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PostPosted: Mon Oct 26, 2009 11:54 pm 
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ejenner wrote:
...It's not an old wives tale... but the reason (which I'm sure you know) does not contradict your statement with regards flow through the combustion chamber... The difference is that a long throw stroke adds a lot more weight to the end of that piston as it flings to the end of its run around the crank. That's what creates the higher torque lower down. The oversquare engines rev better because the engine does not mind reving so high as the inertia is lower by design and not just the weight of the individual components.


I'm not going to get sucked into one of these endless "yes it does, no it doesn't" debates or hijack this thread so this'll be my last comment on the topic but I'm afraid none of the above makes any mathematical or mechanical sense and for a GIVEN ENGINE CAPACITY the bore/stroke ratio plays no part in the power or torque calculations as any textbook will tell you. The bore length and stroke length both cancel out of the calculations to leave the total swept volume as the only thing that matters.

For a given engine capacity you can make the bore bigger and the stroke shorter or vice versa and as long as you keep the head flow the same in all cases (plus everything else obviously such as burn speed) then the shape of the torque curve will not alter a jot.

Clearly if you make the engine capacity bigger, either by increasing bore or stroke, then that's an entirely different issue.


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PostPosted: Wed Oct 28, 2009 12:18 am 
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I had always thought that for a given engine capacity, one with a longer stroke would give a higher torque value when compared to an engine of the same capcity but with a shorter stroke and wider bore.

Being a stubborn so and so, I decided to do a few 'fag packet' calculations to prove it one way or the other

:corn:

Firstly, a few values and assumptions:

Pi = 3.142
1 Pascal (Pa) = 1 Newton (N) per square metre
Force = pressure x surface area
Stroke = diameter of crank throw
Radial distance = half stroke length
Torque = force x radial distance
Max torque figure delivered when con-rod at 90* to crank – this figure is taken as ‘at that very instant’ so as to avoid complicating matters.
Piston faces are flat
A force of 1,000KPa (arbitrary figure, but somewhat realistic-ish) will be generated in the cylinder of each engine – they each have the same head and therefore airflow capacity.


For an engine of stroke 90mm and bore 90mm

Capacity per cylinder:

Vol (in cc) = Pi x (radius x radius) x swept volume
= 3.142 x (4.5 x 4.5) x 9
= 572.63cc
Total disp = 2290.5cc

Area (in square metres) of one piston surface: Pi x (0.045 x 0.045)

= 0.00636255sqm

1,000KPa applied to piston

1,000,000 x 0.00636255 = 636.255N

Radial distance = 0.045m

Therefore, torque produced = 6362.55 x 0.045 =
286.3Nm


For an engine of 82.504mm stroke and 94mm bore
= 3.142 x (4.7 x 4.7) x 8.2504
= 572.63cc
= 2290.5cc

Area of piston: 3.142 x (0.047 x 0.047)

= 0.006940678sqm

1,000KPa

1,000,000 x 0.006940678 = 6940.678N

Radial distance = 0.041252m

Torque = 6940.678 x 0.041252 =
286.3Nm

:geek: :geek:


Thus it turns out that Dave is entirely correct, and I stand enlightened (happily so!) and a little surprised!


I think what Emmett may have meant when comparing the two engines, is that the one with the shorter stroke & larger bore may well be more willing to rev higher and pick up the revs more quickly owing to the lower interia of the pistons - peak piston speed will be lower owing to the shorter stroke.
As a result, for a given rpm, the con-rods and pistons in the short-stroke engine will experience lower forces/stresses when compared to the long stroke engine. So for the same rod and piston design, the short-stroke engine could rev higher before it would suffer a failure.
As shown above, the torque values for both engines will be the same.........however, on the short-stroke engine, if this value is held to higher revs, it will develop more power.

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PostPosted: Wed Oct 28, 2009 10:03 am 
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Alright, just one more brief comment.

Your calculation simplifies down readily to the following.

Torque = pressure x piston area x stroke/2 (crank radius) ==>

Torque = P/2 x piston area x stroke ==>

Torque = P/2 x swept volume.

In your calculation this must always be 1000/2 x 0.57263 = 286.3

As I said originally, stroke and bore size always cancel out of the calculation leaving swept volume as the only relevant factor. The full and proper calculations are obviously much more complex being the integral of the pressure v crank and rod angle over a full cycle but essentially the basic point stays the same. Only when the pressure changes because breathing or burn speed change can the torque and power change for a given engine capacity. So yes there may be incidental effects which mean the real world performance of two engines with the same capacity and same head but different bore/stroke is not completely identical but it's incidental to rather than directly because of the stroke length or any extra supposed "leverage" a long stroke produces. Any benefit from this must be exactly offset by the reduced piston area.

The real reason small bore long stroke engines are "thought" to have good low rpm torque is simply because they're usually old designs with limited breathing, short duration cams and they do have most of their torque low down because they don't produce enough power to rev. However stick a high flowing 4v head on them like a Honda VTEC and they'll rev out as well as anything else with the same head.


