Ive read alot of threads abot turboing an na engine, and how na rotors are high compression, but isnt high compression good? explain why not please, I a bit confused.

 

When u compress the air in the turbo it gets cool by an intercooler.
When u comrpess the air in the engine, there is nothing to cool it.
Compressing air heats it up.
Therefore when u run a lot of boost into a high compression motor the air will be getting a lot hotter, and detonation( the igniting of the mixture before the spark plugs) will be more likely to occur. This is how I understand it simply put. Im sure if im wrong someone will let you know.

 

High compression is better than low compression for motors in general how high is the difference in turbo and naturally aspirated motors. s4 turboII's have 8.5 compression where as s5 tII's have 9.0 so yes higher compression is better but only to a certain degree.
-Gabe

 

An engine with low-compression rotors allows a bit more lee-way in terms of running boost and avoiding the pitfalls of detonation. Of course, stupidity can negate any benefits of building an engine with low-compression rotors.

 

it depends...most moder motors have much higher compression than older cars...even motors that aren't very old like the Mitsu 4G63 (8.5:1). For a turbo engine in 2003, this would be considered low compression in my book. high compression and low boost make for a great car in terms of driveability...good off-boost power, good spool etc...the problem is they can't handle high boost very well. This is why most NA - turbo conversions can't handle much more than 7-8 psi...compression on modern NA motors since you are staring with very high compression (think integ. GSR).

 

This is not always true, there are people running 4G63's with Weisco 10:1 pistons in them, still pushing 20psi daily and 30psi on race gas.

Daniel

 

Once again, and as always - It's all in the tuning.

 

Exactly. Going to supercharger & intercooler my 1989 Series V NA convertible using a Haltech ECU for management. Hopefully moderate boost under this setup should net 250HP with 1-2 degrees of retarded timing.

The question I have is.... If the V6 Supercharged piston sedans are designed on 87 octane, then why can't a rotary engine??

 

They probably run low boost, runs rich and is timed accurately/well for boost.

 

I'm curious if once I tune the haltech for 93 octane, could I just say "ok, now lets tune fairly conservatively for 87 octane" and see what HP #s I get ??

250 HP, moderate boost, good air/fuel ratios & 87 octane would be very nice.

 

but wait a second, with good tuning from a haltech you cold run about 13psi with an turboed na motor with higher compression, and if you could, wouldn't it run even harder?

 

in other words, would a high compression turboed na motor running 13psi be quicker then an original tII with low compression running 13psi

 

Yes but youd also be running some serious (fuel) mods to cover for that compression.
-Gabe

 

this is an issue I've been struggling to understand in my own mind. I hear a lot of ppl say you shouldn't turbo an N/A engine because it has a higher compression. But geek mind keeps thinking...

Turbo RX7 running 10lbs/intercooler with various mods kicks 300hp while on the typical 9lb compression rotors

VRS

NA-T with factory 12lb comp rotors is pulling a third less boost at 7lbs/intercooler and will make similar HP as well as being fed cooler air.

The heat of the air being compressed is an invalid point because who's going to through the trouble of turboing a car and not intercooling?

What I don't get is why is it considered such a bad thing to turbo an N/A engine when there isn't going to be any more stress on the block than what is put on the stock turbo block? In fact the cooler compressed air being forced in by the intercooler would almost see benificial in my mind. Feel free to argue with me, I'm tired of trying to figure out why it is so bad and want to be set straight with VALID points.

 

My saab has 9.1:1 ratio and runs 10 psi of boost stock. Some people kick that up past 20 psi with proper mods.


And my car is designed to run on 87 octane.

 

Lower CR allows for higher maximum boost, ALL ELSE BEING EQUAL. This is because the engine can only handle so much pressure before it blows itself apart. With increased boost pressure, you have a higher EFFECTIVE compression ratio. The limit for effective CR is probably the same for a low-boost/high CR engine as it is for a high boost/low CR engine, and it seems reasonable to me that if both had the same effective CR, they would have the same fuel and ignition requirements, and would both produce the same amount of power.

Then again, I'm not done with my engineering studies, so I could be wrong.

Ren

 

Well yes the air going into an boost n/a and t2 engine will be the same heat(if there are no other variables, ofcourse) BUT the heat in the engine will be a lot hotter on the n/a motor because of its high compression, the higher compression will heat up the air a lot more; since compressing air generates heat. Therefore its a lot hotter and a lot more likely to detonate, since heat + fuel/air mixture = ignition.

 

an NA w/ a stock TII turbo (like aaron's) @ 12 psi would RIP with the right fuel!!

 

Yep, that it does.... but gotta be very carefull!
I start to hear detonation around 9 psi if I am running 87 octane, but it can go up to around 11-12 with 92.
I ran 94 for a bit, and only got detonation intermittantly at 13 psi.
That's with timing backed off around 2-3 degrees.
I am also leaning out badly too, (stock TII fuel system, doh!) which leads to higher combustion temps, causing the fuel/air charge to be more pre-ignition prone. With proper enrichment, (lowering combust. chamber temps, temps, and unfortunately, a bit of power) I'm sure 13 psi would be perfectly safe on 92 octane, maybe a little more on 94. Thats on my high comp series V rotors too.
There's always h2o injection too!