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 Post subject: Webers on a......
 Post Posted: Wed May 30, 2012 10:10 pm 
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Im interested in running webers on my smallblock (326 ci mopar)but it seems that sizing of the the throttle bores on webers is the limiting factor for hp on an IR manifold.
I cant really afford the big 44/48mm IDA's or IDF's atm so was wondering if there would be any benefit in going smaller say 4 x 36-40 IDF's or DCNF's on a plenum style manifold?

Thoughts?

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 2:07 am 
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Once you go to a plenum setup you dont need anywhere near as big a carby venturi as on Individual runner.


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 2:24 am 
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Thats what i thought.

Is there a formula for sizing carbs like webers on a plenum Shrinker?

If the 36 - 40mm carbs are too restrictive for my cubes and rpm per inake port would running 1 weber for 2 ports be a better option? It would mean building a manifold with 4 plenums to acheive this but if it may be a fun experiment to see what it could make.

Hysteric.


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 2:41 am 
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There is a formula for calculating CFM flow with a depression. I have it on excel, its complicated.
Either build a IR setup or do a proper plenum setup. Dont do a one weber into 2 cylinders deal, the pulsing isnt going to be even and one cylinder will suffer a fill percentage and the AFR will be a mess.
A 36mm venturi flows 201cfm per venturi at 1.5" Hg depression.
A 40mm venturi flows 249cfm per venturi at 1.5" Hg depression


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 4:21 am 
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So 2 x 36mm webers on a single 4 would flow 800cfm? Any benefit to running webers over holley on a single 4 manifold?

Weber manifolds dont exist for small block chrysler so i have to build one. Im looking to build a base from which i can change different set ups and even adapt it to run chevy intakes.

Quote:
Dont do a one weber into 2 cylinders deal, the pulsing isnt going to be even and one cylinder will suffer a fill percentage and the AFR will be a mess.


I take it you have tried this? Were the the 2 cylinders shared from the same bank or both sides?

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 4:46 am 
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Ive never tried it because it requires you to build a phase manifold. See every plenum to be correct needs to be at least 8 to 10 times the volume of 1 cylinder. So how can you make 4 plenums that are each that size and get it all to fit and do it in a even firing order pattern, 360 degree phasing? There are other things to consider too like wetted area etc and then make the manifold strong enough that it doesn't collapse with the vacuum. Its all too hard. So people just build a single plenum and feed all 8 from that, its a satisfactory way to go. The advantage of a correctly sized plenum is the carby receives smooth air flow, carbys cant regulate instant air flow rate changes, EFI cant either. Nothing likes pulsing flow smooth flow is preferred. Leave the pulsing stuff to be inside the runners not in the plenums or the venturi.
Yes 2 weber's that size will flow 800cfm.
Weber's are a far more sophisticated carby than a Holley but you wont find many to help you in USA. There are a lot more emulsion options and mixing options with Weber's but unless your going all out for a top line performance engine I'd just stick to a Holley.
The closest Ive ever come manifolding wise to what your talking about was back in 1969 with a 318 Chrysler in a Super modified speedway car. It had 6 double barrel Stromberg 97's on it. Now there's a setup for the history buffs. It was 2 logs, one for each head. It went alright, was a top car in Oz.

You could certainly build a twin log type manifold easily and feed each log from one of your webers. That will work and look cool. Stick em out over the rockers like the old max wedges.


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 8:59 am 
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Thanks Shrinker,

Got any pics of that log manifold?

Im in OZ too! Got a mate who did his apprenticeship on Lambos and Ferraris in the 80's so hes not scared of setting up webers. Also got 40 dcoes on my 67 Alfa Romeo so i gotta learn sometime.

So a single plenum with 4 x 36 IDFs will be a better alternative? Are there any good books on manifold design?

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 8:45 pm 
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Scientific design of exhaust and intake systems by Smith and Morrison. ISBN 0-8376-0309-9 Is the definitive bible on manifold design and pulsations etc. Done in the 50's and never been beaten because it was done right.

