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Stroking A Engine


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#1 dennismini93

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Posted 11 May 2014 - 12:29 PM

iv read about stroking an engine on here and in the yellow bible but there are a few things that i am still unsure on.

 

it says in the yellow bible you can stroke up to a 91mm stroke and it shows you the varying capacities of different stroke lengths to a 1380, but what would the effects be on a standard 70.6 bore engine. is it not worth stroking a 1275 until you get to 1380 or is it just something that isnt done due to cost?

 

Another question iv got is that a longer stroke will increase mid range torque and reduce the maximum rpm ability but by how much. the standard stroke is 81.3mm (correct me if im wrong) so how much % of your max rpm do you loose for every 0.5mm of stroke you add. i know other factors will affect this as well. also what effect on torque will the increases have?

 

sorry for all the questions but iv got a 1293 to re build and i want to learn as much as possible before i do anything.

 

thank you

 

EDIT: Stroking AN Engine


Edited by dennismini93, 11 May 2014 - 12:31 PM.


#2 vx220

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Posted 11 May 2014 - 12:38 PM

Been thinking about this with the recent fashion for not going straight to 1380 to save blocks

I guess stroking good for longevity, as same power for less revs?

Interested to see how this goes

#3 mini13

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Posted 11 May 2014 - 12:47 PM

stroking is a good way of boosting torque, but it can be costly,

 

you can use cooper S rods and offset grind a 1275 and get to 84mm stroke, but for more than that you need a new crank,

 

I would not say a stroked engine is good for longevity, in fact the opposite, a longer stroke engin will  have a flexier crank, and you will be loading items like the rod bolts more for the same rpm, ie the pistons will be punning higher G forces.

 

also, there is a law of diminishing returns in that, as you increase the capacity it is harder to fill the cylinder with the new charge.



#4 dennismini93

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Posted 11 May 2014 - 12:59 PM

I would not say a stroked engine is good for longevity, in fact the opposite, a longer stroke engine will  have a flexier crank, and you will be loading items like the rod bolts more for the same rpm, ie the pistons will be punning higher G forces.

i was aware of the cost and the law of diminishing returns and had guessed that a new crank would be required. you have confirmed my fears there tho lol

however i hadn't thought about the effects of the longer stroke. what you say above is a very interesting result and it may be the reason i dont go down this route. i remember david vizard said something about a 1600cc a series that could only rev to about 1500rpm and it failed quite quickly. he said it was incredibly torquey but the effects of this type of engine made it un-usable



#5 Vipernoir

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Posted 11 May 2014 - 03:40 PM

I've seen a 1500+ (near 1600) engine racing and it went like stink.  Certainly wasn't restricted to 6000rpm, but it wasn't designed for the road...

 

The difficult bit about a long stroke is sourcing decent pistons at sensible money.
Don't forget that the piston crown needs to be reduced by the same amount as the offset grind (half the stroke increase) which is why 1098 pistons are .150" shorter than 998 ones.



#6 dennismini93

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Posted 11 May 2014 - 04:40 PM

yeah iv heard of race engines stroked but never road :( instead of shorter pistons could i not get shorter rods?



#7 mini13

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Posted 11 May 2014 - 05:06 PM

th most sucessful stroker I know of was swiftunes yellow racer, iirc this was a shade under 1500cc, and revved well, but the key to that engine was the head with around 300 hours of flow work done on it.

 

you also have to consider the working life of the engine, a 1500cc racer is going to be rebuilt pretty often, and is going to be less  tollerant of any out of ballance parts.



#8 dennismini93

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Posted 11 May 2014 - 05:13 PM

yeah the head is very important. i wanted a road everyday engine with more low-mid range torque but i dont want to bore it out more than i have to. i would have to do the maths but i was hoping that a small stroke increase would give me more torque without sacrifising too much reliablity, revability and wear. 



#9 Tupers

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Posted 11 May 2014 - 05:42 PM

I've got an offset ground 84MM stroke crank from MED for my engine. I'm going to use it with +0.60 pistons to give a 1374cc engine. The idea is to get the power and torque of a 1380 but without the associated risks of offset bores. 

IMG_4527_zpsda9a579b.jpg

 



#10 Vipernoir

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Posted 11 May 2014 - 05:58 PM

What pistons are you going to use with that ?



#11 dennismini93

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Posted 11 May 2014 - 06:02 PM

sorry to bombard with questions but what are the other modifications you have carried out? did MED give you any idea of peak rpm changes? why 84mm and not 83 or 85? just want to learn as much as i can of different engines and their characteristics 



#12 Guess-Works.com

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Posted 11 May 2014 - 06:16 PM

Don't forget there's two ways to stroke an engine..... f-nar f-nar...

 

You can also short stroke, which is basically what the cooper S 1071 and 970 engines were....



#13 dennismini93

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Posted 11 May 2014 - 06:50 PM

Don't forget there's two ways to stroke an engine..... f-nar f-nar...

????

 

You can also short stroke, which is basically what the cooper S 1071 and 970 engines were...

ahh ok, thats interesting. whats the consequence of short stroking? higher and easier reving but less torque



#14 Guess-Works.com

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Posted 11 May 2014 - 06:52 PM

basically... yes, higher RPM's, more power, less torque



#15 dennismini93

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Posted 11 May 2014 - 07:09 PM

i want to try and get a more torquey engine between idle and 3500-4000rpm i never really go over that so was looking at stroking for this reason. i dont want to bore it more than it needs though. 






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