High Compression .vs Low Compression

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High Compression .vs Low Compression

Postby jason » Sat Feb 28, 2009 9:58 pm

This is a topic which I'm really interested in.

I've always ran 8.5:1 compression in my rotary engines when building for a turbo application.
Recently I have thought about using 9.7:1 rotors on a weekend warrior engine.

So, here's my thought (nevermind being rotary or piston):
- Why don't we typically see higher compression engines which are forced induction?
- What are the drawbacks of both?

I think the big key here is the dynamic compression... not the static compression.
with overlap (porting in my case) the dynamic compression could be adjusted to the same levels as a lower compression setup.

Lately, Tuning:
- Is it really so with higher compression that the "safety margin" is narrowed .vs lower compression?
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Re: High Compression .vs Low Compression

Postby Neel » Sun Mar 01, 2009 1:26 pm

This is an excellent topic worthy of discussion. Its also one of those things where answers are not cut and dry.

Assuming that an engine is mechanically capable of handling the cylinder pressures resulting from your compression ratio, you're on the right track thinking we need to consider is the resulting effective compression ratio. This is the compression ratio that results from the mechanical characteristics of the motor plus boost pressure. There are a number of excellent articles on the internet regarding this; if you Google "effective compression ratio" you'll find a number of them so I won't go into detail here.

Your assumptions are correct. Ideally, a motor with higher static compression ratio and lower boost will make more torque. It will also, however, be detonation prone at lower boost and fuel octane levels so you have to be more careful when tuning it. There's another thing that gets overlooked however, which is cylinder volume. When you have a higher compression engine, it has less cylinder volume at TDC, so the drop in pressure as the piston descends is greater. Some people argue that this results in lower torque. I can see why, but some cylinder pressure data versus crankshaft angle would be very interesting to compare two setups.

On the other hand, running a lower compression ratio and higher boost has its drawbacks. Your intercooler efficiency becomes more important the more you compress air. Your exhaust gas backpressure could go up. But then again, these are partially system design issues - most of the time you don't have two optimized setups to compare.

So in the end, most people go the lower compression route because its easier to tune and for that reason safer.
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Re: High Compression .vs Low Compression

Postby EfiOz » Tue Mar 31, 2009 5:04 pm

I guess what we're really trying to do with the turbo is keep the VE (volumetric efficiency) up. So when you run a higher static comp ratio (say 10:1) and then use the boost to maintain the cylinder filling and dynamic compression to a consistent value. Consistently high Dynamic CR equals flat torque. Hence the flat torque curves you see in late model Euro turbos like Golf GTi's.

Now, how that works in a racing application is another thing. Some of the late 80's F1 engines ran as much as 9.5:1 with 3.5 bar absolute manifold pressure and could make 700 odd HP for nearly two hour long races out of 1.5 litres. The thing that let them down at the time was the cave man injection of the day. The engines themselves were also the strongest thing I've seen this side of heavy duty diesel.

I don't know how a boosted high CR rotary would go. Wouldn't the apex seals be the weak link?
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Re: High Compression .vs Low Compression

Postby Neel » Tue Mar 31, 2009 6:16 pm

Now, how that works in a racing application is another thing. Some of the late 80's F1 engines ran as much as 9.5:1 with 3.5 bar absolute manifold pressure and could make 700 odd HP for nearly two hour long races out of 1.5 litres. The thing that let them down at the time was the cave man injection of the day. The engines themselves were also the strongest thing I've seen this side of heavy duty diesel.


And you've had the privilege of playing with those things!

I don't know how a boosted high CR rotary would go. Wouldn't the apex seals be the weak link?


That's why I would shy away from a high boosted CR rotary. Rotary + Detonation = rebuild.

Actually, we've had great success with high CR boosted motors IN HOUSE for once! Rhys, our lead engineer, took his DAILY RIDDEN 2006 Hayabusa to the Texas Mile. His bike did a standing mile at 206 MPH! It has a stock, 11:1 CR motor and is running 10psi of boost. Rhys has put a lot of work into mapping it properly and sizing components to avoid detonation, and it shows.

The bike is AMAZING. He honestly rides in to work and back every day, and it has little to no lag. But at Texas, it beat a lot of very built bikes. I think credit goes to his tuning on this one.

-Neel
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Re: High Compression .vs Low Compression

Postby EfiOz » Tue Mar 31, 2009 7:10 pm

I guess you'd call it a priviledge.

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Re: High Compression .vs Low Compression

Postby jason » Fri Apr 10, 2009 7:20 pm

As far as the rotary & apex seal goes.. there is a plethora of new technologies / metals that can hold up to high boost and compression.
Some apex seals do not break, rather they bend/warp saving the housing(s), turbo(s), etc.

I've seen some low budget racers detonate these `bendable seals` and actually reuse 'em.
One guy even went as far as using a table-vise to straighten 'em up some :oops:
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