Oversize exhaust valves

[jono]

Are there any advantages to be gained in using larger exhaust valves on a big valve head fitted with QED 360 cams?

Jono

[msd1107]

Rohan should probably weigh in here.

Especially if you go to a long stroke crank, the TC becomes under valved. So you would benefit from both larger intake and exhaust valves, with proper porting and exhaust system. (The kicker here is "proper", since "proper" generally translates to dollars (or GBP or other currency).

And use as short a duration cam as possible to minimize the loss in low rpm torque you could get with a long duration cam. If this is a street driven car, you will spend much more time at 2000-4000 than at 5500-7000. The skill and experience of your engine man is important here.

David
1968 36/7988

[john.p.clegg]

Jono
There is a mathamatical relationship between Lift and Valve diameter (don't have it handy at the moment) but suffice to say that fitting a 420 cam gives no better airflow than a 360 cam with a 1.4" valve..

John

P.S. will dig it out if you need it.

[john.p.clegg]

Just found it in my old notes:-

Maximum airflow when Valve lift is 0.27 Valve Diameter

John

[rgh0]

The QED 360 cam is basically a sprint profile cam as i understand it.

In general the exhaust ports on the twin cam breath very well and even in racing engines need little modification. One of the reasons short duration high lift cams work so well on the twin cam is that they stop you loosing to much of the incoming charge down the exhaust port on overlap due to the efficient exhaust porting arrangement

I would leave the valves at sprint standard sizes and not change them if running with QED 360 cam as I dont think it will make much if any difference. I would change both the inlet and exhaust valve in combination and not do it unless goign to a higher lift cam.

The biggest constraint on the exhaust side is the standard cast iron manifold or the small diameter lotus original tubular headers. A signficant improvement can come from fitting a larger bore sete of headers even in an otherwise standard engine.

cheers
Rohan

[steveww]

I did a lot of research in to this when I rebuilt the head on my twinc. In the end I decided for street use the standard sprint cam (QED 360) is the best compromise. A wilder cam may give more power at the top end about 6000 rpm but at the loss of low down torque. Remember power gives you top speed and torque gives you acceleration.

I got good improvements in power and torque across the rev range using sprint cams and valves with porting the head and fitting a large bore manifold and exhaust. There is a lot of material around the exhaust ports so these cam be opened up to match the larger manifold. Generally the standard head is quite neat but there is certainly room for improvement in some of the radii and around the valve seat.

Remember that after tweaking you will need to double check the carb jetting and timing and adjust to match.

[jono]

Thanks for all your replies. My engine builder has actually fitted sprint size inlet as well 33.7mm exhaust valves, re worked the ports appropriately, and fitted QED 360 cams timed with vernier pulleys.

I will be using fuel injection on the engine via 40mm Jenvey tb's with distributorless ingition and probably a Megasquirt ECU with a 'get you running' map to be followed by a rolling road session

The stock exhaust headers certainly look very weedy - can anyone suggest the best exhaust and header system to use on this engine? The car is for road use and I don't want a very noisy system, but do of course want a nice sound

many thanks, Jono

[steveww]

The TTR system is very good but costly
PM is also offering a large bore system

A good custom exhaust maker should also be able make something but space is very tight.

I was lucky to get a NOS ashley off flea bay. See http://www.steveww.org/index.php?id=131

Remember to use large bore exhaust gaskets to match, available from QED.