i'd really like some explanation about this-

everyone tells me bore is a better way to add cubes than a longer stroke because you can rev higher and create more power up top.

I can see more cubes creating more vacuum and hence more air at lower rpms and the whole chain of effects...

now would a longer stroke create more vac than a larger bore (assuming equal displacement through this whole thread)since it has a higher velocity now? I'm not real familiar with air flow charactoristics so i'd really appreciate some help with this.

Also- if this is all true,
why did this LS1 get a longer stroke and shorter bore. I'm sure it wasn't for ****s and giggles.

I'm COMPLETELY lost with this one.

 

Let me see if I can break this down a little.

1. For your first statement, "everyone tells me bore is a better way to add cubes than a longer stroke because you can rev higher and create more power up top."

It depends on your situation. Yes increasing bore within reasonable limits is always a great way to increase power because if your heads are ported to take advantage of the extra bore size you can pretty much always flow more air and make more power. However cubic inches are great, especially on a street car and there is usually a lot less displacement to be gained from overbore than there is from stroking. You are usually limited on most small block stock blocks to increasing the bore by around .040-.060 max without risking too many problems yet you can easily increase your stroke by .250 up to .500 without running into any problems other than clearancing issues with the block and cam. As you can see there is much more displacement to be gained from stroking.

Now if you have a racing class that you are building an engine for that has a displacement limit your best bet for high power output is to use the largest bore you can get within the limits of reliability for what you are doing. The benefits in airflow potential and cylinder pressure to surface area of the piston out weigh the benefits of a longer crank arm. Airflow is the major factor in power production and the bigger bore engines will allow the same heads to breathe better.

2. Your second question " now would a longer stroke create more vac than a larger bore (assuming equal displacement through this whole thread)since it has a higher velocity now? I'm not real familiar with air flow charactoristics so i'd really appreciate some help with this."

You are kind of headed in the right direction.
For a given bore size your cylinder head will support a certain piston speed before the port becomes velocity limited and locks up.
Piston speed is a function of stroke and rpm. (stroke x rpm)/6
So if your head and bore size will support a given piston speed before locking up and assuming that bore remains constant, then each different stroke you try will have to turn a different rpm before it reaches that limiting piston speed.
If you increase the stroke then you will reach the limiting piston speed earlier.
If you decrease the stroke then you will reach the limiting piston speed later.
So we can say that since the airflow of the heads on your bore will limit piston speed, and peak power, (generally about 1.9 to 2 hp per cfm on well built street engines) then as you increase your stroke the rpm where you reach the limiting piston speed will get lower.

3."why did this LS1 get a longer stroke and shorter bore?"

Even though bigger bores allow for better breathing heads to be used and higher overall power numbers to be attained they do continue to get less efficient as they get bigger.
As the bore gets bigger the flame front has more distance to travel in the same amount of time. This causes more emissions reduces fuel economy. (a MAJOR concern at the factory)
The small bore of the LS1 burns very efficiently and has heads that are designed to flow well on that bore. Coupled with an extreemly well designed intake manifold you get an engine that makes cleaner emissions and has a nice flat torque curve that makes for more power all over the rpm range. Stick in an efficient drivetrain and you have an extreemly fast car that has emissions that were unimaginable for a V-8 making that kind of power 20 years ago.

 

To elaborate on the previous excellent post: When dealing with a street car with a relatively limited budget, based on a contemporary block, it's not like there are a huge number of choices for bore/stroke ratios. Rules and budgets aside, when dealing with a race setup, there is a lot more freedom. Contemporary stock blocks cannot be overbored easily beyond 0.030'-0.040". Careful block selection and preparation may allow up to 0.060", but even this only gains 10ci on a 4.00" bore. So when you consider that the best route to better street or street/strip performance is displacement, you are pretty much left with building a stroker.

Rich Krause

 

cant big blocks gain a significant amount by borning?

 

Stock big blocks go 0.060" over easily. A 60 over 502 gains ~13ci. Bowtie and aftermarket big blocks (and small blocks) can take much larger overbores. If you are building to a displacement limit, do the max overbore your block will tolerate and chose a stroke to give the desired displacement. If it's a street motor, save some cylinder wall for the next rebuild and use as long a stroke as is practical.

Rich Krause

 

In general, the larger the original bore size, the larger the increase in displacement would be with the same amount of overbore. However, the increase eventually becomes static as the overbore becomes a smaller and smaller percentage of the original, and would start to taper off if bore size were increased further.

