Benefit of longer stroke

[kongracing]

What are the pros or cons of a longer stroke crankshaft.
4 cyl 80 mm bore 66mm stroke at moment, possible to go
to 70 mm stroke.

 

[Lou Scannon]

It would help if you described your goals, included all constraints, and listed some alternatives you might consider for reaching the same goals.
Assuming there is a mechanically sound package (crank, rods, pistons) for the 4mm stroke increase, I don't see much drawback (other than the cost) if increasing torque is your goal. You may or may not get an increase in peak power depending on where you are with breathing at the current displacement.

 

[patprimmer]

All other things being equal (which they will not) you will increase displacement, make more power, reduce rpm at which maximum power is developed but increase piston speed.

You will need pistons to correct pin height and compression ratio.

You will probably end up with different rod to stroke length ratio.

You will probably end up with different piston weight, quench area and skirt length.

The engine will respond to bigger ports and valves and more cam duration.

Regards

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[reidh]

To expand on what Pat pointed out regarding piston speed, it has been shown in many long term durability tests that engine durability varies indirectly with piston speed. I do not know what 4 mm will do to your durability.

Reidh

 

[thundair]

There is a good and bad thing that happens that I can add.

The bad is it will shorten the rod if deck height is locked in. This in turn will put an undesirable angle of attack on the crank. I know there is a lot of big inch motors running the long stroke with the short rod, but at what attrition rate I don't know.

The good is you can up your compression ratio without going to domed pistons..I don't know how this relates to the dynamic compression ratio.

Cheers

I don't know anything but the people that do.

 

[dcasto]

I'd go ahead at this point and have it bored out too since you have to replace almost everything rotating.

 

[patprimmer]

Some engines can accommodate longer stroke without shortening the rod or raising the deck by raising the pin in the piston.

It really depends on the original layout and where spare room is left.

Regards

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[Tmoose]

Heavier pistons and shorter rod/stroke ratio make the secondary vibration worse. Inline fours already have plenty of that, and some buzz and drone obnoxiously at some speeds.
The first US VW GTI engine included lighter pistons and better rod/stroke ratio, probably to make the engine reduce the secondary vibration and thus less four-y and more civilized with the taught engine mounts. Mine was a very willing performer. A young lady described it (but, sadly, not me) as a frisky pony.

 

[chucksk]

If the compression ratio is upped, then the octane rating needs to go up at some point as well, or you're liable to see some pre-ignition and ping. What you get at the pumps is actually an average octane fuel (if you're not aware of this, as I was once, you'll see a little note printed on the pump - "R+M/100 method"). The fuel companies guess what the refinery will produce (ideallized as R for Rated octane); what actually comes out is M for Measured. So, we get an average - actually it's almost always less than the real octane rating for the fuel, and I hear it can vary quite a bit. So, if you increase the compression ratio, you might need better fuel! Just one thing to keep in mind. You might see better mileage, though..

 

[mattsooty]

Chucksk

Thats not right.

The R is short for RON which is an acronym of 'Research Octane Number' and the M is short for MON which is 'Motor Octane Number'.

RON & MON apply to two different measurement techniques for determining the 'Octane Number' of a fuel and, therefore, its preponsity to autoignite.

The (R+M)/2 method, as used in the USA, provides a number refered to as the 'Anti Knock Index'.

MS

 

[reidh]

To expand on what Mattsooty has correctly stated, both the research octane and motor octane numbers are measured values. The Motor method uses variable ignition timing and preheated intake mixtures, as well as a greater engine speed compared to the Research method. This is why the values for MON are lower.

-Reidh

 

[chucksk]

Well, news to me! My brother works as an engineer for firms that revamp refineries, and this is basically what he told me!! He says that's really what it means - the refineries guess at what they can make, tell the government one number, and then usually fall short of that stated octane goal in the final fuel being produced. Well, but what you both have stated makes good sense - but why then does he tell me these things?? Maybe the one rating method allows them to fudge a lot.. either way, the project engineer says what you get at the pump is a lot less octane than what it's supposed to be!
I'm beginning to think electric fuel will be nice - there's only one way to measure it!

 

[Lou Scannon]

There's no doubt about it. Mattsooty and reidh have provided reliable information. One further criterion derived from RON and MON measured values is a fuel's "sensitivity", which is a function of how far apart these two values are.
I have heard that gasoline retailers can be audited for octane, and are subject to punishment if what they are selling is lower antiknock index (R+M/2) than advertised.

 

[patprimmer]

chuck

Read these sites and you might get a lot of news.

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

http://naca.larc.nasa.gov/search.jsp?N=17&Ne=2

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

I should have written that (R+M)/2 !