what octane?

[red57]

Andy,

I would be very careful about removing the 'eyebrow' - that is the "squish" area and it helps create a swirl in the incoming charge, helping to insure a complete mix and burn. Second, if you do remove the squish area you may well have to have raised dome pistons to get a reasonable compression ratio. I have seen serious Healey race motors with head modifications like you are contemplating but they have raised top pistons and are probably running 12 or 13:1 (maybe more). If you remove the eyebrow and still have flat top pistons, I'm not sure you could skim the head enough to get the compression ratio where you want it. Point I'm getting at is that if you make that radical a change, there may be some downside.

I run about 9.5:1 and still have the eyebrow, have unshrouded the valves and skimmed over .060 off the head and still have 9.5:1. My chambers are about 48cc, and my guess is you would need to be arouond that, 48-50cc, with flat top pistons to be in the ballpark. I think unshrouding the valves and smoothing the eyebrow (rounding all edges) will help, and for sure help your air flow. I suggest you read the David Vizard book "Theory and practice of cylinder head modifications". Great info and very helpful.

Good luck,
Dave

 

[RAC68]

Hi Andy,

Although other may have something useful, I remember having a tracing of competition Healey’s combustion chamber a great while back. I will look for the tracing and forward it via e-mail if found.

First, as I understand, run-on has often been the result of excessive carbon in the combustion chamber built up from running rich. The Healey’s chamber design does not help matters for unless you port, polish and relieve the chamber, there are inherent edges that can linger as glowing hot spots and cause the problem you are having. I would suggest that before you commit to extensive head work, check your mixture and timing and try a produce (such as SeaFoam) to clean the manifold and de-carbon the combustion chamber.

Sorry for repeating something you already know.

All the best,
Rat (64BJ8P1)

 

[57_BN4]

Hi Dave and Ray,

Thanks both for the information.

My situation is that I have never actually driven the Healey, it arrived last year in two trailer loads of 'stuff' and I have this grand plan to reassemble it all into a car again over the next few years. The PO had the engine reconditioned to a reasonably high standard of bored/sleeved to std, new pistons, rings, bearings -10 main & -20 rods, new cam & lifters, new rockers and shaft etc. It then sat for 13 years with only ~1000 miles on it. I have fully disassembled the engine for a complete clean, balance, flywheel lighten and general check over.

So, this part of the work is just a preemptive strike on what is a relatively common occurrence for these engines but isn't such an issue in the more open chambers of the Ford and Triumph engines I have owned- hence questioning the beak in the chamber.

Re the squish concept- I'm not entirely sold that it could work very well in the standard engine given that the pistons are about 2.5mm away from the head at tdc due to the gasket and space above the pistons. Even if I managed to cut away the beak completely, there is still a lot of squish area that will remain and the shape will then be similar to many other engines. According to some SAE papers I read a few years back, a great deal of HC emissions come from fuel being trapped in unburnable spaces in the head and the space between the top ring and piston crown. This is why modern engines have reduced distance to the top ring and no or very little squish area. Not that I'm planning on removing the squish area, just an interesting observation.

Triumph six is similar to the Ford and my Vitesse didn't run on when the Stromberg carbs where working correctly, although it needed the clutch-stall every time when the throttle spindles were worn.

So unless somebody has done this and had a disaster, I will try this plan:
Get the valves seats dressed, potentially fit hardened exhaust seats as they are very low already.
Deck the block until pistons are flush or just proud which will lose about 10cc
Measure combustion chamber volume and reshape the beak as much as possible while keeping the total volume under 52cc = 9.5:1 CR.
Cross fingers that I haven't just wrecked a very difficult to replace cylinder head!

An interesting fact I just learned is the petrol auto-ignition temp is only 250 degC and with a coolant temp of 100degC that doesn't leave much headroom once the heat of compression is added. Certainly doesn't need anything close to glowing to start dieseling.

Andy.

 

[RAC68]

Hi Andy,

Well, it seems you have a plan. Although I have only limited knowledge in compression chamber configuration, and although your plan seems logical and likely, I would suggest you not be too drastic when you relieve the peak and other areas of the chamber and place more emphasis on porting, polishing and balancing. I would suggest differently if you had a spare head, but since you don’t and fully reconfiguring the chamber could pose a substantial risk, I would suggest a little conservatism.

It is a fact that our Healey’s do present a technology from another era and to my delight the carburetors are not connected to the tail light. In some ways we can luck upon our car’s as Donald’s experiment and in others a study in simplicity. What is true is that today we have many areas we can provide performance improvements and are relatively simple and reversible. However, if you are intent on challenging the gods, I would suggest you start a thread addressing the specifics of advanced Healey Engine Performance Modifications and begin a conversation with those in the forum that are into racing an this specific area of activity. I look forward to this new thread and will be a very interested follower.

All the best,
Ray (64BJ8P1)