l20b thicker headgasket?

[bredatto510]

Im trying to lower the compression a bit on my l20b bored to 87mm with oversized 280zx flattops and a u67 head ..damn u67 and its pinging

 

So is there such thing and or has anyone ever done this... i know certain older engine builders would put 2 gaskets although i doubt thats a good idea... is it safe?? if i can get this damn thing to run 91 im keeping it

[dvboth]

nissan motorsport head gasket is thicker! it is $72 though!

[bredatto510]

stock is 1.2mm and i believe that one is 1.5mm would that make a huge difference... i believe my compression ratio right now is 10.292 to 1

[datzenmike]

An '87 bore flattops and open chamber head is 10.29 compression!!!

 

 

Doubling the gasket will add an extra 7.12cc of head volume and this will reduce compression to 9.22.

 

I'm not aware the Nissan gasket is thicker, I doubt it as this would disrupt the timing chain length.

 

 

I don't see why you couldn't open up the chamber with a grinder. Plenty of room to unshroud the valves. You only need to cc the head and get to about 53cc, 7-8 cc taken off.

[bredatto510]

An '87 bore flattops and open chamber head is 10.29 compression!!!

 

 

Doubling the gasket will add an extra 7.12cc of head volume and this will reduce compression to 9.22.

 

I'm not aware the Nissan gasket is thicker, I doubt it as this would disrupt the timing chain length.

 

so would it be safe to do this and be able to return my timing to 12 degrees btdc and run 91 octance?

[bredatto510]

well doubling gaskets is not recommended so does anyone know of a thicker gasket or someone who can make one or what i should retard my timing maybe some hotter plugs? any ideas

[kneesamo]

RockAuto.com sells a cylinder head spacer shim, don't know how thick it is, but it sells for 15.00 bucks

FEL-PRO Part # 21178Sp

[datzenmike]

well doubling gaskets is not recommended so does anyone know of a thicker gasket or someone who can make one or what i should retard my timing maybe some hotter plugs? any ideas

 

Pinging is caused by concentrating the latent heat in the air into a smaller space. The more it is compressed the hotter it gets and the higher risk of it self igniting (ping). ANYTHING that will add heat to this will make it worse so just like an equation, anything that will reduce the heat will lower the chance of pinging....

 

A colder range of spark plug might help. If they are too hot they will act like glow plugs in a diesel. Like a self cleaning oven, the plugs have to run hot enough to stay clean so don't run too cold.

 

The air under the hood is very warm even very hot at times. Drawing intake air from in front of the rad is always a good idea on a high compression motor. Try to vent under hood air through inner fenders.... be creative.

 

A heat shield between the exhaust and intake manifolds will reduce radiant heat absorption. Header wrap insulation works too. Painting the intake white or silfvr or polishing the aluminum will also lower heat absorption.

 

A lower range thermostat may help, although I'm not a fan of this practice as the motor is designed for best efficiency at nearer to boiling point.

 

Insulate the fuel line to prevent it absorbing heat. If you don't have the later L20B return line, install one. It allows gas to be constantly circulated past the carb, returning the excess warmed gas and replacing it with cooler gas from the tank.

[bredatto510]

wow thanks mike... ok so im gonna buy and install that shim from rock auto... with a new head gasket... intake manifold is already silver so im gonna wrap the manifold with header wrap..

 

im gonna see how it runs there if its still a problem ill try a colder plug... is there any u recommend?... i do know that in ngk the higher the number the colder so like originally they call for a 6 series like bpr6es or so... should i try bpr7es?

 

also should i try taking out the driver side high beam.. get some colder air in the engine bay?

[bredatto510]

also... just to clarify... some confusing because its Below top dead center... is 8 degrees more advanced than 12 degrees below top dead center or more retarted?

[hang_510]

Before Top Dead Center

 

try the colder plugs!

[datzenmike]

12 degrees Before Top Dead Center (BTDC) is more advanced than 8 BTDC degrees.

 

 

If the ignition timing is set at 12 degrees BTDC then this is the point when the cylinder gas and air is ignited. Because the gas and air doesn't explode instantaneously but does take a very short time the crankshaft will rotate another 12 degrees to Top Dead Center (TDC) while it is burning and starting to expand. TDC is the upper most position of travel before it begins to move downward. The crank will travel an additional 17 or so degrees After (ATDC). This is the 'sweet spot' where cylinder pressure reaches it's maximum and exerts most of it's effect pushing down on the descending piston. The 'sweet spot' cannot be moved* so everything has to be 'timed' in 'advance' for this to happen just right at this point.

 

Generally the tighter the gas and air is packed the less time it takes to burn completely. A shorter burn time means the timing of the start of the burn must be started later to hit the 'sweet spot' at about 17 degrees ATDC. This may be perhaps 8 degrees, perhaps 2 degrees. It would depend.

