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More on SU Carburetors

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OZ_BN1

OZ_BN1

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Hi All

I am new to the list. I have a question on SU carburetors. I have been thinking about making an adapter for my existing 100/4 manifold so that I can fit 1 3/4" SU's. I could buy the 100M type manifold but thought why not make an adapting block. I have a set of 1 3/4 HD6 SU's. What are your thoughts?

Some photos of my restoration, hope you can access, at https://members.optusnet.com.au/~hamgr/
 
Welcome to the Forum! I'll have to defer to the technically talented for a good answer to your question, but unless you're just plain looking for more things to do, why would you not use an off-the-shelf manifold?
 
Your project looks very nice so far.

As you may know, the LeMans conversion uses H6 carbs, not HD6. HD6's were commonly used on the old Volvos.

Not an easy "adapter project". Your manifolds are 1.5" Dia. with two vertical mount bolts for the H4 carb. H6 or HD6 carbs are 1.75" dia. & have four mount bolts in a square pattern.

The existing manifolds would need to be tapered from 1.75" at the inlets to about 1.5" at the ports.

You could possibly adapt H6 carbs if you matched the diameters & bolt patterns.

Considering the extra space that the adapting might require, You could not likely use the LeMans cold air box with it. There is limited space between the carbs & the vertical shroud support bracket. If you wish to also use the cold air box, this support must be bent outboard & a special offset bracket between the support & ths shroud must be used.

If you adapt HD6 carbs, the HD's have a larger bottom & the rear carb would interfear with the steering gear.

The only really clean, original way is to use the Lemans H6 carbs, manifolds, & air box. Done correctly, this conversion, in conjunction with the LeMans cam, pistons, valve springs, & distributor can give a noticeable boost in power. Beware of increasing the head port sizes.

Best of luck on your project.
D
 
Hello and a big welcome to another OZ Healey person. Are you a member of any of the OZ Healey clubs?

That's a mighty fine job your doing there, did you use the modified Kilmartin main chassis rails as well?.

I have to admit that getting hold of all the 100M carb stuff can be expensive but as you have the HD carbs already then your 75% of the way there. Using HD carbs instead of H type isn't going to make that much difference anyway.

The adaptor for 1 3/4" SU's can be fairly easy depending which carbs you use. If you get hold of a piece of alloy plate 1/2" thick (12mm), the same vertical distance as the original manifold (measured through the centres of the stud holes) by the horizontal distance of the 1 3/4" carb through the stud holes your off to a good start.

Basically the original studs need to be removed and the holes in the adaptor counterbored so a socket head capscrew will sit down clear of the face of the adaptor.

The adaptor can then be drilled and tapped with a 3/8" UNC thread to suit the 1 3/4" carb mount holes.

The bore can be drilled to suit 1 1/2" diameter then tapered out to 1 3/4" diameter either by hand or any half way decent machine shop can do it.

Sorry Dave but this car is right hand drive so no steering column worries.

Are you going to use an exhaust extractor as well? I can recommend it if you are thinking of it. I don't know if anyone in OZ has reproduced the old "Sonic" extractor but if you can get one to copy then it really makes a difference.

What else are you going to do to the car as you go along ?.

Does the car have a 4 or 5 stud diff ?. I see a 4 stud diff centre in one of the pictures, is that going back in ?. The 4 stud one may be lighter but the 5 stud is better all round.

Regards

bundyrum
 
I still have my decades old sonics Bundyrum, used them as a template to get some new s/s ones made up, how did you know the name?, or with that monicker are you an expat qld sandminer?
Cheers
 
On this particular engine design, gas velocity/inertia through the ports is important to good cylinder filling. Increasing port sizes reduces the needed velocity. Larger ports "might" provide some benefit at engine speeds over 5,500 rpm - but the stock engine will not safely turn over 5,000 rpm.

In any event, any port modifications must be verified by actual flow & dyno testing. Port shape is much more important than size. By just enlarging/polishing there is likely to be reduced cylinder filling, fuel air mixing, & power.

By way of illustration, my engine is extensively modified with very special DWR crankshaft, rods, cam, harmonic balancer, & pistons so that it can safely turn 6,500 rpm.

I have a modified stock BN2 cylinder head with noticeably enlarged/polished ports. This head was modified by a previous owner. I also have a DWR "race" head. The DWR head has much smaller but better shaped ports. The DWR head has been gas flowed & dyno tested to optimize the port shape & size.

This engine puts out about 30% less power using the "ported" stock head than it does with the smaller port DWR head. I have no doubt that it would have had less loss with stock ports.

Over the last 40 years, I've modified many race engines from motorcycle to big block Chevy. Nearly always, there have been torque & power gains by judiciously filling certain areas of the intake ports to increase gas velocity. In some cases the port size was decreased as much as 25%.

Side note - Never increase the compression ratio on the Four by milling the head. The stock deck is too thin to start with & milling just guarantees that there will be more than the usual head gasket sealing problems.
D
 
Don't quite know how we managed to get to port airfow at this time but we were bound to get there anyway.

Your first sentence Dave would probably apply to about 90% of all engines built I'd say.

It's always nice to read what you have to say Dave more so when I agree with most of it (actually it's nice to find someone to agree with for a change).

Yes a stock 100 engine will need a lot of work to allow it to rev over 5000rpm time and time again. But then it's not stock after all the work.

