3G Go Kart Project , Intake

[FyreDaug]

Since Ive got the exhaust all planned out (thanks mike) its time to work out the intake side of things. I (and by I, I mean Mike) have decided to use a 34/34 weber for this a20 motor (kart project, not my 3g). But Im quite unfamiliar with the fact that there are EFI and carb intake manifolds. I know the EFI ones arent designed to flow fuel aswell as air, and fuel settles in the manifold before it even makes it to the motor.

So here is a horrible photoshop drawing of what im thinking may work.



This way, I dont need to purchase a conversion kit or anything for this weber to work.

However, this leaves me with the fact that again I dont know how to design a carb manifold. If it were just air it wouldnt be a problem. Im sure ill be getting mikes input on this aswell (thanks buddy), so I might aswell explain what im trying with this.

The runner tubes will be approx the same size as what the port size is on the head, and whatever length they should be (not really a limit to how long they should be, if longer is better, then by all means...) and a custom 3/8" plate ontop drilled to accomodate the 34/34 weber. No conversion kit neccesary because the manifold will be built to take this specific carb. Think of it kinda like a reverse 4-1 header, flowing into the head instead of out, but using the same length runners with a carb up top.

[A20A1]

I like that you have the carb up high like that.
Remember that you need to keep the carb level, and that the stock motor has a 15 degree tilt, the motor must have a 15 degree tilt to make sure oil is picked up from the oil pan properly.
I'm sure scheduled pipe has some texture to the surfaces... not sure if it's both inner and outer. You can try and make a stainless manifold or an aluminum one.
To weld with aluminum try and propane torch and alumaloy.
Anyways building a custom manifold will cost you more then buying a adapter plate.
I would not use smooth surfaced or mild steel for an intake manifold.
I think you can do alright with the stock manifold, a 1-2-4 will simplify mixture distribution but you can make a 1-4 manifold. I would say 4 pipes all located at the front of the carb.
This way you can turn the carb so either the primay or secondary barrel is closer to the runners. This will help a little.
Your plenum size will effect performance as well.
Try and stay away from favortizm... by that I mean you don't want it easier for one runner to get air and fuel then the other ones. Don't place the runners directly under the carb unless you have the carb really high above the mouth of the runners.
Since the carb is progressive you don't want runners under the primaries to have an easier time getting fuel then the ones under the secondary.
Also the carb shoulnd't face sideways like that. Either the primary or secondary should face the engine the other barrel should be behind it.
You might try 1-2-4 and try and take advantage of intake resonance, thereby simulating longer runners and boosting low to mid range torque, not sure if it would work though with a single carb, usually its for divided plenum manifolds.
But 1-2-4 gets rid of the mess of having all 4 runners meeting the plenum which if done wrong can create irregular fuel distribution.
I like the tunnel ram like design though, but I still think that having air fuel pulled through 2 seperate runners first before spliting into the intake manifold will help with mixture distribution, even if the transition is short.
BTW you can check out this thread for ideas on intake runner lenght.
https://www.3geez.com/forum/showthread.php?t=8193

http://www2.cip1.com/ProductDetails....41&Click=18774

not what I would like to see but there is something like 4-1.

It uses hose connectors to clamp the plenum to any sort of pipe of the correct diameter.




http://www.dansperformanceparts.com/...ggyintake4.htm

more manifold parts weber related.



This one I like : http://www.vintageford.com/sect_sear...281&PageNum=15

http://www.vintageford.com/Products/00394-10.jpg


more manifolds parts.

http://www.jameng.com/products/index.phtml?section=8

http://www.jameng.com/products/images/99003.803.jpg

interesting, it has an optional intake manifold heater plate.

Oh and any DGV manifold will work, but you'll need to port the smaller area to allow room for the larger barrel of the 34 to open up.



http://www.jameng.com/products/index.phtml?section=9

Volvo manifold, not sure on the port spacing.
http://www.jameng.com/products/images/99004.095.jpg

thought the port below the carb seems big enough to fit any side downdraft.

[FyreDaug]

Is the stock manifold as bad as they say it is? If I did a perfect intake manifold that didnt play favorites, AND was tuned properly, would it be worth the time and effort? Would just porting the manifold and putting a 2" spacer or something between make a bigger difference or similar? Since I dont have a hood or anything, Ive got about an extra 9-12" where I can use for an intake, so ill definately use a big custom spacer in there.

