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Old 06-06-2008, 03:43 PM   #1
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Lightbulb High Compression / Low Boost

I had a thought...
  • JRSC is a roots-type blower, so the air it produces becomes hotter as the boost increases.
  • We all know about the "multiplier effect."
  • Strong high compression pistons exist... 10.8:1, 10.2:1

Why hasn't anyone tried to run a JRSC at lower pressure on a higher compression engine? The same "effective compression" could be reached using fixed geometry instead of compressing more air, right?

For example, I have an SVT with a JRSC and BBK. I didn't know much, so I bought a thicker HG when I had to repair a toasted exhaust valve. I now wish I had kept the SVT's 10.2:1 CR. Before I burnt the valve, I had the non-BBK SVT JRSC with the factory 10.2:1. I now have the BBK & Cometic's thicker HG (0.060"), and the car doesn't really feel more powerful than before (butt dyno).

I have been toying with the idea of refreshing the motor sometime (maybe not till next summer). I have the right safety equipment (J&S Safeguard & Monitor, Wideband, Pyrometer) to play with things, and I'd run the BBK tune softly to drive it over to see Tom in NC. I was thinking about going with those +.007 JE Pistons, since they'd set a nice 10.8:1 CR using a HG of the normal SVT thickness. At least initially, I'd run some underdrive pullies like these to keep from getting into very much boost. Not sure what the ratios are, but if things seemed safe, I could consider going back to the normal pullies, with the full 11psi that I see now... but I would think that would require water injection, or at more than the 94 octane BP pump gas that I use now. If it was safe and possible, I'd probably have one of the more powerful JRSC'ed Foci ;)

Thoughts? Why hasn't anyone tried this? Too expensive, or just a bad idea? Theoretically, you'd have a VERY drivable car with the underdrive pulleys on. The higher CR/bigger bore/lighter pistons would give you more all around NA power, and the boost would be more effective and efficient because the base engine's better CR, and because the air temp would be colder, you'd get more gains and be even safer than the high-boost low-CR setup.


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Old 06-06-2008, 06:54 PM   #2
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On a turbo you can make "safer" power by lowing the CR and upping the boost, BUT on a SC its not really a good idea....

I have seen person after person do it (after I pleaded with them not to) and then wonder why it didnt make any more power.... well duh you dropped the CR and you only upper the boost a couple of pounds... which really doesnt even effect it until higher in the RPM's.

SC's LOVE Higher compression especially on something like the JRSC which is very limited to its output. I wouldnt go any higher than the stock SVT CR.

NOW something else to think about when buying pistons (other than stock) a 9:1 zetec piston will be closer to 9.5:1 in the SVT.. so if you bought 10.8's you would be closer to 11.3.........
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Old 06-06-2008, 07:39 PM   #3
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ooh, the SVT head is different, wasn't thinking about that.

I pretty much feel really stupid for getting that thicker HG... live'n learn, I guess. :'( not looking forward to pulling the head again, but at least its not an emergency.

Grabbing some high CR pistons should be fine for the SVT if a slightly thicker HG is used, right? Or can pistons be bought with custom CR's?

i.e. 10.5:1 JE's with a Cometic 0.030" or 0.040" HG?

I just think it'd be pretty sweet to have a JRSC BBK with something like 10.6:1 on forged pistons and rods . Obviously a 93 octane tune would be in order, and with the J&S Safeguard's retard switch, couldn't you still be safe if you had to fill with 87 at a lonely pump about 200 miles from the nearest premium?

These ideas are getting expensive, but I'm just wondering. Also, where can I find the cylinder and head specs for the Zetec VS the SVT (such as bore, stroke, head volume, etc)? That'd take the guessing right out of this... :) I'm aware that the most cost effective way to fix my booboo would be re-ringing the stock pistons (I'd hate to take it all apart again and have those croak on me), and going back to the stock HG...
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Old 06-07-2008, 07:40 AM   #4
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I really dont think your going to pop a stock piston with the JRSC... no offence but they just wont produce enough boost to hurt them (with a good tune)

And yes you can run 87 if the need rises... just stay out of boost and it should do fine (until you can put good gas back into it)

SVT and Zetec bore and stroke are the same the only difference is the size of the combustion chamber on the SVT head.

The biggest problem with your idea is that the JRSC can not cool its boost so going with a higher CR on top of air thats already heated (and cant be cooled) is basically detonation just waiting to happen.
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