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PostPosted: Wed Oct 28, 2009 9:11 pm 
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I can't really argue as I don't have the same knowledge or experience. But a bit interested in the 'real world' 'incidental' bit...

This wasn't really the original argument though... it's sort of veered off on a tangent... originally I was wondering why you shouldn't bore it out a bit.

Can increase from at least 90mm to 92mm – I may have heard someone somewhere going on about 94mm pistons? Are you sure that wouldn’t make any difference to output bearing in mind bore increases happen all the time in OEM land without a corresponding valve-size increase?


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PostPosted: Thu Oct 29, 2009 2:15 am 
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Car Model: One or two...
What the guys are saying is quite simple, the increased bore will increase power and torque, but ONLY as a result of the larger engine capacity you end up with.

I have to confess I was always of the view that long stroke = more torque too, but the sums say otherwise...

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PostPosted: Thu Oct 29, 2009 8:39 am 
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No, I think the assertion was that only increased flow through the head increases output? Maybe increasing the bore size would mean the flow would increase even if the valve sizes stay the same and this is why Saab and other manufactures did it?


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PostPosted: Thu Oct 29, 2009 9:03 am 
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Not quite,

For a given swept volume, only increasing the flow can improve the power/torque.

A bigger bore will give a larger swept volume, and will increase the power and torque.

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PostPosted: Fri Oct 30, 2009 9:19 am 
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If my engine was going to be used for serious racing etc, i probably would have gone for the largest lightest forged pistons and rods, but for the small increase in power/torque it just wasn't worth the extra expense with this particular fast road engine.
I took Birdie for a spin in the car last night and i even let him have a go, I've asked him to post up his non-biased opinion about the engine, i'm sure he will post something later.

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PostPosted: Fri Oct 30, 2009 9:30 pm 
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sonett wrote:
i'm sure he will post something later.
after spending a few hours in the shed grinding bits off internal engine parts on the white rally car. :lol:


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PostPosted: Fri Oct 30, 2009 10:06 pm 
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Location: REMAINING UNFAITHFUL TO FACTORY STANDARD SPECIFICATIONS SINCE 1988
Car Model: OLD SAABS....NUMEROUS
APOLOGIES FOR THE DELAY,I HAVE BEEN BUSY...........

''PROJECT TURBO KILLER'' HAS BEEN IN THE MAKING,FOR AROUND FIVE OR SIX YEARS NOW........
AS USUAL ''LIFE'' GETS IN THE WAY...OF ALL GOOD INTENTIONS..........I DO HOPE YOU FIND IT INTRESTING,THE BUILD OF A 'NORMALLY ASPIRATED' ENGINE,AS AN ALTERNATIVE, TO JUST BOLTING A TURBO ON........?

I ONLY USE 'REAL LIFE' SCENARIO'S......
I USE ANY FINDINGS, FROM MY OWN PERSONAL EXPERIENCE.............

NOW JUST TO QUANTIFY,THE OVERBORE OF A 2LITRE SAAB ENGINE TO A 2.1,IN A NORMALLY ASPIRATED MANNER,COULD HOPE TO LIBERATE AROUND 4-6 BHP,IN MY EXPERIENCE.
NOT MUCH ,UNLESS YOU HAPPEN TO TO INCLUDE IT AMONGST, ALL OTHER ENGINE UPGRADES ,SUCH AS BIGGER VALVES,SUITABLY FLOWED HEAD,LIGHTENING AND BALANCING ETC....

HOW THIS OVERBORE WOULD AFFECT A 'TURBO' ENGINE,IN TERMS OF INCREASED POWER/TORQUE,AS A RESULT OF FORCED INDUCTION,I CANNOT TELL YOU,AS I HAVEN'T TRIED IT ...............YET :!: (GIVE ME SIX MONTHS OR SO.......I AM ON IT)


SO,''TURBO KILLER''HOW DOES IT DRIVE THEN..............?

DAVE ,TURNED UP LAST NIGHT,I DIDN'T EVEN HEAR IT COMING.....THE ENGINE, IS THAT QUIET ON TICKOVER.........

NOT LONG HOWEVER,BEFORE A BLIP ON THE THROTTLE LETS YOU KNOW,EXACTLY WHAT IS LURKING BENEATH THE BONNET......

A TEST DRIVE OF TEN MILES OR SO,THE ENGINE PULLS STRONGLY FROM LOW REVS(1500 RPM) IN ALL GEARS,REVVING QUITE FREELY.........AS DAVE SAYS,THE FRONT END IS DEFINTLEY LOUDER THAN THE BACK :lol:

THE MOST NOTICABLE DIFFERENCE,IS THE CAR WILL PODDLE ALONG AT 30 MPH IN FOURTH,OR 35 MPH IN FIFTH..........THIS I CANNOT DO IN THE RALLY N/A EMS..........

PERSONALLY,I THINK THE CAM IN MY N/A EMS IS WILDER, WHICH ALONG WITH MY FOUR SPEED 'BOX,MEANS THAT ANYTHING UNDER 40 MPH IN FOURTH,REQUIRES A DOWN SHIFT....
HOWEVER THE BENEFIT OF THE FOUR SPEED BOX, ELIMINATES TOTALLY,THIS 'GAP' THAT BOTH DAVE AND EJENNER,SEEM TO HAVE BETWEEN FIRST AND SECOND GEAR.......

