Let's talk about stroke vs dwell

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clutchdust

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It seems to me there's a couple of competing philosophies regarding displacement. Of course, the really popular thing to do today is put a "stroker" kit in a rebuilt engine, thereby increasing displacement. The age old adage, "there's no replacement for displacement" holds. However, I read Smoky Yunick's book "Power Secrets" where his testing proved that given the same displacement, a shorter stroke will yield more dwell time at TDC, which results in higher cylinder pressures and more power.
Now I get that we want the most displacement that will fit within the package we're working with. However, let me ask this question. Let's say you were building a race motor, you wanted it to make a lot of power but it still had to hold up (as in a road race motor instead of a 1/4 mile motor). And to complicate things, you were competing in a category that limited you to, say, six liters, or approximately 365ci. So with that in mind, would you look at taking a 350ci (gen1) or 5.7 (gen3) engine and stroking it? Or would you look to take a 400ci (gen1) or 6.2 (gen3) block and destroking it?
Keep in mind the goal. Within the class displacement limit, you want to build the most powerful engine you can that will hold up under long overhaul intervals.
 
Given what is above, from everything I have heard over the years...DEstroke for race....

Stroke for street....

:mobeer:
 
I always heard as a kid that "square" engines last the longest.(Bore=stroke):quote:
 
A longer stroke has more degrees of crankshaft rotation at both bottom and top dead center. The crank pin has to swing more degrees of rotation before changing direction. Displacement is unrelated to this part of the question. A long stroke works to build low end torque and it's "pull" on the intake tract when the intake opens is helpful at low rpm. At high rpm the piston is actually in the way - the valve is opening but the piston has not started to move away from TDC.

Everyone has an opinion...but if I was going racing with a reasonable redline to keep parts in the engine I would buy the best cylinder head I could afford. Then get the cam to support that rpm/air flow. Then build the engine around that with the money left over!

I would build the largest bore I could then tailor the stroke to keep within the displacement rules. A large bore allows the biggest valves to flow the most air. In a road race engine torque in mid to high rpm is king and you get that with good cylinder filling in this rpm range.

383 strokers were a huge rage in the days of TPI - super long runners with 400 ft-lbs at 2,200 rpm because the old L98 heads and intake run out of steam by 4,400.

Just one guys opnion! Pile on the opinions too!
 
Depends on what you want. I have a destroked 400 with very long rods. I'm trying to get my dyno to work and then I will tell you how the thing runs.

I had it running for a short while when assembled and it would rev real easy. That's about the only thing i can tell about it.

Anyway, about low end torque, remember that you can always go to a higher numerical rear end, which would enhance torque...of course a 5 speed is almost mandatory to keep top speed high enough.

A car is not just the engine its all about the combination.
 
Of course Smokey can be only right !!!!!!!!

He talk about "POWER" wich is not torque!
To have more power you really need very big bore with small stroke..... in order to reach higher RPM.
With bigger bore (assuming constant the Average Pressure in the chamber) you will have more force (the force is equal to pressure X surface....).
Of course the lever you will utilize to apply that force is smaller (less stroke, the stroke is the double of the lever) but you can apply that force many more times in the time unit.

A small 2.4 liter F1 engine can rev 19000 RPM (limited by rules) with a power of some 900 horses!

An other important parameter is the stroke/rod ratio...... allways longer rods are preferred because their ability to work with more effective angle with the crank and the lower side load on the cilynder walls.

Of course..... power is for race and torque is for street!
 
An interesting discussion to be sure. I've always thought of torque as kind of the density of the horsepower, or the ability of the horsepower to do work. Even if you can make 900hp but you have to turn 19000 out of a small displacement engine to get it, what kind of torque is it making and when? It would seem you'd have to keep the engine in high revs all the time to stay in the power band. Plus I'd imagine any failure at those kind of revs would be catestrophic. (I confess I really am ignorant of the small displacement high horsepower engines) If you can get 900hp with half the revs, doesnt' that give you a more useable power band?

I do agree torque seeems more applicable on the street, therefore I tend to lean more towards the torque side of things. I think it was Carroll Shelby who said "Horsepower sells cars, torque wins races".
 
big2bird said:
I always heard as a kid that "square" engines last the longest.(Bore=stroke):quote:
Click to expand...

HUMM....I can say one thing a old Poncho 400 engine would run forever with any decent care....so would a 350 poncho.....

:clap:

and YES, torque moves the freight, ask any locomotive driver....
 
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Guys, keep in mind this is just a theoretical discussion. I'm just kind of playing with the idea. It's like why Ferrari can make 500hp with a <5.0L engine that turns 8000 or 9000 rpm and Corvette makes it with a 7.0L that turns ~6500. I don't even want to get into forced induction!:goodnight:
 
When looking at a Oscilloscope, in electronics with all that waveform stuffs, in movies i'ts always a sine wave that's displayed for some reason...looks like a bunch of U shapes alternately inverted and upright across the screen scale...

those curves can represent the power, as in power under the curve, I look at it like that on a dyno chart readout...

power is under the curve...the more area under the curve the better....

if you not into 'lectronics, maybe someone else can 'splain it better....

:cool:
 
Let DCR Decide?

I first heard of extreme rod/stroke ratios in F1 cars. I realise that both methods will work, if the parts chosen compliment the build. For years many have said that the Chev 327 was the best engine 3.25 stroke/5.7 rod.
I built my sb w/6" rod. I get best power w/32*, 19* initial works fine.
I have never had any ign issues w/8.4 DCR and 91 oct.

