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Tech Talk: Big Bore or Long Stroke: Which Is Better?

 
tuffnuff
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Joined: 12/09
Posted: 12/19/13 11:39 AM

By David Reher, Reher-Morrison Racing Engines

“An engine produces peak torque at the rpm where it is most efficient.”

Recently I’ve had several conversations with racers who wanted to build engines with long crankshaft strokes and small cylinder bores. When I questioned them about their preference for long-stroke/small-bore engines, the common answer was that this combination makes more torque. Unfortunately that assertion doesn’t match up with my experience in building drag racing engines.

My subject is racing engines, not street motors, so I’m not concerned with torque at 2,000 rpm. In my view, if you are building an engine for maximum output at a specific displacement, such as a Comp eliminator motor, then the bores should be as big as possible and the stroke as short as possible. If you’re building an engine that’s not restricted in size, such as a heads-up Super eliminator or Quick 16 motor, then big bores are an absolute performance bargain.

I know that there are drag racers who are successful with small-bore/long-stroke engines. And I know that countless magazine articles have been written about “torque monster” motors. But before readers fire off angry e-mails to National DRAGSTER about Reher’s rantings on the back page, allow me to explain my observations on the bore vs. stroke debate.

In mechanical terms, the definition of torque is the force acting on an object that causes that object to rotate. In an internal combustion engine, the pressure produced by expanding gases acts through the pistons and connecting rods to push against the crankshaft, producing torque. The mechanical leverage is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC).

So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously there are other factors involved.

The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.

An engine produces peak torque at the rpm where it is most efficient. Efficiency is the result of many factors, including airflow, combustion, and parasitic losses such as friction and windage. Comparing two engines with the same displacement, a long-stroke/small-bore combination is simply less efficient than a short-stroke/big-bore combination on several counts.

Big bores promote better breathing. If you compare cylinder head airflow on a small-bore test fixture and on a large-bore fixture, the bigger bore will almost invariably improve airflow due to less valve shrouding. If the goal is maximum performance, the larger bore diameter allows the installation of larger valves, which further improve power.

A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don’t travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced. In a wet-sump engine, a shorter stroke also cuts down on oil pressure problems caused by windage and oil aeration.

The big-block Chevrolet V-8 is an example of an engine that responds positively to increases in bore diameter. The GM engineers who designed the big-block knew that its splayed valves needed room to breath; that’s why the factory machined notches in the tops of the cylinder bores on high-performance blocks. When Chevy went Can-Am racing back in the ’60s, special blocks were produced with 4.440-inch bores instead of the standard 4.250-inch diameter cylinders. There’s been a steady progression in bore diameters ever since. We’re now using 4.700-inch bores in NHRA Pro Stock, and even bigger bores in unrestricted engines.

Racers are no longer limited to production castings and the relatively small cylinder bore diameters that they dictated. Today’s aftermarket blocks are manufactured with better materials and thicker cylinder walls that make big-bore engines affordable and reliable. A sportsman drag racer can enjoy the benefits of big cylinder bores at no extra cost: a set of pistons for 4.500-inch, 4.600-inch or 4.625-inch cylinders cost virtually the same. For my money, the bigger bore is a bargain. The customer not only gets more cubic inches for the same price, but also gets better performance because the larger bores improve airflow. A big-bore engine delivers more bang for the buck.

Big bores aren’t just for big-blocks. Many aftermarket Chevy small-block V-8s now have siamesed cylinder walls that will easily accommodate 4.185-inch cylinder bores. There’s simply no reason to build a 383-cubic-inch small-block with a 4-inch bore block when you can have a 406 or 412-cubic-inch small-block for about the same money.

There are much more cost-effective ways to tailor an engine’s torque curve than to use a long stroke crank and small bore block. The intake manifold, cylinder head runner volume, and camshaft timing all have a much more significant impact on the torque curve than the stroke – and are much easier and less expensive to change.

Smile  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/19/13 12:21 PM

all the oem factories need to quit building small bore engines because big bore engines are better! i mean all street cars NEED to convert to drag race engines and that means big bores for big valves!  

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Dave632
I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 12/19/13 09:29 PM

I am glad you liked the Reher and Morrison statements.
I always thought they were some of the best when it came to
engine building.  
[[SUPPORT AMERICAN CARS, STOMP A RICE CAR TODAY.]]

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/20/13 06:57 PM

pro stock nhra championships aint passed out to idiots !!  

