cutting a manifold in half

Post by **XDB** » Mon Jul 07, 2008 9:44 pm

My minor is running a treat after I chucked the wrong head off, I have seen somewhere that a small power increase is possible by separating the inlet manifold from the exhaust. Is it straight forward to cut the existing manifold or am I better getting a second one and hack the two parts I don't want from each one. Any thoughts please.

Post by **les** » Mon Jul 07, 2008 10:14 pm

If you cut one up then bolt the separate parts on, they will only be parted by a saw cut. I think the idea is to cut it keeping the bulk of metal on the exhaust, throw the inlet and fit an alloy manifold, or the other way round

Post by **XDB** » Tue Jul 08, 2008 5:31 am

Thanks Les, that makes sense.

Post by **XDB** » Tue Jul 08, 2008 5:32 am

Thanks Les, that makes sense.

Post by **Peetee** » Tue Jul 08, 2008 7:37 am

In my experience it was impossible to separate the two manifolds with one cut. The inlet came off in 3, if not 4 pieces. It's doubtful if you are going to end up with an efficient gap if you did cut up two cast units and as a noticable amount of power can be achieved with an alloy inlet you'd be wise to buy one. I just don't know if you can get one for a 1 1/4 carb.

Post by **rein** » Tue Jul 08, 2008 7:40 am

After separating the inlet from the exhaust I found it difficult (not impossible) to align the inlet manifold.

Post by **alex_holden** » Tue Jul 08, 2008 8:29 am

Peetee wrote:In my experience it was impossible to separate the two manifolds with one cut. The inlet came off in 3, if not 4 pieces.

I almost managed it - the hacksaw blade only just broke through into the inlet. I patched it up with a bit of brass and some "chemical metal" epoxy and it's still running like that many thousands of miles later!

Post by **bigginger** » Tue Jul 08, 2008 2:18 pm

It's a job (if you're keeping the exhaust side, at any rate) that can be done with an angle grinder, though you have to sacrifice the inlet to give room

Post by **bmcecosse** » Wed Jul 09, 2008 6:33 am

The alloy inlet from an MG Metro is the way to go - a simple adapter plate would be needed to fit a standard 1 1/4" SU - but a slightly larger (and much better) HIF 38will bolt straight on.
Although eliminating the hot spot does help power - it may not be the best idea if you want the very best economy!

Post by **MoggyTech** » Wed Jul 09, 2008 10:14 am

Why not go the full hog and split the manifolds, and then fit an EGR valve

Post by **Packedup** » Wed Jul 09, 2008 1:04 pm

Peetee wrote:I just don't know if you can get one for a 1 1/4 carb.

You can - I've got one

It's actually a 1.5", but has threaded holes for the usual 2 diagonal/ 4 normal studs, and also the two vertical ones (with the right sized holes for the HS2 size studs). Chances are it could be filed out to go to 1.75" as well if so desired.

I'm using it on my Midget with a HIF38 as it's a little better angled for the carb to clear the bonnet (although it still fouls a little) compared to a Metro one. Why anyone would go to the trouble of chopping around and swapping manifolds to keep the strangling HS2 carb I don't know - I agree get a Metro one (or any of the aftermarket ones) and stick a HIF38 on there. Or at least drop an HS4 on, given HIF38s can be a bit tricky to source.

Post by **wanderinstar** » Thu Jul 10, 2008 9:06 am

In my ignorance. What is an EGR valve??

Post by **bmcecosse** » Thu Jul 10, 2008 10:41 am

Exhaust gas recirculation - but I can't see the logic in MT's answer here! It's a sure-fire power sapper - used to cheat early cars through the emissions regs.