48 quad Dellorto vs big 750 Holley?

[Al Weyman]

On my black 3rd gen Camaro fitted with a stout all roller engine and a 750 Holley on a single plenham manifold I find it is a bit boggy at the bottom end and does not really start singing till I get the revs up a bit (it is built to rev to 7800).

Want I want to know is will matters be improved if I fit a downdraught 48 DHLRA (?) Dellorto setup, and I have read some of the theories and I know I should get a lot more hit out the corners and more torque and low end power with this set up, but will it harm the top end performance too much and will one advantage outweigh the other. Any one got any views or experience on this?

[Tim Falce]

edited cos I didn't read the title properly.

These are very expensive, wouldn't you be better with a couple of 4 barrel Holleys on this although I have no idea of prices of these now, the last one I bought was £20 new, in 1976

[Al Weyman]

No the Holleys are not cheap anymore, a 750 double pumper is around 400 quid now I believe. Also by a quirk of fate the Eldenbrock manifold and big Holley I have on the car at the moment belongs to the Penske IROC car I am renovated and was the actually one fitted so i have to buy something else.

Holleys don't work very well into an independant runner manifold as far as I am aware and work best into a single plenum manifold. Trouble a dual Holley set up would mean the one and a bit cylinders on the downstroke would see a massive 1400cfm hole and would not start working till revs got very high which is what I am trying to get away from. A quad Dellorto or Webber set up (and I have my eye on a good 2nd user set up) each cylinder sees just the one choke hence the low down torgue and low end power improvement especially with a stout cam like I am running.

It is just the top end that worries me, I know a lot of the Rovers are running a quad carb arrangement and I am sure some of these guys must have tried a single Holley so i was interested to see their or any SBC users experiences in a circuit race environment as oppossed to a Drag race.

[johnh875]

Al my first thought was that single plane manifolds favour top end and dual plane favour bottom end. One development around now as a "best of both worlds" compromise is the "air gap" type - basically a rectangular port at the top of the plenum divider just under the carb, something for you to look into.

Having said that, in Group C touring car racing out here in the late 70s a lot of the cars were running twin side draught Webbers with adaptors for a 4bbl manifold.

[Mackmot]

I cant remember the sizes but people normally try to use too large exhaust primaries which would have the effect your talking about. It might be worth researching into the other side of the engine.

[Al Weyman]

No not in this case in fact they are on the small size at 15/8" as they are the only ones I could find off the shelf for the chassis, it is very tight for space on these later cars that use the fletch panel as a suspension mounting point.
Now you may be right on my other car I have just finished rebuilding, the 2nd Gen Camaro as that has whopping great 17/8 competition Hookers that I orginally bought when I ran it with a 400ci motor. I have yet to run this one yet and hope to on Wednesday at Mallory.

[graham bahr]

go for it, with the dellorto's that is, they might not provide quite the top end power of the holley, due to the non plenum type manifold, however if they are set up right the throttle responce they will give will be awsome, and they will also provide better torque just about everywhere, personnaly i would of thought that you would gain more than you loose going with the dellortos.

heres another angle,
niether 60's racing GT40's or Gerry marshalls Baby bether were fitted with holleys but instead webbers like the dellortos were talking about, there was a good reason for it!

[graham bahr]

something every one seems to forget is that what actually does the work for us is torque and the better spread of the stuff we get the easier and faster our cars are to drive, a couple of years back i found a modifiaction that added 12% more power and torque at 5,000 rpm, that made a substancial difference to the cars lap times and its ability to pull a couple of car lengths out of the opposition when exiting corners, infact that mod made far more difference than the previous half dozen mods which all chased and added to the top end performance.

[graham bahr]

i've just done a bit of reading of a Vizard book, he was talking in terms of 4x48's MIGHT loose out a bit at the top end when compared to a 850 holley, but suggested as i said the 48's gave many benifits. and although i know very little about Yank V8's im sure a 850 holley is bigger than the 750 one we were talking about here

[275 GTB-4]

Originally Posted by Al Weyman

On my black 3rd gen Camaro fitted with a stout all roller engine and a 750 Holley on a single plenham manifold I find it is a bit boggy at the bottom end and does not really start singing till I get the revs up a bit (it is built to rev to 7800).

