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Compression ratio v ULP
https://ausmini.com/forums/viewtopic.php?f=5&t=26948		 Page 1 of 1
Author:  Mowog [ Fri Jan 19, 2007 9:03 pm ]
Post subject:  Compression ratio v ULP
Hi all.
I'm rebuilding a Jap import A+ at present to go in my clubby & thinking about running it on everyday 91 octane ULP.
Can anyone tell me what compression ratio I can run using ULP, no engine management system, just a good old SU & Pulsar dizzy.
I know some new cars run maybe 10:1, but their engine management continually adjusts timing when it detects pinging.
Thanks
Scotty.
Author:  drmini in aust [ Fri Jan 19, 2007 9:18 pm ]
Post subject: 
I run a 1293 A+ on 95 usually, it will run on 91 but pings a bit.
C/R is around 9.3:1.
Carbs are twin HS4, cam is a 270* Tight 104..
Dizzy is a Pulsar D4R85, with advance slots modified to give 10* dizzy advance.
What you can run is VERY dependent on the camshaft duration and overlap.
Author:  miniobsessed [ Fri Jan 19, 2007 11:33 pm ]
Post subject: 
yeah exactly what the doc said. It's your cam more than anything which determines where you can set your compression ratio...

this happens basically because the more 'radical' your cam is the later the inlet valve closes.... so you effectively have less fuel/air mixture in your cylinder (yeah I know... not always the case but lets not go there)

here's a rough guide running 91 octane fuel...

mild road cam (cooper s type spec) aim at about 7.5:1

fast road cam (270 deg type spec) aim at about 8:1

raod/rally cam (280 - 290 deg type spec) aim at about 9:1

race cam (300 deg plus type spec) anywhere from 10:1 plus

If you went up to 96 octane fuel you could add about half a point to each of these...

I would consider these safe numbers :)

Good Luck
Author:  drmini in aust [ Sat Jan 20, 2007 6:14 am ]
Post subject: 
miniobsessed, I think your numbers are a bit low, by 0.5 at least. :wink:

I'm wondering WHY anybody would build a performance Mini motor to run on 91 RON, they are are throwing HP away with low C/R. :shock:

My other car with a 1360 and RE282 cam, modified Pulsar dizzy too is running nice at 10.7:1. It only drinks 98, but apart from Tassie, that's available in most places anyway now.
No it doesn't ping- GR says with this cam I could raise it a bit more. 8)
Author:  Mowog [ Sat Jan 20, 2007 9:25 pm ]
Post subject:  Octane v compression ratio
Thanks for you input guys.
I 'm not actually building a performance motor as such, just refreshing a standard A+. So if the standard c/r is I think about 8.8:1, then it should run happily on 95 if not 91RON?
Kevin, what total advance would you recommend for this motor with auto trans & standard cam?
Cheers
Scotty.
Author:  drmini in aust [ Sat Jan 20, 2007 9:57 pm ]
Post subject: 
With 8.8:1 C/R I'd definitely run it on 95, not 91.
Be careful though, some A+ motors were higher than 8.8.

re the advance, I'd mod the advance plate to reduce centrifugal advance to 10* (dizzy degrees), and set full advance with vac line off to about 28-30* using a timing light.
Total advance is much more important than static.... :wink:
Author:  richarde [ Tue Jan 23, 2007 5:05 pm ]
Post subject: 
Sorry for the (minor) hijack, but I know that increasing CR improves efficiency and power, with the disadvantage being pinging if the CR is too high. Has anybody actually looked at the effect it has on efficiency and whether it is worth increasing CR and running on higher octane, more expensive fuel?

Basically, does anyone know if the decrease in fuel consumption and the increase in fuel cost offset each other, or will it still work out more expensive?

Not that fuel consumption and price is necessarily a major factor in a car such as a mini :D
Author:  jbeenz [ Tue Jan 23, 2007 5:20 pm ]
Post subject: 
hmmmm, some a+ engines were 10.5:1 standard, with a cam similar to the 544.
Author:  MRClubby1098 [ Fri Feb 02, 2007 8:38 am ]
Post subject: 
i have a lightly modifyed 1098 and it pings on 91 but i use 98 now and it runs alot better and doesn't ping. After i build it up the CR will be 10:1.. :lol: 8)
Author:  Lillee [ Fri Feb 02, 2007 9:03 am ]
Post subject: 
richarde wrote:
Sorry for the (minor) hijack, but I know that increasing CR improves efficiency and power, with the disadvantage being pinging if the CR is too high. Has anybody actually looked at the effect it has on efficiency and whether it is worth increasing CR and running on higher octane, more expensive fuel?

Basically, does anyone know if the decrease in fuel consumption and the increase in fuel cost offset each other, or will it still work out more expensive?

Not that fuel consumption and price is necessarily a major factor in a car such as a mini :D


I think this topic is covered pretty extensively in Vizard's yellow book under "Tuning for Economy". Can't remember what he recommends for economy tuning as I really don't care... :P
Author:  MRClubby1098 [ Fri Feb 02, 2007 9:21 am ]
Post subject: 
lol, people that worry about fuel prices, shouldn't have a car
Author:  richarde [ Tue Feb 06, 2007 4:13 pm ]
Post subject: 
Lillee wrote:
richarde wrote:
Sorry for the (minor) hijack, but I know that increasing CR improves efficiency and power, with the disadvantage being pinging if the CR is too high. Has anybody actually looked at the effect it has on efficiency and whether it is worth increasing CR and running on higher octane, more expensive fuel?

Basically, does anyone know if the decrease in fuel consumption and the increase in fuel cost offset each other, or will it still work out more expensive?

Not that fuel consumption and price is necessarily a major factor in a car such as a mini :D


I think this topic is covered pretty extensively in Vizard's yellow book under "Tuning for Economy". Can't remember what he recommends for economy tuning as I really don't care... :P


yeah, i never really got around to reading that chapter as there are more important things to worry about then fuel economy :) I was just wondering is all.
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