Engine compression ratio please?
30 August 2018, 07:47 PM
#1
Engine compression ratio please?
Just would like to confirm my options on compression ratio of my engine (clarification really).
my block is 8.5cc and heads are 48cc.
is anyone good at working out what CR options from that and what thickness gaskets.
thanks
my block is 8.5cc and heads are 48cc.
is anyone good at working out what CR options from that and what thickness gaskets.
thanks
31 August 2018, 07:58 PM
#4
You need to check your 8.5cc piston, has that been measured as fitted at tdc, or is it the bowl volume quoted by Omega,un fitted ? as the two are very different.
With the figures you gave a 79mm stroke & 93mm gasket dia & 1.1mm thick gasket your at 9.38:1 comp ratio.
With the figures you gave a 79mm stroke & 93mm gasket dia & 1.1mm thick gasket your at 9.38:1 comp ratio.
Last edited by MOTORS S GT; 31 August 2018 at 08:02 PM.
31 August 2018, 10:04 PM
#5
You need to check your 8.5cc piston, has that been measured as fitted at tdc, or is it the bowl volume quoted by Omega,un fitted ? as the two are very different.
With the figures you gave a 79mm stroke & 93mm gasket dia & 1.1mm thick gasket your at 9.38:1 comp ratio.
With the figures you gave a 79mm stroke & 93mm gasket dia & 1.1mm thick gasket your at 9.38:1 comp ratio.
ive been advised 9.0.1 on a 0.78 gasket or 8.7.1 on a 1.1mm.......
i have 10.70 lift cams so just asked if I will have enough clearance to go 0.78mm hg.
01 September 2018, 10:46 PM
#6
Scooby Regular
I'm pretty sure the 8.5 cc was fitted figure, but can't say for sure without talking to builder (tdc?) sorry I'm not experienced in this field lol. I'm led to believe they are a 9 piston already?
ive been advised 9.0.1 on a 0.78 gasket or 8.7.1 on a 1.1mm.......
i have 10.70 lift cams so just asked if I will have enough clearance to go 0.78mm hg.
or Whiskey if you can spare some, if you still get a slight meniscus you can add a drop of fairy liquid. use a 10-20 cc syringe, or failing that pipette it out of a beaker standing on a set
of accurate kitchen digital scales.
The CR I see as less of an issue even at 9.0:1 , if in doubt a splash of meth would give piece of mind, personally Im happy because I have 102 Octane available,
However If you are concerned about the fuel you get on 9:1 CR that's one thing, where I would have thought on normal Optimax/Tesco 99 you still ok but on 98 or 95 you would probably better to look for closer to a 8.5-8,7 ratio You can reach that with either the >1.1mm or achieve it with a little bit of chamber cc vol increase thru careful removal with a grinder which is always a good thing to remove any machined edges to smooth things out and prevent hot spots or if you don't fancy that just use a 1.3 mm , that would help you keep the valve clearance. While you have higher lift cams I would start anyway just to lay both 0,78 gaskets that you have now together and place some blue tack in the piston dish and do a dry build run on it without a torque down and turn the engine through 720° and look for contact issues, Theory and measurement is one way , but a lot can be said for physical checks too. Repeat that for second head, if it looks at all like you are likely to make any contact, if its too close to call (bear in mind things expand with heat ), then repeat with just the 0,78 Gasket if this does not make contact you can even have a go "gently" with no gasket with loose bolts as soon as you feel any resistance check if you can see light thru block and head. Then Its thicker gasket time or other precautions.
Id certainly like more than a Rizzla paper thickness of clearance , so if its very close re-seat the valves and use thinner cam follower bucket shim or pocket mill pistons (id rather not) that drops the CR a touch too.
Ps , Do check all 4 cylinders with bluetack , I had just one valve on last head build that almost touched, it turned out that Id swapped a bucket /shim
and the cam clearance was wrong (too high) on another. no 8 and 5 looked almost the same when the edding was smeared.
Last edited by Linksfahrer; 01 September 2018 at 10:58 PM. Reason: ps
02 September 2018, 07:51 PM
#7
you wont regret the extra C/R mine is a 9.0-1 compression and its very torquey off boost- mine also runs cams and circa 600/500 numbers and its only ever been run on/tuned with vpower, probably about the limit for vpower (reliably) but its quick enough for the road.
found this out a lot lately as i,ve had the same boost pipe pop off half a dozen times lol -still feels nice and grunty even without any boost at all.
found this out a lot lately as i,ve had the same boost pipe pop off half a dozen times lol -still feels nice and grunty even without any boost at all.
