HP versus comp ratio and boost pressure?

[JohnEd]

There is a current post about a 4-71 rebuild.  In that post the re-builder opted for using 18.7 to 1 CR pistons in stead of 17 to 1 CR pistons.  My understanding is that the lower compression allows you to run more boost safely.  The more boost, the more power, right.  Lower compression means lower compression temps for starting and hard starting in cold weather AND at high altitude.  My question is:  without regard to hard starting, what is the difference in HP between the 17 to 1 and the 18.7 to one?   What did he give up in terms of HP?

Thanks,

John

[bs4104]

More Low end torque & Better jakes too. with 18 to 1

[HB of CJ]

Probably nothing much because of the injector size, turbo type and the limited amount of boost the engine will see.  The guy knows his stuff: appears the mill was set up with a specific goal in mind including HP, durability, economy, starting, smoke, etc..

Now if he choose to run 90 or 100 injectors with a turbo designed to see 30 35 psi boost, yeah then the high compression pistons would be a poor choice.  Pyrometer temps would be off the gage.  Melt down time.  However, as built, little. 

Specifically, jumping engines here, with the older Big Cam Cummins line of mills, apparently for reliable big power, one must markedly reduce the compression as well as retarding the valve/injector timing.  Then custom pump and injector...

...settings really work with the proper turbo, intake, charge cooler and exhaust.  But that is a different engine.  With the 471TI alluded to, the changes are designed to work together with a specifc goal in mind.  Very cool indeed.  HB of CJ

[poppi]

Well look hard enough and it can be found

    hp calc. with compression change only.

Your old Compression Ratio of 17 and HP of 318 is now calculated
as a Compression Ratio of 18.7 and 323.65 Horsepower.

   As far as accuracy I haven't a clue.

  Skip

[JohnEd]

Skip,

Thats it!  Now, each has a max boost with reliability and that would raise the HP to what?  Then, with the additional air realized from the additional boost, what are the max injectors that can be used and what is the hp associated with those?

TomC got 350 or 375 from high compression pistons running 10 or 15 psi from a wastegated turbo.  Injectors?  The pistons speced for turbocharging in conjunction with supercharging are lower compression and you can get lots more air in there and I think I heard that 25 psi is optimum but the injectors would be???...I haven't a clue.  Is that military surplus engine that Don put together that made 600 HP running 17:1 compression and what PSI did it run?

Thanks,

John

[JohnEd]

I found it!

Like my setup, using 75 injectors is a moderate increase from the 65 injectors-so the turbo pressure will not be that high with the high compression pistons.  If you want to go to 90 injectors, then you'll be in the 20-30psi range of turbo boost-most likely too high for the high compression pistons.  Don Fairchild had a 8V-71T with military pistons in it that only had 16 to one, and he was running 120 injectors-making for 600hp.  Smoked on starting, but you should have felt the power!  Good Luck, TomC

It would seem, as in hunch, that the 17:1 pistons with 25 PSI boost would yield somewhere in the 500 HP range with injectors sized for max power.  No wonder the "Old Schoolers" love that 8V71.

John