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Author Topic:   Rod Ratio ?
Kromulous
Member
posted June 15, 2004 09:10 AM
How do you figure up rod to crank ratio's? rod length divided by the total stroke of the crank?

Example: 6.00 / 3.48 ? = 1.724

I have heard alot of engine builders say the perfect ratio is between 1.8 and 1.9, now when i figure up the ratio's on some popular engines such as a 355, 377 and 406 i notice the ratio is quite low.

Now what exactley is the benefit of getting your rod ratio in this magic window so to speak? Is there any performace gains to be had?

One engine did figure up really good by the way, a 358, a 3.25 stroke crank (327 size) and a 6.00 rod, figures out to be 1.85 which by what i have read should be very good.

Although to me a 3.25 stroke crank would be rather weak knee'ed (Not alot of torque) coming off the corners but maybe thats the key to the rod ratio.

Anything i'm not figuring right here?

outlawstock17
Member
posted June 15, 2004 01:37 PM
.....according to an recent article by david vizard, 1.9 for a restricted engine and 2.2 for an unrestricted engine. my thinking is "unrestricted" means forced induction...


dirtbuster
Member
posted June 15, 2004 01:41 PM
by 'unrestricted' i think they mean not required to run something like a restrictor plate, or small carb, intake etc.


outlawstock17
Member
posted June 15, 2004 01:45 PM
i'm not so sure about that dirtbuster. most nextel cup engines are around that 1.9 number......even the unrestricted engines.


Kromulous
Member
posted June 15, 2004 03:19 PM
to get a 1.9 ratio your stroke could never get ove 3.30 or so, and your rods would need to be 6.2 or better. Seems kinda extreme to me.

Thats my question thou, is this ratio a good thing to consider when building a modified or late model engine ? to me it seems un-attainable in most realistic applications.



outlawstock17
Member
posted June 15, 2004 03:54 PM
yes, 1.9 would be nice. i believe F1 and indy car engines are in the 2.2 rod/stroke ratio range.....and although i don't know much about motorcycles, i'd guess that some of the japanese "crotch rockets" have rod/stroke ratios that are getting up there...

a 302 chevy has a rod/stroke ratio of 1.9.....now you know one of the reasons why they run so good.

[This message has been edited by outlawstock17 (edited June 15, 2004).]

stockcar5
Member
posted June 15, 2004 06:10 PM
if you look at some of the used nextel stuff on ebay you'll find that most of their cranks are in the 3.2-3.3 range and their rods are 6.2-6.3. so it looks like they are shooting for 1.9 also.

luke

------------------
www.geocities.com/dirtstockcar5


bigcityracer
Member
posted June 15, 2004 08:20 PM
They all seem to be shooting for 8,000 thru 12,000 rpms too. Would get stuck on it too much.


Kromulous
Member
posted June 16, 2004 12:28 PM
Not really stuck on it, just something for me to think on while bored at work.

Question, whats the best running engine out the follwoing:

358 (400 blk, 327 crank)
377 (400 blk, 350 crank)
9/16' motor (3.5625 crank) 363 cubes

Out of these three, with comparable parts what do you all think would be the best running engine in a mod?

I know the 377 is popular, at least down here in the Cincy Ohio area, not sure about elsewhere.

Just looking for opinions.

dgb
Member
posted June 17, 2004 08:16 AM
A 377 with a 6.125 rod has a 1.75 Ratio. Which allows you to run a low comp height piston.

I'm thinking this combo would be best out of the 3 myself. Trying to talk myself into building one of the three.

By the way the 358 has the best rod ratio at 1.85 with a 6.00 rod, and the 9/16'ths motor the worst at 1.68 with a 6" rod.

Considering the stroke and rod ratio i think the long rod 377 would be the best, but would like to hear from someone who runs one to be sure. Wish i had the cash to build all three and find out myself.

dirtbuster
Member
posted June 17, 2004 08:48 AM
I would also agree that i feel the 377 is prob the best for most of us saturday night guys. The bigger bore lets it breathe better but you dont give up the cubes and torque to the 353(400 bore/327stroke). The short stroke 353 would work good if you could gear it to turn 8000+ all the time, otherwise I think you would get beat off the corner because of the loss of torque.


2nd2none
Member
posted June 17, 2004 01:39 PM
i had a 377, 3.48 stroke, 6.250 rods. It was an awesome motor once I figured out it needed a whole lot of gear in it (over 8000 rpms). I will be building another one this winter.


(I sold it....thats why I no longer have it )


flatfoot27
Member
posted June 17, 2004 08:50 PM
On a short track would a 406 be better than any of these combo's? I run against 350's,377's,383's and i tried a 410 this year and went from a 308 gear to a 273 in my Malibu at 3100 lbs, and I am absolutely dominating the class with 28 cars. But I have found in the last 4 years it doesn't really matter what motor I run my car works very well. I have really been gifted with one of them cars that just work. Everyone said the 410 wouldn't work, but WOW does it run and I'm only turning 5000 to 5200. But I do haft to say the 377's are really tuff to deal with. They get good tork and have plenty on the top side.


lazyracer5
Member
posted June 27, 2004 01:23 PM
JUST GOT MY FIRST 410 .I HAVENT GOT THE BUGS OUT YET.IVE ONLY RAN IT TWICE.IM ONLY TURNING 6200.TELL YALL SOON HOW IT WORKS OUT. LAZYRACER5


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