16 replies to this topic

[Anthony30]

I was looking at the Maniflow inlet manifold for a set of twin HS2 carbs. In the options list it mentions servo take-off, which I want, then 1.4 or 1.5 port. What does this mean.

 

http://www.maniflow....54&product=1100

[nicklouse]

How big are the ports on your head?

Or what engine/head have you got?

[Anthony30]

998CC engine with standard CAM4810 cylinder head. I will be upgrading, or getting the standard head modified to 'stage 3' in the future, for mild/fast road application.

[Anthony30]

I'm not intending on going any bigger than the standard inlet port size on a 12g295 head. Would the 1.4 be better suited to this application? I take it the 1.5 is for 1275cc engines, or 998cc race spec heads?

[matty...]

on the 998 race heads ive seen and made inlets for have been 35mm inlet ports these are using the 12G2940 heads. It wouldn't be the most difficult job to open out a 1.4" inlet port to 1.5" if you decide to get he ports opened up in the future.

[Cooperman]

You will be best waiting until you have the fully gas-flowed head, then getting the manifold to match to it. If the locating rings are not going to be used it is necessary to fit two location dowels to ensure the manifold lines up accurately with the inlet ports.

 

It is not worth fitting twins unless the head is also improved together with the exhaust system as there will be no benefit, but the twins will require setting up differently for a flowed head from a standard one.

[Anthony30]

 I've got a new maniflow cooper freeflow manifold, link pipe, 1.75'' single box pipe, just about to buy the back box. I was planning on a set of twin HS2 carbs on a maniflow manifold. I understand my head is now a BIG restriction. I don't fancy skimming my 12G295 head by 80 thou, so might buy a 12g202, and get that modified to suit my application, or moidify my CAM4810 unleaded cylinder head currently on the car.  

[Cooperman]

In order to see how much the 295 head can be skimmed you measure the thickness of the head and subtract from it the depth of the rocker pedestal oil feed hole. Then you can skim safely to within 0.050" of that difference. i.e. you ideally need 0.050" of metal left. It's just engineering really - measure, calculate, measure, calculate, etc.

 

Until you fit a better head, all that other stuff will gain you virtually noting as the head will already be flowing at its limit. The only thing is that if you don't do the head at the same time you will be paying twice for a set up and that will cost more than the cost of a head skim and new valve guides.

 

Don't worry too much about gas-flowing the 295 head as it's still a 998 and will never have big power or torque. It is possible to modify an engine but lose performance. This happens when the modifications are intended for better power at higher revs, but this can't happen because of an engineering limitation, and there is a loss of bottom end torque caused by the modifications. I've seen this often when an engine is 'over-cammed', but other mods. can have the same effect. A modified engine is a combination of all things, not just bolting on a few 'nice bits' and hoping for the best. 

[carbon]

I'm not intending on going any bigger than the standard inlet port size on a 12g295 head. Would the 1.4 be better suited to this application? I take it the 1.5 is for 1275cc engines, or 998cc race spec heads?

Even 1.4 inch will be on the large side for twin HS2s. The bore of the HS2 is 1.25 inch, so there's not a lot of top end horses to be gained in going any bigger and larger bore inlet manifold will likely reduce low and mid range torque.

[Cooperman]

The same applies to inlet ports and carb sizes as applies to exhaust systems.

It's the flow velocity of both inlet & exhaust which is critical and it has to be optimised for torque and power. Too large a diameter means the inlet mixture flows too slowly and the inertial ramming effect it reduced. With the exhaust too large a cross-sectional area leads to slower than ideal gas velocity with reduced scavenge effect due to low pressure and low molecular velocity down the pipe. For a 998 the optimum exhaust pipe diameter is around 1.625" (approx. 40 mm) internal diameter.

With the inlet tracts it is removal and smoothing of the valve guide bosses and the 'short side radius which is most vital, plus de-shrouding the valves a bit.

Then the C.R. needs to be set accurately.

[86mayfair]

Cooperman, I'm a few weeks away from putting my engine in, and I'm in the boat were my 295 has the locating rings, but the manifold for my HS2s doesn't. I can only see the full size rings online, do you know where to get smaller ones just to go in the head?

[carbon]

I stand to be corrected here, but think there is only one size of manifold locating ring for standard A-series heads.

[86mayfair]

So my options are to either machine mine down, or buy a different manifold?

[carbon]

Machining one of the early twin HS2 alloy inlet manifolds without locator rings to take them could be done, but you will either need access to a decent mill (and a fair amount of patience) or pay a machine shop as tolerances are pretty tight.

 

Locator rings are handy for getting alignment, but probably not essential.

 

If you want to use locator rings with your 295 head then getting another inlet manifold may be the simplest option.

[Dusky]

Just don't fit the rings.