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tuffnuff
Moderator
Posts: 7841
Joined: 12/09
Posted: 12/21/13 07:12 PM

Icon Quotedond1965:
all the oem factories need to quit building small bore engines because big bore engines are better! i mean all street cars NEED to convert to drag race engines and that means big bores for big valves!

Now I know you don't mean one word of that post.,. you're saying that with tongue in cheek.,. Dave, dond just got busted,,,,,,, chuckles.

Grin  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

Post Reply
dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/21/13 07:30 PM

Icon Quotedond1965:
all the oem factories need to quit building small bore engines because big bore engines are better! i mean all street cars NEED to convert to drag race engines and that means big bores for big valves!

Icon Quotetuffnuff:
Now I know you don't mean one word of that post.,. you're saying that with tongue in cheek.,. Dave, dond just got busted,,,,,,, chuckles.

Grin


Shocked  heck tuff what would make you say something like that?!?! honestly i really do have great respect for them, but the grump was my bow tie hero. Smile  

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tuffnuff
Moderator
Posts: 7841
Joined: 12/09
Posted: 12/21/13 07:49 PM

dond, many years ago in the early 80's, I ran a 327 SB (out of our gasser) in the unlimited class in 4 wheeling.,. Olympia WA.
I arrived late after a long haul and made the last run of the day.,. lo and behold, I qualified second,,,,, against all the BB Chevy, Ford and Mopars.,. with a well ironed out lil mouse.
I ran a Ford toploader 4 gear.,. launched off planks in 2nd lo range.,. shifts were just under 9000 R's and the paddle tires were grabbing big arm fulls of gravel and dirt.
I had no idea where shut down was and when I saw those huge redwood trees ahead, I geared down on marbles, or so it felt,,,, the tach touched 10 K.
I was very lucky to walk away from that close call.
There you have my view of what can be done with a hi winding SB.
Jenkins was very innovative and knew how to make power.,. great guy.  
When The Flag Drops.,.

tuffnuff

The Bull ***t Stops.,.
tuffnuff

P. Engineer, Engine Builder

Post Reply
dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/22/13 12:45 PM

the real question should be..... big bore or long stroke, which is better for a 2-valve racing engine, not which is better?!?! not to be insultive but the heading is a bit judgemental and misleading. Confused  

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joe-
My first time was in a Chevy
Posts: 143
Joined: 11/11
Posted: 12/22/13 01:09 PM

big bore and short stroke seems like the way to go for high winding power.  
joe-

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/22/13 02:24 PM

and only a FOOL would dispute that FACT joe.... see ya bro  

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76Skylark
Big Block power for the win
Posts: 868
Joined: 12/11
Posted: 12/30/13 09:13 AM

In a Race Engine It is Paramount to rev through its torque and Power Range QUICKLY. But in what I would Call A 'Stationary Platform' IE an airplane Engine or Large Boat/Ship engine. it revs to an rpm that is Very Slow(compaired to race) and then Does not Increase or Decrease for Long periods, so they Employ the Long Stroke Theroy, And in this case Intake and Exaust becomes Taylored to the Needs of the Power Unit. I have Always Said : HorsePower will Get You OVER The Moutain, BUT Torque Will MOVE The Mountain.  

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Dave632
I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 12/30/13 07:32 PM

Icon Quotedond1965:
all the oem factories need to quit building small bore engines because big bore engines are better! i mean all street cars NEED to convert to drag race engines and that means big bores for big valves!

Icon Quotetuffnuff:
Now I know you don't mean one word of that post.,. you're saying that with tongue in cheek.,. Dave, dond just got busted,,,,,,, chuckles.

Grin


Icon Quotedond1965:
Shocked  heck tuff what would make you say something like that?!?! honestly i really do have great respect for them, but the grump was my bow tie hero. Smile

Factories see how low they can bow to the govt. EPA, (environmental pinhead agency), lackies. They all cowered down when the idiot govt. started to put all kinds of stupid restrictions on cars.  
[[SUPPORT AMERICAN CARS, STOMP A RICE CAR TODAY.]]

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 12/31/13 08:48 PM

lets all go buy a japanese car!! YOU first!!!!!  

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76Skylark
Big Block power for the win
Posts: 868
Joined: 12/11
Posted: 01/01/14 12:27 PM

I Insist, after YOU.   I do not Speak Japanese.  

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dond1965
I have an SS396 tatoo
Posts: 294
Joined: 12/13
Posted: 01/01/14 01:24 PM

want rice? got rice.  

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