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Discussion Starter · #1 ·
I know there are ratio's that seem preferable for either rev's or others that generate torque, but beyond that there just seems to be some sizes thta work better. Like 4.25 bore in a 383 and a 426. Just seems to be the right size. The 400 has the same stroke but that extra width of the bore....am I losing it or is possible that it dosent help. I know at 100% efficiency bigger bores and longer strokes (to some point) would be best because you are filling the cylinders to 100 % efficiency. But in the real world you are going to get that so I think certain sizes just have a natural advantage...ie suction or vacumn or whatever. I have had 350 chevs and a 400 chev sb and the 350's were just plain better. More power at the right place or something. So I guess I am asking...all things being more or less equal inside like stroke , compression ratio, weight of assembly, etc., is it possible that the 383 would make more power than a 400? huh? would it?
 

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NO. What your overlooking is rod ratio's and air flow and other things. I could write a page why the 350 seemed stronger then the 400. There might be certain size engines that make more power per cubic inch but that don't mean it will all around producemore power then a bigger engine. Cubic inches will always rule. Back in the 50's Chrysler already had it figured out that stroke being 92% of the size of the bore made the best power. This paticular article I read illustrated the ratio's of all the mopar engines verses GM and Ford and Chrysler stayed within 10% of that and the original Hemi's stayed less then 5% of that.
 

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Mopars use big rod ratios, all of them are better suited for high rpm use, the 60 is the only 1 that is nuetral in that respect.
This is why when doing the 440 crank in the 400 block you use the 400 rod not the 440 rod, otherwise all you did was build a 10 inch bigger 440 in lighter block with a slighlty less weight piston, and to get technical all you really did was build a 4inch bigger motor, without changing any real power bands
 

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the bigger the bore the better the heads can breathe
the cylinder walls have less interference to the airflow
a 400 chev is a far better motor then a 350 chev
Chev just didnt build a hipo version like they did for the 350 327

look at prostock NHRA engines, huge bore very short stroke
pretty sure Nascar is same deal within the rules
I think the Nitro motors are the exception as the fuel carries so much of its own oxygen the valves dont really matter as much
and being Hemi the valves move away from the cylinder walls by design as they open
 

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Actually the bigger the bore just means the valves are unshrouded, it doesn't have as drastic an effect as rod ratio does, the 400 chevy is only better than the 350 do to it's massive and drastic rod ratio which is like 1.4, the 327 is a big RR of 1.75 making it a rpm motor.
The same problem the 18 has is a 1.85 RR and is a rpm motor so is the 73 and the 40, the 60 is a 1.71 RR and is border line lower rpm motor and can benefit from bigger ports and better intakes more so than the others unless your going to really spin them otherwise they stick to the "small port velocity" to keep torque theory.

Pro stock motors are very technical and they launch high in the rpm and run on hp, hp is upstairs rpm and they leave on hp and they stay on hp with there gear differences keeping them within i think it is like a couple hundred rpm from start to finish.
For us anything pro-stock isn't worth getting into
Fuel motors on the other hand have a long stroke and stay at 10,000 rpm from start to finish, the regular Hemi's need rpm to utilize those heads or a big stroke, otherwise there a rpm motor also.

This is why i always push the strokers to move the weigth, this is also why i disagree with the big port theory for torque it is wrong in these cases.
 

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I do not believe the rod ratio theories that abound
long rods help restricted intakes
motors with good flowing intake tracts use short rod
you say long rods are good for rpm yet 500" Prostock engines use deck hieghts around 9"
you will never see gains from rod ratio (if there are any at all) that you would from bigger bore or better heads
 

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Actually the rod ratio is what makes the need for port size, port size and shape and flow are less with long rod motors at low rpm, reason for it is look how long a piston hangs at top dead and bottom dead before moving, less movement more hang time = no air movement, so for it to get good movement, flow and hp we need to spin it high in the rpms, now you run into piston speed problems so now you make a tall deck motor put in a big rod.

motors with good flowing intake tracts use short rod
Yes short rod motors are better with big intakes and head ports, the reason is the pistons have less hang time at top dead and bottom dead, there pulling in more air then a long rod motor, here take a 40 motor with a 1.85 RR and compare it to a 60 with a 1.71 RR or take the 451 and compare it to a 440 and use the 451 set up with the short rod vs the 440 and the 451 spanks it, makes more torque and carries it better through it's midrange, all you changed was the rod ratio.
You put in 1 400 block the entire spining assembly of a 440, in the other 400 you put in the crank only and used the 400 rods, only difference is the RR, and now you know why everyone likes the 451.

RR plays a huge roll in a motors ability to make power and where, remember RR effects piston speed and dwell time at top dead, you want fast and strong cylinder filling and low rpm power you build a stroker and go with a low RR, you want high hp like a pro stock motor you build a long deck block install a very long rod and spin it very fast, like i said earlier forget the pro stock motors here, there is nothing useful about a pro stock motor.

Heads and intakes are designed around a motors rod ratio or where and how it is used
 

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you want high hp like a pro stock motor you build a long deck block install a very long rod and spin it very fast
I will say it again
this is exactly the opposite of what Prostock motors use
they use extremely low deck motors, 9" big block deck hieghts
if the tall deck thing were true then Fords and Dodges would have an advantage with their over 10" deck hieghts
rod ratio importance is a myth
 

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Ok RR is useless, everything is heads, have you ever seen the inside of a pro stock motor?

Leave anything you heard about pro stock motors out of this because if you haven't seen it and are listening to what people are telling you, im telling you they aren't going to tell you anything true.

A pro stock motor is so altered from any factory design it's not even funny.

RR plays a huge role, if you don't want to listen or believe then go try it.

