VCCA Home Forums TECHNICAL - 1945 AND EARLIER Chevrolet Speedsters 28 special Carby options

I've decided this group needs a bit of activity, I'm putting together my 28 Chev special finally , and first race is Sept 22nd. I'm not going to get all I want done by then, but I do want to try a pair of 1-1/4" SU carbies on her , would I be best with a common inlet manifold or just go one Carby per port? As the 4 cylinders fire 12-43 I wondered if the second cycle would not be pulling enough fuel because of this configuration. Ray

Some pics of the progress. Ray Raycycled.com

Ever hear of carb venturi sleeves to control the air/gas intake on more than one carb intake?
Frank Whitby makes them (he resides across the pond). I have them on a pair of one bbl Strombergs. He is very knowledgeable.

Venturi sleeves from Frank Whitby ( mango235@yahoo.com.au).

While I havent read the article on the sleeves what I know of them they are for fixed venturi carburettors but the SU and CD Strombergs are variable venturi controlled by the throttle so I doubt they would work in this case.
Tony

Not heard of them, have U got a link to some technical info on them? I'm more concerned with getting even amount of vapour to the 2 and 3rd cylinders as they are the second draft on each port due to the Chev 4's unique firing order. My thinking, Pros; if I have a dual manifold then there is 2 carbies worth of vapour for every stroke, but the con is that getting the balance on the carbies is much harder so that they draw an even mixture . Also if the carbies are mounted direct to each port they won't protrude past the bonnet, Ray

If you plan on using twin carbs I would use 1 carb for cyl 1 & 4 and the other for 2 & 3 with all tubes the same length or both carbs fully linked for all cylinders.
Tony

Hi Tony, it would have to be for all cylinders then, there are only two ports , one for 1-2 and one for 3-4, the Great Fangio won his first series in A 1928 Chev and had it doing over 100 mph, though I've yet to find details on what was done to the motor. But Carburation is most definitely a big factor. Ray

(Opinion only, I have never built a dual manifold for a 1928 4-cylinder).

I believe you need a single manifold with two interconnected plenums. Each carb attached to a plenum, and each plenum attached to a port. The interconnection tube should be roughly half the diameter of the runner diameter. Attaching each carb directly to a port, I think, will require significantly larger carbs. Research the difference in CFM requirements for a plenum intake manifold versus an individual runner manifold.

The old standby equation:

CFM = [CID times RPM] divided by 3456

for CFM is for a four-stroke multi-cylinder engine of at least 4 cylinders. Running the carbs attached directly to the ports would require carburetors capable of twice the CFM of those used in a plenum arrangement.

Jon.

Thanks for that Jon, what's CID stand for , Cubic Inch Displacment? here are a few pics of the manifold I made I've got to make up some linkages but getting there , Ray
www.shop.raycycled.com/wp-content/uploads/2017/08/IMG_4088.jpg

That manifold looks good though I would have used plates about double thickness where it bolts to the head, to reduce the possibility of warpage.
CID is Cubic Inch Displacement
Tony

hmm Yes Tony you might have a point, I made it from a bronze sink waste pipe and some bronze plate I had. Ok so I'm running around 140cfm at 2000rpm I'd doubt I'll get more than 3000 rpm, these are not a high revving motor. I've not got the capacity for a pair of
1-1/8" SU Carbies as yet but anything is an improvement on the 1" updraft Carter! I'm also guessing my vacuum tank might not be up to the task though, I've run her at 70mph for over 5 miles with the Carter and no fuel issues , Ray

If the plates are brass they may not warp as much, I was looking at the weight and heat issue allowing the carbs to droop pulling the top of the plates away from the head though it wont happen on the first run.
Tony

Evening one and all, just a pic of today's work starting to shape the scuttle, seats installed,
4 weeks to race day !
[img:left]http://www.shop.raycycled.com/?p=96...rd&_thumbnail_id=-1&preview=true[/img]

Getting message " Sorry you are not allowed to view drafts" when clicking on link.

Oh that's odd I'll fix it, Ray

Try this link folks, pics of body progress, and shaping scuttle. Ray
[img:right]http://www.shop.raycycled.com/?p=961[/img]

OK Carbies working , Sort of, after 2 different size main jets and 3 different main needles, have it running, revs out nicely bit rich but getting there. Ray

Morning All, I think I have a big problem, 4 days till race and my rear main bearing is very sloppy, dropped the pan and removed the rear cap, to find this Cracked and loose Journal, Im not prepared to take out the crank to get to the top one at this time so. The big question is : I have a bearing half from another motor should i risk putting it in and getting as snug a fit as possible, as there is no time or budget to do the right job before Saturday, Thoughts please , Ray

I believe that if it's worth doing then it is worth doing right.

True Steve but time does not permit, its been a year getting ready for this race and all i want is to run, if it claps out so be it, I can pull it down and rebuild next week!, Ray

With your situation I would try it. You will find out quickly if the replacement bearing is too tight. May not find out if it is too loose. Might need to take out shims to kinda get an idea of the gap. You will know if you take out too many as the crankshaft wont want to rotate. Put back just enough shims so it rotates and run it. Besides the majority of the force on the main bearings is on the bottom of the bearings. Top for rods but bottom for main bearings.

You are somewhere between the falling rock and a hard place. Personally I would not risk either option considering the damage that could result if it lets go. I dont think 28 engines are all that common these days and at least what you now have is definitely repairable but it may not be if it all goes pear shaped.
Tony

Well two days till race day and Murphy struck with a vengeance! In the last 3 days I've had to replace the rear main bearing with engine in car as I can't use my hoist! All day job . Then apon getting the gear box out found my needle roller universal joint conversion was slipping. Back to the original sloppy version, then on reassembly couldn't crank it over , my mind went nuts, it would only start or crank over with Clutch depressed, much like its owner at this point, turned out the bush where the top gear box shaft fits in to the spline shaft had decided to cease meat minutes before,! so gear box back out , throw the bell housing and uni on to a old gear box and reassemble for the 3 time in as many days, fired her up an hour ago and picked up the kids from school , so far so good. Not got the twin SU carbies right just yet but that was another day operation getting the needles and main jets somewhere close even to get it to start, More pics to come

Stay in there. We often do our best when challenged.

I like those SU carbs. There is a fellow in Portland who has cast aluminum flange adapters similar to those for sale. He laid out the castings for a Model A but since they come un drilled they could be drilled for other sizes It is also set up for a balancing tube between the two bases.