A single plenum will be convenient to fit. See each plenum has to be the same size so how are you going to fit 2 plenums that are each 318 cubic inch volume? That's why tunnel rams were born. By the way not many manifolds available ever reach the correct volume dimension, the Chrysler M1 manifold for your engine is correct from memory. Chrysler (at least they used to) put more effort into those things than others. That's why Chrysler engines respond and burn better. A properly done Chrysler is a very nice engine they drive the best Ive ever experienced of the US V8's.

Sorry no pictures. I remember making it, we even had to make our own surface grinding rig to flatten the head faces because machine shops couldn't do it for us. It was in 2 separate halves joined by a sheet-metal valley cover that was welded to each inlet face. The runners were sheet-metal we folded to fit the port so it was like a straight rectangular tube, no taper. The log was sheet-metal it was only a small volume log, I think each log was about 120 to 140 cubic inch volume with no balance tube, rectangular shape. The runners crossed over so the left bank carbs were on the right side of the engine, that made the runners nice length and straight for max flow. A bend really inhibits flow, no bends is best. It had a total of 1133cfm on it. It was methanol. The cam was stellite welded to build the lobes up to get lift and duration, flat top pistons, stock rods and crank. Ported 318 heads, 13:1 compression.
We even had to alter the oiling system. High volume pumps weren't available in those days so we drilled into the side of the oil pump put in a fitting to run copper tubing down the side of the pan rail and drilled through the side of the mains caps and plumbed the oil straight into the bearings that way. It fixed all the issues and never blew up. We ran 7500 RPM on that engine every week with perfect reliability.
They were the days of home grown innovation and proper hotrodding.


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 10:19 pm 
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Hmmm.................Lots of info to process and that raises more questions.

I would have thought that each plenum would only need to be as big as the cylinders it fed as essentially its now say a 2 cylinder being fed from 1 weber. IR manifolds seem to have no plenum at all so how does that work? Are the runners considered part of the plenum. What am i missing here?

On another note you raise an interesting line of questioning regarding using the same manifold on different cube engines say a performer rpm on a chev from 283 to 400 ci. Would you not need to size the manifold to the engine ci then?

Thanks again Shrinker.

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 11:01 pm 
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If you read the book it will explain it all. Its been 30 years since I opened that book but the conclusion is the plenum needs to be sized at 8 to 10 times the size of a single cylinder in order to reduce the pulsations and increase the air flow to the cylinders. Because on a V8 2 cylinders draw with reasonable effort at one time the pulsing is smoother to start with so a smaller plenum can be used with satisfactory results. The word satisfactory is not perfection, its just satisfactory. To be perfect the plenum still needs to be 8 to 10 times the volume of one cylinder irrespective of the number of cylinder feeding from it. So V8 perfection would be a plenum the same as the cubic capacity of the engine. 4 cylinder perfection is usually 10 times. The V8 gets away with less because of the intake draw consistency of the basic design. Plenty of examples are made that dont follow that rule but just because you can buy something doesnt mean its designed right.

With IR setups the runner is not considered a plenum, its still just a runner but the carby venturi size need to be around 80+% of the valve size in order to get the air in in the time available. A plenum doesn't choke the runner like a IR carby does. The plenum has to be that large too act as a shock absorber for the runner flows. The plenum delivers air to the runner in whatever fashion the engines demands it and it smooths out the air flow so the carby can function perfectly. Carbys cant accelerate the fuel at the same rate that the air acceleration in the venturi happens with a pulsing flow like an IR setup. this is due to the mass difference of fuel and air. Even though the average AFR of the chamber might be say 13:1 the AFR thats actually traveling down the runner will be varied, it will be lean at the start of the flowing section of air then rich at the trailing end of the air flow. This results in distribution issues within the chamber. Plenums correctly sized eliminate that. Some people think that adding spacers is enough to increase the volume of the plenum and perfect flow etc. yes it does improve it, but its nothing like designing the correct sized box to start with.


