BP DOHC block overboring limit?

[Speedyj]

Does anybody know how much could be BP block overbored ?

[Silentbob343]

1mm isn't a problem from what I've heard from miata guys and you can do 2mm, but then the OEM head gasket will have to be modified. And that is about all I know when it come to over boring the 1.8

going over by 2mm gets you pretty close to 2.0L, I don't remember the exact number 1.9XX.

[Geoffwhite18]

So are we talking a .030 over or less?

[Mazda_Powered]

isnt 2mm .020?

JACK

[FE3-323]

i reccomend 1.5mm & thats also what flyin miata uses in their stroker kit. Its a good combo & puts you technically @ a 1.9L

[bmikedaman]

.5mm is .020 inches, give or take, so 1.5mm is a .060 overbore, not exactly small. Standing logic was always that the C Spec Miata stroker kit used 85mm pistons, and that not all blocks were up to par to build a race motor at that bore size, 1 in 4 were cast and bored accurately enough for mazda to consider them "safe". Really on an NA street motor I think 85 is fine, but for all out racing Mazda checked each block for thickness. 84.5 should work without issues, and there are plenty of off the shelf pistons in that size.

[Speedyj]

.5mm is .020 inches, give or take, so 1.5mm is a .060 overbore, not exactly small. Standing logic was always that the C Spec Miata stroker kit used 85mm pistons, and that not all blocks were up to par to build a race motor at that bore size, 1 in 4 were cast and bored accurately enough for mazda to consider them "safe". Really on an NA street motor I think 85 is fine, but for all out racing Mazda checked each block for thickness. 84.5 should work without issues, and there are plenty of off the shelf pistons in that size.

I found this pistons Wiseco 84.5 mm with 9:1 compression ratio (good for turbocharging).

[bmikedaman]

I found this pistons Wiseco 84.5 mm with 9:1 compression ratio (good for turbocharging).

I'd run the 10.5:1s for a turbo car personally, I'm all about high compression turbo, its the wave of the future. easy example, there's plenty of turbo stock bottom end GSRs making high 200s, and the first thing that breaks is the weak cast pistons, with forged 10:1s you'll see well into the 300s on those cars when properly tuned. I think Wiseco sissied out on their shelf stock of these pistons, I'm personally going to custom order the 10cc domes (11:1) for my NA build, I'd like more, but I think thats the limit of the forging. (I know a Wiseco engineer, so I can get lots of cool info on the pistons)

[Tazman2]

More base compression = less room for tuning errors/crappy tuning, etc.!

[bmikedaman]

Unless you're a real hamfist tuner in this day and age it shouldn't be a huge problem. Engine management is pretty advanced now a days, and with a well prepped combustion chamber compression isn't that bad on the motor. I'm not suggesting you run a 10.5:1 piston in a motor with tuning that consists of a rising rate regulator and a boost retard box.

[Tazman2]

Unless you're a real hamfist tuner in this day and age it shouldn't be a huge problem. Engine management is pretty advanced now a days, and with a well prepped combustion chamber compression isn't that bad on the motor. I'm not suggesting you run a 10.5:1 piston in a motor with tuning that consists of a rising rate regulator and a boost retard box.

That may be true but alot of cars blow up that were "tuned" properly even with 9.0:1 and turboed!

[bmikedaman]

That may be true but alot of cars blow up that were "tuned" properly even with 9.0:1 and turboed!

Must not have been tuned that well, we saw 538 fwhp on a 10.5:1 civic that didn't blow up.

[Lex]

How long did that motor last?

Must not have been tuned that well, we saw 538 fwhp on a 10.5:1 civic that didn't blow up.

[bmikedaman]

How long did that motor last?

I dunno it made 6-7 trips to the drag strip and got driven daily for like 5-6 months this summer. Its comming back apart to increase the stroke and bore and go to a PT76 turbo. The goal is 650 plus, but we did have to knock the compression back down now, probably going to be 9:1 this time.

[Lex]

Nice, the tune on it must have been excellent.