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Author Topic: How much effect does Compression ratio have on the BHP?  (Read 11459 times)
Lids
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« on: May 30, 2010, 22:02:39 pm »

This is a question for the experts.

I know a high CR will(maybe) reduce the life of an engine, due to heat etc.

But what are the gains of a high CR.

Can you get decent power from a modest 9.5 or 9 to 1?

Lets use an 82 x 94 as a base running 44 x 37 valves, with a lary cam like an FK89 or 86C?
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Fiatdude
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« Reply #1 on: May 30, 2010, 23:32:17 pm »

If you want to make HP u got to have high CR (NA we're talking about)

If you want modest HP you have a modest CR

It comes down to a complete package for your engine -- some cams work great at high CR and aren't worth anything at lower CR
So it is a complete package with head/cam/rockers/cc's and TURBO's
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« Reply #2 on: May 31, 2010, 14:58:15 pm »

generally a high CR increases your cylinder pressure a lot. But it is also depending on ignition angle, cam specs and alot others. But a retearded ignition angle can reduce power very much..

there are a few basic formulas which help to calculate you approx. Pressure and torque output.. - HP is just torque*rpm

cheers max
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John Maher
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« Reply #3 on: May 31, 2010, 16:37:12 pm »

Unfortunately there's no simple answer...

For maximum power output and engine efficiency, you should use the maximum CR your engine can safely handle. Major determining factor is the fuel you intend to use.
Higher octane fuel is more resistant to detonation.

Personally, I think in terms of each engine having three compression ratios.... Static, Dynamic and Effective.

1. Static CR is the mathematical ratio as measured on the assembly bench. Determined by deck, chamber volume, piston notches/dishes(if applicable) and engine capacity.

2. Dynamic CR.... you calculate this by figuring piston position at the point the intake valve closes on the intake stroke. In theory, this is the point at which cylinder pressure changes from negative (intake) to positive (compression). The intake valve usually closes somewhere between 50° to 70° after BDC. If you measure how much stroke is left before the piston reaches TDC, input this stroke figure into your normal CR formula and you find Dynamic CR is much lower than Static CR. The longer your cam's duration, the bigger the difference between Dynamic and Static CR. Rod length also plays a small part because different length rods will alter the piston's position at a given valve closing. There are various DCR calculators on the web.

Take a street engine as an example... for best efficiency and decent power output, you probably want to run Dynamic CR somewhere in the region of 8:1.
A stock (short duration) cam has a dynamic CR fairly close to it's Static CR, therefore Static CR should be pretty close to 8:1 (pump fuel).
Fit a longer duration cam eg FK10 and you'll find the Static CR needs to be up around 9.5 to 10:1 to achieve a dynamic CR close to 8:1

But there's another factor at play which is much more difficult to pin down....

3. 'Effective CR' is the term I use, others may call it something different. It's a way of better estimating the actual CR the engine actually 'sees' at any given point in the rpm range (talking about wide open throttle testing here). Yes, Effective CR changes throughout the rpm range, generally maxing out at peak torque.
Static and Dynamic CR don't take into account what happens under running conditions. Both formulae assume the intake charge stops entering the cylinder when the piston hits BDC on the intake stroke. They also assume the cylinder gets a 100% fill i.e. 100% volumetric efficiency (VE).
Few engines (n/a) achieve 100% VE.
E.g. on the dyno, say an engine achieves 90% VE @ 6000rpm and Dynamic CR was mathematically calculated as being 8.0:1.
What the engine actually 'sees' is... Effective CR = Dynamic CR x 90%.
ie 8.0 now becomes 7.2:1
In other words if my plan was to hit an Effective CR of 8.0:1, I have to raise Static CR, which in turn raises Dynamic CR.
E.g. a low CR engine fitted with an FK89 under performs - dynamic CR is way too low for the system to function efficiently.

A race engine with a well developed tuned length induction and exhaust system can easily exceed 100% VE by optimising manifold lengths and exhaust primary pipes. Engines of this type manage to 'ram' high pressure intake charge into the cylinder such that it continues to pile in after the piston has hit BDC and starts it path upward on the compression stroke. Ideally the intake charge will keep coming in up to the point the inlet valve closes. An optimised engine can be tuned to push VE above 120%. Using the same example engine as above, Effective CR now becomes...
8.0 x 120% = 9.6

That's all a long winded way of coming back to what I said at the start.... there's no simple answer.
To summarise.... the milder the cam, the lower the CR (Static CR).
The wilder the cam, you need more CR.
Higher octane fuel can tolerate more CR.
Intake and exhaust systems should also be taken into account because they have a major influence on VE, which in turn effects Dynamic CR etc etc.

In the real world you either stick with a tried and tested combination, relying on the experience of others, or you experiment on the dyno if it's a unique combination and you're looking to exploit the engine's fullest potential.

Having said all that, minor increases in CR make very little difference to power output. It's very much the law of diminishing returns.
If for whatever reason you'd built a 2276cc engine with FK89 and set CR (static) at 8.0, you'll see a healthy increase in power by stepping up to 10.0
However, the difference between 10.0 and 10.5 will be very small. The exact number to aim for will be a judgement call based on all that stuff I discussed above.
« Last Edit: May 31, 2010, 16:42:09 pm by John Maher » Logged

John Maher

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« Reply #4 on: May 31, 2010, 20:40:41 pm »

Thanks, John. I always look forward to hearing what you have to say Smiley
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« Reply #5 on: May 31, 2010, 22:09:34 pm »

nice discusion ...
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Black Sheep
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« Reply #6 on: May 31, 2010, 22:14:19 pm »

This is a question for the experts.

I know a high CR will(maybe) reduce the life of an engine, due to heat etc.

But what are the gains of a high CR.

Can you get decent power from a modest 9.5 or 9 to 1?

Lets use an 82 x 94 as a base running 44 x 37 valves, with a lary cam like an FK89 or 86C?

Got you thinking after this weekend eh  Wink Bang the compression up Mike you know it make sense
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« Reply #7 on: May 31, 2010, 22:45:16 pm »

Thanks John, some good reading.
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Lids
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« Reply #8 on: June 01, 2010, 06:07:07 am »

This is a question for the experts.

I know a high CR will(maybe) reduce the life of an engine, due to heat etc.

But what are the gains of a high CR.

Can you get decent power from a modest 9.5 or 9 to 1?

Lets use an 82 x 94 as a base running 44 x 37 valves, with a lary cam like an FK89 or 86C?

Got you thinking after this weekend eh  Wink Bang the compression up Mike you know it make sense

Yeah, but I need to balance BHP with lifespan, at the moment I need it built right once can't afford to rebuild every 6-12 months just for the sake of  10 bhp!  Got a kid to feed Smiley
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Lids
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« Reply #9 on: November 07, 2011, 07:53:50 am »

An old thread but i am the resurrection.

My head chambers are 49cc on an 82x 94 with 5.5 rods.

Is 10.5 CR to high for 95 octane?
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Peter
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« Reply #10 on: November 07, 2011, 08:57:46 am »

re-read John's reply

You didnt say what cam you'll use
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Lids
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« Reply #11 on: November 07, 2011, 09:12:43 am »

re-read John's reply

You didnt say what cam you'll use

its a raptor cam similar to webcam 86c.
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Torben Alstrup
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« Reply #12 on: November 07, 2011, 11:01:41 am »

IŽd say most likely OK. But Johannes is about the only one who can answer that question. The public does not have enough experience with these cams as of yet.
T
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