Dual Hitachi 26/30 vs Weber 32/36 on an EA71

[Dirk]

From a performance perspective which do you suppose would be better and why?

[Dirk]

Bueller? Anyone?

[cubastreet]

If you work out the area of the chokes, the duallies will have more flow potential, but both will be restricted by the stock manifold and heads so it's going to depend on how much work you've done to the rest of the engine.

 

single weber will be easier to set up and probably a little more economical but the twincarbs will look cooler.

[Dirk]

Hey.

 

Heaps of porting and polishing to heads and manifolds.

Lightend flywheel.

Fully ballanced

Cam grind

 

Pretty much the works.

 

I have a dual carb manifold and also a single carb manifold that takes the larger EA81 hitachi.

 

Flow should not be a problem for either setup.

 

Like every greedy person I want good top and low end power.

 

So far as I have read, it would seem that the single weber would give me better low end and the duals would give better top end.

 

I would like to test this theory with some of you guys who have one or the other.

 

I would try both myself but I only really have the cash and time to complete one of these setups.

 

Cheers

 

Dirk.

[cubastreet]

I've just got a standard ea81 with 32/36 weber atm.

 

Real nice low and midrange, no top end but that's most probably due to the manifold and heads.

 

I think the weber's good for 120hp or so, but the manifold is narrow and has 2 sharp 90 degree bends which is not good.

[DR DLEX]

the 26/30 for the hitachi refers to the throttle plate size,thus two 26/30's are accually 52/60 total carb bore. compare this to the webber 32/36 and then do the math. hope this helps

[Dirk]

the 26/30 for the hitachi refers to the throttle plate size,thus two 26/30's are accually 52/60 total carb bore. compare this to the webber 32/36 and then do the math. hope this helps

 

 

Not really. spell it out for me.

[Dirk]

I've just got a standard ea81 with 32/36 weber atm.

 

Real nice low and midrange, no top end but that's most probably due to the manifold and heads.

 

I think the weber's good for 120hp or so, but the manifold is narrow and has 2 sharp 90 degree bends which is not good.

 

 

Cool. Thanks.

 

The other apsect is that the long manifold promotes good torque. The dual carbs technically shorten the manifold lengths so I could be sacrificing the low end torque for breathier top end power.

 

I wonder if the boxer design will get the most out of the dual carbs or if the weber is better suited.

 

I am crrently running the EA81 28/32 carb of a stock Ea71. It feels well matched at the moment. Certainly no benefit to a larger carb without the further mods.

[Dirk]

Not really. spell it out for me.

 

 

Sorry man. Was having a little moment when I wrote that.

 

I get wnat you are saying but unless I get my heads flow tested I won't know if I can make the most of the duals. Hence the dilema.

[cubastreet]

two 26/30's are accually 52/60 total carb bore

 

Not true, as increases are exponential

 

area = pi x r squared

 

so for the hitachis (26/30)

3.14 x 13 x 13 = 530 square millimetres for the primary choke

3.14 x 15 x 15 = 706 for the secondary

 

= 1236 per carb / 2472 for both.

 

a 52/60 carb would be

3.14 x 26 x 26 = 2123

3.14 x 30 x 30 = 2826

 

which would make total area 4949 square millimetres

 

32/36 is 804 + 1017 = 1821

[DR DLEX]

o-kay let me slow this down abit.

 

1. On a non turbo intake fuel/air system, the cylinder volume creates the vacuum in the intake thus sucking the fuel/air mix at the carb(s).The smaller the carb., the less availible mix=longer time to fill the cylinder=less acceration(i.e.resrictor plates in NASCAR).The larger the carb.( or # of carbs.) more volume of fuel/air=quicker acceleration(i.e.drag racing,six-pack hemi's,etc..)

2.Torqe is made by the the ratio -fuel/air to comrpession.The more fuel/air mix available, and the higher the mix is compressed,the bigger the bang thus more torque.

 

Now with that being said;the dual hitachi would be the choice,,,,BUT

 

,,,,hitachi's are hiachi's and chasing problems one is often a headache and two=weeks of nightmares.Having run duals on stock EA-71s for over 100,000

miles i've been there.

 

I would suggest that you try each set-up and see which one fits for you

[Ekvh21]

I know this is an old thread, but--

 

Actually the math is a little flawed comparing carbs because it's venturi size that matters. With a 306 there were different venturis. One was a 20/26 which is what I'm going to attempt this summer. I'm going to try it on a Goldwing, 1200 with bigger valves and solid lifter heads which should allow 10k rpm.

 

The 20 venturi is only 316mm sq and the 26 is 530 mmsq so one Hitachi would be capable of 846 mm sq.

 

The Weber have different venturi options but most 32/36 had 26/27 venturi size, so 530 mm sq on the primary and 572 mm sq on the secondary.

 

So WFO the two Hitachi have 1692 mmsq vs the single Weber 1102 mm sq. for higher rpm it might be better with a built engine that can rev.

 

For me it's this little 20 mm venturis that I think can feed two cylinders better and more directly. I am hoping to elevate them a little to allow a straighter feed, but like Subaru there is still the 90 at the head. Comparing the primaries, the Weber has 530 mmsq feeding four. The Hitachi is 632, ( 316+316 to feed four) not really a huge difference. (16%)

 

I'm new at this, a late in life hobby, so take what I say with a grain of salt followed by tequila. My guess is I won't be fully opening the second barrels. Until it's ready to blow.

 

Love to hear opinions especially from any who've tried it. My reading tells me the progressives are a pita to get tuned. I have committed myself. Will likely get started in May.

[Ekvh21]

I should have added I'm looking for feed back on the purpose of the "secondary air flap" circled in pic. Is it to slow the draw of fuel/air when the second barrel is suddenly opened, to stop the dreaded bog that progressives experience? I thought I would gut that, but since I may be over-carbed anyways, I'll leave it to start with. It might help with lean pops when you slam the throttle down too.

 

Also wondering about the powervalve. Where does the fuel go when the valve opens? Is it just a short cut to the main jet?

 

Again, I'm new, and although I bought two carbs to do this, I haven't laid hands on them so all my knowledge is from surfing here and there. My carb experience is mostly with bikes, cv types.

 

Thanks in advance for any response.

[Ekvh21]

Just found some explanations of how powervalve and air flap works from a Mazda site. This picture is of a dcg306 mechanical secondary. In it you can see how much more restricted it is with this flap. This would drop the total mm sq by quite a bit. If the pic is to scale, I would guess it obstructs the 26mm venturi to make it a 21-22mm opening.

 

If true, 21mm would drop the secondary cross sectional area down to 346mm sq and the total carb to 664 mm sq. I think it will work great for my application, but also think it would give better top end than a single Weber 32/36.

Edited March 5, 2017 by Ekvh21

[Ekvh21]

I found an Australian site that says Aisan 20/26 had cfm ratings of 134cfm. Pretty small, but actually might be nice for NA dual progressives. I'm looking at getting rid of the secondary air flap. I have arrow in the first pic ( beautiful huh?) going to the step system to help the carb progression to the secondary. The second is of a vacuum secondary and the step jet for progression is just above the throttle plate. The mechanical secondary step jet is considerably higher.( first pic) I think I will try and drill one just above the lower plate after I remove the secondary air flap. Anyone else ever do this???