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MG MGB GT V8 Factory Originals Technical - Which engine

I am upgrading my factory GT V8
So far I have sorted the brakes and suspension so its
time for the engine. Is it best to keep the 3.5 or go for the larger capacities. I intend to fit the weber 500, Rv8 manifolds and stage 3 heads.
Mark Rawlins

The anser largely depends on the condition of your existing engine, your budget and what you want from the car.

I have retained a 3.5 which is overbored by plus 0.040" that takes the capacity to just over 3.6. If I were in the market for another engine then I would be tempted by the availability of 'turnkey. 3.9 litre engines. There is some intersting discussions in other threads.

Roger Parker

I re-built my 3.5 approx 25k miles ago.
New mains, big ends and rings etc. using an sdi engine. I was hoping to sell this engine for not much less than it has cost me ( 200 ).
I would like my V8 to have effortless performance, be nice to drive on a long run and be able to hold its own at a sprint or hill climb.
The spec so far is.
Adjustable konis all round.
Composite rear springs.
Panhard rod.
Anti tramp bars.
Willwood superlite front callipers & Vented drilled discs.
Lowered fast road front springs 8.1 inch free height.
3/4 inch front anti roll bar with rose joints.
Compared to the standard setup the handling is superb.
I am seriously considering a 3.9 from RPI to stage 3 with webber 500 and RV8 manifolds.
Any suggestions!


Mark Rawlins


Having recently uprated from 3.5 to 4.6 from RPI a note of caution. The Heads from RPI were not cc'd, the block core plugs leaked, piston rings were broken and journals scratched. On the basis that it was always my intention to blueprint and rebuild the engine these did not turn out to be major problems. Also to be fair RPI did reimburse me for the problems re the block or exchange the block, but not the cost of equalising combustion chambers which I assumed to be included in the cost of the heads after reading the Web Site.

All that is in the past and I now use the car every day for work, however, also be prepared to uprate cooling system as a 4.6 generates a lot more heat, clutch ( I'm using RV8) and adding LSD. I also use Willwood front discs. The other problem is Insurance and Insurance valuations.


I am un-decided as to which way to go with the engine size. What is the 4.6 like to drive? What mods have you done to the chassis/suspension? and would you recommend RPI as my supplier?
Could you possibly send me a photo of your car direct.
Many thanks
Mark Rawlins


Chris Crane of RPI is very helpful has a very good web site and he (?) did reimburse me. At the time of my conversion Real Steel (my first choice) did not have engines available and MG owners were not advertising 4.6 V8 either, as they are now.

I bought the Block and Heads seperately as the engine was always going to be hand built and it is the origins of the block which was dubious in that as far as I can now ascertain the Block was produced by Land Rover sold on to Morgan then to another supplier before RPI (It was the other supplier that reimbursed me). During this process someone had pushed into the engine 5 Core plugs replaced then with new core plugs but did not take account of the fact that the original core plugs are staked, this resulted in leaking, which was lucky because the 5 Old core plugs were left in the engine which if not removed could have led to overheating and worse.

The moral is to check the origin of your engine.

I also got the Stage 1 Heads from RPI. If you read the web site it says that standard replacement heads are cc'd. I assumed this to be the case with stage 1 but they were not. Of course if the Heads had not have been cc'd after purchase I would be none the wiser.

The moral is to check exactly what you are getting for your money.

If you need head work done I do have a couple of contacts as I,m not sure whether Real Steel are carrying out this work at the moment.

As an aside Real Steel and RPI are probably poles apart on the subject of V8 engines.

Spec etc to follow.



Following another thread (if I were to do it again) I would start with MG Owners as they offer a full spec.

The set up
Negative camber wishbones
3/4" anti roll bar
Anti tramp bar
Rally type rear spring
Front Spring 8"x600lb/in
Koni on rear (at softest setting)
Uprated lever arm on front

The handling is neutral with 4.6 Power so your current set up should not need changing.
Engine size and tuning is more a matter of budget (RPI can offer 5.2) and we probably all would, given the chance, put in a F1 engine until you end up rebuilding it each weekend.

My own view is more cc and less tuning should improve reliability and driveability for a road car.

I'm not convinced that taking exhaust through flitch panels is that good an idea as it probably weakens the chassis. The idea is also sold on the basis of reducing under bonnet heat but the problem I have with under bonnet heat is stuck in a traffic jam on a hot day, hence I've uprated radiator.

Sorry no digital photos.

I also drive a standard midget owned 27years from new with 60bhp and both are fun cars to drive.


Further consideration.

The 3.5 and 3.9 (and the current 4.0 for that matter) use a 77mm throw crank. All larger displacemnt engines operate on a longer throw crank and I can ALWAYS feel the difference in throw through less smooth engine balance. This is not a problem, but I personally prefer the smoothness of the original throw and subsequent engine smoothness.

The engine was designed as a 215CI (3.5 litre engine and this includes the head details. Therefore the air flow design limits in the heads are balanced for the 3.5. Increasing the cubic capacity reduces the ratio or airflow to capacity and the result is an engine that is less efficient per cc. This is most obvious as the revs rise in the upper 1/3rd of the rev range and again is not a problem but does affect the character of the engine.

Going from 3.5 to 3.9 involves a 4mm larger bore which has a quite significant effect on unshrouding the inlet valves. (Bore wall is now further from the edge of the inlet valve. Exhaust valves are approx 20% smaller so are less affected.) The unshrouding actually allows a very useful increase in airflow through the inlet which offsets the restriction that the extra capacity would otherwise have.

