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MG MGB Technical - Overbore: How Much?

I'm getting ready to buy another 1800 block and head to rebuild. A lot of people talk of overboring +.060 which would increase displacement to 1850cc(I think), but some suppliers (APT) sell pistons (same price for the +.060) for +.080 (1900cc). I know that going to 1950cc has known problems, but since it's very little trouble to overbore to +.080, does anyone see any problems for +.080 vs. +.060?

thanks,
Tyler
Tyler

One problem: if you use.o6o, you have room for another bore next engine rebuild- you can take it to .080 then. If you use the bigger bore now you will have to install liners next time at much added expense. I don't think .080 will give much more power than .060. I believe the .060 will give you 1868 cc.
Terry

I think +.080 is getting into the thin cylinder wall area where you might start to have flex and subsequent oil control problems, but I'm not a professional engine builder. Perhaps Peter B or Chris from Octarine will comment. +.060 also allows a rebore down the road. Just my $.02.
Paul K

I have over bored my engine to 84mm.( havn't bothered to calculate the increase in "thous"). takes the capacity to 1980cc. The proceedure is to bore all cylinders "offset", ie;no's1 and 3 to the front, and 2 and 4 to the rear, this maintains a good clearance between the cylinders and stops blowing head gaskets, I had the cylinder bores sonic tested first and ended up fitting one sleeve, In hindsight I should have sleeved all bores. The engine produces greatly improved torque, as well as maximum power. I sprint my car, so it has all the other engine goodie's, big Valves,roller rockers,scatter cam and fully programmably EFI, but I would recomend the over bore even on a standard engine.
Laurie Houghton

Laurie:
I would be interested in learning a bit about your EFI setup. What's the story?
Marvin Deupree

I bore most blocks these days to + 60 thou (1868cc).

There is enough metal in all but the very early blocks to go up to about +110 thou (1950 cc) without offsetting bores.

Lotus TC pistons are a readily available alternative in standard, + 20 and +40 sizes, but work out more expensive than MGB pistons.

Sleeving is necessary if you run into porosity problems.
Chris Betson

Marvin. I had gone down the road of 1-3/4" SU's and then 45DCOE Weber and decided EFI was next, the first effort had 1 injector mounted in a flange at each inlet port, long inlet runners ending up in a plenum chamber with a throttle body to atmosphere, all controlled by a "Haltech F9A" programmable computer,TPS as load sensing. Wouldn't rev over 3000, no matter how much more we increased the injector duty cyle.Back to the drawing board, Mk11, fabricated two throttle bodies by welding an 1- 3/4" ID Alum. tube to the butterfly section (cut off ) of a pair of 1-3/4" SU's and welding the injector boss upstream of the butterfly, Joined the two SU butterfly shafts and fitted the TPS on the end of one of them.
A fuel rail and two air horns finished the manifold, the whole thing mounts directly onto my original 1-3/4"
SU manifold. High pressure, lift pump and surge tank in the spare battery box. Two hours on the Dyno with a laptop and a 10% increase on power over the Weber, starts instantainously, 78Kw at rear wheels.
Laurie

Engine with 83.5mm bore (1930cc), HS6 carbs with KNs, oselli 271 cam, extractor manifold and a 43D dizzy plus of course a big valve head give 106 hp (78 KW) at the rear wheels of my 64 mgb.

The same engine had 112 hp @ 5000 rpm when using the CAEH 714 cam.

So whats the point in building and using EFI and ending with the same power???
joern

joern. 1-It will never go out of tune. 2- If I wish, I can change the tune with a keystroke. 3 - It is more economical than the Weber. 4 I can Data Log a run, after a event and observe engine conditions. ( I sprint my car ) 5 - And in my case, I was told it couldn't be done sucessfuly
Laurie

Laurie,
all your points taken, especially #5. Reminds me of 1984, when I had the plan to fit a 5speed Rover-SD1 gearbox into my MGB. Everybody from the local parts dealer to Ron Gammons from Brown&Gammons told me that this would be impossible to do – it would simply not fit I was told. This was enough motivation to go on with my idea to fit this box with a modified original MGB-bellhousing to the engine. After the gearbox was fitted the same people now said to me that the modification looked like a standard setup even from underneath the car!

Joern
Joern

Laurie: Quite fascinating idea, your EFI setup. I wonder if something similar couldn't be done for more mundane road cars. For road use and to preserve as much of the stock under hood look why not convert a pair of SUs into TBI units as you did, but try to keep as much of the look of the original carbs as possible. One could eliminate the jets and the piston, and hide the injector under the dashpot dome? No wait! Better: mount the injector under the carb body in place of the jet assembly to spray upwards? In this case the piston could be left in place to deflect the fuel spray. Just as the TBI was an electronic replacement for convential downdraft 2 and 4 bbl carbs, create a TBI SU, using electronic fuel metering.
Andrew Blackley

Fascinating stuff from Laurie.

Am I correct in assuming the EFI setup is partly a fabrication you have constructed, but also containing proprietary parts.

If so, do you have any pics, or failing that drawings.

Finally, can you name the proprietary parts for we lesser mortals.
Tom

TWI makes throttle bodies for webber dcoe carbys. Allot of ZX guys run them
Bob

Tom . . The throttle bodies, fuel rail, TPS mount were fabricated, the hi-pressure pump and regulator are Delco,the Computer is from Haltech, F9A ( fuel only )secondhand cost $AU 400.00, and they have a web site.Add $150.00 for sensors, wiring, relays etc; and it is cheaper than a new 45 Weber.
I'll have some photo's soon, as Haltech have asked for these to put on their web site, so I'll e-mail you a set. Andrew, If you would like a look as well send me your e-mail address.
Laurie houghton

Joern

I'm fitting a Rover 2Litre O Series and using twin SU HIF6/44's. I haven't decided on air filters yet and was wanting to fit K&N's to standard Coopers Air filter bases. Obviously the MGB filter box base is too small to fit on HS6/HIF6/HIF44, so what did you use.

I've search the web trying to find a car that uses 1-3/4 inch - 44mm SU's with Cooper type Air boxes, without success.

May eventually use the Box from the Rover SD1, with feed pipe to a universal K&N infront of the radiator.
Steve Williams

Steve W.:
Why donīt you use the complete K+N filters? they are excellent, look good and give very good performance. I am using K+Ns with the ram air stacks inside. a pic of my engine is in the MGB-section of mgcars.org. There is a thread about moving the battery to thwe trunk. Here you find a comment from me with a direct link to my< engine compartment.
Joern

Laurie


How are you timing the firing the Injector -- Did you fit a Crank Position Sensor? Did your controller come with a management startegy built in or did you do it from scratch? Are you still letting the distrbuter control spark timing or is it handled by the micro as welll ---- just curious to see how you are handling advance.

Sounds Pretty Cool though

kevin
kichterman@motorola.com
Kevin Lichterman

Kevin. The Haltech Computer controls the on-time for the injectors (2) and controls fuel Only, gets it trigger signal from the distributor. Threw the Lucas " Prince of darkness " dissy out and replaced it with a Bosch unit custom built for competition use c/-Porsche points, still uses the centrifical advance mechanism. Check their web site,www.haltech.com.au/
L.J Houghton

This thread was discussed between 14/06/2002 and 22/06/2002

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