Turbo a 4cyl with 10.5 CR ? (Subaru )

[afterburn 549]

I am asking for a friend of mine-
I said I would take the question to the brain trust - A noteworthy bunch.
He has a subaru engine with 10.5 pistons. All fresh all new. ( 4 cyl )
This is not in a car
The Rpm is always the same 4800 ( within a 200rpm) and at times just needs more torq
He wants to boost it to about max of 8 lbs.
I know other engines do it /have done it. But no use in reinventing the wheel here.
When he wants more power he wants it now, no spool up time, run it right on the boost thresh hold .
He is a smart person , no dummy. has IC and all parts.
*Should he change back to 8.5 pistons ?
* thats the BIG Question*

[LakeCleElum]

I have friends that turbo their snowmobiles and are always having to be towed out.....It's all about Detonation and computers to adjust fuel and timing. Does it have a knock sensor?

CR in a two stroke is different than a 4-stroke, but I think he can do more boost and reliability with the 8.5....

[oldE]

What will he be running for fuel?

Best
Les

[sammyg2]

Running 91 octane pump gas, he MIGHT be able to get away with 8 and 10.5 psig boost if his intercooler is over 90% efficient.

But he'd definately be pushing it.

I've turbocharged several engines, the latest were a 2 liter 914 and a 3 liter 911SC. Neither were inter-cooled but on pump premium the 914 was ok with 7 psig and the 911 could take a little more.
The SC was 9.3 to 1 and the 914 was around 8 to 1.

But why does he insist on 8 psig? Why not start low and gradually increase it until you find the sweet spot and then stop?

That's what I did.

[afterburn 549]

I am not sure why he puled the numbers 6 to 8. i am guessing a hip shot guess.?
I will pass this along to him maybe in the form of try 6 pounds and see if that works. After that drop in the 8.5 CR so he does not hurt himself.? ( he can not park where he drives)
He has to have reliability.....Part of the problem right now is it can be under powered . I know suby can be be jacked way up in Tq and HP .....But even in track cars that is intermittent power bands.
Maybe he should ask one of the race prep people that specialize in Subaru arithmetic?

[cockerpunk]

typically in a turbo engine, reducing compression and increasing turbo size (not necessarily boost) will yield more power. it will however hurt transient performance when off boost. this is because the turbine in the turbo is far more efficient at compressing air than a piston is.

sadly, unless he is willing to kill his exhaust valves with anti-lag, almost all street car applications, you are trading between peak power, and spool. can't have both. power, spool, reliably, pick two.

[asphaltgambler]

Really depends on the combination if running an aggressive camshaft where some cylinder pressure is bled off. The advertised mechanical compression ratio is just approximated with this piston. Also, very high cylinder pressures from forced induction and domes on a piston do not mix well. Crazy turbulence in the combustion chamber, hot spots, lots of quench areas.

Modern pump gas engines that are turbo'd have piston and combustion chambers that have more in common with a diesel design. Pistons are hanging out of the hole to minimize quench and deck height, usually dished concentrating the flame front dead center. The combustion chambers V-shaped, very shallow, perfect valve (stem) angles. The cam timing is variable, PLUS the fuel and ignition delivery is tightly controlled; per cylinder.

VW / Audi - somewhat 'old school' 1.8 turbo 4cyl engine ala early 2000's. Very basic but very good engine. 2-cams with somewhat crude variable timing on the intake cam only. The cranking compression was @210psi. The stock boost was @9lbs. Do the math. They can do this because of the total engineering that went into it

[greglepore]

Sounds like some kind of an aircraft application, in which case no fi'in way.

If its in a hydro, why not? He can always get a tow.

[masraum]

Just for another data point.

The Mazdaspeed Miata which is a factory turbo from 2004 and 2005 has 9.5:1 and stock runs 8-8.5psi, but seems to be fine up to about 9.5 or 10 psi.

The mazdaspeed miata system seems to be fairly crude by today's standards.

[afterburn 549]

OK, then for safety factor put the 8.5 back. Maybe put a ceramic fire wall on them?
I will tell you guys what this is latter, as it always becomes a subject we are not on .
Thanx

[David]

From one of A. Graham Bell's books:



Of course direct injection changes things completely.

[motley911]

If constant speed and instant power delivery is required have you considered spraying with NOS? Seems like an ideal situation for NOS and by changing nozzle /jet size you can dial in the amount of additional power.

[slow&rusty]

Two simple rules: You manage fuel for reliability, you manage timing for power.

On a 10.5:1 engine, I recommend starting at 6psi and installing a AFR gauge, for 8-10psi water or methanol injection is your savior.

Lots of guys running 10.5:1 on forced induction 16V twin cam motors. I just finished building a 9.5:1 16V engine that will see over 20psi on a daily driven car.