A simple stroke vs. revs vs. capacity comparison

[ed]

I wrote this for my site tonight, and thought it was worthy of reposting here for interest's sake

original article here:
http://www.v-eight.com/tech_forum/viewtopic.php?t=143

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I did the following simulations simply to investigate the releative effects of stroke vs. revs vs. capacity

engines are always a compromise, with optimisation needed for any individual setup to maximise performance without going beyond some fundamental mechanical and materials limits. In all of the engines below, there will be limiting factors imposed on a given configuration that will limit how far you can push the components in any one direction.

for this first example ive taken piston velocity as the limiting factor and have simply shifted stroke around from the 1uz baseline configuration of 88.5mm bore, 82.5mm stroke and 4.0L capacity. cam 257 @ 50thou. the capacity is not fixed and is free to vary with the changes in stroke observed.

in one comparison stroke has been taken down to 75mm and a capacity has dropped to 3.6L. cam is 277 @ 50thou

in the next comparison the stroke has been taken up to 90mm and a capacity of 4.4L. cam is 237 @ 50thou

in all cases a red dot has been placed on the curve indicating a piston velocity of 4900-5000fpm. this is an extremely high limit, but provides a round number for these comparisons. cam duration and timing has be altered appropriately to put the peak power immediately prior to this piston velocity limit.

IMPORTANT NOTE: All other engine parameters have been kept the same, including rod length - thus implying that the deck heights of the engines in these comparisons would be different, ie the blocks themselves would need to be different to accomodate these changes. this obviously ignores the real world situation where if modifying any given stock stroke value, that the rod length would have to change accordingly with stroke to keep the the assembly within the stock block deck height. such changes in rod length would have an impact both on frictional losses, engine longevity, piston design, intake flow characteristics etc etc... but lets ignore that for now and concentrate simply on stroke and rpm...

obviously other factors such as port size and runner/primary sizes would need to be further optimised to suit each individual assembly, but given the simplicity of this comparison, i have chosen to ignore these also.

so, here are the comparisions...

the original baseline 82.5mm stroke 4.0L 1uz. 5000fpm limit is reached at 9200rpm. * note - original baseline engine is GREEN in this example


* note - original baseline engine is GREEN in this example

the following demonstrates a drop to 75mm stroke and 3.6L capacity compared to the stock 82.5mm stroke and 4.0L capacity. 5000fpm limit is reached at 10,000rpm. * note - original baseline engine is now in RED

note the increase in revs required to reach was is ultimately the same peak power, but with a significant loss in overall torque (which requires that said increase in revs to compensate total power).


* note - original baseline engine is now in RED

this is the 90mm 4.4L capacity comparison to the stock 82.5mm stroke and 4.0L capacity. 5000fpm limit is reached at 8300rpm. * note - original baseline engine is now in RED

note the big increase in available torque, but due to limits imposed by piston velocity, the revs are unable to be brought up to a point where total power is recovered.


* note - original baseline engine is now in RED

[ed]

the following comparisons now take the perspective of a fixed capacity at 4.0L and vary the strokes again (75mm and 90mm) but with respective changes in bore size to maintain a fixed 4.0L capacity (93mm and 85mm)

again, this is the stock 88.5mm bore, 82.5mm stroke, 4.0L configuration running a 257 @ 50thou cam as noted in the first example above. 5000fpm limit is reached at 9200rpm. * note - original baseline engine is GREEN in this example



this is the 93mm bore, 75mm stroke, 4.0L capacity running a 277 @ 50thou cam. again 5000fpm limit is reached at 10,000rpm. * note - original baseline engine is now in RED

notice how the increased capacity (back to stock) has increased the tourque from the previous 75mm stroke example, but this time the engines ability to rev without hitting peak velocity too early has given it an overall increase in total HP - this is reflective of the net increase in total CFM passing through the engine



the following is now the 85mm bore, 90mm stroke, 4.0L capacity configuration running a 237 @ 50thou cam. again 5000fpm limit is reached at 8300rpm. * note - original baseline engine is now in RED

notice now how we have lost capacity, and despite the modest (but reduced) increase in lower rpm torque, we have again lost the ability to rev, and thus completely fail to reach the baseline HP we have achieved with our stock engine configuration. again, this is a simple reflection of decrease net CFM passing through the engine

[rob]

You must spread some Reputation around before giving it to ed_jza80 again.

[ed]

You must wait 1/2 an hour and recover before giving it to ed_jza80 again.

fixed for you

[The Witzl]

This topic gives little valuable analysis to The Witzl without the deck height remaining fixed and rod lengths/pistons being adjusted.

Ed is also smelly.

[takai]

Sweet analysis, very useful for shutting up some nobends. Pity about not being able to give you more rep again.

[Enchanter]

Excellent to see this kind of stuff, I love it.

must spread the love though

[Ben Wilson]

Ed - Did you have a look at inlet air speeds (and mach numbers) at peak power for the different bore/stroke ratios?

