L18 Long rod or L20b motor?

[DanielC]

King Rat, correct me if I am wrong.

Boring a L-18 .020 over stock size adds about 5.3 CC to the volume of the cylinder, at the bottom of the stroke.

A stock L-18 has about 442.5 CC per cylinder.  Adding 5.3 CC to 442.5 is a little over 1%.

 

Unless you are building a race engine (not a street racer wanna be) I do not think that is enough of a difference, to make a difference.

 

If you are that close to the ragged edge, you need a burette, and other equipment to CC the cylinder heads, and the piston dish.  Before you CC the head, you need to have a valve job done and complete, so you know what the combustion chamber volume is.

You need assemble the engine, measure deck height, positive, or negative, and take into account torqued head gasket thickness.

 

Burette, and plates.

Burette, close.

 

 

Here is my math.

Cylinder displacement equals the radius of the cylinder, squared, times Pi, times the height. (stroke)

L-18 bore size is 3.346, stroke is 3.071

Displacement per cylinder, stock L-18, 27.0036 cubic inches, or 442.507 CC.

Displacement per cylinder, .020 over L-18, 27.3273 cubic inches, 447.813 CC.

One cubic inch  equals 16.387 CC

[DanielC]

If you are still deciding between a long rod L-18, or a L-20-B, it boils down to this.

If you have all the parts for the L-18 long rod, including pistons, it will have more power than a L-16.  I believe there are two different rods used in L-16 engine, later rods have larger diameter bolts.  I think these rods were also in the L-24 engine,used in the 240Z

 

If you do not have all the parts for a long rod L-18, build a L-20-B.

 

Generally, the L-18 will rev higher, possibly be more "fun" to drive.  But in most situations, the L-20-B will be faster.

[datzenmike]

Stock 85mm bore  L18 with crushed gasket thickness of 1.2mm......

Stock with open chamber head............................ 8.44

Stock with closed chamber head.......................... 9.00

Stock with L16 head............................................ 9.38

 

Stock L18 with L20B pistons

Open chamber head........................................... 7.65

Closed chamber head......................................... 8.10

L16 head........................................................... 8.37

 

L18 over bored with 86mm L28 flattop pistons

Open chamber head............................................ 9.1

Closed chamber head......................................... 9.85

L16 head.......................................................... 10.30

[datzenmike]

King Rat, correct me if I am wrong.

Boring a L-18 .020 over stock size adds about 5.3 CC to the volume of the cylinder, at the bottom of the stroke.

A stock L-18 has about 442.5 CC per cylinder.  Adding 5.3 CC to 442.5 is a little over 1%.

 

Unless you are building a race engine (not a street racer wanna be) I do not think that is enough of a difference, to make a difference.

 

 

Here is my math.

Cylinder displacement equals the radius of the cylinder, squared, times Pi, times the height. (stroke)

L-18 bore size is 3.346, stroke is 3.071

Displacement per cylinder, stock L-18, 27.0036 cubic inches, or 442.507 CC.

Displacement per cylinder, .020 over L-18, 27.3273 cubic inches, 447.813 CC.

One cubic inch  equals 16.387 CC

As the L18 is in mm I stayed with mm to calculate and there's less 'rounding off' converting back and forth.

 

85mm x 78mm is..... 442.386cc or a 1,769.544cc motor

85.5mm x 78mm is... 447.606cc or a 1,811cc motor

 

The difference is 5.219cc

 

Our differences can also be attributed to the mm being 0.0394" So half a mm isn't 0.020" but slightly less. Not nearly enough to matter for a compression ratio.

[DanielC]

"As the L18 is in mm I stayed with mm to calculate and there's less 'rounding off' converting back and forth."

 

That is cause you're Canadian, Ehh, I am in Mericua, we measure in inches!

 

 

 

 

Kidding.

[datzenmike]

:lol: ... well you continue to convert then.

