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View Full Version : Stroke/Bore RELATION TO Torque/Horsepower



Stallion
04-08-03, 02:24 PM
I was just wonder, if you happen to build/rebuild/modify your own block and you change the combustion changes and the specs of bore and stroke, how does this affect the torque and horsepower?

What I'm asking is what is the relation to different combinations of stroke and bore for different torque and horsepower?

Thanks! :)

TR

Stallion
04-11-03, 09:49 AM
Anybody?

78SilvAnniv
04-11-03, 10:42 AM
...but I think that is a great question.

Perhaps the information is available somewhere with graphs showing the power and torque curves of differently built engines?
Heidi

Do a search of Grumpyvette's (Grumpy Vette?) posts as he explains things in great detail and I think he may have discussed this previously...

Edmond
04-11-03, 12:11 PM
Originally posted by Stallion
I was just wonder, if you happen to build/rebuild/modify your own block and you change the combustion changes and the specs of bore and stroke, how does this affect the torque and horsepower?

What I'm asking is what is the relation to different combinations of stroke and bore for different torque and horsepower?

Thanks! :)

TR

bore and stroke is a direct relation to engine size and it's assumed that the bigger the motor, the more power you're going to make.

What kind of pistons you will use also change the compression ratio for different power.

Stallion
04-11-03, 12:22 PM
Originally posted by Edmond
bore and stroke is a direct relation to engine size and it's assumed that the bigger the motor, the more power you're going to make.

What kind of pistons you will use also change the compression ratio for different power.

Yes I know...there is no replacement for displacement!!! :D

I just figured what Heidi was talking about, maybe an analysis on different bores and strokes resulting in different hp and torque outputs. I'll look for his posts and see if he has anything to say on the subject.

Edmond
04-11-03, 01:11 PM
Originally posted by Stallion
Yes I know...there is no replacement for displacement!!! :D

I just figured what Heidi was talking about, maybe an analysis on different bores and strokes resulting in different hp and torque outputs. I'll look for his posts and see if he has anything to say on the subject.

Yes, there are a lot of articles out there in regards to different bores and strokes for the small block Chevy. I think the most popular is the 383 stroker. You see most guys go with that combo. It's only a 33 cubic inch gain in terms of number and if you were to just bore out the block and not replace the crank, I believe the displacement would be around 355.

But when you put that stroker crank in, you get more torque. I believe that more stroke equals more torque.

Ask the guys and they'll ask you what you want to do with the motor and how much money you want to spend. Maybe this should be moved to the C3 mods forum?

KOPBET
04-11-03, 01:24 PM
Rob has some good formulas:

http://corvetteactioncenter.com/tech/tools/formulas.html

Torque is directly proportional to displacement:

Torque = MEP x Displacement / 150.8

(MEP = mean effective pressure)

MEP = HP x 792,000 / Displacement x RPM

Displacement is proportional to the cylinder volume, the length of the stroke and the number of cylinders.

Stallion
04-11-03, 03:36 PM
Thanks for those equations!! What units are the displacement in? CI? Liters? But I get the general idea now. Thanks!! :)

TR

KOPBET
04-11-03, 03:48 PM
Depends on what you want to use. English or metric.

Stallion
04-11-03, 04:01 PM
Okay, I see. We'd probably want to use cubic inches so we get results in ft. lb. because if we use Liters then we would get Newtons per Meter I think.

Thanks again! :)

TR

KOPBET
04-11-03, 04:14 PM
Actually, the newton/meter is an S.I. (International System) unit. Metric would be kgf/m (Kilogram Force/ Meter) or derivitive thereof. But you can use whichever you are comfortable with as long as you don't try to cross systems.

MaineShark
04-11-03, 06:06 PM
I'd watch just plugging numbers into those equations. The "150.8" and "792000" are probably conversion factors, for a particular system of units (I would assume that cubic inches, horsepower, ft-lbs, etc. would be the ones they use).

Alright, stroke vs. bore... engines come in three flavors:

Under-square: stroke longer than bore
Square: stroke and bore equal
Over-square: bore larger than stroke

Really, only two, since under-square engines are extremely uncommon, and impractical for modern use. Any of them can make the same torque, and the same power (within the same operating range). The more over-square the engine is, however, the more the engine will like high rpms. The power band will be shifted upwards, as you go further and further over-square.

The limiting factor is the result of piston speed. While the rotating components operate in rpms, the pistons and connecting rods are moving up-and-down, at some number of feet per minute. The longer the stroke, the greater the distance the piston moves per revolution. So, at any given number of rpms, the longer-stroke engine will have greater piston speed. Get too high, and you'll lose more and more power to increased friction, not to mention increased wear, and eventually having a mechanical failure.

So, that puts a practical limit on the operating rpm range of the engine. And, since horsepower relies heavily on rpms, an engine which is limited will not produce as much peak power as an engine that can turn more revs.