[ed]

*thread sanitised to keep this thread on track for what it actually is - a simple comparison

ben - i did look, but cant remember. the baseline engine has port speeds of 306fps @ 8300rpm (peak power) with 340fps noted at abs redline 9000rpm (piston velocity 4900fpm)

as i metioned, i didnt optimise the other egine parameters as that would just take wayyy to long to figure out simply for these comparisions.

im just doing a run based on fixed deck height now, the examples arent very extreme though, so im not sure if the subtle differences in loads and frictional losses with show themselves... stay tuned

cheers
ed

[Billzilla]

Thought this would be of interest.

[ed]

ok, so the following are a more realistic simulation based on running a stroker/de-stroker crank in a stock block. this means a fixed block deck height, and fixed bore size.

pistons pin height has been fixed at stock height of 33.75mm and calculations are made based on an assumed deck height of 221mm.

the stock basline engine runs a 146mm long rod, yeilding a rod stroke ratio of 1.77:1

the 75mm de-stroked engine can run a 149.75mm rod, and we get a rod stroke ratio of 1.99:1

the 90mm stroker thus is required to run a 142.25mm long rod, giving a final rod stroke ratio of 1.58:1

the following is simply a graph of the two stroked/de-stroked configurations put against each other in this comparison. both are cammed as in the previous models, with peak piston velocity of 5000fpm being the limiting factor (8300rpm ans 10,000rpm respectively



note the following losses generated for the 90mm stroker engine taken at 7500rpm:

for the 90mm stroker with 142.25mm rod:

torque lost to friction - 65.4
hp lost to friction - 93.4
mechanical efficiency - 83.9%
hp losses to the cooling system - 139

for the 90mm stroker with 146mm rod (modeled previously)

torque lost to friction - 64
hp lost to friction - 92
mechanical efficiency - 84.5%
hp losses to the cooling system - 139

for the destroker configuration its interesting to note the following losses comparison taken at 9500rpm:

for the 75mm de-stroker with 149mm rod:

torque lost to friction - 59.6
hp lost to friction - 108
mechanical efficiency - 82.2%
hp losses to the cooling system - 150

for the 75mm de-stroker with 146mm rod (modeled previously)

torque lost to friction - 61
hp lost to friction - 110
mechanical efficiency - 83.6%
hp losses to the cooling system - 150

so, despite a 2000rpm difference in sample point, and having basically the same mechanical efficiency and piston speeds (and thus distances, net cfm etc) at these points, note the significantly (about 10%) increase in frictional losses experienced by the stroker engine due primarily to the poor rod stroke ratio.

even within comparisions, the small changes in rod/stroke ratio can be seen to manifest themselves by small, but measurable frictional losses

now looking at the graph above, its notable that the stroker config doesnt reach a comparable level of hp output, despite both of these engines essentially functioning at the same level of performance in their peak power conditions (ie same VE, same mech efficiency, same piston speeds, same displacement per min etc)... if one looks at the increased frictional losses incurred by the stroker engine (4.4ft/lbs torque) at this point and overlay this loss on the stroker engine at this peak power rpm, we can say its 'missing' 6.3hp at this point. this comes close to making up the witnessed deficiency noted between the two output curves...

thats a bit of an over simplification, but its a demonstrative example, and is designed to be simply that. these engines can both be further optimised for use in their desired operating conditions, but the above priciples will still apply

the other thing to keep in mind is that this discussion is ONLY designed to look at efficiencies in design, with the goal of demonstrating why some aspects of an engines design are important to optimise. but which would be the 'better' engine? who would win a drag race? which one would be more fun? very close call. area under the curve of these engines is very tight. subjective driving experience, gearing, intended purpose and driving style are for a different discussion...

for a fuller discussion on the impact of rod/stroke ratios on engine dynamics and output characteristics, go here:
http://www.v-eight.com/tech_forum/viewtopic.php?t=110

cheers
ed

[Enchanter]

Thought this would be of interest.

So using this as a snapshot, if your displacement was restricted by class regulations but your number of cyl was not, a short stroke engine is a far better choice.

[The Witzl]

i've already repped you for this thread Ed, so consider this a confirmation of that repping.

Good posts, and just the sort of info I wanted to read about

[JustenGT8]

For us mere mortals that will only ever play with bore, want to run some UZ sims up to a 100mm max? 100mm would net us 5.4litres from memory...be nice to see what the maths says a way oversquare UZ would do.

[rupes]

I think the real advantage of using a shorter stroke to gain more rpm (reliably), becomes apparent when a large turbo is installed. This would keep the torque up (or better yet, boost the torque) where it would normally start falling off. With the torque figure not falling off, then we gain alot more hp up top, with the rpm advantage.

Of course, in the real world, the high-rpm loads plus boost induced torque can really stress the engine.

Great thread.