XKE 4.2 S1-S2 Dual Stromberg vs Triple Carburetion:

This post covers the various discussions from 2011 jaguar postings (summarized), as well as my experience with a 2014 conversion of a smog’d 1968 S1.5 from a Dual ZS CD175 (“stock”) to a Triple ZS CD175’s installed on a 420 manifold. Shortly after the conversion, with children gone, we down sized our home from 3 car garage and 3 driveway parking spaces to a single garage condo, in town, with restrictions against working on cars. Sold the XKE to a buyer in UK.

In a few days I will post the details of fabrication, preparation and installation, for conversion to 3 ZS CD175’s on a triple 420-intake manifold.

To answer the first two questions up front: No, I did not do a DYNO check, for which I apologize. "Seat of the pants" performance improvement after adjusting shifting points was very noticeable. Second question: to keep the top of the forward carburetor from hitting the bonnet, I had to machine and install the 420 manifold with a 7-degree tapered intake shim, and use three overhauled ZS CD175 carburetors. (SU’s are too tall)

From Factory ligature: With the change from Gross HP to Net HP, it became hard to compare later performance deterioration from emission requirements to earlier non-emission engines.
Jaguar XKE performance peaked in 1967 with the triple SU HD8 setup: 266 HP gross and 283 ft-lbs. torque.
1968 XKE S1.5 European cars with 3 SU’s were down to 255 HP and 275 ft-lbs. torque
1968 XKE S1.5 North American cars with 2 ZS’s were down to 246 HP and 263 ft-lbs. torque (any one who has owned a 2+2 with 3 speed Automatic, knows these figures are quite generous)

I reviewed the changes between 67 and the smog’d 68. Main items were: 3 SU vs. 2 ZS carburetors, triple “balanced intake runner” manifold vs dual “log” manifold and a vacuum advance distributor vs. a mechanical advance only. My “project target” was to get back to the 266HP performance level economically.

1. I mimicked the 1967 distributor curve with the Vacuum Advance Pertronix distributor. Cost effective and easily done.

The cost of the “Triple SU HD8 kit” was one-third the market value of the car at that time. Och!!!

2. After some research (inadequate as it was), I decide to go with 420 or late Mark 10 triple and ZS CD175’s carburetors. A pristine 420 manifold was available locally (original buyer thought he was getting a XKE triple for SU HD8’s !!!!) so I purchased it for $165.

3. Found third ZS from a XJ6 S1 for $75. Because I had tried to tune my ZS’s for years with much frustration and the new carburetor looked extremely neglected, I decided to check out the professional Carburetor shops. Any of the top three would probably been fine, but selected Joe Curto in NY. He understood what I was doing: Full overhaul, conformed all three by position, converted them to adjustable needle, adjusted the initial jet height for new air flow, provide numerous missing parts for carburetors and linkage. All brightly polished and ferrous metal parts plated. They looked better than new. At least I wasn’t going to be embarrassed to raise the bonnet for a serious car guy.

4. I did the Header calculations and found that 1 ¾”od tubing was optimum (with growth), however only found available off-shelf 1 5/8”od mild steel and 2”od SS. The 2” SS headers were on sale, so went with them. The exhaust system calculations indicated the dual 1-7/8" OEM exhaust was fine until needing replacement, at which time I would go to 2".

At this point I was focusing on numerous cautions about the carburetor to bonnet clearance.

From my 2014 Notes: Overview: The XKE Tri-SU carburetor OEM Intake Manifold has a 10-12 degree down-slope intake-track to give the SU HD8s hood clearance. The Mark X/420 Tri-SU Carburetor OEM Intake Manifold did not have a hood clearance, but a wheel well clearance problem, hence the 7 degree up-slope track configuration. The ZS CD175 is not as tall as an SU HD8, but still has negative clearance with the XKE Hood (bonnet) if installed on Mark X/420 manifold. A 7-degree Shim, between the cylinder-head and the intake manifold, lowers the forward (critical) CD175 carburetor to a positive clearance of 5/16” (this assumes the Hood is properly shimmed vertically at the front hinge point). SU HD8s will not work with a 7-degree correction. The Mark X/420 manifold with the 7 deg correction, is a straight vertical intake track from intake valve to air filter, while the OEM SU manifold intake track has a ~10-12 deg down-slope in the manifold, through the SU Carburetors, then back up at a ~10 angle to the center line of air filter.

