Mikuni 40phh intake runners

[madkaw]

I broke out my Mikunis to look them over for the future install on my Z. I wanted to check out jet sizes and such to get my base line for future tweeks. I went thru great trouble to gasket match my SU's to my e88 before install and thought I would do the same for the Mikuni set-up. But WOW, i couldn't believe what I saw when I layed an extra intake gasket against the manifold. Am I missing something here or is this right? That is a huge difference-not sure what to do here guys, I just seem baffled over this. These are 40phhs by the way.

Comments please.....

[Xnke]

Yep, that's how that manifold comes. It's set up for L20A ports, I think, which are much smaller than the L24/26/28 ports.

[madkaw]

Okay, I was told this kit(came in a mikuni box) was for the 240z specifically. So they intentionally make the runners small to apply to all the L6 motors? I noticed quite a lip behind the mounting flange area- in the runner, so gasket matching shouldn't take it beyond the actual runner diameter?

[blue72]

Who made that intake manifold?

I've got an SK triple intake with that exact same lip on the cylinder head mounting side. Like Xnke said, it was made for an L20A.

I measured mine and the lip had an ID of 30mm while the area just behind the bump actually measured an ID of 34.75mm. Just cut that lip out.

[madkaw]

Mikuni

[TimZ]

There's a fair amount of room for porting - here's mine:

 

 

I did have to weld some material back in at the point where the relief is for the thermostat housing.

[madkaw]

Thanks Tim, I'm not really interested in doing any "porting " as much as doing gasket matching. Looks like you went beyond gasket matching, but your intake ports might be enlarged. I only enlarged my e-88 ports to match the stock gasket and SU manifold.

[Tony D]

If you read the applicable Mikuini Material regarding their manifold, they intentionally made the runners smaller for boring up to size as dictated by head build. If you make a manifold too big, you're screwed if the ports you have are smaller. As a manufacturer, it only makes sense to make the SMALLEST runner possible and then let the user tailor it to the build.

 

I'd not screw with it, port matching will gain you nothing if the manifold is smaller than the intake port. That manifold bolted on with a set of 44PHH's was a 22% bump in HP for the L24, and somewhere in the area of 8-12% over a comparably EFI equipped L28. BOLT ON JET CORRECTED.

 

Dyno pulls were done at the Mikuini Facility in Northridge, and verified at several SoCal Dyno Facilities at the time. It was a rock-solid modification.

 

That unported 'doesn't flow well' manifold BTW, pumped 350HP to the rear wheels on a non-ported N42 Head in Blowthrough Turbocharged Form, and was 165 to the rear wheels in N/A form.

 

You may want to reconsider 'improving' it until you do real head work and cam changes. That was with a 44PHH on them, with a 40PHH, and a smaller carb...I'd leave it alone!

[Tony D]

Yep, that's how that manifold comes. It's set up for L20A ports, I think, which are much smaller than the L24/26/28 ports.

 

Incorrect, the manifold was set up for L24 ports. The Mikuini Manifold Ports are larger than an L20A by approximately 1mm radially.

 

The 'port matching' phenomenon has shown to be a red herring for the most part. What people are now realizing that having STEP from the intake manifold to the head (manifold SMALLER than the head by some margin) actually HELPS with low speed drivability by creating an anti-reversionary effect in the intake pulse.

 

I have mine unaltered, I was pleased with the results. Once you make it larger, it's hard as hell to bring it back to a size smaller! If anything, I'd not go any larger than 1mm diametrically at the extreme outside unless the head is being worked. The whole theory behind the Mikuini manifold was the ability to RAISE THE PORT ROOF to allow a better shot at the back of the valve. If anything, ADDING material at the bottom of the runner is what is required, in addition to welding in the floor of the stock intake port!

