effect of rod/ stroke ratio on detonation

[datsphilly]

hey all, been reading here and abroad and occasionally find mentioned that a larger rod/stroke ratio ( typically 1.8:1 or greater) somehow reduces detonation... im a pretty smart guy but dont immediately see how the two things correlate, some essplaining would be nice from some of the seasoned L motor builders on the board as to not only the science behind it, but the piston/rod combinations that can achieve a ratio in this range. the project i have in mind is a fuel injected L28 but im sure the theory applies to all four stroke internal combustion engines.

 

ive noticed also from reading here that the threashold for pump gas L motors seems to be 11:1 compression ratio and im curious to what extent carbs vs fuel injection was used to come to that medium.

 

also i recently moved to california (thanks to the us army) and theres only 91 octane available here, as opposed to 93 back home in miami. can two octane points really make a dramatic difference in detonation?

 

damn, that turned into a much more complicated post than i originally hoped... any insight would be greatly appreciated. thanks in advance.

 

-philly

[MAG58]

The point of a longer rod/stroke ratio is to increase dwell at the top and bottoms of the cylinders. The more dwell it has at the bottom, the longer the time the cylinder has to fill under it's full capacity. What goes with a long rod is less side loading, with less rod angle, more of the stroke is dedicated to up and down movement, thus more efficiency. Along with this lower rod angle, you see less friction from side loading and a slightly cooler running engine.

 

Onto your question, increasing rod/stroke ratio allows for the piston to decel and accell slower at the relative maxes of it's movement, and allows for a longer compression event at TDC. This slower decel (the deceleration is spread out over a longer time) gives the gasses more 'time' to compress, and reduces detonation.

 

Reducing detonation is where efi really shines over carbies. Using a knock sensor, efi has the ability to only advance timing as far as the engine allows without knock. (Though in most cases, it starts at max timing and pulls timing as it detects knock)

Carbs do not have this benefit and rely on your skill at timing and tuning to reduce knock.

 

I cannot tell you how much cr you can run in CA since in IN you can get 94 octane from sunoco, but you'd be surprised how much 2 octane numbers can change things.

 

Though if you're dedicated, you can switch to E85 which iirc is about 103-105? octane.

[datsphilly]

thanks for the explanation, that makes perfect sense. any insight as to what rod lengths are readily available for the L28? is the L24 the "go to" rod for increasing my rod/ stroke ratio, or are there other ones i can use?

[datsphilly]

also, has anyone used an L24 crank instead of longer rods to achieve a better rod/ stroke ratio? some thing like a destroked small block chevy? the reason i ask is the L24 crank is about 5.5mm less stroke than the L28 while the L24 rods are only about 2.5 mm longer than the L28 ones... i guess this would require custom pistons to keep the same deck clearance at TDC ?

[datsphilly]

crap. sorry. again thats alot of questions but its just stuff either im too dumb to find or hasnt been asked... probably the dumb me part.

Edited September 28, 2009 by datsphilly
spelling

[Evan Purple240zt]

also, has anyone used an L24 crank instead of longer rods to achieve a better rod/ stroke ratio? some thing like a destroked small block chevy? the reason i ask is the L24 crank is about 5.5mm less stroke than the L28 while the L24 rods are only about 2.5 mm longer than the L28 ones... i guess this would require custom pistons to keep the same deck clearance at TDC ?

 

The motor that Garrett76zt had was a stock turbo crank, L24 rods and custom pistons. Very smooth revving motor.

 

Evan

[datsphilly]

im curious why more emphasis isnt put on turbocharging L24's if this rod stroke ratio is such a factor in detonation, i know in the early days prior to the L28et and the F54 and the P90 guys were turbocharging L motors and making good power, why wouldnt a flat top L24 with head tricks makes as much, if not more, power safely than the L28?

[datsphilly]

400 cc of displacement is neither here nor there when we talk about turbocharging.

[RTz]

im curious why more emphasis isnt put on turbocharging L24's if this rod stroke ratio is such a factor in detonation, i know in the early days prior to the L28et and the F54 and the P90 guys were turbocharging L motors and making good power, why wouldnt a flat top L24 with head tricks makes as much, if not more, power safely than the L28?

 

400 cc of displacement is neither here nor there when we talk about turbocharging.

 

 

 

A slight improvement in det. resistance won't make up for a 16% increase in displacement.

[johnc]

Reducing detonation is where efi really shines over carbies. Using a knock sensor, efi has the ability to only advance timing as far as the engine allows without knock. (Though in most cases, it starts at max timing and pulls timing as it detects knock)

Carbs do not have this benefit and rely on your skill at timing and tuning to reduce knock.

 

Uuuhhhh, timing and fuel are separate items. You can have a knock sensor and a crank trigger as input to computer controlled timing on a Weber Carb'd L24 and achieve exactly what you're describing above. Been there, done that - although its so 1980s.

