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Power increase from twin 1.25su to twin 1.5 Su
https://www.ausmini.com/forums/viewtopic.php?f=2&t=98470		 Page 1 of 2
Author:  AWOODY [ Thu Apr 18, 2019 4:54 pm ]
Post subject:  Power increase from twin 1.25su to twin 1.5 Su
What would the power increase be from twin hs2 to twin hs4
Author:  Matt I [ Thu Apr 18, 2019 7:25 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
Depends on the spec of the rest of the engine... Mine picked up a bit, but I think that was more down to the tune being better than the carb change itself.

I only went to the HS4s because I had them and the new engine build will need them if I stay with a 5 port head and SUs, so the money was better spent on them than the HS2s.
Author:  Kennomini [ Fri Apr 19, 2019 3:48 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
IMO I don't think you will get any benfit.
If the current set up is correctly tuned and in good condtion, replacing one element that can possibly flow air a fraction better won't give you any noticable improvment (maybe a couple of HP on a dyno sheet).
Like Matt I said unless your engine is a fire breathing monster that is struggling to get enough air through 2 1/2 total inches of carbies the extra air flow won't do diddly.
Cooper S's had twins and made great power, the rally cross minis were punching orer 100bhp and still used twin 1 1/4 SU's.

If on the other hand you are just curious what the difference is I'd take a guess and say Less than 10hp gain but maybe the low down torque would pick up enough (or same torque would come on earlier) that would get an engine moving a tiny bit better, (but I don't think it's worth it, service/optimize everything else first).
(Just my thoughts).
Author:  AWOODY [ Fri Apr 19, 2019 4:55 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
Thanks that's what I thought .
Author:  cooperess [ Fri Apr 19, 2019 4:59 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
Didn't the police use 1.5" SU's?
Author:  winabbey [ Fri Apr 19, 2019 6:19 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
cooperess wrote:
Didn't the police use 1.5" SU's?

Yes, but matched with other performance mods.
Author:  Mick [ Sat Apr 20, 2019 6:08 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
Feels really good up top, especially on the highway if you have a bit of a road cam and good exhaust.
Author:  Besser [ Thu Apr 25, 2019 8:19 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
The required sales pitch dictates "easy 20% mate"
Reality is SFA on their own.
Author:  mickmini [ Wed May 01, 2019 7:45 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
you might lose driveability at the bottom end due to slower intake velocity due to the larger throat. so it would actually feel slower most of the time! Only with matching compression, valve, cam changes would you see overall benefit.
Author:  AWOODY [ Wed May 01, 2019 7:53 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
Thanks guys .
Author:  phillb [ Thu May 02, 2019 7:32 am ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
I went from 1.5 back to 1.25 on my Cooper S (for originality) and really didn’t notice much difference. Possibly even a little more at the bottom end. The 1.5s certainly sounded nice though. I think as others have said the benefits are at the top end at high speed and revs and I’m sure that’s the reasoning behind the police pack using this setup.


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Author:  mickmini [ Fri May 03, 2019 6:22 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
phillb wrote:
the reasoning behind the police pack using this setup.


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the police pack had camshaft and other changes that could take advantage of the 1.5s
Author:  drmini in aust [ Fri May 03, 2019 10:23 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
mickmini wrote:
phillb wrote:
the reasoning behind the police pack using this setup.


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the police pack had camshaft and other changes that could take advantage of the 1.5s

Head work (by Lynx Eng?) too.
They did 106MPH downhill with a tailwind, as I found out... back in 1969. Open road speed limit in NSW then was `/' ;)
Author:  winabbey [ Fri May 03, 2019 11:06 pm ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
drmini in aust wrote:
Head work (by Lynx Eng?) too.

The head sometimes gets mentioned as being modified by Lynx Engineering, with some suggesting the chambers were probably worked on, plus some extensive porting and polishing. As far as I'm aware with the research I've done the only change to the head on the SPO41 Cooper S was fitting Special Tuning valve springs and collar, otherwise it was a standard S head. For the SPO28 Clubman GT some porting was done "to improve performance" as a Service Letter says. I understand without the porting the cars were not meeting the 100 mph clause in the contract. It doesn't say how much work was done, just that "the ports were modified".
Author:  9YaTaH [ Sat May 04, 2019 8:39 am ]
Post subject:  Re: Power increase from twin 1.25su to twin 1.5 Su
As I have said before here, some police Cooper S engines were modified by local BMC dealers and others...I suppose at the discretion of the local commands...
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