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Old 25 Apr 2006, 17:27 (Ref:1594775)   #1
Tim Falce
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Throttle bodies or Carbs?

Would I get any advantage from throttle bodies over Webers? they are a bugger to tune and I have not found anyone who seems to know enough about them (or more likely can be bothered) to get the best out of my engine.
It's a 4.2 Jaguar on triple 45DCOEs, 310degree cams with 11mm lift. I can either get decent top end and nothing low down or good torque midrange and it runs out of breath over 5000 revs.
Apart from the expense of an engine management system I could probably sell the Webers to pay for the bodies and fittings.
Are the benefits worth the hassle of doing it?
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Old 25 Apr 2006, 17:49 (Ref:1594785)   #2
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Yes.

Not quite the same thing but I originally ran my Corolla on carbs and a purpose built dizzy.

It had a huge hole in the power around 5000RPM which it wouldn't accelerate through in ANY gear.

Mapped ignition at least allowed my to drive through it, throttle bodies pretty much cured.

We then played with 40mm, 42mm, 45mm and 48mm throttle bodies. 48mm gave the best power but lost little or nothing compared to the smaller bodies.

And by best I am talking 10% plus improvements (48mm compared to 42mm).

The car also starts and ticks over, which is nice.

I am on similar cam dimensions to you, but only have 400cc per cylinder.
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Old 25 Apr 2006, 18:36 (Ref:1594811)   #3
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graham bahr should be qualifying in the top 10 on the gridgraham bahr should be qualifying in the top 10 on the grid
yes yes yes without doubt in my opinion
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Old 25 Apr 2006, 19:10 (Ref:1594832)   #4
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Originally Posted by graham bahr
yes yes yes without doubt in my opinion
Calm down Graham, it's only an induction system!
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Old 25 Apr 2006, 19:59 (Ref:1594859)   #5
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How do you work out what diameter you need and what injectors to use?
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Old 25 Apr 2006, 21:49 (Ref:1594956)   #6
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graham bahr should be qualifying in the top 10 on the gridgraham bahr should be qualifying in the top 10 on the grid
throttle body size is'nt as critical as carb size. in most instances 45 bodies are plenty and if a bit on the big side dont really suffer a downside anyway, a pair of 45's on a 4 cylinder will have enough flow potential for 260bhp so 3 will be more than up to supplying your jag.

with injectors you can get technical and work out how much much power you are likely to have and find injectors big enough to supply that much bearing in mind you dont want the injectors running at 100 duty cycle,
or what most do and just find a set of a std engine which is in the power ball park that your looking for, that said almost all production engines have loads of reserve capacity in their injectors typically 30-50%

injectors dont really wear so loads of engines are out there running on injectors sourced from scrap yards.

yellow bosch ones as fitted to cossies will supply enough fuel for around 280bhp
Xe 16v vauxhall ones are good for about 240bhp

so using 6 of either the about will be more than adiquate for your jag,

or a set of from an old 6 cylinder bmw 535/735 ( i had 200bhp using 4 of these)
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Old 26 Apr 2006, 00:36 (Ref:1595036)   #7
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If your Webers are optimised your Max power will be similar - but the torque, the shape of the whole curve, you will not believe the difference. The minimum size, well all the air in your DCOE needs to go through a Venturi of X mm. Go any smaller you will loose power. If you have a really big TB you will loose gas speed at the bottom end, costing torque (still better than Webers, though). Not knowing your power potential I can see no reason to go bigger than 45mm, and if you can find something cheaper that is bigger than the Venturi start with that. I had a Suzuki GTi that I made TB's for from a BMW K series, it had more than 170 hp from 1300 cc, from memory they were only 34 mm

Biggest costs can be things you don't expect. TB/Computer are easy to budget for. Pumps, swirl pot, regulators, fittings, return lines and such are the things that sneek up on you. If your car came as injection in the first place that can save a heap as you will have access to OEM parts. (Don't know what model Jag you have) A lot of stuff in good condition can be picked up s/hand from the street guys who upgrade based on it is the "done thing"

As Graham said selecting a second hand injector with the same power per cly as you have is a great place to start, just get them cleaned and away you go, by increasing cycle time (max 80%) and also fuel pressure you will be able to easily get the 50% more than standard, The Suzuki above had standard injectors still, standard power was 100 bhp.
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Old 26 Apr 2006, 07:38 (Ref:1595179)   #8
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my understanding ( very limited, sorry) is you should be ably to map glitches , flat spots etc out with management and TB's, top end poweer is achievable with either, as is mid range, but you basically have infinite variation with a fully managed system, something even the sacred Webers struggle to do !

