Miata Mailing List: March 1997, Message #106
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   From: (none)

Subject: sebring intercooler

   Date: (none)



Percy writes...



> Hey, I got a line on an intercooler I'd like to rig up for the eaton based

> 

> blowers. BUT.....

> 

> 

> 

> What kind of effciency do you all think I can achieve with a roots type

> 

> blower vs the success of the autorotor's compressor (right?)?



I don't see how the efficiency of the sc unit themselves would change.  The 

Sebring would still lag the autorotor significantly in terms of thermal 

efficiency.  What would change, more importantly, is the intake charge.  A good 

intercooler would remove almost 90% of the heat generated by the 

blower/compressor in question.  The intercooled Eaton's intake charge would 

still be higher than the intercooled autorotor's, but not by much.



>From talking to people much more knowledgable than myself, I'm beginning to 

think it is, in fact, possible to intercool a Sebring.



However, a few issues.  An intecooler usually has a boost drop of around 1 psi 

(I measured it myself from reading boosts before and after the IC -- 11.5 psi 

after, 12.5 psi before.)  Or in the case of a stock setup (8psi after, around 

9psi before)



To get equivelent power from a Eaton based unit, you would have to run it at  

around 10psi (to compensate for reduced thermal efficiency).  Quite a bit more 

than the stock 5.5-6psi application.  



However, increasing the speed of the rotors by 60-75% should have some further 

impact on thermal efficiency.  In other words, does thermal efficiency start to 

decrease dramatically as boost levels increase?



Also, what is the published redline on the Eaton unit?  I think the published 

redline for the autorotor is high enough to support around 14psi at 7k rpm on a 

cruising basis (signicanly more on short term peaks).  Needless to say, the 

8psi stock setup is well short of the full potential of the SC itself.



Further, to raise my boost levels from 8 psi to 11.5 (a 43% increase), I hade 

to install a signifantly larger crank shaft pulley and a significantly smaller 

sc pulley (among other things).  Much more and I think belt slipping would be a 

problem (even with the stock 6 ribbed belt).  



I forget how the eaton is set up, but to increase boost by 70-75%, you would 

have to do some serious pulley reworking.  I think that getting the SC pulley 

much smaller would definately result in serious slippage with the stock 4 

ribbed (??) belt.



In summary, it probably is possible to intercool a Sebring with some serious 

modification.  



Issues to be consider by anyone attempting to conduct R&D on this project would 

probably be:



1.  Cost of pulleys that would have to be reworked (not just to increase boost, 

but to solve likely slippage problems).  COST:  perhaps $200+



2.  Cost of intercooler and associated plumbing.  COST:  approx $1000



3.  Cost of fuel pump to support fuel requirements at higher boost levels. 

COST:  approx $200



4.  Cost of ignitial retard devise (MSD)  COST:  approx $150



5.  The reliability of the Eaton when running well beyond it's 6psi stock 

configuration.





Total addition material cost is likely to be more than $1500.





> I think the driveability factor is naysaying myself. I talked with Jim and

> 

> he mentioned that Peter Farrell pulled the intercooler of their Miata

> 

> because it "had lost its driveability and didn't make any real gains." BUT

> 

> I think if you twisted the body and suspension and put what 225+ tires on

> 

> the thing......geez.....what is driveable and what kind of rolling

> 

> ressitance are we talking here? Not to mention the added weight of all that

> 

> gadgetry.



Methinks some cost/benefit analysis had something to do with PFS's decision to 

pull the intercooler out when it came to production.  I don't think rolling 

resistance had to much to do with it ;)



Comments??



Faithfully,



Shiv