cylinder head question

[BRE_dAtSuN]

i have a l20b with a u67 head and flat tops now im told the u67 is the most prone to detonation... i've gotten it to a smallest amount of pinging .. but i dont like it... now i have a cam and dual springs... my question is if i were to buy a used w53 or a87 and have the rebuilt can i use my cam and springs or not??... also which head would you guys recommend more?

[DanielC]

Retard your ignition timing about 2 degrees.

[datzenmike]

The detonation is caused by the increased compression. Yours is over 9.9 to one. Both the U67 and the open chamber A 87 are the same size so there is no improvement doing this.

 

Detonation is caused by the latent heat contained in the air that is concentrated into a smaller space by the compression of the motor. Since removing the flattops isn't helpful and there is no lower compression head available the only thing left is to reduce the intake air temperature to the bare minimum. If the outside air is say 85 degrees it can easily gain 30-40 (or more) degrees by the time it gets to the combustion chamber from contact with the heat of the motor, carb, intake, fuel.

 

Some things that will reduce heat gain or remove heat:

 

Draw intake air from in front of the rad by ducting air with a piece of insulated cloths dryer hose.

Get or build a sheet metal heat shield between the intake manifold and the extremely hot exhaust pipes. This will reduce radiant heat being absorbed by the intake. I see you have SUs and these should come with a heat shield.

Paint the intake a reflective silver that doesn't absorb as much heat.

Install an L20B fuel return line. It will keep cooler fuel from the tank flowing past the carbs. No sense heating the gas you're going to use.

Run a cooler thermostat. Perhaps a 165 or 170. I'm not keen on this as it reduces engine efficiency but pinging is worse.

Get a large oil cooler. Can't stress this enough. Oil temperature is always higher than the coolant because it goes to places that are much hotter like the under sides of the pistons. Cooler oil will cool the critical hot spots.

You may want to take a look at water injection when under heavy load. A small amount of added water absorbs heat in the combustion chamber by turning to steam. If not over done and properly tailored it removes pinging and does not affect power.

 

 

Reduce your static timing.

[ggzilla]

Yep, reduce the timing. You'll still make more power with the closed-chamber head.

[datzenmike]

On a stock L20B a closed will work but with 9.94 a closed will up it to 10.68... that's gotta be worse for pinging.

[ggzilla]

In that case, just use premium gasoline. It's only 10% more expensive, and a lot of guys say you will save 10% in fuel economy.

[BRE_dAtSuN]

i run 91 my timing is like at 7 degrees btdc so its retarted any more and it backfires like crazy... i have dual mikuni's phh44's... silver mikuni intake manifold... im going to wrap the exhaust manifold and find a heat shield... ill buy an oil cooler good idea... hadnt thought of that... i was looking into buying a water injection kit but for the life of me no1 had any idea as to where to tap the nozzle or nozzles to evenly spray each runner... i have colder plugs helped out a lot... i have a 165 thermostat... electric fan... runs like at 175 and 190 in traffic so no overheating... ill try header wrap and oil cooler and let u guys know.. however the reason i ask about the heads is i have a friend who has almost the same setup as me but with a peanut head closed chamber and his doesnt ping on 91 he also has a u67 and it pings with the u67... and from what i have read the u67 is just really prone to detonation

[datzenmike]

Forgot about cooler plugs.... as long as they don't foul. Try running slightly richer. You may be a slight bit lean and that will run hot. The added fuel does the same job as water injection and absorbs heat when it vaporizes.

 

The p nut head has a very large quench or pinch area and with flattops promotes good turbulence and swirl that homogenizes the burning mixture and prevents hot spots. I'm afraid it will raise the compression way too high for even that to help.

[banzai510(hainz)]

BRE

The mistake is using flattop pistons on pump grade gas for the street.

 

going from 11.35 cc volume and them removing it by getting 0 dish is just bad news in my opinion.

 

Gettnig a closed chamber head helps with detonation but your increasing the compression even more and I think 91 isnt going to cut it.

 

To me cheapest is finding stock bottom end and bolting all your stuff to it. and try that.

 

To me the perfect street L20 is stock bottom with a closed chamber head. but I would be ashambed of just a stock L20 with a U67 head either. youll still want to run high octane as you want the most adv right before it pings. aesp since sidedrafts want more adv.

 

I think your wasting your time with the oil cooler.

On a cool/cold day try running a 160 stat(cheap trouble shooting) and if it still pings then just forget about it.You cant get that head any cooler and if still PINGs will you know the awnser. No need to plumb another oil line and cooler.

as for the dual springs. All L motor have dual springs. Just depends if you put aftermarket dual springs. this isnt going to matter anyways. Of coasre the stuff will fit another L motor ,why would it not.

[DanielC]

My thoughts. You are running a lot of carburettor on a small engine. If you open the throttle too fast at a lower RPM, two things happen. Manifold vacuum goes very low, and because of the size to the carburettor venturis, air speed in the venturi really goes down. Both of these factors make it hard for the carburettor to control the fuel to air ratio. The engine goes lean, until the engine RPM builds up to a point that the carburettor can accurately meter fuel into the engine.

The lean fuel to air ratio may be increasing the tendency to detonate.

 

You need this:

 

Here is a picture with the burette and some acrylic plates.

