Max C/R you can get away with on 99 fuel..?
Discussion
Any engine experts out there, who would like to tell me what C/R you can safely get away with on 98/99 fuel, and run without 'det' risk in 10-25C ambient temps, and without taking too much spark out of the timing..?
I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
RatBoy M3CSL said:
Any engine experts out there, who would like to tell me what C/R you can safely get away with on 98/99 fuel, and run without 'det' risk in 10-25C ambient temps, and without taking too much spark out of the timing..?
I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
Do you not realise that the CR you can run on a given fuel is totally dependant on the engine design and cam duration, neither of which you specify?I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
OP seems to track an E46 M3 CSL, so a fair bet that it's that motor.
http://www.esstuning.com/products/E46-M3-VT2%252d5...
A supercharger kit for the same engine using stock internals, and it runs 11.5:1 as standard AIUI. I should think you'll "get away" with 12.5:1, but for there are probably better ways to get the horsepower without stripping down the engine completely to put the pistons in.
http://www.esstuning.com/products/E46-M3-VT2%252d5...
A supercharger kit for the same engine using stock internals, and it runs 11.5:1 as standard AIUI. I should think you'll "get away" with 12.5:1, but for there are probably better ways to get the horsepower without stripping down the engine completely to put the pistons in.
Pumaracing said:
Do you not realise that the CR you can run on a given fuel is totally dependant on the engine design and cam duration, neither of which you specify?
Exactly. To the OP I can say I've ran 14.2:1 on 98 pump. Not in his application, but that should tell a thing or two.. 
.. I see you spotted my deliberate information ommision, it's for my E46M3 track car BMW S54 engine, 288/280 cams.. I like proper atmo engines..!!
My concern is that I won't be able to get enough spark into it on 98/99 fuel, and if the knock sensors back it off too much, theres the advantage gone..!
ref force feeding these engines, yes you can do it, with a procharger, which I dislike, it's a 10 grand job to do, with bigger injectors, and you will get 4-5 mpg on track (80-100 miles) on a tank of fuel, I have explored this route, before deciding to do a proper atmo engine, AKAIK no one makes a roots type proper blower for them anyway, of course track days / racing has nothing to do with economy, but @ 9-10mpg on track it's a tank in the morning, and the same in the afternoon which is fine by me, filling it up after every 10 laps would be just painful.
My concern is that I won't be able to get enough spark into it on 98/99 fuel, and if the knock sensors back it off too much, theres the advantage gone..!
ref force feeding these engines, yes you can do it, with a procharger, which I dislike, it's a 10 grand job to do, with bigger injectors, and you will get 4-5 mpg on track (80-100 miles) on a tank of fuel, I have explored this route, before deciding to do a proper atmo engine, AKAIK no one makes a roots type proper blower for them anyway, of course track days / racing has nothing to do with economy, but @ 9-10mpg on track it's a tank in the morning, and the same in the afternoon which is fine by me, filling it up after every 10 laps would be just painful.
Edited by RatBoy M3CSL on Wednesday 22 February 13:01
more often than not you can run up to 13:1 on super but 12.5:1 is a safer bet. make sure you do the necessary CC checks tho! if it ends up a bit on the high side, ask the piston supplier how much can safely be machined off the intruder and then work out how much needs to come off to get you where you want.
The VANOS will also have an effect on the amount of cam/compression. You may be able to re-map the cam advance and retard, to make it work, but just like the knock retard on the ECU, you may end up stealing some of the advantage gained.
You can try running a tighter squish to help with the mixture propagation towards the plug. BMW motorsport used 0.8mm crown to head clearance on the s14 DTM engines revving to 10k. Don't know what the s54 runs.
Detailed chamber work may reduce hot-spots also.
I think you will be fine at 12:1, but may run in to trouble at 13:1. And like has been said, measure the compression ratio properly. Don't take it as gospel that the pistons will give the compression quoted.
Steve
You can try running a tighter squish to help with the mixture propagation towards the plug. BMW motorsport used 0.8mm crown to head clearance on the s14 DTM engines revving to 10k. Don't know what the s54 runs.
Detailed chamber work may reduce hot-spots also.
I think you will be fine at 12:1, but may run in to trouble at 13:1. And like has been said, measure the compression ratio properly. Don't take it as gospel that the pistons will give the compression quoted.
Steve
Pumaracing said:
RatBoy M3CSL said:
Any engine experts out there, who would like to tell me what C/R you can safely get away with on 98/99 fuel, and run without 'det' risk in 10-25C ambient temps, and without taking too much spark out of the timing..?
I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
Do you not realise that the CR you can run on a given fuel is totally dependant on the engine design and cam duration, neither of which you specify?I have a set of pistons that are 12.5/1 .. and hoping I don't need to use 102 or C12 race fuel as it can get horribly expensive..!
