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MG Midget and Sprite Technical - Shave my head?

No, not my physical head, my cylinder head. I have a mk3 Sprite, 1098 overbored 40 thou, 731 cam with 4 degrees advance on a Duplex chain, HIF44 carb, LCB exhaust manifold and straight through silencer. I want to have the head skimmed to increase the compression ratio. So my question is, how much should I have taken off? It's a 12G295 head and if I remember correctly, Onno mentioned that there are issues with an oilway if you take off too much on this particular head. Also, do I need a different head gasket than standard to take the extra compression and do I need special head studs and nuts. Do I also need to alter the static ignition timing? Sorry for so many questions.
Thanks as always for any input.

Bernie.
b higginson

when your block was bored, was the top decked / skimmed too?
what size piston dish do you have?
what chamber size is your head currently?
what CR would you like to end up at?
David Smith

How to detremine how mutch you can shave of your head is in Vizard page 184 figure 9.13
simply said measure down the oilfeed to the rockers and add 60 thousands thats the max that can be taken off.

But that may be to mutch....
So you realy need to meassure (with a burret)the volume of your head now and than do the math to figure out what you want it to be.

A good payen HG should be fine.

Minispares do a good set of head studs at a resonable price.
http://www.minispares.com/Product.aspx?ty=pb&pid=34310&title=
Always a good idea.

Timing it staticly is not going to cut it.
You want to time it with a strobe at max advance.
It should be some where around 30-32 degrees of advance.
But you can only find out what is best on a RR
I would set it up at 32 degrees and see how it drives if you get anny pinking than retard it a few degree's
Onno Könemann

David. The head is still on at the moment, so don't know chamber size other that it is standard. I don't think the block was skimmed, but can't be sure,it was a long time ago and was done by Burton engineering on Eastern Avenue London.
Only shallow dish on the pistons and as regards CR I would like something that would give a little more low down power as the engine really sings when it come fully on the cam around 4000 to 5500 rpm.

Onno. Thanks for such a comprehensive answer. I'll print it off for when I get it done. I hope to have it done soon, as a bunch of us from N. West area of MASC are going to Peter Burgess' on the day before midget 50 for a group session on the rolling road. I don't think he will be able to get too deeply involved in carbs and needle changes etc. but he has said he will do a bit of ignition timing adjustment.
Thanks again to both.

Bernie.
b higginson

Bernie
Make sure you know what needle and jet is in your carb.
Also list all of the other details of your engine ignition.
Then e-mail PB and explane when you are comming and if he has any advice for the set up.
If he has make sure you bring/fit those.

Adjust the valves check the ignition timing/carb adjustment and relace all that even remotely looks worn.

Being prepared is most important and you do not want to waste precious RR time.
Onno Könemann

Bernie as Onno says, Vizard (and other places) will tell you how to calculate CR but you need to find out the numbers first; without them you cannot calculate and are simply groping in the dark. Aim for 10:1
David Smith

>>>>>>>I would like something that would give a little more low down power as the engine really sings when it come fully on the cam around 4000 to 5500 rpm.<<<<<<<


That seems a bit late or to high for the cam to be coming on line.... Just my personal opinion, bbut Id think somewhere around the 2000 rpm mark would be more fun for the street... what type of racing are you doing with this engine If its okay to ask.

Im sorta wondering if you want more down low torque, you may want to consider a differant cam rather then a Higher CR

If your going to have the head shaved another additional thought to seriously consider while its off and at the machine shop, would be to have the head ported and some bigger valves installed... that way it can breath so much easier.

Prop

Prop

Prop it is a 1098 with a 731cam and low CR!

The 731 is an old style cam that loves high CR.
And as far as i read it it works below 4000 but gets it's best above that not realy a strange situation.

the 731 in my 1380 comes in at 3000 to 3500 but htat is just down to the bigger engine.

2000rpm if i want that i would buy a diesel ;P
Onno Könemann

Prop. I can't really afford to go to bigger valves and porting and the 12G295 is not too bad as regards flow in its standard form. The engine does start to come onto the cam lower down, but seems to really open up higher up the rev range.

