Compression ratio question


#1
Newbie here considering joining the turbo club and weighing up my options.

I know that the standard way of dropping compression to 9:1 on a Gen1 motor is to put in a 0.080" spacer plate (2.032mm), which necessitates re-timing the cams.

I also know that any build over 280ish hp should involve aftermarket rods.

I notice that Carillo make some rods for stroker engines that are 2.13mm shorter than standard (117.37mm instead of 119.5mm).

I'm sure there must be something obvious that I'm overlooking here, but if it's necessary to change to different rods anyway, why can't the compression ratio be dropped just by putting shorter rods in instead of needing a cylinder spacer?
 
#2
Surely someone has an answer to this?

It seems to me that shorter rods would let you drop your compression to around 9:1 while avoiding the need to have to mess around with your cam timing.

I've never heard of it being done that way though, so I think I must be missing something, and would like to know what.
 

icemansid

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#3
Not a turbo guy but a spacer is WAY cheaper than shorter rods.... which i'm not exactly sure they even make for the busa...
 
#4
I mean if you need to replace rods anyway for a 280+hp turbo build.

Carillo do make rods with are 2.13mm shorter than standard. A 0.080" spacer is 2.02mm.
 
#5
2mm shorter rods are like dinosaurs ;), left over relics from an earlier age
You could use them but the trade off is rod ratio (google the disadvantages of a change in rod ratio )
I know one guy in Vic that gets custom long rods from Carrillo for his stroker motors because he can see the advantages of maintaining rod ratio

Are you planning on racing or streetriding the busa when you turbo it ??
generally for street i don't worry to much about changing rods or pistons till around 300hp,
but if i change to turbo pistons i always do rods ,
 
#6
Thanks for the answer.

It will be a primarily street bike, with the odd trip to the drags, but will be kept at stock wheelbase.

Currently looking at a second hand ghetto kit with the air/air intercooler, which are reportedly good for 350-380hp on pump fuel (US 93 octane, i.e. equivalent to our 98 octane). It would be nice to put that potential to use without worrying about the stock rods giving way.

I know that an air/air cooler isn't functional until the bike is moving at speed already, but my plan would be to build my own multistage boost controller anyway based on a modified gear position sensor, and do something like 6psi first gear, 10psi second, and full boost in third onwards (with a manual switch to disable that stage if I were forced to fill up with lower octane fuel). With a stock wheelbase I don't want high boost in the first couple of gears anyway.
 
#7
Thanks for the answer.

It will be a primarily street bike, with the odd trip to the drags, but will be kept at stock wheelbase.

Currently looking at a second hand ghetto kit with the air/air intercooler, which are reportedly good for 350-380hp on pump fuel (US 93 octane, i.e. equivalent to our 98 octane). It would be nice to put that potential to use without worrying about the stock rods giving way.

I know that an air/air cooler isn't functional until the bike is moving at speed already, but my plan would be to build my own multistage boost controller anyway based on a modified gear position sensor, and do something like 6psi first gear, 10psi second, and full boost in third onwards (with a manual switch to disable that stage if I were forced to fill up with lower octane fuel). With a stock wheelbase I don't want high boost in the first couple of gears anyway.
 
#8
Planning a street build that will see the odd trip to the drags, but will be kept at stock wheelbase.

Currently looking at a second hand intercooled (air/air) ghetto kit, which are reportedly good for 350-380hp on pump fuel (US 93, i.e. our Australian 98). It would be nice to be able to put that potential to use.

Yes, I know that air/air coolers aren't effective until the bike is already moving at speed. My plan would be to make my own multistage boost controller via a modified gear position sensor, and run something like 6psi first, 11psi second, and full boost in third onwards - with a manual switch integrated into the third circuit so that boost could be capped at 10psi if I were forced to fill up at a country servo where no 98 octane was available.
 
#10
Gah, sorry for the triple posts. The forum let me post just fine the first few times, then said something about a moderator having to approve my posts, then let me submit posts and later disappeared them, etc.

I think it may be Busashot that I've been in contact with already, just by another name on another forum
 

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