I tried drawing this to scale with what I think would advantages DeShrouding.




The black area is where deshrouding could perhaps be caried out, and for optimum flow into the chamber should be ground with a radius eaquil to the lift of your cam from the valve seat outward.

Gee I hope some of that makes sense??

Oh and it is based on a GX chamber with standard valves and 77mm bores.

Modifications are based on Modifications By 'Graham Bell' To a 'BMC mini' combustion chamber, witch is verry similler to our a series chambers. and I have re interprated them to suite.

I suggest anyone who wants to look into this further read '4 stroke Performance Tuning' by Graham Bell. Its a verry good bok

Hey Connor,

It makes sense, but looking at it in 2d makes it seem easier than it is. Unfortunately, to undergo this sort of procedure on my head it'd mean working the entire chamber as it is already so shallow. I also couldn't see that much material coming out. With a plane-jane GX head without any surface removed, this sort of work would certainly be easier.

I understood de-shrouding but when you mentioned recessing valves earlier on in the thread I thought you had some other crazy idea! I understand this.

My head has had 4.6mm shaved off it. In theory this is what needs to be replaced to get it back to stock CR. So I'm reckoning on something around 3mm.

Considering how much has been machined off your head, the potential for increasing chamber size is minimal (relatively speaking). They can and do'sink' the valves, but it will then have to have the short turn radius re-done, and considering its been optimised as it is, you'll only end up losing some flow. You are far far far better off not ruining that head imho.

When you go with the thicker gasket, just be sure to go a little lower on compression than you otherwise think it wil need because the big gasket option removes any real quench effect. It doesn't mean that the whole thing is pointless, you just need to go a bit lower in compressio for any give octane ratio than you would with a closed chamber head and tight quench clearance. Some of the heads have chambers that are more or less like having a thick gasket on there already, so it's clearly not the end of the world.

If starting from scratch, you'd have the closed chamber style head, and run custom pistons with a dish shape to mirror that of the chamber, it gives you nice wide quench pads but deep enough dish (in chamber and piston combined) to run a low enough comp ratio to survive on more modest fuels.. Basically that is the shape that will allow the highest comp ratio for any given fuel (and interestingly also produce the most power, more than a flat top piston and a more deeply dished chamber.

Have you weighed up the setup costs of a water injection setup? It'd allow you to run the head more or less as is, and run progressively more water at you get out past 1/2 throttle and up in the rpm range. The up front costs aren't dirt cheap, but the water itself is essentially free thereafter.

IIRC, the head saver shims are available off the shelf in two thickness's. 30 thou, and 60 thou. (.75, and 1.5mm for the new schoolers) As said above, don't sink your valves, it'll harm your flow drasticly. Since your having your head done up, (And you have such a considerable amount of compression to shed) then having a small amount of deshrouding performed can only be benificial. That said given how small you chambers are and how little material there is to remove, just unshroud them, and no more.

Having just now properly read this thread, I'm heartened to hear how much has been removed from your head. (4.6mm) I'm planning an A14et build, using e15et pistons, which have massive dish. (Like 10x the dish of a stock a14 piston) And
I was wondering how much could be removed without ruining the head, when the compression ratio wasn't an issue. Looks like I can safely reach my target CR after all.

Thanks fellas.

The head saver shims from permaseal are $104.00 ordered, so I figured I might as well go for a custom gasket with twice the thickness and probably twice the cost.

I'm in between a rock and a hard place as the head is pretty tired, but it has all the port work and bigger valves. I almost decided to get a H89 head worked instead but decided that the cost of port work would have probably exceeded that of getting a custom made gasket. Anyway it's in for a recon and I'd say this'd be the last time it get one, after that I'll look at getting another head worked. Thanks for the info guys, really helpful stuff.

a15 head is decomped already
they machine a 2mm recess into head same size as the bore
usually easier to get as noone wants them

If I change the head I'll have to spend a mint getting it ported and match ported to the manifolds. Cost is pretty significant here.

Dazza thats the H75 head more commen on the "econamy" a14's. most a15's came with the H89 witch has the same chambers but larger valves then the GX head. The advantage of a GX head is the aproach angle and lack of a water galery over the intake ports.

I think in a street going 1000 this motor will still move it along quite nicely!

I've finally drawn up the gasket to suit the head, the original one I had drawn up using a off the shelf gasket [as a reference] was surprisingly inadequate in terms of clearing the vessels of the head.




Basically rustling all the quote-requests together now.

The H89 chamber isn't as good as the GX one. It's a bit less heart shaped, and a bit deeper.

Also, $104 is a rip off! I got mine for between 50 and 70 (Can't remember exactly) AND it had to come from Aussie too!