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Gtir Motorsport club » General Discusion » Gtir related Discussion » Compression Ratio

Compression Ratio

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1 Compression Ratio on 8th November 2015, 7:05 pm

pulsarmoley

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Ok so i wanna have a discussion about compression ratios, i know in standard form our engines run 8.3:1, just interested to see what other CR people are running on their cars as to power and mods etc.
Im interested as i think from my workings out im only running approx 8.2:1. With my plan to only run the car around 330hp im wanting to up the CR to improve on torque, so my question is would i be ok to run closer to 9:1?


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2 Re: Compression Ratio on 8th November 2015, 9:11 pm

Indy


Iirc naturally aspirated cars run high comment ion ratios where as its ment to be better to run lower compression on turbo cars and then up the boost by using a thicker head gasket

3 Re: Compression Ratio on 8th November 2015, 9:36 pm

pulsarmoley

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Yeah ive been researching and found that for example running a big turbo such as a 3071, people were runner a thicker h/g to lower CR for more boost overall. Some people however ran a lower boost and higher CR for better off boost response. My thinking was that running a small turbo; like a GT2860RS, i could run a CR close to 9:1 if only running the turbo at circa 1.3bar?


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4 Re: Compression Ratio on 8th November 2015, 10:16 pm

shroom

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Speak to Stu as his brother built a high comp engine exactly how you explain above to give a more responsive car. The thick headgasket is quite an old skool method to whack ip the boost (very dp motorsport) and although OK I dont know how happy I would be with the car.

Stu's brother did a lot of work on this subject although Danny Fennel brought it off him and you know the rest of the story it now has some big bhp

5 Re: Compression Ratio on 8th November 2015, 11:09 pm

pulsarmoley

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Ok cheers, hopefully Stu will check this thread and give some more input


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6 Re: Compression Ratio on 9th November 2015, 5:16 pm

Indy


I believe running lower compression is ment to be 'safer' for the engine but suppose thinking logically about it when you run high boost it work out the same in the compression chambers

7 Re: Compression Ratio on 9th November 2015, 5:28 pm

pulsarmoley

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Something like that, i think to make higher boost or higher cr you retard the timing


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8 Re: Compression Ratio on 9th November 2015, 8:35 pm

johnny gtir

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Higher boost on lower compression is the way Norris work there sr20 Det lumps I think they say it's better to blow a turbo than engine. I could be talkin crap

9 Re: Compression Ratio on 9th November 2015, 8:41 pm

pulsarmoley

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johnny gtir wrote:Higher boost on lower compression is the way Norris work there sr20 Det lumps I think they say it's better to blow a turbo than engine. I could be talkin crap

I wonder if this is applied to all power specs they have done; i.e 300hp spec compared to 500hp


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10 Re: Compression Ratio on 9th November 2015, 8:56 pm

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Alot of the new engines run high comp with turbo .... not sure what cr is though ...


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11 Re: Compression Ratio on 9th November 2015, 9:02 pm

pulsarmoley

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nomad wrote:Alot of the new engines run high comp with turbo .... not sure what cr is though ...

Didnt think about that, another thing to look into


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12 Re: Compression Ratio on 9th November 2015, 9:23 pm

Indy


On the subject of cr how does one check cr without knowing what the engine spec is?
Is there a calculation to measure it?
Sorry for the hijack

13 Re: Compression Ratio on 9th November 2015, 9:38 pm

nomad

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With something like this .....
http://www.csgnetwork.com/compcalc.html


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14 Re: Compression Ratio on 9th November 2015, 10:08 pm

pulsarmoley

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nomad wrote: With something like this .....
http://www.csgnetwork.com/compcalc.html

Yeah this is what i used

87mm bore
86mm stroke
87mm h/g bore
1.5mm h/g thickness
46cc head combustion area
-14.3 piston dome area
0.2 mm deck clearance (a guess?)

Equates to 8.26:1 CR


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15 Re: Compression Ratio on 10th November 2015, 8:02 pm

pulsarmoley

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nomad wrote:Alot of the new engines run high comp with turbo .... not sure what cr is though ...

Had a brief look Lieghton, found astra vxr runs a 8.8:1 and that new civic with the turbo vtec run 9.8:1 but i reckon they get away with that through the setup of the vtec


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16 Re: Compression Ratio on 10th November 2015, 8:45 pm

nomad

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Intresting i know corsa vxr's are also .... i wonder what size headgasket they use .....


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17 Re: Compression Ratio on 10th November 2015, 8:57 pm

pulsarmoley

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The astra vxr runs a square block design like the sr20; bore x stroke 86mm x 86mm

I think from what i have found h/g is 1.3mm


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18 Re: Compression Ratio on 11th November 2015, 4:53 pm

pulsarmoley

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shroom wrote:Speak to Stu as his brother built a high comp engine exactly how you explain above to give a more responsive car. The thick headgasket is quite an old skool method to whack ip the boost (very dp motorsport) and although OK I dont know how happy I would be with the car.

Stu's brother did a lot of work on this subject although Danny Fennel brought it off him and you know the rest of the story it now has some big bhp

Stu?


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19 Re: Compression Ratio on 11th November 2015, 5:34 pm

Stu

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Ffs. Stupid fecking android and wifi has deleted my reply grrr
I will ask my brother to pop on and give his advice


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20 Re: Compression Ratio on 11th November 2015, 5:47 pm

pulsarmoley

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Stu wrote:Ffs. Stupid fecking android and wifi has deleted my reply grrr
I will ask my brother to pop on and give his advice

Top man Cool


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21 Re: Compression Ratio on 11th November 2015, 8:20 pm

johnny gtir

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Stu wrote:Ffs. Stupid fecking android and wifi has deleted my reply grrr
I will ask my brother to pop on and give his advice
Look forward to this

22 Re: Compression Ratio on 11th November 2015, 9:32 pm

Mr B

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I would be careful with comp ratios on the old det's as the engines don't have the best knock limit or knock control standard.
In petes scenario of running lower boost of a bar to say 1.2 with perhaps a GTX2860r the suggested comp of 8.8 to 9 would probably be ok but I would choose 8.8. Running higher comp increases chance of det so you want best fuel you can get & good ecu/mapping to control it, proper closed loop det with dynamic ignition map trimming would be advisable to make most of it on the R safely .
early evo's had 8.8 comp, later 9 . You will get slightly better initial response from higher comp ratio but don't go to far. evos have tried anything up up to 10.5 but late evo engines have good knock limit.

23 Re: Compression Ratio on 11th November 2015, 9:35 pm

pulsarmoley

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Thanks for that Mr B, good info there thumbsup


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