8.4:1 is a high boost set-up. Why not 9-9.3:1 instead? At 8 psi, the Eaton is working really hard in the upper rpms, right? Hell, LS1s are capable of running safely at 6-8 psi on stock engines with compression ratios of 10.1:1. I can see 8.4:1 if monster boost is involved; but, the stock H-rods aren't up to spec for that kind of power. It doesn't make sense.

Has anyone upped the the compression when they rebuilt their motor? Sounds like a good idea for a daily driver to me. Am I missing something?

 

Your right it would make it more efficient, but more prone to detonation.

 

The lower the compression ratio, the more boost you can run without fear of detonation. My guess would be that Ford wanted to make sure they "covered the spread" in terms of a supercharged production vehicle and keeping it safe and in the process keeping their warranty costs down.

Just my .02

 

The LS1 you cited was not designed to be boosted. 8.5@ is a long time standard of production forced induction, Buick GN, Typhoon etc... Making more boost will always yield more power than a point of compression.

 

The Buick GN's make 14-15psi of boost in its stock form.

You make power by burning fuel. In order to burn fuel you need air and spark. A higher compression ratio yields a more explosive (for lack of a better word) combustion and thus more power. However you must not forget that cylinder pressure is also a very important factor in all of this. A normally aspirated engine with a 10:1 compression ratio will have less cylinder pressure than a forced induction engine at 8psi and 8.4:1 compression. High cylinder pressures also make the ignition system's job a lot harder. In the case of the Buick GN, they flow twice as much air through the intake manifold @14 than a n/a motor. At sea level normal atmospheric pressure is 14.7 psi but we actually consider that 0 psig ("g" means on the gauge). At WOT a n/a motor is inhaling its air with the aid of atmospheric pressure. With the 14-15 psi of boost (over atmospheric pressure) that the turbo supplies the manifold is seeing twice as much air as a n/a counterpart. If the cylinder heads can handle the flow AND the same cam profiles are used a forced induction motor @14/15psi with the SAME compression ratio as a n/a motor will have twice as much cylinder pressure as the n/a motor. In the case of our Lightnings, at 8psi we are still increasing cylinder pressure to over 150% of what it would be if we were n/a at our same compression ratio. Thats is plenty enough to make good performance gains and keep the setup safe. This is only a scratch in the surface of this topic but I didn't want to bore you all to death. Also remember Ford had to make a truck that could deliver reliably on 91 octane in the climates everywhere it is sold. HTH


ps. that little fact about the GNs flowing twice as much air is why that little 231 cu motor performs like a 462 cu motor in its stock form.

 

One of the more important things that I left out is VOLUME. If you acheive the same cylinder pressures with a lower compression ratio then that means you have a larger volume of fuel and air in the cylinder at ignition. And of course a larger volume of combustibles has more potential then a smaller volume.

 

Ummmm ... Larger Mass ... same volume ... providing the only
thing you change is the compression ratio

 

I'm not quite sure what you are saying/asking/pointing out SVT_KY.

 

I did a little extra reading on the subject. There are few interesting points in this write-up from Street and Strip Motorsports.

Engine Preparation

The extent of the engine preparation will depend entirely on how the engine is to be used. A supercharger can be installed on a stock engine with cast pistons and a cast crank as long as moderate boost (below 8 lbs.) is maintained and any detonation is strictly controlled. Engine speed should also be limited to 5,000 rpm. Detonation on cast pistons can easily break ring lands. Too much boost and/or detonation on a stock or worn engine can cause piston damage or burned valves.

Most late model "smog" engines work well with a supercharger due to their lower compression ratios and smaller cam profiles.

Supercharged Engine Guideline

1) 7.0:1 to 9.0:1 compression ratio: The optimum compression ratio is 8.0:1.

2) 4-7 psi boost level: This range of boost has proven to be the best compromise for power and reliability.

3) Engine rpm: When using stock cast pistons, the engine should be limited to a maximum of 4,500-5,000 rpm. Exceeding this limit may over-stress the cast pistons causing failure. Blueprinting an engine using the proper components will allow higher rpm reliability and will maximize a supercharged engine's potential.

4) Detonation (pinging): Detonation is the single most destructive force in a supercharged engine and steps must be taken to eliminate it. This may include lowering boost level, retarding timing, installing a boost timing master, increasing fuel flow to prevent leanout, and/or using a fuel additive to raise octane level. The cooling system also needs to be in good condition to prevent overheating, which may lead to detonation.

If an engine is to be driven hard or under load, as in towing, a thorough blueprinting should be considered. Forged pistons, with their inherent strength and ability to withstand higher temperatures, are recommended. Follow the piston manufacturer's recommendations for piston-to-cylinder clearances.

A compression ratio exceeding 8.0:1 is not recommended, nor is it necessary for brisk performance from a supercharged engine. If raised to this level, fuel, ignition timing, and total boost become critical factors.

Next consideration would be the piston rings. They should always be the best quality available because the piston rings take as much abuse as any other component in an engine. "Moly" or "Double Moly" piston rings (iron piston rings coated with Molybdenum Disulfide) are an excellent choice for supercharged street engines. They seat quickly and wear well. For hot street or competition, where higher boost will be used, chrome or stainless steel piston rings should be considered.

Consideration should be given to using heavy-duty fasteners especially on the connecting rods and main caps for added durability and strength. If the engine will be run with a high boost level (12 psi or more), high-performance head gaskets with built in stainless steel O-rings are recommended because they can withstand the higher combustion pressure and temperatures encountered in a supercharged engine.

 

Now do the aftermarket pistons keep the compression ratio the same or lower it to 8.0? Or maybe even raise it?

 

they raise it to 8.5

 

Nasty Wendy & Odin's wrath,

Very good information thanks

I lowered my compression to raise my boost!


Suavy