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Post by dave1800 on Feb 1, 2015 4:57:39 GMT
Calling all SU experts! The standard Landcrab uses a HS6 carb (1.75")and the 1800S 2x HS6. The MGB uses 2x HS4 (1.5") and the 1622cc Austin Cambridge a single HS2 (1.25"). Des Hamill's excellent book on the SU Hamill SU book suggests that a 1.5" would be the most appropriate size for the standard 1800. Does anyone have any idea why the 1.75" was chosen for the 1800. It's a big jump from the 1.25" of the later Farina Cambridges and may possibly detract from the low /mid range performance. I believe the P6 Rover 2000 TC used 2x HS8for show but this was later "corrected" on the 2.2L engines that used 1.75" SU carbs. David |
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Post by snoopy11 on Feb 1, 2015 8:15:51 GMT
I would not say I am anything but a casual by stander when it comes to the technical details of carbs but years ago I had a mini 1000 that had a 1.25 carb as standard. On long runs I found it gave fuel supply problems almost as if the engine was not getting enough.
A friend had an 1.5 carb sat on a shelf so for a bit of fun we put it on. I never had the issues again.
I can only think the chopping and changing of carbs on different cars was purely financially led.
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Post by dave1800 on Feb 1, 2015 11:32:30 GMT
I thought I had a good handle on this but looking at the attached SU data I am less clear. SU carbs etcThere seem to be anomalies such as the Mini 848/998cc manual g/box cars use a HS2 but automatics a HS4. 1275 single carb Austin 1300s use a HS4 for manual and auto transmissions and the same needles. The 1622cc Cambridge / Oxford uses the single smaller HS2 (1.25") but with the stronger yellow spring (as used by the Crab) the others mentioned use the lighter red spring as used by the MGB and 1800S. Any more thoughts? David |
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Post by andrewa on Feb 1, 2015 11:36:18 GMT
I'm certainly no expert but the rolling road chap who did the final set up on my "relatively" tuned car and had owned landcrabs in the past reckoned twin carbs were excessive and my engine didn't have quite enough puff to make full use/the best of them. It wasn't sour grapes - he was just making an observation. He believed I might have been better with a single 1.75 with my set up - which sort of leads to 1.5 making sense for a standard car.
I reckon it was more marketing than anything else - the old man had a 2000TC and he was dead chuffed to have a twin carb car - he sold that to buy a second hand 1800 and I remember we were not impressed! Those were the days - even in affluent sw London most blokes would be outside tinkering with their car at the weekend. Not sure if this is a reflection of the unreliability of the cars, how (relatively) easy they are to work on, or just a different age when people did more stuff themselves - or more likely a good way of hiding from the missus before going to the pub at lunchtime and then home for Sunday roast!
Cheers
Andrew
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Post by dave1800 on Feb 2, 2015 1:05:29 GMT
Andrew that’s an interesting comment from your expert tuner about the SU carb size.
I had been under the impression that in the ideal world the intention was to ensure that the SU just fully opened when the engine was developing maximum power (not rpm). If it fully opened too soon then it would go into non constant depression mode and no longer function as intended and this may lead to a weak mixture; if it didn’t fully open it would act as a smaller carb. The opening is determined by the height of the piston which in turn depends on the mass of the piston and the strength of the spring. The profile of the needle is then calculated to suit.
It would appear that the 1622cc Austin Cambridge with a single 1.25” HS2 and strong yellow spring probably fully opens, whereas the 1798cc single 1.75” SU carb in the Crab that uses the same spring doesn’t. Both have similar camshaft timings. I don’t know the comparable piston weights so this is not a fully informed assessment.
I appreciate there are many other factors to be considered including the way the engine is likely to be driven, eg Mini high rpm, 1800 more use of low down torque, vehicle weight, manifold and cylinder head design.
Given the lack of computer assistance, I believe the SU engineers did a remarkable job and that there must be good reasons for the Cambridge running what appears to be too small a carb and the 1800 too large.
Can anyone here help me understand this better?
David
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Post by dave1800 on Feb 3, 2015 10:38:20 GMT
I have now found this on the Burlen SU website
"Piston springs
On 1 1/4 in., 1 1/2 in. and 1 3/4 in. diameter horizontal carburetters, the red (4t ounce) spring is normally used for initial testing and in most installations this load of spring will be effective, assuming that the size of carburetter has been correctly chosen. When the correct spring is fitted it is usual to obtain full piston lift at full throttle and at approximately three-quarters of the maximum rev/min. If the absolute maximum of power is required, however, it is more usual to choose a somewhat larger carburetter, which will not obtain full piston lift until nearer maximum rev/min."
Am I any clearer why this is, not really.
David
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Post by indianajones on Feb 3, 2015 23:50:35 GMT
I have read into this matter a few times (As some know, I have looked into converting to twin carbs), and it seems a lot of Mini and MGB (Austin A55 sports car....For Chris lol) owners seem to think a single HS/HIF 4 or 6 makes for good low down power/around town, but one seems to lose some puff at the top end.
I can't make any comments based on experience as I haven't driven a 'crab with twin carbs.
