MGExp

MGB & GT Forum

Bored 40 over how much power increase?

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Page 12
Topic List Forum List Newer Older Start New Topic
Bored 40 over how much power increase?
#1
Cubfloater Gold Member Jim Tice
Monroe, NC, USA   USA
Sign in to contact
1973 MG MGB
1974 MG MGB
Hi all, I just picked up my freshly rebuilt B engine that was bored 40 over and wondered if nothing else was changed how much power increase I could expect?

Thanks,
Jim

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Bored 40 over how much power increase?
#2
ohlord Avatar
ohlord Platinum Member Rob C
A tiny Island off the coast of Washington State, N.W., USA   USA
Sign in to contact
1957 Land Rover Series I "EYEYIYI"
1957 Land Rover Series I "OVRLND"
1971 MG MGB
1971 MG MGB "Bedouin 2"    & more
about 2-3%
Nothing to write home about.


LNDRVR4X4.COM
Home of Project "INCARN8'


1957 Series 1 Land Rover electric VEHICLE CONVERSION

FIXITUPCHAP.COM
FIXITUPCHAP INCORPORATED

RD3 Radar/ Electronic Warfare Technician
VIETNAM 1969-1972

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#3
ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
Sign in to contact
1971 MG MGB
The increase in displacement would be very small. If there were any measurable effect at all, it would likely be due to the slight increase in compression ratio. When I rebuilt my engine I went through the calculations to see what the increase was with my 0.030" oversize pistons and slightly planed head (I actually measured the combustion chamber volume). I'd have to look it up, but I think I went from 8.8:1 up to about 9.1:1 or something like that.

I'm sure Hap or one of the wizards will know about how many HP gain you get for a couple of points of compression ratio - my guess is about 5 or 6 HP.


Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Bored 40 over how much power increase?
#4
spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Canton, IL, USA   USA
Sign in to contact
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1976 MG MGB "Cecil"    & more
I disagree.
Brag all you want!
I would LOVE to have just picked up my freshly rebuilt engine bored over .040!

I'd say 3-4 HP. sounds a LOT better than 3%.

Show me anyone here that wouldn't love an additional 3 HP... Rob included!


Now... the fastest way to lose 10HP???

see this in your mirror:




Michael J. Caputo
'79, '77, '76, '74.5 (rubber dual SU), and '73 owner. Extensive experience in 12v Audio System design and installation. Vendor of Regalia and Promotional Products since 1993. Supplier of Accessories to MOSS. Forum Member since 2009; with a warped sense of humor since birth. Publisher of the annual MGB & GT Calendar, mailed worldwide.


Member Services:
2024 MGB & GT Calendar www.mgbcalendar.com
Please send photos for next year's calendar to photos25@mgbcalendar.com. If you need help with your Weber downdraft carburetor, feel free to call me at 978-249-5760, USA, Central Time!
Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#5
joron Avatar
joron Silver Member Michel Joron
Roxboro, QC, Canada   CAN
Sign in to contact
1977 MG MGB "Victoria"
2018 Jaguar XE "Meghan"
Based on a certain power, and stroke the increase of bore has an effect to the square... between 2% - 3 % increase of power.

Displacement = Pi/4 * Bore ^ 2 * Stroke * 4 cylinders
if you accept the power increases proportional to Displacement, you will have at least 2.5 % of increase.

MBG Bore 3.16 in. (80.26 mm.)
Stroke 3.5 in. (89 mm.)

3.20^2
--------- 1.025
3.16^2
It is not necessary exact you might have more because the compression will increase also with the increased compression you have more power.


1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#6
pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
Sign in to contact
1958 MG MGA 1500
1961 Austin-Healey Bugeye Sprite
1971 MG MGB GT
1973 MG MGB MkIII
Not enough to be able to detect it without measuring equipment.

Adrian


Home built Eaton M62 Supercharger with 8psi boost, 8:1 compression, custom "supercharger" cam from Schneider Cams, Mikuni HSR48 Carburetor, custom ground high ratio "stock" rocker arms, Maxspeeding rods with Teflon wrist pin buttons, custom aluminum cold air intake, CB Performance computerized ignition, Fidanza 9 pound flywheel, 1.44 exhaust valves in 48cc chamber head, matched manifolds, 2 1/4" exhaust system.


Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#7
ohlord Avatar
ohlord Platinum Member Rob C
A tiny Island off the coast of Washington State, N.W., USA   USA
Sign in to contact
1957 Land Rover Series I "EYEYIYI"
1957 Land Rover Series I "OVRLND"
1971 MG MGB
1971 MG MGB "Bedouin 2"    & more
40 over brings displacement up about 45cc a 2.5% increase compression add a tad more.
Having done nothing more performance wise to his low compression 18V engine he isn't going to see any 3-4 hp increase
2.5% is being generous on an 8.0:1 build with a 40 over bore being the only change. Maybe 2hp


LNDRVR4X4.COM
Home of Project "INCARN8'


1957 Series 1 Land Rover electric VEHICLE CONVERSION

FIXITUPCHAP.COM
FIXITUPCHAP INCORPORATED

RD3 Radar/ Electronic Warfare Technician
VIETNAM 1969-1972



Edited 1 time(s). Last edit at 2015-01-28 10:59 PM by ohlord.