 

I was brought up with feet, inches, rods, furlongs and miles. Had to adapt to the metric measurements  when I started working in the surveying business. I think surveying changed over in the late '70s early '80s to metric so there are still some legal plans out there in feet. When we have to work with one I get told 'Mike, pace down the road 66 feet to find the next property line" I have to convert feet to meters and pace 20 long steps. It's so weird not being able to look down the road and roughly know where 66 feet is, but if told 20 meters... oh that's easy. 

 

20 meters is the width of a road allowance p/l to p/l. It used to be 66 feet or one chain of 100 links or (4 rods or 22 yards) 10 chains makes a furlong. 20 meters is not 66 feet but close at 65.62 feet so new road allowances are now slightly smaller. A mile is 5,280 feet... try dividing that in quickly into thirds. The kilometer is 1000 meters. You can quickly figure 333.333 meters. :lol: :lol: :lol: :lol: :lol: :lol:

[DanielC]

1760 feet = 1/3 of a mile.   And I do not have to find that .333 of a foot!  but if I did, it's 4 inches.

But we are getting way off topic here.

[Doctoraudio]

Alrighty then?

If I stay out of the secondary, my mileage seems pretty good still.
But if I stump on it, I am down to about 18 mpg. Which is about 8 mpg off.
 

I can also testify to this.
Once those rpm's start climbing.
It is a little punchy, lol!
I thought my clutch was slipping. Nope it is breaking loose pretty quick.
I love this engine!

[metalmonkey47]

Okay this is going to sound stupid seeing as my motor is equipped with what I'm about to ask...but I honestly have no idea...

 

What's the benefit of running longer rods if it doesn't change displacement, or such?

 

 

 

I can also testify to this.

Once those rpm's start climbing.

It is a little punchy, lol!

I thought my clutch was slipping. Nope it is breaking loose pretty quick.

I love this engine!

Sounds like my LZ man! I got on hills and just spin if I'm not careful. In the rain, I'm pretty much a grandma, otherwise I'm sliding and the tires have zero traction.

[datzenmike]

Displacement cannot be changed by changing the rod length.

 

 

On the same bore and stroke motor, a long rod is moving slower as it comes up to and past TDC than a shorter rod. I would imaging that at extreme RPMs when time is short the piston top is able to capture more of the downward force of the exploring gasses. Long rods also exert less side thrust on the pistons.

[metalmonkey47]

Displacement cannot be changed by changing the rod length.

That's what I'm getting at. If it doesn't change that, what does it actually change? What's the benefit?

[datzenmike]

 

On the same bore and stroke motor, a long rod is moving slower as it comes up to and past TDC than a shorter rod. I would imaging that at extreme RPMs when time is short the piston top is able to capture more of the downward force of the exploring gasses. Long rods also exert less side thrust on the pistons.

.

 

I could be wrong but maybe this above. I know if you run a longer stroke you have to shorten the rod. Too short and the piston gets pulled sideways too much and this increases friction. Too much importance is put on rod stroke ratios. It might mean something at 8K but for the most part is not that important

[DanielC]

I think the advantage of a long rod engine is that the piston is slowed down close to TDC, in comparison to a shorter rod, and this allows the engine to breathe slightly better at high RPMS.

But I still think that if this combination of parts was so good, Datsun would have done it.

[620Turbo4X4]

 

That's what I'm getting at. If it doesn't change that, what does it actually change? What's the benefit?

 

 

 

The rod ratio changes how the power is delivered to a certain degree. A longer rod requires less cam duration and operates in somewhat of a peaky power band. Shorter rods do the opposite and will offer a wider power band, but require a bit more cam duration to operate in the same RPM ranges. What it all boils down to is the piston speeds and length traveled at TDC and BDC (not to be confused by a stroke change) VS., valve closing and opening times (degrees of crankshaft and camshaft rotation when these events are occurring).

 

Long rods would be fun for the weekend racer or race car with low gears and close ratio trans. Short would be more drivable and forgiving for a daily driver.

