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Old 11-17-2004, 19:33   #1 (permalink)
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Question GT40P's before instalation...

I have myself some GT40p's that are nearly ready to slap on to the old 5.0.
They have been professionally ported and then polished up a bit by myself.
They now have 61cc chambers, measured after minor intake valve unshrouding.

Should I keep the current chamber volume at 61cc (for aprox 9.3:1), or try and get it down a bit for more compression & power (say 58cc for 9.7:1)?.
What would be the best compression ratio for this setup, on say BP ultimate with a 2031 cam?
Maybe just a surface grind before the instalation to lower chamber 2-3 cc's?
(I am keeping in mind also, like most people I have dreams of 9 lbs forced induction, but this is likely never to happen).

Also should I keep the stock 1.84/1.46 valves?
I know these heads are a quick burn, high velocity head, but I have seen lots of corral boys using a 1.84/1.54 valves, or bigger with there combos.
Is it worth the extra expense in changing to bigger valves (or at least the exhaust valve), or will the total cost outweigh the benafits?

I have to also keep in mind that bigger valves may help the compression ratio out too, so I have a bit of thinking and planning to do.

All experiences are welcome.
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Old 11-17-2004, 20:33   #2 (permalink)
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Re: GT40P's before instalation...

The benefit of putting larger valves in would be outweighed by the cost IMO. The heads should flow fairly well with just the minor port work for any 5L application. Same goes for increasing compression. The 2031 and most other 5L cams are designed to work with around 9:1 compression. Going up will give you a bit more power but again IMO is not worth the cost. It will also give you problems if you do go forced induction later.
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Old 11-17-2004, 21:27   #3 (permalink)
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Re: GT40P's before instalation...

Light clean up then latter if you want f/i then use copper head gasgets .080 thick or so..Or use Cometics if they have the correct thickness....
On hot days and with air conditioning etc, compression can be your worst enemy, unless you can twEECer the advance curve you'll have issues with detonation..Up to 9.5 should be fine...
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Old 11-17-2004, 23:17   #4 (permalink)
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Re: GT40P's before instalation...

Thanks heaps fellas.
So I will be scrapping the idea of new valves, and any thoughts of supercharging (for now).
I will be aiming for a compression ratio of 'up to' 9.5:1, being my best bet.
With a light head surface clean up to prepare them for installation and to lower the chamber volume between 1-2cc's giving around 59-60cc's, to approach the 9.5:1 mark.

So how much is taken off the surface of a head to remove 1cc from the chamber?
I know a shop should know this, but I was curious.
I have read 0.006 for every 1cc, and I have also read 0.5points for every 0.010 off.
Anyone know of other rules of thumb for doing this?
Just want to be armed with a bit of knowledge for when I take the heads to the shop.
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Old 11-17-2004, 23:39   #5 (permalink)
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Re: GT40P's before instalation...

would just be a little maths invoving the diameter of the chamber and a formula like 3.142 x Rsquare x whatever depth gives 1cc. wouldnt it?
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Old 11-17-2004, 23:49   #6 (permalink)
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Re: GT40P's before instalation...

I don't know how to get the square area of the chamber surface shape.
If I had the area then I could figure the depth out with the volume.
Unless I get some grid paper and my crayons out.
I going to let a shop do it, or just guestimate.
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Old 11-18-2004, 00:06   #7 (permalink)
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Re: GT40P's before instalation...

dont we just need to figure out the height of a really small cylinder that gives a volume of 1cc.we know the diameter of the really small cylinder that measures 1cc by measuring the chamber diam at surface level.i could be outsmarting myself but thats how i see it.
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Old 11-18-2004, 00:09   #8 (permalink)
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Re: GT40P's before instalation...

ok. chamber is not a perfect circle as i just looked at a spare set of 40p,s i have so my formula and mindset are rubbish.disregard previous dribble.
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Old 11-18-2004, 02:53   #9 (permalink)
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Re: GT40P's before instalation...

Would there be anything wrong with just cleaning the face up with say 800 paper, and leaving it at 61cc's?
Or is it best to grind the face flat again?
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Old 11-18-2004, 03:06   #10 (permalink)
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Re: GT40P's before instalation...

Quote:
Originally Posted by dodgie
Would there be anything wrong with just cleaning the face up with say 800 paper, and leaving it at 61cc's?
Or is it best to grind the face flat again?
As long as the surface is straight (cast iron heads don't often warp), then cleaning them up will be fine. With mine I just gave them a light sand with wet and dry, and then cleaned them with contact cleaner (can use carby cleaner or rubbing alcohol).
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