David. Yes, 10:1 that's what I now will be aiming for. Besides all the great help Iv'e had from you guys on here, my freind Les Robinson has been doing some calculating for me and with his help I came up with 10:1.

Onno. Thanks for the "be prepared" advice. I've got all the details of engine, needle and ignition etc to hand and I'm just about to email PB and ask his advice. I wasn't aware that the 731 liked high CR more than other cams, so the decision to skim the head looks like it should pay off. I wish I could afford a 123 dissy as well!

Bernie.
b higginson

Hi Onno

The biggest problem I will have is I am checking 8 cars or so with a sort of dyno shoot out. To be honest I prefer setting cars up rather than just power test them, that is why I said I will give them all a little +/- power testing with altering the dizzy. I will not have time to play with fuelling. I have set the charges way down low so the dizzy stuff will be a bonus. I will also take af ratios under WOT conditions and a hint of cruise AF. The cars will have to run as driven in :(

Peter
P Burgess

ahh did not get that.

My experiance here is RR are bloody expensive so having the RR operator happy is very important so he will not charge that extra 15 minutes ;)
Onno Könemann

Hi Onno

We charge about £80 plus any bits for a dyno session, it takes as long as it takes, usually a couple of hours (I do not rectify big faults as that is really beyond the scope of dyno time...it should be used for fine fettling). If someone wanted to mess about with their laptop we would charge around £100 plus any bits. With programable stuff I would tend to do the testing and see what had changed and maybe offer some suggestions. The session may take all day but as long as I can do other work as well I am happy. We do not charge by the minute or hour :)

The Midgets are coming in to have bhp runs/power loss through transmission runs and a little messing with ignition timing.

You are right though, it is a waste of time if the cars are not prepped for the rollers! And annoying! Especially for the quick run cars....EG full throttle, points in good condition,etc etc. That is why I prefer full session tuning...we usually only book in one per day in the mornings.

Peter
P Burgess

Bernie
ALL high overlap cams love/need High CR It is not only the 731 or mystical but is commonsense when you understand the reasons (unfortunately 99% of Pub BHP experts don't understand why)

Well it is simply this

CR should be as high as possible in any engine to ensure full efficiency

CR is limited by pre ignition and so has a high limit

With a high performance/high overlap cam the trapping efficiency is reduced so that at low/medium RPM the actual CR is reduced. This means a higher CR can be calculated before detonation is encountered and thus this higher CR will make full use of the lower amount of trapped mixture at lower to medium RPM

Simple really :)
Robert (Bob) Midget Turbo

I think i need to go to the UK for RR next time!

last time i was on the rolers it spent 1.5 hours and it cost me €200,-
Here you have to pay for:
getting on the rolers
each run
or
you pay every hour.

So here it realy is no cheap deal

If i can attach a holiday to it the UK rout would certainly be cheaper.
I'l have to see next year when the new engine is finished.

Onno Könemann

It's time for some genius to come up with a set of plans for "build your own dyno".
JM Morris

>>>>It's time for some genius to come up with a set of plans for "build your own dyno".<<<<<


Hmmm, im thinking way back in the early to mid eighties ither car craft, hot rod, or super stock car magizine had an artical on how to build a dyno


Car craft or Hot rod did have a great tutorial on how to build and operate a clyinder head flow bench.

Prop
Prop

that said... theres a big differance in reading about building something like that and actually building a contraption like that then strapping the car down and racing it to 100 mph in your garage...LOL
Prop

Bob. Thanks for that. I just didn't want to go too far with the CR as I didn't want to create problems with the starter not being able to cope. There's another thread on here where the writer is having problems with a newly built engine with a high CR,
not that mine is newly built.

Bernie.
b higginson

Had a 12:1 CR with a 731 cam and my std starter has no problems.

I would be surprised if you get it up to a 12:1 CR with a 1098
Onno Könemann

This thread was discussed between 06/03/2011 and 11/03/2011

MG Midget and Sprite Technical index

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