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High compression head identification

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warwick-steve

warwick-steve

Jedi Hopeful
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Hi all
I have discovered that my 1961 MK2 BT7 was fitted with a gas flowed high compression head and a MK3 camshaft at the time the car went for an engine rebuild when the car was restored about 15000 miles ago. Due to his illness I cannot contact engine rebuilder to ask whether the engine still has these two items. When I originally spoke to the rebuilder at the time I bought the car he told me that the engine and carbs were completely rebuilt "everything you would expect". There are unfortunately no written details. I also know that the original "standard" head was also supplied with the car but was probably in a very poor state.

The engine runs really well, smooth, responsive, good oil pressure and cylinder compression figures and no oil leaks.

Wishful thinking I guess but is there any way of determining without taking the engine apart whether the engine has the high compression head fitted? Would the compression figures provide a clue?

thanks
Steve
 
Most likely it is just a normal head that has been skimmed, polished and ported. Skimming it will have increased the compression ratio. I am assuming it is not a new aluminium head from DWR.
 
Hi Steve,

My father, a man who had no interest in cars or mechanics, often told me "If the car runs good, don't lift the hood."

When rebuilding a non-Mark 3 (BJ8) motor, it has become almost common for the owner to want more power as a result and takes the opportunity to have his original cam ground to, or replaced with one having, the more aggressive BJ8 profile. Additionally, it is also opportunistic to have the work Derek mention as a part of redressing the original head. In both cases, I would expect the head and cam to still be in place.

To get some sense of verification, I would suggest you do a quick compression test to see if your pressures come closer to the original BT7 book number or the BJ8. Additionally, if you have an adjustable timing light, I would see where the dynamic timing has been set and also compare it to the book settings for the BT7 and BJ8.

All the best,
Ray(64BJ8P1)
 
Thanks Derek and Ray
When I bought the car I asked for a professional compression test and leak down test to be carried out, the figures were
1 - 175 leak down - 30%
2 - 170 40%
3 - 165 50%
4 - 165 30%
5 - 165 45%
6 - 170 30%
I was informed that these figures were good to very good. I wouldn't know if these figures would be for a standard or high comp head. Anyway, the engine runs really well so I will live with that and assume that the high comp head and MK3 camshaft have been retained.
Appreciate your help
Steve
 
Mine is 10 to 1 and it makes the car more prone to pinking. Local to here is Spamspeed and his friend worked in the Competition Dept in the sixties. Under his guidance my head was modified to 1965 Works spec and it pinks and I've had to back off the ignition timing a little, so that it doesn't pull strongly until 1200 rpm.

The BJ7 engine it replaced pulled from 300rpm, but was less powerful. Given the awful, dawdling traffic we have to contend with I'm not convinced that power of the BJ8 is much of a benefit, especially as the gearbox is a clunker.

For the road, they're just a nice old tourer and best left as such IMO.
 
warwick-steve said:
Hi all

Wishful thinking I guess but is there any way of determining without taking the engine apart whether the engine has the high compression head fitted? Would the compression figures provide a clue?

thanks
Steve
Click to expand...

No real way to tell the C/R without removing the head and measuring the volume of the combustion chamber and bores and doing the maths. As has been said above, if it runs great and doesn't pink, just keep doing what you're doing and enjoy it. You may take a run at a rolling road to see if you have any improvement over stock horsepower, but even then there are so many variables that it would be hard to tell if a change in compression ratio is the difference maker.
 
HI Steve,

I would say that your compression is very good. I shaved my BJ8 head back in 1968 or 69 by 0.060" resulting in a higher then standard BJ8 compression. a few years ago, I did a compression test and my results are quite close to yours.

------------My Test results
1 - 175 ---- 170
2 – 170 ---- 162
3 – 165 ---- 165
4 – 165 ---- 171
5 – 165 ---- 171
6 - 170 ---- 172


Since my compression ratio is higher then the standard BJ8, I expect you have the modified head installed.

Also, EV2239 makes a good point on needing high octane fuel to eliminate pre-ignition (pinging). Pre-ignition is quite detrimental and can cause major engine damage. Steve's piston pictures on his thread "My rebuild - It's amazing how well it ran" is an example of what pre-ignition can do. Assuming, with your BJ8 cam profile, you are setting your dynamic timing to BJ8 specs. Although you can reduce pre-ignition potential by reducing the degrees of advance. However, this can both reduce performance and only be somewhat effective. Unless you are going to use your Healey as your daily driver, I would suggest not reducing the timing and use High Octane Fuel.

All the best,
RAy(64BJ8P1)
 
Last edited:
Hi Ray
Thanks for that, - really useful. Looks like the rebuilder did use the high compression head.
I use BP ultimate which has 97octane with Millers VSP additive which has an octane enhancer and ethanol protection. On the basis that the high compression head is fitted what spark plugs shouldI ideally be using, - a colder type?. I am currently using NGK BP6ES. - The engine runs fine with no pinking or over run. I am not using the car for frequent long distance motoring just occasional trips between 30-150 miles.
Thanks again for your help
Steve
 
Hi Steve,

I very infrequently use an additive other then a led additive in the fuel, however, many do. Since all is operating nicely, I would leave well enough alone and keep doing what you have so far. Having had a long history with my Healey, I love putting the Beach Boys on my radio and driving back to my younger days and I always come home with a smile.

Enjoy your Healey as You Will,
Ray(64BJ8P1)
 
Thanks Ray.
Having spent the months tidying up and tinkering with my new toy I do intend to use the Healey more. Just looking forward to the spring and some decent weather.

Your help much appreciated
Steve
 
Just a thought. Someone told me a gas flowed high comp head would have hardened valve seats, is that so? The head fitted to my car would have been in the late 60's early 70's
cheers
Steve
 
warwick-steve said:
Just a thought. Someone told me a gas flowed high comp head would have hardened valve seats, is that so? The head fitted to my car would have been in the late 60's early 70's
cheers
Steve
Click to expand...

Depends on when the head had the work done on it. If it was in the last 30 years or so any good machinist would have built a 'lead-free' head with hardened seats and (at least) stellite exhaust valves. Prior to that, it's a crap shoot.
 
Just noticed you're in the UK; don't know when you went lead-free so the time frame may be different. Note we also called the modifications a 'no-lead' head.
 
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