BIG BORE OR LONG STROKE ?

Friday, 24-Mar-2000 02:40:27

213.228.61.11 writes:

BIG BORE OR LONG STROKE ?

Here is an old debate. What theory do you prefer ?

Some say big bore gives you power with free revving response.

Some say long stroke gives you power with torque.

I know the problem is not that simple as rods length, piston weigth, c/w crank (when 69) count but give your comments.

I have two or three examples to start with. They are a bit extreme considering this debate but they are interesting because all these cars are 12 second quarter milers.

EXAMPLE N1 : James Lutz' race car. It's a 96.5 x 69 giving 190 HP. Bore / stroke ratio is an impressive 1.33 !

EXAMPLE N2 : "Thumper", Gary Madrio's show car. It was first at Bug In 30 (1971 blue with stripes Bug with polished Porsche wheels). Barely seen capacity of 1855cc thanks to 87 x 78 (giving 1.115 bore / stroke ratio). I found this car in a 1984 French magazine. Some details : MSD, 11:1 CR, 48 IDAs, 150 HP and 12.04 @ 187kph.

EXAMPLE N3 : World famous Dave Rhoads green wonder. Engine is a very square 88 x 82 (1995cc and 1.07 bore / stroke ratio) with 180 HP.

So, with the same ancillaries (carbs, cam, CR...) would you prefer :

- 92 x 69 (1835) or 87 x 78 (1855) ?

- 94 x 76 (2110) or 90.5 x 82 (2110) ?

I know that some will say : "Hey ! Just go for big bore AND long stroke and that's it !" but I think 2500cc type 1s and 3000cc type 4s don't have that nostalgia feeling (like a free revving 1679 for example...) even if they are often well engined and potent workhouse.

Thanks all in anticipation for answer and further examples.

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It only matters if you are in a LIMITED DISPLACEMENT racing class....

Friday, 24-Mar-2000 07:16:22

216.190.11.161 writes:

increasing the bore increases displacement.

Increasing the stroke increases the displacement, AND the leverage the piston has on the crank.

But, increasing the bore also unshrouds the valves more (or allows you to).

Both have advantages, but the question only matters if you have say, a 1900cc limit. For the street or unlimited, bigger is better almost all the time. The only exception is if there is another limitation on the engine, such as carb size, restrictor plate, etc. Then, a larger engine won't offer as much (if any) of an advantage, and will certainly cost more.

John Connolly

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Re: BIG BORE OR LONG STROKE ?

Friday, 24-Mar-2000 07:19:34

202.146.251.2 writes:

Go with the tried and true 2110cc!! The Berg's did! The 87s, 92s, and 94s (some will argue for the 94s) have a thin wall and won't last as long as the 90.5. Longer than 82 stroke will do, but it'll widen the motor too much (that's trouble), unless you're willing to cut some tins. I'll definitely stick with the long stroke theory and keep using 90.5s or thick wall 88s.

Satch

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It's called "Cubic Dollars".Build the best engine you can AFFORD.I didn't build my 69x96.5 becuase I'm a genius,It's becuase I was on a budget & that's what was lying on the floor.

Friday, 24-Mar-2000 11:10:59

208.130.241.123 writes:

The most important factor in building an engine is MONEY.The more you have the more "Bullet Proof" parts you can buy.Simple as that.

James Lutz

Re: BIG BORE OR LONG STROKE ?

Friday, 24-Mar-2000 12:10:12

142.107.52.15 writes:

I agree with the other comments - it really depends on what your budget and intended uses are. If you want a relatively inexpensive engine that will tear-up the streets on Saturday night and be fun at the track, build a Sweep-The-Floor 1914 special and have some fun. Spend some time on the heads, raise the compression, strip the interior, remove the bumpers, and it'll haul ass.

