Dynamic Compression ratio

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pre65racer
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Dynamic Compression ratio

Post by pre65racer »

If you have ever wondered why just swapping cams for a longer duration item can often have the opposite effect on the cars overall performance google Dynamic Compression Ratio (DCR).

I always wondered why with my race engines (using Avgas 100 mon fuel) we got much better performance at 12.5:1 static compression ratio and even as much as 13.5:1 if the really long duration cams such as when the R23 was being used. It then dawned on me that the compression ratio when an engine is moving is only relevant from the point the values are closed.

A quick google later then the light bulb came on.

My best race engines all had dynamic compression ratios around 8.9/9.1, the ideal range for the high octane fuel I was using. If I fitted cams with longer duration (late closing valves) and had not increased the static compression ratio I was effectivley running a low (dynamic) compression engine and lost the performance I was looking for unless I advanced the cam (closed the inlet valve earlier).

I went back to my text books but found nothing about DCR. Maybe I need some new text books or is DCR the engine builders dirty little secret they don't want to share!
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Re: Dynamic Compression ratio

Post by Lotus-e-Clan »

Search this forum on dynamic compression ratio 'cos myself and others have written stuff on this and indeed use an appropriate static compression ratio to compensate for late inlet valve closing. I also calculated and produced a table for some cams to show what compression ratio you need to bring the DCR back to the std engines 9.08:1 ....does wonders for the low and midrange torque on an otherwise "cammy" cam. I use 11.8:1 with a GB72 (like a frazer B). 8)

Image

I just realised you might not have seen the previous threads on DCR if they are in forumareas inaccessible to non-members :oops:
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Re: Dynamic Compression ratio

Post by ImpManiac »

I want to determine my engine's dynamic compression ratio when the car's back together again. Reg Patten built my cylinder head for me but I cannot remember that much about it. I should try to find the build sheet, really.

But yes, DCR will have a huge impact on engine characteristics.

IM 8)
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Re: Dynamic Compression ratio

Post by pre65racer »

I agree with the table posted earlier provided that the actual closing of the inlet valve is actually known along with the other charaterisics that make up the Static compression and the fuel octane is approrpiate.
I've yet to see any two performance cams that are the identical some so called GE3 are so different they can't be the same grind, even standard cams vary by a degree or two. Also a DCR for old style 5 star-100 MON (110-115 RON) is 9:1 if the engine is using lower octane fuel then the DCR is way too high resulting in the spark advance needing to be backed off.
I've yet to see an engine make more power with a retarded spark (32-34 degrees) with a higher than optimal DCR compared to a slightly lower than optimal DCR and an advanced spark (38-40 degrees). Yes the mid range is greatly improved as a reuslt of the increased compression but peak power will be down. Depends what you are looking for I suppose?
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Re: Dynamic Compression ratio

Post by Lotus-e-Clan »

I found the same peak power mid range trade off with the higher comp ratio, but as you implied, it doesn't necessarily translate into poorer performance (especially if gearing is matched to suit). BTW regarding fuels, I've kept an eye out for signs of detonation using the higher comp ratios but haven't seen any evidence (stripped down) for detonation on 11.8 static / 9,08 DCR using the 98 RON Shell V power we have over here...do you have it over there?

For most "race" cams a driver with a feel for the cam would never instinctively use WOT when the revs are "off cam" so the important area of the ignition curve/map to tweak for WOT performance is that above 4 krpm and using a cr box that's where you find yourself most of the time anyway.

I use logs of on road/on track AFR, rpm, tps and ignition advance data to find stuff out. By tweaking parameters like AFRs and ignition advance you can see the effect on in-gear acceleration rpm gradients. I ran a map with increasing advance beyond the 4 k rpm on WOT then logged the above parameters. The aim was two fold. 1) to see the effect increasing advance above 4k and 2) to select the best absolute advance number above 4 krpm. The section of map looked like this:
  • krpm-----ign
    4 ---------34
    6 ---------35
    8 ---------36
From the resulting logs I noticed when each 1 degree increase came in, the rpm gradient decreased, so I removed the ignition increases completely and kept 34 degrees all the way to the top then ran the log again. The rpm gradients were now more linear without the drop off seen with increasing advance. This clearly indicated that I didn't need anymore advance than 34 degrees above 4 krpm for my particular set-up. Now I must add an obvious point here that when using mapped ignition 34 degrees on the map only means 34 degrees in the combustion chamber IF your trigger wheel and sensor has been set accurately.

But obviously people have to determine these numbers for themselves. In this day and age of user friendly engine data monitoring / management gizmos you can discover for yourself what''s best for your engine under varying conditions without the need for a rolling road which is only relevant for the conditions that prevail on the day anyway.
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Re: Dynamic Compression ratio

Post by pre65racer »

Your numbers seem to make sense when reading the text books all things being equal there should not be a need for large increases in max advance over a standard road car. Our measurement of the advance will certainly not be as accurate as your mapped system. We are setting advance on the rolling road and by feel on the track for the conditions on the day so advance requirements could quite easily vary day to day, circuit to circuit. In the end everything is a compromise with a mechanical distributor.

Re fuel, we do have something called V-power but it is 95 RON. There has been a court cases over hear regarding the way the fuel was advertised.

We are still allowed to use 100 Mon (110-115 RON) leaded Avgas as race fuel. They tried to ban it but had so many engine failures in classic cars using 98 pump fuels that they relented. The failures were nothing to do with the removal of lead. The lower octane necessarily combined with the combustion temperature increase caused many head/gasket/wills ring failures. The problem may have been that the exact composition of these road fuels is not consitent here with summer and winter blends and differences within refined batches. Even when advance and mixture adjustments had been made the next tank of fuel caused problems, no one wanted to continue experimenting as it was getting expensive so everyone went back to what they knew.

I am still surprised that you have no issues with 98 RON and a high DCR but that may be part of the reason why your advance is so different in practice from ours, the Avgas burns comparitivley slowly and at a much lower temp. Its very hard to make it preignite and sometmies even harder to get it to start!
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Re: Dynamic Compression ratio

Post by ImpManiac »

Paul Harrison
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Re: Dynamic Compression ratio

Post by pre65racer »

Very comprehensive list of fuels and properties.

The fuel we use is listed as EXON Mobil Aviation fuel. 99.5 MON minimum.
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