 

But the Mikuini Manifold was specifically designed with bolt-on application to the L24 and L28 in mind. The L20A was not a consideration, and Mikuini really didn't sell a lot of these castings domestically. Which is strange. My manifold was ALWAYS a source of curiosity amongst the Japanese. EVERYBODY had the original Datsun/Nissan Competition Parts, or knockoff copy therein. Or FET/SK which were smaller by a great margin than the runners on the Mikuini Manifold. Visibly so!

 

Matter of fact, the L20A with a Mikuini Manifold running 44PHH's will start to 'breathe' at 180kph in 5th gear and start responding snappily. Below that speed, it's a bit soggy in performance...

[madkaw]

TonyD,

thanks for the response, but confused about this line

I'd not screw with it, port matching will gain you nothing if the manifold is smaller than the intake port

I think you meant that I don't want the intake manifold port larger then my head port-which I understand that. My E-88 head is modified with the larger valves, mild porting, unshrouding of the chamber area and a stage 2 cam.

 

I was just taken back by the size difference in comparison to the gasket-as in the photo. If there was some opening up to do I wanted to do this before I mounted these up. This is all a learning curve for me and I just ASSumed that gasket matching was prudent, but worth while effort towards performance. I have no proof, but I believe my SU's are choking my engine performance in their stock form. With my humble head improvements and dual 50mm exhausts it seems my engine needs to 'breathe more', but I might be just talking out my a$$.

 

I will probably just bolt on as they come for now, and see how the improvement goes. These 40's came off of a car running a L24. I believe the brochure for the Mikunis stated that this was a 23% hp increase!

Edited October 18, 2010 by madkaw

[Tony D]

No, I meant EXACTLY what I said. Read it again and the logic behind the statement.

 

IF the MANIFOLD is SMALLER than the port in the head, the resultant STEP exists in a REVERSE-FLOW direction, which will stop intake tract REVERSION during the overlap portion of intake open/exhaust open valve events. The flow on the manifold is PROVEN to give a 22% bump in performance to an L24 AS-CAST (that was with 44's not 40's). That means WITH THE SMALLER THAN HEAD PORT ON THE MANIFOLD SIDE. You open it up and 'port match' it, you gain NOTHING. Most people have realized 'port matching' in a Datsun head is wasted effort and that a small step as exists is BENEFICIAL to low speed operation. Cut that step out by match porting, you LOSE this benefit.

 

IF the HEAD side is SMALLER than the Manifold Port, there will be an impeding of the airflow caused by the bump.

If the HEAD side is BIGGER than the Manifold Port, there is a 'check valve' action during overlap, and no real impedance to flow whatsoever.

If they are MATCHED in size, you have no overlap anti-reversionary step, making for worse ariflow, and chances are that the porting will not MAINTAIN a taper from throttle plate to valve. It will OPEN UP in the 'match ported' region, and then begin to taper again in the head. This is TERRIBLE for flow, because it kills velocity.

 

There is absolutely NOTHING wrong with using the Mikuini Manifold AS CAST on a non-ported head.ESPECIALLY without a cam!

 

Remember this is how the thing was DESIGNED AND TESTED! NO PORTING OF ANYTHING JUST BOLT IT ON! Jetting was the only thing open for change depending on trackside conditions.

 

Mess with it, and chances are unless you taper that intake back almost to the throttle bore you will make the flow through the manifold and into the head worse than if you just left it alone and drove it!

[madkaw]

Okay, I understand the logic behind the reversion better now, but again I was suprised of the size difference. And when refer to overlap, are you refering to a cam that has an overlap grind? Does my stage 2 270/270 with 480 lift qualify that statement?

 

Also, does the fact that this is a Mikuni short manifold change the effective velocity as compared to a long manifold? In other words do changes as far as the possible negative effect of gasket matching to velocity lessen, because of the less distance for the charge to travel?

 

Lastly( I hear a sigh of relief from the crowd), when using this manifold later on for ITBs, would this be a mute point as far as fuel injection? Would gasket matching be a better course at that point?

 

I hope someone else is getting something out of all these questions!!!!!!