I have a set of 4 Kawasaki 36mm TB's and injectors I'm thinking of using on a precrossflow head . . .just for a bit of fun !
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Old 26 Apr 2006, 10:17 (Ref:1595301)   #9
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phoenix should be qualifying in the top 5 on the gridphoenix should be qualifying in the top 5 on the grid
I think your carbs are too small to work well on such a large engine at both high rpm and in the low-mid range. Don't forget that each cylinder is about 700 cc and at 6000 rpm the engine needs about 680 cubic feet per minute of air - and with those cams maybe more. Many people, including me, have used 45s on engines as small as 400 cc per cylinder very successfully, and they 2 litre four pot boys use 48s to feed 500 cc pots if they have revvy engines don't they? So I feel 45s are too small.

This is because ideally air speed through the carb should not fall below about 14 metres per second nor go above 75 metres per second.

On your engine at 5000 rpm through a 45 mm carburettor without a choke the air speed would be already be 70 metres per second. Obviously you run chokes, so the airspeed would be higher and almost certainly exceeds 75 m/s.

With large chokes you can tune the engine for higher rpm - as it can breath more easily - but the large chokes will give poor mixing of the fuel, so the low and mid range will suffer.

Larger bodied carbs with relatively smaller chokes would give better fuel mixing and therefore improved low and mid range and flow more air altogether to improve the top end.

48s may give you 6000 rpm but if you want to run at say 6500 rpm or beyond I think you will need to run 50 DCOEs. If that involves buying new carbs, then I would seriously consider going for throttle bodies, or even a single 80mm throttle body, plenum chamber and an injector per cylinder setup which may work out cheaper than one throttle body per cylinder but could perform as well - definitely better than the 45mm carbs.

Martin

p.s. when comparing throttle body sizes and carb sizes, the choke size on the carb is what you need to look at, as gas speed through the throttle body depends on it's given diameter whereas in carbs max gas speed is achieved in the choke.

Last edited by phoenix; 26 Apr 2006 at 10:27.
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Old 26 Apr 2006, 12:48 (Ref:1595406)   #10
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Thanks for all the help guys. I will be looking into getting some TBs for my winter project, that'll give me plenty of time to read up and find some sources of the stuff I will need..
Ta

Last edited by Tim Falce; 26 Apr 2006 at 12:51.
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Old 2 May 2006, 08:40 (Ref:1599196)   #11
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knighty should be qualifying in the top 5 on the gridknighty should be qualifying in the top 5 on the grid
have you got 5 progression holes????

hold your horses guys, before you bin your webber carbs and spend a minimum of 2 grand converting to throttle bodies, you need to do one quick check......its with regard the the "big" dip in mid range power......you need to unscrew the brass cover just above each throttle butterfly......then count the amount of progression holes.......most webbers only have 2 or 3 progression holes.......the later webbers have 5 holes.......5 holes basically kills that notorious dip in power and makes your engine drive much better, the progression holes ease the transition from the idle circuit to the main fuel delivery circuit......5 holes delivers more fuel into the "dip"

WebCon (Webber UK) in Sunbury on Thames have a special drilling jig which thay can attach to your carbs and drill the extra progression holes.......do not attempt to do it yourself - as you will scrap the carbs for sure.......phone Alan Collins on 01932 750252 (Carb god at Webcon) and organise for your carbs to be drilled, cost is about £40 each I think......I know loads of people who have done this mod and have now forgotten about EFI and throttle bodies.......
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Old 2 May 2006, 09:00 (Ref:1599212)   #12
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That sounds like my kind of money saving Mod Knighty.
I've also been advised recently to stickk to a good refurbed Italian DCOE rather than newer Spanish ones ( I've been running fine on old ones thus far )
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Old 2 May 2006, 09:42 (Ref:1599233)   #13
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knighty should be qualifying in the top 5 on the gridknighty should be qualifying in the top 5 on the grid
progression holes

Hmmm......I honestly cant say I know the difference between the Italian and Spanish carbs, but I do know that Magnetti Marelli (Webber) basically sold off all their carb business to the spanish company, so putting two and two together would mean all the original tooling and assembly equipment is now being used in spain.......I honestly cant think what could be wrong with the spanish versions, but I'm all ears if someone wants to reveal all......

