40 DCOEs anygood for Sprint?

[p collins]

Multiple throttles are by far better if matched to the engine correctly
Give better response and torque
40s will give you brilliant throttle response
Then restrict it like a noose

[Dollyboy]

in my experience with bike carbs, in simple terms the effect you get with smaller carbs in relation to engine size is good power and throttle response lower down the rev range n running out of puff higher up.

the bigger the carbs you fit, the higher up the rev range the power/throttle response moves. it stands to reason that people race dolo sprints on weber 48's with lairy cams as the cars run at high rpm. the disadvantage with bigger carbs is that you're bordering on flooding (engine cant accept the amount of fuel/air available fast enough) if you open the throttles too much at lower rpm, so you need to open them gradually. therefore the optimum is something somewhere in the middle for road/fast road cars

thats my little proven theory anyway...

[Raider]

in my experience with bike carbs, in simple terms the effect you get with smaller carbs in relation to engine size is good power and throttle response lower down the rev range

You know, that's probably just what I want.

My car is being built for tarmac rallying - the Manx in particular.

Unlike track days or racing, there is no way I can learn the corners etc sufficiently well to carry much speed through them and I wouldn't be good enough anyway

So, I'll be slow into corners probably braking way too early and will then need good throttle response to pull away again.

I rather suspect I won't be flat in 5th for any length of time either

So, throttle response from low speed is what I am after. The engine is not being built for ultimate horsepower anyway as I can't afford it and probably couldn't make the best use of it.

Reliabaility based on the "to finish first, first you have to finish" principle is a priority.

There is one other thing - my co-driver is likely to be my daughter so there's no way I'll be taking unecessary risks. Here's us on the Speedfair rally at Knockhill this year http://www.youtube.com/watch?v=lwkVeXr5fvI

And here's a chance I had to go up "Rest and be thankful" on this year's "La Carrera"

http://www.youtube.com/watch?v=mg4RkNb0qcc

Now I have a Toledo I feel it's OK to inflict a TR7 on you all

[DavePoth]

I reckon the HS6s will be able to do the job for you. They are also a much nicer carb to drive "gently".

[Raider]

Well, I also have a pair of 2" SUs on a manifold too

[Mr.Speedy]

Well, I also have a pair of 2" SUs on a manifold too

That's what I'll be using when I eventually get round to building my next engine!

[iandollysprint]

2 inch su's for me aswell. In combination with a TT10105 fast road cam should do the business. Cam is active between 2500 and 6250 rpm so it's not a lairy cam.

6 weeks to get the engine back in with the above I hope. Just waiting on an adjustable cam sprocket from Sprintparts.

[SPRINTPARTS]

Back in the past when we ran my Broadspeed Sprint as an FIA Group A car I removed the 48 DCOEs and fitted 2" SUs with no other changes. The engine performance was totally transformed. The 48 DCOEs were tractable from 2500rpm whilst the SUs had no torque or power below 4500rpm and then reved like stink to 7500rpm. We had similar experiences with a Mini Cooper S when we converted it from SUs- no driveability below 3500rpm with SUs but would pull from 1500rpm with a 40 DCOE.

For low speed driveability I would choose Webers over SUs every time.

Philip

[Lee Flintoft]

Sorry to re-raise an old topic, but i've read through comments and have taken points on board.

Firstly, all I intend to do with my car is give it a bit more poke, lets say from set off.

I have a spare engine, but do'nt know it's history. We have it turned over but not started.

Perhaps look to change pistons? Re- bore? New bearings? Any advice on compression testing would be appreciated.

What i've got locked in the cupboard, new vernier, separate head currently having what we think is a TT10106 (cam bought from Dave AKA sprint36) installed with shims (some upto 4mm thick!! being made at Jigsaw) 10 or 11 thou inlet, 11 or 12 thou outlet?. Competition head gasket from Jigsaw (looks like the head has been skimmed). Manifold with 40 DHLA dellorto's (should I be looking at 45's) Would they be better for the cam for RPM. Also got Facet fuel pump.

Any advice appreciated

[Mad Mart]

If you can turn the engine over then yes do a compression test on it before you dismantle it. Then strip it & check the condition of the bores & pistons. These may well be OK.

So the cam is a reprofiled jobby (I used to have a few shims at 5mm.) Are you sure you need a comp. h/gasket? depends how much has been taken off the head.

With that cam I would run 45s, it will run OK with the 40s but you can unleash a few more horses with the 45s.

[tinweevil]

My impression is that you might be over egging the omlette. 106 cams and sidedraughts don't really equate to 'a bit more poke'. A lot more poke yes but not a bit. I agree with Mart, that cam is more suited to 45s but is that what you want? It's not gonna be a nice machine to pootle to the shops in. Blast there very noisily on empty roads at 3am, oh yes please, but not pootle. Idling will be lumpy, you'll need to rev it for it to start producing power, fuel economy will be down as will reliability if you use what its got.

Stick with standard carbs and get a billet TT10104 if you want it a bit quicker but still very user friendly.
Go for a billet TT10105 with standard carbs and the 40s later on to make it a bit of an animal.
Keep the 106 for when you fancy building a track day weapon.

IMO.

[trackerjack]

I have 45 Dellorto's on my sprint and I find them very very good.

Loads of low down torque (when compared with 2" SU's which is what they replaced).

Not a lot of difference at top end just a lot more drivable.

28 mpg is a normal fuel consumption.

I have also tried bike carbs Kehlins 38mm (Kwacker XR6) these carbs are brilliant but hard to start car and needed sorting on a rolling road, others who have kit cars using them say they are more economical.