RoyM
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| posted on 15/2/16 at 04:16 PM |
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Xflow induction
Looking for some thoughts / guidance please.
I have a Marcos GT I am rebuilding. Standard fit is a Xflow and I have a 1700, 234 cam, Vulcan Maxiflow coming which I want to fit. Basically
I'm, unsure how to fuel it!
I've collected over the years the following:
Twin 40 DCOE webers (matched) - condition unknown but look fine and throttles etc smooth
Twin 40DHLA Dellortos (matched) - known good, all smooth etc
Twin 45DHLA Dellortos (matched) - known good, very low miles
For the above I have a matching intake manifold but I also have a Lynx crossover manifold for a single one of the above.
28/36 DCD Downdraught on suitable manifold (Low miles)
For those of you who don't know it the wooden chassis Marcos has a narrow engine bay and low bonnet line - I will probably have to play about
with engine mounts for twin sidedraughts (to avoid steering linkage) but that is not an issue. The exhaust manifold is a 4-2-1 Marcos original which I
will retain - fairly good system but probably not the greatest flow. Car has achieved 'classic' status so feeling is to go for something
'period' rather than ITB's etc (and my pockets are not that deep). Main aim is tractability rather than outright power but I also
want it all to be reliable - don't want to spend valuable driving time getting it started. Car will probably be about 700kg when finished.
Haven't decided on ignition system yet but likely to be a Nodiz or Megajolt.
Any thoughts on which system would be best to use? - I quite fancy tying the Lynx manifold but have read differing reports and have no idea how you
jet a single sidedraught whereas there's loads of info on twins everywhere.
Thanks
Roy
[Edited on 15/2/16 by RoyM]
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britishtrident
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| posted on 15/2/16 at 04:30 PM |
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ISTR many years ago the great guru of head mods David Vizard did a test on the Lynx manifold and was not impressed ISTR he regards the standard Ford
Weber downdraught was better.
Perhaps someone will confirm ?
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Paul Turner
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| posted on 15/2/16 at 04:53 PM |
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I ran a x-flow that was an almost identical spec to yours for several years and fuelled it using 2 x 40 Webers fitted with (I think) 32 mm chokes.
Produced about 130 bhp and ran very well in a Caterham. In your slightly heavier car I would expect it to be fine.
Only know one person who ran a 28/36 on his Caterham which was a 1600 with an A2 cam. He raced it and all his competitors were using twin 40's
with 30mm chokes. He won the championship at a canter, a brilliant combination.
But with a 234 there is no way you will get the best out of it with anything other than the 40's.
No experience of Dellortos but a set in good condition is every bit as good as Webers, some will say better.
Forget the 45's, totally unnecessary on your engine.
No experience of a Lynx manifold but a 28/36 will have virtually the same choke sizes as a single 40.
Whatever you use make sure you take it to a rolling road where the operator is totally familiar with your carbs, a waste of money if they are not.
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steve m
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| posted on 15/2/16 at 07:17 PM |
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I run a 28/36 dcd on a 1660 xflow. and thinks its fine
ive got a pair of 40 webers as well, but fuel economy was around 10 mpg at full throttle, and starting , balancing etc was always
a pita, so back to the 28/36 dcd, and happy days, plus economy of around 28 now is soo much better
as for driving, I prefer the downdraft, as its easier to drive, and doubt there was much difference in the power
I had a Marcus GT, and had an Essex V6 in it, nice car!
steve
Thats was probably spelt wrong, or had some grammer, that the "grammer police have to have a moan at
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sprintB+
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| posted on 15/2/16 at 07:34 PM |
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I run a crossflow on a 28/36, good old carb. Have a Lynx swan neck with a 45 DHLA Its now an "ornament" in the mancave, it didn't
impress me much. Drank juice though !!
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rusty nuts
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| posted on 15/2/16 at 08:20 PM |
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Similar spec to my Xflow which is running on a set of Kawasaki TBs on a home made inlet manifold controlled by an Emerald ECU , pulls like a train
from 10MPH in fifth gear and totally tractable . I for one would not go back to carbs after running full engine management . I believe it's
possible to use Weber side draughts as TBs o look period but with all the benefits of engine management?
