L18 Long rod or L20b motor?

[datzenmike]

Generally, similar equipped larger displacement L20B equals poorer gas mileage than L18.

[Doctoraudio]

Okay, second question is.
Which motor will respond quicker and have more snappy punch.
I am not looking for spin the tires faster on take off. 
I guess I am asking, which one will come on step quicker?

[datzenmike]

IMHO...L20B.

 

Longer stroke and 11% larger.

 

I had a 521 with L16 and transplanted an L20B from a car in. It pulled nicely at low and mid range... much firmer.

[Stoffregen Motorsports]

The L20B will have more off-the-line perofrmance because of the longer stroke. But the rod-stroke ratio of the L20B does not compare when considering a long rod L18. If it were my motor, I would build an 87mm bore, L16 rod, L18, but if you can't do the work yourself, a stock L20B is the obvious choice.

[68Datsun510]

Say whaaaaaaaaaaaaat???

 

 

 

My L19, exactly what Stoffregen Motorsports described, and built! Will be 3 years old in march '14.... Incredibly reliable "happy" engine, revs freely (for a single carb) and does everything I ask. 1868 cc, 10.1:1 cr.

 

Indeed it lacks bottom end off line grunt, but its still perfectly livable.

[Dime Dave]

Rod/Stroke ratio is actually pretty good on a stock L20B.

145.9/86 = 1.697

 

The L18 with an L16 rod is basically the same (a 0.47% difference)

133.0/78 = 1.705

 

And the L18 with a stock rod is lower

130.2/78 = 1.669

 

But if you want a better rod/stroke ratio, just go with a stock L16 combo

133.0/73.7 = 1.805

 

Or you can run Z20 6" rods in the L20B and it is almost as good

152.5/86 = 1.773

 

All in all the benefits of a long rod L18 will not offset the 182cc displacement advantage of the L20B.

[68Datsun510]

I built mine simply because I scored a used one from a friend that had been built in the 90's by the same Stoffregen Motorsports. Through sheer luck i found him years later and he helped me out majorly in resurrecting it. Its a unique talking point, watching peoples questioning look when you say "L19"... =P

[datzenmike]

Rod/Stroke ratio is actually pretty good on a stock L20B.

145.9/86 = 1.697

 

The L18 with an L16 rod is basically the same (a 0.47% difference)

133.0/78 = 1.705

 

And the L18 with a stock rod is lower

130.2/78 = 1.669

 

But if you want a better rod/stroke ratio, just go with a stock L16 combo

133.0/73.7 = 1.805

 

Or you can run Z20 6" rods in the L20B and it is almost as good

152.5/86 = 1.773

 

All in all the benefits of a long rod L18 will not offset the 182cc displacement advantage of the L20B.

 

 

 

 

So everyone argues about the r/s ratio. What about an infinitely long rod? how would this improve a motor... or will it?

[68Datsun510]

Z24

165/96 is 1.72...

 

Z22

145.9/92 is 1.58....

[Dime Dave]

What hasn't been talked about is what the rod/stroke ratio effects. 

 

People generally associate ratios above 2.0 as having limited benefit and below 1.5 as extremely RPM/performance limiting. Small rod/stroke ratios on long stroke engines create extremely high acceleration rates when the piston changes directions as RPM increases. 

 

The Nissan QR25 when it was first raced in World Challenge had many durability issues. Chuck Cunningham's Real Time Racing was running the program for Nissan and was not keeping the engines together in race conditions. The 100mm stroke and 1.43 rod to stroke ratio was ripping rods and pistons apart. It wasn't until they got together with Carrillo Rods and J-E Pistons that they solved the issue on the racecars. The 2006 and earlier OE QR25 had a 6100 rpm redline. The 2007 on SER Spec V used better reciprocating parts and the redline was increased to 7000 rpm. 

 

Here is a good article by Mike Kojima about the QR25 comparing it with the SR20 and the stroke/rod ratio issues.

Jim Wolf Technology's Turbo Nissan QR25DE Powerhouse - Part 1

 

Here is a detailed tech article by Jere Stahl (Stahl Headers) talking about rod/stroke ratios and their effects

http://www.stahlheaders.com/Lit_Rod%20Length.htm

[datzenmike]

Z24

165/96 is 1.72...

 

Z22

145.9/92 is 1.58....

:blush:  whoops.

 

 

 

 

So from the reading, the rod stroke ratio mostly affects race engines? Few street engines exceed 6K.

 

"In general, most observations relate to engines used for some type of competition event and will in general produce peak power higher than 6000 RPM with good compression ring seal as defined by no more than 3/16 CFM blowby per cylinder.

"

 

 

So from the reading, the rod stroke ratio mostly affects race engines? Few street engines exceed 6K. This is somewhat like arguing the merits of a smaller or larger spoiler on a daily driver. Would you say that there is an effect between longer and shorter rods but this comes into play only at high or extreme RPM? Simply put, in every day driving, is there any advantage one over the other?

[68Datsun510]

Its just another number to throw around when bench racing. =)

[Ranman72]

since Rebello builds long rod L20b I would have to say longer is better 

everything he builds has Dyno numbers to back it up 

so my opinion LONGER IS BETTER 

[68Datsun510]

So is rebello gonna fix you?

[68Datsun510]

oooooooooooooooooooooooooohhhhhhhhhhhhh snap! hahahah jk ranman

[Ranman72]

damn maybe when I go back it will get bigger again 

[Dime Dave]

Most of the more modern engines are over square or longer stroke than bore. This helps with emissions and generally doesn't help with high end performance because they tend to have smaller rod/stroke ratios (like the QR25) and huge acceleration rates on the rod/piston as you crank up the RPM's.

