NHRA Pro Stock 500 ci bore, stroke, rod ratio...

This seems to have come up on one or two threads recently. Based on some tech articles I found online going back a few years, here are some generalized answers to these questions, NOT based upon current information.

Block deck heights under 9.0"... probably close to 8.6" or 8.7"

Bore sizes 4.70"+

Stroke range 3.5" to 3.6"

Rod ratio 1.6 to 1.7, depending upon how good the heads are (better heads used with shorter rod ratios)

RPMs... before the NHRA mandated the RPM limit (10,500?), sounds like some engines were turning 11K+, maybe even close to 12K

Interesting, but really hard to talk apples-to-apples with the majority of what's being run or built on here.

Google "NHRA Pro Stock bore stroke" and you should see a couple of articles that discussed this stuff.

Some Mopar guys seem to really fixate on rod ratios, when in reality, most of the stuff they're running it would make very little difference changing it greatly.

The interesting thing about Pro Stock engines is everyone will be slightly different in it's make up, but they are often making very, very similar power.

3.6x4.7 is what I have seen. What heads do they use on those engines? Thanks for posting Brad.

The interview w/ the Elite Motors engine guy has some interesting comments re rod ratio and head choice.

At the level those engines are, he said they were using upwards of 1.8 RR before the heads had evolved to where they could "keep up" w/ the faster acceleration from TDC of the lower RRs. As the heads continued to improve, they could lower the RR because the better heads weren't the restriction. No mention, at least in that context, of the downside of increased piston-to-wall friction w/ the increased angularity of the rods. However, given their pistons probably look like hockey pucks with valve reliefs, there would be less skirt surface area to be pushed against the cylinder wall, too.

At least this is in sync w/ the general idea I've heard that a longer RR is better when the heads are limited.

The WJ article I read was older, but confirmed some of the "where we came from" context of the later article.

I read an article last night I can't find today about a big (900 plus CID IIRC) Sonny Leonard engine and it had over an 8 inch long Rod in it and that was near a 2:1 ratio.

I've said it many times, had a pretty heated discussion with a well known PS engine builder about this very thing. Rod to stroke ratio makes a BIG difference when you are induction limited. I don't even know why that is questioned.

And the FACT is virtually every Chrylser based engine we discuss here is induction limited.

Talking about Rod ratio in a PS is a different discussion because the induction system is more than capable of feeding a paultry 500 CID at 10,500 RPM.

To make the blanket statement that Rod to stroke ratio doesn't matter is rediculous when the entire combination isn't considered.

That was my next question, what type of pistons are used in PS.

Fair enough but most of these discussions on rod ratios here revolve more around piston side loading concerns even tho every BBC on the planet has what many here consider an unacceptable ratio.

Kevin

The best ones they can get that day

Ok, but what I ment was short skirt, long skirt and what kind of pin height?

Full article: https://www.enginelabs.com/engine-t...s-the-secret-to-those-big-power-numbers/

Piston:

That article is over 4 yrs. old, I wonder what the hot parts are today with EFI?

Looks like it uses a piston-guided connecting rod set up.

Yeah, those are short.

There is a Pro Stock Tech thread in the Naturally Aspirated section over on Yellowbullet. Nick Ferri posts over there quite a bit and is surprisingly forthcoming with answers to questions and what goes into those engines.

Thanks, I'll check that out . Brad's link was very informative

Piston compression height calcs for 10.700 RB W/ zero deck height:

3.750 stroke w/ 6.760 rod = 2.065
RR = 1.80

3.910 stroke w/ 7.100 rod = 1.645
RR = 1.82

4.150 stroke w/ 6.760 rod = 1.865
RR = 1.63

4.150 stroke w/ 7.100 rod = 1.525
RR = 1.71

4.250 stroke w/ 7.100 rod = 1.475
RR = 1.67

Not "fixated", just observant...

Nick is one of the most brilliant engine builders you will meet or ever know. Man has taught me stuff you would never think of.