 

 

Reading this article will give you a much better idea how Rod/Stroke Ratio all works...

 

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

 

Chapter 18 Rod/Stroke Ratio. I highly recommend reading it all if you have the time….

[datzenmike]

I think the advantage of a long rod engine is that the piston is slowed down close to TDC, in comparison to a shorter rod, and this allows the engine to breathe slightly better at high RPMS.

But I still think that if this combination of parts was so good, Datsun would have done it.

 

Well they did in the Z20E vs. the L20B motors. The Z20E has a 152.5mm long rod. The L20B which uses the same basic block, same crank and has the same bore and stroke uses a 145.9mm rod.

 

 

That's what I'm getting at. If it doesn't change that, what does it actually change? What's the benefit?

The rod ratio changes how the power is delivered to a certain degree. A longer rod requires less cam duration and operates in somewhat of a peaky power band. Shorter rods do the opposite and will offer a wider power band, but require a bit more cam duration to operate in the same RPM ranges. What it all boils down to is the piston speeds and length traveled vs. valve closing times.

 

Long rods would be fun for the weekend racer or race car with low gears and close ratio trans. Short would be more drivable and forgiving for a daily driver.

 

 

Reading this article will give you a much better idea how Rod/Stroke Ratio all works...

 

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

 

Chapter 18 Rod/Stroke Ratio. I highly recommend reading it all if you have the time….

 

.

All except the rod length does not change the distance the piston travels.

 

The shorter the rod the more side loading on the piston.

 

 

The Racer Brown article is very good, but involved. Increasing the rod length alone probably doesn't do that much. (other than reducing side loading... if it was a problem in the first place) To take advantage of the changed piston acceleration rates and increased dwell at TDC you would also have to change cam profiles. It's all very esoteric and somewhat above street driving.

[metalmonkey47]

Dang thanks guys  :thumbup:  I'm gonna bookmark that. Lots of good info.

 

 

Only reason I asked is because I have the long Z20 rods in my LZ and I was curious if they were really doing me any good. 

[Doctoraudio]

From what I understood that having the long rods reduces the side to side load on the piston.

Also the longer rods changes the engine's rpm range.
Short rods equal low end torque.
Long rods bring a higher rpm torque range.

Also they say short rods for daily driver is best.
And long rods makes the motor a street racer.

If any or all of that is wrong, someone please say so.
I am just checking that I got it right.

 

[datzenmike]

Well they did in the Z20E vs. the L20B motors. The Z20E has a 152.5mm long rod. The L20B which uses the same basic block, same crank and has the same bore and stroke uses a 145.9mm rod.

 

.

What we need is someone with a Z20E motor with an L20B head on it. This would give us a comparison with a regular L20B motor. The difference is then between the 152.5mm and the 145.9mm rods. I think a long rod motor has more high RPM potential than a short rod motor.  Potential....  but nothing to speak of in a street car. If I was building a race motor I would go with longer rods if they were available.

[620Turbo4X4]

Hard to say if the additional or reduced side loading would be much of an issue in anything less than all out 8.5K race engine. Pistons purposely have the pin bores offset to reduce piston slap, which is noisy and increases wear. (That's why there is a front and back, even on flattop pistons.) Since peak combustion cylinder pressures typically occur in the 17 Deg. ATDC range for best torque on most gas engines the amount of actual angle at peak pressures would be minimal compared to being at 90 ATDC. Would be hard to say if it would be an issue. Who knows... It might make for a longer piston life by reducing piston rocking? Honda engines have very short rod stroke angles and run forever….

 

Yes the Racer Brown is a bit much to take in. The more times you read it, the more it will make sense. He does do a good job of explaining how a shorter rod ratio has a de-sesitising effect on valve timing and visa versa, and how that effects the power band or torque curve of an engine. A long rod motor with a big cam would be similar to a two stroke power band. fun motor!!, but not so friendly for a DD.