40 DCOEs anygood for Sprint?

Message

Raider · #1 Post by **Raider** » Fri Aug 17, 2007 9:08 am

Hi, I have a Sprint engined TR7 currently on SUs.

I also have an inlet manifold for webers

I now have the opportunity to buy a pair of 40DCOEs for Â£130 - off a Ford Pinto engine.

Are 40s OK only I thought 45s would be the optimum?

tinweevil · #2 Post by **tinweevil** » Fri Aug 17, 2007 11:12 am

You only need 45s for bleeding edge power levels on the lariest of cams.

Raider · #3 Post by **Raider** » Fri Aug 17, 2007 12:14 pm

Oh thanks for that.

My spec is for about 160BHP - fast road really tho I will use the car on tarmac stage rallies.

The 40s should be OK then - they come with trumpets and a Ford manifold which I will sell on e-bay

DavePoth · #4 Post by **DavePoth** » Fri Aug 17, 2007 12:54 pm

The SUs will be OK for 160bhp, a pair of them flows enough mixture to get 165 out of a rover V8 in a P5B.

Raider · #5 Post by **Raider** » Fri Aug 17, 2007 6:46 pm

Oh well I have bought them now as I thought the price was a good one anyway.

This is how they look up against a manifold I bought on ebay a couple of years ago

I am in no rush to bolt them onto the car

[/img]

DavePoth · #6 Post by **DavePoth** » Fri Aug 17, 2007 11:12 pm

I suppose there are few things better than twin dual choke webers under the bonnet.

p collins · #7 Post by **p collins** » Sat Aug 18, 2007 2:01 am

So you have decided to restrict your engine buy fitting these 40s
1.75 su
1.6, 40dcoe
Even though you now have 4 seperate chokes of 40mm there is only one piston at a time drawing air for 2 banks
The su that is fitted only sees one piston drawing on it at a time so you have lost 0.15 of area
You will get better throttle response but it will drive like its been THROTTLED

Raider · #8 Post by **Raider** » Sat Aug 18, 2007 8:21 am

Hmm - interesting.

I can always put them on ebay next time it's free listing day.

Or they could go on my Toledo - they must be an advantage over a single 1 1/2" SU

In the meantime I am interested in learning more - if I understand correctly a 45 DCOE would only be as good as a 1 3/4" SU too

DavePoth · #9 Post by **DavePoth** » Sat Aug 18, 2007 11:21 am

Well, sort of. the Flow through a weber is smoother, so it can flow a bit more air than an SU. But power wise you won't be able to get much more out of 40 DCOEs than you do out of SUs, and fuel economy will drop, especially around town, as that sort of driving is not the DCOE's fortÃ©.

Raider · #10 Post by **Raider** » Sat Aug 18, 2007 12:37 pm

If there is only one piston drawing air at any one time, doesn't the logic of that mean that only one carb is ever needed

I mean why would anyone fit twin SUs? Why not just a big single SU?

At that rate a 2" SU should be more than enough for most tuning levels

DavePoth · #11 Post by **DavePoth** » Sat Aug 18, 2007 1:28 pm

Raider wrote:If there is only one piston drawing air at any one time, doesn't the logic of that mean that only one carb is ever needed

I mean why would anyone fit twin SUs? Why not just a big single SU?

At that rate a 2" SU should be more than enough for most tuning levels

On the intake cycle, the piston moves down and creats a vacuum in the cylinder which sucks the air/fuel mixture through the carb and into the engine. To a point, the bigger the carb, the more air/fuel can get into the engine, and the more power you will get.

However, if you make the carb too big, the vacuum that occurs on the intake cycle will not be enough to suck the mixture through the carb, which will result in bad running, especially at low revs. A pair (or more) of carbs allows the engine to get more air and fuel, whilst retaining sensible sized intakes to keep the vacuum good for better torque.

As regards the SU/Weber thing, with twin SUs, each cylinder will see a 45mm intake. A bit of turbulence is created at the point the intake runner joins the one for the cylinder next to it, which robs some power, but there is a vacuum in that pair of intake runners for the majority of the time, which means that the carb works more efficiently.

For twin dual-choke webers, for argument's sake 45mm ones, each cylinder also sees a 45mm intake. However, because each cylinder has it's own intake, for at least some of the cycle the carb is dumping fuel into an intake with no vacuum, which is going to reduce the economy of the engine. However, because the intake runners run direct to one of the car venturis, the flow is smoother, which makes more power.

Well, that's my opinion anyway.

Raider · #12 Post by **Raider** » Sat Aug 18, 2007 1:53 pm

Blimey - it's complicated this stuff innit

Dollyboy · #13 Post by **Dollyboy** » Sat Aug 18, 2007 10:45 pm

does my head in all this, i couldnt have explained it better than that.

i have twin 45 webers on my sprint engine although the car is still in bits.
some racing sprints have 48's.

want to melt your brain? ok

basically the bigger the throttle body, the more fuel and air you can make available for the motor to suck in. the bigger the volume of fuel/air mix you burn, the bigger the 'bang' therefore more power. also in terms of compression, the more compression you have, the bigger the bang. theres a fine line to get to the optimum, and of course the optimum for a race/sport motor is different to a road motor.

then as your mate dave indicated on CT, efficient gas flow in and out of the head can make a lot of difference, but taking a DIY approach too far can do more harm than good, porting is nice but trying to keep it all evenly matched is important for balance.

adjust one thing, it has an effect on another, getting on to jets and needles and air filters and mixture etc

cold air has more oxygen in it which is why air boxes usually have ducting to the fresh air at the front of the car. sticking K&N's or just ram pipes can have the opposite to the desired effect, they de-restrict the airflow into the carb so you get more air, but if you modify the airflow into the carb and you need to modify the amount of fuel to compensate.

get it right and you will enjoy the benefit of optimum power/torque/efficiency, but get it wrong and it'll reduce your brain to jelly and send you running home crying 'mummy mummy, can i have a few hundred pounds to take my car to a rolling road so i can pay a nice man to set my car up for me' which is what i reckon is the best thing to do in the first place

tinweevil · #14 Post by **tinweevil** » Sun Aug 19, 2007 11:46 am

If there is only one piston drawing air at any one time, doesn't the logic of that mean that only one carb is ever needed

That is a question that has been bothering me all weekend. Only ever one piston on the induction downstroke so why have 2 carbs? You could almost argue that with all 4 pots sucking through one carb the flow would be smoother, less pulsed. I fear the answer will lie in the world of fluid dynamics and that will most certainly make my brain hurt.

DavePoth · #15 Post by **DavePoth** » Sun Aug 19, 2007 11:51 am

tinweevil wrote: That is a question that has been bothering me all weekend. Only ever one piston on the induction downstroke so why have 2 carbs? You could almost argue that with all 4 pots sucking through one carb the flow would be smoother, less pulsed. I fear the answer will lie in the world of fluid dynamics and that will most certainly make my brain hurt.

As I said above, I believe the reason is that at low revs the engine isn't able to create enough vacuum to suck through the carb, which will reduce torque and ruin the low end driveability. Twin (or multiple) carbs give you the benefit of increased intake area, without the problems of intake vacuum. At points of the cycle, I think you will end up with two cylinders on the induction cycle, although at opposite ends of it. Having one carb in this situation will result in the A/F mixture fluctuating a bit too much to be good.