“This straight vertical intake track of the 420 Triple manifold may flow better than a Triple SU manifold, and have better performance; ie. if you put 3 Su’s / 3 ZS's on a 420 manifold with 7-degree correction, you might produce more Horse Power than the original 1967 @266 HP”????

See comments from 2011-member postings, on the value of 45 DCOE Weber straight tracks vs the curved tracks of the Triple SU & Dual ZS manifolds. I have highlighted in red those comment which indicate the “7-degree corrected 420 manifold” with 3 ZS carburetors have a positive performance improvement.

……“ Paul_Saltwick Dec '11
In reply to a message from MGuar sent Sat 3 Dec 2011:
There already are before and after numbers… Done by the
factory. Off the top of my head 265 HP and 220 are the
differences. 45 horsepower…

MGuar, the factory claimed output change from twin to triple carbs is 20
hp, not 45 hp.
Since there were no normal production 3.8’s with ‘‘B’’ type heads and
triples, the only direct comparison is a 4.2 twin carb 420 engine
and a triple carb 4.2 E or 420G engine. The factory claimed 245 for
the 420 and 265 for the E, a difference of 20 hp, which is somewhat
believable. The exact same engine with triples, made the exact
same torque 283 ft/lbs., but at 250 rpm higher, accounting for the
20 hp difference at similar peaks (5400 vs 5500 rpm). This also
implies that the triple SU carbs are not the limiting factor to
output, which I think is true. The factory also claimed a 10 hp
increase for a point of compression (3.9%), which is also
credible. These are gross numbers and are inflated by at least 20%
compared to net or DIN figures.

Personally, these numbers do not reflect the change in throttle
response the driver feels which makes the conversion worth it.
Much like the 70-80’s Porsche 911s with CIS injection. A swap to
carbs only nets between 10-20hp on the dyno, but on the street it
feels like 50 hp. Usually this is all that matters on the street.

The paired cylinder manifold in theory allows
each cylinder to draw from a 50mm throttle, but in practice most
XK’s cannot inhale enough to fully open the pistons. Some of the
highest output XK’s ever built, claiming over 350 Net HP, still ran
2-inch SU’s, showing that bigger carbs are not needed on a road car.

I am not making any ‘‘fair’’ approximations, only comparing factory
figures on identical engines. The post 68’ smog motors have several
other differences and there is no direct comparison for twins and
triples. I think the US rating dropped all the way down to 162 hp
under the ‘‘net’’ hp ratings mandated in 1972.Paul–
MGuar Dec '11 In reply to a message from PS sent Sat 3 Dec 2011:Paul; We totally agree on at least 2 points.
With regard the 1-3/4 Stromberg’s versus the 2 Inch SU’s
I’ve had enough of each on my flow bench to confirm that the
1-3/4-inch Stromberg’s flow right around 200 CFM and the
2 inch SU’s flow right around 315 CFM.
We are in total agreement that the SU’s at 945 CFM are not
the limiting factor in power output.
What is relevant is the effect of Weber’s on an engine…
Please bear with me. If you put a 45DCOE on the flow bench
depending on which chokes you are running, you will be right
around 155-160 CFM per throat; close enough to the 315 CFM
SU that I won’t dispute the difference. Yet dyno experience
has proven that in similar relatively stock engines,
properly tuned the Weber’s are worth 15-20 hp. more than the SU’s.
There can be only one logical reason for that gain…
Better airflow. A straight path. Not a curved one or one
with actual bends like the 2 carb Stromberg.
The reverse has to also be true. Yes, it’s likely that the Stromberg’s flow enough air at
400CFM to feed a little 3.8, or 4.2 engine of relatively
mild state of tune… However, the flow pattern is much worse. Part of that
problem of flow is the different mass of fuel and air…At
every corner/curve the fuel will go to the outside of the
curve relative to the air… if the fuel should land on the
walls of the manifold there is every likely-hood that it
will return from a vapor state to a liquid state… Liquids
don’t burn (hence flooding)
If the slight curves of an XK-E triple SU manifold costs
15-20 horsepower over a straight Weber. The much sharper curves of the
the duel Stromberg manifolds is likely responsible for the
majority of the difference… Not the size of the Carbs…

It would be an interesting experiment to put three 1-3/4
Stromberg’s on a Triple SU manifold and see if indeed makes
the same power… (I suspect that’s likely)” …. End quote

I have much detail on the conversion documented, so will organize it and post it soon.

