Rod length? Piston speed?

[thewrench3]

I run a four cylinder drag car myself. I have a 3.126 stroke and I changed from a 5.205 rod length to a 5.7 rod length. The 5.7 rod is 105 grams lighter and uses a small rod bearing diameter. The piston is 225 grams lighter and uses thinner rings. The longer rod engine makes more power up high- above 5000 rpm. the short rod motor makes more torque at 2700 rpm to about 3700rpm then the long rod engine makes more. The long rod engine is more particular about the cam timing. Overall I'm happy with the long rod engine. In the car the long rod engine runs a quicker et and more mph. My combination maybe different than yours the car is 2400 lbs. with me in it at race weight. Just talking from experience here.

[mattclow]

Look at all the used NASCAR stuff for sale on eBay...... Big bore, short stroke, long rod. Piston weight seems to be the priority. They spent a little money on research and development. So you know they've figured something out. Mind you, all of the secrets won't make it to ebay until it's common. So there could be a secret short rod combo... But I doubt it.
I could be comparing apples to oranges. But you can't ignore decades of homework.

[Adger Smith]

Well, you sure took the rod length at face value. There are "other" parameters in cup engines that have an affect on the "total" engine package. Just like in F-1. Yes, they are very well designed and tested for the purpose they are built for. If long rods are the answer then why did most of the 358 cu in PST engines (making well over 2.5 HP per cu in N/A) use very , very short rods and BB pins?

That's the question I'd like to know the answer to. What is the advantage to a short deck block and short rods in a modified or comp engine? It has to do with the 9,000 to 11,000 rpm range...

[Adger Smith]

Intake manifold design

[Bill Baer]

I believe the thinking behind high rpm short rod motors is that the piston accelerates away from TDC faster which helps initiate intake flow plus you have the benefit of the lower piston / rod (rotating) weight. It appears to me this works well if you have adequate or more than adequate air flow but it’s my guess is that you need to keep these motors buzzing in a narrow RPM band.

Adger, you mentioned intake design....Are you referring to mainly having a sheetmetal intake? Can you elaborate a little more? And at what RPM ranges do you think responds better to the short rod/short deck configuration?