Delloto 45 DHLAe quandary

[vxah]

I have a set of these and am considering fitting them to my S4 instead of the 40 DCOE 31’s it’s currently running.
Reason being that although the engine pulls really well mid to high rpm I just can’t get a proper smooth progression and using a wide band o2 sensor I can see the rich and lean “pot holes” in the lower throttle openings. I guess I could put more time into this to try and improve it but I’m also thinking there might be performance improvements to be had?
I say this because the DCOE’s have 34mm chokes fitted which I understand is about the maximum for 40mm carbs, these are recommended by QED for the Q420 cams. Would the 45’s flow better with the same size chokes? Or could I go to 36mm which is an option listed by QED, without losing mid range?
The 45’s also have six progression holes instead of the three of the Webbers.
Engine is fairly fresh, steel bottom end, Nick Stagg ported head with oversized valves and 45mm intake openings, 10.5 compression, Q420 cams, mappable ignition curve and large air box.
What would you tuning folks recommend?

[mbell]

I think the dhla are probably a better carb. Whether I'd swap works depend on laid off factors including:
- condition of both sets
- how many spare parts I had for each
- How much I might spend on parts to get each carb tuned
- if I felt is exhausted the options on the dcoe
....

Generally low and mid-range is helped by small chokes as increases air speed through the carb. If different jets and other tuning changes haven't sorted it I would be tempted to try a smaller choke but it's a trade between better slower running and high rpm power.

[bitsobrits]

I'm running Dellorto 45s on my 1720cc TC, 10.3 CR, custom cams. The 6 hole progression circuit works well in managing the idle to main transition. Once I got all the jets/air bleeds squared away I have a hiccup free engine from idle to redline. 35MM chokes, btw. I've had a few cars with Webers, and never managed to get rid of the brief cut out on idle to main transition on light throttle tip in. Never tried drilling additional progression holes, which some say has helped that situation for them.

[rgh0]

I have a set of these and am considering fitting them to my S4 instead of the 40 DCOE 31’s it’s currently running.
Reason being that although the engine pulls really well mid to high rpm I just can’t get a proper smooth progression and using a wide band o2 sensor I can see the rich and lean “pot holes” in the lower throttle openings. I guess I could put more time into this to try and improve it but I’m also thinking there might be performance improvements to be had?
I say this because the DCOE’s have 34mm chokes fitted which I understand is about the maximum for 40mm carbs, these are recommended by QED for the Q420 cams. Would the 45’s flow better with the same size chokes? Or could I go to 36mm which is an option listed by QED, without losing mid range?
The 45’s also have six progression holes instead of the three of the Webbers.
Engine is fairly fresh, steel bottom end, Nick Stagg ported head with oversized valves and 45mm intake openings, 10.5 compression, Q420 cams, mappable ignition curve and large air box.
What would you tuning folks recommend?

The options are endless and generally tuning of Webers easiers than Dellortos. 40's in either with 34 mm chokes is good for your engine if setup right and in good condition and for road use.

Keith Franks Weber emulsion tubes and jets enable better / easier tuning than the standard weber ones if your doing it yourself see his webstore link below. I swapped the Dellortos off my 73 Plus 2 back to Webers as had more bits and dyno people had more bits for Webers to set them up right. Though I see Keith has recently started making Dellorto jetting also

http://Webstore.com/~DCOE_Tuner

cheers
Rohan

[1owner69Elan]

The options are endless and generally tuning of Webers easiers than Dellortos. 40's in either with 34 mm chokes is good for your engine if setup right and in good condition and for road use.

cheers
Rohan

....

Generally low and mid-range is helped by small chokes as increases air speed through the carb. If different jets and other tuning changes haven't sorted it I would be tempted to try a smaller choke but it's a trade between better slower running and high rpm power.

As per the above, going with a larger carb and larger chokes is probably not the right direction for a road car. I run 40DCOE's - 151 with 32 mm chokes. Lots of low, mid range dynamic response and no transition "holes". Also, no sacrifice on high end power/torque. But, the latter might be a consequence of other factors (choice of cams, SAS well ported head, ..)