Want I want to know is will matters be improved if I fit a downdraught 48 DHLRA (?) Dellorto setup, and I have read some of the theories and I know I should get a lot more hit out the corners and more torque and low end power with this set up, but will it harm the top end performance too much and will one advantage outweigh the other. Any one got any views or experience on this?

temporarily get yourself over to AtlasF1 big Al...

http://forums.atlasf1.com/showthread...racing+engines....

ask the engine guy or McGuire...gun tuners

[R59]

I use an 850cfm Quickfuels circle track carb, which gives a faster crisper response than a holley, but I'm going to change to fuel injection with a cross-over body setup to reduce the overall height of the installation. I was looking at going for 50mm throttles to do the job. I did think of 55mm but it'd probably lose me some bottom end performance in favour of top end horses. Like Graham quite rightly said, it's torque that get's you out of the corners and away onto the straight, ultimate top end is only useful if the straight is long enough!

Thinking back many years, I ran a 1600 vauxhall on 40DCOE's, and it had bags of grunt. On 45's it lost the bottom end tractability, but gained lots of top end. The same goes for playing with cam profiles, where a milder profile allowed me to run around Mallory in 3rd 4th and 5th gears, and set the third fastest lap time, only 0.3 off of the fastest lap. With the race profile, it made a 1 sec difference, but by the time I'd gone to that, the fastest guy was still 0.3 quicker (dammit! Watson!), and I had to use 1st at the hairpin, and rev the dooodahs off it.

Back to the V8, another trick is the size of the spacer between the manifold and the carb. Increase the length of the inlet tract and you can increase the torque, as well as increase overall power by increasing the volume of the intake. There's something in one of Vizard's books about the length of the inlet tract, and it showed a Dodge Dart from the 60's where the two carbs were on really long inlets that put the carbs outboard of the Vee. Looked wierd, but gave massive increase in torque.

Still, the old Can-Am trumpets on a V8 look great!!

Rob.

[Al Weyman]

Thanks for the replies gentlemen.

I have tried it with an 850 also and that was worse! I am going to do an experiment if I get to Mallory wednesday. On my 1970 Historic Camaro I am currentl have built it with a 650 but I am going to take a 750 with me and see if it makes a difference. I won't bother trying the 850 as the engine in this car would not be up for it.

I have always been very disapointed with the 350ci engine coming out of slow bends and although I probably have 150bhp more than a fellow competitor I used to race against, he had a Rover SDI on Webbers (quad) he could leave me for dead out of the corners. Now when I ran it briefly (till it let go once too often) with a 400ci that was a different story as the thing made 425 ft lb torque on the dyno at under 2000rpm!

These carbs I am thinking of buying are fitted with 40mm chokes and set up for a 500+bhp small block and I think they will be the business if I can set them up properly. Just got to make sure they will go under the hood (sorry, bonnet) of the 3rd gen car as the rules state a maximum 2" bonnet bulge. If nothing else they will look damned good.

[graham bahr]

Originally Posted by Al Weyman

Thanks for the replies gentlemen.

These carbs I am thinking of buying are fitted with 40mm chokes and set up for a 500+bhp small block and I think they will be the business if I can set them up properly. Just got to make sure they will go under the hood (sorry, bonnet) of the 3rd gen car as the rules state a maximum 2" bonnet bulge. If nothing else they will look damned good.

i'm not sure how big your engine is but if its a long way over 5.0 then 40mm chokes are probably too small for the size of your engine in terms of top end power, i'm not sure as to what sizes you can get chokes for the downdraft carbs in but 48mm sidedraft dellorto's come upto 43mm on all out race 2.0 4cyl engines we used to run 42's

chokes are easily bored in a lathe, but wont work aswell if you dont retain the origonal venturi shape, dont be tempeted to bore bigger than 42-43mm otherwise you loose masses of low end torque, and have to fit massive main jets and still wont gain any top end power

[R59]

Originally Posted by graham bahr

chokes are easily bored in a lathe, but wont work aswell if you dont retain the origonal venturi shape, dont be tempeted to bore bigger than 42-43mm otherwise you loose masses of low end torque, and have to fit massive main jets and still wont gain any top end power

Absolutely, you must be careful not to overdo it, or the venturi effect won't work, and thus the carb won't work. The Weber handbook, and again, some of Vizard's books, give simple advice on how big you can go with each size/type of webster carburettor.