Added an oil catch can in breather system


Added two supplemental heat shields; one for alternator (Jag P/N) and one for brake reservoirs
Rgds David

God Bless Jaguar Owners

 

Jaguar XKE ZS CD175 Tri-Carburetor Conversion

Kit allows a “single piece Mark X” or 420 Tri-carb Manifold to be used with 3 ZS CD175 carburetors to replace the XKE S1-S2 ZS CD175 dual carburetor setup. (30% - 40% of a Triple SU Conversion Kit) !!This assumes you are going to do the work yourself!!

Basics for modifications:
- Late Mark X or a 420 Intake Manifold
- 3 ZS CD175 Carburetors (Manual choke), 2 from existing Jaguar setup
- Custom Throttle linkage (modified forward throttle shaft)
- Custom Coke cable for 3rd Carburetor
- Custom K & N Filters or air horn options

Installation Kit:
1. 1 ea. Machined 7-degree Cylinder Head to Intake Manifold Shim
2. 3 ea. Machined 5-degree Transition Spacer from 1.75” ZS Carburetor to 2” intake manifold
3. 3 ea. Machined 3-degree Transition Carburetor Flange for 2” OD x 1.75” ID intake runners
4. 1 set. Machined Manifold Blanking Plates and Throttle Links
5. 1 set. Basic Fastener Kit, (including 7-degree Tapered Washers)
6. 1 set. Instructions, optional configurations, diagrams, and pictures
a. Aft, Center and Fwd. Carburetor Post-Overhaul Configurations
b. Throttle and Carburetor linkage options (automatic or manual transmission)
c. Third Choke cable options
d. Air Filter, intake runner and air horn options
e. Part Numbers and Suppliers for parts needed for modification and future maintenance

Note: Some fabrication skills and tools are required for Throttle Linkage, Intake Runner, and Choke Cable modification. The Mark X/420 Manifold will require 18 existing mounting holes to be angle-drilled for 7-degree variance (drill press or vertical mill); Aft and Center Carburetor shaft lengths will have to be trimmed to correct length after trial fitting.

Summary: The ZS CD175 is not as tall as an SU HD8, but still has negative clearance with the XKE Hood (bonnet) if installed on Mark X/420 manifold. The 7-degree Shim, between the cylinder head and the intake manifold, lowers the forward (critical) CD175 carburetor to a positive clearance of 5/16” (this assumes the Hood is properly shimmed vertically at the front hinge point). SU HD8s will not work with a 7-degree correction. The Mark X/420 manifold with the 7 deg correction is a straight vertical intake track from “Intake Valve to Air filter”. The desired intake track distance (Intake valve to air filter) was calculated to be 17”to 19” for modified street performance. With a K&N 5.5”od air fiter and a ~2” runner, the actual distance is slightly over 17”. A smaller diameter air filter would allow a longer runner. The 3 air filters have a combined surface area 30% greater that stock OEM filter, so are not a restricting factor in flow performance.

Extra Cost improvements:
1. A correct Distributor with the “1967 advance curve”; can be an OEM distributor from a 1967 XKE, or a new Pertronix vacuum advance Distributor. (recommended).
2. The 2” SS exhaust headers and the oil catch can in the breather system, were not required features, but were done at the same time as desirable improvements.

If you have an XKE with the dual ZS CD175 setup, and the overhaul of the carburetors is long overdue, then you can economically upgrade to a 3-carburetor performance configuration. A good third manual choke ZS CD175 core and a Mark X/420 manifold can be found for under $100 and $250 respectively. If someone in the future wants the OEM original configuration for collector value, the modification is fully reversible (just save all the removed parts). In the meantime, you have a fun driver and you are not embarrassed to raise the hood (bonnet) at your local car show.

