45DCOE Webers

[260Devil]

i have a l26 with a 040thou overbore and 2nd stage cam.

looking to use it on the street/club events

what size tripple webers should i use?

Thanks.

Aaron

[MaygZ]

This is something that requires some calculations:

 

work out the swept volume of your engine

 

multiply that by the maximum RPM you will be running

 

This will give you the volume per minute that passes through your engine.

 

Divide by 6 (for a 6 cylinder engine if you're using dual throat carbies) and that will give you the air volume to be drawn through each throat.

 

Compare this figure with the specs of the selected carbies to find the best match.

 

Now this is where it gets complicated - the three other things that need to be considered that affect the air flow are your inlet manifold, your exhaust system and your head air flow rates (inlet and exhaust).  There is no point working out this great theoretical figure only to find that one of the other components will only carry half that volume.

 

You might decide that you want to be able to spin your motor at 7000RPM, but find that due to the head (or inlet or exhaust) design it wont spin past 6000.  You now have the wrong carbie on the motor and it will run less efficiently than a smaller unit.

 

Having said all this, I would have a guess at 40's if you are running a good extractive exhaust system, but to be spot on you should have the complete set-up flow tested??

[dazzed]

why dont you just wack a set of forties on it

[Roberto]

A few years ago, i had almost exactly the same decision to make. I was wanting webers on my L26 (unbored) with stage II cam.  40's or 45's??

 

All the guys in the USA chatroom (IZCC back then) strongly suggested 40mm, but the calculations with engine displacement and rpm, suggested i could get away with 45's (with smaller chokes).  I was only 25 years old and bigger was better, so i went with 45's and regretted the decision. 

 

It was great at full throttle, but under part throttle acceleration there was surging / hesitation.  I took it back to dyno tuner twice and they couldn't completely get rid of it.  The car drank like a fish and I had all of this un-driveability for just 94rwkw. 

 

Webers ended up coming off.  I will only ever put triples on a road car now if its going on a reasonably hot engine (compression, porting), otherwise your small gains will not be worth the headaches.  But, if you have your heart on them, go for the 40's !!

 

My 2cents....

[GV260]

I had a similar spec L26 bored our to 2.85 ltrs, but the head had 280Z exhaust valves and other enhance porting. Using well tuned original 240Z carbs it had a 105rwkw - it drove really well and was relatively good on the fuel.

[Scoota G]

I run Triple 45mm Dellortos with 36mm (correction 34mm) chokes. I calculated this with the help of a friend, he was going to sell me his 45mm webers and then backed out and then said yes again. By this time i was saying i'll go get my own. I started off with 40's on an almost stock motor then after i put a part of one of the rings out the bore i did it rebuild. It now has,

 

2mm shaved, ported and cc'd N42 Head

Stainless valves, arp stud kit, crow cam and springs

Adjustable cam gear, Electronic ignition, genie headers

Blah, blah blah blah (i could just keep going but you get the idea)

 

I get over 400 kays per tank on highway and on average around 300 city milage with a modified auto. Now what was my point....... a yes the 40MM Dellortos were fine i just wanted 45's out of spite and then bulit the rest of the motor to suit. Once you start modifying it's a full package that will get a good result, bolt on are fine but it depends on what you want and how far you'll go with it.

 

Oh and i'm very pleased with the output of my engine, can't wait to get it on the Dyno.

[peter mc]

hi just  put my 2 cence in that l26 of gv with 45 webers and new exh went 135kw

[260Devil]

Wow thanks for all the great help guys.

And i commend MaygZ for all that writing and what you said did make alot of sense.

Thanks again 

Aaron

[Scoota G]

Anyone know the CFM for 40MM/ 45MM sidedraughts. I've heard 350/ 375

[MaygZ]

Wow thanks for all the great help guys.

And i commend MaygZ for all that writing and what you said did make alot of sense.

Thanks again 

Aaron

 

No problem Devil.

