Are you bored? (Mathematics question)

[RobThomas]

I have a 948 Minor engine here and a 12G295 head. Assuming they were standard, what would be the approximate comp ratio? I haven't yet seen if the new pistons are oversize or what the dish is, nor have I checked the head for skimming.

I also have a Shorrocks Supercharger kit in a box. Can you see where this is going???

[philthehill]

Taking into account the unknows I would suggest (without doing a full calculation) a compression ration of around 8 : 1.

The standard Minor head/pitons/bore (in high compression form) gives a 8.3 : 1 compression ratio with a 24.5cc combustion chamber.

Given that the 12G295 head has a larger combustion chamber volume at 28.3cc the compression ratio will be slightly lower.

Low compression dished pistons are best when using a Supercharger.

[RobThomas]

AE 14396 +20 pistons (4.6cc dish?) for 8.3:1 ratio. Head is standard 2.75 inch deck.

[RobThomas]

Thanks, Phil.

[philthehill]

Rob

Take care not to get blown away with the increase in performance

Phil

[RobThomas]

I sat down and did a bit of Algebra. It suggests about 7.95cc for the dish plus the gasket if you use standard figures for chamber volume and swept volume (237cc and 24.5cc). Does this sound about right, Phil? 3.35cc for the gasket area?
If so, the change to a 12G295 would drop the ratio to 7.54:1.
This all assumes my 35 year old memory of Algebra is right.
If so, that will be quite good for the Shorrocks.

Swept vol + x + Chamber : Chamber + x
237cc+x+24.5cc : 24.5cc+x gives a ratio of 8.3:1
x=7.96=area between top of piston and base of chamber
x-dish (4.6cc)=volume above top of piston edge to bottom of chamber=3.35cc

237+7.95+28.3 : 28.3+7.95
=7.54 : 1 ratio.

Alternatively, I could be out mowing the lawn.


I should add that this is to go in the Series MM car that I got from John Gamble that has previously been fitted with an A-series. The current SV engine has got massive ring blow-by and dumps the entire sump in about 20 minutes of driving.

[philthehill]

Rob
It sounds right to me.

My quick calculation did not take into account the dish of the piston, the volume above the piston but below the bottom face of the head (which including the volume created by the head gasket).

Phil

[RobThomas]

Ta.

I doubt I can blow too much into a 948 without it complaining. Not sure how much the Shorrocks will boost it by. I have a couple of sets of pulleys to play around with. The head will need a light skim, once we're allowed out. I have a couple of 10CC blocks here that I could also have bored out. Both look to have broken rings and scored the walls, although they have little wear. The 948 only cost me £50 for the entire rebuilt engine (Vedette boat engine) so it is the smart choice for now.

[philthehill]

Rob
The poor old 948cc 'A' Series is much stronger than many people think. It will accept a reasonable amount of additional boost without complaining.

Have a read of the following:-

http://www.rjh-heritage.ch/index_htm_fi ... 0B%20C.pdf

I would agree that the 10CC engine is the better engine but if the 948cc engine is near ready to go that is the way I would go.

The one recommendation I would advise to carry out with the 948cc engine is to bore to 998cc and fit fully floating rods and dished pistons. Use 1098cc rods. They are same as the Cooper version.

The camshaft bearing loading is no more with the supercharger than without as a near standard camshaft is preferred. So no need to fit a full set of white metal camshaft bearings.

Fitting a supercharger is one of the easiest ways of increasing the engine power - but Shorrocks superchargers and their fitting kits are not cheap.

Phil

[noidea]

wow
all them numbers seams like an exam to me cant get my head round things like this .think i must have something missing in my head ..
but i suppose i must have some brain as i used program cncs from drawings and could make any part out of any material

dave

[RobThomas]

CNC from drawings? Hmmmmm. I have some drawings here that I wanted to have laser cut but am not yet very good with the CAD freebie that I downloaded. If only I could find someone who was any good and was not very busy at work.....

[Declan_Burns]

Rob,
If you need help with CAD I can probably help as I spent all my working life doing CAD drawings. That is how I make my rubber oil seals for A-series and MG. I can also do 3D CAD.
CNC from drawings from drawings is no problem. Once the drawing has been created it is exported as a so called dxf file which can be imported to the CNC machine-whether it is a milling, a plotter, a laser or water jet metal cutting machine. The dxf file is an international standard which can be exported from all CAD programs. It is all very easy if you are used to it. There are plenty of online companies who will do such cutting and it is not very expensive at all.
Regards
Declan

[RobThomas]

Wow, thanks for the offer, Declan.

I'll try to get the drawing scanned before the weekend.

Cheers

Rob

[Declan_Burns]

Rob,
Is it a HD drawing? (hand drawn )
Regards
Declan

[RobThomas]

Copy of a Factory blueprint with all the dimensions on it, fortunately.

[philthehill]

Blueprint - now that brings back memories.

[noidea]

i must be getting on a bit as there we no files when we programed them, you had a drawing a control panel and bobs your uncle you could programe the machine make the parts but over the years things have changed and it is different now .all you need to get it laser profiled is a sketch with sizes on and it can be produced i have just had the heater dial for a mini done in stainless and also etched with on;of screen heater done with the lazer

dave

[noidea]

hi rob

if you need lazer profiling doing i can help and point you in the right direction

dave

[RobThomas]

Thanks for the offers of help, guys. I have a local laser cutting company who can cut from emailed files and drop them off en-route to work for me. Ideal, really. I'm just trying to use all this spare time to find and fix lots of little 'issues' that I've had on the back burner for a long time.

[RobThomas]

blowog2.jpg (128.6 KiB) Viewed 1361 times

Engine is now well on the way towards completion.


Still working on getting this blueprint turned into a file for laser cutting.
I'm not very computer savvy but have got the files in .pdf format. Do I need to convert them first before I could try sending? Kids are not being at all helpful today.

blowog1.jpg (249.25 KiB) Viewed 1361 times