 

All the historically high RPM engines were over square (larger bore than stroke). Ford 289 (like in the AC Cobra, GT40 & Shelby Mustang) was a 4" bore with a 2.87 stroke. The Datsun L16, 83mm bore/73.7mm stroke. The 302 Z28 Chevy and Ford Boss 302 both 4" bore and 3" stroke. The Nissan SR16VE with a 86mm bore and 68.7mm stroke in the N1 version has an 8600rpm factory redline.

 

 Had a conversation with a cam grinder in Florida talking about the L13 Datsun engines that were being used in hydroplanes, These 83mm bore x 59.5mm stroke race engines were regularly being turned to 10K. 

[DanielC]

Read this.

http://www.datsport.com/racer-brown.html

Somewhere, there is a discussion about bore, stroke, and connecting rod length.  

 

The connecting rod length, compared to the engine stroke, affects the engine this way.  Lets just say we have that infinitely long connecting rod.  The piston would be halfway down in the cylinder 90 degrees past TDC.  We have shorter rods than that.  The change is the piston is already halfway down in the cylinder, about 75 degrees past TDC.  In other words, the piston is moving really fast, close to TDC, and moves pretty slow around BDC.

 

So what does piston speed have to do with anything?  This.  You would like the intake valve to be as wide open as possible, when the piston speed is high.  This creates a high vacuum, and sucks air in faster.  But you really cannot open valves that fast, because it takes time for valves to open.  Do yourself a favor.  Read the lengthy racer brown article.

 

So back to engine choice.  The L-16 likes to rev. best of all three engines (L-18, L-20 )  The L-18 put out torque a little lower than a L-16.  That is because of the bore to stroke ratio, among many other factors.  You can make the L-18 a little happier at higher RPMs, by using L-16 rods, that improves the ration of rod length to stroke, but is is still not as good as the L-16, the L-16 will still REV a little more.

 

The L-20-b engine like to rev the least, but is has about 25 % more displacement.

 

My feeling, if the long rod L-18 would be such a good engine, Datsun would have done that.  Instead of making l-18 rods, when the L-18 engine came out, just keep making the L-16 rods.  it is cheaper to keep making something you already have, than it is to design new parts.

 

Another advantage of longer rods is less side thrust on the cylinder walls, especially in the power stroke.  But to get long rods to fit in any engine, you have to make the engine taller.  Or you have to move the piston rings higher on the piston.  longer rod, higher piston pin.  Eventually, you run out of room between the top of the piston, and the piston pin, for the piston rings. 

 

Almost every design characteristic in an engine has both advantages, and disadvantages.  The trick is finding the right balance of every design parameter.

PS.  Read the racer brown article.  It is really good.

[datzenmike]

since Rebello builds long rod L20b I would have to say longer is better 

everything he builds has Dyno numbers to back it up 

so my opinion LONGER IS BETTER 

.

Yes for racing high RPM power. For street stock it doesn't matter that much.

[EricJB]

Rod ratio aside, more displacement puts more demand on the flow of the head. So it's safe to say an L18 will be more rev happy than an L20, with the same head/intake/exhaust.

 

You could say an L20 revs better than an LZ 2.3. I had to do a ton of top end work to get my LZ to rev like my L20.

[datzenmike]

All things equal the L20B will always make more power even if using the same L18 head. Yes it will have to work a bit harder at higher RPMs than the L18 but still out perform it. Yes the L18 will rev faster and easier because of it's lighter internal parts and because it's smaller displacement needs less air flow. Can it out perform an L20B? absolutely, but it will take a lot to make up the 200cc advantage of the L20B.

[DanielC]

You have to remember when the L-16 engine was made, the USA was just beginning to notice small economy cars.  This was before 1973, and the first oil crisis.  Gas was still around 25 to 35 cents a gallon, for good high octane leaded premium.

 

Datsun pickups had pushrod engines, 1200 then 1300 CC.  The 521 was the first Datsun truck to get a L-16, the 510 came out with the engine, and the 240 Z made the USA notice Datsun.   But if you take a look you can find 521 trucks, and even 620 trucks with the 1300 engine, or the 1500 pushrod engine, in foreign countries, an engine that if you found one in the USA, it was in a fork lift.

 

Why the L-16 engine in a 521?  it was still a small engine, compared to what people in the USA were used to.  One reason, the 521 had to have decent performance at freeway speeds.   J-13 engines were not real comfortable at 70 MPH, with a 4.88 rear axle.  so the 521 got the L-16, and the rear axle gearing put the engine right at it's torque peak, at 55 to 60 MPH, about 3600 RPM.  This made 70 MPH cruising pretty easy.

 

The L-18 engine was a "transition engine".   Something with a little more power, while the L-20-B engine was being developed.  The L-20 engine had enough displacement that it was not as critical that it be able to freely REV as the L-16.  So the L-20 was build with a lower RPM peak torque curve.   The Z-engines that followed the L-engines were even bigger displacement, and especially in the trucks, that got even a lower RPM torque curve.

 

So the L-16 engine was designed to run, and be happy at high RPM's, because it had to.  The bigger four cylinders engines had the advantage of displacement, and did not need to run at as high RPM.

[datzenmike]

That's very true. Emissions regulations were also reducing some power and it was only prudent to simply make motors larger. That, and North Americanos want larger motors. Look at the Titan and the Toyota trucks now.... they look like Chevys of the '70s. Where did the mini truck get to?

[EastBay521]

how much will the compression change if I were to get the block bored .020 over?

[datzenmike]

I'll work it out later this morning for you when I get home.