Low Compression Ratio! Help needed!

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.


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.

easy!

fit a thicker head gasket

you sure you dont mean thinner as thicker will increase the volume even more and lower compression further.

Skim more off the head...   

yep just re read what you have wrote and your figures cannot possibly be accurate seeing as the head has obviously already been skimmed
the combustion chambers in head HAVE to be smaller not bigger unless they have machined those too of course!

So......i have used 50 mililters of liquid.....
I have discuussed with the machinist and he told me that the valve seats were pretty much destroyed.....so he machined alot from the valve seats.
So my new volume of 50cc is accurate.

I am thinkibg in getting an 1.2 mm gasket instead of the 1.5 mm which I have now.

Is this correct??
Please advise!

yes you can do although I think the cosworth hg is actually 1.1mm
but if your planning on running around 2 bar boost (which you would need to get a 3071 to around 470-480 then I would have dropped the comp ratio anyway as the engine will be safer at a lower ratio, less chance of lifting head or torque twisting the block

Just one thing to note is if your machinist has taken a lot off the valve seat you may have issues setting cam clearances as Bob did with my head if I remember correctly.

yes that's true as shim clearances were mega tight even with the smallest shims so had to dremmel some of them

Apexi & cosworth do 1.1mm HG and Apexi even do a 0.8mm
Would of thought a 1.1 HG would work out about right, do the maths & see what you get. Kind of pointless skimming head again unless proves only way get result you need.
As above, deffo check the valve clearances fully, needs be a top job here for top end reliability & healthy sound.

So......the clearances are set not (shim to valve....adn shim to rocker) as per the Service Manual.
I will do also block deck to piston height clearance and also measure the combustion chamber of the piston!

Will keep you guys updated!

bogdan when he did your valves has the guy taken the lead off the valves?
just looked at them again and they seem as they are sunk very low into the combustion chamber
hope you get them shimmed up all ok as like already said ive had this before with another head I was given to shim!
problem is if hes taken the leads off the valves they will burn out very quickly plus wont be opening to full extent so youll be down on power

I would take a serious good look at the work that's been carried out here especially as the comp ratio is so low and it must be caused through those valves being too far recessed : /

DET seats & valves are very hard wearing. Normally if seats excessively badly pitted or worn you would bore them out & press in new seats.
Lead as such was in the fuel originally for octane increase but was also found that reacted with metal valve seat/face surface creating a very hard surface oxide that helped resist wear.
Valve lift will be same but you just starting from a more recessed position which technically not the best scenario.
I never like throwing too much money on a base gtir head as would prefer jump direct to a vvl over extensive standard head work.

Hmmmmmmmmmm......you made me think a lot now:(

Can I run it like this ?
Or...shall I go and ask him to pay me back for the head he just destroyed?

Please advice as I need to close the engine ASAP

I have just spoken to the guy...and he told me that the seats were pretty much destroyed and he just machined the seats and just a small valve job.

I think that he took alot from the seats since I have a combustion chamber of 50 cc's instead of soemthing close to 46.5 cc's.

But anyways I'll do the piston to deck clearance and double check the block combustion chamber and see with what I'll come up.

Please advice.........

Hard call without seeing it.
If everything measures up fine & you can get suitable compression ratio you should be good to go.
Only downside is overly recessing valve loads not ideal & allows for bit of concern on valve spring preload and exactly how he corrected shimming.
GTiR head is one of those areas where I wouldn't mess around with porting, valve work & cams etc as personally would prefer go to vvl & have better design & reliability.

so hes machined the seats rather than fit new guides?

that's fine if just lapping in valves to seats to make a good seal but my guess is hes put the valves in a drill and ground the fook out of them which as said will take the lead off the valve, will also reduce preload on valve spring so could possibly get valve bounce and more risk of throwing a rocker, and you have the valve burning issue if theres no lead and just a thin edge (especially with running big power)

the only way he could shim that engine would be to grind the internal part of bucket on shim or the valve stem head itself which also is not good if hes taken too much off

I wouldn't be happy if it was a head fitted to my car personally as would be worried about whats wrote above but its your call at the end of the day

He machined/recut the seats rather than fit new seats
http://www.ebay.com/itm/EPP-Intake-Exhaust-Valve-Seats-SR20DET-SR20-SR-Pulsar-GTiR-N14-/150813531411
Generally if well worn/pitted you would replace seats rather than cut new seat face  too deep/recessed.
I would assume too he would of shortened valve stem head a fraction to get shimming in suitable range but that all depends on how deep he gone & what thinnest size shims he may of had available possibly :-/
If only a fraction you can get away with it but not ideal practice for vehicles intended performance/usage.

Yeap...He machined the seats and just slighlty machined the valves.
In order to get the correct clearances he just shimmed a little bit of the valve (the part where it touches the shims).
Bob...I have bought a set of rocker stoppers....hopefully that will solve the rocker problem.

yeah rocker stoppers will stop the rockers from shifting off the shims and being thrown
but theres a downside too in that if the valves aren't tensioned enough by the springs they will bounce and the rocker stoppers will force the rocker to push the valve back down whilst it is bouncing then theres a high risk of valve to piston contact

seeing as your pre-load on valve springs is going to be less then obviously that risk is amplified

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

This is my head after having work done on it, mine came out at 45.8cc

Pfff man....thanks alot for the info!