let me get this straight. From what I read in the super charging thread, to get 7.22:1 comp you use an a14 crank with a15 pistons.
since the blocks and rods are the same it shouldnt matter which ones I use , right?
All this being correct, how much would it raise the comp if I used a GX head on this motor? ( Im picking this head up from the machinists tommorow) I already have an a15 engine that only needs a freshen up, and Im getting an a14 (for the crank) this week if my info is flawed and it wont work some one please tell me now . Im gonna put as much boost into the motor as I can.

A15 pistons with a A14 crank will give you 7.22:1 comp ratio .. Just remember that is for a Std engine .. with over size pistons ..faced head and block .. that will increase the compression ratio up slightly again but should be still below 8:1 depending on how much over size you go ect ..

SWEET!!! Thats what I wanted to know, how much will the comp rise with a gx head?

Build, measure, calculate.....

OK, let me see if i get this right. You will be using standard Datsun components to come up with a 1400cc engine with 7.2 to 1 compression. You will be turbocharging it with as much boost as you think you can get away with.
With that sort of compression, you should be able to give it quite a bit of boost, but are you using stock type cast pistons?

Has anyone had much experience with cast pistons & high boost? I suspect that these might be a weak link in the chain.

please tell me how is too much

my guess is 1.3 bar

from what i have gathered from turbo people on the site from turbo forums. stock cast pistons are fine to use if the setup is done correctly, and the cast ones can last a fair while too..

yeah bout the high boost im not sure but surely if ur aming for high boost u would have everything done properly , bottom end rebuild etc.....why would u bother with cast ones

I know that when supergharging the hemi engines, with compression ratios around 8 to 1 for street use, they recomend somewhere around 5 to 7 pounds of pressure as a safe max on cast pistons. They also recomend against a steady diet of leadfoot syndrome.
Forgings are recomended for heavy right feet, higher manifold pressures, & higher compression ratio's. I would have thought that a similar situation would have existed for our Datsuns also.

If forgings are used, the longer stroke A15 crank could also be retained as most forged pistons can be ordered with custom specs, re the compression ratio.

Anyone had much experience with high boost & low compression on cast pistons?

weakest link comes down to tuning.

I think I went through this a long long while ago in another thread.
I have driven and seen from passenger seat boosted standard Aseries engines run boost pressures that have sent the guages needle past the 30 PSi limit. And the engines have lived. ( By saying this doesnt mean the engine will last all day running these pressures, probly wouldnt even last 15 minutes of it. But short burst it will survive)

Forged pistons are just insurance on big dollar engines ( They can handle a bit more abuse) If the engine isnt tuned right even forged pistions wont last.

I cant say forged pistons are better, just they tolerate more. And from my experiences I wouldnt say cast pistons wont handle the job. If you boost your engine and let it lean out its gonna get hot inside and somethings gonna give way whether forged or cast.

dodgeman, forcefeeding mighty chrysler hemis is a bit different. The different cylinder area means It will likley survive less boost with castpistons. still regardless, I have some cast pistons I can re use and if I run it a bit rich she'll be fine. Plus the turbo is off an 1800 so it should generate a bit less heat at Xboost than a smaller one (albeit at the expense of maybe slower spooling up.
And if it Sh1ts itself Ill know how much boost is too much for the next motor.
but my signature sums up my aproach!