Hey guys im building up my A12 and was wondering if twin 40 dellorto are going to be too big for this build. I have .040" over flat top pistons, stock head with dual valve springs, 4.1 extractors with maybe a 2" exhaust (not sure if going to be too big aswell) and my cam specs are as follows
| Advance Ground on Cam: Lobe separation: 107 |
| .050 VALVE TIMING |
| Inlet opens: 13 BTDC Closes: 47 ABDC .050 Duration: 240 |
| Exhaust Opens: 47 BBDC Closes: 13 ATDC .050 Duration: 240 |
| ADVERTISED VALVE TIMING |
| Inlet opens: 40 BTDC Closes: 74 ABDC ADV Duration: 295 |
| Exhaust Opens: 74 BBDC Closes: 40 ATDC ADV Duration: 295 |
| Inlet lobe lift at TDC on the over lap:.084 |
| ----------------------------------------------------------------------|
| Valve lift Inlet: .4118 Inlet Rocker Ratio: 1.50 |
| Valve lift Exhaust: .4091 Exhaust Rocker Ratio: 1.50 |
| Valve Lash Inlet:.014 Valve Lash Exhaust: .014
Your help would be greatly appreciated because the carbs are the last thing left for me to order before i start rebuilding, so if you have a different carb setup you think would be better or think i should change anything in the build let me know. Im new to the datsun scene and need your help.
Thanks in advance

I'm not an expert at carbys but if you get the jets and chokes spot on it will be fine(this may take hours and may require a dyno). Twins will be harder to tune than a single dellorto, (once set up they will need a tune now and then) another option is a twin hitachi/su factory setup from a gx engine which uses a oval port head. I used a set of these for around 10 years and found tuning simple and they worked well. I'm my opinion you can't beat the sound of twin webers/dellortos

Twin Dellorto's, Webers or Mikuni's would go very well on that engine. As mentioned you'd need to spend some time on the jetting.

I recommend an electronic ignition kit as well. A stock H89 head would go nice as well and bring the power up in the higher revs but you will sacrifice some bottom end power. It comes down to how you drive and what your fuel economy expectations are.

had twin weber 38's on my a12. setup right will pull hard above 3000rpm.

thanks guys its going to be my daily driver as well, but i do eventually plan on doing head work. im not too fussed on fuel economy i mean how much more is it really going to use atm i drive about 20km everyday monday-friday and it costs me about $15 per week. I was expecting it to double would that be about right. And I will be ordering an electronic dizzy too. thanks for reminding me

With a cam like that (and I'm not suggesting it's 'the wrong cam spec' , but rather it's just a matter of getting the rest of the combo right to get it to work harmoniously) you're going to want to run enough compression to make it 'work' You'd certainly want to be over 10.5:1 (based on the assumption you'll run pump hi octane unleaded, not e85, or else you'd be able to go much high) probably as much as 11:1 given that it's the smaller bore a12 (which also helps avoid detonation vs a larger bore). Given that that is the case, I'd second the suggestion of going for a closed chamber oval port head. They'll outflow the round port head in std form, so if you didn't want to pay for porting at this time, it won't cost any more to reco an oval port head than an a12 round port head).

With such a rise in compression ratio you'll tend to 'get back' a lot of the part throttle crispness, throttle response and even fuel efficiency that is often seen to go hand in hand with modified engines. As long as you can source the various jets and chokes for the dellortos, and enough time is spent optimising the chokes, emulsions and jets/air correctors, you can actually get pretty decent economy out of the italian side draught carbs in general (don't forget they were fitted in various forms to a bunch of smaller Italian cars in the 60s and 70s, although outside of Italy most people only saw them used on race cars and factory high performance cars. A lot of that was also to do with price. But there's no reason you can't get good economy out of that style carb and an engine combo like you outlined. Obviously the other factor is a little harder to control (and I'm as guilty as the next person!) and that is the fact that this engine will outperform a factory a12 quite well, there's going to be a constant temptation to drive it to access that performance. So the fuel efficiency will also be strongly tied to whether or not it can be driven in a manner that will result in fuel economy in the first place :)

