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MG MGB Technical - headers

Have recently installed a re-built engine. Everything was balanced to within an inch of it's life, apart from the crank/flywheel, by me. So I know how accurate the reciprocating bits were done. I also gas flowed the head and balance the combustion chambers. A HR270 cam accurately timed. It also has an LCB, performance exhaust, isolated from the chassis by poly doughnuts.
On a rolling road it delivers 115bhp at the flywheel and is very flexible and torquey to boot, BUT, at certain rpm it sends vibrations through the chassis. Question....could these be generated by the header/exhaust system? The other point of transmission might be the new!!!!! engine mounts, not made from entirely compliant rubber.
Any views appreciated.
Allan Reeling

Allan, I'm interested in your thread because I have the same problem with an MGA 1622 engine that I had rebuilt and balanced a year ago. I now have over 7000 miles on the engine but still have an annoying vibration between 2400-2800 revs. I wonder if it's just simple harmonics of the engine. My A engine also has the later B front pulley/harmonic balancer fitted.

My other Bs are stock engines without any modifications and don't have the same vibration.

55 TF
60 A Coupe
67 B
74 B GT
Andy Preston

This is spooky! The vibration is in the same rev range on my "B". Are you on an LCB header, or the stock Cast Iron manifold? My suspicion is that the vibration originates from the tubular header.
Allan Reeling

Some of the aftermarket engine mounts are too hard and send nasty vibes through the car. Make sure it is rpm related and not speed related.

Peter Burgess Tuning

Oh it's a rev thing Peter. You can even switch it on and off with the O/D!!
I've been waiting forever, for a firm who make "better" mounts to get back to me. They say they already make V8 mounts, but haven't seen them yet!!
Allan Reeling

Engine mounts promised......soon!! Will keep you informed. But I am almost on the point of going back to the C I manifold and the old system just to see if things improve. A shame though to risk the performance gains! Maybe its the "tuning" of the LCB header, anyone out there with experience of this "black art"?
Allan Reeling

Allan- A knitpick: the 115 was at the flywheel or was it RWHP as implied by "BHP"? Sounds like a super engine rebuild. Did Peter do the head, the rolling road? That's a trunk load of power at the wheel for specifications described. Congratulations! What sort of torgue curve did it show? Can you post the dyno sheet for us to drool over? Cheers, Vem
vem myers

At Flywheel! 91.5 at wheels with 119ft/lbs also at wheels
I did the head, and built the engine. The rolling road session was at Peter's.
Allan Reeling

Still great numbers. Isn't that where the stock 62-66 MGB was off the production line?
vem myers

The flywheel figures of torque and HP are based on the rolling road calculations of transmission losses, mostly friction. i.e., 115.5 BHP and 131ft/lbs.
Standard 18v outputs at F/W were 95bhp and 105fl/lbs. Max BHP was seen at 4600rpm but the max torque was at 2650rpm! The tractability of the engine is superb and that all important power to overtake at motorway speeds is excellent. Never any need to drop out of O/D!
Allan Reeling

Here is the readout for those interested. But, of course, figures only tell part of the story. It's how your mods affect how the car drives. It's also good when you have done most of the work yourself!! Not forgetting Peter's 3 angle valve seats, that is!!

Allan Reeling

This thread was discussed between 06/04/2012 and 14/04/2012

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