that isn't advisable (especially for beginners). Also need to consider piston to valve clearance. Long rod, thin gasket, hot camshaft, ratio rockers, milled cylinder head,... all decrease piston to valve clearance. I try to not run any less than 0.065" piston to valve clearance. Google how to measure piston to valve clearance. I use the clay method.
Also consider the compression ratio vs the octane fuel you want to use. Dynamic CR is what is important here. Need to consider camshaft event timing here.
Lots of things to consider when putting an engine together. My first build thread was like 80 pages cause I had to learn all of this.
Taping the blower housing prevents new air from entering the housing. It is easier (less drag) on the flywheel to keep the same air spinning in the housing than to keep cycling new air in and out. It is a quick and dirty alternative to cutting the fins off of the flywheel fan. That is essentially eliminating the cooling system. Great for methanol builds that run cold, not great for gasoline builds that might overheat.
I've never tried it, so I don't know the technique. I assume that you just cover the holes in the housing with tape. Simple. Probably best to test with a cylinder head temperature gauge. I wouldn't bother with it unless racing competitively in a class that allows it. Even then, an aftermarket flywheel with smaller (or less, or no) fins would probably be better. Billet flywheels don't grenade.
bob58o
SuckSqueezeBangBlow
Clearance meaning how much space before two things contact each other. For piston to head clearance... if you have a piston that sits 0.010" in the hole at top dead center and a 0.009" head gasket, you have 0.019" P2H clearance. So they don't touch. Great. Until you fire the engine up and find out that the 0.019" clearance disappears when the engine is turning and now you have a piston slapping against the cylinder head.
I don't know the numbers for a gx160... I am pretty familiar with Hemi-predators and could give you a good idea of what you could do without measuring on a Hemi 212cc. With a gx160, I just don't know - so measuring stuff is going to be important.
I don't know the numbers for a gx160... I am pretty familiar with Hemi-predators and could give you a good idea of what you could do without measuring on a Hemi 212cc. With a gx160, I just don't know - so measuring stuff is going to be important.
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