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MG MGB GT V8 Factory Originals Technical - Power Output

Appologies in advance for what is a pretty stupid question, but I'm a bit puzzled.

My roadster has a 10.5:1 3.5 motor, Offenhauser dual plane mainfold, and Edelbrock 500 carb. What sort of power should it be producing (assuming it's all working properly)? Ball park figure of what - 120, 150, 180 HP?

I ask as it doesn't seem quick. I don't have much experience with performance cars, but it doesn't _feel_ as fast as (say) an old Golf GTI.



Nick Wilson

Should be on the "good" side of 180bhp, standard Sd1 was rated at about 160ish , so with a few goodies you should be 10-15% up on that figure.What heads are you using? The old 10.5comp motors (Rover P5B) used small valve heads, identified by short reach plugs, these heads will make the motor feel out of breath as the revs climb..Have you had the engine set up by someone with Rover experience or rolling road??It's a common mistake to build an engine full of goodies only for it to dissapointing on the road because it's not set up properly, carb jets, timing etc..Good luck.
John :)


May be worth having a rolling road session - There is a place in Ivor Bucks. For a road V8 I would say between 140-220 depending upon mods - anymore details?.

An alternative to the rolling road is to try the easydyno software wich should give a ballpark number.


Here is a formula for working out horse power.
The site is from

And they have lots of other calculations as well
(You will have to use a scientific calculator)

My Rover 3.5 work out as follows

Atmospheric 14.7

Compression ratio 9.75

Volumetric effeminacy
(with aluminum heads) 1.1

CI Displacement 215

RPM 5500

/5252 /150.8

14.7 x 9.75 x 1.1 x 215 x 5500 / 5252 / 150.8 =235.39HP

But you have to keep in mind this does not take the carb and exhaust into consideration. It does not take the cam etc into consideration as well. But it is interesting

5. Horsepower
HP = *Atmospheric pressure X Compression Ratio X *Volumetric effiency
X Cubic Inch Displacement X RPM at HP peak / 5252 / 150.8
(example: 14.7 x 10 x 1.05 x 306 x 5800 / 5252 / 150.8 = 345 hp)

*Atmospheric pressure-nominal is 14.7
for cars with blowers or turbos try adding amount of boost to the AP#,
(example 14.7 + 10lbs of boost = 24.7)
*Volumetric efficiency -a well tuned stock motor should be able to get 90%(.90),
a car with bolt on parts should be able to get close to 100%(1.0)
and a car with aluminum heads and cam should easily achieve 110%(1.1


I built a V8 with a similar specification and whilst it was quick, it required a session on the rolling road at Oselli's in Oxford to unleash the hidden horsepower. They spent a considerable time experimenting with the timing and the carb. The end result was fantastic - it was like a different car. Seven years ago this cost me about 300, but without it all the money I had spent beforehand would have been wasted. Incidentally, 181 HP (with a rather restrictive exhaust), but more importantly over 200ft.lbs. of torque all the way from 1200 revs onwards. On a 3.07 rear axle in fifth gear (SD1 box) it does not just pull from 20 mph - it accelerates like a bucket of bricks dropped out of a third floor window!
David Daw

Bruce, I think your VE of 1.1 is a little ambitious.The Edelbrock Performer manifold, which is probably the best one available for the BOP/Rover engine only claims 75-90%. Most stock manifolds are about 70%. Edelbrock Victor & Victor Ram manifolds can make theses figures, 105-115% & 110-122%,but they're not available for BOP/Rover engines. However, my info could be outdated by now. If so I stand corrected. Barrie E
Barrie Egerton

It's a P6 motor. It was in the car when I bought it. It's probably been rebuilt at some stage as it's very clean inside the lifter gallery and there is very little "step" at the top of the bores. Modifications are restricted to the manifold and carburettor mentioned. (I did it as the HS6 carbs were shagged, rather than to improve performance.) Once the car is "sorted", I suppose I'll treat it to a rolling road session.


