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Guess-timating Compression Ratio
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Author:  wvenable [ Sun Jun 15, 2008 5:42 am ]
Post subject:  Guess-timating Compression Ratio
First I would like to thank tophat,sandy in BC, and DusterIdiot for the pics and info on how to cheaply cc heads and determine deck height - your suggestions worked well!

My combustion chambers are at 57cc's. TDC to top of deck looks like .176 in. :shock:

So I found an online comp. calculator and entered the above info plus bore and stroke figures for a stock 225 as well as .040 for a compressed gasket. The only thing I was unsure of was a figure for head gasket bore size so I entered 3.40 . Pushed the calculate button and out popped 7.8852 : 1 :shock: Not exactly 8.4 : 1! I did not calculate stock shim gasket which would bump it up a bit.

Question #1 Do these figures sound about right (first time I've messed with something like this) or did I goof up something?

Question #2 With a 60 thou overbore, what would I have to mill off the block/head to reach 9 or 9.5:1 ? Thanks for your help.
Author:  sandy in BC [ Sun Jun 15, 2008 7:35 am ]
Post subject: 
You are not gesstimating...you are calculating. Good one.

Your figures sound right.

.066 mill will get you one point......mebbe

80 thou will get you to 9:1.....mebbe

see why lots of folks do a .100 cut?
Author:  DusterIdiot [ Sun Jun 15, 2008 9:10 am ]
Post subject:  Yep...
Sadly enough that doesn't sound to far from the truth...

Sandy's pretty close given the 'probabilities'/ guesses...


If you're using a felpro comp gasket, and you can get the head milled to give you a 54cc chamber, and get .066 knocked off the block deck... that would net you a 9.19:1 compression ratio (if you're off by a CC or a little on the deck milling that you're still in the ball park)...


If you just knocked .100 off the block deck with no change in the head and used the fel pro gasket you'll be in the 9.4 ish range...

Good Luck on your choices,

-D.Idiot
Author:  wvenable [ Sun Jun 15, 2008 1:38 pm ]
Post subject: 
Thanks guys for all your help! Did a little refigurin' on that online compression calculator - entered 54 cc chambers and .076 (.100 off the deck) and 9.6126 : 1 came up. This is sounding better. I know I read it on the forum but I can't remember where - How many thousandths off the head equals a 1cc chamber reduction?
Author:  1969ronnie [ Sun Jun 15, 2008 4:43 pm ]
Post subject: 
your comp number should be even lower since fel-pro lists the bore size as 3.520 and not the 3.4 you used. try 3.520 and post back. ron
Author:  terrylittlejohn [ Sun Jun 15, 2008 5:02 pm ]
Post subject: 
0.0066" of the head surface to reduce chamber volume by 1cc.
Author:  wvenable [ Sun Jun 15, 2008 6:46 pm ]
Post subject: 
Guys, Thanks again for all your help! terrylittlejohn & 1969ronnie thanks for the response! gettin' a bit closer to the goal! :D
Author:  Adrenalin [ Mon Jun 16, 2008 7:21 am ]
Post subject:  0.1 milling cause pinging?
Dear All,

I also want to mill my 225 head 0.1". Does this cause pinging or any other misfiring issues.
Author:  terrylittlejohn [ Mon Jun 16, 2008 9:35 am ]
Post subject: 
on a stock engine not necessarily but when you increase your compression you must be careful with timing curve, total mech / vacuum adv settings, also you might need to use supreme gas.
Author:  wvenable [ Mon Jun 16, 2008 11:03 am ]
Post subject: 
Entered the gasket bore size and reduced chamber size to 56 cc and came up with 9.5808! If I put in the oversize valves (flat surface not concave like stock) this should bump me up slightly but sounds like 9.6 : 1. This should work well with the Erson 270 and be a good streetable situation, at least on paper. Thanks Ron and Terrylittlejohn!
Author:  Dart270 [ Mon Jun 16, 2008 11:19 am ]
Post subject: 
That should be a very nice setup as you describe. Probably be OK on 87 octane if timing curve is right.

Lou
Author:  sandy in BC [ Mon Jun 16, 2008 11:56 am ]
Post subject: 
One thing to note. If you plan on a little relieving around your oversize valves you will increase chamber volume.

Do your head.....cc it ....then make a block - head decking plan.

After head work I had to mill .160 to get the CR I wanted.
Author:  wvenable [ Mon Jun 16, 2008 12:21 pm ]
Post subject: 
Sandy thanks for the tip! I had planned to rework the cylinder head first as the budget calls for baby steps right now. :x Polishing the chambers may also increase volume slightly,which I plan to do as well as some gasket matching and bowl work. A friend of mine has some 340 valve springs, I hope he lets em go cheap. After that I will probably get the oversize valves from that engine builder guy. Thank you for your help!
Author:  Adrenalin [ Mon Jun 16, 2008 10:11 pm ]
Post subject:  head milling heeeelp
I mostly use LPG and 95 octane gas. For the advance setting, is there anything more to be carefull other than the adjustment of the timing? Do I need to change something in the vacuum thing? Also I wish I could have 10:1 comp ratio, if I mill 0.1" does it only increase CR to 9.5:1? 0.1" is a large value ( it is 2.5 mm!!!). I am afraid if the valves hit the pistons but as I read the posts, this is not an issue. So I am only left with the pinging possibility after milling. If this is prevented by advance adjustment than its fine I guess.
Author:  sandy in BC [ Tue Jun 17, 2008 8:00 am ]
Post subject: 
You need to adjust cam timing as well. ..

Search dynamic compression ratio.
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