Before I go knee deep into a huge project (again) I wanna be doing it right.

Those B-series EFI IM's wont work well would they? Was I right about the fuel pooling in them before it reaches the head? Maybe with my lack of experience with IM I shouldnt be building on lawl.

Ill look through those links right now

EDIT: Since initially Im gonna be using the factory intake system, if I throw a weber on there, I may not see its full potential until the motor can breathe. Damn cheap ass 20 year old motors, oh well, atleast it wont break on me.

The factory manifold looks pretty thick, im sure I could shave off a fairly significant amount in it to help it flow a little better with a weber. But let me make sure I got this right, without using intake pulses in this, the shorter and smaller the tubes the better low end, and the longer and bigger the tubes the better top end correct?

Since I intially wanted to build the motor for high revving, I may scrap that if I can just create a little more power in the low end and use the cam for the top end power. That way I dont NEED to rev it to fly

[rjudgey]

What weber are you using?? and why only a 34/34 i would go with say a DGV 38/38 or even IDA 40/45 or what might be better a single DCOE 40/45 and have it side mounted. The IDA may help a little with tumble and also with the fact that it's using gravity but you'll have a serious bend to get it at the right angle not to mention needin one seriously high hood scoop. With a DCOE or IDA you could group a pair of ports up and feed each pair of one choke which would be much easier than what you have in mind. Also with carbs longer is better but there is a limit to how long, longer normally produces better torque curve and shorter less torque more bhp but it's a trial and error thing really no real solid answer i've experimented with short ones and they really work well and i've had long manifolds too and they equally work well, also having trumpets with varying lengths makes it easier as it's cheaper to change those than make up a new length inlet manifold each time. Also the tubing should not be the smae width all the way along you need to maximize flow by creating vacuum and this needs to be done by creating a trumpet like shape with the tubing at both ends, so you have a nice trumpet shape at the carb end that meets the choke, and then it narrows down reasonlably gently to a round pipe say around 35-40mm and as it nears the inlet port expanding back out to the inlet port shape, the inlet port entrance should then be bell mouthed and made so that it sits around the outside diameter of the inlet manifold port that mates to it like a trumpet again, this will reduce spit back and help with getting the most from the carb and mixing the air and fuel. What kind of mods can you do to the engine i would have thought that a 34/34 would be very restricitve even for a stock a20 which has a 36/32 as standard?? with some minor headwork and a mild cam with a DCOE or IDA attached i would have thought it would put down some good figures 150-160bhp should be achievable with a good exhaust system bolted on.

[A20A1]

yes that is correct, longer larger tubes top end.

Though I would say just larger tubes are for top end. and leave the longer out of it because I'm sure there are some instances where it is not always the case.

Because Longer tubes are good for low rpm.

Larger plenum good flow high rpm

smaller plenum low rpm

It's not going into a car, this is on a cart. So the sky is the limit.

His target was around 140, I think it's reasonable you can get that on a 34/34.
Then there was nitrous about a 50 shot.

I gave him 2 choices 34/34 and a 38/38.
I leaned more towards the 34 because it was progressive, nothing is set in stone though.

I agree the 38 would be nicer at WOT. Getting there is my concern. With the 32/36 it still seemed a bit off, with a 34 that has a larger primary barrel it should be better, I was reading something about idle and off idle mixture settings being a problem with primaries being that much smaller then the secondaries. The solution was to have larger primaries.

His plans for a cam was the stage 2 tri-flow.

"it would be nicer at WOT" how so? Just more power? Better sound?