Nick Wilson

Ok, the plot thickens..The P6 Rover also had small valve heads(and 9.25comp), which are Ok on a heavy saloon where low rpm power is needed , but on a light weight (ish!)MGB that is gonna work for it's dinner,you need the bigger valved SD1 heads, cos you can put the biggest carb and best manifold in the world on your engine, but if the mixture can't get past those small valves then you may as well have saved your money, and spent on beer!!:)
I recomend you take a plug out and see how long it is(ooeer), that will prove head type, if it is bad news then a second hand set of SD1 heads can be bought for less than 100, or a complete seconhand engine for 150-200. :)
(optional but probably worth it)The heads can then be reworked cheaply(there are some good Rover tuning sites around) to give a few more horses and then it's yeeha GTi bashing time!!(one of my fave's)
If they are already SD1 heads then a rolling road will be the only cure, are you sure your getting something as basic as full throttle at full throttle??

John, they are the older style heads. The engine number tells me that it is from a 3500s and would have had 10.5:1 CR. (It's stamped 10.5:1, anyway.) I put in a pair of composite head gaskets to reduce this (no idea how much of a reduction this gives), as it was pinking a bit.

I'm not really seeking more power (I seldom drive it - it's my wife's sunny weather car). It has an MGB o/d gearbox that would probably break (I'm told) if I fitted fettled SD1 heads.

I only asked, as I'm surprised that it doesn't go like stink. Though it may just be the absence of frantic revving that makes it seem like that. (My BMW bike seems much slower than my Ducati, but it's actually a fair bit faster.)



Nick Wilson


Just going off the information provided. Does any one know of another formula which would more accurately reflect the horsepower?

Hi, I had a V8 roadster, P6 bottom end and SDI Heads going through a adaptor plate and MGB box, no probs, with SU on adaptor plate was producing 165BHP at the wheels on standard cam, also lower compression engine (9.35) if my memory serves correct. Car did feel pretty quick and would certainly keep up with the hot hatch brigade at the time of ownership.
Ian Sanders

Well, i recon you must have a new BMW KI200 and an old 750ss or 600 Monster, cos most BM's ain't the quickest thing on two wheels, and a 748 upwards would shurley kick the ar*e of most of BMW's offerings:)
I've been out on my T595 Daytona tonight, bloody frightening in the damp, still the quickest way to get across town though...
Back to the B...
With a bit of decent setting up and fine tuning,it really should go well,and i think your wife will be able to frighten herself silly all day long and get a few more MPG's as well..I doubt if it will make much difference to the old girls box (ooeer again) as it's probably a bit loose anyway (sorry),well overstretched, erm, i think i better go.


I can't spell surley....

The formula is OK its just the numbers you use.
Vol Eff for a rover V8 at 5500rpm is nearer 80% on standard SU, nearer 90% with EFI and heading for 100 if you have flowed heads and cam designed for peak power at 5500rpm. In the above example you get 171HP std, 192 for EFI and 213 for tuned. On a properly designed engine peak Vol Eff is at peak torque.
Difference between stoich and LBT fueling acounts for about 5% variation in performance. Most engine tuning focuses on either changing the Vol Eff characteristics, combustion efficiency or basic friction.

Should have added more.....
The Vol Eff numbers for the manifolds mentioned earlier are almost irelevant.
Vol Eff is a function of the amount of air you can get in the cylinder and is thus limited by the smallest restriction in the SYSTEM and the tuning effect.
A manifold may be capable of handling a Vol Eff of 120% - this means if the rest of the system is modified the manifold wont be restrictive untill 120%. But you can't bolt it on and get 120%. So the numbers quoted may be true but be carefull what you interpret it to mean.
To get these big numbers you need to ensure the valve size and porting are not restrictive, the cam is correct in terms of lift (and timing for the speed range) the carb and air cleaner are not restrictive and all the parts fit together accurately without to many other losses.


Here is a really good British site for all types of tuning theory, calculations etc etc,

If anyone can give me the motor specs for a 215, a cam, and flow #'s on a set of heads I can run them through desktop dyno and get some baseline numbers..

Larry Embrey

John, not far off the mark. They are k100RS 16v and 900 Superlight. On paper the performance is quite similar and, possibly, with a better rider, the Duke would be faster. On the road, though, the Duke feels _fast_ at 90 but on the beemer it feels like you could get off and walk faster.


Nick Wilson

How do you like DD Larry? :)
You should be able to copy most of the results from my run. The cam specs are for a crane cam.