 

By the way, does anyone know who is knocking off all my Karma??  :'(

[260Devil]

By the way, does anyone know who is knocking off all my Karma??

 

I know right! ive got -2. Didnt even know that was possible

[chris240]

I agree with all the comments above..

Im my humble opinion go for the 40's without a doubt...(mine are still sitting on a shelf in the shed  !)

hehe, someone knocked off a karma or 2 from me as well for no apparent reason.....I thinks it run its course Gav.

[MaygZ]

What a fine story it was too.

 

http://www.viczcar.com/forum/index.php/topic,7587.60.html

 

for those that missed it.

 

Still working on a sequel .... any ideas or theme suggestions anyone?  Looking to change the main characters as they don't seem to be friends anymore  :'(

 

MaygZ

 

Sorry about the thread Hijack so back to the discussion regarding choise of triples.  On that subject I think it was Rev who posted something about OEM (or similar) carbys.  Appearently these carbies have taken all that is good about the different brands of carby out there and put them in one unit.  I don't know anything about them (probably even got the name wrong) but might be a consideration other than the increasingly hard ($$$) to find webers.

[luvemfast]

but might be a consideration other than the increasingly hard ($$$) to find webers.

I may have some triple 40PHH Mikuni's for sale by Christmas 

[260Devil]

Well im not even sure what the karma even does for me so im trying to forget about that for now.

Also, concidering the AUS dollar is soo freekin' high at the moment i dont really mind buying them from US ebay.

I also met a guy at Lakeside raceway the other weekend who had a rather interesting fuel setup.

With hoses leaving the firewall to the carbies it was meant to keep fuel pressure up during accelleration (from G forces pushing fuel back) by keeping all the hoses the same length.

Im sure theres much more to this setup but i cant remember off the top of my head.

[MaygZ]

maybe a prequel, seems to be the thing? some ideas...........

when were these characters ever friends? if not, why not...........

what got them to this point?

 

Done.

 

http://www.viczcar.com/forum/index.php/topic,7587.60.html

 

[Scottz]

Aaron,

 

Air flow calculation for carbs, using cubic centimeters as the units: -

Engine Capacity x RPM/2 x Volumetric Efficiency.

VE = 80 to 85% typically for 1970’s OEM spec carbs and engine.

VE = 85 to 95% for performance enhanced. (Choose what you think is close to your engine spec).

VE = 95 to 100% for competition spec.

VE = >100% for extreme performance (blow'n).

RPM/2 = Max rpm you expect to achieve divided by 2 revs per air intake stroke.

That is; 4 strokes per cycle every 2 revolutions = intake / compression / power / exhaust. (2 revs every 4 strokes).

So for every 2 revolutions there is only one intake stroke. Therefore air required is only once per 2 revolutions. Clear like mud Yes?

Example:

Performance stroker engine L series 6 cylinder, 3100cc capacity, mild head work & mid range cam, with expected max rpm of 7000.

3100cc x 7000/2 x 0.90 = 9,765,000 cc per minute air required.

 

Now convert cc to cfm. Why? Because OEM's like to spec their carbs in cfm.

cubic centimeters to cubic feet; 1,000,000 cc = 35.315 cf

9,765,000cc = 345 cfm Total.

 

Using this website I got 365 cfm.  http://www.carburetion.com/after.htm

And using this website I got 346 cfm.  http://www.csgnetwork.com/cfmcalc.html

So I’m prity close to the mark.

 

That's for all cylinders so divide by 6 and you get 57.5 cfm per cylinder.

Or divide by 3 (because you are wanting 3 carbs to handle total air flow) = 345/3 = 115cfm per carb.

 

The Stoichiometric ratio (AFR) for air to gasoline is 14.7:1, meaning that for each weight unit of gasoline, 14.7 units of air will be consumed. Stoichiometric ratio is different for different fuels (Av gas, methanol, ethanol, diesel, etc).

