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MG MGB Technical - Engine detonation

I have built an MGB engine for racing in modified road class. With a Fast Road Cam (285), gas flowed head, fully balanced etc. Compression Ratio is 11.25:1 and it runs on Webers.

It ran nicely on the road and rolling road, giving 134 BHP at the wheels and good emissions. But once on the race track, serious pinking/detonation ending up with broken No 4 piston & cracked Cylinder Head.

After rebuild and changing to Aldon distributor, timing set at 28 degrees., car agin ran well on the road. At Brands Hatch meeting, 2 sets of practice (40 laps in total( car ran well but there was still a slight pinking. Checking the timing, it had moved to 32 degrees, so reset to 28.5 degrees, thinking that was the problem. 3 laps into the 1st race, the head gasket blows. On removing the head, No 4 piston is destroyed and has obviously been very, very hot.

I have run out of ides. If anyone can shed light on my problem I would be very grateful as I now have to build another engine. I have built many B engines in the past for race & road and never had a slightest problem.
R L Tinkler

Is there a chance you have some type of blockage in the cooling passages in the block resulting in improper cooling of #4. Since you have blown the same piston twice and if you have no damage to the other three it would appear that it is something other than the timing.
John H

What sparkplugs are you running at the track? You are timing the ignition at 3500+ rpm, right? Running on the road is much different than running at the track as far as load to the engine.

When you had it on the chassis dyno (rolling road), I assume that you had some way to monitor the air to fuel ratio. 12.5 to 1 is ideal but almost impossible to obtain with a carb throughout the power band.

I run a full race MGB with a 45DCOE and 14:1 CR and I run 32 degrees total advance with no detonation problems.
Glen Popejoy

I agree with John. I once rebuilt a 76B on which, after the block had been cleaned and hot-tanked by the machine shop, I found that the rear cylinder water jacket passages were almost completely blocked by residual core sand. This was clearly a foundry error; the sand was nearly vitrified and hence welded to the metal, normally a result of too-hot pouring. It took a couple of days of probing and scraping with sundry crooked chisel/scraper tools I made to get around all the corners through the available holes. Most of the hard sand had to be broken up by hammering on the chisel tools, followed by scraping to bare iron. By probing all the water jackets, you can get a good idea of what the contours are supposed to be like; there were times when I thought I had it to bare iron, but more banging resulted in more baked sand. In this case, I'd say 80% of the rear (exhaust) side of #4 was totally without cooling, but I now check all blocks and have found lesser degrees of this problem on others. Usually it's at the bottom/rear of #4 water jacket, which may not cause trouble in normal use, but under modified/race conditions, even a little could push it over the edge.
FR Millmore

BTW, I forgot to ask about the octane of the fuel you are running. With 11.25:1, something in the 103-105 range should be required. Also, was the cylinder head professionally ported or DIY?
Glen Popejoy


I was working along the lines that the only cause of the problem must be the cooling. I have now probed down the water jacket and there is no passageway between the No. 4 Cylinder & the push rod holes. I checked against a spare block and there is clearly a problem. It's not core sand, it's solid cast iron.

It now seems logical that there was enough water getting around at basic road speeds and for short bursts on the rolling road. Under racing conditions, the No. 4 Cylinder overheated and caused detonation which then accelerated the overheating problem and rapid meltdown of the piston.

We had the block caustic washed with the core plugs out and the company that did that did not pick up the problem. Improving the engine cooling and fitting high tensile studs has succeeded in making things worse!

Many thanks to F R Millmore & John H. Perhaps you would both like to accept an invitation to be honorary members of "Church Lane Racing". We are a pair of retired engineers and my son - also an engineer and race pilot. Contact me at and I will send pictures & keep you informed of progress.

I will now go ahead and build a new engine, saving whatever expensive bits I can from the old one.

Out of interest to others, we were running on Shell Optimax with octane booster & NGK plugs (7EX).
R L Tinkler

RL, I would be honored.
John H

Remeber that there are different castings for the block of the B series engines available.
It allways makes sense to go for an 18 V block for building a race engine.
I run mine at only 11.5 CR with a usual Leyland ST distributer as quoted in the 'Special Tuning Manual' and on 100 octane fuel. While porting the head, the water ways inside were also ported a little, as far as you can reach inside. I found out that there are benfits to keep the heater valve open all the time, as it helps to flush the rearmost end of the block and helps to cool down # 3 and 4 a little.
Sparks fitted are NGK BP 7 EVX, carbs are HIF 6 with BDR needles and red springs otherwise the same engine like the one in RL's car.

R.S. Ralph Siebenhaar

This thread was discussed on 17/07/2006

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