[MAG58]

True statement. The better and correct phrasing I should have used was, this is where full engine management shines over a carb and dizzy setup. I'm going to stick with my dizzy statement because the work required to run that kind of ignition control would put you well on your way to full engine management and much more easily adjustable fuel curves (unless you just like swapping jets in your weber's).

[datsphilly]

A slight improvement in det. resistance won't make up for a 16% increase in displacement.

 

well then i guess that begs the question, how much detonation resistance is evident with higher rod stroke ratios and is it in fact significant enough to build a street engine towards this goal?

[wondersparrow]

Uuuhhhh, timing and fuel are separate items. You can have a knock sensor and a crank trigger as input to computer controlled timing on a Weber Carb'd L24 and achieve exactly what you're describing above. Been there, done that - although its so 1980s.

 

I would be extremely interested in hearing about that one day. Is megajolt capable of doing that? If its not that complicated, I would really be intrigued enough to try that...

[Daeron]

well then i guess that begs the question, how much detonation resistance is evident with higher rod stroke ratios and is it in fact significant enough to build a street engine towards this goal?

 

Unfortunately, that is where it breaks down. To the best of my knowledge, detonation resistance is not a quantifiable characteristic (I am sure it is quantifiable in some means, but not for comparison/planning purposes) but the general consensus seems to be that the displacement is worth it.

 

If you REALLY wanted to maximize this idea, you would want to grab an L20A crank (Japanese and Aus market 2.0 liter L6) crank, LD28 rods, and 240SX pistons (maybe custom pistons very close to the KA in deck height, i forget) and bore your N42 out to 89mm. R/S ratio around 2.0:1, high potential redline engine, compression can be whatever you want.. flat tops kissing a closed-chamber head, I wanna guess around 10.5:1 or maybe 11. The thing is, THIS motor is going to be a picky picky picky picky bi-atch. It will take LOTS of time on a good dyno to tune it in properly to eke every bit of power out of it, it will need to be geared to whatever track you are running on (because this is a race motor I am proposing here, and a durned specific, oddball at that.. this is all a hypothetical to take your idea to its fullest extent. I have been where you are.) AND it would need to be governed by a pretty good standalone ECU if you wanted any reliable longevity. (In other words, without strict engine control, this engine may easily blow its top as frequently as an old Jag...)

 

Here. http://www.ozdat.com/ozdatonline/enginedesign/ Have fun. Most of the info is accurate, but don't go spending $3-5K on parts and put them together assuming they fit because the website said so. (Thats my standard disclaimer when distributing engine builder programs like this.)

[MAG58]

I'm still not sure why people want to make a high winding monster on a compromised crank (unless nissan added 4 counterweights for posterity on the L20A?) .

[Daeron]

I'm still not sure why people want to make a high winding monster on a compromised crank (unless nissan added 4 counterweights for posterity on the L20A?) .

 

Huh??? How is the L20A crank compromised? (Aside from the two facts that A: I was just speccing out the furthest this gentleman's idea could be taken with existing parts, and B:

)

 

Edit: 1. The question was a legitimate one, and not intended to sound as smart@ss as it might.

2. Point B above was just to illustrate how totally wicked-badass 9500 RPM sounds Don't take me TOO seriously.

[MAG58]

Unless the L20A is fully counter weighted (none of the other L6's are so I don't see why It would be) it would still be missing the same 4 counterweights on 2 and 5, leaving the surrounding counterweights to make up for it. I was reading in my "how to modify you're 510 & 240Z" books that at a bit lower rpm than what that L20 was running the cranks still died fast due to harmonics above 8k.

 

I'm not saying that it is physically impossible to do, or shouldn't be done... but what kind of gains would you get in that rev band and what kind of longevity would you see?

[datsphilly]

thanks for the input guys, youve been very helpful, as usual.

[Daeron]

Unless the L20A is fully counter weighted (none of the other L6's are so I don't see why It would be) it would still be missing the same 4 counterweights on 2 and 5, leaving the surrounding counterweights to make up for it. I was reading in my "how to modify you're 510 & 240Z" books that at a bit lower rpm than what that L20 was running the cranks still died fast due to harmonics above 8k.

 

I'm not saying that it is physically impossible to do, or shouldn't be done... but what kind of gains would you get in that rev band and what kind of longevity would you see?

 

Thank you for clearing that up. I knew the points you brought up in response, but when I read your initial post it just... went over my head.

[nissan2ner831]

haha well.....if your going for a race application then why not have a custom crank built?? I mean if your going to take it all the way go have a company like SCAT build you a custom counterweighted crankshaft. I mean if we are talking race motors here we could go this far =P

 

I like Garrett76zt's build as well...overbore to 89mm, L24 rods and custom pistons with a stock L28 crank. I wonder if you could actually go further than that...f you can get close to 138mm rods you'll approach that "perfect" rod/stroke of 1.75. It just depends on how short you can get the custom pistons I guess (I wouldn't know the minimum you could go, I guess it would depend on whether you were NA or boosted as well).