Bore/stroke ratio vs rod/stroke ratio

Hi andrew and all how u guys doin ?

I have a qoustion when you are bulding an engine which ratio is more important for engine rev bore to stroke ratio or rod to stroke ratio how they affect the engine ? Sry for my english

Sam

Both ratios are important, however if you're looking for the ability to rev to very high rpm, the bore to stroke ratio probably becomes more critical first. You'll generally need to reduce the stroke as your rpm ceiling climbs to control or limit max piston speed. If you want to maintain a certain capacity then this requires the bore diameter to increase.

Once you've decided on a stroke, then the rod to stroke ratio needs to be considered. For very high rpm operation, rod to stroke ratios greater than 2:1 are typical.

Thank you andrew

So imagine an engine with 78.5mm bore and 82mm stroke if I want to build this engine for drag race which on of this options are better

1.80mm bore and 82mm stroke

2.80mm bore and 85mm stroke

3.80mm bore and 89mm stroke

And will please explain it for me so i can undrestand why thank you so much

Thank you andrew

So imagine an engine with 78.5mm bore and 82mm stroke if I want to build this engine for drag race which on of this options are better

1.80mm bore and 82mm stroke

2.80mm bore and 85mm stroke

3.80mm bore and 89mm stroke

And will please explain it for me so i can undrestand why thank you so much.

First of all you haven't listed a rod length for these combinations so it's not possible to calculate a rod to stroke ratio. Secondly it really depends on what rpm range you expect to operate to. Lastly there's usually little flexibility in a production engine to make wholesale changes to either ratio so you're going to be working at close to the stock specs in most instances.

You also need to also weigh up the rod to stroke ratio vs the capacity. To give you an example of what is possible, we ran a 4G63 with a 88 mm stroke and a 156mm rod to 10,500 rpm regularly and occasionally 11,000 rpm.

Oh sry about that

I forgot to say The rod lenght is 133.6mm in the 2 first one but in the last ine is 137.75mm.

So if the stroke/bore ratio reduces is better for reving and performance ? And if the rod/stroke ratio increases is better for reving ? Am I right ? Does it matter for turbo and natural aspired engine ?

In the first option that I said with 80mm bore and 82mm stroke and 133.6mm rod lenght we have stroke/bore:1.025 rod/stroke:1.62 but in the 3rd option 80mm bore 89mm stroke 137.75mm rod lenght we have stroke/bore:1.11 rod/stroke:1.54mm what its the pro and cons of this two for example if u can please explain it I'm really trying to learn.thank you

A shorter stroke reduces piston speed for a given rpm, so yes it's better for high rpm operation. Likewise an increase in rod to stroke ratio will also favour high rpm performance. Really the limiting factor is what you're able to do around the stock engine design.

Ultimately the only way you're going to be able to definitely decide which combination is better for your use is to build the two engines and test them back to back - This sort of testing is what race engine development programs are all about. In once instance you've got a less than ideal rod to stroke ratio but more capacity and in the other you've got an improved rod to stroke ratio but less capacity. You haven't suggested what rpm you want to run the engine to so there's still really not enough information to really make a decision. If I had to take a guess, I'd say the larger capacity would offer more gains than the improved rod to stroke ratio given the info you've provided.

I'm building this engine for a 1/4mile drag races so I'm looking for best and quickest reving I'm talking about 8000 to 8500rpm.

Yes in one option we have more capacity which whith the rod to stroke ratio and bore to stroke ratio numbers maybe it won't rev fast I don't know because this one is a very expensive kit in my country so thats why I'm asking how can I deside which one will be my best option. Best option for a 1/4 mile which one will be ? Thank you

As stated above, the only way to get a decisive answer to your question would be to build both and dyno them to see which gives you the best power in the rev range the engine will operate in. Given that 8000-8500 rpm isn't exactly a high rev limit, I'll stick to my recommendation.

Thank you