do I need valve reliefs?

[sneax]

Hello comunity!

 

May I introduce myself since this is my first post here!

 

I'm German, lived in BC Canada for some years and now traveling in Mexico since 18 month.

I had old cars my whole life and worked on them myself.

My newest project is an 1960 MGA Coupe rally race car in Mexico for La Carrera Panamericana! I raced it last year with an original Datsun 1800 J-engine.

This year I am swapping an L20B in it! For the rules I have to stay under 2liter with 8 valves, carburated and wet sump!

 

So I have read a lot about the L20B and my options of making a decend Rally engine out of it!

I bought 2 engines and 2 5-speed transmissions and an W53 peanut head.

Determend an 150mm forged H-beam rod that fits in and will eather get custom forged pistons made or use forged pistons from an Mitsubishi with 86mm and 10.5 to 10.75:1 c\r.

 

I have many questions about tuning the L20B and the first one is:

From witch cam lift on do I need valve reliefs (pockets) in an 0 deck flat top piston with the W53 head???

(I actually might end up 0.2mm above deck hight - depends on the cc's of the pockets)

Can not mesure it myself rightnow because the head is not in my hand yet ;)

 

Thank you guys!

[datzenmike]

Without knowing the piston you are going to use this is almost impossible to answer or guess even. The stock L20B piston does not need valve reliefs as the exhausts are almost closed and the intakes barley starting to open at TDC on the exhaust stroke AND there is a large dish on the top. The highest valve lift is happening while the piston is well down the cylinder bore and not anywhere near TDC.

 

If I were you I'd assemble the engine (but not set at TDC) with an old gasket and carefully turn it up to TDC with a small amount of clay or plastercene on the piston top. Remove the head and measure the thinness of the impression the valve made. If the valve meets or touches the piston top... stop.

 

I doubt the smaller valves on an A87 will touch even a flat top piston.

[distributorguy]

Assemble your engine, then install the head with a weak hardware store spring in place of the #1 intake and exhaust springs.  Dial in the cam timing, and press down on the valves to check piston clearance from 30 BTDC to 30 ATDC on both strokes.  You'll likely find you have TONS of piston to valve clearance.  I had something like .120" clearance between my domed Rebello pistons and custom valves on the Bonneville motor.  

[scooter]

Four variables here. Pistons, valves, camshaft and the height the valves are installed.

 

In an l20b head with 280zx valves your valves will be deeper in the head than l20b valves.

 

Assuming stockish pistons, stockish head. Any of the cams that will work with stock valve springs, you will probibly be fine. Race cams require valve reliefs due to longer duration and lift. Distributorguy's pistons will have valve reliefs in the pistons even though their domed.

 

If your building a hot motor, do what Distributorguy said. I would advise against running under .100 piston to valve clearance.

 

You can check this by doing Distributorguy's technique or the ol' clay on the piston method.

[scooter]

Where in bc are ya? If your close to me I can help you put it together

[hang_510]

Yes,dished/notched pistons will be required at that deck height. No way to accommodate increased lift otherwise.

I am at equal height and have partially dished and notched forged pistons.

Easier to change piston shape than the head. Especially with larger valves.

[sneax]

thank you guys for the replys!

 

@scooter (and datzenmike) = I was living on Vancouver Island for 5 years... in Cobble Hill to be more precise. but now I am in Mexico City with that race MGA and building that race L20B here as well!

 

 

the plan for the engine is an full rally race engine: forged H beam rod 150mm, forged pistons, W53 head full ported, most likely bigger 44mm&35mm valves, 10.5 to 10.75 compression, 2x 45 weber, full length header, turbo oil pump, adjustable cam sprocket, 270 to 280 cam with a max of 0.6 lift (I guess), spray bar, alu 280z flywheel, race clutch, has to have a wet sump for the Panamericana rules, electronic ignition (not sure witch) , 5 speed gearbox...

 

  

unfortunately I can not go with that "try and error" method (yet) to determined the valve pockets. I do have the engine here but not all the other parts like the W53 head, rods, pistons and camshaft. 

 

I need to get the pistons made for me or use those Mitsubishi pistons that would fit (~about).

