Outer Limits 4x4 Board

G'day I want to put dual sidedraft carbies on my 1.3 zook motor but I'll need to have a custom manifold to do it. What type of thickness will I need for the flange to prevent warping when I weld it or from engine heat? I have plenty of 3mm steel lying around but I have a feeling it may warp slightly where the welds are. I would modify the standard manifold but it's alloy and I have no idea on how to weld alloy...

I wouldnt use anything less than 5mm steel on the flange side........ But I would more likely use say 10 or 12mm in ally, so I could machine it after the build up and get it flat as.........

Made a custom exhaust manifold for a mates turbo Patrol from 50x50x6 shs and 12mm fms for the flange.
Needs to be faced afterwards regardless Kitika you dont want any irregularities at all.

Mate I got a 10mm flange here that I had lasercut as a spare. I was originally going to turbo my 1.3 and also create a custom inlet manifold, so i designed the flange and had it cut, but as it turned out i just went a 1.6.
Its way overkill and could have used half the steel, but I wanted to minimise warping as much as possible. If you want it PM me.
Also remember that you will have to fab up the coolant feeds to the head too which i also had cut into plate.

I've made a couple of different styles at work recently and we tend to use 12mm ally as depending on size of flange there could be over 1-2mm of warpage to get machined out.

For steel I would go 6mm to partly reduce warping and to allow plently of meat in the mounting bolt holes after you get it machined. Depending on how well you weld it you could still loose a mm in some spots. I would drill and tap the carby flange to insert studs (even if they are just cut off bolts or thread). Much better than mucking around with nuts and bolts to hold the carbs on. Also make sure you use the o ringed gaskets between carb and manifold and the spring loaded washers for bolting them on. You can get plastic oring mounts with renewable o rings but they are a pain get the gaskets with the bonded o ring much easier to use. Also get the rubber spring/damper washers for fixign rather than the spiral spring versions as they break.

Alright so as thick as I can get is the general consensus.
What about the length of the intake runners?
I have read a few places that the length of the runner behind the carby can affect performance in different rev ranges because of the way the fuel mixes and flows. And spacers can be bought for this reason.
I can make them from about 1inch to 8inches long in the space I got, how long do you reckon I should make them?

10mm will so the flange but i suggest 12 mm for my turbo manifold for my vitara i used 3 mm wall steam pipe, tacked with a mig then had a tig welder tig it all together, i only run a maximum of 7 pound boost and i have had no problems

No matter what size steel you use for the flange it is still going to warp if you're welding it with a mig. You will still need to get it machined afterwards.

Kitika wrote:Alright so as thick as I can get is the general consensus.
What about the length of the intake runners?
I have read a few places that the length of the runner behind the carby can affect performance in different rev ranges because of the way the fuel mixes and flows. And spacers can be bought for this reason.
I can make them from about 1inch to 8inches long in the space I got, how long do you reckon I should make them?

Now we are getting into technical stuff here and manifold design is an art and no doubt there are a few programs out there for estimations, don't know. Not sure how relevent in a 1.3 offroad vehicle. for my money I wouldn't use side drafts or twins on this application even though I love them on old road cars. What are the carbs anyway, even twin 40's are way overkill unless you ar erevving the tits off it all the time but fun to play I suppose.

Short runners promote top end power, long runners promote bottom end. Hoeveer it is the total inlet length that influences so would include the bellmouths on the outside of the carb. So decide where you want the power and make them to suit. really long will put a lot of load on the whole design when thumping offroad and really short may not be practical to make. Remember your carb ports will not match the spacing of the ports on your head so you need sufficient length to be able to taper or angle them which ever way to get them all equal. Also bear in mind the size of the carb ports in relation to the head ports, They are likely differnet sizes as well and if a fair difference you should really have tapered tubes otherwise one end wil have a step one way or the other. I would say a 5" manifold with say 2" bellmouths would be a nice compromise. You really should run bellmouths on sidedraft as airflow does not turning sharp corners. Even with full air cleaners over the carbs you should still run bellmouths inside the air cleaners so this shold be taken into working out overall set up length and available space.