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 Post subject: Re: Webers on a......
 Post Posted: Thu May 31, 2012 11:32 pm 
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Just ordered the book from Pitstop in Perth.

Quote:
With IR setups the runner is not considered a plenum, its still just a runner but the carby venturi size need to be around 80+% of the valve size in order to get the air in in the time available


Is the 80+% a guideline for performance engines? Im looking to run a small valve....say 1.90-1.96 11/32 so 40's would meet that requirement or is rpm a factor here?

With the "ideal" sized plenum for a given ci what sort of %'s are we talking about in performance improvements? Are the improvements mainly from a smoothed intake charge better filling the cylinder and better carb signal? I would have thought the larger plenum size would dampen the signal to the carb?

Is this similar to your manifold in design?

http://www.summitracing.com/parts/EDL-2150/?rtype=10

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Fri Jun 01, 2012 12:00 am 
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Yeah thats a cast version of what we did. pretty close to it actually.

The benefits of plenum over IR are easier tuning and more consistant AFR distribution thoughout the intake stroke of each cylinder. NOTE thats not a statement of AFR distribution per cylinder; its within the one cylinder. Plenums (when wrong shape or too small) can cause AFR distribution issues to the different cylinders at least you dont get that with IR. So a bad designed plenum is not as good as an IR but a good designed plenum is better than IR. So there is no way I can quantify the performance change from IR to a plenum. There are not many plenums out there that are correct to compare things with. Ive never changed from IR carby to plenum other than going from EFI to carby on a plenum. The EFI system was IR and the change to a carby beat it hand down, there was heaps more power on the carby. No comparison really.


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 Post subject: Re: Webers on a......
 Post Posted: Fri Jun 01, 2012 12:10 am 
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40mm would be about right for a performance engine as an IR setup. For street driving Id go a bit smaller maybe 38mm, there's really not a lot in the differences for top end power but a lot of ease in the tuning with the smaller carby. Your better off building an engine that has good tune in the midrange, people rabbit on about their top power but its pretty useless because you only just touch top end power and then shift. better off getting power in the rev range that you run building up through the gears , then the car is a quicker car. Drag racing is different because of stall convertors and such. Other racing is much harder to optimize. So whatever your doing good luck.


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 Post subject: Re: Webers on a......
 Post Posted: Fri Jun 01, 2012 1:51 am 
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Quote:
So a bad designed plenum is not as good as an IR but a good designed plenum is better than IR.


This jumped out at me.......Can this be interpreted as the current offerings of manifolds are a compromise at best due to space(fit under the hood) and carb limitations(cost considerations) that oem and aftermarket companies are limited by?

LOL.....so now my quest is the Ultimate plenum design!!!!!!

Im basically looking to run high 11's with as streetable car as possible with 324 cubes @ 3150 lbs(Ap5 Valiant).

Hysteric


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 Post subject: Re: Webers on a......
 Post Posted: Fri Jun 01, 2012 2:23 am 
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Go the mighty Valiant.
Yes most manifolds on offer are not correct but they work due to the evenness of a V8 induction. Technically they should be larger but its a compromise to fitting under bonnet etc. Full race manifolds like the Mopar M1 are the right size and shaping etc for a single 4 application. Personally I'd just put one of them on (M1 or modern eq)and concentrate on other things like how to build the engine with good seal and strong bores and experiment with the cams to find what works best etc. There are more important things to learn than a manifold plenum.
Ok so you chase the ultimate plenum then what carby do you put on it, how do you line up the barrels for optimum feeding into the plenum huh? Custom stuff like what I make is very expensive. Personally I would spend the money first on getting a port job that has the right velocity profiles everywhere in the port, with no turbulence or chokes upstream from the valve and no fuel separation issues. The motor starts at the heads.


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