I have had bore motors and I have had stroke motors, and in my opinion, for a regular driver, nothing beats a stroker motor. Stroker motors produce awesome torque and are so easy to drive. The leverage of the crank will effortlessly pull the car around everywhere, and will pull stock gears or a heavier car with little effort. Further, as Ratto, Herbert, and others have pointed out, you can get away with more cam with a stroker motor.

Engines that require you to wind them out into the higher rpms to produce power tend to wear out faster, and can be a bit tiring to drive on a daily/regular basis. It all depends on what you want out of it. I personally like motors that are durable (able to drive cross-country), require little maintenance, and are easy to drive (ie: produce lots of low-rpm torque). My personal all-time favorite combo is the 2110 with stock-length rods.

My 2 cents;

John

John Plow

Re: I like to look at valve size,motor size and rpm limit

Friday, 24-Mar-2000 16:16:15

209.179.146.76 writes:

There are people selling heads nowadays with huge valve sizes like 48x40. If you think about it,you only need enough valve to feed a motor to highest rpm your going to rev it to. Even if you built a 2300cc motor I think you would have to rev it to 8500 rpm to use such big valves. A set of 42x37.5 heads can make 200 HP @7000 with less than 9to1 compression . And this is within the limits of a VW valve spring. The larger spring motors start to have more frequent problems with flat cams and ovaled out lifter bores. You can put bigger valves in the same motor but then it starts to rev higher and becomes even more lame in the lower rpms. Also the higher revs go beyond the limits of a VW spring. So basicly if you want your motor to stay together with VW springs then your limits are 42x37.5 valves and 7500 RPM,aproximately. I try to shoot for just over 7000. Now a motor that is smaller will want to rev higher with these same valves and a bigger one ,lower. You don`t want to rev any higher but a bigger motor with bigger valves will rev a little lower with VW springs but it might make more power. I don`t know what the curves are. If you use the 42x37.5 ,VW spring , 7500 rpm example then a motor of around 2100ccs will fit the bill. Otherwise , with a motor this size your 42x37.5 heads will run out of air at just after 7000 and stop making power just before your valve springs loose control of the valves and they float. So with info in hand I think the best motor would be ,the smallest bore that will not hinder the flow of your valve size and whatever stroke that will equal your 2100cc limit. I think an 88x86 would fit the bill. You need to fit the other parameters of your engine like rod ratio and cam size,carb size,exhaust size,etc to the RPM limit and your all set. Using RPM only this is the motor I would build but it would cost more to build than a 90.5 x82 and I would rather build a 94 x 82 because its the cheaper to build and I like lower reving motors for daily street driving and longevity. Good luck on your choice,

Mark H

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... Big bore goes with big valves and BIG rpm. The F1 engines are running bore/stroke ratios around 1.6, a 100mm bore and a 60mm stroke (just an example), this allows them to cram several large valves above the bore. Ferrari did 5 valves several years ago. This coupled with their springless pnuematic "springs" helps to allow 17,000 rpm now. And a power band as wide as a hummmingbirds beak.

If natural evolution was allowed, no displacement limits, I think you would see bigger motors that turn less rs than the curent drag motors. The happy street motor is big and not overly stressed, which leads to a happy owner.

In the quest for Torque AND Horsepower, Gary Berg built "Merlin", the 90x95 motor that set the PRA SS record a few years ago. That engine is around 315 hp at 9500rpm, quite impressive for '94.

The old adage "There ain't no subsitute for cubic inches" is very true for street engines, and "the longer the stroke, the more the poke" is what I believe in.

just my ramblings

Tom H.

Since I'm relatively new at this, please excuse my obviousness...

Friday, 24-Mar-2000 16:56:16

207.51.224.13 writes:

If you think about the actual physics envolved a longer stroke does more work for roughly the same amount of energy, like using a longer cheater bar to get the axle bolts off. Where as a larger bore takes more energy to do more work, this time we got up every morning and did push-ups so that we don't need a longer cheater. I'm still thinking this stuff out but you can see where I'm going.

Paul Semrad


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