 

But in my eyes it just does not make sense to ship the engine to the producer of the pistons to the Usa to get the pockets grinded. 

I will use 86mm pistons to stay under 2000cc and would like to have an very close squish of less than 1mm ... so let say 1mm! that gives me a deck height of -0.2mm.

so that gives me an compression height of 34,25mm for the pistons! 

with the ~42cc chamber volume, 150mm rod length, 1.2mm gasket thickness, ~87mm gasket bore and the 86mm original stroke...  for flat top pistons give me a c\r of 11.41:1! Too much for the premium pump gas I will use in the Rally here in Mexico!

With 4cc pockets in the pistons gives me an c\r of 10.6:1 ...what would be ideal in my eyes!!

 

so would you recon me just to order 86mm flat top pistons with an compression height of 34,25mm and ask for more material to be able to grind 4cc valve reliefs - NO MATTER WHAT LIFT I WILL HAVE LATER???
and then I could measure the possible maximum lift with an 0.100in clearance and order an custom grinded cam afterwards!?!?!

 

Or do I do some mistakes here?
(for this calculation I take the squish of 1mm as an given and go from there to determine the chamber volume incl the dish volume!)

 

additional question: would you recon an smaller squish than 1mm? I have read some do 0.03in=0.76mm squish!  
(I do not think that the engine will rev over 8000) 

[sneax]

Yes,dished/notched pistons will be required at that deck height. No way to accommodate increased lift otherwise.

I am at equal height and have partially dished and notched forged pistons.

Easier to change piston shape than the head. Especially with larger valves.

Thank you hang5

that was the experience I was looking for!  :thumbup:

[datzenmike]

150mm rods, 43mm stroke, 34.45mm piston pin height gives you a zero deck height. Note you can easily run 0.3mm above the deck safely and there is still about 0.9mm of gasket space between the piston tops and the underside of the head to allow for any 'rod stretch'. This would produce 11.2 compression

 

Closed chamber heads are 41cc and if it's been milled will be less, so 41cc is a maximum combustion chamber volume.

 

 

 

Using you numbers... 150mm rods, 34.25mm pin height, 1.2 gasket, 86mm flattop pistons and 41cc head your compression is 11.18

[sneax]

150mm rods, 43mm stroke, 34.45mm piston pin height gives you a zero deck height. Note you can easily run 0.3mm above the deck safely and there is still about 0.9mm of gasket space between the piston tops and the underside of the head to allow for any 'rod stretch'. This would produce 11.2 compression

 

Closed chamber heads are 41cc and if it's been milled will be less, so 41cc is a maximum combustion chamber volume.

 

 

 

Using you numbers... 150mm rods, 34.25mm pin height, 1.2 gasket, 86mm flattop pistons and 41cc head your compression is 11.18

Youre goddamn right!  :thumbup:

 

I meant to write 34.65 and not 34.25mm pin height! that would give me a squish of 1mm.

 

thinking about it .... I think i go better with a pin height of 34.85mm for a squish of 0.8mm! (milling down the pistons by 0.2mm can be done easy if I have to.) 

that give me a deck of 0.4mm, an c\r of 11.67:1   WITHOUT POCKET!

so to achieve an c\r of 10.6:1 ish .... I need to grind 5cc pocket (or less if I use bigger 44mm\35mm valves that sit deeper in the head!)

 

I think thats an good approach?!?! 

or am I missing something here...??? :confused:

[datzenmike]

Youre goddamn right!  :thumbup:

 

 

 

 I agree within 0.02 on all your compression numbers.

[G-Duax]

A word to the wise.

Valve pockets are machined in, not ground in ('ground', past tense of grind, not 'grinded').

You walk into an automotive machine shop, and sound like you don't know what your talking about, you are more likely to be screwed over.

Seen it way too many times.

[sneax]

A word to the wise.

Valve pockets are machined in, not ground in ('ground', past tense of grind, not 'grinded').

You walk into an automotive machine shop, and sound like you don't know what your talking about, you are more likely to be screwed over.

Seen it way too many times.

thank you! 

everyone is smiling\greening anyways because off my german accent! haha

and all the machine shops I am walking in these days are located in Mexico City and my spanish is way worth then my english is! 

​but I appreciate your correction!  :thumbup: