15 replies to this topic

[zero_wlv]

Hi all,

 

I'll be taking in my car for rolling road tuning shortly  (engine spec below),  and did have some doubts about whether I should be taking it with standard 95 RON.

Obviously I'll be taking it to the RR with the same octane fuel that I intend to use long-term,  and I'd pretty much decided on 95 RON,  but something I just read in another current thread rekindled questions in my mind:

 

"It might be wise to use 97 RON petrol from now on as well with the CR you appear to have."  (here)

 

According the engine builder's test certificate,  my compression ratio is:    "9.5:1"  and  "10.6:1 (calculated)".    I'm not sure exactly what this means but I think he's worked out the 9.5:1 from the compression test and the 10.6:1 from the engine geometry.

 

Does anyone have any advice on whether it's fine to use standard fuel with this compression ratio  (9.5 or 10.6,    I'm not sure which is the more relevant  -  otherwise I might have been able to figure out the answer from other threads).

 

The engine does run-on really badly after ignition is switched off,  but I'm not too bothered about this as I've got used to stalling it before it has a chance to run on  -  however,  I did experiment with a tankful of Shell 99RON and noticed that it made a big difference to the running on  (although it didn't eliminate it).

 

I haven't noticed any pinking but having said that I haven't been running the engine under heavy load anyway due to other tuning issues  (I think the needle's too lean under heavy load),  and I'm not sure I'd recognise pinking if I heard it anyway.

 

I'm not averse to paying for premium high-octane fuel if it's necessary but then again I wouldn't want to be shelling out for it for years unnecessarily.   I'm not bothered about squeezing out an extra BHP or so if the engine can be tuned to run happily on 95 RON.

 

So the real question:    is 99 RON fuel necessary to be able to get this engine to run well,  or would it be an unnecessary luxury?    I'd just like to know so I know what to fill up with before the RR session.

 

------

 

Engine spec:

 

998 + 0.060" = 1047cc

Cam:   MG Metro clone

High compression pistons

12G940 large valve head with some porting

Standard rockers

Hif 38

Compression ratio 9.5:1

Compression ratio 10.6:1 (calculated)

 

 

[Spider]

How has the Cam also been timed in?

 

It could well be that the run on issue is from a hot spot(s), eg, a bur from machining is a common one.

 

Static CR is usually derived from careful measurements and then calculated. His two figures here, I think need a bit of explanation so we can better understand them.

[zero_wlv]

I'm not sure how the cam would have been timed in,  as I got the engine as a whole package with everything fitted.

 

As you mention hotspots,   it's worth mentioning that the engine's running on the lean side right at the moment,  which I suppose could exacerbate the effect of any hotspots (if any).

 

It's not clear from the test certificate what he means by the two different compression figures,  but:

 

  9.5:1     I think this is the dynamic ratio which he's worked out from the compression test results.    It's written right underneath the compression test figures so this would make sense.

  10.6:1    He calls this "calculated compression ratio" so I think this is the static ratio calculated from the combustion chamber volume etc.

 

This is one reason why I'm unsure about which fuel to use,   because on the threads where I have found some advice,  I'm not sure whether they're referring to static ratio or dynamic ratio.

 

For example,  I found this graph on another thread (I've just realised it's from one of your posts so I guess you're familiar with it!     ) but the recommended minimum octane for 9.5 is very different from that for 10.6

(Bit unsure whether it refers to UK-style octane ratings)

 

Edited by zero_wlv, 03 February 2018 - 08:28 PM.

[zero_wlv]

Also,  I think many A-Plus engines in the 80s were as actually as high as 10.3:1 out of the factory?  (e.g. here)     I'm not sure what RON leaded petrol was but this does indicate that nothing too exotic would be required for 10.3:1.

 

On the other hand,  ACDodd says (here):

 

  If you are intent on building a modern version of the mg spec engine, limit the compression ratio to 9.75:1 with the MG metro cam. It will run smoother and give more power. The fuel we use now simply won't perform with that cam at 10.5:1. 

 

I'm not sure but I would guess that in both these instances it's static compression that's being referred to  (which I think is 10.6:1 for my engine).

ACDodd's quote is particularly interesting for me because he's discussing the same cam (almost) as what I have fitted.

[absx2]

The mini city E of the 80`s had a CR of 10.3:1 static and the owners manual stated 98 RON fuel.

 

I have a bog standard 70.000 mile city E engine in my daily drive and I can confirm it is extremely temperamental regarding fuel quality with the odd 10 second run on if the super market fuel is particularly bad. If it wasn`t such a sweet motor I would have chucked a gas flowed head on by now to lower the compression as fuel quality is such a joke now that we need to get sensible with comp ratio`s unless its a weekend warrior that you are going to run on super with additives.

 

In comparison my morris minor with an 8.5:1 CR will run perfectly on anything that will catch fire if you throw a match at it, a little too low for performance use but a smooth and happy motor.  

[zero_wlv]

Thanks absx2,  that's just inspired me to look in my owners' manual   (which of course isn't directly relevant to my engine but interesting nevertheless to see what fuel Austin Rover recommended).

 

It says for 8.3:1 compression,   91 RON (2 star)

And for 10.3:1 compression,   97 RON (4 star).

 

(I distantly remember 2 star from childhood as being something really mysterious and exotic that only Yugos ran on  )

 

So maybe I really am pushing my luck by trying to run 10.6:1 with standard fuel and expecting the rolling road guy to be able to tune it properly.   Maybe I should ask him before I go.

 

I'm really after smoothness and reliability more than eeking out every last bit of power.

[Ethel]

Dynamic CR should be from when the inlet valve effectively closes. Can't you ask the engine builder how he came up with the figure?

 

Running a higher CR should also be better for fuel economy as the charge has a proportionally bigger volume to give up its energy in. It'd depend on the relative fuel prices whether you can make it pay off.

[zero_wlv]

Can't you ask the engine builder how he came up with the figure?

 

To say he's not the easiest man to get hold of would be a diplomatic understatement 

Edited by zero_wlv, 03 February 2018 - 11:14 PM.

[Turbo Phil]

Bear in mind these RON figures recommended by the factory for a certain compression ratio were when leaded fuel existed. Removing lead from fuel reduces its knock resistance. So a 97 octane UL is not as good as a 97 octane leaded at supressing knock.

 

Phil.

[Spider]

  9.5:1     I think this is the dynamic ratio which he's worked out from the compression test results.    It's written right underneath the compression test figures so this would make sense.

  10.6:1    He calls this "calculated compression ratio" so I think this is the static ratio calculated from the combustion chamber volume etc.

 

If this is right, then at 9.5:1 DCR it is Sky High, well past 'Hand-grenade' territory. I doubt you'd even on that on 100 RON.

 

8.8:1 DCR is as far as I'm 'happy' to go and that's with fresh 98 RON Fuels, but I feel much happier at 8.4:1 DCR.

 

I don't have a lot of time right now, but off hand, I feel given your Cam Profile, these are two figures (the SCR and DCR) are in error.

[mikal]

"Bear in mind these RON figures recommended by the factory for a certain compression ratio were when leaded fuel existed. Removing lead from fuel reduces its knock resistance. So a 97 octane UL is not as good as a 97 octane leaded at supressing knock."

 

Sorry, but that is simply not the case.

The Octane rating (RON or MON) of a fuel is a measure of a fuels resistance to 'knock" or detonation. It is determined by running the fuel on a single cylinder variable compression Octane Engine and comparing it to lab formulated test fuels of known Octane ratings. An unleaded fuel of say 97 RON has exactly the same knock resistance as a leaded 97 RON fuel by definition...

[Turbo Phil]

You're right of course. What I should have said is that old leaded four star was generally higher octane than today's super unleaded, mainly due to the lead content.

Phil.

[Spider]

I think the leaded fuels hold their Octane Rating much long than these new fuels do.

 

BP (here in Aust) only guarantee the Octane Rating for 4 weeks from delivery to the Petrol Station.

[zero_wlv]

If this is right, then at 9.5:1 DCR it is Sky High,

 

That very likely means I've guessed wrong then.

 

The engine builder's clearly calculated the compression ratio with two different methods and come up with 9.5 and 10.6.

Naturally the easiest way to find out would be to ask him but he appears to be a busy man    and I'm pretty sure I wouldn't get a reply before Monday 13th when the car goes in for RR tuning!

 

I think from these two figures I'll have to stay on the safe side and assume that my engine's 9.5:1 static.

From what I've read I'm fairly certain that a 10.6:1 static compression engine should be run on at least 97RON.

As for 9.5:1  I'm still uncertain,   and wouldn't want to be paying for premium fuel for the next few years if it's unnecessary.

[carbon]

Zero,

 

Looking at your spec:

998 + 0.060" = 1047cc

Cam:   MG Metro clone

High compression pistons

12G940 large valve head with some porting

Standard rockers

 

The static compression could be as high as 11.5:1 using a 12G940 head with standard (21.6cc) chambers, flat top pistons set level with top of the block, and no exhaust valve cut-outs.

 

If you cannot get definitive data about the engine build (piston to deck clearance, any block cut-out volume, and combustion chamber exact volume) then you will always be guessing. Taking the head off and making these measurements will at least give you the correct info.

Edited by carbon, 04 February 2018 - 09:16 PM.