[Tony D]

Argh...

 

The manifold needs to taper from the throttle plate to the back of the valve.

 

This can't happen when they are 'gasket matched' in the traditional sense.

 

Can we stop using the word 'port matched' or 'gasket matched' now?

 

Proper inlet manifold will taper from the throttle plate to the back of the valve. The mikuini manifold will allow you to do this and the larger ITB you use the more the veolocity will increase as the taper will be more over a shorter distance (to a point.) A longer manifold would only mean any matching would mean a more gradual blending over a longer distance...

 

 

As for being a MOOT point with ITBs... It is because GASSKET MATCHING IS USELESS THE MANIFOLD NEEDS TO TAPER FROM THE BACK OF THE THROTTLE PLATE TO THE BACK OF THE INTAKE VALVE CAN WE PLEASE STOP USING "GASKET MATCHING" AS A TERM NOW BECAUSE GASKET MATCHING DOESN'T WORK!

 

In terms of the reversionary step, NO, this will still be true. The intake manifold will benefit from being slightly smaller than the intake port in the head.

 

The key is unless the manifold continually tapers from the throttle plate to the back of the intake valve you are FAR BETTER OFF having a smaller intake manifold so you have the reversionary step than to make it the same size. If you are not willing to properly taper the manifold to match the port in the head, then LEAVE IT ALONE. Hogging out 2" of port for the sake of making them even is useless and a waste of your time. You will GAIN nothing, and likely will hurt reversionary tendencies of the setup, if not overall flow.

[Tony D]

Port matching got a BIG NAME from DOMESTIC heads where the ports are misaligned and in some cases the gasket blocked half the port, or were off by hald the cross sectional area.

 

In an old Chevy, port matching is almost MANDATORY because of the piss poor casting alignment.

 

This ain't a Chevy. They don't need an overhaul at 100K. They don't need manifold port matching. They don't need ridge reaming.

 

Let the domestic myths die!

[madkaw]

Okay, but what about gasket matching!

 

Only kidding:)

[Leon]

Intake velocity is much more important in a carburated engine where the air-fuel mixture must travel from the throttle body all the way to the cylinder, through the manifold. This is because at higher velocities, pressure decreases and the fuel is better atomized. As velocity decreases, pressure increases and the fuel begins to pool in low spots and on manifold walls. The need of the fuel to vaporize is also another reason why manifolds are heated, at the detriment of performance.

 

If you go with ITBs, manifold velocity will be less important when compared with carburated engines. That is another advantage of port fuel injection, you can have big runners that promote flow yet fuel delivery is not affected as it would be with a large-manifold, carburated engine.

[Tony D]

If you go with ITBs, manifold velocity will be less important when compared with carburated engines. That is another advantage of port fuel injection, you can have big runners that promote flow yet fuel delivery is not affected as it would be with a large-manifold, carburated engine.

 

That is true to some extent, but the more velocity the better even in ITB's. The atomization is taken care of mostly by the higher pressure injection through the nozzle, but the higher volocity acts the same way for homogenizing the mixture same as with a carb. "port injection" is more for emissions purposes---real high performance injection has the injector in the air horn, or throttle body, and the manifold is still 'wet'!

 

The thing ITB's give you is that you don't need to RESTRICT the airflow through the ITB body to PULL fuel into the airstream (venturi and jet well arrangement)--therefore they flow better. You INJECT the fuel atomized, and the higher velocity airstream you inject the fuel into---the better results you will have in mixture homogenity!

 

ITB's flow better (SCADS BETTER) in the SAME runner than Carburettors for this reason. Carbs are usually measured at 28 or 14" mercury restriction. EFI at 9". The flow increase for the same runner size is astronomical. With ITB's you don't NEED larger runners. The above statement has it exactly backwards---CARBS need the big runners for flow because of the inherent restriction needed at the front end. They then taper radically from carb to intake to keep velocity up.