350 CID = 4.00" bore, 3.480" stroke
355 CID = 4.03" bore, 3.480" stroke (+5 cu. in.)
360 CID = 4.06" bore, 3.480" stroke (+10 cu. in.)

454 CID = 4.25" bore, 4.000" stroke
460 CID = 4.28" bore, 4.000" stroke (+6 cu. in.)
467 CID = 4.31" bore, 4.000" stroke (+13 cu. in.)

502 CID = 4.47" bore, 4.000" stroke
509 CID = 4.50" bore, 4.000" stroke (+7 cu. in.)
516 CID = 4.53" bore, 4.000" stroke (+14 cu. in.)

572 CID = 4.50" bore, 4.500" stroke
580 CID = 4.53" bore, 4.500" stroke (+8 cu. in.)
588 CID = 4.56" bore, 4.500" stroke (+16 cu. in.)

632 CID = 4.60" bore, 4.750" stroke
640 CID = 4.63" bore, 4.750" stroke (+8 cu. in.)
648 CID = 4.66" bore, 4.750" stroke (+16 cu. in.)

The amount of displacement gained by increasing stroke is more significant, although it also eventually becomes relatively static as the increase in stroke becomes a smaller percentage of the original.

350 CID = 4.00" bore, 3.480" stroke
377 CID = 4.00" bore, 3.750" stroke (+27 cu. in.)
390 CID = 4.00" bore, 3.875" stroke (+40 cu. in.)

454 CID = 4.25" bore, 4.000" stroke
468 CID = 4.25" bore, 4.125" stroke (+14 cu. in.)
482 CID = 4.25" bore, 4.250" stroke (+28 cu. in.)

502 CID = 4.47" bore, 4.000" stroke
518 CID = 4.47" bore, 4.125" stroke (+16 cu. in.)
534 CID = 4.47" bore, 4.250" stroke (+32 cu. in.)

572 CID = 4.50" bore, 4.500" stroke
588 CID = 4.50" bore, 4.625" stroke (+16 cu. in.)
604 CID = 4.50" bore, 4.750" stroke (+32 cu. in.)

632 CID = 4.60" bore, 4.750" stroke
648 CID = 4.60" bore, 4.875" stroke (+16 cu. in.)
665 CID = 4.60" bore, 5.000" stroke (+33 cu. in.)

Combining an increase in stroke and bore diameter nets the greatest increase in displacement possible, although it too tapers off as the increases become a smaller percentage of the original measurements.

350 CID = 4.00" bore, 3.480" stroke
383 CID = 4.03" bore, 3.750" stroke (+33 cu. in.)
401 CID = 4.06" bore, 3.875" stroke (+51 cu. in.)

454 CID = 4.25" bore, 4.000" stroke
475 CID = 4.28" bore, 4.125" stroke (+21 cu. in.)
496 CID = 4.31" bore, 4.250" stroke (+42 cu. in.)

502 CID = 4.47" bore, 4.000" stroke
525 CID = 4.50" bore, 4.125" stroke (+23 cu. in.)
548 CID = 4.53" bore, 4.250" stroke (+46 cu. in.)

572 CID = 4.50" bore, 4.500" stroke
596 CID = 4.53" bore, 4.625" stroke (+24 cu. in.)
621 CID = 4.56" bore, 4.750" stroke (+49 cu. in.)

632 CID = 4.60" bore, 4.750" stroke
657 CID = 4.63" bore, 4.875" stroke (+25 cu. in.)
682 CID = 4.66" bore, 5.000" stroke (+50 cu. in.)

Note that the examples listed above are not necessarily viable combinations, but were used to illustrate the trend in displacement gained by each method.

Formula: bore^2 x stroke x # cylinders x 0.7854 = CID

 

This is predominantly for two reasons...

1. A larger bore will usually help to unshroud the valves and/or allow running larger valve diameters. This would tend to support the "more power up top" theory.

2. An increase in stroke increases piston speed, and at higher rpm this becomes a much bigger issue as it increases the stress on internal parts. This would tend to support the "rev higher" theory, although that's not necessarily the case.

My stroked 396 is a good example of an engine with a relatively low rod to stroke ratio (1.55:1 = 6.0" / 3.875") that can rev as high as you'd want for a street engine, but the components in the rotating assembly are as strong and as light as possible to reduce stress, and that means more expensive as well.

Since the crankshaft is the heaviest component of the rotating assembly, it also stands to reason that the closer the rod journals are located to the centerline of the crankshaft, the less effort it will take to spin it, and the faster it will "rev", all things considered.

