MGExp

MGB & GT Forum

Engine compression question

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Topic List Forum List Newer Older Start New Topic
Engine compression question
#1
Russ Johnston Avatar
Russ Johnston
Toronto, ON '73 Supercharged MGB, flowed head, c, Canada   CAN
Sign in to contact
Just got my engine back from the machine shop. He tests all engines on an electric jig to ensure oil pressure, lash, etc. He said I have 175 lbs across the board.

I'm using 8:1 pistons, block didn't need decking, the head had a minimal amount of metal removed and the valves were enlarged. Does 175 seem high?

This is a purpose built engine for a higher boost supercharger (7.5 -8.5 lbs of boost).

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Engine compression question
#2
1977 MGB Supercharged Avatar
1977 MGB Supercharged Ray Wyberski
Pago Pago, AS, American Samoa   ASM
Sign in to contact
1977 MG MGB "Jazzy's Car"
The 175 is more in line with the 8.8:1 CR, so yes it seems high. With some fine tuning it should be ok maybe add some sort of boost retard. Regards, Ray

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#3
Speedracer Avatar
Speedracer Platinum AdvertiserAdvertiser Hap Waldrop
Taylors, SC, USA   USA
Sign in to contact
1967 MG MGB Racecar "The Biscuit"
Are you sure you got the 8 to 1 pistons? Still got your piston box with part #.


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: Engine compression question
#4
Basil Adams Avatar
Basil Adams Platinum Member
About 12 miles from Sears Point, CA, USA   USA
Sign in to contact
I sent Russ the pistons. They were factory low compression pistons in the BMC box. I'd question that reading unless the machinist milled the crud out of the head. 175 is high even for a 8.8:1 engine. Basil 707.762.0974 basiladams@yahoo.com


Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Engine compression question
#5
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
Sign in to contact
1972 MG MGB
175 is about what my HC engine runs.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#6
1977 MGB Supercharged Avatar
1977 MGB Supercharged Ray Wyberski
Pago Pago, AS, American Samoa   ASM
Sign in to contact
1977 MG MGB "Jazzy's Car"
Russ,

8.0:1 approximately 130-140 psi is normal for a LC MGB (Factory Specification for LC MGB is 130 psi)
8.5:1 approximately 150 psi
8.8:1 approximately 160-170 psi is normal for a HC MGB (Factory Specification for HC MGB is 160 psi)
9.0:1 approximately 170-180 psi is normal for a HC MGB that had some kind of changes, ie. camshaft, decked head, head gasket, etc..

I was reading in the archives where Hap in Feb 2012 built a 9.5:1 CR street engine pumping 175-180 psi.

The more I look at your 175 psi with 8.0:1 pistons there has to be something wrong. Deck milled to much, Combustion Chambers need to be CC'd again, Head Gasket Thickness, Deck Height, Camshaft that increases cylinder pressure, etc...


Regards, Ray



Edited 1 time(s). Last edit at 2012-04-23 01:34 AM by 1977 MGB Supercharged.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#7
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
looking for the popcorn eating smiley again...

175 is high.

Adding 8 pounds of boost in there?
scary


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: Engine compression question
#8
Ogre1 Avatar
Ogre1 Allan Ogilvie
Melbourne Victoria, Australia   AUS
Sign in to contact
1977 MG MGB "Greenb"
Run it in then worry about 175. Tight tolerances in a new build can lead to high comps and high temps. No other way to get high comp unless you got the wrong head/piston combo. Also check the instrument that measured the comp.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#9
1977 MGB Supercharged Avatar
1977 MGB Supercharged Ray Wyberski
Pago Pago, AS, American Samoa   ASM
Sign in to contact
1977 MG MGB "Jazzy's Car"
Russ, With a High Boost pulley installed and 8 psi of boost I am not sure I would take it to the limit just yet until I found out what the Compression Ratio really is. Ray



Edited 2 time(s). Last edit at 2012-04-23 05:50 AM by 1977 MGB Supercharged.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#10
Steve5412 Avatar
Steve5412 Steve Wareing
Nanaimo, BC, Canada   CAN
Sign in to contact
1970 MG MGB GT
1972 MG MGB GT
Hi Russ,