Reply
Was this post helpful or interesting?
Yes No Thank
  makka45 thanked ohlord for this post
Re: Bored 40 over how much power increase?
#8
Ogre1 Avatar
Ogre1 Allan Ogilvie
Melbourne Victoria, Australia   AUS
Sign in to contact
1977 MG MGB "Greenb"
Don't forget that he will also see an increase in efficiency by getting back any hp the engine had lost through wear. . Depending on how worn out the engine was, he could see 20 or 30% more from were the car was at. I when I drive a car with a freash engine, it's enough to put a smile on your face and thats what counts

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#9
davester Avatar
davester Dave Diamond
Berkeley, California, USA   USA
Sign in to contact
1965 Austin-Healey Sprite
1971 MG MGB GT "Dad's Car"
In reply to # 2907349 by ohlord 40 over brings displacement up about 45cc a 2.5% increase compression add a tad more.
Having done nothing more performance wise to his low compression 18V engine he isn't going to see any 3-4 hp increase
2.5% is being generous on an 8.0:1 build with a 40 over bore being the only change. Maybe 2hp

I don't think you can even buy LC pistons any more, or if so they are special order. A bore job of any kind automatically means that you will jump to HC pistons unless you are trying really hard to do something dumb, so a standard LC engine will see much more than a 2 hp increase.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#10
ErnieY Avatar
ErnieY Ernie Y
Albatera, Alicante, Spain   ESP
Sign in to contact
1976 MG MGB GT
Presumably the rebuild includes a new cam and depending on what that is, and the state of the old one, it's possible that a 40 thou bored engine might actually produce LESS power then before !

It's unlikely I admit but I make the point to illustrate that you've asked a question which unanswerable !

Also 40 thou rebore, what happened to 20 ?

Unless very badly scored or ovaled 20 thou would be the first oversize and of course if all that's actually happened is that it's gone from 20 thou to 40 then that effectively halves any potential power gain.

Bottom line is that of itself a 40thou rebore will not make enough difference to the power to be noticed.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#11
Speedracer Avatar
Speedracer Platinum AdvertiserAdvertiser Hap Waldrop
Taylors, SC, USA   USA
Sign in to contact
1967 MG MGB Racecar "The Biscuit"
Street guys always try to justify a gain, by how much it is, it makes sense, not like most street guys are going to do 30 upgrades like a racer. CR bump and better cam are the two biggies. Now I understand a street guy will not go anywhere near as far to make power as a racer, but I do remind my street customers to stop thinking in terms of "what one thing should I do to get a gain" and start thinking terms of what 3-5 things can I do to make gains, it all adds up in the end.


Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
Acme Speed Shop www.mgexp.com
MG/ Triumph Performance Street Engines - Cylinder Head Porting for street performance and race - DIY Engine Rebuild Kits With Free Tech Advice - VTO alloy wheels for British Sports Cars, and others
Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#12
tomkatb Avatar
tomkatb Larry Baygents
Dayton, Ohio, USA   USA
Sign in to contact
1963 MG MGB
Was it Hap that said, "horsepower is expensive"?

My observation is what Hap said, you have to do a bunch of stuff to make horsepower. There ain't nothing you can do for $100 that will affect the butt dyno after the car is brought up to spec!


L.W.(Larry)Baygents
63B
77 Spit

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#13
Speedracer Avatar
Speedracer Platinum AdvertiserAdvertiser Hap Waldrop
Taylors, SC, USA   USA
Sign in to contact
1967 MG MGB Racecar "The Biscuit"
In reply to # 2907520 by tomkatb Was it Hap that said, "horsepower is expensive"?

My observation is what Hap said, you have to do a bunch of stuff to make horsepower. There ain't nothing you can do for $100 that will affect the butt dyno after the car is brought up to spec!


Not really, performance always seem to work the same way, in the beginning the gains are big for not a lot of money compared to stock items, a higher CR piston set, and performance cam, machine work to get CR up in some motors, aluminum flywheels, all bargains for what you get in return. Then at the end of the HP trail, things start to get a lot more expensive for smaller gains.


Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
Acme Speed Shop www.mgexp.com
MG/ Triumph Performance Street Engines - Cylinder Head Porting for street performance and race - DIY Engine Rebuild Kits With Free Tech Advice - VTO alloy wheels for British Sports Cars, and others
Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#14
ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
Sign in to contact
1971 MG MGB
HP obeys the law of diminishing returns in a big way. The first 10 HP are fairly easy. The next 10, more difficult (expensive). The third increment of 10 comes at great cost and effort!

Burgess' book really shows this well. He has HP values for various stages of tune. Near the end, the most extreme measures net very little increase in HP.

But Hap - didn't you have a rule of thumb about how much power increase you get for a given increase in compression ratio?


Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Bored 40 over how much power increase?
#15
Steve Lyle Avatar
Steve Lyle Gold Member
Tulsa, OK, USA   USA
Sign in to contact
1972 Lotus Elan
1972 MG MGB
In reply to # 2907645 by ingoldsb
a rule of thumb about how much power increase you get for a given increase in compression ratio?

Didn't I read some knowledgeable poster hear say the rule of thumb is 5 hp per point of compression increase?


Documenting my project in my journal at https://www.mgexp.com/home#journal

Reply
Was this post helpful or interesting?
Yes No Thank
Page 12
Topic List Forum List Newer Older Start New Topic

To reply or ask your own question:

or

Registration is FREE and takes less than a minute

Forums  

Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster





Join The Club
Sign in to ask questions, share photos, and access all website features
Forum List
MGB & GT MG Midget Buy, Sell & Trade Vendor Market MG Performance MG Engine Swaps Factory Correct MGA MGC MG Magnette 1100 & 1300 T-Series & Prewar EV Conversion Modern MG MG Motorsports Events & Meetup MG 2024 Katy Texas Other Vehicles Off Topic Clubs Archived
Your Cars
1980 MG MGB V8 Conversion
Kent Gavey   GBR
1980 MGB V8 Conversion
Text Size
Larger Smaller
Reset Save