Porting an intake for MAP

AndreGT6 · Post by **AndreGT6** » Mon Dec 22, 2008 3:12 pm

Looking for guidance on the subject.

1, closer to the carbs or closer to the head?
2, how many ports? 2 or 6? I have a twin SU carb setup, running on a 6 cylinder engine.

Thanks.

NITROPIXIE · Post by **NITROPIXIE** » Mon Dec 22, 2008 5:53 pm

Whats the threaded hole for in the middle of the manifold nearest the front of the picture??

AndreGT6 · Post by **AndreGT6** » Mon Dec 22, 2008 6:09 pm

Used to vent fumes from the valve cover back into the intake.

So yeah its a vacuum port.

Considering using it as a test.

I also have a vac port on my rear carb.

So it would be interesting too see under load if they gave similar readings.

What I might do this week is see if I can adapt a plug for the port on the intake and then record the results at var RPM ranges between the carb's vac port and the manifold.

NITROPIXIE · Post by **NITROPIXIE** » Mon Dec 22, 2008 8:15 pm

The port on the back of this carb is on the atmosphere side of the butterfly when fully closed I believe. So you can't use this.

What you need is a port on the manifold side of the butterfly when closed so it will have a vacuum for the Map sensor during idle.

What i would suggest is to have a vacuum port on the manifold by drilling and tapping a hole in the same tube as the crankcase vacuum on the manifold, or you could use the same threaded hole as the crankcase vacuum but you will need an anti run on valve between the junction for the map sensor and the crankcase port on the block.

I think tapping the manifold would be the better option personally??

Is that a HS6 carb?? What manifold are you using and is it the same as the stromberg manifold??

Gilesy998 · Post by **Gilesy998** » Mon Dec 22, 2008 10:06 pm

NITROPIXIE wrote:The port on the back of this carb is on the atmosphere side of the butterfly when fully closed I believe. So you can't use this.

Correct, they show some really wierd vac characteristics thos ports.
I'd suggest driiling and tapping for a take-off right next to that existing vac connection.

AndreGT6 · Post by **AndreGT6** » Mon Dec 22, 2008 11:11 pm

NITROPIXIE wrote: Is that a HS6 carb?? What manifold are you using and is it the same as the stromberg manifold??

Yes they are HS6's, SU 1.75" carbs on a GT6 MK1 intake manifold, same one I ran my SU 1.5's or stock would have been the stroms.

I have some adaptor plates from Triumph Tune, they help the bolt pattern and make more room for the butterfly to move about.

After looking at my options I think I will tap 2 map points as discribed above.

Reason 1, 2 balanced inputs.
Reason 2, I still think I want to vent the valve cover into the intake, from experimenting this summer the fumes can get a little strong other wise.
Reason 3, the intake is off and accessible. If I went with the centre point option and it was too spiky I would have to drain the cooling system.

A.

AndreGT6 · Post by **AndreGT6** » Mon Dec 22, 2008 11:12 pm

Gilesy998 wrote: I'd suggest driiling and tapping for a take-off right next to that existing vac connection.

Centred on both sides?

NITROPIXIE · Post by **NITROPIXIE** » Mon Dec 22, 2008 11:37 pm

In all honesty where you have put the port on the manifold in the newer picture will be more than adequate and optimal really!!! I am assuming that what it is drilled into goes to both branches of the inlet manifold???

No need to have 2 ports drilled now or anything like that. This wont affect the performance/reading of the map sensor in anyway and does not need to be centred/balanced in anyway, nor does it need to be smoothed with a vacuum reservoir due to the fact all the inlet ports are connected through the manifold by that tube which would also act as a vacuum reservoir/smoother.

On my mini engine i have just one port which is by no means in the centre of the manifold, its mainly on the right hand branch, although only a single carb, it works perfectly well.

What you have here would be the ideal setup and you would not need to deviate from this. Infact your engine being a straight six cylinder engine will make the map vacuum smoother than it would my 4 cylinder engine by design.

Another good thing from not using the rocker cover breather for a map port is any oil fumes coming from the cover wont get up to the MAP sensor tube.

I think i understand what your trying to achieve by having 2 ports and balancing with your design but there is real no need. But by all means thre is no harm in running some tests to see what can be achieved.

Gilesy998 · Post by **Gilesy998** » Tue Dec 23, 2008 12:08 am

It would seen that the exact position is unimportant - I use the takeoff on the right-hand runner of my inlet, and it's perfect. Too se what an odd position this is, see here - http://i20.photobucket.com/albums/b247/ ... age004.jpg
I'm yet to try the other union in the centre (the hose will foul the airox), but it's perfectly fine where it is for now, even on a pulsey A-series.

AndreGT6 · Post by **AndreGT6** » Tue Dec 23, 2008 11:34 am

Sweet, well then I will port on the left side as it will be a cleaner run to the bulk head into the car and to the MJLJ box.

I am looking at breather port options, but still feel doing a small tap job for the port will be a clearer looking install.

Leaves the breather port alone to either be used or capped.

A.