Numbers seem a little steep
Michael Hartwig

DD is pretty cool! I have been having fun looking at how a particular part will affect the power. That is a HUGE help when looking to buy a new cam, heads, or any other power adder.

Your CR seems a bit low man, is that a calcualted humber based on your pistons and heads or is that just a "blind #" you put in? I was a bit dissapointed when I had DD calculate the CR for my 302, a "lowly" 8.5:1, this robs me of some power but at least I know it will run perfectly on pump gas and has room for other power adders down the road..

MY DD results for my 302 are here;
Larry Embrey

Michael, (and others)
Which "model" cam are you all using? Each Mfg will make numerous cams for each power plant. I will start making Cam files for the ones I can find. And actual flow numbers for some stock heads would narrow down the true power output.. Cam and heads are what really govern a large portion of a motors power....
Larry Embrey

I have entered all the exact Cam specs from the crane cams site into DD. The specs can be found here:

Larry Embrey

The engine had 8.13:1 with tin gaskets, going to composite gaskets got me.

I think it's safe to say I can use regular gas :)
Michael Hartwig

Interesting DD data guys.
Larry seems to have a race cam on a street motor - probably better with a smaller event cam or increasing the head/manifold/carb if you really want power at 7000rpm.
Michaels numbers seem high. If you have stock valves, i thought the intake was smaller (1.57)- you will find that will have a significant effect and probably pull the numbers down about 5%. I don't know how DD handles intake losses - these programs are not usually very sophisticated. Most times they have a bunch constants that make the model of one engine work but get very unreliable on other engines. The 390carb is probably OK but it will only need small changes in flow coefficients to change the numbers another 5%.
I don't use DD but I do use more sophisticated packages.
Also remember wether your calculations include losses for water pump, pas pump, alternator etc if you ever compare with vehicle data.

Actually, the 260H cam is pretty small with 212 Deg at .050 in. lift and .447 in. lift. I had one (still in my garage) in a 351 Windsor that I changed out for a Crane cam. I know that a 302 will think it is a larger cam than the 351, but even so, it will probably peak at 5000 rpms and not at 6000. It is a good low rpm torquer and very appropriate for the 1.94 intakes, dual plane intake and small tube headers. Will make a terrific street engine and will over power the tires at will (unless the rear end ratio is in the twos).

Wayne Pearson

I wonder if you could run some numbers for me? '63 Olds Turbo engine with original cam (I think the Turbo cam was only slightly different from standard, probably not enough to worry about.) CR is between 8 and 8.5, .030 over, positive displacement blower at 5 and 7 psi, free flowing tubular headers and header mufflers, sequential injection, and I'm not sure how to describe the manifold. Maybe a large plenum log manifold with an internal intercooler? I'm just curious if DD will come out in the neighborhood of what I'm expecting to see.
Jim Blackwood

Larrys DD print shows the 260 event @ .05 lift. Which is how Crane would quote a 260H cam. Worth double checking the model?
As you say a 212 event @ .05 is a good street cam which should give peak power between 5000 and 6000rpm.
Wayne is probably right so the DD model may need to be adjusted. That would make the engine spec fairly good.

Well, Just so everyone know this is a 302 not a 215 that I am building. I only mentioned it so that people could see the kind of detailed output I could get with the DD program.

The numbers I used in DD come directly from the MFG for both the Cam and heads. The cam I have is called an RV cam, it is a copy of the CompCams 260H and is designed as a mild street cam that will pass emissions testing. The truth is that I have not found out it is a "weak" cam for a 302. I would be better off with a FMS B303 cam, but lack the time and $$ now to go changing things. Long term (after my last emissions test next year) I will be rebuilding things with larger headers, and the 280H Magnum cam which is good for 40hp!
Base Cam catalog page:
Cam SPEC sheet:

It is the Spec sheet details of Valve open/close timing that I used to enter the cam, so I cannot see how the info could be wrong? BUT I changed the setting from .050 to Seat-Seat and that set things to were they should be in terms of power/rpm.. I think I will try emailing Comp Cams to see what they think..

The head flow #'s I got from a website that has flow #'s for all sorts of Ford heads. I have a set of GT40X alum heads with basic port and polish. These X heads flow a touch better than the more common Y heads. The use 1.94Int and 1.54exh valves.
Larry Embrey

This thread was discussed between 07/01/2002 and 14/01/2002

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