The carbs are single butterfly Mikuni Solex's off a 800cc suzuki Van/Lj 80.
So I will have 1 carb/butterfly per 2 cylinders.
I was planning on running pod filters (I know oiled filters are shitty at filtering but it looks like the easiest way to do this and may give me a little bit more flow) straight off the end of the carbs with a box made up to hook it up to the snorkel. What is a bellmouth by the way? Is that what they have coming straight off the carbies in drag cars etc?
I was thinking 5inch would be a good compromise size and hopefully not put to much leverage on the manifold while bouncing around. Altho the stock 800cc manifold is about 10 inches long going by eye but they only use a single carb not two.

No more water crossings...

If you are running pod filters off the end of carbs you may be better off running the socks. They are oiled foam cleaners shaped like stubby holders and get cable tied on. cable tied to the bellmouths or ram tubes they get called. Basically you should have a tube of some sort that can bolt to the mouth of the carb but it should be trumpet or bellmouth in shape and the larger the bell radius the better. They can be bought off the shelf for webers and dellortos and you may be able to modify them or you might be able to flare some tube yourself which would be better than nothing. We are talkign here about getting that bit more out of them and maybe you would not notic the difference but ideally you should not have any sharp corners in your inlet design as the air overshoots the corner before it can start turning in. You may be better off putting like a K & N box filter inside your box which is bolted tot eh face of the carbs and then just running an air pipe into the box but still either way it is preferrable to have a bellmouth entry even if it is only an inch or so long.

How are you going to transition your single carb opening into twin manifold runners to the 2 inlet ports on the head.

I was going to have a play with the manifold on the weekend but the zook blew an axle bearing ( not even 10,000ks on it either!!!) So was too preoccupied with that to start cutting the 6mm plate I got.
I like the sock idea I may give that a go to start off with but a standard type filter into a box could be on the cards eventually.
I have been thinking about the best way to do the intake runners and i'm not quite sure which is the best way to do it. I was thinking of running slightly bigger pipes than the head ports( will countersink the manifold on the runner side to help with the flow) running on diagonals so it all meets up even to give even flow. Better described in a dodgy paint picture!


So what do you reckon about that idea? May not need bigger pipes it all depends on the size of the carb outlet compared to the head ports.

You do not want bigger runners than the head ports or you will have a step from the runners to the head. This is bad. Smaller runners would be better than bigger if you cannot get the correct size. Unless you go bigger runners and don;'t stick them through the flange and you shape your flange openings so the runner side matches the runners and head side matches the ports. So the flange holes are tapered. Bit of rooting around for you application, get the right size or slightly smaller. If I make a manifold or similar flanged pipe for anything else generally I will poke the pipe though the flange so it sits a mm or 2 back inside the flange and weld it from both sides. but this is when tigging. We do a light sealing weld on the inside that just fuses the pipe to the flange to ensure sealing and then a proper weld on the outside for the securing strength. Proabably not relevent if you are migging it together.

I would also not be joining my 2 port runners just before the carb flange eg within about 10-20mm. I would try and get it a bit further back but that would depend on the angle of the port offsets etc. Allow a bit of room for the gas flow to settle and not make it split immediately out of the carb. You could also use a very short section of pipe that matches the carb opening size to get your transition from the carb to the runners where they split, again depends on the angle and the size difference from carb to the 2 runners.

Try to make the flange for the carbs as one piece so this will help to keep them in line and at the same angle etc. Alos means they won't flex differently and have influence on throttle actuation so they balance better. You also only need one fitting for brake booster vacuum, I've seen people feel they need to take vacuum off all runners for balance or something but this is extra hassle that is not necessary.

