Please Explain Benefits of Stroking VS Destroking

The engine I'm talking about is a flathead, with 3.5" bore, 4.5" stroke, and 8.75" rods. With a .040 overbore, and .25" stroke, I was looking at 373.5 CI.- I really liked that Idea!

If I use the same overbore, and destroke to to 4.0625, I'll be looking at 326 CI.

So please tell me, what is the main benefit of each? I figure probably more RPM's out of the short stoke, less torque. This engine will be ran mostly at WOT,in a LSR car.

 

I always thought that stroking was mainly a way to get either more displacement once the bores were maxed out, or to get more displacement with better low RPM torque.

I also always thought that destroking was usually a way to get into a displacement restricted cl***. Also, for v8s that have been converted to flat plane cranks, destroking helps with the vibrations, i.e., the rotating ***embly has a smaller moment of inertia so that the vibrations inherent in a flat plane crank v8 are less severe. The smaller moment of inertia on a flat plane crank of short stroke is obviously from the smaller stroke as well as from the lighter counterweights. Flat cranks are known to rev quickly; good for road racing.

I am only magazine smart, though. Some of what I said may be a misperception, or the repeating of myth.

 

To quote Tex Smith, regarding strokes of 4 1/4 inches and more: "...the rod angles become severe and the piston speeds too high for practicality. The rings and the wrist pin must be moved so far up on the piston that stability in the bore becomes a problem."

I think I read somewhere else that every 1/8" of additional stroke lowers the maximum revs by 500 rpm.

 

All good thoughts. If it were me I'd worry more about induction,exhaust,timing and lubrication more than displacement. Use the best aftermarket rods,pistons, valves and springs with a cam that lets it breathe at high RPM. A high rpm flat head is mostly about flow, lubrication and spark. Also I ***ume you mean 273 and 226 C.I.

 

for a given cubic inch less stroke = hp , more stroke = torque, Larger bore for unshroding valves. if longer stroke is used generaly better to use longer rods. This is all real general. Roger

 

STROKERS, PROS AND CONS
What are the advantages of a longer stroke?
Are they real or just hype?
These questions are often asked but the answers are not always as straight forward as we might like.
It is generally thought that the ideal bore stroke ratio is 90%. That means that the stroke should be 90% of the bore. This combination gives a reasonable amount of piston travel per degrees of rotation without being excessive. It also allows the bore to be large enough to carry valves of sufficient size to properly feed the engine. A Hemi, because of its inclined valves, can cheat this a bit but in a wedge it is a severe limitation. If the 90% rule is followed then all should be well. To properly understand this, one should ask, "Are there any disandvantages to a short stroke?"(Looking at the opposite problem will often clarify a point) The answer is yes. A short stoke motor requires a high dome to obtain a high compression ratio which comes with its own set of problems. Sometime the dome is so high that flame travel in the cylinder suffers. The piston does not move away from TDC as far (travels a shorter distance per degrees of rotation) so it may be plagued with valve to piston clearance problems that will be difficult (but surmountable) at best. It will have to operate at higher RPM to obtain the same HP because it will be smaller in cubic inches than say the 90% motor. RPM is expensive. Valves ,valve springs, cams, retainers , pushrods etc etc must be of the very best (read expensive) quality to survive this. If the engine is only making the same HP as the larger one can at a lower RPM, then all one has accomplished is to build a difficult ,expensive, high maintainence "gernade". However, the same problem exists at the other end of the scale. If the stroke is too long then the cylinder heads cannot keep up with the sheer volume of air being pumped . Often a stroker will produce the same power but at a lower RPM. Is this because of the torque characteristics of the long stroke, as some would suggest, or is it because the cylinder heads have reached their peak at a lower RPM than would have happened on a stock stroke engine? In many cases the latter is true. Also in a long stroke engine the rods are stressed as piston speed in feet per minute increases to the danger point. Even a high dollar engine with the very best parts will require constant replacement if pushed to those limits. The best reason to built a stroker is because you want one. What are the guidelines? Like we said at the start. Stroke should be 90% of the bore and no more. This is not the total sum of the story, by any means, but these are some of the things worth thinking about before you part with your gold.
I wrote this several years ago . It is directly from my web page and it is entirley mine so i can share it . Several years later i still think this way.
don.

 

ford flatheads can use some stroke, but not everyone does it...ive got one motor that will be over 300ci. but im not sure the cost is worth any benefits in context to LSR. the cad has plenty of stroke from the factory. your goal is to be pulling (rpm) at each mile marker... 1-2 miles for the short/combo course.
id focus on improving any known shortcomings of flow concerning the flat cad, wouldnt worry about a stroked crank...you need rpm....camshaft, ports/valves, induction... blower would help overcome some of those flow issues. drag racing on the other hand is a different deal... torque/stroked crank etc etc.

hows the car build coming?

 

Thanks guys!

plan9, I received the 4-link parts, and will have pics and updates later this week.

