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BMEP numbers

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    steelcomp was here
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    Default BMEP numbers

    I'd like to start a discussion on what maakes the difference between good BMEP's and not so good BMEP's.
    Things we've touched on so far in the other thread:
    Effeciency, effeciency, effeciency.
    BMEP's ar related to torque
    They are calculated like HP
    They are a common denominator between all engines.

    My thoughts are that they start with a planned out engine combinatioin, before a part is ever purchased. For me, the key is thinking of every part in the engine as they're related to eachother, not as individual parts. That's basic.

    I think where good BMEP's start are in the combustion chamber. I had a thread on "what makes an engine accelerate" on HB, and I tried and tried to steer the conversation in this direction. No one ever mentioned BMEP's, or combustion chamber effeciency.

    Camshaft design.
    If you have a bunch of mismatched parts, the best cam in the world isn't going to help much, but with a good combinatioin of parts, the right cam will make all the difference. I don't believe there's any "one cam fit's many engines" thinking here. I'm a firm believer in having a cam designed specifically for the application intended. It dosen't cost that much more over all, and the difference can be huge.

    Induction.
    A good induction system is huge to BMEP's. Again, the right intake and carb(s) for the application. With carbs, it's the little details. Just about any setup will run, but really getting the carb(s) dialed is a big producer of usable power. If what goes in the chamber isn't right, the combustion will suffer, no matter how good the chamber design and work. Fuel inj. and EFI are other options, but I'm keeping it simple, since most of us are working with carbs.

    So, again, I think BMEP's are a direct sign of combustion effeciency. Whatever is related to that, however you can improve that, IMO, will yield improved BMEP numbers across the board. That sounds failry generic...almost duh, but when you start thinking about what effects combustion, it gets a little more interesting.

    Those are just a few thoughts, and just a start.
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    Steel, I like this definition of BMEP:

    The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.

    Note that BMEP is purely theoretical and has nothing to do with actual cylinder pressures. It is simply an effective comparison tool.

    If you work through the arithmetic, you find that BMEP is simply a multiple of the torque per cubic inch of displacement. A torque output of 1.0 lb-ft per cubic inch of displacement equals a BMEP of 150.8 psi. in a four-stroke engine and 75.4 psi. in a two-stroke engine.

    (The discussion on the remainder of this page is with respect to four-stroke engines, but it applies equally to two strike engines if you simply substitute 75.4 everywhere you see 150.8)
    If you know the torque and displacement of an engine, a very practical way to calculate BMEP is:

    BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

    This formula is easier than the above one of:

    (HP x 13,000)/(LxRPM) L= liter of displacement.

    Here are some solid base line numbers to use:

    At the end of the 2006 season, these F1 engines made in the vicinity of 750 HP at an astonishing 19,000 RPM. Assuming peak power is around 18,500, the torque at peak power would be 213 lb-ft and peak-power BMEP would be 219 psi. Peak torque BMEP would likely be at least 10 psi greater. There can be no argument that 219 psi at 18,500 RPM is truly amazing.

    The 2006 Nextel Cup engine is a severely-restricted powerplant, being derived from production components. It is based on a production cast-iron 90° V8 block and 90° crankshaft, with a maximum displacement of 358 CID (5.87 liters). A typical configuration has a 4.185" bore with a 3.25" stroke and a 6.20" conrod (R/S = 1.91). Cylinder heads are similarly production-based, limited to two valves per cylinder. The valves are operated by a single, engineblock-mounted, flat-tappet camshaft (that's right, still no rollers as of 2007) and a pushrod / rocker-arm / coil-spring valvetrain. It is further hobbled by the requirement for a single four-barrel carburetor. Electronically-controlled ignition is not allowed, and there are minimum weight requirements for the conrods and pistons.

    How does it perform? At the end of the 2006 season, the engines were producing in the neighborhood of 840 HP at 9000 RPM (and could produce more at 10,000 RPM, but engine RPM has been restricted by means of a rule limiting the final drive ratio at each venue). 840 HP at 9000 RPM requires 490 lb-ft of torque, for a peak-power BMEP exceeding 206 PSI. Estimating peak torque to be 550 lb-ft (probably in the neighborhood of 7800 RPM) yields a peak BMEP of nearly 232 PSI.



    In reading the definition it real apparent that cam timing would make a huge difference of the BMEP numbers. The same cam set straight up or advanced or retarded could change the number's a bunch.

    Or different cam's one for nitrous vs a N/A engine. If the cam opens the exhaust valve earlier (BBDC) it will bleed off energy used it the calc wouldn't it ?

    I would also ask, since BSFC is also an idicator of combustion effecancy would an engine have a higher BMEP because it has a lower BSFC ? Do the two go hand in hand ?

    I also wounder what the number's would look like on the 565 if the nitrous cam came out and a straight up N/A max power at 7,000 rpm cam went in it.

    What produces a higher BMEP, Turbo, blower or Nitrous?

    It seem's I have more question's than answer's.

    Sleeper CP

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    Last edited by Sleeper CP; 01-19-2008 at 02:07 PM.