Why not run motorbike carbs? proberly easier to adapt, 1 throat per cylinder and better for running at angles I would assume. There would be a great choise of sizes from 24mm to 40mm in side and semi down drought available and jetting would proberly be easier to sort out.

Also if you run too big a dia. and too long an inlet runner from carb to port you will loose air speed and loose acceleration.

Possably look at 36mm CV gsxr carbs. should gat a set for $300 quite easily. I would imagine rpm and displacement would make jetting quite similar between the two.

nastytroll wrote:Why not run motorbike carbs? proberly easier to adapt, 1 throat per cylinder and better for running at angles I would assume. There would be a great choise of sizes from 24mm to 40mm in side and semi down drought available and jetting would proberly be easier to sort out.

Also if you run too big a dia. and too long an inlet runner from carb to port you will loose air speed and loose acceleration.

Possibly look at 36mm CV gsxr carbs. should gat a set for $300 quite easily. I would imagine rpm and displacement would make jetting quite similar between the two.

Yep there is at least 1 mob in NZ that modify bike fuel injection I think, to match a particular car motor.... I don't know what you could use for your Suzi tho... the big bikes would need way more carb than this thing... maybe something off a 600/750, no bigger I would think, 'cause the bikes all rev to >14,000 at least.

The only problem with bike carbs is their spacing is often too tight for many car set ups and splitting them can be a pain or getting the piping to fit with the difference in angles can be tough. biggest headache can be fuel delivery as bike carbs (most) are designed around gravity feeding of the fuel so some are better suited to pump delivery pressure than others.

How would a set of carbs off a ZZR1100 do in this type of application Semi flatslide 42mm x 4 with all rubbers..Off track a bit but just curious.....

Bugga, should have read the post above first :-(

I'll be sticking with the carbs I got seeing as I got them for free with a rebuild kit. The jetting shouldn't be too far out as they are set up for a 800cc so 2 of em equals 1600cc. I'm running a 2 litre carb off a astron motor with out any dramas with the jetting (the mechanicals of it I am tho bloody vacuum secondaries).

Troopy93 wrote:How would a set of carbs off a ZZR1100 do in this type of application Semi flatslide 42mm x 4 with all rubbers..Off track a bit but just curious.....

Bugga, should have read the post above first :-(

ZZR1100 are 40mm semi down draught CV's. Max power is at 10,500. ZZR1200 used the same carbs but max power at 9,800. 110kw/100Nm and 116.5kw/164Nm claimed from factory.

Most 1000's redline at 12 max.

GSXR750 run 38mm on certain models.

Should be no harder to make an inlet manifold to adapt bike carbs to a sierra then anything else. Webbers and other side draughts are not much different in spacing to bike carbs. Linx manifolds made a Webber manifold to suit Z900/Z1000 so I can't see it being difficult. Would proberly be easier to mount bike carbs due to the rubber clamp set up.

Bike throttle bodies are easy and cheap also but most are semi down draught.

Just out of curiosity how does a semi downdraught work?

Sits on a steeply angled manifold and as the name suggests is somewhere between downdraft and side draft. Bore of carb usually runs through it at an angle downwards. Suits the heads of modern bikes which are angled downwards as well. Can't say I'd see any real benefits in a basic car configuration unless you needed height for clearance of manifold.

I made the flange didn't even take me an hour! Much easier than I anticipated even with the old bunky drill press and holesaws. Didn't have any 30nb pipe for the manifold pipes tho so I'll have to get some pipe to continue on with the project. I may have to use curved pipes to allow enough room for the water outlets to fit in between as it's all spaced close together. So far so good!

Pics as you go would be good.....

Here's the pic of the flange I cut. I been busy with the Ls1 conversion so haven't looked at the manifold for awhile.




I might go down to the local exhaust fitter and see if I can get a donut or something in 30nb. As I don't think the original plan of the angled pipes will work out as I still need room for the water outlets.
Do you reckon having a curved manifold like this will bugger up the flow?


I know that is a very crappy paint pic but I hope you get the gist of it!