RR and heads work together, a big port head on small RR motor will make more torque than a big port head on a big RR motor, to compensate you use a smaller port head or round port rather than large rectangular head port design.

Trust me RR plays a huge role, forget things you hear about pro stock motors.
 

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ok we can use SCCA Transam motors if you wish
again the deck hieghts are cut down, to SBF hieght of 8"
Chev cast a special block to be competitive with the short deck Fords
now those are 2 naturally aspirated, unlimited funds, motorsports
both using the shortest decks they can to the point of having custom blocks created

RR plays a small role, it can be used as a crutch if the rules you run under require a restricted intake tract.
thats it IMO
to spend any money changing the rod ratio of a street motor would be a complete waste of time
the money and effort could be spent elsewhere with greater results
pretty much cannot beat money spent on heads that match your expectations or more displacement
 

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mj said:
ok we can use SCCA Transam motors if you wish
Ok
again the deck hieghts are cut down, to SBF hieght of 8"
Chev cast a special block to be competitive with the short deck Fords
Ok so what does the deck height have to do with RR, do you think because a deck height is low they can't use a long rod, have you ever seen the piston ring spacing on these motors, the thickness of the rings.

both using the shortest decks they can to the point of having custom blocks created
Using the shortest block they can to install the crank and rods needed to make the motor perform to where they need it to perform without making the piston weak.

RR plays a small role, it can be used as a crutch if the rules you run under require a restricted intake tract.
thats it IMO
RR plays a huge role, forget rules, who has rules on RR's, rules on bore spacing, rules on bore size, rules on max disp, rules on max min strokes, rules on max and min deck heights, rules on head styles and port shapes, but there is no rules on RR's.

to spend any money changing the rod ratio of a street motor would be a complete waste of time
the money and effort could be spent elsewhere with greater results
pretty much cannot beat money spent on heads that match your expectations or more displacement
I don't know how else to explain this, yes heads make a big difference but what is it that makes the heads work?

Ok lets say you want to build big torque, are you saying your gonna use a 318's lower end specs? and then add W-9's? and a huge intake? a RV cam? and you think that is gonna make a awesome torque motor?

Or are you going to change the RR so it can use the heads properly to get more air in the motor and change the torque curve in the motor to work lower in the rpms to make more torque?

Let me ask you a question have you built a stroker motor before?
What was it you liked about it?
What was it that changed?

Compare these 2 motors, get a 440 take out it's internals and install it in a 400 block (using the correct piston) now if you didn't bore it you just made a 444 now take another 440 apart again throw out it's rods use the 400 rods again you just built another 444 let both of them use INDY heads since you like big heads and the same compression, and set the pistons the same in the bore, same cam, same intake, same carb.

Then get back to me about how the short 444 does it sooner and better than the long 444

Then i'll tell you what it was that is different, and by how much between the 2, and then tell me about the myth about RR's

RR's play a huge roll.
Huge heads either work well in a high rpm motor or small RR motor.
Huge heads don't work well in big RR motors.
 

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Also RR plays the role in head choice for the intended use for the motor and also dictates the cam for such use, soon im going to do a test that expells the theories on velocity and proves quite a few points
 

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Discussion Starter · #13 ·
I do remember reading in the Big Block Mopar Performance book that the primary reason for the 440 crank / 400 block stroker combo is the lighter piston weight. ut I also remember the 383 and ?371 combo's possible from the 350 / 400 cheby's depending upon the rods primarily if I remember right. One (383) was considered a torque motor and the other was considered a easy reving engine. 383 is popular but the other isnt too common.
 

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Sure you will lose piston weight by going the entire rotating assembly of the 440 in the 400 block but you also lose the weight of the block, but that isn't the reason you use the shorter rod, like i said go and do both and you tell me what happens.

If you look at how the stroke and rod lenght interact with each other to make the rod ratio and how it translates into how the piston will move in the cyl and how it will dwell at top dead and bottom dead center you will understand how to put everything together for a said motor to do a particular thing perfect.

Piston speed is what makes the head work, so if you have a small RR the piston is moving faster and you can use a big port head and it won't kill the low end like it would on a motor with a big RR.
IT will fill the cylinder much better and make more power than it would if it had a big RR at low end since that piston isn't sucking nearly the same or as fast as the small RR motor (small port head), so if you can start sucking in fuel and air sooner at a greater volume you will make more power at a lower rpm, that is why Some bore and stroke combo's are just better
 

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Low reving motors are short rod motors with a low rod ratio.
1.45 to 1.75

High reving motors are long rod motors with a high rod ratio
1.75 to 2.1

So if you are interested in low speed power and lots of it you see what you want, and with the low end power in that RR you can use bigger ports and bigger cams.
 

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Ok so what does the deck height have to do with RR, do you think because a deck height is low they can't use a long rod, have you ever seen the piston ring spacing on these motors, the thickness of the rings.
a taller deck makes it possible to get the rod ratios you are recommending, not possible with the short decks that are actually being used

why are there no rules limiting rod ratio? well because it really doesnt matter what ratio you run would be my interpretation

prostock IHRA mountain motors use rod ratios around 1.3:1
I hardly consider them low rpm

rod ratio just isnt as important as you and many others are trying to make it seem.
make the engine as large as possible for more torque and ignore the rod ratios
 

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I don't know why your stuck on these pro stock and mountain motor theories, and i don't know who is feeding you this information about RR being useless.

rod ratio just isnt as important as you and many others are trying to make it seem.
make the engine as large as possible for more torque and ignore the rod ratios
Well you can only bore a motor so far and the few inches gained aren't giving you the increase in torque, it's the stroke decreasing the rod ratio giving you all those torque gains you feel.

Bottom line RR is important, i don't know who is telling you it's not.
 
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