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Old 03-02-2011, 04:31 PM   #1
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Default Compression for an early BP block w/99 head

I did the math using the 49cc combustion chamber measurement, and all the stock 94 measurements that I found (rods, crank, compressed hg ect) and came up with stock compression but then there was a large sub discussion in wes65's thread about the combo of an early BP (94/95) bottom end coupled with a 99-00 head = lower compression engine than one would expect. Can anybody weigh in on this?

Here's the sub discussion:

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Originally Posted by hustler View Post
It's even lower if he's on an early 1.8 bottom end and a 99 head.
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Originally Posted by wes65 View Post
I am running a 96 bottom end and 99 head, does that mean my compression is lower than 9:1?
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Originally Posted by hustler View Post
John and I put a few 99-heads on early 1.8 bottom ends and they never seem to make more power aside form a slight increase in mid-range. I'm not sure if the cams mount higher, altering the cam timing (VW issue) or if the deck is higher but something strange is going on. The next time we have the two lying around maybe he and I can measure.
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Originally Posted by MartinezA92 View Post
Does anyone have any more insight on this? It would explain a lot of things. I'm assuming its the difference in cam profile but I never got any better than 165 psi on a compression test (all 4), 94 block/99 head, 9:1 pistons. Also, cylinder leakage was 1-2%. No complaints as far as power (other than the fact that I'm N/A), but I found it kind of odd.
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Old 03-03-2011, 08:35 PM   #2
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Ok!
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Old 03-03-2011, 11:48 PM   #3
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I'm just assuming the miata bp's are the same but the earlier bp's in fwd applications were 8.8:1 CR when they were labeled as 9:1 this was corrected to a true 9:1 for 96+ bp's.

//////SIDE NOTE:
I can't remember for sure if the valve relief cuts line up (iirc they do) but a sohc b8 short block with a bp head gives an 8.6:1 CR. The sohc crank is substantially lighter though not forged.
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Old 03-04-2011, 01:56 AM   #4
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I'm interested in the total CC's of the combustion chamber in a 94-97 head compared to a 99 head. I should have done this when I had both heads on a bench

Also, if anyone has cam specs for both, that might explain a lot. I do remember reading long ago that the 99 head was more "aggressive".
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Old 03-04-2011, 08:24 AM   #5
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Quote:
Originally Posted by MartinezA92 View Post
I'm interested in the total CC's of the combustion chamber in a 94-97 head compared to a 99 head. I should have done this when I had both heads on a bench

Also, if anyone has cam specs for both, that might explain a lot. I do remember reading long ago that the 99 head was more "aggressive".
From what I read the 94-97 and the 99-00 heads have the same 49cc combustion chamber. When I get my 99 head back I WILL be verifying it. Here is a link to solomiata's cam page:

http://www.solomiata.com/cams.html
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Old 03-04-2011, 10:24 AM   #6
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Quote:
Originally Posted by astroboy View Post
From what I read the 94-97 and the 99-00 heads have the same 49cc combustion chamber. When I get my 99 head back I WILL be verifying it. Here is a link to solomiata's cam page:

http://www.solomiata.com/cams.html
While they might have the same size combustion chamber, that chamber is formed completely different. The top shape of the '99 piston enters further into the head than does the 94-97 piston, which only comes to the top of the bore. This leaves a much larger combustion chamber area, which in turn drops the compression quite a bit.

It would be nice to know what the effective compression ratio is by combining the older bottom end with the '99 head.
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Old 03-04-2011, 12:18 PM   #7
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Hmmmmm this has me wondering if this maybe part of the reason that my 94 bottom end with 140k miles on it and my MSM head is taking 25psi without blowing up
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Old 03-04-2011, 05:27 PM   #8
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Quote:
Originally Posted by olderguy View Post

It would be nice to know what the effective compression ratio is by combining the older bottom end with the '99 head.
Definitely this. 165 psi compression with 1-2% cylinder leakage must mean its quite a drop.
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Old 03-04-2011, 07:51 PM   #9
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Quote:
Originally Posted by olderguy View Post
While they might have the same size combustion chamber, that chamber is formed completely different. The top shape of the '99 piston enters further into the head than does the 94-97 piston, which only comes to the top of the bore. This leaves a much larger combustion chamber area, which in turn drops the compression quite a bit.

It would be nice to know what the effective compression ratio is by combining the older bottom end with the '99 head.
When I get my 99 head back I will be measuring the combustion chamber and taking lots of measurements. I can even do some side by side shots to try to get to the bottom of this caper.
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Old 03-05-2011, 07:16 PM   #10
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^ I would greatly appreciate it. Looking forward to the results.
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