18V cam timing

[Dave Whitehead]

Hello all... I am new to this forum, but have been reading it for a while; just registered today. I have a 1974 MGB with the 18V engine. I am in the midst of a restoration, and will have the engine apart to refresh bearings, rings, etc. I have been reading the Power Tune you MGB book bu Peter Burgess, in which he talks about how the 18V had basically the same cam as earlier versions, but the timing was shifted. He states that in older version, intake max lift was 110 deg ATDC, but in the 18V it was changed to 106 deg ATDC, which improved off the line torque, and decreased emmisions slightly to the detrement up upper band performance. I am considered changing the cam timing back to 110 ATDC by way of an offset cam key. Has anyone tried this? I'd love to hear anyones experiences!

Thanks, Dave

 

[Dave Russell]

Hi Dave, good name.

on any engine, the intake valve does not close until the piston is already rising. Eg somewhere after BDC.

When the factory changed the cam timing from 110 to 106 atdc this in effect causes the intake valve to close earlier in the cycle, earlier on it's way back up from BDC. This allows the piston to begin compressing the charge sooner & raises the available compression pressure.

The actual engine compression ratio is never the calculated ratio because in reality the piston cannot begin to compress anything until the intake valve gets closed. If the intake valve were to be completely closed at BDC, which it never is, the actual & rated compression ratios would be the same.

Because of this, the point of intake valve closing is the most significant in the sequence of valve events as far as noticeable effect on the engines torque characteristics.

At higher rpms & thus higher gas velocities, some advantage can be taken of the gas inertia in the intake tract by closing the intake valve later to provide some inertia filling of the cylinder even though the piston is moving upward.

It is all a compromise between low & high rpm torque. 106 degrees gives more low end & less top end. 110 degrees gives just the opposite. If you feel that you can live with less low end torque to gain a little at the top end, go with the 110 degree timing.

D

[ 02-26-2003: Message edited by: Dave Russell ]</p>

 

[Dale]

Hi Dave!
I can't add anything to Dave's excellent discussion of cam timing and valve overlap, but I can add a hearty welcome to the forum. Good luck on your rebuild. I have 8200 miles on my TR7 since its' rebuild and am sure happy with it, as I'm sure you will be with yours.

 

[Dave Whitehead]

Thanks for your input guys. I think I'll give it a go. Since it is still bitterly cold here, it wont be for a while though. I know how to advance cam timing... now how do we advance warmer weather

?

 

[Gary Lloyd]

Welcome aboard Dave, nice to have another canuck on board!! I was in Ottawa in Sept! I can't add anything to the other comments!! I just know that everything is 'gain and loss'!! For example Smaller valves apparently give more bottom end, but you loose on the top end!!

 

[Bruce74B]

May I also welcome you to the BCF and commend Dave on his analysis...I too have a 74 B with the stock setup and like the low end torque...it is noticably better than my 69 was at the low end, but doesn't have the power band in the high end...but then I am middle aged now and don;t need to crank it like that anymore.

Bruce