ALFAGTV6.COM

5yearplan · Post by **5yearplan** » Wed Jan 24, 2007 7:19 pm

30 PSI, thats why

Alfettish · Post by **Alfettish** » Wed Jan 24, 2007 10:27 pm

SydneyJules wrote:My mechanic has built himself a 12v 3.0 with alloy rods, 7.6:1 comp, custom cams, and serious balancing and valvegear, and he will not be happy until he has it revving to 8k under 30psi.

I will let you know if he splits the block... but its been a long running project.

Fark me

5yearplan · Post by **5yearplan** » Thu Jan 25, 2007 10:45 am

What kind of HP figures is he looking at with that setup? It gotta be well over 300, depending on how much power he lost with the low compression, tunung, and temperature ranges.

Greg Gordon · Post by **Greg Gordon** » Thu Jan 25, 2007 3:55 pm

His effective compression ratio will be so high he won't loose anything with the low static compression ratio. The boost will make up for it! I would guess this motor will make an easy 450!

5yearplan · Post by **5yearplan** » Thu Jan 25, 2007 4:31 pm

I have a pretty rough turbo calculator on Microsoft excel, it's more for plotting points on a compressor map, to help pick out a turbo/supercharger. According to that, it should make 470 to the rear wheels!!! With a little tuning. I've cross-checked it's accuracy against an 86' 535i, and it was pretty darn close, within 5-10Hp, So we'll see. If you have anything you want me to check it against, let me know.

twinspark6 · Post by **twinspark6** » Thu Jan 25, 2007 9:23 pm

I'd rather go with higher compression for street use. Helps the turbo spool faster. In my turbo truck. 8:5.1 is where is wanted my compression to be. My turbo is only efficient to 22psi. Although It can be pushed to 26. You need to have a progressive alky kit.
With higher compression ratio you can run less boost and make mad power. It really makes the car more streetable.

Greg Gordon · Post by **Greg Gordon** » Thu Jan 25, 2007 10:07 pm

The current school of thought does lean toward the more compression and less boost method.

twinspark6 · Post by **twinspark6** » Fri Jan 26, 2007 7:54 am

Yes Greg. Guys that are building turbo engines.(at least here in the U.S) are going that route. A friend and a member from another BB was running his vehicle with 28 or so lbs of boost. That engine went south. He decided it was time for his new one. He is making more Hp right now at 20 then he was making at 28. Of course turbo sizing and other things do play a role. There are guys out there running over 20lbs even with a compression ratio higher than 10:1
It does sound cool when someone says It's running 30 lbs of boost, but in reality it's not necessary. Power does hit really hard when it comes into boost. I like the power to be more linear.

5yearplan · Post by **5yearplan** » Fri Jan 26, 2007 9:41 am

To me it really depends on the characteristics of the engine and turbo(s) you're using. Depending on the spool time, it will change where in the powerband you'll make power. Where and how much torque the engine makes in stock form, as opposed to the turbo. How streetable you want the car to be, etc. There's alot to consider. I really don't even look at boost as a main factor, but what power i want and where, plug in all the other variable and see what boost figure i need to meet those criteria, then see what turbo(s) meet my RPM criteria. That seems to work pretty well

Greg Gordon · Post by **Greg Gordon** » Fri Jan 26, 2007 5:29 pm

I do wish to point out that it's a different situation with a Roots blower. The higher compression, less boost idea is more of a turbo or centrifugal thing. With a Roots the only real drawback to the low compression high boost formula is a loss of off boost fuel economy. That is except for cases where an increase in boost starts to result in a loss of charge air density due to heat.

5yearplan · Post by **5yearplan** » Fri Jan 26, 2007 5:45 pm

Interesting. On that note what are the major differences between the roots and centifugal style blowers? I've always thought that the roots (in general) spooled quicker and made more low/mid end torque, whereas the centrifugal acts alot like a turbo (Bad comparison). Anyway, after reading your article about forced induction myths, I'm not sure what to think.

Greg Gordon · Post by **Greg Gordon** » Fri Jan 26, 2007 6:30 pm

The Centrifugal blower uses a compressor section exactly like that of a turbocharger. It can be thought of as a belt driven turbocharger. Of couse because its belt driven it doesn't have the ability to vary its speed independently from engine rpm. That means its boost curve will be very very steep (ie no boost at low rpm, little in the mid range and all of it at the high end). A turbo with the same compressor section can have boost as soon as the exhaust flow allows it, so it will have full boost at a relatively lower rpm. The centrifugal supercharger will only have full boost at redline (with the exception of those driven by some sort of transmission).

A Roots blower by comparison has a lot of its boost at low rpm. However with all other factors equal it will heat the air up more under boost. So with both set up for say..10psi, the centrifugal will put out more power at redline due to its lower discharge temps and drive power requirements. The Roots will make more power until within about 500-1000 rpm of the redline.

5yearplan · Post by **5yearplan** » Fri Jan 26, 2007 7:36 pm

When you put it that way, it makes the centrifugal setup almost ridiculous. How does vortec stay in business. Other than that, what you said makes sense, by the way the redline you refer to is the compressor's, not the engine's correct?

Greg Gordon · Post by **Greg Gordon** » Fri Jan 26, 2007 10:46 pm

No, it's the engine's redline, not the compressors. On a centrifugal the boost increases roughly as the square of the engine speed. So, if you set it up for 9 pounds of boost at the engine's redline of 6000 rpm it will have only 3 pounds at 3000 rpm and almost nothing below that.

The Centrifugals are effective in certain cases. For example a high powered light weight car with very closely spaced gears can do very well in drag racing with a centrifugal. If you have a 408 Ford in a Fox body Mustang with a 4.57 rear end and a 6 speed you will be traction limited through most if not all of first gear anyway. Then with 6 closely spaced gears the engine can stay within 1000 rpm of redline the whole way down the drag strip. In this case a centrifugal will probably clobber a Roots.

On the other hand if you have a small displacement motor and widely spaced gears a centrifugal would be a bad choice. The Roots is also a better choice for road racing.

The real reason centrifugals are so popular in the aftermarket is that it's a lot easier to make a kit with a centrifugal and they will provide higher peak horsepower. Few buyers look at power under the curve which is what really counts on a street car or for road course racing.

5yearplan · Post by **5yearplan** » Sat Jan 27, 2007 6:41 am

Ah, I see. I've never heard it put that way, so trying to get 10psi anywhere close to 4000 RPM out of a centrifugal would be pointless. it's funny, because I can't quite tell just by looking at the compressor maps... On another note, i've heard all types of reports on whether a liquid to air intercooler is more efficient than an air to air intercooler, and under what conditions. I've come to the conclusion that the liquid to air intercooler is more effecient in short term ,or drag situations but after awhile the water/coolant gets to the point that it isn't being cooled down enough, because there isn't enough wter/coolant in the system based on weight and space limitations. Making the air to air intercooler more effecient in the long run, especially whe outfitted with a liquid sprayer, similar to the Subaru STi models. Any input on this?