Holden 304 Banana Manifold

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Holden 304 Banana Manifold

Postby brindo » Sat Jan 15, 2022 4:19 pm

I think we have all read or know about the Holden 304 banana manifold and its problems. For all its faults, the factory manifold does look good, and they’re cheap. Or they used to be. But they’re clearly not ideal if you are chasing horsepower, and the dual/single plane manifolds with a 4 barrel throttle body seem to be the logical choice rather than investing hundreds of money into the stock manifold only to end up still not as good. On the flip side though, I was involved with installing a Harrop dual plane on an EFI 304 in a VK Commodore in the early to mid 2000s, and remember it giving us a lot of heart burn. There was no provision for the IAT, and the radiator hose fouled on the fuel rail so I ended up machining a spacer to lift up the thermostat housing for clearance. I completely remade the wiring harness but the IAT, IAC and TPS are in completely different positions that a stock loom won’t reach. Maybe they have come a long way since then, but there would still be a lot of expense involved and it’s not exactly plug and play. And they still suck hot air in from on top of the engine, unless you want the air cleaner above the bonnet. Or go the large single throttle on an elbow out the front to suck in hot air from somewhere else.

From the threads I have read, extrude honing seems to be a fix for one of the many banana flaws. But extrude honing is not cheap – and that’s if anyone can still do it in Australia. The weld-on necks for the 90mm throttles are also touted as a partial cure for the banana asthma. But the weld-on neck mod is not cheap either. Neither are 90mm throttles. Put the extrude hone and the larger neck together and you are looking at well over $2k for what is still a performance compromise, but you get to keep the factory look and save some heartburn particularly for those running MAF on a VT or VS Series 3.

To be honest, you can jack hammer what I know about manifold design on a Panadol. I am not an expert in anything, but I had an old banana manifold from a poorly maintained engine that had bad corrosion around the water jackets (from what I suspect was accelerated by the use of acetic cure silicone around the water jackets). I thought I would sacrifice this manifold and have a crack at acid porting manifold runners in an attempt replicate the extrude hone. So I invested $17 on 5 litres of hydrochloric acid and have cut this manifold up to try and learn something.

This is what it looks like inside.
Banana Manifold.jpg


I measured the fluid volume of each runner and Cyl #1 and #2 hold 500ml, and all the rest hold just a fraction more. The diameter is around 41.5mm, and they are oval by around 0.2mm. I did read that the runners were only 30mm in diameter, which I now know they are not.

I set the manifold up upside down, level on the work bench and my intention was to just add 500ml of a water/acid mix to a runner to see what happens. That way it would just fill the runner only and not the plenum and hopefully only port the runner and nothing else. The last few inches of the runner where it turns into a rectangle to enter the head isn’t touched by the acid mix. First runner I tried to acid port I mixed 250ml of acid with 250ml of water - it ate a hole through the runner after about 5 minutes.

Next runner I tried I diluted it down to 50ml of acid to 450ml of water. It certainly reacts with the aluminum, and I left it there for a few minutes before draining and rinsing it out with water. But it didn’t show an increase in runner diameter or volume that I could measure. So I did the same again and left it overnight before draining and rinsing it out again. Still in no increase in volume or runner ID. But after the 4th acid treatment with water flush, I measured an increase of about 0.2mm in the runner ID. After 6 treatments, the runner was a bit over 0.70mm wider in ID. If I keep going should be able to get somewhere over 1 mm bigger which is probably all that extrude honing would do. Only not as smooth.

I also tried another runner with 100ml of acid mixed with 400ml of water, but it bubbles too much that it spills out into the plenum where I didn’t want it. So looks like a ratio of 10% acid or less works best. Using hot water from the hose to mix with the acid also seemed to make a difference. But it would appear that patience is the essence here and multiple small doses of acid are the go rather trying to take more off each time with a stronger mix.

So I am not suggesting that this acid porting will yield a guaranteed Brazilian killerwasps on a stock 304. It won’t help solve the problems of the runner length or plenum size, or reducing heat soak or any of the other flaws. Maybe a small increase in runner ID will make a positive difference on a mild stroker engine. Or maybe the hydrochloric is best left for the swimming pool. But if increasing runner ID is worthwhile, then acid porting may well be a DIY option that doesn’t require the manifold to be cut and shut, and no TIG welding. Just add 500ml to each runner, rinse and repeat. One of those giant plastic storage containers from the Reject Shop filled with water might also be the ticket to let the manifold soak in to neutralise the acid after you’re done. Probably a teaspoon of bicarb in each runner wouldn’t hurt either??? Perhaps protect anything you don’t want affected by acid (like the steel vacuum ports or machined faces) with grease or Vaseline? The manifold would still need port matching to the head but you can access this easy enough with a decent burr and blend it in to runner to where the acid has started to work.

