L18 flat top vs l20 flat top compression

[Tonellius_620]

Guys, 

 

I know this has been well covered but I’m still not sure on which is the safest route to go.

 

For starters, I have two motors lying around. A l18 and an l20B.
 

I bought flat top pistons and have a w53 head I plan to reuse. Im also going to get a mild cam from Schneider.
 

I’m not sure which block would be better to use because I do not want to run too high of a compression ratio. 

PS: the motor I chose will be driven 3-5 days out of the week when completed.

 

Thanks

[Crashtd420]

9 hours ago, Tonellius_620 said:

Guys, 

 

I know this has been well covered but I’m still not sure on which is the safest route to go.

 

For starters, I have two motors lying around. A l18 and an l20B.
 

I bought flat top pistons and have a w53 head I plan to reuse. Im also going to get a mild cam from Schneider.
 

I’m not sure which block would be better to use because I do not want to run too high of a compression ratio. 

PS: the motor I chose will be driven 3-5 days out of the week when completed.

 

Thanks

What size are the pistons?

 

I dont think compression ratio will be an issue with either one....

Flat tops with the w53 closed chamber head is a good combo....

 

Personally I like the L18 but the L20b would be the way to go.... more displacement is always good. ...

 

I run and L16, 84mm 9cc dome pistons with the w53 head and a mild Schneider cam... compression is around 10.7.... I drive my truck 5 to 6 days a week 9 months out of the year....

 

My guess is you would be around the high 9s or 10 for compression ratio... mike is better for the actual #..

 

[datzenmike]

 Are these pistons over size???

 

If these are from an L28 then they are already 1mm larger than the L18/L20B so just over bore the block to fit. The finish hone should reflect the rings style you plan to use so discuss this with the shop that does the work.

 

Yes... use the larger displacement L20B.

 

The W53 has tiny 1.25" intake ports better suited for an L16/18 than an L20B so port them out and gasket match to the intake you plan to use. The best intake for an L20B is the '78-'80 L20B. They have larger intake runners than the earlier '68-'74 L16/18s. The early '75-'77 co -joined intake/exhaust I don't even consider. Use the L16/18 cast iron header.

[Tonellius_620]

58 minutes ago, datzenmike said:

 Are these pistons over size???

 

If these are from an L28 then they are already 1mm larger than the L18/L20B so just over bore the block to fit. The finish hone should reflect the rings style you plan to use so discuss this with the shop that does the work.

 

Yes... use the larger displacement L20B.

 

The W53 has tiny 1.25" intake ports better suited for an L16/18 than an L20B so port them out and gasket match to the intake you plan to use. The best intake for an L20B is the '78-'80 L20B. They have larger intake runners than the earlier '68-'74 L16/18s. The early '75-'77 co -joined intake/exhaust I don't even consider. Use the L16/18 cast iron header.

Mike, yes, these are 1mm overbore pistons. 
 

I’m assuming if I go the l20B route, I won’t be able to use regular gas?

 

Thanks!

[Tonellius_620]

1 hour ago, Crashtd420 said:

What size are the pistons?

 

I dont think compression ratio will be an issue with either one....

Flat tops with the w53 closed chamber head is a good combo....

 

Personally I like the L18 but the L20b would be the way to go.... more displacement is always good. ...

 

I run and L16, 84mm 9cc dome pistons with the w53 head and a mild Schneider cam... compression is around 10.7.... I drive my truck 5 to 6 days a week 9 months out of the year....

 

My guess is you would be around the high 9s or 10 for compression ratio... mike is better for the actual #..

 

They are 1mm overbore pistons. 

[Crashtd420]

3 hours ago, datzenmike said:

The W53 has tiny 1.25" intake ports better suited for an L16/18 than an L20B so port them out and gasket match to the intake you plan to use. 

This is something I still need to do... I have new valve's for it too ... bigger intake I believe..... 

[datzenmike]

By oversize you mean '86mm L20B pistons or are they 87mm L28 pistons.

 

86mm flattop pistons and closed chamber head is 10.87 compression. I would run a higher octane gas for this.

 

 

[Stoffregen Motorsports]

57 minutes ago, datzenmike said:

By oversize you mean '86mm L20B pistons or are they 87mm L28 pistons.

 

86mm flattop pistons and closed chamber head is 10.87 compression. I would run a higher octane gas for this.

 

 

I might even cut the pistons to bring the CR down to 10:1, to be on the safe side.