You'll be fine running a 2 inch exhaust system. they always tended to work on 998cc-1275cc minis even though on paper teh smaller 1 litre variants ought not to have needed quite that diameter exhaust, they just never lost any power, and for some reason the larger engines also similarly just 'clicked' with a 2" exhaust. The a12 isn't identical by any means, but in practice it also tends to show the same result. It's a handy size as most off the shelf extractors/headers come with a 2 inch main outlet past the final collector anyway.

thanks jmac i was planning on running 95 or 98 octane with the higher compression from the flat top pistons. But how else can i raise the compression to 10.5:1 or even 11:1? would that involve shaving the head, and decking the block? and the closed chamber oval port head your talking about is that from an a14/15? Head work is in the pipe line but just not yet. Ive already gone over budget on the first stages of this build and im getting all the machining at a discount, parts at trade and doing the work myself. I am a qualified mechanic im just new to modifications and have never really had the opportunity to do this sort of work.

If you could find a gx head, they are the ones to mod, with arguably the best flow potential, but they are difficult to find and relatively expensive. There's a couple of heads for the a14/15 that are only modestly less in full flow potential (but you won't be porting either to the max anyway, so it's not as big a deal) but you want to look specifically for the closed chamber head version. This will give you more compression from the get go, but additionally the closed chamber, which results in decent quench areas, will allow you to run higher compression ratios than you could on an open chamber head, all else being equal, so it is surely the go.

As to increasing the comp via block/head machining, ideally you'd want to dummy assemble the engine and see how far up the pistons come at TDC, and if they sit below the deck get some careful measurements done, and the block can be decked to the point that the pistons are dead flush with the block deck. Theoretically with a small bore like thia and short enough stroke, and a combo in general that won't see astronomical rpms (that cam will rev out, but not quite full race territory) with all those factors, you could probably get away with the pistons sitting as much as 5 thou above the deck. But for a streeter you don't need to push it so close to the edge where piston to head contact is only just being avoided. The higher the piston is in the block at tdc (but again, don't go above deck height) the more it closes up that quench gap, and that leads to the highest possible comp ratio being safe with a given fuel. The further you drop the piston, the bigger the quench distance, and the less effective it is at what it does (which is mostly to improve mixture motion and hence lead to quicker cleaner more consistent combustion, so that the whole lot burns early, and max power is extracted, and since the burn is over sooner, it doesn't continue to heat the exhaust valve up as much on it's way out the open ex valve and out to the exhaust, and all that = best possible power (even if only by 2-5% at best) and most resistance to detonation.

SO based on that, the first thing is to meaure and machine the block deck. Then CC the head and do the math and calculate the exact comp ratio and work on it from there as to whether or not the comp ratio can be safely (and beneficially) raised, and then and only then do it by machining teh head. Obviously don't go too quick, better to need to machining cuts on teh head, to creep up on the required CR, as it's a lot easier to machine 3-5 more thou than it is to work up a band-ad fitx if you take it too far with the first cut. There are options if that were to happen (head is machined too far) but none of which are ideal

The only issue I see potentially is the fact that if you run the std a12 head, for whatever reason, then to run the dellortos on it, you'll need an intake manifold to suit the round port heads, around 3-400, then you'd need an oval port suitable intake, for another severaql hundred bucks so your costs go up to per
severe with the round port head, so you're lookin ok, Aside from the manifold costs, with teh changes in flow rates due to the head swap, it'll mean you'd have to recalibrate the dellortos to suit, again a doubling of the effort and expense. An alternative might be to considerleaving teh dellortos off it and ideally only fitting and tuning then once that last head swap is done. just an idea.

Ok I see what your saying about having to buy 2 manifolds if I change the head later on. But today i had a somebody offer me a set of what i can only assume are twin hitachis (he wasnt sure). would I get similiar performance out of these carbs to the dellortos? considering i could probably save $800-$1000 (the dellortos i have found from www.recarb.com.au are $1299) would i see $1000 worth out of the dellortos compared to the hitachis?

Ive just had my pistons, gasket and bearing set, and exedy heavy duty clutch turn up today so im getting pretty excited haha. just waiting on extractors and all the cooling system gear and ill be set.

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If I decide to buy an A14/15 engine is everything interchangeable between the engines. I realise the pistons wont be, but the cam, headers and clutch?

Quote:

No. But more power at high RPM than the stock carb.