Where the capacity is increased further with crank throw lengthening you do not gain such airflow efficiency improvement.

There is a great deal of untapped potential in the standard V8 cylinder heads. The best being the SD1 and HPD Land Rover injected engines. (HPD being High Performance Derivitive. Effectively this applies to all injected 3.5, 3.9 and LSE 4.2 heads.) Earlier non SD1 (Rover P5B, P6B and Range Rover to engine suffix F) do not have such a good design and lack ultimate high valve lift flow. The latest heads as found on post 1994 engines are actually worse still in flow terms!!!

The last finding actually raises questions about standard 4.6 Land Rover engines, at least for non Land Rover applications as the emphasis is very much on torque and loss of higher rpm power is not an issue, hence a 3.9 peaks with 185 bhp, a 4.2 has 200 and the 4.6 has 225.

The fitting of a pair of expertly modified heads will on average provide between 30 and 45 bhp more at the wheels of 3.5 and 3.9 litre engines, with a linear increase throughout the whole rev range. I don't know about 4.6 as I haven't seen any, but I have genuinely seen in excess of 25 cars produce these results on a before and after basis. My own 3.5 V8 injected B showing a rise from 145 at the wheels to 186, although some of that increase would be down to the engine freeing up from the start at about 2000miles to the latter at 9000 miles.

Engines with quality heads are often much smoother due to equalised firing pulses due to even port shapes to equalise airflow in each cylinder followed by chambers that provide equal compression. As an example I can have full throttle acceleration from 400rpm in 5th gear with smooth unflustered progress. This to illustrate that low down torque is not compromised. At the other end of the scale the engine has an appetite to run and positively encourages the driver to use it. Subjective but one of the features that makes a cars pleasures.

With a 4.6 you would definately need to have the heads modified by a ture expert this would extend the breathing capacity to just above standard 3.9 levels. To assist further a cam change would be an advantage as the one other effect of capacity is to 'soften' the effect of a cam and substantially bring down the peak power rpm point. The reality that will be seen on a rolling road dyno will be about 4200 to 4300 for that peak. What would have been a 'hot' cam for a 3.5, such as a Crane 248, would be a pussy cat in a 4.6 and be not much more than a 'soft road profile'. This would also usefully extend the engine rpm band to around the 4600 to 4800rpm range.

Large displacement engines in the MGB do work and diferent people have a different view. However not many have had the opportunity of being able to drive so many different MGBs and other cars with different capacity and spec Rover V8 engines. I have been fortunate enough to drive behind literally dozens of differnt cars, some for very extended periods and it is the experience that has provided the different characteristics I have mentioned.

In addition to the two companies listed I suggest you may like to speak with Peter Burgess, a friend of mine so I am biased, whose heads and engines were over 10 years ago providing Championship winners. (Cox an Buckles TR Championship 3 conseq years with a 3.9 TR7 and well over 300 fully road useable bhp.) Peter has a web site hosted here at

I have gone on far too long again, but I hope that you find the info useful.


Roger Parker

.........and dont forget to fit an RV8 exhaust as that can free up around 12 hp against a block hugger!
bob p

Further to Roger's considerations I thought I would look at the numbers (sorry boring accountant)

Engine BHP Torque
3.5 140 190
3.9 200 220
4.6 220 280
Stage 2
3.5 220 200
3.9 230 250
4.6 265 300

Following Rogers analysis that the Heads are designed for the 3.5 the table above shows a 80 BHP increase with stage 2 Heads. Also changing from stardard 3.5 to standard 3.9 gives a 60 BHP increase. So per BHP the 3.5 and 3.9 are winners.

Also Roger points out that the 4.6's emphasis is on Torque. The table again prooves Rogers point with the Standard 4.6 increase in torque over the 3.9 is 60 with only a modest 20 BHP increase.

So the 3.9 are smoother and free reving engines.

However, why does the 4.6 show a 45 BHP increase when taken to stage 2?

Also the 4.6 takes advantage of the latest 4.6 Crossbolted block with stronger castings, Bigger journal crankshaft, fully enclosed main bearing caps, stronger rods and rod stud fixings.

Just when you thought it was all over the 3.5 and 3.9 make a comeback because they are cheaper which allows the cash saved to be spent on the heads.

The choices from RPI the Ultimate Big Valve Heads 1,500 and Peter Burgess Rally and Race Heads at 1,000 or POA.

Perhaps Roger and Peter can give the expected pro's and con's of this route and expected cost BHP and Torque figures.

The conclusion,if you have a V8 whatever the spec you are onto a winner.

One further option is a stroker kit from Real Steel (see article buy Bob Hitchcock Enjoying MG Oct 98).


Paul Wiley

The figures in the table were from MG Owners' Accessories. I was not happy with a couple of the numbers so checked with RPI web site which indicates the following

Standard 3.5 170BHP
Standard 3.9 190 BHP
Using these figures would smooth out increase in BHP when comparing standard to Stage 2.

RPI give BHP for stage 1 and stage 3 - Stage 3 for 3.9 140 BHP and 4.6 185 BHP.

Bob mentioned above that RV8 Exhausts give additional 12BHP. MG Supersports article indicates you lose 10BHP if you have twin exhausts. Time for a game of Snakes and Ladders.

Paul Wiley

This thread was discussed between 29/10/2000 and 05/11/2000

MG MGB GT V8 Factory Originals Technical index

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