Well anytime you add a larger carb you'll get more power untill you don't have enough vaccum in the carb for the carb to operate. Usually around 0.5 Inches/Hg.
You could increase the jet size to flow fuel more readily under less vaccum but then the car would probably run poorly untill WOT.
The 38 isn't going to have trouble making vaccum though. I keep getting mixed facts... one says the diameter of the throttle bore is 38, the other says it's the diameter of the venturi. Its easily checked though, just measure the throttle bores.
I's go with barrel diameter.
Does the DCNF have removeable chokes? I was thinking of that one inplace of the 38.
Nevermind it does.
So I'm not sure on part avalability like choke sizes and jets, also I've never tried to tune with that carb so you're almost on you're own if you choose a DCNF.
http://www.euronet.nl/~wilaben/bagtip1.html
well you're still going to need to jet it to get the full potential, when a carb is jetted right it sounds louder and the throttle is very crisp and responsive.
The 38 is the largest carb that will fit with the adapter plate, the DCOE will fit but that would require an additional adapter. All other carbs need a custom manifold.
weber makes it easy because their jets are stamped on what they actually flow and not the size of the drill hole like domestic jet sizes. I also found a good write up and reposted it in the carb tech forum, very easy to follow and know wether to size up and jet or down.
I'm sad that I didn't find it before I gave my carb away, I ran around for years with a weber that was never jetted right because I didn't bother to buy all new jets. I bought a few and made due. I knew where my holes in the throttle response were but I was happy with WOT and thats all that mattered at the time.
Do you have a O2 Sensor?
http://www.racetep.com/halmeter.html
It'll help with tuning.
Man oh man are they throwing curve balls...
With the highest CFM ratings I found for each carb, you're looking at:
38
115 hp - 181 hp
34
115 hp - 136 hp
32/36
115 hp - 150 hp
I've seen sites rate the 32/36 at 125 hp

Heres just browsing the net.
32/36 good fuel economy, responsive when secondary kicks in.
34/34 good fuel economy, more responsive on primary and still responsive when secondary opens
38/38 poor fuel economy, more responsive. Not progressive.
Others will say more low end torque for the 38 which makes some sence, and then says its less responsive.
Then goes on to say neither carb will perform in the top end... which brings me to the statement below.

It would be seriously troubling but not that odd if all the carbs flowed the same CFM, since we know the throttle bores are changing, you can speculate if the choke is the same so whatever the choke is limits CFM to a set value, but I'd like to say that the bore will affect it too aside from a smaller bore having a higher velocity. It's been so long I'm doubting if the chokes on the 32/36 were even of different diameters.

[FyreDaug]

Yeah Mike's got it on the ball, would a 38/38 hurt my bottom end? The cam doesnt start to 3500, so thats already shifting what little power this motor makes below that, up higher. I need to keep a drivable lower rpm but I dont wanna step down to a stage 1 cam.

Nitrous was about 50-75, not sure, and it wont be on right off the bat anyways. Im expecting 210 on the bottle from this motor. 140-150ish was all motor (hopefully... with some bottom end work done later).

"it would be nicer at WOT" how so? Just more power? Better sound?

This is the progress thread on the kart, so view this for more info on it https://www.3geez.com/forum/showthread.php?t=53448

Maybe this cam will hurt more than it will help. I definately want pull to the redline (and above apparantly... bottom end work will fix that later) but I still need some street drivability at lower rpms. I imagine with a good exhaust and good intake, the lower end rpms should be about equal to stock, which is good enough, and thats what im gonna shoot for. This thing weighs less than 900lbs (maybe even less than that) so the power I make in this would be equal to nearly double in the 3g.

[rjudgey]

Hmmm maybe go for an IDA you can alter the choke sizes then 34mm-40mm and then play around with them, from my experiences with the DCOE's on ET's i'd say that 34 will strangle the engine and better off going with the 38 loss of torque will be very minimal at the low end of things. But at least with IDA/DCOE you can have bigger chokes when your track is more open and have smaller ones when it's tight and twisty.

What is a Kart we takling go kart type thing or something a bit bigger with space frame and fibreglass shell? Of those weird winged dirt racers that go round circles??

[FyreDaug]

Basically like an atom

Theres too much to carbs that I just dont understand with jetting and such, I just want an upgraded carb, capable of not bottlenecking me at 140-150hp thats "simply" bolt on without having to re-jet things, I still think the 34/34 is good enough

[A20A1]