But this is where I should probably stop and say if anyone is going to try this at home, invest some time on googling safety precautions on working with this acid. I have done all this “porting” at my workplace with no dawg or kids about, with the luxury of a powder coated work bench in the open air, along with an eye wash station, hose reel and ready access to as much PPE as needed.

I have only got 1 runner successfully ported so far so will keep going with this when I get a chance, just to see what can be done for my $17 investment. As above, the surface finish of the runners could be better, and is a bit rougher than what you’d expect, but arguably smoother than the factory finish. Was thinking that a flex drive extension with one of those star flap discs drowned in CRC could be used through the runner at the head end to polish the surface the entire length of the runner. It might even clean up the entry into the runner and make it more like a bellmouth. But I don’t have a flex drive extension. Or any star flap discs. But they could be options if you can get them through the port. But if you have this gear, it may be possible to have the same performance compromise as extrude honing for a fraction of the cost. Maybe a bit of porting to the neck and a 70mm throttle body will be as good as it gets before trying to chase down someone to weld on a larger neck – if you can find one.

Anyway, I have never seen the inside of one of these manifolds, or ported anything with acid, so nothing lost at this stage.

Posts: 147
Joined: Mon Jun 29, 2009 9:43 am
Location: Western Queensland

Re: Holden 304 Banana Manifold

Postby brindo » Sat Jan 15, 2022 5:36 pm

This is an action shot of the acid port in progress. Cyl #2 has the 50ml acid 450ml water mix. Cyl #3 has the 100ml acid 400ml water mix which has bubbled over and spilled into the plenum Once it settles down, the level is lower than it should be. Cyl #5 was the runner that had I tried first.
Acid Port.jpg

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Re: Holden 304 Banana Manifold

Postby vlad01 » Sat Jan 15, 2022 6:23 pm

From my dyno testing I only had 2 manifolds to compare on a mild/moderate 355. Stock banana and twin throttle COME racing manifold, stock was much better. Very bottom and peak remained the same between the 2 but the COME manifold lost 75Nm in the mid range.

Not sure what the dual or single plane manifolds are like but I heard the single plane ones do well on more serious builds up top.

I honestly think the banana manifold is the best one for mild to moderate builds and not given enough credit where it's due, the engineers knew what they were doing.
I'm the director of VSH (Vlad's Spec Holden), because HSV were doing it ass about.

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Re: Holden 304 Banana Manifold

Postby immortality » Sun Jan 16, 2022 6:36 am

The stock manifold is good for a stock motor or a very mild cam. Not only do you have to deal with very smallish runner diameter, also runner length as well as the hot oven type plenum under the manifold.

I know one guy that had a smallish cam on a stockish motor with banana's, then stepped up to a modern air gap dual plane and noticed improvements all around. Then stepped up to a bigger mild cam.

I like the OG banana's look under the bonnet but not the handicap it brings when you start to cam it up. It's a shame no one developed a modern type banana manifold like the LS motors where the plenum is between the runners.

Posts: 147
Joined: Mon Jun 29, 2009 9:43 am
Location: Western Queensland

Re: Holden 304 Banana Manifold

Postby brindo » Mon Jan 17, 2022 4:22 pm

I seem to remember reading somewhere else that the runner diameter becomes much smaller at the top where it is curved. So today I just tested this with a ping pong ball, which is 40mm in diameter. In an untouched runner, the ball can easily run all the way around to where it changes shape to become a rectangle near the injector boss, so that clears that up.

Have done another 3 acid doses today, and leaving them to react for around 2 hours each time, then rinsing out. The runner is approx. 1.1mm wider in ID. So this is now 9 acid doses all up for this one runner, mixed at 50ml acid to 450ml water. But I would have to say now that the port wall finish is rougher than original, and would no doubt benefit from being smoother.

I might still keep going with it, then repeat it on another runner to know exactly how many treatments give what increase in ID.

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