Thanks
RB..
Do look at or calculate DCR or cranking compression when looking at this?
Stan
Stan Weiss said:
OK, it is the chap from the wrong side of the pond again. 
What method do use to rate Octane? In the US we use [ROM + MON] / 2 for automotive fuel.
Stan
RONWhat method do use to rate Octane? In the US we use [ROM + MON] / 2 for automotive fuel.
Stan
http://en.wikipedia.org/wiki/Octane_rating
Stan Weiss said:
Dave,
Do look at or calculate DCR or cranking compression when looking at this?
Stan
I don't bother with DCR much but I like to see 200 psi min cranking on a 2v engine and somewhat more, maybe 220 psi on a 4v one regardless of cam duration on pump fuel.Do look at or calculate DCR or cranking compression when looking at this?
Stan
http://www.pumaracing.co.uk/comp.htm
These are very similar, as I use dynamic compression ratio to calculate cranking pressure. If I have an engine at sea level and than take it up to Denver the DCR does not change. Let's say I have an engine with 7.6:1 DCR and at 70 degrees no humidity and Barometric Pressure of 29.92 it cranks 186.4 psi. Then I change locations and have 25.95 Barometric Pressure it will only crank 161.5 psi.
Dynamic compression is the actual physical compression which takes place after the intake valve closes and this generates the cranking pressure. NOTE - this is all happening at starter motor RPM's
Roughly this is how I do it.
1) Calculate displacement of one cylinder
2) From CR and displacement of one cylinder #1 above I calculate volume above piston at TDC
3) From IVC I calculate piston position and then dynamic stroke
4) From dynamic stroke #3, bore and volume above piston ATDC calculated in #2 I calculate Dynamic compression ratio
5) From Dynamic compression ratio #4 and atmospheric variables I calculate cranking pressure.
Stan
Dynamic compression is the actual physical compression which takes place after the intake valve closes and this generates the cranking pressure. NOTE - this is all happening at starter motor RPM's
Roughly this is how I do it.
1) Calculate displacement of one cylinder
2) From CR and displacement of one cylinder #1 above I calculate volume above piston at TDC
3) From IVC I calculate piston position and then dynamic stroke
4) From dynamic stroke #3, bore and volume above piston ATDC calculated in #2 I calculate Dynamic compression ratio
5) From Dynamic compression ratio #4 and atmospheric variables I calculate cranking pressure.
Stan
Hello Stan
Sounds like a good approach to me, I also use DCR calculations using the static compression ratio, rod length, bore, stroke and the .010" before intake closing point (since the valves hardly flow at all up to this lift point) as measured in the engine with lash, what calculator do you use?
I am using the kb-silvolite calculator here: http://www.kb-silvolite.com/calc.php?action=comp
From testing it seems 9.1 to 9.2 to 1 DCR makes best power with 99 octane fuel, very close to sea level, and an unrestricted IR intake, I know this sounds like a high DCR value and believe that calculator is producing high DCR figures, hence why I am getting figures above 9 to 1 for best power
Would be interesting to hear which DCR calculator you use and how you account for rod length
The DCR calculations certainly help with finding a good static compression ratio to use, cuts out the guess work
Regards
Jason
Sounds like a good approach to me, I also use DCR calculations using the static compression ratio, rod length, bore, stroke and the .010" before intake closing point (since the valves hardly flow at all up to this lift point) as measured in the engine with lash, what calculator do you use?
I am using the kb-silvolite calculator here: http://www.kb-silvolite.com/calc.php?action=comp
From testing it seems 9.1 to 9.2 to 1 DCR makes best power with 99 octane fuel, very close to sea level, and an unrestricted IR intake, I know this sounds like a high DCR value and believe that calculator is producing high DCR figures, hence why I am getting figures above 9 to 1 for best power
Would be interesting to hear which DCR calculator you use and how you account for rod length
The DCR calculations certainly help with finding a good static compression ratio to use, cuts out the guess work
Regards
Jason
Jason,
I use my own calculator which is part of a program that I wrote and yes it does use rod length.
Lets see what a wide difference in rod stroke ratio does.
Bore = 86 mm, Stroke = 86 mm, CR = 11:1, Intake Closes ABDC = 90
Rod length = 120.4 mm (1.4:1), DCR = 6.9233
Rod length = 172.0 mm (2.0:1), DCR = 6.6351
Stan
I use my own calculator which is part of a program that I wrote and yes it does use rod length.
Lets see what a wide difference in rod stroke ratio does.