On the other hand, torque is basically force exerted on a lever. In this case, the pressure in each cylinder pushes on the piston's surface area (pounds-per-square-inch, multiplied by the square inches of the piston, results in pounds), which is dependant upon the bore, and that force is multiplied by the lever length (stroke) and the number of cylinders, to get the torque. Increasing either the bore (which increases the area of the piston top) or the stroke (which increases the lever length) will increase the torque. However, the practical limitations of piston speed apply, so the long-stroke, small-bore engine must make its power down low, before it runs out of revs, while the short-stroke, large-bore engine can move its powerband upwards, and make more ultimate power, with more rpms.

It all depends on what you want. Some people actually under-stroke engines. Replace the 3.5"-stroke crank you have in your 350 with a 3.25" crank, and you have 327 cubic inches. This engine will be farther over-square than yours. You have a 1.14 bore/stroke ratio. That engine will have a 1.23 bore/stroke ratio. So, it will be happier in the upper-rpm range. A 383 typically has a 1.07 bore/stroke ratio (4.030" bore, 3.75" stroke), but GM's HT383 has a 1.05 (4" bore, 3.8" stroke). Both of these engines are very close to square, so they have limited upper-rpm power, but lots of low-end grunt.

Other odd engine combos: put a 3.25" crank in a 400-block (4.125" bore), and you get a 347 (close enough to 350), with a 1.27 bore/stroke ratio. That's even further over-square than the under-stroked 327. Put a 3.5" crank in the same engine, and you have a 374, and a 1.18 bore/stroke ratio. That's about halfway in-between the stock 350's 1.14, and the under-stroked 327's 1.23, with a large gain in displacement. The stock 400's ratio is 1.1, putting it between the 383 and the stock 350, again with a large gain in dispacement. Personally, that's what I've decided on (406 = 4.155" bore, 3.75" stroke = 1.11 bore/stroke ratio). Sort of halfway between the 350 and the 383, with a big increase in displacement.

Hope this helps a bit...

Joe

Stallion
04-11-03, 09:18 PM
That's exactly what I was looking for, Joe!! Absolutely perfect explanation and easy to understand. Thanks a lot!!! :D

TR

KOPBET
04-11-03, 09:40 PM
Originally posted by MaineShark
I'd watch just plugging numbers into those equations. The "150.8" and "792000" are probably conversion factors, for a particular system of units (I would assume that cubic inches, horsepower, ft-lbs, etc. would be the ones they use).


Great explanation Joe! The formulas and conversion factors were given only for representative purposes only, since factor like iMEP cannot be determined without an indicator, not normally available the average fellow.

MaineShark
04-12-03, 12:30 AM
Not a problem... I used to teach, and I can't quite get it out of my system :)

Joe

grumpyvette
05-06-03, 02:24 PM
first the math
(1)youll be limited to about 1.25-1.5 hp per cidN/A by engine size
(2) YOULL BE LIMITER to about 6400rpm with HYDROLIC flat tappet cams and about 7500rpm-8000rpm with SOLID LIFTER flat tappet cams
(3)piston speeds that exceed 4500fpm usually lead to trouble
(4)dynamic compression ratios of over about 8.3:1-8.5:1 with aluminum heads or about 7.8:1-8:1 with iron heads can cause detonation problems

http://cochise.uia.net/pkelley2/

(5)the formula for matching POTENTIAL HP to INTAKE PORT FLOW is (.257 x port flow at max cam lift x 8(# of CYLINDERs)= POTENTIAL hp
(6)http://www.babcox.com/editorial/ar/elements/30228b.gif look closely at the duration used for each MATCHING rpm range. ALSO KEEP IN MIND THE DCR AND OVERLAP MUST MATCH look here these are the valve timeing overlap ranges that are most likely to work correctly
trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap
but all engines will need the correct matching dcr for those overlap figures to correctly scavage the cylinders in the rpm ranges that apply to each engines use range

(7)youll be limited by the standard chevy block sizes, and bore sizes
(8)youll be limited by airflow in the ports by both high speed flow and loss of low rpm velocity if the ports become too large,
(9)enter all those factors and youll find that the best compromise is a stroke to bore ratio of close to .8 to 1. or if your looking for combos, in a small block thats
a 327,with a 4" bore (3.25" STROKE )
a 377 with a 4.155 bore(4.155 bore x 3.48 stroke)(
a 477 with a big block (4.5" bore x 3.75" stroke)

now you can make far more power with a larger engine but youll have a good deal of trouble makeing MORE HP PER DISPLACEMENT, it comes down to TIME available to flow air thru the valves .and CYLINDER FILLING EFFICIENCY, and AVAILABLE AIR FLOW SPEEDS.



http://users.erols.com/srweiss/calchpm.htm


http://www.prestage.com/carmath/calc_ETMPH.asp

http://users.erols.com/srweiss/calcrpm.htm

http://cochise.uia.net/pkelley2/
BTW START HERE, http://www.theengineshop.com/techinfo11.shtml IF YOUR BANK ACOUNT IS REALLY IN NEED OF A FAST DROP IN THE BALLANCE

now if you want to get crazy and have a large bank account, a twin turbo, intercooled , methonal/nitrous injected big block with about a 4.6" bore and 4.375" stroke.,7.5cpr with about 12-18 lbs of boost and a little computer controls might do nicely at just over 2500hp/3500hp