-Andrew
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Post by Penguin45 on Feb 4, 2015 0:36:17 GMT
Yes, but I threw a spanner in the works by fitting a pair of HIF44s..... Cheap, manifold included, but meant for the A55 sports car (MGB  ). It's a lighter car, so the carbs will be jetted and sprung accordingly. Being a "sporty little number" they'll be required to demonstrate "brisk" acceleration as well. In retrospect, they may not be the best fit for the 'Crab, although I will say that the performance of mine is definitely a bit peppier. I do get good MPG on a long run (Leeds to Cornwall twice a year), but they can be appalling around town. TBH, I'm not all that bothered. It's my hobby car and I don't expect it to behave like a "modern". No doubt some time spent on a rolling road doing a proper set up could set the HIF44s such that they are better suited to the 'Crab's extra weight. Trouble is, a rolling road makes no allowance for the weight of the car, so road testing would be the only realistic way of improving the situation. No doubt that was how it was resolved by BMC in the development stage of the car. All those jets, needles and springs to play with....... Chris. |
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Post by dave1800 on Feb 4, 2015 3:49:53 GMT
The garage owner in Leeds who used to do my MOTs worked on the 1800s run by one of the taxi firms back in the late 60s/70s. He commented about the really good MPG they got driving around the city compared with other vehicles and I noticed the same compared with my Saabs.
As far as the SU is concerned it appears from everything I have read that there must be a sweet spot where the choke size (as it is wrongly termed)determined by the piston lift gives the best compromise between power, driveabilty and economy across the rpm range. If the piston rises too much or too little for a particular engine then this won't be achieved.
Given that the piston lift will be determined by the piston mass and spring strength, I'm surprised that so far I've not seen any mention of adjusting the spring strength - other than changing for another standard one - a a means of fine tuning. Of course any such changes will mean finding a more suitable needle or polishing one to size. Maybe just too involved?
David
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Post by hydrolastic on Sept 25, 2016 13:44:40 GMT
Hello guy's just going to throw my 2 cents out there based on my experience racing minis. first lets look at a Japanese intake system. Most are a plenum manifold with 4 runners and the plenum. generally good for mid range power they can be modified by shortening or lengthening the runners or changing the diameter. For a 1800 cc engine it would require about a single 2" throttle plate and would be getting four pulses every two revolutions or a fairly even pull on the intake. Now lets look at ITB's (individual throttle bodies) on a japanese engine these flow huge amounts of air but have to be bigger to cope with the on and off pulse of the individual cylinder. Now for a siamese port head like the mini or the 1800 it is neither a plenum or a ITB the siamese port acts like an ITB in a sense but since there are two cylinders hooked up to the port it sees one long draw on the carb. This causes the outer cylinders to run at full power but the inners have the air partially robbed.The situation only gets worse with longer duration cams and or higher rpms. The 1800 S engine i take it has a different cam and was probably intended to provide more power at higher rpm. This means more carb and since its kind of a half ITB setup it requires a much bigger carb than it would as a plenum style manifold. Or in a single carb it sees more pulses or even draw so its smaller but as a twin carb it is much bigger. Aaron
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Post by dave1800 on Sept 25, 2016 13:53:14 GMT
I believe it was for the same reason as the Rover 2000, marketing - both were around the same time. In practice the 1800 does have plenty of low down torque but it would be interesting to try a 1.5" SU. I wonder how far the piston actually rises at full power, possibly it doesn't actually fully open?
David
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Post by dave1800 on Sept 26, 2016 8:08:31 GMT
Aaron I wonder with your mini experience if you had the opportunity to analyse how the ECU was programmed for the multi point injection on the last of the classic cars. My understanding is that it fed different volumes of fuel into the manifold for cylinders 1&4 as compared to 2&3 to compensate for the differing requirements of each pair of siamesed cylinders due to the effect you describe. I believe the move away from the relatively simple SPI which acted very much like a carb was primarily to reduce emissions some of which were brought about by the differences between adjoining cylinders.
I haven't been able to track down any helpful details on this that could possibly explain the SU sizing. I cannot readily see how to measure the mixture of each cylinder using a wideband sensor on a single carb installation only an average. Perhaps ignition timing could be used as a proxy by having separate control for each cylinder as there is a relationship between timing and mixture.
David
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Post by hydrolastic on Oct 15, 2016 14:30:36 GMT
Hello David, looking around i have not been able to find a satisfactory explanation to your question, But i did fire up the computer engine dyno. What it shows for a typical eight port head is that the ITB's kill the low end and without increasing the diameter of the throttle body it also kills the top end. When i increase the amount from one to four and increase the CFM x2 of the throttle plates the top end power comes back but just barely and it says they are only effective above 5000 rpm. The Twincam mini i did last year started out with ITB's but i built a plenum manifold using the computer engine dyno. It worked exactly as it said it would and there was a very dramatic increase in power from 3000 and up. In the process of building the manifold i decreased the intake tube diameter and added 4 3/8'ths of length then i put a 50mm single TB on it.
The mini guys have developed a program for the megasquirt that does a timed code that accounts for the siamese ports. I don't know how the factory did it or what the mini guys did.
I have been wondering what a twincam four valve head would do for the 1800 but i expect it would be equivalent to the mini and make a very powerful engine. Aaron
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Post by dave1800 on Oct 17, 2016 9:48:41 GMT
Thanks. I have had a look at the information on the Megasquirt system to address the issue of the siamese ports and unfortunately not been able to find sufficient to draw conclusions about the degree of charge robbing. It will of course vary from engine to engine depending on the cam profile and induction and rpm. I cannot see any way of actually measuring the air: fuel mixture differences for each of the cylinders in the pair but am still considering whether there is a way of balancing the power to some extent through individual cylinder ignition timing when using a carb.
regards
David
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Post by dave1800 on Oct 20, 2016 6:24:04 GMT
I've found this website, not about carb size but an interesting approach to fuelinjecting a siamese port A series engine Siamese ports . MPI MiniTBMINI David |
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). It's a lighter car, so the carbs will be jetted and sprung accordingly. Being a "sporty little number" they'll be required to demonstrate "brisk" acceleration as well. In retrospect, they may not be the best fit for the 'Crab, although I will say that the performance of mine is definitely a bit peppier. I do get good MPG on a long run (Leeds to Cornwall twice a year), but they can be appalling around town.