Jaguar XKE ZS CD175 Tri-Carburetor Conversion Data Package

Instruction and Recommendations (PDF)
· Overview Tri-Carburetor Setup
· Machined Parts & Basic Fasteners Kit
· Supplemental Part Numbers & Suppliers
· Three ZS CD175 Carburetor Overhaul Configurations
· Mark X/420 Manifold Preparation
· Mark X/420 Manifold installation
· Throttle Linkage Options
· Air Filter Options
· Intake Runner Fabrication
· Choke Cable Options for Third Carburetor
· Fuel for Third Carburetor
· Optional Adjustable Trans Kick-down linkage
· Optional Oil Catch Can Breather Kit

I have attached some of the instructions sheets as samples.
If any one is interested in this approach, I have enough parts to put together several kits
Rgds David

 

David, very interesting post, hope you continue with it.

I too have been attempting to improve the performance of my 68 OTS. After an engine rebuild to clean things up and install new bearings, pistons and rings, the engine ran smooth and was pleasant but far from exciting. As many others have done, I removed all of the smog paraphernalia and felt a considerable improvement in throttle response. But the engine now ran rough and hot, in other words, too lean. After some experimenting, the addition of a shim in the carburetor throats richened the AFR to a bit over 13, good I think for performance. The car now ran smooth as new but still lacked excitement. I have owned the car since new and I remember how it felt at the beginning. That prompted a dyno test which gave about 150 hp at the wheels and an estimate of about 180 bhp. The compression is down slightly to about 165 psi, probably need new valve seats, but I do not expect a large improvement.

Time to dump the carbs. After a deep breath, I sprang for the triple SU's, manifold and linkage. Additionally I had the distributor rebuilt and added a vacuum advance. I know the Pertronix can match the centrifugal advance but am not sure about the vacuum. At last, I am approaching happiness.

Now a couple of questions I hope you can help with. The SU's sit level and the only slope is between the air filter and the carbs inputs. Do you believe that is important? Second, is the value of exhaust headers. Do you believe the improvement in exhaust flow more than offsets the power loss due to increased under hood temp? I have not heard that discussed. I would be interested in your thoughts.

 

Bill, the slope & short distance from Air Intake Flange to carburetor inlet is probably the least critical in the intake track. One design consideration I looked at, was to keep the stock air-filter system and just add 3 1" air horns to the inside of the snorkel flange. The project was a series of small improvements with limited expense. Because I was trial fitting the shim, I found significant casting tolerances between intake manifold tracks and cylinder head tracks, cleaning these up with a sanding drum was probably more beneficial than than I realized at the time. I understand these miss-matches exist with the SU tri-manifold also.
The headers expense was slightly more than the cost of re-enameling the two exhaust manifolds; which were very unsightly, so more weight on looking-good than just performance. With the headers I could replace cams with high lift & longer duration units in the future, with stock exhaust manifolds those mods would not be as beneficial.
The hood temperature from headers is away from intake side so I don't think that would be a performance issue, the louvered bonnet is the key. I have an XJ6 now, and after a long drive pop the hood, as the engine bay cooks with out any ventilation. I was concerned with the alternator, brake reservoirs and brake master cylinder, so added two additional heat shields for protection.
Good to know some Dyno numbers from your 68 XKE, would like to close loop on actual improvements, as have been working on theoretical numbers and seat of the pants. Were your dyno numbers before or after SU conversion?
Thanks David

 

Dave, the measurements were after the engine rebuild, the smog control removal and Stromberg mods. I have read in several places that this vintage car was well below 200 bhp but was disappointed that the above changes did not show an improvement. My engine rebuild did not include work on the valve guides and seats; I do not have the tools or expertise for that. I believe that is probably the reason for the slightly low compression and probably worth some gains when I send the head out for machining in the future.

The measurement was the impetus for the triple SU conversion despite the price. The effect of Brexit on the value of the pound also helped, The "seat of pants" results indicate a significant improvement but I do not think I am close to 250 bhp. I have been suffering poor mileage while cruising and some roughness on deceleration. I hope these symptoms are addressed by having the distributor rebuilt with vacuum advance added. I have installed that and am waiting for winter to end here in New England for a final tuneup.

Good point about hood louvers keeping the exhaust heating under control. SS headers are an attractive option to chipping porcelain. Wish they could keep their original color after use. To cope with additional heat,I have considered moving the fluid reservoirs to the firewall but that is decidedly non-original especially after juggling the position of other hardware.