I'm still looking for choke diameters, I think I remember it stamped/casted on the side of the barrels.
It can't be true that they all flow 300 CFM... it's like madness.
Stupid site needs to die for showing me that.
I'm pretty sure I remember the chokes in 27 and 28 or 26 and 28.
Found a picture of my carb.
EDIT: 26mm Primary 27mm secondary chokes
now I just need to find the 38 choke sizes to prove that silly site wrong.
EDIT: 27mm chokes
Maybe the site is right... the carbs just are suited to driving needs, not really offereinf that much more in terms of top end performance
And I was hoping that the 38 would have less restrictive chokes or something.
Sound like they are more or less the same... damnit.
Well a 38 just changes where the torque occurs and actuall starts lower in the RPM
I used to think that it would suffer because it was synchronous... but apparently it offers better low speed torque.
That fits the previous statements that a larger primary helps with low speed performance. And you don't get much larger then 2 38 mm's openeing synchronously.
The big hitter is really fuel economy and possibly throttle response.
Stick with a moderately size plenum and you should be good to go with a 38.
smaller plenum more low end, larger plenum more top end.

some more nice tuning info.
http://www.1stgencivic.com/phpgdv2/hpower.php?c=6
If you plan to do a lot more to the engine though which I see you might then go with the 38.
By that I mean really working on getting the head to flow.
Oh I was just reading this site that said with certain cams you want to have your exhaust flow less then your intake to stop from overscavenging.
probably another way of saying they want to have backpressure.
But the suggestion was to open up the intake to increase the gap between what the exhaust and intake flows, not cut back on what the exhaust flows.
So lesson here is open up your intake.
Anyways 34 or 38.
If it's a downdraft carburetor you want to limit the angles, bends, or turns fuel will have to make. If you had the carb up high like in your image but not that high, High enough so that once the air/fuel came out of the carb is was nearly a straight shot into the intake port. Basicly you should be able to see the intake port from where the carb mounts to when looking through the runners. Well plenum to intake port, has the line of sight.
Minimal bends if any, straight shot for the carb plenum to ports for a 4-1 intake manifold

I'm glad this info on the 38 coming out now though. I mean some things remain the same, I can't speak for rjudgey but I was under the illusion/impression that the 38 was really a bigger carb... over the other 32/36 and 34. But it's not.
I retract my first 34/34 statement and say go with the 38.
The 34 still an option.
and of course 40/45 sidedrafts or a 40 DCNF
32/36 never an option.
and lucky you, a whole mess of carb goodies all for $500
- jets
- carbs
- manifold
http://www.fiatparts.com/terrys.htm
Thats hot sh!t in my book, I mean jets can be expensive.
Manifolds also... I would cut the thing up and fit it.
that site is where I found out the choke sizes for the 38 btw...

[A18A]

I didn't read through it properly because there is too much, but wouldn't that hit the hood?

[A20A1]

Nope, its not in a car.

[FyreDaug]

Mike can you give me a custom title reading "3Geez Go Kart Project" or something along those lines? lawl.
Lets talk factory manifold for a second, is it a big limiter? Should I just build one close to its spec, but like you said with a straight shot to the port.
I dont know all the tuning stuff im sorry if I feel ignorant, but I understood your posts mike, dont worry, im not STOPPID.
So if we went with the 38, with the larger bores you were saying something about fuel economy and throttle response... Im guessing the fuel economy would suffer with a bigger carb, but how much throttle response are we talking about losing? I imagine driving the car would be a little harder with a big carb, you cant just floor it and expect it to go, its a matter of DRIVING it right? That I dont mind, same with fuel economy, im gaining a bit by not carrying a ton and a half, so I kinda dropped a ton of weight (ahahah..... ...) so im expecting to drop some more. 20mpg would be nice, but I dont care, Ive got a some-what stock 3g to handle that :p

Give me an estimate (if you can) what size of tubing would I need for each runner, something as small as 7/8" but with runners about 10" long? Or am I thinking wrong here? What about 1" with 6" runners?
If I were to build one, it would be a 4-2-1 design, because I think that would work better. Firing is 1,3,2,4 iirc, so have 1and 2 be 1 set and 3 and 4 be the other, that way each side would be pulsing 1 at a time and it would be equal. And as a single carb with 2 barrels, I would put it sideways, soe that the 2 barrels are opposite the openings on the intake like
...O <-- carb barrel 1
..OO <-- secondaries of intake manifold
...O <-- carb barrel 2
Obviously they would be overlapped, but thats the best way I could draw it.
And hey maybe I can use the infamous "tornado" into each secondary to create a turbulence to mix the air/fuel even more.
So the 2 terms are synchronous and progressive, and to make sure I am understanding things here, the syncrhonous one opens both barrely simulotanouesly(holy fuck that was impossible to spell... damn word) and progressive would be more like our keihns where at a certain rpm, or pedal depression it would open the other one.
So if I do use a sync carb and do it the way I said aslong as both barrels are operating equally there should be no favorites right? This seems like the best idea to me, just like the 7/8" runners 10" long... to give me a better understanding could someone explain the pros/cons of this setup vs the 1" 6" long... doesnt have to be indepth, im smart ill pick up on it
EDIT: Should the carb be completely level while just sitting, idling too? Does tilting do anything?

