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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:
Originally Posted by hustler
(Post 694364)
It's even lower if he's on an early 1.8 bottom end and a 99 head.
Originally Posted by wes65
(Post 694398)
I am running a 96 bottom end and 99 head, does that mean my compression is lower than 9:1?
Originally Posted by hustler
(Post 694745)
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.
Originally Posted by MartinezA92
(Post 695134)
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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Ok!
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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. |
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 :facepalm:
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". |
Originally Posted by MartinezA92
(Post 697036)
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 :facepalm:
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". http://www.solomiata.com/cams.html |
Originally Posted by astroboy
(Post 697072)
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 It would be nice to know what the effective compression ratio is by combining the older bottom end with the '99 head. |
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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Originally Posted by olderguy
(Post 697098)
It would be nice to know what the effective compression ratio is by combining the older bottom end with the '99 head. |
Originally Posted by olderguy
(Post 697098)
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. |
^ I would greatly appreciate it. Looking forward to the results.
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