[vxah]

Thanks for the thoughts on this folks, I don’t think the “holes” in the progression are at the transition to the mains as it seems to early on in the throttle opening, I think it could be moving between the progression holes and not quite having the correct low speed fuel and air jet settings. I guess this is where Keith’s jet system would come into play?

I’m not sure that I would want to reduce the choke size to less than 34mm as I can and do exceed 7k rpm so that might reduce the top end a little?
Does anyone have any thoughts about the effects of the step from 40mm carb bore to the 45mm intake runner opening? This is one of the motivations to swap the carbs even if I keep to 34mm choke size.

[rgh0]

Thanks for the thoughts on this folks, I don’t think the “holes” in the progression are at the transition to the mains as it seems to early on in the throttle opening, I think it could be moving between the progression holes and not quite having the correct low speed fuel and air jet settings. I guess this is where Keith’s jet system would come into play?

I’m not sure that I would want to reduce the choke size to less than 34mm as I can and do exceed 7k rpm so that might reduce the top end a little?
Does anyone have any thoughts about the effects of the step from 40mm carb bore to the 45mm intake runner opening? This is one of the motivations to swap the carbs even if I keep to 34mm choke size.

If going above 34 mm to say 35mm to 36mm in a road engine you certainly need to go to 45's ( I use 38mm chokes in my competition engine with 45's) . At 34mm it sort of a moot point as to whether 40's or 45's are better at it probably depends on how you intakes have been ported and whether the intake flanges have been opened to suit 45's. At 33mm or less definitely stick with 40's

cheers
Rohan

[vxah]

Have to say the head looks really well ported (probably too much for the road) it has 45mm openings to the intake ports, that’s what leans me towards the 45’s

[2cams70]

If the flanges on your head have been machined out and head ported to suit 45’s that step will most definitely have an adverse effect on flow if you are only using 40’s instead of 45’s. Fuel coming out of the progression holes which are located on the circumference of the carburettor bore will hit that step as though it were a brick wall!!

[vxah]

Are you thinking about that the wrong way around?

[rgh0]

Are you thinking about that the wrong way around?

Yes a step up is not that much of a problem, but it does have some effect on the reflected wave patterns, but its a step down you really dont want.

It does depend on how well the inlet tracts have been opened up to suit 45's as its hard to do that well with the standard original head casting as you get very thin on the tracts as you go down the inlets unless you do a taper that is really to sharp. The original Hart 416B heads porting did it well but they did not leave much room for error and I think they were considered to have a very short usable life also .... Much more easily doable in the new cast big inlet tract heads or the McCoy conversion

cheers
Rohan

[2cams70]

Are you thinking about that the wrong way around?

Aaah yes - too many wines! It will still have an adverse effect on flow though and definitely not recommended.

[Andy8421]

Are you thinking about that the wrong way around?

Yes a step up is not that much of a problem, but it does have some effect on the reflected wave patterns, but its a step down you really dont want.

Rohan

Rohan,

I recall reading a David Vizard book where he was recommending a step down in inlet port design. My memory is foggy now, and I can't find the book, but he was suggesting that a step down reduced inlet reversion during valve overlap, and also helped any fuel that had condensed on the port walls to re-enter the airstream.

My own view is that this is all very case specific, and what works in one situation may not work in others.

In these days of CFD, it may be possible to model port design for best flow, but the dynamic flow characteristics are extremely complex multi-wave patterns, and it may just be easier to stick it on a dyno and see what happens.

Andy.

[rgh0]

Hi Andy
Wave effects in inlets and exhaust get very complex and there certainly could be circumstances where a step down in diameter at the carb interface may help but I don't think this would be beneficial in most twin cams. But like you say you can debate lots of the points but sticking it on a dyno will certainly tell you

cheers
Rohan

[c42]

I recall reading a David Vizard book where he was recommending a step down in inlet port design. My memory is foggy now, and I can't find the book, but he was suggesting that a step down reduced inlet reversion during valve overlap, and also helped any fuel that had condensed on the port walls to re-enter the airstream.

I did notice that Vizard now has a series of videos on Youtube if you wanted to find more information.
Cheers
John