Fast Road Use - What Improvements can I make?

[steveww]

The value seat on my twinc head already does protrude on the the face. When I said the seat is only a whisker away I was talking about the area of contact between the valve and seat, you know the bit that gets lapped.

[bill308]

Roy,

I think your analysis of the effects of a lightened flywheel are not quite correct. One would only feel the beneficial effects of the kinetic energy stored in a heavy flywheel if the engine were to free rev to an elevated rpm and then the clutch were engaged. Stored energy in the flywheel will assist the engine output for a second or two. Typically, this can only be performed from a standing start. The down side is that it is brutal on the drive train and mounting features. The second instance where there might be some benefit, is where one might want to down shift to a lower gear, for additional braking without blipping the throttle. Here, an engine turning a relatively low rpm, becomes a transient brake for a very short period of time. Again, this can be quite brutal on the drive train. In all other cases I can think of, a high inerita flywheel is a detriment to torque availability at the rear wheels.

Consider the case where the car is moving slowly, in say second gear. When one mashes the throttle, the engine will respond, but it has to accelerate all the internal engine rotating and reciprecating parts, all external loads, including the alternator/generator and water pump, the flywheel and attacted clutch, and the clutch disk and gear box input shaft to engine speed. The gear box, the drive shaft, and rear end pinion are accelerated to something less than engine speed, except when in 4th-gear. Wheels, tires, brake disks, and half shafts with joints, are accelerated to roughly 1/4 the of the transmission output rpm. What torque is left over, becomes available to accelerate the car. If one can reduce the rotational inertia of any part, partuclarly those that spin at engine rpm, more torque will be available to accelerate the car, rather than the attached components. The flywheel is probably the single biggest inertia load, from the point of view of the engine. One wants to use all available torque to accelerate the car, not the rotating and reciprecating components. Therefore, a lightened flywheel will enhance the torque available to the rear wheels under all conditions, except if one drops the clutch from an elevated rpm. Here, energy is stored, then dissipated upon engagement of the clutch.

As far as reducing the flywheel inertia too much, I think one has to get quite radical before this becomes an real issue on a twink. The inertia load contributed by the flywheel, is likely the single biggest contributor, but is not the onlyimportant contributor. The clutch assembly is also quite significant. In the end, the engine low speed torque characteristics and weight of the car will be important, as is how the car is driven. If the engine is tuned for high rpm torque, at the expense of low rpm torque, and the car is heavy, lightening the flywheel should be reconsidered. It's the overall performance the the vehicle that is important.

I hope this helps clairify the issue.

Bill

[rgh0]

Steve

If the actual valve sealing surface is only a wisker away from the head face then you do have a challenge the next time you have to reface the head.

However its quite practical to sink the valve seats a little deeper into the head if needed. You have to do a redesign of the valve train to accomodate the moved valve position but it all can be done. The Hart 416B engines did this from new to fit in the bigger 1.7 inch inlet valves and avoid the valves hitting each other on overlap.

If the casting letters on the side of the head face have not been totally removed yet than you have plenty of life left in your head for another reface or two. The rebuilds only get a little more complex!

Rohan

[carrierdave]

Hi Rohen,
I started to do a little reshaping today. First of all I am looking at the exhaust ports and taking them out to the size of the gaskets. It?s quite a scary feeling taking a grinder to head!
I have also got a set of larger intake valves; all I need now is a tool to cut the valve seats bigger, then I will look at the inlet side.

Dave

[bill308]

carierdave,

Have you measured the thickness of your cylinder head?

The reason I ask is because I'm unsure of the wisdom of spending a lot of effort on a head which will have little residual life. If the head has substantial life (thickness) left, then go for it.

The head on my S2 has never been, to my knowledge, resurfaced, so it made sense to spend the time and effort to improve it when I last worked on it. I matched the intake port to the the carb gaskets and the exhaust manifold passage to the exhaust gaskets. I also had Sprint specification intake valves and hardened valve seats installed. Part of installing the larger intake valves was to increase the ID of the throat intake valve seats to the desired id, for a given depth, and blend in the local area of the port. The cut through the valve seat was perfomed on a Bridgeport miller with a tool holding special cutter inserts and was indexed to the valve guide. This tool cut the localized throat a little larger in id and cut a radius on the valve seat area. Later, a normal valve seat grinding stone was used to establish the sealing surface location, width and angle. This was followed up with a light lapping to ensure a tight seal. In the end, the sealing surface geometry, widths and locations, were as desired. This kind of automotive machining is hard to come by. I found a local shop that was willing to work with me and do it right. The end result was a very peppy twink, especially considering the modest compression ratio.

Bill

[carrierdave]

Hi Bill,
Thanks for the comments. Yes I have measure the head and it comes up to 4.642? from memory. Apart from a 10 thou Skim last year the head is original so I have lots to play with.
My concern over cutting back the original seat to accept a bigger valve is that the throat of the valve seat is the same size therefore the flow into the head will be the same. If I increase the throat size then I reduce the pressure drop and face velocity at the same time thus helping the head to breath easier.

Dave