Hello,

I'm thinking about building a custom inlet manifold for the A15 in my coupe and I'm wondering what dimensions others have worked to. For a maniflod that comes straight out from the head I would guess at having about 200mm before it hits the strut tower and a little more if it curves up. From what you've all done;

-How much clearance is needed between the manifold and strut tower for engine movement? (~30mm?)

-How long are the twin weber/ lynx combinations and how much space do they leave between the aircleaner and strut tower?

-Those with quad motorbike carbs/ throttle bodies, how far out did your set up sit?

Any help would be appreciated, I'm still thinking/ planning and slow with projects so I wouldn't expect any results too quickly.

Thanks

I've thought about this some more and what I really need to know is the minimum distance between rocker cover and bonnet of an A15 in a 1200 coupe and the vertical distance from the centre of the intake port to the top of the rocker cover. I figure this will be easier to measure if someone already has the intake manifold off.

What I'm planning on doing is fitting up Quad Throttle bodies off a CBR 600RR to the A15 in my coupe. They're 38mm throttle bore with 74mm centre distances and 12 hole injectors. I reckon they'll be roughly the right size. I'm planning on sitting them more or less on top of the rocker and having a single bend radius pipe from them to the intake ports. I've done a few solid works drawings and a few calcs so far but need some better information for input and I'm hoping to avoid pulling the manifold off my car just yet. From memory I calculated the circumference of the intake port to be equal to a 32-33mm ID pipe, which will mean a slight reduction in cross section, needs to be built into the pipes.

If anyone can help with the measurements I'll be happy to post some pictures of the drawings and share the drawings I come up with for flanges to the throttle bodies.

Cheers for any help

You may be able to retrieve some rough dimensions by simply measuring from the A12 block deck or head deck to the underside of the bonnet. Just put a piece of timber or a straight edge across the two front quarter guards to give you a vague idea of where the bonnet sits when shut, don’t forget to include the bracing on the bonnet underside in your calcs. The a15 block is simply taller by the difference of the stroke I think so add that to the equation.

Also have a feel between the sump and cross member, if there is excess space you can modify the mounts so the engine sits lower, be sure to keep and eye on angles through to the drive train. I dropped my engine and it sits very low which I like.

That semicircular single bend radius is a good opportunity to put a slight angle on the throttle bodies which will give the opportunity to setup a prime approach angle of the runners as they lead into the ports. So imagine ¾ of a semicircle rather than exactly half a circle (well that’s the cheapest way anyway). Good idea.

If clearance becomes a real issue you have the opportunity to cut the rocker cover top off and shorten the rocker cover or put a flat top on it. You will have to plumb the rocker breather outlet perhaps coming from out of the back of the cover, be sure to put a baffle in place (as the stock cover has).

Thanks for the input, I'll definately have a squiz at it, I meant to last time I was home but ran out of time (my car lives in a different state for now although this will change when we move later this year).

I'm interseted but not sure what you mean by this

"That semicircular single bend radius is a good opportunity to put a slight angle on the throttle bodies which will give the opportunity to setup a prime approach angle of the runners as they lead into the ports. So imagine ¾ of a semicircle rather than exactly half a circle (well that’s the cheapest way anyway). Good idea."

, are you saying the input ports are angled and the runners should meet the head at an angle? I've never had it apart to look and it's the first Datsun I've ever owned, and probably the best looking small car there is in my opinion.

I've attached a draft of the design to give an idea of what I've got in mind. I was under the impression that the intake ports were directed more or less prependicularly from the manifold face.

Cheers

Attach file:

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  A15-CBR600-Mani.jpg (31.44 KB)

That would be cool but yeah clearance will be an issue.

Sorry the approach angle of oval port heads is a bit different, for round port heads what you have is fine, although there is one last very sharp turn from port runner to valve inside of the A12 round port heads. Don't forget the oil filler hole in the cover. It might be worthwhile putting a join mid way along the 'U' of those runners so you can disassemble relatively quickly to fill oil or check tappers/rocker gear.

With an oval port head, if the approach angle of the runners can suit the angle of the ports and lead down into the cylinder you (I would think) gain better flow.

Right, I'm with you now. Do you happen to know what the angle is? A mitre of roughly 8 degrees or less wont matter much if the cross secion is consistant before and after. I've been hoping to pick up a second hand head cheap to play with, I guess I'll look up that section on the forum and have a guesstimate, the angle will mean the port cross secion is less than the flange face area. It's not too hard to project the profile onto a plane at an angle though. My maths is pretty good.

I do remember seeing another custom manifold that bent over the rocker cover made of stainless on the forum which is a good reference. In any case it's early days design wise.

Ummm approach angle on my modified GX Head was 22 degrees but that is a number that has just popped into my head. I don't recall the angle of the other oval port heads off the top of my head but I think they are around that mark maybe more like 17-20 degrees.

I'm pretty sure the designs you were thinking of did have the join, think it was for a turbo setup,not sure.

I have a few old round port heads somewhere all dirty and crap. You can have one for free if you want it.

Having looked at the sections again I see what you mean, I'd forgotten about that. Does it depend on what head you have or is the basic design of the oval port the same. I don't actually know which one I have at this stage. WHile there are no oil leaks I'm a bit reluctant to pull it apart just yet but this will have to happen before I finalise the design. Measuring the H89 head section off the computer screen it is 14-15 degrees but there is no way to compensate for parallax.

Next time I have a spare day in Adlaide I'll have to drop you a line.

The design I'm thinking of did split half way through the bend, had orings there so it was a quick on off, great idea. I'm still thinking about the ideal place to do that, a necessity I think it the tappets ever need adjusting.

In any case it's fun creating different designs and then making stuff.

Thanks again.

This manifold only had about 15mm tower clearance. You will find that the engine will move only a little closer to the tower when driving. It more rotates a bit but not realy moving it too much closer.

I like the idea of having a straight section before the port. But the tight radius bends to come back over the rocker cover are less than ideal on this manifold.

It also has a 1.5 degree taper leading to the port over the last 220mm, ive read that this taper maximises velocity at the port. much more taper than this starts reducing the gains from a taper. http://datsun1200.com/uploads/photos/12251.jpg


If you go with quads, moving the throttle bodies further from the head enables the use of slightly larger throttle plates without losing midrange because a good taper can be included in the design. Giving less restriction at the plates and good air speed at the ports.

Cheers for that! All useful input. So 15mm clearance is sufficient with the datsun engine/ mount setup. That will be useful to remember.