Outer Limits 4x4 Board

... and can anyone see a problem with machining 20mm or so off it, as the guy who took this photo is doing?

Saves me from getting a "bonnet bulge" done...

IS it a Manifold or Plenum. I can see it may be a bit more responsive to the throttle!

I can also see heat issues with the fuel, due to less metal or distance from source.

Also, clearance issues with other components hitting what sits on top

I could be wrong though, cause it may not be what I think it is!

chimpboy wrote:

... and can anyone see a problem with machining 20mm or so off it, as the guy who took this photo is doing?

Saves me from getting a "bonnet bulge" done...

Its the lower half of the Plenum chamber. I would think the throttle body would hit the rocker cover if you took 20mm off a stock set up.

RUFF wrote:Its the lower half of the Plenum chamber. I would think the throttle body would hit the rocker cover if you took 20mm off a stock set up.

If I took off as much as could come off without that hitting, can you see any other issues with it? It's just an idea. This guy took off nearly an inch, apparently.

Would you also shorten the trumpets, as they would be too close or interfere with the top. That will give less torque.

bigbad wrote:Would you also shorten the trumpets, as they would be too close or interfere with the top. That will give less torque.

Would definitely shorten the trumpets to suit. That would be the easy bit for sure.

It just seems a bit whacky to have to have a hole in the bonnet if there's some scope to lower the profile of the engine sufficiently.

edit: actually there's room under the throttle body (without that cold weather warming thing on there anyway) but there might be some interference issues with the fuel injection rail.

Anyway, assuming I can solve physical fitting issues, the exact thickness of this part would not be some highly tuned thing that has a huge influence on performance or something would it? I mean it is just a bunch of holes that air passes through, basically?

chimpboy wrote:

bigbad wrote:Would you also shorten the trumpets, as they would be too close or interfere with the top. That will give less torque.

Would definitely shorten the trumpets to suit. That would be the easy bit for sure.

It just seems a bit whacky to have to have a hole in the bonnet if there's some scope to lower the profile of the engine sufficiently.

edit: actually there's room under the throttle body (without that cold weather warming thing on there anyway) but there might be some interference issues with the fuel injection rail.

Anyway the exact thickness of this part would not be some highly tuned thing that has a huge influence on performance or something would it?

I gave that some thought and came to think that it may only make a difference if it was direct injected from above that point, as the injection timing per cylinder may need adjustment. Also for an atmo motor, the cam timing may need a tweek, if at all, because we are talking about 1000ths of a second difference.

Saying that. you would need to do complex comparative testing to determine any difference and adjustments needed.

if its is four your 4wd i think torque is very important,

maybe another way would be to use a dry sump and lower the whole engine??????????

serg

uninformed wrote:if its is four your 4wd i think torque is very important,

maybe another way would be to use a dry sump and lower the whole engine??????????

serg

Its in a triumph

Do it, just suck and see, beats cutting a hole in the bonnet, it'll alter airflow, with an end result of ???????????, the trumpets will remain the same length....Do it

^^^^^ x2

Should give beta response and rev faster with shorter trumpets tho may loose a little top end tourqe tho a super charger will fix that

lokka wrote:^^^^^ x2

Should give beta response and rev faster with shorter trumpets tho may loose a little top end tourqe tho a super charger will fix that

at the worst you'll have to find another manifold (not as hard as finding a new bonnet I'm assuming)

Shortening the trumpets will be the hard part , i'm pretty sure they are stainless and will be hard to cut down cleanly. Not sure how they are held in though ?

Loanrangie wrote:Shortening the trumpets will be the hard part , i'm pretty sure they are stainless and will be hard to cut down cleanly. Not sure how they are held in though ?

I think they are just an interference fit actually.

or just do away with the trumpets if the torque isnt a necessity, other otions here http://www.mez.co.uk/ms12.html

bigbad wrote:http://www.mez.co.uk/ms12.html

Wow, there is some fun stuff there.

Shortening the intake runners will move your peak torque up by a certain amount of rpm - basically a loss in the low down torque department. Probably won't make too much difference in your car though.

Why would you need to shorten the trumpets? You are machining material off the bottom not the top.

Why would you need to shorten the trumpets? You are machining material off the bottom not the top.

Correct. Trumpets will have exactly the same clearance to the inside of the plenum as before.

Anyway, assuming I can solve physical fitting issues, the exact thickness of this part would not be some highly tuned thing that has a huge influence on performance or something would it? I mean it is just a bunch of holes that air passes through, basically?

Ultimately it will have a minor effect on the air velocity which would have a minor effect on how the atomised fuel from the injectors gets picked up. However, beyond a upwards change in the rpm at which your peak torque comes in, it shouldn't have a massive effect on performance. You have to remember the system was designed in the late 80's, and wouldn't be as strenuously flow tested as any new manifold design - and the same manifold was run (on 3.5s, 3.9s and 4.6s) with very minor changes up until the Thor manifolds in '99.

actually the front two and rearmost two trumpets are seated only 5 mm from the bottom mounting face, and the interference fit is only 12mm deep so taking 20mm off will mean no fitting for those trumpets.

Thanks guys.

I was just wondering if there were any big gotchas like, "no way if you do that it will kill the carefully calibrated air flow" or something. But it sounds like basically it's not a big deal to machine this part (or maybe the upper part of the plenum as well) as long as everything still fits together okay at the end of it.

RUFF wrote:Why would you need to shorten the trumpets? You are machining material off the bottom not the top.

You are absolutely right. When I first looked at this I was thinking of skimming some height off the bottom of the top part of the plenum as well, which would reduce the amount of space above the trumpets. Now it sounds like a bit of both might be in order, which might still be the go if it keeps the bonnet flat.

Measuring the one here it is actually 8mm from the bottom before you cut into the trumpets.

From a fluid mechanics perspective, shortening the intake runners and/or moving them closer to the valves will make the airflow less uniform, (i.e. less laminar) especially at low engine revs.

So it may mean that the airflow is not as even/balanced at low revs, and the engine may not drive quite as smoothly. But overall it should not make a big difference.