Yes on the comments regarding porting although I have reworked ports and felt a big difference.

It's all a matter of finding the happy balance with port size/flow rate and valve size. There is a limit for how large a port you have because you cannot physically get the ideal size valve in there anyway. The larger the valve the more it interferes with the combustion chamber wall which reduces the amount of flow you get through the valve as a result.

Of course when we start talking flow bench figures it's very difficult to have a benchmark as not everyone uses the flow bench the same way and to the same standards.

Yes on compression ratio too. If for whatever the reason you want a higher compression ratio (and I understand the reasons for having one) then it's better to change the shape of the top of the piston to have either less dish or none at all. If you have a piston with a flat top then if it has less compression height (the distance from the centre of the wrist pin to the top of the piston) then it will keep the compression below the "limit". The other problem with higher compression is the extra load it puts on the bottom end and you don't need any extra load on your bottom end (some people have enough of a load on their top end let alone on their bottom end).
 
Hello again OZ BN1

Just a bit more here for you this time on SU's. The adaptor plate should be about 3 3/8" (86mm) X 3" (76mm) X 1/2" (12mm). I was thinking today (yes it has been known to happen, usually with bad results) that the studs in the original manifold are a BSF thread so the socket head capscrews (unbrako in OZ speak) I told you to use will also have to be BSF. A UNF screw will not work as it has a different number of threads/inch than the BSF. You can either bore the adaptor out to 1 3/4" and then grind the first part of the manifold out to match this size tapering it down to the original diameter about halfway into the manifold. This is probably the better of a number of ways to do this.

My comment on the "Sonic" extractors brought a comment from zblu so maybe he will let you borrow his and you can copy them. Just a thought. I know if I were in OZ I'd let you borrow mine but that's not going to happen just yet.

Anymore questions/thoughts lets hear them. This forum is all about sharing and learning and that's what so good about it.

Regards, bundyrum.
 
BUNDYRUM said:
Don't quite know how we managed to get to port airfow at this time but we were bound to get there anyway.
-----------
Yes a stock 100 engine will need a lot of work to allow it to rev over 5000rpm time and time again. But then it's not stock after all the work.
-------------
Of course when we start talking flow bench figures it's very difficult to have a benchmark as not everyone uses the flow bench the same way and to the same standards.
-------------------
Yes on compression ratio too. If for whatever the reason you want a higher compression ratio (and I understand the reasons for having one) then it's better to change the shape of the top of the piston to have either less dish or none at all.
--------------------
(some people have enough of a load on their top end let alone on their bottom end).
Click to expand...

I got "there" from ZBLU's question "Why the beware of increasing the head port sizes?"

I'm quite sure that DMH would have built the "stock" cars to a much higher performance level, had he not been under strict price constraints.

Flow bench numbers can be very misleading if wet flow is not an important part of the consideration. There can be high flow rates accompanied by air-fuel separation which leads no where.

On my engine, increased compression ratio was able to keep the low rpm torque as high or a little higher than original in spite of the reduction caused by the later intake valve closing of the more radical cam.

We used have a saying, "don't let your mouth overload your tail". EH?
D
PS - I'm not sure what a "Sonic" extractor is, but The DWR version of the "Derrington" 3-2-1 system, with the 2's being quite long, works very well for me.
D
 
Hi Dave Russell and BUNDYRUM,
As I've previously posted my BN1 head was 125 thou planed and had about 12 cracks in it, had a magician weld the cracks and build up the heads lost material (125 thou), took the hi comp pistons out and installed stock, and resleeved back to stock bore, this left highly modified head with large capacity in head chambers with oversized valves installed (100/6)from memory (puts comp ratio below stock)
were out to 1 34" for LM carbs already, not sure if they were a straight cut or a taper from 1 34 to 1 1/2, cam is a special road/race (think its a special somewhere between M and full race, very tractable)and with the sonic offtakes works well with the ambient temps and octane ratings over here, had a couple of other Healey ex M owners drive it and comment on its grunt, spins out to 5k+, was interested in the flow comments of yours Dave, wasnt sure you meant the intake ports, and/or the head porting
In view of your thoughts, I might get a dyno check done
Cheers
Zblu






'as I got it,
 
"Sonic" was a brand of exhaust extractor manufactured in Australia in the sixties, possibly later. They made a range of products for many types of car.
 
Hi All

Thanks for the replies, I am new to all things Healey so this information will give me plenty to think about.
Answers to a couple of the questions asked, member of the South Aust. Healey Club, main chassis rails are original, however I have replaced all the floor, footwells, outer rails and boot section. Also modified for a 4 speed gearbox and added extra chassis bracing as well as the camber modification. All from kilmartin.

The original diff was 4 stud but came with a 5 stud as a spare. I plan to use extractors but have not tried to source them yet.

Have just recieved my new wiring harness so have been mainly looking in to all the electrical side of things, all the electrics had been removed and came in a box.

Look forward to any comments
pictures here https://members.optusnet.com.au/~hamgr
 
Hi Oz, in investigating the electical side of things, you might want to look into using an alternator to replace the generator when things go back together. I purchased from the North Texas A.H.Club a bracket kit which looks very well engineered. I have not used it yet, but I believe many on this forum are using this or something similar. It greatly increases electrical capacity. Vertually eliminates dead batteries.
Good Luck.
 
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