Bore = 86 mm, Stroke = 86 mm, CR = 11:1, Intake Closes ABDC = 90
Rod length = 120.4 mm (1.4:1), DCR = 6.9233
Rod length = 172.0 mm (2.0:1), DCR = 6.6351
Stan
Hello Stan
Would you be willing to share your DCR calculator with me? I could make my own excel spreadsheet but it would obviously take considerable time to complete
Have been searching for a better calculator than the KB-Silvolite one but haven't found any so far
I will send you an email
Best regards
Jason
Would you be willing to share your DCR calculator with me? I could make my own excel spreadsheet but it would obviously take considerable time to complete
Have been searching for a better calculator than the KB-Silvolite one but haven't found any so far
I will send you an email
Best regards
Jason
Rwdfords said:
Hello Stan
Would you be willing to share your DCR calculator with me? I could make my own excel spreadsheet but it would obviously take considerable time to complete
Have been searching for a better calculator than the KB-Silvolite one but haven't found any so far
I will send you an email
Best regards
Jason
Email replied toWould you be willing to share your DCR calculator with me? I could make my own excel spreadsheet but it would obviously take considerable time to complete
Have been searching for a better calculator than the KB-Silvolite one but haven't found any so far
I will send you an email
Best regards
Jason
You have to include the rod lengh as part of the calculations.
With everything the same except rod length you will see the piston position at a given crank degree of rotation is different. Which means a different volume above the piston.
Stan
Bore = 86.0 Stroke = 86.0 Rod Length = 120.4
Wrist Pin Offset = 0.0
Crankshaft Degrees at which Rod and Crank are 90 Degrees 70.3462
Crank Piston Crank Rod Cylinder Cylinder
Angle Travel Rod Bore Volume Volume
Degree mm Angle Angle CI cc
-ATDC
180.0000 86.000000 0.00000 0.00000 30.48485 499.55721
185.0000 85.894714 -3.21626 -1.78374 30.44753 498.94563
190.0000 85.578494 -6.44439 -3.55561 30.33544 497.10877
195.0000 85.050281 -9.69627 -5.30373 30.14820 494.04048
200.0000 84.308381 -12.98380 -7.01620 29.88521 489.73093
205.0000 83.350576 -16.31888 -8.68112 29.54570 484.16723
210.0000 82.174287 -19.71344 -10.28656 29.12873 477.33440
215.0000 80.776783 -23.17936 -11.82064 28.63335 469.21657
220.0000 79.155451 -26.72842 -13.27158 28.05863 459.79856
225.0000 77.308124 -30.37224 -14.62776 27.40380 449.06780
230.0000 75.233467 -34.12216 -15.87784 26.66838 437.01653
235.0000 72.931418 -37.98906 -17.01094 25.85237 423.64437
240.0000 70.403668 -41.98326 -18.01674 24.95634 408.96117
245.0000 67.654181 -46.11430 -18.88570 23.98172 392.98993
250.0000 64.689707 -50.39070 -19.60930 22.93089 375.76988
255.0000 61.520283 -54.81982 -20.18018 21.80740 357.35932
260.0000 58.159666 -59.40763 -20.59237 20.61615 337.83815
265.0000 54.625676 -64.15851 -20.84149 19.36344 317.30989
270.0000 50.940407 -69.07517 -20.92483 18.05710 295.90288
275.0000 47.130282 -74.15851 -20.84149 16.70651 273.77061
280.0000 43.225923 -79.40763 -20.59237 15.32251 251.09095
285.0000 39.261846 -84.81982 -20.18018 13.91734 228.06440
290.0000 35.275975 -90.39070 -19.60930 12.50445 204.91125
295.0000 31.309010 -96.11430 -18.88570 11.09826 181.86793
300.0000 27.403668 -101.98326 -18.01674 9.71392 159.18256
305.0000 23.603844 -107.98906 -17.01094 8.36697 137.11012
310.0000 19.953733 -114.12216 -15.87784 7.07310 115.90734
315.0000 16.496941 -120.37224 -14.62776 5.84775 95.82751
320.0000 13.275629 -126.72842 -13.27158 4.70588 77.11554
325.0000 10.329707 -133.17936 -11.82064 3.66162 60.00325
330.0000 7.696103 -139.71344 -10.28656 2.72808 44.70516
335.0000 5.408107 -146.31888 -8.68112 1.91704 31.41464
340.0000 3.494815 -152.98380 -7.01620 1.23882 20.30070
345.0000 1.980660 -159.69627 -5.30373 0.70209 11.50527
350.0000 .885027 -166.44439 -3.55561 0.31372 5.14095
355.0000 .221969 -173.21626 -1.78374 0.07868 1.28938
360.0000 .000000 -180.00000 0.00000 0.00000 0.00000