 

It is a lot of work, but maybe it is time to take the engine apart, and check what your compression actually is. With the tools to measure your cylinder head CC, you can then maybe add some volume to your combustion chamber, by unshrouding the valves. If your valves have not had a recent valve grind, doing a good three angle, or more valve job could give you flow benefits at the lower valve lifts, and that might improve the vacuum signal at the carburettors.

 

My advise, check the compression ratio, so you know what you have got. Get that where you want it to be. Make sure the engine is not temporarily going lean. There is information on using O2 sensors to monitor air to fuel ratios, on the internet, or you can just buy a kit, from some companies like Summit, or Jegs.

[datzenmike]

Tha right. The stock head combustion chamber is 45.2cc at best. Over the years it may have been plained which decreases the size and raises the compression even more. You're looking at just over 11ccs difference between stock L20B dished pistons nd your flattops.

[banzai510(hainz)]

you could try a gas cool tank for the gas line but its just a hail mary to fix this.

11 cc and a open chamber head is just not good unless you got 100octane.

 

I have a cool can. its a round alum with a inlet and outlet. the gas comes in then go thru the can in s coil like fasion then out. What you do is put Ice or Dangerous dry ice in there. hopefully is cools the gas. But to me this is just a stop gap from the real proplem.

[BRE_dAtSuN]

yes... i know my solution would be replacing the pistons... problem is money and i cant have it down as im always driving it to do side jobs since i dont have a job... so things like oil coolers colder plugs thermostats... fuel coolers i can i can invest in slowly as i get paid for every job... i should only have to hold off another month or so till i can get a job... once that happens im going to advance it back to 12 or 15 run some good hot plugs put and msd box and merit oil company is like half a block from my house and they sale 110 there i can run half that and half 91 i did this for a while and my build was good... just in a tuff spot right now with no money...thanks for all the great advice guys...like i said im trying every little thing to minimize it as much as possible

[banzai510(hainz)]

you dont need MSD. Ping is preignition. meaning if igniting before the plug even sparks

[zed]

what about quench height? i'm running 11.1 compression on 97 octane - i get very little pinging. i believe the key is my .035 quench distance

[69FJWagon]

What Jetting do you have in your carbs??? have you had it tuned??

[datzenmike]

Quench thickness should be crushed gasket thickness or 1.2mm or about 50 thou. Might be very slightly more as the L20B piston doesn't quite reach the deck.

[BRE_dAtSuN]

well im going to get my fuel cooler out of storage... and warp the manifold... it barely pings... like for example today it was cloudy and like 60 degrees out and it didnt ping at all ... so i know doing little things like that can help alot. i still have to check what jets i have its the ones rebello put on when they rebuilt them

[BRE_dAtSuN]

you dont need MSD. Ping is preignition. meaning if igniting before the plug even sparks

 

i mean after i start working and can pump 100 octane without a problem

[datzenmike]

A fuel cooler isn't practical day to day, ok for racing 1/4 mile. If you don't have a return line, put one in. It prevents the fuel in the lines near the carb from heat soaking.

[metalmonkey47]

Wouldn't it just be simpler and cheaper to reduce the timing a few degrees and run octane booster?

[BRE_dAtSuN]

A fuel cooler isn't practical day to day, ok for racing 1/4 mile. If you don't have a return line, put one in. It prevents the fuel in the lines near the carb from heat soaking.

 

oh ok well its not one of those canister ones it looks like a oil cooler with an inlet and outlet and fins... where, how and to where do i mount the fuel like to?... back the tank?

[BRE_dAtSuN]

Wouldn't it just be simpler and cheaper to reduce the timing a few degrees and run octane booster?

 

it is retarted like 6 degrees any more and it backfires and barely runs... octane boosters dont really work... only ones that do cost alot per tank and i might as well pump half 110 half 91 but the issue here is no money...

[HRH]

Okay, here's my thoughts: I have an L20B with U60 head (same as U67), flattop Z20 car pistons. Literally flat except for two little tiny valve reliefs. Head is shaved .030, have a rally cam, did some portwork, and she runs like a raped ape. I had a little detonation issues with the dual SUs, which was solved by adjusting the mixture more rich at idle and disconnecting the vacuum advance.

 

On all the high compression Ls I've had and others have had, I always disconnect the vacuum advance. It just ramps up the spark curve too quickly with it, and generally causes mid-range detonation. Can't remember if I reconnected it with the EFI and the matchbox distributor at the moment, but with the SUs I ran about 14 degrees timing at 900 rpm or so, and with no vacuum advance didn't have a problem with detonation.

 

I'd say adjust your spark curve by either disconnecting the vacuum advance, seeing how that goes, or putting heavier springs in the flyweights. Or you have a lean condition in your carburetors, possibly not big enough accelerator pump jets, etc. Something has to be off though unless you have a lot more compression than I do.

 

The way my setup is right now with EFI, I run around 12 degrees at about 1000 rpm. Basically screw with it like the Datsun manual. Get it to ping, keep backing off until it doesn't.

[dimedriver]

Have you tried Taluline or what ever its called? You can buy it at home depot or any other hardware store next to the paint thinner and acetone.

 

Its what "octane" is made of pretty much. I have herd of people using it as octane booste. But using it is a lot like a high school Chemistry experiment(we need an icon for this).

 

But look into getting your jetting right it will fix the problem correctly.

 

-Dime