304 My Bananas so far..

[simmyz1]

i've been racking my brain about a decent manifold for my 355, the old banana just doesn't cut it for me any more, i know there is so much more power to be gained from a new or modified manifold. i haven't got the $2500-$3000 for a new set up so i'll be modifying an old one i got for $65 at the wreckers....now the fun begins, i've been reading a lot of old threads about this subject and i'm not the only one who has/is doing mods on old stockys so hopefully you, that have/are, can give me some input as to what i'm doing.
A few questions to the brains out there....
-the length of the runners seems to be an important factor, longer for low down power and shorter for top end?..am i on the right track?
-the plenum chamber should be around 80% of engine size...true or false?
-would a side mounted twin throttle set up be better for flow than a front mounted one?
-would some sort of air vane help airflow in the plenum or leave it bare?
I'll have plenty more questions to come so hopefully this thread will answer some of them in the next few weeks...
I've done a fair bit of grinding and cutting so far and (if the pics work) you can see i've cut the bell mouth area of the manifold completely out and all i'm left with is the 8 runners, i'll be welding the plenum on the top of the runners in some sort of shape when i work out the final runner length.
I cut the excess ally from in between the runners, i thought that this would aid in cooling the tubes (some drop in temp would be better than none).
I was thinking about flow differences between the runners (i've read that sometimes there is quite a bit of difference in the castings) and i think all i can do is to get the top of the runners machined off with a mill, polish the internals and maybe get a flow test done before the plenum goes on to ensure all cylinders are getting the same amount of air,is there some sort of tolerance anybody might know about?

hope to hear your comments!!:idea3:

 

[greenfoam]

Runner length is the main thing along with runner cross sectional area.
The flow through the factory banana runner is about 190 cfm, which is pretty bad but all 8 runners are very close to each other.

Runner length is an easy one, it can just be 10 inches and it will work, that's the magic number you want so that's easy. The cross sectional area is easy too, the banana is allready a very good size for a mild 304, you can go a little bit bigger for a 355 but honestly the risk of buggering it isn't worth the potential gain so try it first with standard size runners on the inside.

Plenum can be 80-100% (4.5-5 liters will be fine), if you can keep the entry into the runners low in the plenum then side mounted throttles will be fine, from the front should work too but either way try and give the bit that transitions from throttles to plenum a nice shape so the air car turn well into any runner it's heading towards. All that and you should have a very nice manifold for a 355.

Also try and get a bit of a taper into the entry on the runners for the first inch or 2 and some kind of bellmouth shape.

The thing is to find the problems with the banana and then fix them the major problems of the banana are

1. Runner length < it's good for a stock motor but bad if you need to rev over 5000
2. The short distance between the runner entry and the plenum floor
3. The extreme heat inside the manifold
4. Possibly the angle the runner has going into the intake port
5. The angle the neck is on causing a lean condition on cylinder 2 and maybe 4 and 1

You'll be able to fix 4 out of 5 so that will be a good gain

 

[VrWagz1]

So far the runners are def too short, though im no expert on this but i have done some reserch on this before and i think i come up with i only wanted the runners a couple inches shorter then the stock legnth. They are a perfect legnth if you want to dump on a turbo or somthing(see ebay for people who have done what you have just done and weld a plenum ontop and flog them offf for around $600). I think you will have trouble getting the runners the right legnth and still have it fit under the bonnet, if you are going straight up. They will still need to curl over.

 

[delcowizzid]

depends what its use is. if its a street car runners that hold a cylinder volume each and a plenum the size of the whole engine is the perfect mid way type manifold

 

[simmyz1]

cool, this is all good info, thanks boys..
greenfoam...
your a champ and i have read some old posts of yours on this subject from 2007, so it seems you have the know how. i'll try to keep temps down by way of some heat coating, powder coating..is it possible to dip the whole manifold in something to coat the inside of the runners as well as the outside?
i've got the bellmouth design worked out (in my head) and from what you've just told me i wont worry about the flow test for now, if this thing works i'll do a proper job on the next one.
the 10" runner your talking about, is the measurement taken from the valve seat to top of runner or just the runner?
vrwagz...
i've seen the stack injection manifold and the runners are bugger all?, thats not the sort of thing i'm aiming for but i'm thinking that my length will still work...won't it?
delco...
from what you are saying i need 5800 /8 (=725.00cc) in each runner..which is 725 ml..correct? i'm going to measure the volume now....i won't sleep otherwise

 

[delcowizzid]

measure an inlet port too if you have one or if not foam will have the cc of a stocker im sure .

 

[simmyz1]

i'm getting a calculation of 142cc volume for each runner when i should have 725cc?...somethings not adding up...am i doing this right?...even with a 10' runner it still doesn't add up...i should have listened at tech instead of choofin in the torana!

 

[simmyz1]

the runners now are about 140mm long and diameter of 35mm approx...

