L20b ideas

[OldSkool]

I want to build up an L20b, and I have been doing some research already. But I do have a few questions unique to my ideas.

I daily drove a 71 wagon with an L20b with L28 flat top pistons, racer brown cam(forgot specs), and A87 peanut chamber head, twin SUs, on 93 octane. That was a few years ago. I didnt remember it detonating or pinging though. Looking around, it seems that most people think that using a A87 head peanut chamber on an L20b with L28 flat top pistons is WAY too much compression for the street. I want to hear your thoughts on this because I am looking to do a somewhat similar L20b build, but with more cam and different carburetion.

 

 

BTW, just a quick rundown. I want to run an L20b with L28 flat top pistions(supposedly it yeilds around 10:8 compression?), A87 peanut head with Isky z-273 cam adv. duration 306 with .595 valve lift(too much cam for street?), with matching springs, and a 44mm twin mikuni setup.And yes, I know it wouldnt wake up until like 3,000-4,000rpm. Opinions(without bashing)?

 

 

Thanks

[banzai510(hainz)]

try to find L18 4cc pistons and 93 might be OK.

[albyneau]

I want to build up an L20b, and I have been doing some research already. But I do have a few questions unique to my ideas.

I daily drove a 71 wagon with an L20b with L28 flat top pistons, racer brown cam(forgot specs), and A87 peanut chamber head, twin SUs, on 93 octane. That was a few years ago. I didnt remember it detonating or pinging though. Looking around, it seems that most people think that using a A87 head peanut chamber on an L20b with L28 flat top pistons is WAY too much compression for the street. I want to hear your thoughts on this because I am looking to do a somewhat similar L20b build, but with more cam and different carburetion.

 

 

BTW, just a quick rundown. I want to run an L20b with L28 flat top pistions(supposedly it yeilds around 10:8 compression?), A87 peanut head with Isky z-273 cam adv. duration 306 with .595 valve lift(too much cam for street?), with matching springs, and a 44mm twin mikuni setup.And yes, I know it wouldnt wake up until like 3,000-4,000rpm. Opinions(without bashing)?

 

 

Thanks

 

Do a search for a free download of a dynamic compression ratio calculator. It does what it's called~ calcutating your compression ratio based on when the intake closes rather that just from bottom dead center. You're going to find that you might even be a little under the optimum CR with that much cam. You'll likely also be looking at cutting valve reliefs in the pistons with that much lift. Not bashing here, just the facts~ you're treading on dangerous and expen$ive ground trying to build a motor from the cam out. From what I've found for L-series head flow data (and that's very little) they benefit very little going above .500" lift~ especially for the cost involved.... Stick to that .500" max lift, and between 220-230^o @ .050", and you'll have a fun streetable combo that you won't grow weary of in 6 months.

 

M2C's

 

P.S.~ as much as the first 75% of detonation is inaudible to the human ear....

[albyneau]

Saved you the trouble~ here you go!

 

http://www.empirenet.com/pkelley2/DynamicCR.html

 

Peace~

Scott

[datzenmike]

Peanut heads are more detonation resistant than open chamber heads. In fact you can actually increase the compression and still be as good as or better than an open chamber head. This is due to beneficial turbulence or 'swirl' induced by a design which has a large quench area. Quench is the thin area above the piston top (at TDC) and the underside of the head surface and is usually the thickness of the head gasket. About 1.2mm. Turbulence prevents lean hot spots or over rich deposits that remain hot like a glow plug. To work best a flattop piston is best to maximize the pinch effect between piston top and flat head surface.

 

L20Bs have a large 11.3cc dish and are not flat, so the pinch is lessened. L28 flattops would work best but the compression is 10.87!!! High even for 93.

 

87mm (mm oversize) L20B pistons with 11.36cc dish and open chamber head........ 8.55

87mm (mm oversize) L20B pistons with 11.36cc dish and closed chamber head...... 9.05

87mm L28 flattop pistons and closed chamber head............................................. 10.87

 

At low speed the cam overlap would reduce the compression some.

 

 

Scott you got me beat...

[OldSkool]

Thanks for all the replies guys. Really I'm not building the motor around the cam. I'm just trying to build a motor that pulls healthy up top. It will be street driven daily but I like to rev.lol. But anyway I was hoping I wouldn't have to cut reliefs in the pistons for the valves. So the l28 pistons are a no go? So do you guys think I will be better off with l18 pistons? I want as much compression as possible but be able to use 93 octane. I use 93 anyway.

[docbainey]

Kinda depends on how much dist. advance you want to run also. The setup you desrcibed, if your out of the power band (and that cam will have a hi rpm power band) in taller gears and get heavy on the go pedal, you'll have detonation. I think your better off to make a wider power band (lower toque, less cam, read racer Brown section in HP book) but have the head , intake & exhaust be able to flow @ hi rpm so you can winder up. Carbs will also depend on what size choke (venturie) you run for rpm power band. Also with increased rpm, comes more abuse on all the turning parts. Gotta think about rod & main bearings but more importantly, oiling them. Stock crank, block & cam, oiling is not condusive to hi rpm use. But if you want to blow stuff up, its all good!!

