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·what exacatly are rod ratios? and whats a good ratio to stay in?
rod ratios
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Here are some common rod ratios of both stock and stroker kits. Although they are using figures from Coast High Performance (sorry Ken) the numbers are common. When this was published the 331 wasn't as popular as it is now, but the ratio is 1.66 I believe.
Also, someone smarter than I am will have to explain the effects of rod ratios that are less than stock. Read: side loading.
http://a6.cpimg.com/image/3C/8E/10551356-4560-01D30200-.jpg
Also, someone smarter than I am will have to explain the effects of rod ratios that are less than stock. Read: side loading.
http://a6.cpimg.com/image/3C/8E/10551356-4560-01D30200-.jpg
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2,155 Posts
Rod ratios and their effects are subjective when applied to an engine, mostly with a stroker engine. It goes something like this:
The rod ratio is determined by the length of the rod as compared to the stroke of the crank and a longer rod is considered to have more HP potential because the piston stays at the top of the stroke for a longer period of time. That therefore enables the engine to take advantage of the combustion burn for a longer ammount of time. In simple terms = the same burn over a longer ammount of time will give more HP.
All that sounds like the holy grail of engine building, and when building a NASCAR engine or a Pro stock, well, maybe it is. The problem is that changing just the rod ratio from a standard to a long rod motor will net maybe 2 or 3 percent, big stuff in an Indy engine, but nothing worth noting on the street.
In practical terms a stroker engine will have a shorter rod and make a bunch more HP. The reason is that you have increased the Cubic Inches and that is what makes the HP. So you shorten the rod with a stroker and loose 3 percent because of that, and add 30 percent because of the increase of Cubic Inches (and other factors like voluumetric efficiency).
So, will a better rod ratio make more horsepower? Sure. Is it economicaly feasable? Not unless you are on an unlimited racing budget and it is the last place left that you have to go to make more horsepower.
One last thing, the longer the rod you run the farther it pushes the wrist pin up into the piston (design-wise), at least with a finite crank CL to deck height. Past a certain point and that will severely compromise the stability of the piston in the bore, unseal the rings, and rock the piston in the bore creating accelerated wear and a higher chance of catastrofic failure at high RPM. On the flip side of that, Too long of stroke will give a much higher side load on the pistons in the bore, also rock the pistons, and wear the bores out much quicker.
So is it worth it to buy a long rod 302 or 351 kit? hahahahahaha, (oh, sorry)................... *G*.
The rod ratio is determined by the length of the rod as compared to the stroke of the crank and a longer rod is considered to have more HP potential because the piston stays at the top of the stroke for a longer period of time. That therefore enables the engine to take advantage of the combustion burn for a longer ammount of time. In simple terms = the same burn over a longer ammount of time will give more HP.
All that sounds like the holy grail of engine building, and when building a NASCAR engine or a Pro stock, well, maybe it is. The problem is that changing just the rod ratio from a standard to a long rod motor will net maybe 2 or 3 percent, big stuff in an Indy engine, but nothing worth noting on the street.
In practical terms a stroker engine will have a shorter rod and make a bunch more HP. The reason is that you have increased the Cubic Inches and that is what makes the HP. So you shorten the rod with a stroker and loose 3 percent because of that, and add 30 percent because of the increase of Cubic Inches (and other factors like voluumetric efficiency).
So, will a better rod ratio make more horsepower? Sure. Is it economicaly feasable? Not unless you are on an unlimited racing budget and it is the last place left that you have to go to make more horsepower.
One last thing, the longer the rod you run the farther it pushes the wrist pin up into the piston (design-wise), at least with a finite crank CL to deck height. Past a certain point and that will severely compromise the stability of the piston in the bore, unseal the rings, and rock the piston in the bore creating accelerated wear and a higher chance of catastrofic failure at high RPM. On the flip side of that, Too long of stroke will give a much higher side load on the pistons in the bore, also rock the pistons, and wear the bores out much quicker.
So is it worth it to buy a long rod 302 or 351 kit? hahahahahaha, (oh, sorry)................... *G*.
Rod ratio is the relationship between the length of the connecting rod and the stroke of the motor. Generally speaking, it is desirable to have a higher rod ratio than a lower ratio. You start running into a lot of ware issues when the rod ratio is below 1.55 and a stock 302 ratio of 1.69 is considered OK for a street motor. Most stroker motors have a lower rod ratio due to the longer stroke and limited room for a longer rod. Again, generally speaking, it is easier to increase HP with more displacement on a NA motor then any other modification. There is a direct relationship between cylinder pressure, displacement and HP. A long rod motor increases cylinder pressure with out affecting compression ratio and displacement.
Why would one want to build a long rod motor?
1) If you run in a class where displacement is limited then you can gain extra HP by increasing the rod ratio.
2) For a given displacement, you can increase the efficiency of the motor. You can make more HP on the same grade of fuel and maintain or even get better MPG.
Again, it is easier to make more HP with more displacement but I’ve selected to build a more efficient 302. As soon as I get my car back together, I’ll chassi dyno it and post the results and MPG results.
Hope this helps
Why would one want to build a long rod motor?
1) If you run in a class where displacement is limited then you can gain extra HP by increasing the rod ratio.
2) For a given displacement, you can increase the efficiency of the motor. You can make more HP on the same grade of fuel and maintain or even get better MPG.
Again, it is easier to make more HP with more displacement but I’ve selected to build a more efficient 302. As soon as I get my car back together, I’ll chassi dyno it and post the results and MPG results.
Hope this helps
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The stock 302 ratio is about as low as I'd like to go in a high-rpm racing engine....I have run such ratios into the 7500 rpm range with no ill effects but I share the other poster's concerns about thrust side-loading on the pistons and bore...
I've always built within the 1.7 to 1.8 range, due to parts/budgetary constraints and have gotten good perfomance and service life from such combinations.
As I believe Hal said, on a street engine of modest HP, it isn't that big a factor...
I've always built within the 1.7 to 1.8 range, due to parts/budgetary constraints and have gotten good perfomance and service life from such combinations.
As I believe Hal said, on a street engine of modest HP, it isn't that big a factor...
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