Low Compression Ratio! Help needed!

R

RO_SUNNY

Active Member
Hello everyone,


I have bumped into a problem....or so it seems.


So.....I have went to a machinist which repairs cylinder heads.


After that I have made this test.........to find out what my cyl. head volume is and the results were 50 cc's.


Enlarge this image






Enlarge this image




That's kinda sad because the standard head volume is either 46.1 cc's (according to SR20 forums) either 46.5 cc's


I have used 3 comp. ratio calculators and all came up with the same results.....7.95 to 1 comp. ratio.


This is too low.........low low!


Please help me .......to come up with a solution.
What shall I do in this particular case?
1. Shall I buy another head
2. Shall I buy a bigger turbo (I have an GT3071R now) to make 480 BHP which is my aim as we speak.
 
red reading

red reading

Active Member
These are your choices, skim the head more, weld up the combustion chambers and re machine them, get higher compression ratio(less dished) pistons, have you had the valve seats machined? The valves look very recessed in to the seats.....if they are you will not get the flow you want and it will be a pig to shim up to spec.

tbh 8-1 won't be as bad as you think.
 
Fast Guy

Fast Guy

Moderators
Staff member
It'll make it exciting. ;)


Let's face it, 480bhp isn't exactly going to be kind to a standard box.
 
R

RO_SUNNY

Active Member
I will also do the deck to piston clearance and also measure the compression chamber of the piston......after that I will re-do the calculation ans see

Will keep everyone updated!
 
R

RO_SUNNY

Active Member
I studied into this issue ....
So the Gtir head has a round cutout and that's the reason why it has 8.3 to 1 comp ratio....comparing to the 8.5 to 1 on other sr20det engined cars.
The main reason is that the combustion chamber from the gtir is much bigger than the 200sx one (talking in tenths of milimeters).

So....since nobody was able to give me an exact number on the cubic capacity of the Gtir I will take that 50 cc's is quite good....taking into consideration that the 200 sx head is somewhere between 45 and 46.5 cc's ( this is what I found through the internet....because I didn't found any exact numbers).

Just for the info ...this is an 200 SX head.


This is the comparison between the two....right is Gtir and left is 200 sx


And this is the Gtir Head......

.........In which everybody can clearly see that the reason for the lower comp ratio on the Gtir head

I have made also the deck to top of the piston clearance and it's 0.25 mm
I have also measured the combustion chamber of the block and the result was 15.5 cc's

So for the future reference of others that will build their engines:

-Head combustion Chamber : 50 cc's
-Deck to top of the piston clearance : 0.25 mm
-Block cc's : 15.5 cc's
 
red reading

red reading

Active Member
There is 2 types of 200sx heads, one has quench pads on the inlet and exhaust side and the other has them on just the inlet, quench pads will give better burn in lower powered sr20 and therefor better emissions, but will ultimately restrict how much power you can make...hence why on the sr (inc vvl turbo)and RB engines people machine out the combustion chamber to the hemispherical shape like the gtir..like mazworks .
 
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