Stallion
05-07-03, 07:08 PM
Wow!! Thanks for that information. I'll definitely have to read that over many many times to fully comprehend it. :) I'll post back if I have any questions. Thanks!! :D

Stallion
05-12-03, 07:59 AM
"now you can make far more power with a larger engine but youll have a good deal of trouble makeing MORE HP PER DISPLACEMENT, it comes down to TIME available to flow air thru the valves .and CYLINDER FILLING EFFICIENCY, and AVAILABLE AIR FLOW SPEEDS."

So this is why we are always looking for a high lift and longer duration cam shaft?

grumpyvette
05-12-03, 10:17 AM
only in a vague way,the reason we use higher lift and longer duration cams is to allow the valves to remain open loner so that more time is available for the ports to flow air/fuel mix. read this, on cams it will help a great deal

look carefully at this chart

torque is basically the result of cylinder pressure (dynamic compression) and leverage (stroke) plus the NUMBER OF POWER STROKES PER MINUTE (rpm) from the cylinders that are efficiently filled (volumetric efficiency). up to a certain rpm level the cylinders don,t efficiently fill due to low port air speed, above that level the valves and pistons move too fast to effectively fill the cylinders due to lack of time. your highest torque will be at the point where the engine spins the fastest it can while still packing the cylinders to the max efficiency.
here read this info on cams
http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm
(lessons 1-8)
then look at the chart to get a rough idea as to the duration necessary to fill the cylinders effectively . duration , lift and LSA are all a combo that must match compression, displacement, rod length to stroke ratio,port size and length,exhaust scavageing effectiveness, ETC.
look here
http://www.babcox.com/editorial/ar/elements/30228b.gif
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
then look at your cam spec sheet, the piston compresses nothing untill the piston has reached the point where both valves are closed, from that point on your compressing the voluum in the cylinder. look at this cam

http://www.compcams.com/information/search/CamDetails.asp?PartNumber=12-433-8
the pistons not compressing anything untill 77 degrees past BDC, looking at the other chart we see that the piston is about 2.4" down the bore on a 350 chevy not the 3.5 inches that the engines static compression in theory could compress

if cams are a mystery please take the time to read these, it will get you a good start

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm
(read LESSONs 1-8)

http://www.mercurycapri.com/technical/engine/cam/lca.html

http://www.wighat.com/fcr3/confusion.htm

http://www.cranecams.com/instructions/valvetrain/camfail.htm

http://www.mercurycapri.com/technical/engine/cam/index.html

http://www.idavette.net/hib/camcon.htm

http://www.cranecams.com/master/adjustvt.htm

http://www.centuryperformance.com/valveadjustment.htm

http://www.totalengineairflow.com/tech/valvelashing.htm

http://www.chevytalk.com/tech/engine/Cam_Selection.html

http://www.chevytalk.com/tech/101/Cam_Theory.html

http://www.babcox.com/editorial/ar/ar119736.htm

http://www.mercurycapri.com/technical/engine/cam/vtg.html

http://www.n2performance.com/lecture1.shtml

http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://home.wxs.nl/~meine119/tech/camqa.html

Edmond
05-12-03, 10:33 AM
Grumpy,

Won't a cam with too much lift and duration give you idle problems for street driving?

grumpyvette
05-12-03, 02:40 PM
lift by itself has very little effect on idle
duration can have a very pronounced effect but within limits its changes in the LSA that increases the overlap area that has the quickest effect as the duration increases , if your in the 220 @ .050 duration or lower range. its the overlap period when doth valves are open that in most cases causes the reversion in the intake ports that causes the rough idle,as the rpm levels increase the engines cylinder scavageing increases due to both inertia in the intake and exhaust tending to keep the airflow thru the cylinders going in the same direction, thats why the engine tends to smooth out as the engine rpms increase.
on a C-4 / TPI corvette your limited to about a 210@.050 duration and at least a 112 LSA to keep everything idleing fairly smooth, increase the LSA to 114 and things smooth out further, drop the LSA to 110 and the idle becomes noticably rougher
look at these two cams

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card
.
http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=110921&B1=Display+Card
http://www.newcovenant.com/speedcrafter/tech/camshaft/images/twolobe.gif
.
they are the same cam EXCEPT FOR THE LSA
the first cam idles with a fairly rough idle
the second cam idles MUCH rougher
read this
.
http://www.newcovenant.com/speedcrafter/tech/camshaft/3.htm

http://www.newcovenant.com/speedcrafter/tech/camshaft/6.htm

other info
http://www.iskycams.com/techtips.html#2003

http://www.mercurycapri.com/technical/engine/cam/lca.html

http://www.victorylibrary.com/mopar/intake-tech-c.htm

http://tru-442.tripod.com/camselect.htm