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02 September 2018, 09:40 PM
#8
Id imagine you'd want to be real sure about the piston cc though and non swept cc, and as you cannot use water due to the meniscus , Id advise you to use Alcohol from the chemist ,
or Whiskey if you can spare some, if you still get a slight meniscus you can add a drop of fairy liquid. use a 10-20 cc syringe, or failing that pipette it out of a beaker standing on a set
of accurate kitchen digital scales.
The CR I see as less of an issue even at 9.0:1 , if in doubt a splash of meth would give piece of mind, personally Im happy because I have 102 Octane available,
However If you are concerned about the fuel you get on 9:1 CR that's one thing, where I would have thought on normal Optimax/Tesco 99 you still ok but on 98 or 95 you would probably better to look for closer to a 8.5-8,7 ratio You can reach that with either the >1.1mm or achieve it with a little bit of chamber cc vol increase thru careful removal with a grinder which is always a good thing to remove any machined edges to smooth things out and prevent hot spots or if you don't fancy that just use a 1.3 mm , that would help you keep the valve clearance. While you have higher lift cams I would start anyway just to lay both 0,78 gaskets that you have now together and place some blue tack in the piston dish and do a dry build run on it without a torque down and turn the engine through 720° and look for contact issues, Theory and measurement is one way , but a lot can be said for physical checks too. Repeat that for second head, if it looks at all like you are likely to make any contact, if its too close to call (bear in mind things expand with heat ), then repeat with just the 0,78 Gasket if this does not make contact you can even have a go "gently" with no gasket with loose bolts as soon as you feel any resistance check if you can see light thru block and head. Then Its thicker gasket time or other precautions.
Id certainly like more than a Rizzla paper thickness of clearance , so if its very close re-seat the valves and use thinner cam follower bucket shim or pocket mill pistons (id rather not) that drops the CR a touch too.
Ps , Do check all 4 cylinders with bluetack , I had just one valve on last head build that almost touched, it turned out that Id swapped a bucket /shim
and the cam clearance was wrong (too high) on another. no 8 and 5 looked almost the same when the edding was smeared.
or Whiskey if you can spare some, if you still get a slight meniscus you can add a drop of fairy liquid. use a 10-20 cc syringe, or failing that pipette it out of a beaker standing on a set
of accurate kitchen digital scales.
The CR I see as less of an issue even at 9.0:1 , if in doubt a splash of meth would give piece of mind, personally Im happy because I have 102 Octane available,
However If you are concerned about the fuel you get on 9:1 CR that's one thing, where I would have thought on normal Optimax/Tesco 99 you still ok but on 98 or 95 you would probably better to look for closer to a 8.5-8,7 ratio You can reach that with either the >1.1mm or achieve it with a little bit of chamber cc vol increase thru careful removal with a grinder which is always a good thing to remove any machined edges to smooth things out and prevent hot spots or if you don't fancy that just use a 1.3 mm , that would help you keep the valve clearance. While you have higher lift cams I would start anyway just to lay both 0,78 gaskets that you have now together and place some blue tack in the piston dish and do a dry build run on it without a torque down and turn the engine through 720° and look for contact issues, Theory and measurement is one way , but a lot can be said for physical checks too. Repeat that for second head, if it looks at all like you are likely to make any contact, if its too close to call (bear in mind things expand with heat ), then repeat with just the 0,78 Gasket if this does not make contact you can even have a go "gently" with no gasket with loose bolts as soon as you feel any resistance check if you can see light thru block and head. Then Its thicker gasket time or other precautions.
Id certainly like more than a Rizzla paper thickness of clearance , so if its very close re-seat the valves and use thinner cam follower bucket shim or pocket mill pistons (id rather not) that drops the CR a touch too.
Ps , Do check all 4 cylinders with bluetack , I had just one valve on last head build that almost touched, it turned out that Id swapped a bucket /shim
and the cam clearance was wrong (too high) on another. no 8 and 5 looked almost the same when the edding was smeared.
you wont regret the extra C/R mine is a 9.0-1 compression and its very torquey off boost- mine also runs cams and circa 600/500 numbers and its only ever been run on/tuned with vpower, probably about the limit for vpower (reliably) but its quick enough for the road.
found this out a lot lately as i,ve had the same boost pipe pop off half a dozen times lol -still feels nice and grunty even without any boost at all.
found this out a lot lately as i,ve had the same boost pipe pop off half a dozen times lol -still feels nice and grunty even without any boost at all.
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