 

In an ITB, they are smaller by a factor, and need not taper as much because they don't start out as large. They operate at less differential, and therefore flow more for the same size port.

 

The Bonny car gained 40HP and 700rpms of operational speed when going from 45 Webers to 45 ITB's--same head same cam, same manifold. And that head went on our 2 liter and has run to significantly higher rpms. The Webers idled at 1700 rpms to keep running relatively smoothly. The EFI would idle wherever we wanted it to, from 400 to 1700rpms. WE normally idled it at 900 just because.

[Leon]

That is true to some extent, but the more velocity the better even in ITB's. The atomization is taken care of mostly by the higher pressure injection through the nozzle, but the higher volocity acts the same way for homogenizing the mixture same as with a carb. "port injection" is more for emissions purposes---real high performance injection has the injector in the air horn, or throttle body, and the manifold is still 'wet'!

Sure, velocity in an ITB system will promote mixture homogenization but so will the shape of the runners, combustion chamber and pistons. Thus, velocity is not as important in an injected engine versus a carburated one as I said in the original post.

 

When you say "real high performance injection" I'm assuming that you're speaking of F1 and other race-only engines, where rpms are usually (very) high and there is plenty of velocity in the intake to keep the mixture atomized. I don't think they are very concerned about low speed operation and fuel condensation, at least compared to street applications. Plus, those runners are usually pretty darn straight. Port injection is just fine, and much better than carbs (as you've shown), for street and race engines as it has proven in countless applications.

 

 

ITB's flow better (SCADS BETTER) in the SAME runner than Carburettors for this reason. Carbs are usually measured at 28 or 14" mercury restriction. EFI at 9". The flow increase for the same runner size is astronomical. With ITB's you don't NEED larger runners. The above statement has it exactly backwards---CARBS need the big runners for flow because of the inherent restriction needed at the front end. They then taper radically from carb to intake to keep velocity up.

Notice I never said anything about needing larger runners, I think you've misconstrued my statement. Yes, it is correct that carbs do need bigger runners to flow as well as ITBs, but that's not what I said. Here is the last part of my post:

 

you can have big runners that promote flow yet fuel delivery is not affected as it would be with a large-manifold, carburated engine.

 

As you make a runner bigger in a carburated engine, the velocity of the mixture decreases, therefore you have more fuel condensing in the tract. With a port injected ITB setup, fuel atomization is not dependent on manifold velocity, therefore you can utilize larger runners without detrimentally affecting performance or driveability. That's what I meant by that statement, nothing about matching performance.

[Tony D]

Port injection is a compromise for Emissions, don't kid yourself.

 

Runner velocity is critical in either application. Making it sound like it's something you can pooh pooh because it's injected at the port is poor judgement IMO.

 

Most ITB's will inject not at the back of the intake valve, but in the ITB itself, and all testing has shown the further away the mixture is injected the more power it will make. If you want low end drivability, you keep velocity up.

 

That is why carburettors always seemed to suffer when sized for maximum power, there just wasn't enough velocity nor manifold vacuum for them to function properly below several thousand RPM's.

 

If you port inject, you are giving up power under the curve in a big way. If you inject at the TB you aren't giving up any drivability if you have a proper runner design.

 

Like I mentioned, we gained BOTH drivability, idlability (?) AND top-end performane going to EFI from carbs on the exact same head, intake runners, and etc We simply replaced the 45 Webers and went from a low of 1700 for idle, to anywhere we wanted from 400 to 900 for warm idle. We gained 40HP and somewhere around 700rpms on the top end, power peaking now at 8250, and shifting at 8500 instead of a power peak of 7500 and shifting at just before 8000. Torque curve was flatter, broader, and higher.

 

We had good velocity before, with the ITB's injecting in the body we picked up all across the board. Our pumping losses decreased quite a bit not having to draw through those venturis.

 

And we did have an anti-reversionary step at the head/manifold juncture---it was NOT 'match ported'...

[madkaw]

I think that last line was for me