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big and nasty headwork


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Seeing as this thread has some great info in it already I'll ask another question to hopefully add to the content. Is it worth attempting your own polishing with a port and polish kit from SA? I have a spare head to practice on but I'm not sure of the benefits in my case. I'm using an otherwise stock L28et with a hybrid turbo shooting for 400rwhp. Is it worth spending the time and effort? I won't be trying any porting, I will be upgrading to a 60mm tb and I have a full mandrel exhaust 3" in the works. I want to order this kit and try it if the experts chime and say it's workable if not I won't bother and look elsewhere to upgrade power.

 

Bernard

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As usual, I went crazy and wrote up a novel so this is my disclaimer. I apologize for taking up so much band width. When I get babbling about engines, I tend to ramble on and on… When someone posts questions like these, I just can’t help myself

 

I realize that there is a lot of info here and for some of the new guys, this might be a bit thick and hard to absorb an in some cases may cause headaches, ear bleeds, bulging forehead veins etc. If any of these symptoms arise please, drink 2 beers and call your physician.

If anyone finds any of this information or incorrect or misleading, please feel free to post corrections or additional info...

 

 

Ok, to the question at hand…

Opinions vary on just how effective just “polishing” alone of the ports of an EFI head really is. As for the Datsun L-series, on the intake side, the injectors are aimed directly at the back side of the intake valve so there is very little contact of the fuel spray with the port walls so polishing the ports can aid in keeping the boundary layer just that much closer to the port wall, though how much HP is that worth on a stock engine or mildly wild street/track engine? Probably not much, though the more radical the engine is, the greater the gain, though I don’t feel that it will offer any more than 1-2% gain. (On carbed engines, I would prefer to leave the port surface rough like the “as cast” finish. This roughness causes the boundary layer in the port to be slightly turbulent which helps keep the fuel from falling out of suspension). On the exhaust side, it doesn’t matter whether it is carbed or EFI here, just polishing alone will help about as much as it does on the intake, Not very much and for the same reasons.

 

 

As for polishing the chambers, this could go either way. I will give you the theories as I understand them to be. Any other tech-geeks, please feel free to chime in…

 

 

My opinion is just use which ever one of these theories that fits your particular application and budget and you should be just fine.

 

 

Chambers AS CAST, i.e. not polished. Being as the Datsun heads are aluminum, they conduct heat from the combustion chambers rather well as compared to cast iron heads, (sorry no Iron heads that I’m aware of for the L-series GAS engines.), though this heat loss from the combustion cycle is detrimental to power production as this heat directly affects cylinder pressure. (Torque and HP 101…. Remember, it is the pressure difference between the top and bottom of the pistons that causes the piston to go down the bore and turn the crank which in turn rotates your tires and turns them into a foggy haze which inturn will upset you neighbors if you try this infront of your house. The greater this pressure differential over as long a period of time during the power stroke, the more torque your engine will produce, and the more torque you can produce at higher RPMS, the more HP you will generate. Easy right?) Any how, the “AS CAST” surface is quite rough and has lots of surface area compared to a polished surface, so the “as cast” surface would absorb more heat from the chamber than the polished chamber. Now this can be bad or it could be good. If you are running a high octane fuel, then your engine most likely can withstand more pressure and heat before the air fuel mixture ignites itself, in this instance, polished would be good as it would keep more heat in the chamber therefore building more pressure, hence more torque. Now on the other hand, if you are using a sub grade fuel and you are already having pinging/detonation issues, polishing the chambers will only make things worse in that instance. (A fuels Octane rating is essentially that fuels ability to handle pressure and heat before it chemically changes state and releases its potential energy in the form of heat which is measured in BTU’s, i.e. the higher the octane rating, the more stable the fuel is, or, more heat is required to ignite it). Static Compression ratio has a little something to do with this as well. As you already know, the higher the compression ratio, the more likely your fuel is to ignite al by itself, so we try and get as much compression ratio as we can for the type of gas we intend to burn.

 

 

Now I’m going off on tangent and I’ll throw another wrench into this mess, Dynamic compression! Dynamic compression is what really counts. Dynamic compression takes into account static compression AND the intake cam timing, to be more precise, the point at which the intake valve closes. You see, if you look at any cam card you will notice the intake valve closes some point after the pistons has passed bottom Dead Center and is on its way up the bore on the compression stroke. At very slow RPM’s, this just means that less air is being trapped in the cylinder cause the valve is closing at some point during the compression stroke. The more radical the cam, the later the intake valve closes. That is why if all you do is swap in a big burly cam, your cranking compression will be noticeably less than with the stock cam. Now as RPM increases, the dynamic compression will change, for the better that is, in regard to making power, (ever notice that big cams don’t make much power down low?) The pressures being developed by this dynamic compression is essentially the “volumetric efficiency curve” of the engine, i.e. the torque curve.

Other factors that play into this is intake runner length/dia, header length/dia, and all the other factors that affect cylinder filling. This increase in cylinder pressure is greatest at the point where torque is at its peak. Torque output peaks here cause the ramming affect of the inlet charge is, in essence, super charging itself. In some race engines, like Nascar, engine builders have achieve greater than 100% VE, (super charging without any mechanical help), with out turbos or blowers. This is due to intake and exhaust tracts tuned exactly and specifically to the cam, etc. (This is why I only put a little faith in a flow bench numbers as those numbers only show “static” flow not the dynamic flow that is taking place within the intake and exhaust tracts of a running engine, and thee “static” flow numbers only should be used to compare modifications performed on that one head only to see if the mods helped or hurt “static” flow and should not be used to compare other heads flowed on other flow benches,).

