fotoboy

So people say a lower compression boosted engine will last longer.

Why if the hp is the same would it matter over say a higher compression lower boost engine.

02402ramair

I would say the short answer for that is a lower compression high boost motor is less likely to detonate vs a high comp low boost motor making the same power. Detonation is the killer. JMO

fotoboy

Wy when the over all pressure is the same? My combo for instance is running e85 which burns over 200 deg cooler and seems to be pretty resilient to detonation.

BLOWNBLUEZ06

Because the over all pressure isn't the same.

fotoboy

I understand 1-3 deg per lb of boost but how does higher say base off 110 oct over 92. How much more timing can a boosted car handle?

BLOWNBLUEZ06

Too many variables to say. Depends not only on the dynamic compression, but also on the quench and octane tolerance of the combustion chamber. Then you get into the fuel chemistry. This can get quite deep in the technical arena.

Turbo-Geist

There are two compression ratios at work here. Let's take a very simplified approach and ignore any pressure drops between the inlet and the outlet. Both Engines are on 93 octane and using meth injection.

Engine A is 10.5:1 and receiving air with a 2.22:1 pressure ratio (18 psi)

Engine B is 8.8:1 and receiving air with a 2.36:1 pressure ratio (20 psi)

Engine A theoretical compression: 23.3:1

Engine B theoretical compression: 20.7:1

Engine A is done unless higher octane fuel is added. Engine B wants more pressure and will remain on pump gas. It's all about compression and expansion ratios and how they relate to the tendency for detonation.

Both engines will last if the tune is correct, but which one do you think will be more forgiving and easier to tune without detonation?

By the way, Engine B makes more power with less pressure.

robert miller

Dentonation is the killer here...

nj02vette

Yep.

Pre-detonation is the killer. Octane rating of fuel is it's resistance to pre-detonation (extreamly oversimplified explanation).

Under normal pressures (NA), there is basically a max of ~11.5:1. That's the limit of compression when the fuel will ignite before the spark. Instead of having a controlled clean wavefront that pushed the piston down, you get several wavefronts that impact the cylinder wall, piston, etc. That pre-detonation is commonly caused knock, and the pressure wave will damage the cylinder wall or other parts.

Higher octane fuel will help control that, which is why leaded cars from the 60's were able to run a higher comp ratio.

Now when assisting the air into the cylinder, you still have to keep the fuel air ratio the same (technically richer for safety). So with more air, you'll have a higher chance to get more pre-detonition.

As mentioned, way too complex to completely describe everything, and of course, the engine operates non-linearly across it's RPM range. But that's the quick explanation.

jobberone

I like low compression for the better safety. But there are so many variables.

Its Bruce

No one mentioned the efficiencies of the blower/turbo yet so I'll just add that it does take power to turn the impeller(s). More turning = more power. In stating this, I'm not trying to contradict what's been previously mentioned; it offers more to the point that it's a pretty friggin complex answer.

fotoboy

talking to my tuner he isconfident that he can tune with higher compression so Im staying w 11 to 1 for now. I have the 317 heads on standby if needed later

Turbos are def more efficient bu boost being unpredictable when it will come in it def not the right direction for me for road coarse.

ZMX

What Turbo-Geist said.

...

If the higher/lower compression engines aren't detonating, there shouldn't be a significant difference in longevity.

When running pump gas, it's easy to "hit the wall" with power. Power, limited by the fuel's propensity to detonate. The amount of power lost by lowering compression is much less than the amount of power gained by increased boost.

High compression pump gas engines just start detonating at much lower power levels. And, detonation is sometimes intermittent and hard to pick up.

With E85, you have a much higher threshold of greater of boost and compression can be run.