I WOULD EVEN GO AS FAR TO SAY,THE FOUR SPEED BOX WITH TYPE FIVE PRIMARIES,IS AS CLOSE TO A COMPETITION SPEC,THAT YOU ARE LIKELY TO GET,WITHOUT SPENDING LOTS OF MONEY..........OR,OF COURSE IT COULD JUST BE,I HAVE BOTH CARS SET UP, SUCH THAT THE POWER IS ALWAYS THERE, WHICHEVER GEAR I AM IN.........

IT IS FITTED, IN BOTH MY N/A EMS AND 99 TURBO..........

AGAIN,JUST MY EXPERIENCE......

DAVES CAR ,IS THE CULMINATION OF HARD GRAFT,AND APPLIED THOUGHT PROCESS,AMONGST OTHER THINGS.......ECU'S, THROTTLE BODIES,CRANK POSITION SENSORS........AND A LOT OF CAREFUL WORKING OF THE CYL.HEAD......

IT ALL COME GOOD, IN THE END.........AND IT AIN'T FINISHED YET........

I HAVE ASKED, IF DAVE CAN SCAN THE POWER/TORQUE GRAPH,FOR MY 99 TURBO........I HAVEN'T GOT A CLUE......WE JUST LIKE MAKING SAABS GO FASTER THAN THEY SHOULD..........NOT, JUST ON PAPER :wink:

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PostPosted: Sat Oct 31, 2009 9:17 am 
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Car Model: lada
Mines a four speed as well. But difference is that I drive to circuits so I wanted a bit of motorway legs on it. But bearing in mind how much it spoils the fun having that big gap between 2nd and 3rd... it is obvious that I need to change the primaries... but I want to see what difference decent mapping makes to the whole situation before going mad taking the gearbox to bits again.


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PostPosted: Tue Apr 13, 2010 1:27 pm 
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Car Model: Saab 900 and 99
The spare MSS exhaust manifold has been modified, the exit pipe has been increased to 2'' so it couples straight onto the the Jetex system and the primaries have been reduced, next step is to get it on the car and off to the dyno to see what effect it has.
The 99GL has been off the road during the winter months so i haven't used it much, i have however fitted some new parts i bought in Sweden, complete new front corner lamp assemblies, the originals had seen better days and the area around the lamp holders was burnt/discoloured. I also managed to get two NOS rear louvres, i know they aren't everyones cup of tea, but at least they are period accessories, to go with the EMS steering wheel, NOS grey rubber mats and funky seat covers :)
Image
Image
Image

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PostPosted: Tue Apr 13, 2010 3:26 pm 
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As far as Brown cars go; she's a beaut! (If a little accessory-heavy IMHO.)

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PostPosted: Tue Apr 13, 2010 5:32 pm 
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Car Model: Saab 900 and 99
The Marchal fog lamps will be coming off soon because i never use them.

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PostPosted: Wed Apr 14, 2010 7:54 pm 
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sonett wrote:
The Marchal fog lamps will be coming off soon because i never use them.



That looks magic, t really does,, how mc did you pay for the rear louvre if you don't mind me asking


Rob

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PostPosted: Wed Apr 14, 2010 8:17 pm 
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Location: NETTLEHAM, LINCOLNSHIRE
Car Model: 2x 99GL, C900i & Volvo V70
Never a fan of the rear louvres on C900's but that looks good on the 99 8)

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PostPosted: Wed Apr 14, 2010 8:44 pm 
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Location: Maidstone
Car Model: 9-5 Aero
I'm a fan of the rear louvres on the c900, as well as on this 99 - which looks amazing 8)
However, from this angle, I can't help thinking that it has a passing resemblence to a certain skoda from the 80's.........

Image

Image

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PostPosted: Wed Apr 14, 2010 9:21 pm 
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I wasn't too sure about the louvre myself, mainly because most of the ones i have seen were on 900's, but they do look better on the 99, i think it's because the 99 is more compact and the black trim on my car also helps.

I bought two in Sweden last year off a bloke from Denmark, think i paid about £50 each for them. We had travelled by plane to Norway and then ferry to Stromstad, of course getting on the plane with all the parts i bought wasn't going to happen, so a nice man from Belgium, Ettiene, took them back to Belgium in his 99 and then delivered them to me at our national.

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PostPosted: Wed Apr 14, 2010 9:33 pm 
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Arkwright.
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Location: NETTLEHAM, LINCOLNSHIRE
Car Model: 2x 99GL, C900i & Volvo V70
I have a 'new' 99 to play with, I picked it up last week.

I will certainly be keeping a lookout for one of those louvres, but I should think they are like rocking horse s** :wink:

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PostPosted: Thu Apr 15, 2010 7:48 am 
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It just so happens there is a photo of me arranging delivery with Ettiene, he had purchased the red 99 to take back to Belgium.
Image

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