[A20A1]

When the carb is level the carb will operate best.
It should always be horizontal unless noted, as in the case with a DCOE which uses a 5* tilt.

You don't want turbulence, turbulent flow disrupts fuel distribution.

When fuel is pulled from the carb it comes out in a stream, as demand in one runner increases over the other the stream is pulled to the other runner. Having runners scattered around the plenum pulls mixture all over the place disrupting flow.

With runners restricted to one side of the plenum the stream doesn't have to make any drastic changes.

Spacers below the carb can help if you have such a manifold with runners at every angle... otherwise try to limit the use of spacers because the increase the plenum and may make more problems then they solve.

[rjudgey]

check which way the butterfly's open as to how you mount it and how the pipes connect. I still think a single DCOE would be better on a side mount.

[FyreDaug]

Maybe someone who thinks they have an idea and wants to see it work should build one to a certain spec and ill build it. I can build pretty much anything, but apparantly not understand carbs too well. If we do go with the bigger 38/38 it would capable of having more power, and from what im understanding now, its sync and increases bottem end power so im not seeing a reason to use a 34/34 then.

If I built a manifold kinda like the design I made excellently in photoshop above, compensated for the 15* engine slope and made it level. Here ill actually draw another one, a little better. Hang on

[FyreDaug]

well I never said I was good with perspective and photoshop



that should read [increasing it to 1 1/4"] not 1 1 1/4"

[FyreDaug]

Better or worse than a stock manifold?

[rjudgey]

anything will be better than stock too many tight bends adn it has to immediately negotiate a 90 degree downward turn!!
What you need to do is make sure that each pair of runners gets to suck on directly a choke. And to make sure that each butterfly on the choke is opening onto a pair of tubes in the right direction so that it doesn't impede flow when at anything less than full throttle.

Mike how do the butterflys open on a 38/38??? Is it like the 32/36 or are they different?

[A20A1]

They are connected by a gear, so they are on seperate throttle shafts and open like a 32/36... unlike the DCNF which I think uses a single throttle shaft to open both plates at once.

[FyreDaug]

I think ill probably just experiment, would square tubes be a bad idea? Its different... Ive got 2 complete motors, so I can build this on my spare time. Seems like something I can do, play with. Plus anything is a bit of an upgrade I guess

[rjudgey]

okay in that case would be best to mount it long ways like the 32/36 will get the best airflow then, if it was like DCNF then sideways would be better.

[mkymonkey]

man all of this isnt english to me....nor spanish! wtf are you guys talking about???!?!?!?!?!?111/!?1/1/!?!?!?!!?!?!

lol.

[A20A1]

I think ill probably just experiment, would square tubes be a bad idea? Its different... Ive got 2 complete motors, so I can build this on my spare time. Seems like something I can do, play with. Plus anything is a bit of an upgrade I guess

I dunno about square tubes, but Square runners are used in some custom manifolds I've seen. Its easier to fabricate tapers in the runner. It usually consists of flat bars though. You have one solid base plate that spans all 4 runners and you build the dividing walls on the base. Then add the individual roofs to each runner to close them off. cut between the runners if you desire and you got your manifold almost done.

If the plate it large/long enough, after the runners you can tilt the plate -15* down so that you form the floor of the plenum.
Or
you can choose to build it upside down... and bend the plate 15* up, flip the manifold over and you have the mounting surface for your carb. Build each wall for the plenum and then a floor.

in any case the runners extend from the head straight out so you don't have to build in any fancy bends into the runners for the carb to sit straight, you do that at the carb.

also if the carb is close enough to the runners and high (not too high) enough the fuel will have less of a bend to make.
I'll try and draw a picture to represent this:

Gimme a few hours.

[FyreDaug]

Sure thing

[rjudgey]

i'd stick to round with flared ends at each end will help increase the speed of mixture through pipes and create venturi effect