How to lower Compression?

[Paul Garvey]

One of my Mk2 Spitfire engine options is a 1147 with an 8-port head.  The only problem is it runs on avgas (aviation gas) with the head skimmed to within an inch of it's life running 11 or 11.5 compression - basically a race-engine.

To get it 'reliably road and service station-friendly' aside from the race cam I really need to get that compression down to 10.

Options I've thought of include:
- thicker head gasket made
- increase the combusion chambers upwards - assuming I could...not keen
- dished pistons

I've seen these on the web:

http://www.canleyclassics.com/?xhtml=xhtml/product/lowcomppiston1500.html&xsl=product.xsl

or

http://www.rimmerbros.co.uk/Item--i-155167

Will they fit a 1147?...these are stock and I'm not sure how many thou o'size the current pistons are.

Which is the best option?

Any other options I haven't thought of?

Many thanks,

Paul

[junkuser]

For the Nissan 2L engine you could get a decompression plate, though it was not called this.
It was just an aluminium plate, to pattern of head gasket, and they said to only use a head gasket on one side.
I used one but with a gasket on each side in an engine I modified for a particular reason.
Would be simple to make/have made if you work out what thickness you require.  Should work.

I have had two head gaskets on my Spit for about 30 years with no problems but that was to overcome the sealing problem due to Std 1300 pistons in a 1147 engine. It had been bored and worn too much for largest 1147 oversize available.

Just some thoughts that may save a bob or two.

[CHRIS211083]

Paul, Those pistons etc wont work unfortunately. Also you don't want to remove any material from the already thin 8 port heads. These are race heads ment for power. There are a couple of things that might help. If it is for road use only and want it to have some manners then changing the cam is the first step. This will make it more driveable. Then you need to run the engine up on Super unleaded and remove any vaccum advance(if present) and then check the timing. Max advance on these engines when tuned is 28 deg at 5,500.  Then the next step is to drive the car and see if it pinks. If it does then stop move the Dizzy back a deg and try again. Unfortunately there is no way round this as these engines are race engines and wont ever tick every box unless you use EFI.

Alternatly you could use a standard 6 port 1147 head and skim that for the compression that you want?

I certainly would be very interested if you didn't want that 8 port head anymore.

Hope this helps,
Chris.

[James]

I wouldnt change to a lesser cam when I already have high compression - youre just going to increase your dynamic compression and itll pink even more.

I dont see why junkusers solution wont work - or else you can get copper head gaskets made - I presume to any thickness you want. You dont need anything that much thick as youre adding an entire bore to the capacity.

Just be sure that your compressiom ratio matches your cam - the more wilder the cam the more compression you can use and will need to use.

[Paul Garvey]

Thanks for the suggestions.  Checked and it's 11.1. I think I'll look at a TR5 cam profile & have a new thicker copper gasket made to bring the CR down to 10.

Cheers  

[James]

Paul — what's the current cam profile?

[Paul Garvey]

Sorry but I don't know but is a race profile - was built as a track engine.

[rotoflex]

junkuser wrote:

For the Nissan 2L engine you could get a decompression plate, though it was not called this.

One of the GT6 engines I bought had one of those - a thin steel sheet punched like a head gasket, fitted next to the head gasket.  Possibly the factory ran low for a while on lower-compression US heads & sent out a few with standard heads & those plates in the interim.  

[junkuser]

Steel would be better to reduce any complications due to coefficient of expansion.
The one for the Nissan was made by a gasket company and called a head saver  to be used after a head has been over skimmed but I just used it to lower the compression.

Another point is that the two gaskets on the Spit are the old style, copper sheathed asbestos type.   Doubling up other types may not work.

[Paul Garvey]

Mny thanks for your thoughts,

I've just been on the blower to a local specialist auto gasket makers - they recommended either a solid copper or 'sandwich plate' (copper on both sides with a layer in between).  Suggested when I take the head in to determine the thickness required for 10CR to ask which type they'd prefer used given the thickness required.

As it's 11.1 reducing to 10 I expect it's a comparatively minmal thickness increase and a slightly thicker copper gasket would be fine - equivalent of 50 pounds.

Head specialists I've spoken this morning all agree not to try and increase the head chambers to reduce the CR - especially as it's already skimmed, ported and bench-flowed and likely to be comparatively 'thin' as a result.  

Sounds likce cheap and effective option.

Thanks again.

[Gt6s]

Is there enough meat on the top of the pistons to allow them to be machined down a bit ?

[Gt6s]

Is there enough meat on the top of the pistons to allow them to be machined down a bit ?

[drofgum]

Gt6s,
My thoughts exactly.
Paul

[Paul Garvey]

Hi Gt6s and drofgum - none of the machine shops today recommended skimming the pistons (assuming there is enough) as preferred option - thought more expensive, more labour and more likely to cause other potential set up 'issues' - recommended the same as the head specialists - thicker gasket - cheap fix.

Thanks

[nang]

Have you seen the cost of copper at present  :D

[Paul Garvey]

I know!!! Still $100 is pretty good eh?

[CHRIS211083]

Custom made gasket sounds the way to go. Good luck.

Chris.

[junkuser]

Straight, annealed copper sounds interesting but why I used the 1300+1147 gaskets was that I was not sure how perfect the head and block surfaces had to be to get a seal (and the gaskets were cheaper). Figured the old style gaskets would be nice and compliant.

Very interested to know how this works out, as if the copper works and it sounds as though the gasket makers know what they are doing, you should have no worries about blowing.
Is any special attention required arond the oil passage, such as an inserted "O" ring?

Once the head is off and you can measure the bore, it will be easy to calculate the gasket thickness required for a standard sort of cam with no wild overlaps.

[Gt6s]

Paul_Garvey wrote:

Hi Gt6s and drofgum - none of the machine shops today recommended skimming the pistons (assuming there is enough)

Typical machine shop answer.
It wouldnt be the first time I have had to argue with a machine shop to do what I asked, not what they think, they do'nt like unconventional practices. If the crown of the piston is thick enough I would not hesitate.

I had to argue to get a 60 thou overbore on a standard block.
To get a motorbike cylinder bored by 2mm (465 Suzuki motocrosser out to 500cc)
To get an incert put in the exhaust of my Lister CS they told me the head was scrap.
Regularly to heavily skim cylinder heads.

[JohnD]

Paul,
You have a Spitfire engine, with an eight-port, alloy head, and it's "an option"????????

Complete waste of time trying to de-tune it for road use, like re-painting the Mona Lisa with enamels, 'because she's a bit faded'!   That engine, and especially that head, is Holy GRail to some performance and racing Triumpheros, would spend a lot of money to aquire it.   If you have other options use them, unless you want to use the engine for what it was made for, racing.  Spend the money you could get that engine on a really top-notch, high-performance road engine, a completely diferent beast.

John
Ps Johnson Gaskets of Bradford, if you must have a bespoke gasket. http://vintagegaskets.com/
Mr.Johnson is a craftsman, I can praise no more.  J.

[CharlieB]

Fitting a thicker head gasket or compression plate would reduce your squish. I would have thought machining a dish in the piston (as Triumph did on the US 1500) if possible would be a better solution.