 

 

 

If you are running 10.29 compression you should not be anywhere near the stock timing. Have you tried reducing it (timing) to remove the pinging? Higher compression = shorter burn time = less ignition timing. Since there is no exact number, just turn the dizzy counter clockwise to retard (lower) the advance. Trial and error. Loosen the dizzy bolt just enough so you can turn by hand. Take for a drive and see how your adjustment is working. Stop and turn by hand and try again. Keep retarding it until it goes away or is just barely noticeable under load.

 

 

*Actually it can be moved by using a longer or shorter rod and a corresponding piston pin height change. Unlikely to be done here.

[bredatto510]

thanks mike again... yea see someone had told me that the further away from zero the more retarted because its btdc and not atdc so its set to 15 btdc damn... ima try it at 8 or 6 and let u guys know what happens thanks again

[datzenmike]

Yes, retard the timing by moving it towards TDC by turning the dizzy counter clockwise slightly. You don't need a timing light for this. If this were a stock motor you could because it has been worked out by Nissan to be 12 degrees. For your application it will be trial and error to find a spot that works.

 

What you want is a timing setting where the maximum cylinder pressure from combustion occurs at or very near the ideal

'sweet spot' at about 17 degrees after TDC or ATDC to push it's hardest on the piston. This may not be possible because of pinging and so you may have to retard it more than necessary just to prevent pinging. It's a compromise where you have to accept some loss of some efficiency to prevent engine damage. One cure is to run much higher 110 octane aircraft fuel. This will prevent the fuel from igniting by itself and allow you to run the proper ignition advance... but this is expensive and not realistic for a daily driver.

[banzai510(hainz)]

Bredatto

I think your waisting your time.Mike's tryin to help but this is a lost cause.

stock L20 is 11.36 cc dished and putting a zero cc piston is alot. that head shim isnt going to do much if anything.besides blow it inbetween the gaskets.

Best you could do is find 94 octane and see if that helps.or race gas.

Maybe water injection?

 

I always recommend installing stock pistons in L20s and install a closed cahmber head. thats it from now on.

[bredatto510]

no im just going to install that spacer with the 1.5 mm head gasket not the 1.2 and retard it till it doesnt ping thanks mike

[banzai510(hainz)]

I still think your waisting your time on this for the amount of work.

butlet us know whats happening.

once it get hot or summertime it will still ping.

 

a guy here in washington had his ping even with L18 4cc dished pistons

[bredatto510]

about the octane theres a place by my house that sales 110 leaded fuel and i pumped 50/50 with 91 to obtain 101 octane and it run perfect... 110 aircraft fuel is not recommended because its meant for high altitude low rpm and it burns soo fast... but that one i bought isint aircraft fuel... the only downside is its 7.35 a gallon... so 5 gallons of that and 5 gallons of 91 is like 50 bucks for one tank... i do know where there is a pump that sales 100 octane for 3.50 a gallon but its in corona 30 something miles away... my best bet would probably be go and fill all my gas cans and keep them at home

[Skib]

start looking into water/methanol injection

[datzenmike]

I don't think higher octane burns faster as such. The closer the gas and air molecules are pressed together by compression sets a burn speed. Octane raises the temperature at which gas will self ignite, specially from compression heat. It doesn't add power itself but simply allows a safer high compression free of pre ignition. (on a car engine) Higher compression allow you to get more power out of your gas more efficiently.

 

I don't know what airplane compression ratios are generally, probably not high enough to warrant 110 octane.... but. Any chance of engine damage from pre ignition on an aircraft engine must be avoided at all costs. After all you can't just pull over if there is right?

[hang_510]

I don't know what airplane compression ratios are generally, probably not high enough to warrant 110 octane.... but. Any chance of engine damage from pre ignition on an aircraft engine must be avoided at all costs. After all you can't just pull over if there is right?

i think it has to do more with reduced O2 at elevation...

 

 

obviously you've never flown with any A&P mechanics.. they WILL!

[bredatto510]

ok so i loosened the dizzy and move it clockwise quite a bit all the way i believe while it was running of course at running temp and with the vacuum advance taken off... then i adjusted the carbs idle screws to keep it running... im gonna take it for a test drive to see how it is

[bredatto510]

i currently dont have a timing light in my possession so im just retarting it and trying it... does anyone know what degree im at if its fully retarted?

[bredatto510]

im also starting to look into water injection... has anyone on here done it to side drafts that could help me...and should i buy a kit for 200 or make one

[nismo dr]

soooo your going to put a head shim in and do all this work, but you've never actually put a timing light on it

 

:huh:

 

retart?