Miata Mailing List: March 1997, Message #147
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   From: (none)

Subject: Re: sebring intercooler

   Date: (none)



Well being a low mail day I may as well add my two cents worth.



Shiv's right in that the efficiency of the SC unit won't change, just the

air temp.



I've never had the pleasure of seeing either the Autorotor or Sebring first

hand. Here in Australia we've got a few turbo MX-5's and the kits are

readily available but other than mine I believe there is only one other

supercharged MX-5 in the country and it's over 1000km away. Therefore I

don't have a lot of experience on what's "normal". I'm also relatively new

to forced induction with my race experience limited to "normally" aspirated

vehicles.



My SC install was a custom job, originally fitted without an intercooler.

Since then I've added a IC, one of the BEGI units to be exact adapted to my

installation. Now before the IC was installed I was running a max boost of

4psi. The pulleys haven't been changed and therefore boost won't have risen

and I expect it would actually have dropped by about 1psi. I came to this

figure after discussions with various people both on and off this list.

This happens to be the same amount Shiv lists.



As I've yet to dyno the car since the IC was installed I can only judge by

the SOTP method but despite the slight boost drop power hasn't dropped and

may in fact have risen. It certainly feels stronger at the top end. Another

point to note is that I've also installed larger injectors.



The SC unit I use is a Sprintex unit which is a twin screw design similar

to the Bell unit I think. I believe the efficiency between the Bell and

Sprintex is very similar so I can't say how any of this will relate to the

Sebring but as I said it's a slow mail day.



Now assuming that the Eaton reacts in a similar way it shouldn't loose

power from the slight boost drop but may not gain any either. The real

benefit will be in that the boost can then be increased with an increased

safety margin to avoid detonation. What's involved in a boost increase on

any SC is a new pulley setup. How easy this will be on any unit other than

my own I can't say. If the pulley (either on the crank or SC) becomes too

small the risk of belt slippage will increase. To avoid this a toothed belt

could be the solution.



As Shiv points out increasing the boost on the Eaton may overcome any

benefit of the IC due to it having a lower efficiency to begin with. ie For

each additional 1psi on the Eaton, air temp may (probably will) increase

more than for the Bell. A larger intercooler maybe the answer or possibly

water spray on the IC to increase it's efficiency further. If the IC

becomes to large however how will throttle response be effected? Probably

negatively.



Actually doing it is possible (anything is if you're willing to spend the

$$$) but where do you draw the line? For Percy it probably is worth

developing it. I'm sure there are a few Sebring owners who have become

accustomed to the power and want more (that's always the way isn't it?).

Instead of a totally new installation, be it turbo, another SC, engine

swap, nitrous or whatever they could and would be interested in adding to

what they already have.



Does any of this actually matter? Who knows, but at least it'll help fill

the mail hole a bit.



Matt.

Sprintex SC '94 Laguna Blue Clubman



----------

> From: Shiv Pathak 

> To: Multiple recipients of list 

> Subject: sebring intercooler

> Date: Monday, 3 March 1997 6:01

> 

> 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

> 

>