-------------------------------------------------------------------------
Bore = 86.0 Stroke = 86.0 Rod Length = 172.0
Wrist Pin Offset = 0.0
Crankshaft Degrees at which Rod and Crank are 90 Degrees 75.9638
Crank Piston Crank Rod Cylinder Cylinder
Angle Travel Rod Bore Volume Volume
Degree mm Angle Angle CI cc
-ATDC
180.0000 86.000000 0.00000 0.00000 30.48485 499.55721
185.0000 85.877206 -3.75149 -1.24851 30.44132 498.84393
190.0000 85.508886 -7.51189 -2.48811 30.31076 496.70443
195.0000 84.895245 -11.29010 -3.70990 30.09324 493.13991
200.0000 84.036692 -15.09493 -4.90507 29.78891 488.15274
205.0000 82.933937 -18.93512 -6.06488 29.39801 481.74706
210.0000 81.588133 -22.81924 -7.18076 28.92095 473.92954
215.0000 80.001043 -26.75570 -8.24430 28.35837 464.71045
220.0000 78.175257 -30.75265 -9.24735 27.71118 454.10481
225.0000 76.114422 -34.81793 -10.18207 26.98066 442.13382
230.0000 73.823511 -38.95902 -11.04098 26.16859 428.82636
235.0000 71.309095 -43.18291 -11.81709 25.27729 414.22061
240.0000 68.579632 -47.49608 -12.50392 24.30977 398.36570
245.0000 65.645743 -51.90437 -13.09563 23.26978 381.32331
250.0000 62.520467 -56.41291 -13.58709 22.16194 363.16919
255.0000 59.219484 -61.02601 -13.97399 20.99183 343.99443
260.0000 55.761280 -65.74712 -14.25288 19.76598 323.90639
265.0000 52.167250 -70.57878 -14.42122 18.49199 303.02937
270.0000 48.461716 -75.52249 -14.47751 17.17847 281.50465
275.0000 44.671856 -80.57878 -14.42122 15.83506 259.49009
280.0000 40.827537 -85.74712 -14.25288 14.47234 237.15919
285.0000 36.961046 -91.02601 -13.97399 13.10177 214.69950
290.0000 33.106735 -96.41291 -13.58709 11.73551 192.31056
295.0000 29.300573 -101.90437 -13.09563 10.38632 170.20131
300.0000 25.579632 -107.49608 -12.50392 9.06734 148.58709
305.0000 21.981521 -113.18291 -11.81709 7.79190 127.68637
310.0000 18.543776 -118.95902 -11.04098 6.57331 107.71718
315.0000 15.303239 -124.81793 -10.18207 5.42462 88.89353
320.0000 12.295435 -130.75265 -9.24735 4.35842 71.42178
325.0000 9.553968 -136.75570 -8.24430 3.38664 55.49713
330.0000 7.109948 -142.81924 -7.18076 2.52030 41.30030
335.0000 4.991468 -148.93512 -6.06488 1.76935 28.99446
340.0000 3.223126 -155.09493 -4.90507 1.14252 18.72251
345.0000 1.825624 -161.29010 -3.70990 0.64714 10.60469
350.0000 .815419 -167.51189 -2.48811 0.28905 4.73661
355.0000 .204462 -173.75149 -1.24851 0.07248 1.18768
360.0000 .000000 -180.00000 0.00000 0.00000 0.00000
Hi all
This is where I got the Dynamic CR calculator I've been using (the links are at the bottom of the page, there is a good description of the method in the text) : http://cochise.uia.net/pkelley2/DynamicCR.html
Again it's mainly based on USA V8's but so long as you can convert to Imperial (i.e. Cubic inches etc, thou of inch) then it works fine. This is what I was using to gauge how much static CR I could run on my A-Series Mini engine by working out the dynamic CR on the hottest standard cam (MG Metro) then recalculating the static CR I'd need to maintain the dynamic CR with a hotter cam profile.
Using Dave's "rule of thumb" I come up with 187psi calculated, so I've got some room for inprovement there at the next rebuild.
Phil.
This is where I got the Dynamic CR calculator I've been using (the links are at the bottom of the page, there is a good description of the method in the text) : http://cochise.uia.net/pkelley2/DynamicCR.html
Again it's mainly based on USA V8's but so long as you can convert to Imperial (i.e. Cubic inches etc, thou of inch) then it works fine. This is what I was using to gauge how much static CR I could run on my A-Series Mini engine by working out the dynamic CR on the hottest standard cam (MG Metro) then recalculating the static CR I'd need to maintain the dynamic CR with a hotter cam profile.
Using Dave's "rule of thumb" I come up with 187psi calculated, so I've got some room for inprovement there at the next rebuild.
Phil.
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