Thread: Thinking about a custom manifold

thanks for the links!
will do some calcs today

here is the photo of my sr20V plenum

its still got finished just all in pieces

reusing the standard flange of the intake manifold
hacking of the standard flange plate and welding on all custom runners
the plenum is going to be made as a 2 piece plenum so i can unbolt the plenum from the runners for cleaning and testing out other types of plenum cambers on it with out having to make a whole new intake manifold

based on the formulas on that page, have come up with the following specs

peak torque @ 7000rpm

plenum volume: 1.2L (give or take)

runner length: 30cm from opening to bellmouth - from that formular i take it this means from intake valve to bellmouth?. I would think 30cm (12") runners would be too long.
Going on the 7" figure people recommended here, thats 177mm long.

runner diameter: works out to 50mm, so I will likely use 50mm ID x 150mm long velocity stacks. Add about 30mm of flange/std runner and should get to my calculated length.

Thoughts?

hmm, further reading indicates 1.2L may be a little too small for high rpm operation
dont want it to run out of air!

All this talk about volume, length and stacks/bellmouths but no discussion on taper...hmmm

I'd be adding taper to my runners as donttazmebro says, any taper is better than none but I beleive 3-4deg is the accepted norm. If your using 150mm long bell mouths as your runners then they will likely have taper in them which will be good to increase velocity.
All runner lengths are calculated from the valve
It is approximately 4" from valve to head face so according to your calcs they recommend a length of approximatel 8" from headface to bellmouth. Maybe you could design it so you bolt you runners to your flange and make it 6" length runners and make a 1", 2" and 3" spacer and see which works best on the dyno. Does that make sense?
These calcs really are only to get you ball park as your exhaust will effect its length too, it really needs to be finetuned on the dyno so the more adjustable ou can make it the better.

Whats the volume of the std sr20ve plenum. You don't want to go too large but I would have thought 1.2L is a little small. But I'm not sure on the size of the std one so its hard to say. My gut feeling would be to go slightly larger than std.
Maybe look at volumes for aftermarket B18 intake mani's to get an approximate value. Research research research

I haven't run any numbers on making a 20ve intake (my VE is still sitting in pieces in the corner of the garage and will for a while yet!) but if I was making one I would try to aim on increasing velocity, for that reason i'd probably go for 1 3/4" ID runners at head end with about 4 deg taper and as big a radius on the entry to the bellmouth as physically possible.
50mm seems a litlle large to me, Area of standard runner at head face is 1508mm2, 50mm runner = 1963.5mm2 and 44.5mm runner = 1555mm2, Sr20ve ports are to big and velocity is too slow so anything you can do to increase velocity will go a long way.
Take this with a grain on salt though its not based on any dyno testing.....

runners will definitely have a taper, the 150x50mm ones have a 104mm bellmouth
i need to make sure that the bottom of the bellmouth actually fits into the std runner, so i wont make a choice on runner size until we get some measurements in this area.

7-8" runners could cause an issue with space to the firewall, or i wont be able to have them flush with the bottom of the plenum, is this an issue?

b18 manifolds have tiny plenums, hence my thoughts on going that way.

the port spacing on sr's is 97mm so you'd have to trim 3.5mm of each side of the bell mouth so they fit together, not really a big deal, you could file a flat on the side of the bell mouth so they join to an edge. If your using part of the standard manifold you could chop it off about 5-10mm behind the injector bosses. This is just after the manifold starts to curve up (approximately 15deg) but close enough to the flange that the runner hasn't really started to narrow (the sr's infamous reverse taper)
That will give you a little but more room and if your having a small plenum too that will help. I'm a RWD VE so I have more room to play with (marginally)

The other thing you could do is have your bell mouths sit say an inch off the floor of the plenum that way you gain an inch off space between the plenum and the firewall without reducing your plenum volume. I don't know how much of an effect this would have on performance though I have seen it done before.

Have you thought about doing a center mount TB for your plenum, I reckon that would be a good way to get more even cylinder filling with the space limitations in a FWD). Not sure whether its practical though (don't own a FWD VE)

yeah i'll be using the flange/injectors/bit of runner, with the bellmouths pushed in and shaped to suit.

Good point, the upwards angle should help with the clearance.
if i go slightly smaller with the runners, then the bellmouths will also be smaller, so there may be no need to modify the bellmouths.

center mount will be a bit too tricky, and blow out costs past what i want to spend on this little project id say.

fair enough, keep us posted on how you get on. If I didn't have so much on at the moment I'd offer to run your idea's through CFD software to see how they perform. I've been dying to try it out but haven't had anytime to use it yet!