[OldSkool]

For oiling I am going to run a 280zx turbo pump. And also have the crank oil holes chamfered. Crank polished also. I'm probably better off running l18 pistons. The head is already setup for the cam. Bigger valves,matching springs,titanium retainers, 1"1/2 intake ports. I can put up with some ruckus on the street. Lol. Also, don't need emissions so I want to take advantage of that. Lol

[DAT510]

mess with this

http://www.ozdat.com/ozdatonline/enginedesign/

[banzai510(hainz)]

first find some L18 pistons if they exist. i got some Sealed Power(sterling) 12159P was the part number if i remeber right.

If not just but the l20 piston from Silvolite or Rockauto.

 

You say you like to rev? Does it matter wht the comprssion ration? Thats a Cam thing. also you dizzy would only be so good before it might loose soem accuracy. I say 6500-7000K rang is a good street rpm range.

 

Scheinder used to make Asymetric cams where the closing rate was slower and prevent valve train float but they do only regrinds now.

also you look at the cost of rocker arms, springs lash pads of unknown sizes. Some of this stuff is druying up.

 

silver seal makes cam tower shims if needed also

 

Be honest most Datsun fuys hate buting expensive gas. In my closed chamber heads and stock pistons I can run regular gas.

[OldSkool]

Do you guys think the compression ratio with the 280z flat tops would still be too high even if I used the NISMO 1.5mm thick head gasket?

Im having a hell of a time trying to find L18 pistons. I kind of give up.

 

 

 

Thanks

 

 

[datzenmike]

87mm L28 flattop pistons and closed chamber head............................................. 10.87

 

A 1.5mm gasket drops it to 10.50

[OldSkool]

Thanks

Ok, so it drops to 10.5 with a 1.5mm Nismo head gasket. I could see 10.8 being a little far out, but I think 10.5 is just fine on the street with 93 pump gas. Opinions?

[ggzilla]

Will 10.5 compression work L20B with premium gasoline? I was using 10.9 but with a much smaller bore.

 

If you use too thick gasket you lose the quench effect. The quench needs to be 0.75 to 0.50 mm to be effective from what I understand. Of course if you are using dished pistons there's no telling if it will have a proper quench effect.

[banzai510(hainz)]

Dude !

max is using the L18 pistons with 4cc. Be on the safe side

 

there was 2 guys on here with Flatops on L20s and they didnt like buying 8$ a gal gas. Then blown headgaskets are also more likely.

Reliability!!!!!!!!!!!!!!!!

[flatcat19]

L18 pistons can be had.

Some on eBay right now, and Summit sells them as well.

[datzenmike]

Each point of compression (properly camed) adds about 4% more power. So from stock 8.5 to 10.5 is about 7-8 hp on a 90 RWHP L20B. That is, if with proper cam, ignition advance and gas and revved way up. For driving on the street it will be no fun unless summer Saturday afternoons. 99.998% of all driving is less than half throttle.

[ggzilla]

Each compression point adds 2-4% HP even with the same cam and same ignition timing. For max HP you might have to reduce the timing a little.

[OldSkool]

Well then I guess I better keep looking for L18 pistons. And I decided to come one step down on the cam to the isky z-196 cam. Not much different than the z-273, but it's a tiny bit smaller.

[zed]

The posters above know much more about L motors than I do. But... I'm running an L18 with a V912 (peanut) head, L28 flat tops (86mm bore) AND a .020" skim off both head and block, .040" gasket. Maybe 10.5:1 CR. It runs really well, but I have to watch the timing. Also, we have 95 octane 'Lead Replacement gas here, which probably helps

[docbainey]

Bore out to 86mm

 

use the pistons below, with 6" rods. Thinner rings & longer rods, good for revs. To be sure, you better chk my math!!

Sealed Power Z12150P100MM - Sealed Power Cast Pistons

 

 

 

 

[Farmer]

Im running an L20b with +1mm L18 pistons with closed chamber head and a mild cam. Was able to run mid grade gas with some pinging. Premium is best though. Not planning to be a daily driver.

 

Now my other truck has to run atleast 110 oct.

 

Larry

[Boaty]

My L20B is +.030", stock pistons. Closed chamber head, shaved -.015"

I pinged with regular, was perfectly fine with supreme. Would only ping with advanced timing on warmer days.

 

Ran race gas a couple times, was pretty peppy and smoother.

 

According to posted info above, I would be at or around 9:1 CR, not sure on what the .015" made it. I have always guessed 9.5:1, looks like I may have been close. That is what the machinest agreed with me on for a number.

 

I know somebody with a Ford 390 running 12:1 on pump gas without pinging. Guess it depends on the setup.

[hang_510]

I pinged with regular, was perfectly fine with supreme. Would only ping with advanced timing on warmer days.

 

I have always guessed 9.5:1

my LZ22 is ~9.5:1 and cruises fine on 87, not good under load.

run 91 most of the time.

[DARIN 510]

I have an l18, a87 head, flat top pistons, iskey cam, 44 mm mikunis tri y header. I'm running 93 oct that's the highest you can get in Cali unless you go to a racing fuel place and fuck that. I believe I'm at 10.5 comp and don't have any ping.