 

Small correction to the above
"396 CID = 4.06" bore, 3.875" stroke (+46 cu. in.)"
Actually the commonly used "396" SBC is:
395 CID = 4.03" bore, 3.875" stroke

Your .060 over would be 401 CID (+51 cu. in.)

Cheston

 

Good catch, and I should have caught that one myself, especially since I have a 396.

 

Great thread.

Since friction is such a huge factor in horsepower loss. has anyone ever sat down and figured out which adds more friction. adding X cubes by bore vs. X cubes by stroke. It would make sense to me that stroking the motor adds more friction area per cu. added then bore.

 

I did a quick simulation on 383 SBC.

Large bore/short stroke (4.17 x 3.50) had about 5 fewer friction hp@ 6000 than a small bore/long stroke (4.03 x 3.75) engine.

It was about 2 hp difference @ 3000. Output of the engines was almost identical.

With typical bearings and rings, friction hp @ 6000 calculated about 83 hp for the short stroke engine. That's a lot, but you aren't going to reduce it by a whole lot with lower friction tricks. Reducing it 15% would be about 12-13 hp, and would cost big bucks, IMO.

FWIW, simulating a ProStock engine @ 9500 gives about twice the friction hp.

 

Friction is a good point to bring up.

While it is true that at the same rpm the short stroke engine will almost always have less friction than the long stroke engine the real consideration is piston speed.

Ususally your big bore short stroke engine will at least have the "potential" to breathe a little better than the small bore engine of the same displacement. So if it can breathe better it should support a little higher piston speed.
The same piston speed on a larger bore will draw harder on the intake port making it reach critical velocity faster.
Since the head will also breathe a little better lets assume that both combos will support the same piston speed.
Now even though the shorter stroke version has less friction at the same rpm its friction will increase with the square of the rpm increase it takes to get to the same piston speed
Since friction increases at an exponential rate rather than linear it is easy to eventually cancel out the friction reduction from reduced stroke and even end up with more friction in the long run.

This is why the pro's dont spin their engines any tighter than they have to to achieve their power goals.
Even if you can make the same power at a higher rpm (which some people think is a good idea) you will have to run a lower rear end gear and you will lose efficiency through your drivetrain and lose power to inertial losses from accelerating the engine through its rpm range at a faster rate.

 

i'm sure there is some way to find the optimal stroke to bore ratio for a given cubic inch motor using related rates. As a matter of fact, I just took a test using 4 variables to find the volume of something- thats probobly why i refuse to work it out right now

i guess there would be a few things to take into concideration


more stroke = more extreme angels on connecting rods to wrist pin = more force on piston rings. Friction is directly related to force if co-effecicent of friction is contstant

bore bore = more ring to sidewall contact- but would that make a differance? Lay a block on its side and drag it then stand it up and the frictional losses are the same.

more stroke- if it goes 10% further, there is more friction.

 

 

Dave,

I've seen the opposite of what you are saying.

MY view of it is that the volume of the cylinder is what counts not how it gets there. The one thing I think people think, (I'm not saying you do) is that the piston travel down the bore is directly related to the velocity of the intake port. It's not, the air/fuel in the port is moving due to the pressures acting on it. The low pressure in the cylinder is one thing, but the tuning pulses in the intake track, and the tuing pulses in the exhaust tract seem to play alot more of a role in the presure changes than the rate at which the piston goes down the bore at a given RPM.

In my research I've looked at the optimial bore/stroke relationship for a high output street motor. From what I have seen the biggest advantage to a larger bore motor for the same cubes is the airflow factor.

There is a very good test of this on the same head, same bench different test bores here: http://www.airflowresearch.com/perfo...ch_figures.htm

With the larger bore you see 5-11% gains in the flow bench data. On a motor that will work out to about a 2-2.5% gain in average output, that's a ton. It's higher at the top end vs the botom end of the RPM range, so it's going to be a good improvement at the track too.

The lower drag is important, but piston rings and crankcase presure have more to do with that than about anything (bearing size too, but that's big money)

From what I have seen a "stroker" is usually the only low cost way to get cubes into a motor, but it is certainly not the best.

One more thing, we got talking in another thread about the higher loss rate of a higher gear ratio. I got thinking about that, one thing that you see is that the engine and car will accelerate faster thru that given RPM band on a chassis dyno, meaning that you have a faster engine accelleration rate and less TQ output due to the faster acceleration rate. My feeling is that the faster acceleration rate of the motor is the major contributing factor in the lower output at the wheels and not the higher ratio gears. It would be interesting to see a comparison of two motors step tesed to show the same HP at steady state but with one having a high inertia and one having a low inertia. I guess a aluminum flywheel shows us the same thing.

Bret