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arrow-engineering
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| posted on 15/2/16 at 10:40 PM |
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always makes me laugh when i hear people complaining about fuel economy when they fitted dcoes, its not the carbs fault that the economy is poor, its
the setting up/jetting/ignition. ignition is very important especially on light throttle, dcoes generally run vac-less dizzys and this kills economy
more than anything.
if the mixture is correct then the economy will be the same no matter what, in fact dcoes are able to distribute the fuel much more accurately than a
single carb.
my advice is if you want the period look then you have two choices, go to northampton motorsport and ask about the dcoi they do, its a set of throttle
bodies that look just like webbers. or go for a set of 40 dcoes with 32mm chokes and mapped ignition.
just bought the engine off ebay then???..... if its the one im thinking of he was selling all the ignition set up as well
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Adamirish
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| posted on 16/2/16 at 12:00 AM |
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I agree with the above.
My Xflow gets around 28mpg. 1700, 40 dcoe's, 34mm chokes, big valves, headwork, 244 cam. Edis coilpack run by an omex 100. Would no doubt do
more if I stayed off the loud pedal. I wouldn't care if it did 10mpg, it just sounds so good!
MK Indy 1700 Xflow
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johnemms
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| posted on 16/2/16 at 10:34 AM |
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Bike Carbs
I used to have twin 40's and 32 chokes on my 1700 Xflow...
However... if you got some 40 + Bike carbs with slide n needle ... say £60..
The interesting thing is the choke venturi size varies as the slider moves up and down..
So.. You have a variable choke to suit all conditions giving great torque across the range ..
I could be talking b***cks lol...
[Edited on 16/2/16 by johnemms]
Own chassis & Build - First time pass!!
"7's" aren't really "cars", they are 'experiences"
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David Jenkins
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| posted on 16/2/16 at 10:59 AM |
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I used to have a Weber 32/36 DGV down-draft carb on my x-flow, but it was old and worn out so I took the plunge and fitted bike carbs. In many ways
the engine now runs better - smoother running, more torque at low revs, better acceleration overall - but I do miss the way the engine responded when
fitted with the Weber carb. In particular I liked the way the power kicked in when you put your foot down sharply, due to the acceleration jets in
the carb; on several occasions I squeaked the tyres at 40mph in 3rd! The acceleration with bike carbs is better overall, coming in smoother, over a
wider rev band.
Also - getting bike carbs tuned properly is far more expensive than a single down-draft carb, by a factor of 5 - around £250 for the bike carbs
against £50 or so for the Weber.
Sometimes I wish I'd just bought a new Weber carb as a direct replacement, and got it tuned properly...
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Paul Turner
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| posted on 16/2/16 at 11:14 AM |
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My 1700 x-flow, 235 cam etc using 2 x 40's 32mm chokes from memory averaged about 28mpg. But that included some sprints. On a day out with good
traffic flow it would do 35mpg.
Balance on Webers is not an issue, if it is the carbs are knackered. Check it once a year and adjust if necessary. A simple airflow meter is all
that's needed. Once set up correctly the jetting will never need touching provided you don't mod the engine.
A 3d mapped ignition makes a huge difference over the dizzy set up. No more ultimate power but far better drivability and fuel economy. Wish it had
been available when I had my 1700.
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RoyM
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| posted on 20/2/16 at 12:04 PM |
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Thanks everyone - looks like the 40 dellortos with 32 chokes is the way to go. I am looking at the possibility of bike carbs- got some good info from
Dan at danst so will be checking clearances etc once the block is on and set to final position. Really appreciate all the help
Roy
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mark chandler
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| posted on 20/2/16 at 12:19 PM |
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Bike carbs or even a set of SU's should not be underestimated.
Webers usually feel great as the airflow is low in the Venturi until you are flat out so the fuel is nit atomised as well as could be, the result
being the cars acceleration builds unless they are set up perfectly.
Put on a 32/36 and partial throttle provides high velocity air through open Venturi.
Bike carbs or SU's have a constant velocity as the damper moves up and down so no flat spots but feels slightly less entertaining to drive
because you do not get the jolt when booting it.
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RoyM
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| posted on 20/2/16 at 12:30 PM |
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Issue with bike carbs for me is bonnet clearance and cost (as I already have the dellortos). Will check bonnet clearance once engine is in and then
see how the funds are! Would actually prefer the sort of progression you discuss with bike carbs. I fortunate in having two ( and shortly 3)
crossflows of different specs and conditions so could have a "classic" looker and an everyday (or every other weekend!) Performer.
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britishtrident
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| posted on 20/2/16 at 02:01 PM |
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Proper modern mapped ignition makes a huge difference to starting on these engines. Ford used 10 degrees centrifugal advance distributes (20 crank
degrees) on tuned engines this usually resulted in the static ignition timing being to advanced at cranking speed.
Jetted correctly Weber carbs give good economy a Lotus Plus2S/130 would give 40 mpg plus on the motorway.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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