 

Remember, when bringing the CR up, you also need to play with your distributor to get the timing curve where it needs to be. Datsun distributors always have too much spring tension, not allowing full centrifugal advance, and the differential between base timing and full advance is always way too much. You need to free up the centrifugal advance to get more timing just off idle, then limit the total timing so when the initial base timing is set around 12 degrees, the total won't exceed 32 or so.

 

Oh, and delete the vacuum advance, also to be on the safe side.

[Crashtd420]

1 hour ago, datzenmike said:

By oversize you mean '86mm L20B pistons or are they 87mm L28 pistons.

 

86mm flattop pistons and closed chamber head is 10.87 compression. I would run a higher octane gas for this.

 

 

I'm not saying you are wrong but why does that seem high for the combo? 

Is it the bore and stroke coming into play?

My L16 is 84mm bores with 9cc domes. I did increase the combustion chamber from 41cc to 43cc in my w53 head... from what i have been told and my rough calculations I'm around 10.7..... 

[datzenmike]

An L20B with a 1mm oversize is only 2.8cc more displacement per cylinder.

 

A flattop piston removes 11.36 cc volume from the combustion chamber that a stock piston dish would have.

A close chamber head removes an additional 4.2cc from the combustion chamber.

 

1/ An L20B cylinder that is 1mm oversize has a swept displacement of 499.3cc

2/ The L20B piston does not rise level with the block deck and is short by 0.45mm. This remaining cylinder volume is 2.61cc and is added to the combustion chamber volume.

3/ The crushed gasket thickness is 1.2mm and holds 6.96cc of volume. Also added to the combustion chamber.

4/ Closed chamber head has a 41cc combustion chamber

5/ piston dish is zero

 

Add 1,2,3,4 and divide by 2,3,4.

 

499.3

2.61

6.96

41

549.87

 

 

Divide by...

 

2.61

6.96

41

50.57

 

OR

549.87/50.57 = 10.8734

 

Never hurts to check the math.

 

[datzenmike]

48 minutes ago, Crashtd420 said:

I'm not saying you are wrong but why does that seem high for the combo? 

Is it the bore and stroke coming into play?

My L16 is 84mm bores with 9cc domes. I did increase the combustion chamber from 41cc to 43cc in my w53 head... from what i have been told and my rough calculations I'm around 10.7..... 

 

I get 10.18 and calculated it twice.

 

 

[Tonellius_620]

2 hours ago, datzenmike said:

By oversize you mean '86mm L20B pistons or are they 87mm L28 pistons.

 

86mm flattop pistons and closed chamber head is 10.87 compression. I would run a higher octane gas for this.

 

 

Mike, the flat top pistons are 1mm over sized from l18 pistons. They are the hypatec pistons. 
 

After reading the discussion with Crash, I’m leaning towards using my L18 to keep the compression at a low 10.  
 

Unless you think a l20B is safe for me to drive with such a high compression. 
 

I do have to reiterate, I am no mechanic and very new at motor rebuilds.

 

 I do not want the motor to ping or detonate from such a large compression. 

[datzenmike]

Higher compression improves efficiency and power but it risks detonation. This can be avoided by using a higher octane blend of gas. The power gained may increase the mileage enough to pay for the added cost of the gas.

 

An L18 with 1mm oversize flattop pistons and a closed chamber head has a compression of 9.86.

[Tonellius_620]

And I’m assuming a l20B with those specs is going to be 10.7ish?

 

Thanks a bunch for the help!

[datzenmike]

Yes

 

1 hour ago, datzenmike said:

An L20B with a 1mm oversize is only 2.8cc more displacement per cylinder.

 

A flattop piston removes 11.36 cc volume from the combustion chamber that a stock piston dish would have.

A close chamber head removes an additional 4.2cc from the combustion chamber.

 

1/ An L20B cylinder that is 1mm oversize has a swept displacement of 499.3cc

2/ The L20B piston does not rise level with the block deck and is short by 0.45mm. This remaining cylinder volume is 2.61cc and is added to the combustion chamber volume.

3/ The crushed gasket thickness is 1.2mm and holds 6.96cc of volume. Also added to the combustion chamber.

4/ Closed chamber head has a 41cc combustion chamber

5/ piston dish is zero

 

Add 1,2,3,4 and divide by 2,3,4.

 

499.3

2.61

6.96

41

549.87

 

 

Divide by...

 

2.61

6.96

41

50.57

 

OR

549.87/50.57 = 10.8734

 

Never hurts to check the math.

 

 

[Dusten]

12 hours ago, datzenmike said:

 Are these pistons over size???

 

If these are from an L28 then they are already 1mm larger than the L18/L20B so just over bore the block to fit. The finish hone should reflect the rings style you plan to use so discuss this with the shop that does the work.

