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rogerguzzi

Compression Ratio

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Hello All

              I am about to take my cylinder head in to have the valve seats cut.

I am wondering what compression ratio to have. at the moment it would be 9.3 to 1

I was thinking of 9.5 to 1  but do you think a bit higher would be better as I am going to run a 280 deg cam.

Roger

 

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10:1 or even 10.5:1 is pefectly runnable, if you use Shell V-Plus or similar.

John

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No idea of the results yet, apart from it will start and run, but went through the same conundrum when getting head machined in Jan.  Was happily running 9.3, (but who is ever happy?) thought of going 10, but with a few inputs regarding head studs, cam duration etc opted for 9.75:1 in the end.  As they say taking metal off is easy, putting it back again not so.

Alan

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Hello All

               I collected my cylinder head today and the seats look good and all valves are within 0.002" of each other in depth!(and been lapped in!)

I have checked the volume and the biggest will give 9.66 to 9.7 to 1 so my calculations were correct for having 0.030" off the head!

They are all within 0.4cc at the moment so I will take a bit out of the others and get them within 0.1cc to 0.2cc

Then I will double check my porting!

I made the 0.150" spacers for under the inner springs as recommended by Chris Witor.

Roger

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Hello All

               That,s it not messing with this cylinder head anymore!

Chambers are within 0.1cc to 0.2cc and the compression ratio works out at 9.68 to 1 to 9.7 to 1 or 9.72 to 9.74 depending on how much I allow for the head gasket(2.8cc to 3cc)

The ports all look as good as I can get them!

So I am going to fit the valves etc now and forget it!

Roger

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Hello All

                All done and dusted!

But now I find one of the head studs is b***ered (105121) it is the short one the nut was seized on and the treads look rough!

Does anyone have a spare? not sure I trust modern offerings.

Roger

 

 

DSC06740.JPG

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Hole box of 1500 studs, bolts etc here, as your close you can either come and dig through, or give me dimensions and I'll find and post

Alan

 

sorry Whole, but may have holes!

Edited by oldtuckunder

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Hi Roger

Found the box, I have 18 studs, but slightly perplexed when you said it was the short one! of the 18, 16 are 4.25 and 2 are 4.5 which implies to me that rather than there being an odd short one in the set, there is an odd long one!

Anyway just confirm which you require, PM me your address and its in the post.  All the threads look good! Actually if you don't know which I could always post one of each.

Alan

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2 hours ago, Nick Jones said:

I think there are a couple of long ones.... the ones that go through the lifting eye?

Nick

Hello Nick

                    That is correct.

I have fitted the head and just nipped it down until I get a new stud but I wanted to check the valve lift and clearance.

There is plenty of space between the spring coils but I only get 0.385" lift on the valve! not the 0.405" as quoted!

I think this is because as Alan pointed out the rocker arms only give 1.45 to 1 ratio not the 1.5 to 1 quoted everywhere!

Still this is still going to be more than before and with a bigger overlap so should be OK for a quickish road engine?

This means all the specs in this chart are wrong as they quote lift at 1.5 to 1!!!!

http://www.auskellian.com/paul/links_files/spitfire_cam_specs.htm

Roger

 

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Yep! web is full of misinformation, or as we now call it "Fake Clues! Although to be fair to the chart all the figures are correct if the ratio was 1.5:1 

Course you could call Newman's and say you want a 5% partial refund as its 5% under spec :devil: 

Alan

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Good luck with that.......  I'm sure it'll be fine - it's more than you had before, as you say!

I'm generally content if all valves show the same lift (or all inlets/exhausts if not equal lift) meaning the cam hasn't eaten any lobes.    Was going to check this on the Vitesse today but having drained the oil without finding anything but the finest fur on the magnetic drain plug I decided I couldn't be bothered.

Nick

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Hello All

              I thought I would check the valve lift on the engine that,s in Spitty!

I checked 1 ex and 2 inlets and they all measured 0.310" !!!!  not the 0.350" as shown in all the charts!

This a reground Canley Classic,s Cam 25 65 65 25 with 25,000+ miles on it and clearances are steady

So it still means I will get an extra 0.075" more lift! 

So I feel happier now as I know it is a 280 deg duration.

Roger

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