I know some of the aluminium castings can feature nasty external flash, but thats actually quite acceptable in the high-pressure-die-casting industry, they are actually minor surface cracks in the moulding tool - part of life - but yes, it dosent look nice - but its actually OK
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Old 2 May 2006, 09:49 (Ref:1599236)   #14
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Quote:
Originally Posted by knighty
hold your horses guys, before you bin your webber carbs and spend a minimum of 2 grand converting to throttle bodies, you need to do one quick check......its with regard the the "big" dip in mid range power......you need to unscrew the brass cover just above each throttle butterfly......then count the amount of progression holes.......most webbers only have 2 or 3 progression holes.......the later webbers have 5 holes.......5 holes basically kills that notorious dip in power and makes your engine drive much better, the progression holes ease the transition from the idle circuit to the main fuel delivery circuit......5 holes delivers more fuel into the "dip"

WebCon (Webber UK) in Sunbury on Thames have a special drilling jig which thay can attach to your carbs and drill the extra progression holes.......do not attempt to do it yourself - as you will scrap the carbs for sure.......phone Alan Collins on 01932 750252 (Carb god at Webcon) and organise for your carbs to be drilled, cost is about £40 each I think......I know loads of people who have done this mod and have now forgotten about EFI and throttle bodies.......
Gotta be worth a try for £80!
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Old 2 May 2006, 13:32 (Ref:1599372)   #15
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Originally Posted by knighty
......you need to unscrew the brass cover just above each throttle butterfly......then count the amount of progression holes.......most webbers only have 2 or 3 progression holes.......the later webbers have 5 holes.......5 holes basically kills that notorious dip in power and makes your engine drive much better, the progression holes ease the transition from the idle circuit to the main fuel delivery circuit......5 holes delivers more fuel into the "dip"

....
Got nothing to unscrew on mine, they are plugged.
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Old 2 May 2006, 18:41 (Ref:1599546)   #16
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knighty should be qualifying in the top 5 on the gridknighty should be qualifying in the top 5 on the grid
then look down the choke, open the butterfly and count the holes, you might need a dentist-mirror and torch to help count em.....the later carbs come from the factory drilled with 5 holes......this was a modification that Alan Collins did at Webcon, the Webber factory in Italy heard about it, then converted their production line to drill all sidedraught carbs with 5 progression holes per butterfly, as it was such a good improvement to drivability.......but that was probably the last 2/3 years of production, year 2000 to 2003 ish - I'm guessing......so the vast majority of sidedraught carbs only have the old spec of 2/3 progression holes only - I cant remember the exact amount!.......how new are your carbs?.....
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Old 2 May 2006, 19:55 (Ref:1599617)   #17
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I got mine about 5 or 6 years ago and they were made in Spain. I think mine suffer from finding someone willing to , or who knows how to, set them up properly. Most tuners nowadays just like to sit down with a laptop as a means of tuning.
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Old 3 May 2006, 07:52 (Ref:1599870)   #18
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Originally Posted by falcemob
I got mine about 5 or 6 years ago and they were made in Spain. I think mine suffer from finding someone willing to , or who knows how to, set them up properly. Most tuners nowadays just like to sit down with a laptop as a means of tuning.
Interesting about your spanish carbs......how many progression holes you got then????.....I have to disagree - there are plenty of carb people out there who are quite competent at setting up a pair of sidedraughts.......you just have to ask around!
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Old 17 Sep 2007, 18:00 (Ref:2015988)   #19
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Just an update. I never got round to getting throttle bodies but after 2 years of problems starting the engine from cold and a season of my lap times getting slower and slower culminating the other weekend with me throwing a strop and pulling off the track during my last race, I have stripped the carbs down and found I only had one accelerator pump jet working and two pumps out of three were seized.
The carbs were full of crap and after a good clean and some new seals and filters, the engine starts on half a turn from cold and runs and accelerates smoothly. Just have to check it on the road.
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Old 17 Sep 2007, 20:19 (Ref:2016140)   #20
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Tim as has been mentioned in the posts I also think your carbs are too small.
I even use 50s on my anglebox, OK I might have a low down power problem but you must admit it don't hang about when it gets going. What chokes are you using ?
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Old 18 Sep 2007, 04:28 (Ref:2016410)   #21
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Cold start problems on webber is definitely a warning light, they should fire up pretty easily, big squirt on the throttle and then close it off, wallah
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Old 18 Sep 2007, 13:06 (Ref:2016897)   #22
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What chokes are you using ?
36s. Yes I know they are small but when I went the recommended route of 40s it had no low down power and I don't like screaming the ******** out of it.
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Old 18 Sep 2007, 13:33 (Ref:2016934)   #23
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That's the one main benefit of bodies over carbs, you can go really large without hurting low down too much.

I run 48's on a 1600 and it made about the same low down as 42's.

I think this is part of the reason why motorcycle type carbs are becoming popular, they are better low down than the traditional car ones. Especially if you go for something like a Mikuni flat slide type.
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