 

Yes... use the larger displacement L20B.

 

The W53 has tiny 1.25" intake ports better suited for an L16/18 than an L20B so port them out and gasket match to the intake you plan to use. The best intake for an L20B is the '78-'80 L20B. They have larger intake runners than the earlier '68-'74 L16/18s. The early '75-'77 co -joined intake/exhaust I don't even consider. Use the L16/18 cast iron header.

 

 

Better than a cannon intake?

[banzai510(hainz)]

I would do a L20 with L18 pistons but they are hard to find now as they are a 4cc dished. and  the closed chamber head for its anti ping properties.

 

stock L20 set of 4 pistona are around for 100$ I think.  shit I might stell have L18 and a set of stock L20 sets. saving for what I dont know.

 

I believe Schneider only does regrinds now so youll need bigger lash pads for sure.

this place has the recessed type like the stock ones and can use the stock steel retainers but you need to get close the size you need. some could even vary if machine shop didnt male them the same and valve grinding old vlaves also

http://www.precisionshims.com.au/products/slotted

 

I would go with even a stock L20 set up  ,get  cam and dual carbs still better than a flaltop L18 in general sense of things.  Power is in the HEAD Carbs

 

 

this is just me talking here.(opinion)

[Crashtd420]

9 hours ago, banzai510(hainz) said:

 

I believe Schneider only does regrinds now so youll need bigger lash pads for sure.

this place has the recessed type like the stock ones and can use the stock steel retainers but you need to get close the size you need. some could even vary if machine shop didnt male them the same and valve grinding old vlaves also

http://www.precisionshims.com.au/products/slotted

 

I believe you are correct on the regrind aspect but they also will sell you lash pads, retainers and springs.... they supplied me with the lash pads I'd need based on the grind they did and the wipe pattern was correct when done.... 

 

But that shim website is a good resource....

 

Tonellius_620...

One other thing to remember is with a new or reground cam you want new or to recondition your rockers so you don't damage the cam... this would also effect lash pad choices.... that would also throw off the lash pad choice Schneider might make....

 

[datzenmike]

A reasonably 'high performance' cam will also reduce the compression at low speeds and starting. The stock L20B intake does not close at BDC but at 52 degrees after, well on it's way up the cylinder. So the entire cylinder volume is not compressed. Only at higher RPMs does it begin to approach 100% (or more) volumetric efficiency. So a 'performance cam' will favor later closing of the intake valve with the piston even higher in the cylinder.  

[Tonellius_620]

7 minutes ago, datzenmike said:

A reasonably 'high performance' cam will also reduce the compression at low speeds and starting. The stock L20B intake does not close at BDC but at 52 degrees after, well on it's way up the cylinder. So the entire cylinder volume is not compressed. Only at higher RPMs does it begin to approach 100% (or more) volumetric efficiency. So a 'performance cam' will favor later closing of the intake valve with the piston even higher in the cylinder.  

I heard horror stories about too big of a performance cam for daily driving. I would like something that idles fine when I’m at red lights, lol. 

[datzenmike]

This is why I said reasonably. Yes the stock cam is the most docile for around town.

[Tonellius_620]

2 minutes ago, datzenmike said:

This is why I said reasonably. Yes the stock cam is the most docile for around town.

Thanks again, Mike (and everyone else that helped)! 
 

After weighing the pros and cons, I’m using my l18 with the 1mm overbore pistons, w53 head with larger combustion chamber and a mild Schneider regrind cam. 

if I’m lucky, I’m hoping to push 102 hp 😎.

 

 

 

 

[Stoffregen Motorsports]

4 hours ago, datzenmike said:

A reasonably 'high performance' cam will also reduce the compression at low speeds and starting. The stock L20B intake does not close at BDC but at 52 degrees after, well on it's way up the cylinder. So the entire cylinder volume is not compressed. Only at higher RPMs does it begin to approach 100% (or more) volumetric efficiency. So a 'performance cam' will favor later closing of the intake valve with the piston even higher in the cylinder.  

Yep, installing a cam with more lift/duration will use up some of that added compression and keep it from pinging.

 

My favorite cam for a stock-ish rebuild is a Rebello 27C. It can be installed with stock valve springs, or factory upgraded valve springs.

[datzenmike]

If you don't yet, later upgrade the carburetor to a 32/36 Weber to take advantage of the cams better breathing.

[banzai510(hainz)]

I have a Schneider in my 521 and its a 460 lift 260 /270 duration I think. with just a Weber DGV

Personanlly I think that should be stock on a L motor. Isnt anything special. Its was was avail at the time