I think the real test would be 2 engines, long/short rods both w/very high DCR.Then see which engine will take the most ign advance and make the most power? If that does not show a real winner, reduce the octane of the fuel and see what happens?
Of course a 7K sb is not the same as F1 screamer. This subject is beyond my experience, but I would like to see some comparison testing.

R
 
Ok, my engine has been calculated at desktop dyno to have some 450 lbs of torque and 500 hp which with a peak hp around 6500 and a torque peak around 5500.
(remember this is a 354 ci)

What would a 400 ci do regarding torque, all things being equal, it would not rev easy to 7000. (Maybe it could, but with some really large cam and exotic large runner head.)

My guess is that it could easily produce something like 550 lbs of torque and 550 - 600 hp.
Typically with a torque engine you would run a 3.36 or longer rear ratio. so, in fourth gear, this would be 550 lbs * 3.36 = 1848 lbs of torque at the wheels.

The high revving 354 that I made, will be running a 4.11 in fourth this would mean 1849,50 lbs of torque.

The difference is on my side, although very small.

In the lower rpm's there would be a slight advantage with a long stroke engine.


Of course, my top speed would be low, compared to a 3.36's, but i could run a five speed

Anyway. It's up to personal preferences, and I like running a more free spinning combo that can go from about 2000-7500 with lots of room to accelerate an rip through the gears.
 
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Ricisan said:
I first heard of extreme rod/stroke ratios in F1 cars. I realise that both methods will work, if the parts chosen compliment the build. For years many have said that the Chev 327 was the best engine 3.25 stroke/5.7 rod.
I built my sb w/6" rod. I get best power w/32*, 19* initial works fine.
I have never had any ign issues w/8.4 DCR and 91 oct.

I think the real test would be 2 engines, long/short rods both w/very high DCR.Then see which engine will take the most ign advance and make the most power? If that does not show a real winner, reduce the octane of the fuel and see what happens?
Of course a 7K sb is not the same as F1 screamer. This subject is beyond my experience, but I would like to see some comparison testing.

R
Click to expand...
See, that's kind of what I would like to see. Of course, I don't have a budget to build two engines just to test it, that's why I'm posting here. Trying to get as many different voices of experience as I can.
I think the key though is a displacement limit. Just as the old adage says, the larger the displacement, the more power you can make with it. Kind of like driving a nail with two different hammers I suppose. You can drive a nail with a 16oz hammer in 6 or 7 whacks, or 2-3 whacks with a 32oz hammer. difference is the wear on the arm and time, or something like that.
I'd like to see a comparison like what you described but set a displacement limit, maybe 5.0L or 6.0L.
Kind of reminds me of growing up in the 80's. The 5.0L Mustangs would give 5.7L Camaros, and Vettes, fits. Despite a nearly 50ci disadvantage, they made as good or better power. Not sure if it was in the heads or a bore/stroke issue. But I've heard the same thing about the 327 too. Of course, all the people who remember the Yenko 302 Camaros said those were screamers too.
 
:mobeer: I've had this discussion probably a 1,000 times since I was a kid in the 50s.

One of the car magazines did a test with short stroke big bore and a long stroke small bore engines of the same ci. There wasn't enough difference to call it.

I've always built engines with the idea of a least a 4 inch bore and a smaller than 4 inch stroke.

Over the the years I found bores of over 4.125 inches to allow for valves big enough to get power up in the and over 500 hp for a good street engine.

My 3 cents.
 
There was an old article about something like this. It was called "The small block chevy should have built". Do a search on the net.

As I remember this engine produced lots of power/torque with high CR on low octane gas.
 
I'd go with a 4.185 bore and 3.25 stroke, 6 to 6 1/4" rod length, big heads and cam to feed it well and spin it to 9000. Expect 800 hp and 800 miles before a rebuild.

(SB2 nascar road race engine configuration)

Just get out the wallet and give the Hendricks engine shop a call :phone:
 
Ozzy said:
I'd go with a 4.185 bore and 3.25 stroke, 6 to 6 1/4" rod length, big heads and cam to feed it well and spin it to 9000. Expect 800 hp and 800 miles before a rebuild.

(SB2 nascar road race engine configuration)

Just get out the wallet and give the Hendricks engine shop a call :phone:
Click to expand...

Does it come with a semi truck too?:lol::lol:
 
Looking at the BBC, I've noticed that the 4.6 bore blocks with a 4.25 stroke seem to make some decent power- both HP and TQ. Some of the reason may be that those big valves are not restricted by the bore. If you look at the BBC block, many of them have a relief cut into the top of the bore so the intake valve has more area around the outboard side. Getting the valve, as it's called, "un shrouded" seems to be the key.

On the flip side to that, it also seems possible to unshroud a smaller valve, but then it makes it near to impossible to get that all important airflow through it.
 
I read that article and it was pretty good but it still didn't address exactly what I was saying. It was talking about conventional production engines built by every day street rodders and comparing the "shit to worth" ratio of the additional stroke. While still informative, I still am curious about the limitations. Like I said, and especially with a lot of sanctioning bodies, what if you have a size limit? For instance, if you are building an engine specifically for a class that has a displacement limit of 5.0L. Now, to me, it just makes sense that you would like the largest bore you could get for the reason Tim mentioned. And while the author mentioned piston speed and dwell, he was talking about relatively modest differences. What if you were to increase those differences noticably? So say with a 5.0L limit, you had a 4.030" bore and a 6" rod, what would the stroke length need to be to achieve 5.0L? Now, what would the dwell time and crank rotation degrees be comparable displacement engine with, say, a 3.75" bore with a 5.7" rod and 'x' stroke?
 
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