 

[greenfoam]

Don't worry about runner volume if you were to make the runners that large the velocity would be worth nothing and the car would be a giant slug. I could draw you a picture of how it works but I don't want to today. The problem with the long runners on the banana is that at 3500 they work like magic, the wave of pressure travling in the manifold hits the intake valve just as it opens and you get heaps of free psi. Because it's a long runner it has a very strong tunning effect BUT it can't allways be travling at just the right speed to get in the intake at the right time, at 5000 rpm it starts to be heading in the wrong direction away from the valve and that's where you have your problem of the 5 litre falling on its face with the banana manifold.

Now say you have a....... Trickshift chopped banana manifold these have about a 7 inch runner and are actually stronger than the banana at low rpm 2500? because that 3500 pulse has moved to a lower rpm, then weaker in the mid range then stronger again 5000 up because it comes into the correct pressure tuning in two spots instead of one (because the runners are shorter) but however that wont be as strong as the single go you get with the longer runner (each time it bounces up and down the wave looses pressure), also with the banana the stupid short distance between the plenum floor and runner must stuff this up alot.

With a ten inch runner you get one peak at 3500 which is the natural torque of the motor (win) and one peak at 5000+ ish which is where you want peak power (win again). The group A manifold has 10 inch runners on a 44mm? diameter, little too wide for a 304 or even mild 355 but great on a race 304 at 7500 rpm, it would go at 3500 5000 and 7500 or something like that. The stock banana is about 38-39?mm if you messure it on a bit that's cut straight I'm sure its more than 35 and that's just nice.

The typical X shaped manifolds work like this too, multiple lots of tunning events and high flow on cars with very large cams this kind of manifold works very well. I suspect manifolds like the Harrop involve more than one runner because if you look at it there's allways two runners facing each other I think thats is why that manifold works so well.

Dale on SC built his with 10 inch runners but large diamter runners and it still performs well, but his does have the valves visible from the runner entry (which is perfect) but it's a crazy tall manifold

http://phys.unsw.edu.au/jw/helmholtz1.swf

See this ^ that's how the manifolds work positive pressure - negative pressure over and over again, long runners are win but they cost you twice as bad on the flip side. Of course you could just run a 16 inch runner and have one strong good pulse at 4500 rpm+, that would be great but fitting a 16 inch runner anywhere is a problem and you don't also get the low end kick like the 10 inch runner

 

[simmyz1]

Don't worry about runner volume if you were to make the runners that large the velocity would be worth nothing and the car would be a giant slug. I could draw you a picture of how it works but I don't want to today. The problem with the long runners on the banana is that at 3500 they work like magic, the wave of pressure travling in the manifold hits the intake valve just as it opens and you get heaps of free psi. Because it's a long runner it has a very strong tunning effect BUT it can't allways be travling at just the right speed to get in the intake at the right time, at 5000 rpm it starts to be heading in the wrong direction away from the valve and that's where you have your problem of the 5 litre falling on its face with the banana manifold.

Now say you have a....... Trickshift chopped banana manifold these have about a 7 inch runner and are actually stronger than the banana at low rpm 2500? because that 3500 pulse has moved to a lower rpm, then weaker in the mid range then stronger again 5000 up because it comes into the correct pressure tuning in two spots instead of one (because the runners are shorter) but however that wont be as strong as the single go you get with the longer runner (each time it bounces up and down the wave looses pressure), also with the banana the stupid short distance between the plenum floor and runner must stuff this up alot.

With a ten inch runner you get one peak at 3500 which is the natural torque of the motor (win) and one peak at 5000+ ish which is where you want peak power (win again). The group A manifold has 10 inch runners on a 44mm? diameter, little too wide for a 304 or even mild 355 but great on a race 304 at 7500 rpm, it would go at 3500 5000 and 7500 or something like that. The stock banana is about 38-39?mm if you messure it on a bit that's cut straight I'm sure its more than 35 and that's just nice.

The typical X shaped manifolds work like this too, multiple lots of tunning events and high flow on cars with very large cams this kind of manifold works very well. I suspect manifolds like the Harrop involve more than one runner because if you look at it there's allways two runners facing each other I think thats is why that manifold works so well.

Dale on SC built his with 10 inch runners but large diamter runners and it still performs well, but his does have the valves visible from the runner entry (which is perfect) but it's a crazy tall manifold

http://phys.unsw.edu.au/jw/helmholtz1.swf

See this ^ that's how the manifolds work positive pressure - negative pressure over and over again, long runners are win but they cost you twice as bad on the flip side. Of course you could just run a 16 inch runner and have one strong good pulse at 4500 rpm+, that would be great but fitting a 16 inch runner anywhere is a problem and you don't also get the low end kick like the 10 inch runner

ok....so what about the stack injection design, is it targeted at strip use more than everyday driving?..and these manifolds on ebay (the plenum welded on cut bananas), are they any good?..i guess what would suit me, as it is a work ute, is something like the trickshift specs of strong low, mid mid and strong top. going on what you've seen in the pics would a different cam be the go when i fit this thing?, as the motor will be close to that stage anyway....maybe i'll just save for the COME twin..