1kg of gasoline needs 14.7kgs of air. Considering you can fit a kilo of fuel into approx' 1.25liter container, how much volume do you think 14.7 kilos of air occupies. Answer - lots and lots and lots. Or about 12416 liters or 438.5 cubic feet. ( @ 1 atm and 25°C, air density = 1.184 kg/m³).

Therefore 115 cfm of air flow does not need much fuel to achieve 100% burn, and remember that's at maximum revs for 1 minute, which hardly ever happens or never, on the street.

 

Losses:

Air density changes (altitude & temperature), friction (rough inlet passages, surface finish, material used), back pressure (exhaust restrictions & inlet restrictions), turbulence (velocity and size factor).

All losses cause reduced air supply to the cylinder. Lots of owners reckon you should add 50cfm to the total cfm to compensate for losses, so 345cfm increases to 400cfm rounded off. Divide by 3 carbs=135 (rounded).

 

RPM is the biggest number in the equation so think about what you want at the rpm you are going to use most. No point doing a spec for a high revving screamer if you use it mostly for cruising around at 4000 rpm with short bursts at 6000 rpm. You put the bigger carbs on and it runs great at the track day, but every other day it’s a de-tuned rough idle gas guzzler. :-\

I have ordered 40 DCOE triple Weber carbs. The USA Weber vendor's price was so much better than the OER price, I changed my mind and went for the Webers.

Drop me a PM and I'll give you the vendors contact details if you are interested in a new set. Otherwise the network on this forum is best place to find a used set at the right price.

 

What I’m interested to know is; Has anyone tried the SU’s on a triple setup for the L series engine.

Like there are heaps of used SU’s around from old L series engines, surely there must be an inlet manifold that lets you bolt up 3 SU’s instead of 2. That's got to be a cheap alternative for an owner who already has the twin SU’s and wants to go triples, without the big $$ spend on triple OER’s, Weber’s, Mikuni’s, Dellorto’s etc?

 

Scott.

 

[NZeder]

Re the question on triple su's on the L6 engines. Well people have tried but it will not work very well due to the firing order. The triple su setup you see on some inline 6 engine usually are odd fire engines. There was a threat about the hybridz, CZCC or here a few years back (can't remember which forum)

[Scottz]

RE: Tripple SU's for L6.

Found it, thanks NZeder.

 

A well written paper by Marc Sayer, 2 June 1999. Explains it very clearly. His solution... 6 of 1.25" SU's haha...And if you're going to have individual runners then ITB's would be an upgrade on that idea also.

 

http://www.zhome.com/ZCMnL/tech/TripleSUs.htm

 

Another (theoretical only) solution which will give even pulses for each manifold is to do some radical inlet manifold design and pair up the firing order.  But each carb and manifold would still have reduced air velocity, and how it would all fit under the bonnet with equal(ish) length inlet runners would be impossible.

1 0 0 6 0 0 1 0 0 6 ... Cylinders 1 and 6 paired

0 5 0 0 2 0 0 5 0 0 2 .... Cylinders 5 and 2 paired

0 0 3 0 0 4 0 0 3 0 0 4 ... Cylinders 3 and 4 paired.

(Read Marc Sayer's paper to follow this).

Nissan figured it all out with simple and effective twin SU's and later followed up with EFI.

A classic case of don't mess with something that's not broken.

 

Now, quoting Mark..."If the manifolding is done well it will be equal to, or slightly better than the stock manifold for distribution, but will increase flow. If runner and plenum size is kept small enough to keep air velocities up, the drivability and throttle response should be better than a dual set up."

 

So is this a more important point re: Tripple Webber 40's Vs 45's ? Both can deliver the cfm required, but the 40's (smaller diameter to 45's) will have slightly higher air velocity.

[d3c0y]

If the 40's can supply enough air for what you want go with them - smaller carbs will have better throttle response.

[zedrally]

Follow this link for more on triple SU's, if anyone is interested.

 

http://www.classiczcars.com/forums/showthread.php?t=21410&highlight=triple+su%27s