 

What are you trying to do.....make as
much horsepower as possible, or make
as much horsepower power *per cubic
inch* as possible? For a given displacement
a larger bore and shorter stroke will generally
produce more power per cubic inch.... but the
same bore with a longer stroke will yield more
cubic inches and make more total horsepower.
If you're limited by cl*** rules to a certain
displacement, you generally want the the
biggest bore and the shortest stoke that
will bring you as close as possible to your
cl***-mandated cubic inch limit without
going over it. If you don't have a limit, a
bigger engine will almost always make
more power than a smaller one, regardless
of the stroke, simply because it has more
displacement. Back in the late 60's, for
example, Chevy didn't develop the 4 inch
bore X 3 inch stroke 302 Z/28 Trans-Am
race engine because it made more power
or revved higher than the 4 inch bore X
3.48 stroke 350 they already had. They
made the 3 inch stroke 302 because the
Trans Am rules limited them to a maximum
of 305 cubic inches. The 302 did rev hiigher
than the 350, but it also *had to rev higher*,
just to make the same power of an
equivalently carbed and cammed 350....and
a 350 built with more cam and carburation
to turn the same revs as a 302 would have
made even more power.

Mart3406
======================

 

So what if you bolted on a T-5 trans in back of that 302 with 3.35 first gear ratio instead of the 2.20 from the Muncie??? Wouldn't that trans work better for the lower rpm range for a chevy 302 motor??? Seems that Ford made it work with the 5.0L engines of the late '80's and early '90's.....

 

There is no subs***ute for cubic inches. Or so I have heard for years. Plenty of old Fords run pretty good with a much longer stroke than bore. Dave Dozier's '39 Chrysler straight eight 'liner went 250 on the salt and that was some long arm. With a 500 inch Hemi crate motor he picked up a little more than 50mph. A 326 inch motor in a cl*** that runs from 325-375 is crazy. All unsubstantiated opinion.

 

Food for thought. Nascar runs as big a bore as you get 4.125 + and short stroke to come up with 358ci. 850+ hp they have something figured out so bigger bore = more air bigger valves Just a thought?

 

That's good if you are starting with a clean sheet of paper. Buddy is starting with a dirty old Cad flathead. 4 1/8 inch bore aint happening

 

Nope, I mean 373 and 326- It's a Caddy flathead.


Guess what I'm looking at here is piston speed, and sideload on the pistons. The stock 4.5" stroke is long for any engine. I want power, and I want the bottem end of the engine to stay together.

 

Yes 4.5 is a pretty long stroke, but look at the mountain motors used in Pro mod and AHRA Pro Stock. 800 cubic inch stuff. Very long stroke. Plenty of RPMs. Usually on an 1/8 mile track. But still. And you might also look at how far your crankshaft centerline is below the wrist pin at BDC. Newer engines like the SBC and SBF tuck the crank right up there to make a more compact package and save iron. Nobody ever heard of anything like that at Cad when you block was made. Your rod angles are not nearly the same as if you had a new V8. I am more interested in how your center main bearing will like all those forces and your front will like driving the blower. Only one way to find out. Put the face shield down and leg it.

 

Well, Rich, I hope you're right about the long stroke! Mains are helped by a full crank girdle, and the front bearing further aided by a snout crank support.

I just hope 366 CI and a huffer can pull 5500 rpm in 4th gear!

 

i destroked a 350 sbc using a large journal 327 crank. brings the stroke from 3.50 down to 3.25.

Why?

Cause if im gonna have a sbc, its gonna rev to the moon.

should be a real fun motor.

 

Correct. I always saw de-stroking as less reciprocating m*** which = more revs.

Bob

 

see, i always thought that it was less piston speed = more revs.

 

all i know is id rather be stroked, then whatever the hell de-stroked is.
it just sounds nasty.

 

Did you use the same rods and pistons? I've built SBCs and BBCs, but never swapped cranks. I'm ignorant about how to put a 327 crank in a 350, so school me!

 

i used 5.7 rods and 327 pistons.

Make sure to use the LARGE JOURNAL 327 crank. All 350s are large journal. I lucked out with a steel crank too.

I now have a 4 bolt main 327 that pulls to 7100

 

Thanks for the info.

 

no problem. I also know that you can destroke a 327 to 301 using a small journal 327 block and a 283 chevy crank. Gives 4 inch bore, 3 inch stroke. Not sure of the rods. I think you can shop around and find pistons to match. Might get away with 5.7 rods, but i think it depends on the pistons

I need to find a small journal 327 block, then ill let you know

 

Been there, done that. Since Chevy also did the same thing in large numbers, stock 302 Chevy pistons work just fine.

 

You could also use the 307 crank in a 350 to come up with a 327... Or a 331 with .030" over 327 pistons.. Just make sure you get the crank balanced! And try to keep the revs below 6500 rpm... Yeah right..

 

Just to clarify, you mean a high dome on the piston?

 

Hi Don very good explanation.

 

Unless I missed it, the most important question has yet to be asked. You are racing this right? Will both motors land in the same cl***? Or does it move down a cl*** with the smaller motor?

 

Both engines would be in the same cl***-**O at Bonneville.