Ok, back to dynamic compression. You see, the air is drawn through the intake runners in chunks. The intake valve opens, the piston draws the air into the cylinder, air flow is speeding up, the intake valve closes, the air still has momentum and we all know air has weight/density, the air charge compresses behind the intake valve and surges back, much like a wave hitting a wall and coming back. Then it comes back again with “almost” the same force so as the intake valve opens the air is already moving toward the valve so the piston doesn't have to draw it in, it is quite literally shoving itself in the cylinder. (This is called the Helmholtz cycle and typically most engine designers/tuners tune for the 2nd or 3rd cycle depending on application and design goals). The greater this ramming effect, the greater the VE.

OEM street engines usually don't exceed 85% VE. Hot street engines rarely don't exceed 90%-95% VE, full race engines are around 100%-110% VE.

In short, VE varies with RPM and is greatest at torque peak. Thus more air is being shoved into the cylinder so cylinder pressure is greater, therefore MORE TORQUE.

Now don't be confused, the compression "ratio" never changes, just cylinder pressure.

 

VE= the amount of air actually being drawn into the cylinder versus the actual volume of the cylinder.

 

 

 

Now if too much pressure is present before the pistons reaches top dead center, it will spontaneously ignite as the fuel is only “so” stable and will handle only “so” much pressure and heat before it ignites. Optimizing this pressure/heat level to just under the point that WE want the fuel and air to ignite is a very tough goal to achieve.

 

 

Some people say that polishing the combustion chamber will keep carbon from sticking and building in the chamber, that is BS! Every single cylinder head that I have seen that has been run for any length of time that has been polished still had carbon stuck to the chambers.

 

 

 

 

So should you polish your chambers or not? Well it’s up to you. I’ve laid out some basic theories, hows and why’s and as applied to our beloved L-series engines, if decide to polish or not polish the chambers and/or ports remember, the gains made or lost will be VERY small, so small infact that you might be able measure that gain/loss on the dyno, then again, the gain or loss could be so small, you might not be able to measure it, and I’m pretty sure you won’t feel it in the seat of the pants when you mash the loud pedal.

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Bernardd,

 

If you are at all uncomfortable with doing this, you really should leave it to someone that has done this before. As a novice it is very easy to do more damage by compromising cylinder head material thickness, gasket sealing, or hinder air flow worse than what it was before he/she took the Dremel tool to the head. Now don’t let me discourage you from trying your own DIY head. If you have reasonable mechanical aptitude and have some technical back ground or knowledge in basic Hot-Rodding, I say go for it. How do you think the rest of us got the point we are at? We all started someplace and made a few mistakes along the way. How else do you learn right?

Ok now as for the details you asked for……

 

(here is my sad attempt at humor).

 

Uh… details…well…uh.. you want.. por-ting details… mmmm… yeah….

 

In short, I’m disinclined to acquiesce to your request, or no I’m not going to tell you how much material to remove and where to remove it from. I have my secrets, just as Rebello, Sun belt, and several other fine engine builders and tuners out there, and if we “gave” all our secrets away then there would be no incentive for us to keep pioneering the evolution of high powered L-series now would there?

Now for ONE MEEELION dollas, I would be willing divulge to you ALL that I “know” and “do” when it comes to porting Datsun heads. Until that Meeelion dolla check clears the bank, … LOL

 

In all seriousness,

 

The information is out there, you just have to look for it. Frank Hansowetz book is a great source of info on this subject, in regards to cylinder head porting, and if you read and re-read and study the pictures and have a good technical background especially in the field of fluid dynamics, you will begin to “see between the lines”, so speak, a lot more than most people grasp from just reading it.

 

Good luck,

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Silent,

Check your E-mail and get back to me as SOON as you can tonight so that I can get you that info before I leave for vacation amongst the hot, tan, native babes, oh and my lovely wife of 17 years who has been down there for over 8 weeks already and I’m sure is quite tan as well… …LOL

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  • 4 months later...
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after reading this and another head thread.... Ive come to the conclusion that asking if Braap will give details is like asking is the Pope catholic.
not that im upset... keep em coming.. im actually learning sumthen as my dad puts it.



Where in Portland did you reside?

What in particular are you interested in learning about regarding L-series cylinder heads?
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  • 9 months later...

Maby I have the rong end of the stick,but my understanding of ping or detonation had to do with the timing.as such the fuel does not self combust,just burns quicker after detonation from spark,the ping noise is the cylinder reaching peak pressure before top dead center.Hence the use of water injection to slow this combustion.

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I have a question on manifold / head port matching, exhaust and inlet,

 

Braap said in one of his posts in this thread,

 

"The intake valve opens, the piston draws the air into the cylinder, air flow is speeding up, the intake valve closes, the air still has momentum and we all know air has weight/density, the air charge compresses behind the intake valve and surges back, much like a wave hitting a wall and coming back. Then it comes back again with “almost†the same force so as the intake valve opens the air is already moving toward the valve so the piston doesn't have to draw it in"

 

my question is, if the intake manifold has a smaller diameter port than the cylinder head, thus creating a small step, and the same for the exhaust, been slightly larger, will this help with the "Helmholtz" effect or make it worse ?,

or should you just port match the head and the manifold to the same size ?

 

see picture..

 

Nigel

Picture_thumb.JPG

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By your own definition in the attached thread, detonation always occures after ignition.Peak pressure then rising too fast, peaking before TDC,ping.Thinking of the crank as a clock most leverage is at 3 o clock which would give the best mechanical advantage but its not quite so simple as the cylinder volume is also increasing as piston goes down,mechanicly reducing pressure.So the so called best bet, places PCP at 14deg after TDC.Again i recomend water injection because {as your thread says}fuel load should combust completly ,in a controled burn .Not an explosion.The link has some very interesting info on fast burn chambers that i had not herd of. Thanks Col

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