The compression on my '72 GT is 195 lbs (average) and the car is fitted with a Moss M45 Supercharger. The engine re-build and installation of the S/C was completed before I owned the car and unfortunately I have been unable to obtain the specs on the engine. The compression ratio must be around 10:1 and I use 94 octane fuel. I don't drive the car hard....rarely above 3500 rpm..so the boost does not often kick in. The timing is set a 26 degrees BTDC and the engine runs well with no pinging.
It has gone approx 20,000 miles without problems.
regards, Steve

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#11
Speedracer Avatar
Speedracer Platinum AdvertiserAdvertiser Hap Waldrop
Taylors, SC, USA   USA
Sign in to contact
1967 MG MGB Racecar "The Biscuit"
OK, we know you have 8.0 to 1 pistons, I guessing you guys checked everything compressiom ratio wise, on engine you intended on ruinning 8 psi of boost, for example what are the cylinder head's combustion chamber readings. If one expects to run such a high level of boost, you have to make sure what CR you have first, around here we call that, part of blueprinting a engine.


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



Edited 1 time(s). Last edit at 2012-04-23 04:30 PM by Speedracer.


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: Engine compression question
#12
Russ Johnston Avatar
Russ Johnston
Toronto, ON '73 Supercharged MGB, flowed head, c, Canada   CAN
Sign in to contact
Hey gang, quick update....

I spoke to Sean Brown (he did the head), the machine shop and Basil. About .010" was taken off the head, nothing was taken off the block. Sean did do a cc to the chamber, but it was written inside (it went on the block before being recorded). The pistons were 8:1 (A to B comparison by the machine shop and Basil's confirmation).

The plan:
• Put the engine in and retest using my gauge
• Switch to the stock pulley (5.5 - 6.5 I think).
• Run through a tank of gas and retest

It's clear that on a fresh rebuild, I shouldn't be flooring with the new supercharger. The next 500 or so miles is going to be tough keeping it at 2/3 throttle! I'm just going to have to go for a very very long drive one day.


Thanks for everyone's continued help and support.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#13
ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
Sign in to contact
1972 MG MGB
Russ,
Have you considered running your original carb(s) for a few hundred miles to see how the engine shakes out before adding the SC? (just a thought if you still have the parts)

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#14
Denis Avatar
Denis Denis Hill
Bearii, Nth Victoria, Australia   AUS
Sign in to contact
1968 MG MGB MkII
1973 MG MGB V8 Conversion
Russ the standard compression pressure for a 8.8:1 is 165PSI as has been stated so it does seem a bit high, is there a chance the test gauge is faulty? Or more likely that there has been oil squirted on top of the pistons and some one got over enthusiastic with the oil can. It only takes a little oil to shoot the compression up. Run the engine and test it again. The SC has a bypass so the engine is naturally aspirated until you hoof it so no problems there. Lets know how you get on.

Denis

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Engine compression question
#15
Russ Johnston Avatar
Russ Johnston
Toronto, ON '73 Supercharged MGB, flowed head, c, Canada   CAN
Sign in to contact
You nailed it. Basil and I had a bunch of good back and forth emails. He speculated the same and had the same recommendation. I'll be using that SC bypass (make sure I don't go past 2/3 throttle and make sure my boost gauge needle doesn't move) until it's broken in. I've given my family a head's up that I'll be going on a very very long drive one day (shoot for 500 miles over 2 days or so). Then, I'll start getting on the gas a little more.

Engine should be in in about 3 weeks (got to love going back and forth between baby diapers to supercharged engines!). I'll keep everyone posted.

In reply to # 2048639 by Denis Russ the standard compression pressure for a 8.8:1 is 165PSI as has been stated so it does seem a bit high, is there a chance the test gauge is faulty? Or more likely that there has been oil squirted on top of the pistons and some one got over enthusiastic with the oil can. It only takes a little oil to shoot the compression up. Run the engine and test it again. The SC has a bypass so the engine is naturally aspirated until you hoof it so no problems there. Lets know how you get on.

Denis

Reply
Was this post helpful or interesting?
Yes No Thank
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
1973 MG MGB
Tom Guinther   USA
1973 MGB
Text Size
Larger Smaller
Reset Save