That's the beauty with fool injection. No chokes to worry about - just big holes!!

Rob.

[Al Weyman]

It is just under 5.8 litres or 355ci Graham. As for lathing, I would not be up to that to be honest, I don't have the gear or the skills. Yes I understand the fuel injection situation as it does not need any vacuum to draw the fuel through like carbs.

[graham bahr]

5.8 litre, thats about 725cc per cyl, hmmm 40mm chokes will probably let the engine run to around 7000 before they start to stave it of air, all depending on how highly tuned and what sort of power your expecting, if the rest of the engines upto it it should go over 400bhp but i think you'll find you want bigger chokes if your looking for 500+ brake

i could be wrong as i have now real experience of quad carbs on a V8 but i've lots of carbs on 4 pots and afterall a V8 is only two 4's stuck together

[graham bahr]

just how many cars have you got Al? it seems to be about 20,

when you gonna bring one out to play with the TRC brigade
a nice V8 rumble will make a change from Zillion rpm screaming 4 pots and wooshing dump valves

[Al Weyman]

Actually according to our friend Vizard a V8 in fact is more akin to 4 x V2's stuck together than 2 straight 4 cylinders. I think the difference is important as unlike two 4 pots as I understand it more than one cylinder is on the downstroke drawing air fuel mixture at any one time which puts the equations out a bit. It is a very high spec engine in this car, all steel, all roller, built by Clive Bond of drag car fame and is a proper bit of kit, not like the other car which I have done for the CTCRC pre 73's which is more basic. The all singing engine should be making 500 + bhp as is.

How many cars do I have? Well I have the black one in my avatar, I have the pre-73 2bd gen that I am just finishing off now and I have a genuine Penske IROC spaceframe/tubular chassis racecar from the 70's which is currently in bits. I was going to give the pre 73 spec car a run out with you but to be honest I think I will be swamped and therefore am hoping to get it out where it belongs in the CTCRC pre 73 at Donnington and have now registered with them and the BARC. I don't expect it to win or anything like that but at least I know the spec of the car has the potential to get up there even if I don't and to be honest I could never stand the thought or in fact see the point of entering a race in the full knowlaedge you don't stand a chance and are just going to be making up the numbers.

Having said all that the black car may well be more competitive in the TRC as a class B car so maybe next year but the only thing that worries me a bit is tyre choice and the use of pure race rubber, maybe a control tyre would be better, what do you guys use and what are your thoughts on tyres in the series?

[graham bahr]

Originally Posted by Al Weyman

what do you guys use and what are your thoughts on tyres in the series?

just about all of us run slicks, although some of the guys run sticky list 1B rubber, although if you run in a roadgoing class ( very few do ) its list 1a rubber.

most of the guys on slicks are running secondhand ex TVR, Clio or Touring car rubber

personally i hate the idea of a control tyre, it takes away choice in terms of both what your using and how much you spend, it would also be difficult to arrange with so many different wheel sizes in use from 13inch all the way upto 19inch with just about the same variations in width

[graham bahr]

[QUOTE=Al Weyman]Actually according to our friend Vizard a V8 in fact is more akin to 4 x V2's stuck together than 2 straight 4 cylinders. QUOTE]

yes i can see that, BUT when fitting 4x48's each cylinder gets its own carb in effect so, as far as the inductions concerned the engine actually becomes
8 x single cylinder engines, which what a 4 pot on 48's becomes, single cylinders, so the carb sizing etc should carry over.