Maximum head skim
Discussion
Can anyone tell me what the maximum skim can be done to a RV8 [450/4.6] please? Just had my block/heads measured and it seems whilst they have been skimmed before, my present comp' ratio is in fact 9.3:1 when I thought [was told] it was much higher thinking the standard was 9.35:1 which it wasn't. Now I don't know for sure but it has been suggested that the actual comp' ratio on our motors is less because of composite gaskets used. If the above is true, then my standard engines comp' ratio would've been under 9.0:1 I ask because my newly aquired forged pistons give a comp ratio of 7.89:1 and I would like them to be nearer 9:1 if possible for a turbo application as I don't want monster boost and do want decent pre-boost manners, if that makes any sense. 
PS; my heads at the moment have a volume of 27.5cc, gonna do a dry build using plasticine to gauge piston/valve clearance. Skim advice and effect on squish would be greatly recieved.
PS; my heads at the moment have a volume of 27.5cc, gonna do a dry build using plasticine to gauge piston/valve clearance. Skim advice and effect on squish would be greatly recieved.
Edited by macdeb on Friday 17th January 19:51
Edited by macdeb on Friday 17th January 20:36
macdeb]Can anyone tell me what the maximum skim can be done to a RV8 [450/4.6 said:
please? Just had my block/heads measured and it seems whilst they have been skimmed before, my present comp' ratio is in fact 9.3:1 when I thought [was told] it was much higher thinking the standard was 9.35:1 which it wasn't. Now I don't know for sure but it has been suggested that the actual comp' ratio on our motors is less because of composite gaskets used. If the above is true, then my standard engines comp' ratio would've been under 9.0:1 I ask because my newly aquired forged pistons give a comp ratio of 7.89:1 and I would like them to be nearer 9:1 if possible for a turbo application as I don't want monster boost and do want decent pre-boost manners, if that makes any sense.
PS; my heads at the moment have a volume of 27.5cc
Decided not to sell then Mac PS; my heads at the moment have a volume of 27.5cc
Edited by macdeb on Friday 17th January 19:51
macdeb said:
ChilliWhizz said:
Decided not to sell then Mac
yup, after much thought just can't part after 9 years! and ,, er,, 11k miles 
I saw you'd replied and was expecting something about my hair style! [head skim]
, the car's that is !!Mark
macdeb said:
ChilliWhizz said:
Decided not to sell then Mac
yup, after much thought just can't part after 9 years! and ,, er,, 11k miles I saw you'd replied and was expecting something about my hair style! [head skim]
Anyone?Sounds to me like you bought pistons with completely wrong dish/bowl to start off with. I'd check first how much you can take off the block / gasket so that the piston comes to the top of the bore (lots of these after market pistons don't go to where they should!). Then you can calc what the static CR will be.
You can't really take much out of our heads ~ don't get confused with the v8 brigade chopping s
t out of old 35cc heads..... taking more than a fine skim (27cc) out of the heads is counter productive as you start messing up the combustion characteristics IMHO. Plus.. if you don't leave emergency skim to 26.5cc you might end up with expensive scrap ally in the near future.
All 450's / 4.6's had comp gaskets so whoever told you there was some difference to the std 9.35CR is talking blocks. TVR's maybe a little lower due to the non-std valve cutouts (but HG thickness can quickly correct that ~ they are available in various thicknesses). Obviously the serious minded, scrap the pistons & chuck 4.0 pistons in (just a smaller bowl) to raise the CR for better performance as well as CR being needed for stronger cams ++++.
You can't really take much out of our heads ~ don't get confused with the v8 brigade chopping s
t out of old 35cc heads..... taking more than a fine skim (27cc) out of the heads is counter productive as you start messing up the combustion characteristics IMHO. Plus.. if you don't leave emergency skim to 26.5cc you might end up with expensive scrap ally in the near future.All 450's / 4.6's had comp gaskets so whoever told you there was some difference to the std 9.35CR is talking blocks. TVR's maybe a little lower due to the non-std valve cutouts (but HG thickness can quickly correct that ~ they are available in various thicknesses). Obviously the serious minded, scrap the pistons & chuck 4.0 pistons in (just a smaller bowl) to raise the CR for better performance as well as CR being needed for stronger cams ++++.
Thanks Dave [and Chilli], I'm a little confused at the results given by the guy who has worked out the CR at the moment/with new pistons. Maybe I'm missing something please shoot me down if I've missed something but;
CC result 4550.19 [stroke 82mm?]
cyl CC 568.77
new pistons 44.5
gasket 10.5
head 27.5 [been skimmed]
568.77 + 82.5 = 7.89
The result of the standard set up with a head skim [27.5] is giving 9.3 apparently. This is below what is generally taken as standard [9.35:1] and I understand from what you say about that will be slightly less due to valve cut outs. But as above mine is 9.3 with a head skim. What I don't have to hand at the moment is the cc of standard 450 pistons with valve cut-outs, do you know what they are so I can back calculate as I don't have that sheet to hand.
PS; the new pistons reach block height at circumference.
CC result 4550.19 [stroke 82mm?]
cyl CC 568.77
new pistons 44.5
gasket 10.5
head 27.5 [been skimmed]
568.77 + 82.5 = 7.89
The result of the standard set up with a head skim [27.5] is giving 9.3 apparently. This is below what is generally taken as standard [9.35:1] and I understand from what you say about that will be slightly less due to valve cut outs. But as above mine is 9.3 with a head skim. What I don't have to hand at the moment is the cc of standard 450 pistons with valve cut-outs, do you know what they are so I can back calculate as I don't have that sheet to hand.
PS; the new pistons reach block height at circumference.
Edited by macdeb on Saturday 18th January 19:48
Edited by macdeb on Saturday 18th January 20:29
Have you thought about junking the renewables and starting a valeting company ? I could come and hold your bucket 
macdeb]Thanks Dave [and Chilli said:
, I'm a little confused at the results given by the guy who has worked out the CR at the moment/with new pistons. Maybe I'm missing something please shoot me down if I've missed something but;
CC result 4550.19 [?]
cyl CC 568.77
new pistons 44.5
gasket 10.5
head 27.5 [been skimmed]
568.77 + 82.5 = 7.89
The result of the standard set up with a head skim [27.5] is giving 9.3 apparently. This is below what is generally taken as standard [9.35:1] and I understand from what you say about that will be slightly less due to valve cut outs. But as above mine is 9.3 with a head skim. What I don't have to hand at the moment is the cc of standard 450 pistons with valve cut-outs, do you know what they are so I can back calculate as I don't have that sheet to hand.
PS; the new pistons reach block height at circumference.
Will the pistons be closer at higher revs ,expansion and contraction and all that .CC result 4550.19 [?]
cyl CC 568.77
new pistons 44.5
gasket 10.5
head 27.5 [been skimmed]
568.77 + 82.5 = 7.89
The result of the standard set up with a head skim [27.5] is giving 9.3 apparently. This is below what is generally taken as standard [9.35:1] and I understand from what you say about that will be slightly less due to valve cut outs. But as above mine is 9.3 with a head skim. What I don't have to hand at the moment is the cc of standard 450 pistons with valve cut-outs, do you know what they are so I can back calculate as I don't have that sheet to hand.
PS; the new pistons reach block height at circumference.
Edited by macdeb on Saturday 18th January 19:48
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