Thanks for the advice. Bill

 

I still like the side draft Webber setup the best myself but I also found the Strombergs to be the most pain-free of the bunch so this is a fascinating build to watch. Impressive job, really appreciate the level of documentation you have provided! Thanks!!!

 

"I hope these symptoms are addressed by having the distributor rebuilt with vacuum advance added."

Bill, I hope your distributor overhaul specialist, included advance cure modification for the SU setup. See attached curves. p/n 41207 for Dual ZS CD175; p/n 40617A and Pertronix are for triple Su setup. The power would be down and fuel consumption up in the 2400-3000 RPM range using the ZS distributor on a SU setup. The vacuum advance only comes in at low throttle settings or high RPM with closed throttle. If you look at a cruise RPM of 2800, there is all most a 5 degree of advance difference. The original p/n 40617A distributor was for ~98 octane, with only 91 available today, 5 degrees less than desired will be even more noticeable. Rgds David

 

David, I am slow getting back, some minor surgery. Yes, the distributor restorer did know about the various mods and, besides adding the vacuum advance, did a rebuild. I drove up to visit him in New Hampshire and was impressed. Before choosing this route, I had checked the centrifugal advance and found good agreement with the shop manual. My hesitation with Pertronix is the vacuum response, a lot harder to check. The restorer claimed to be the sole source for SU vacuum advance modules and I am relying on his expertise to get it right. I do use the pertronix igniter. I included a photo because I thought you might bi interested in the air cleaner from ITG. It has a connection for the crankcase vent. It is also quick to swap out when showing the car. Regards, Bill

 

Bill, when the last snow has been shoveled this spring, I think your going see the old zip has returned. Rgds David

 

What a great read

what metering needles did you end up running.
Did you change the Dashpot springs and do you know what colour they are.

 

The original fixed B1E needle was changed with new piston to Adjustable B1E/A needle. I was expecting to go to a richer needle, but unlike SU there are not many choices, as the jet is raised or lowered to achieve the richness needed. The later needles that were available were designed to achieve emission objectives, not performance. So the original needle and dash pot spring are spec for 1968 S1.5 XKE (USA / Ca.). Not sure the color of spring as they were installed during overhaul by the shop. We had lengthy discussion on needles and jet height, no mention of dash pot springs, so I am assuming no change was required. I stayed with a new fixed jet, and had to adjust it twice to get tuning in the mid-point of the Adjustable Needle. I would now recommend the adjustable jet for this project. Many hours and disassembly of 3 carbs required, to lower a fixed jet a few thousands each time.

Rgds
David

 

Hi David,
I 've now a E type 4.2 with Stromberg.
If i follow your adives and very good explanations, i can use 3 HD8 on triple 420 manifold to increase power.
Is it so ?
Best regards
Etienne

 

Etienne

The 420 manifold will work with 3 SU HD8s , the problem is the top of the carburetors will interfere with the bonnet/hood. If you go with correct manifold for the 3 HD8s (which are available), you will be able to get all the correct throttle linkage etc., using the 420 manifold will require modification/fabrication of bits and pieces.

With the 420 manifold, I looked at just cutting holes in the bonnet/hood to let the Carburetors protrude, covering them with an aftermarket racing hood scoop, but decided it was not the best idea..so made a 7 degree shim and use 3 ZS which are shorter.

If you go from 2 ZS to 3 carburetors, you will have to change distributor to correct model, for either of the 3 carburetor manifolds. Using Jaguar adverting numbers, going from the 2 smog'd ZS to 3 SU HD8s would be an improvement in performance of ~10%. Unless you are racing or on a dyno-test, you would not know the difference between 3 ZS CD175s and 3 SU HD8s.

The big difference for me was price, the 420 / 3 ZS solution was 1/3 of the price for a full 3 SU HD8 kit...(Most of the labor was free..me). I Like working on cars as much as driving them and have above average shop tools... If you are going to have a retail shop do this for you, the full SU HD8 retrofit kit will go in quick and easy.

Rgds
David

 

thank you for you response David.
We will try to manage that searching the best Q/P
Have a nice day,
Etienne