Race Engine Forum UK

Hi,
I would like some advice regarding increasing the compression ratio to close to 11:1 on a SOHC 1500 Bertone X1/9 engine. This engine is in a 1988 Australian spec car, and I suspect it's similar to the US spec cars, so I don't know how it compares to UK cars. Australia through the 70's & 80's had traditionally got the same specs as European Fiats, but I don't know about the Bertone.
This Fuel Injected head has a full bore diameter recess in the combustion chamber about 90 thou deep, and pistons with a large cast in valve relief, the C/R measures at 8.8:1, refer photos.
This is the first naturally aspirated SOHC head I have seen with this recess, but apparently it's the norm in the US. What I am thinking of doing is to have the head faced 90 thou to completely remove the recess, this would give a 40 thou squish area (for good swirl & reduce the chance of detonation) between head & piston, if I can get a 40 thou head gasket, otherwise I will have to skim the block a bit as well. I have a new set of oversize flat top pistons with large machined in valve reliefs, so this should give the 10.8:1 or so compression I am after.
As I see it, my only other option is to buy another head without the recess, and a new set of pistons with small valve reliefs (problem is my Piper BP300 cam has 0.440" lift & valves will be a bit close to pistons) or get expensive forged HC pistons. I don't really want to do any of these.
Has anyone faced a SOHC head this much before?
Are there likely to be any reliability or other problems that I haven't thought of?
I know I will have to run a larger cam belt tensioner pulley to take up the belt slack with the cam moved closer to the crank, but that shouldn't be a problem, apparently they have done these mods in the States.
The car will be stripped of as much weight as possible, with aluminium and fibreglass panels, perspex windows etc, and used for club circuit sprints and hill climbs.
The engine will use 98 octane pump fuel, twin 40DCOE's with 36mm chokes, Piper BP300 cam, 39.5 mm inlet valves with ported heads, long primary 4 into 1 extractors and free flow exhaust.

I know this is a budget, not a high tech engine, but any advice on this would be appreciated.

Regards, GregS

Hi Greg,

I would advise against skimming this much material. Could you post a photo where the front water passages of the head are visible (the ones adjacent to 4 M8 bolt holes for fixing the head to cyl. block)?

Usually, after a head gets excessively skimmed, the material thickness in this region becomes unacceptably low, leading to common head gasket failures.

I would suggest importing a high compression head from Europe - since you are already going to invest some money in it, better start properly.

BTW, the CC on photo (clean one) has suffered some detonation damage - do you know some background?

Best regards,
Nik

Hi Nik,

The web across the centre of the water jacket hole is about 3mm below the head face, I don't think it's been machined before. I would have thought the 4 extra small head bolts would be able to help the gasket seal this area even if it is faced a lot. From what I have read they don't have problems leaking on the US modified ones. Have you seen any torque figures for these M8 bolts?

I don't know the history of the car, but I did wonder if the pin holes was detonation, I've calculated the C/R at 8.8:1, so that shouldn't have caused it, maybe the timing was way out.
Here's some more photos.