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    Quote Originally Posted by Sleeper CP View Post
    --------------------------------------------------------------------------

    Steel, I like this definition of BMEP:

    The definition of BMEP is: the average (mean) pressure which, if imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output.

    Note that BMEP is purely theoretical and has nothing to do with actual cylinder pressures. It is simply an effective comparison tool.

    If you work through the arithmetic, you find that BMEP is simply a multiple of the torque per cubic inch of displacement. A torque output of 1.0 lb-ft per cubic inch of displacement equals a BMEP of 150.8 psi. in a four-stroke engine and 75.4 psi. in a two-stroke engine.

    (The discussion on the remainder of this page is with respect to four-stroke engines, but it applies equally to two strike engines if you simply substitute 75.4 everywhere you see 150.8)
    If you know the torque and displacement of an engine, a very practical way to calculate BMEP is:

    BMEP = 150.8 x TORQUE (lb-ft) / DISPLACEMENT (ci)

    This formula is easier than the above one of:

    (HP x 13,000)/(LxRPM) L= liter of displacement.

    In reading the definition it real apparent that cam timing would make a huge difference of the BMEP numbers. The same cam set straight up or advanced or retarded could change the number's a bunch.

    Or different cam's one for nitrous vs a N/A engine. If the cam opens the exhaust valve earlier (BBDC) it will bleed off energy used it the calc wouldn't it ?

    I would also ask, since BSFC is also an idicator of combustion effecancy would an engine have a higher BMEP because it has a lower BSFC ? Do the two go hand in hand ?

    I also wounder what the number's would look like on the 565 if the nitrous cam came out and a straight up N/A max power at 7,000 rpm cam went in it.

    What produces a higher BMEP, Turbo, blower or Nitrous?

    It seem's I have more question's than answer's.

    Sleeper CP

    Big Inch Ford Lover
    I agree with the text book definition of what BMEP is, but let's focus on what makes good BMEP numbers. I think when you get into power adders, there's no use in analyzing the numbers. They're crutched.

    On your build, just my opinion, but I see three areas where you could make it more effecient.
    It seems as if the carb is too small, the heads are too big, and the cam is wrong application. Now if the heads were smaller, then the carb might not be too big, but then the cam would be HUGE!. If you turned that combination 7500, then the heads and cam might be more in line, but the intake and carb are WAY too small. It sems you have apples and oranges to some degree. As it is, the cam might be OK because your intake ports are huge, and the added ex duration may help there, I don't know...just guessing. Your port velocities are probably a little down, and you might be getting some extra scavengine of the intake charge. Like I said, just guessing...these were some of my thoughts on that build. (don't know the overlap) The numbers on that engine are pretty exceptional, so it's not that it's "wrong", just some areas where I see somewhat of a mismatch.

    Anything done to improve torque is going to improve power, BMEP's and hp/ci...they all go hand in hand. The better the combustion effeciency, the further down the cyl the pressure will extend ( depending on cam timing) which develops more power, measured as Q, etc.

    I believe there are things that effect effeciency as well like ring sealing, valve sealing, frictional losses, and it goes on, so when some of the finer details like cylinder finish, valve job and guide clearances, and bearing clearances are over looked, there are losses there, as well.
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    I actually have to go do a few things but I'll get back to you on my 565 as far as being efficient.

    On your 468 what were the flow number's on those heads?

    Sleeper CP
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    Quote Originally Posted by Sleeper CP View Post
    I actually have to go do a few things but I'll get back to you on my 565 as far as being efficient.

    On your 468 what were the flow number's on those heads?

    Sleeper CP
    Big Inch Ford Lover
    Nothing spectcular. Average max intake @.800 (left and right hand ports...remember this is a BB Chev...yuk!) was <345cfm. Ex @.800 was 312cfm. 2.25"/1.88" Manley Severe Duty valves @ 28"H20 on a 4.31" bore fixture (same as engine). No pipe on ex, clay radius on intake. Something to note...these intake ports had over 80% of their max flow at .400" (50%) lift.
    Last edited by scott foxwell; 01-19-2008 at 03:58 PM.
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    Okay I'm back,

    On the 565" there are a few thing's you need to remember about it. It's not a race engine it run's in a "ski" boat that we expect to do alot of things. IE: tow kid's on tube at less than 3,000 rpm's for minutes at a time, run 13-14 miles up river at 4,000 rpms with out eating all our gas, pull water skier's, idle at a reasonable rpm( 900- 1,000) with a lope and start everytime we turn the key. The 855 hp engine logged 45 hrs of running time one summer with nothing more than changing a spark plug or two. This engine should be able to do the same. Oh did I mention it also pushes the boat to over 100 mph.

    This engine is a total compromise: The Carb is too small , but that I knew as the engine was pulling 1" of vac at 5,000 rpm's and 1.4" at 7,000. With a 1,250 it should be able to top 960 hp. The old 855 hp engine was maxing out this carb so the new head's and intake really over stressed it.

    The cam was ground for the old set of head's but works just fine with this combo. That is one reason why I went with the 1.8 ratio T&D's to help this cam out a bit. The cam is a compromise also, if it was just a straight up N/A engine the cam would be a bit different but as it is it does both job's well. (Edit): The cam was ground in 2000 , I gave Danny Crower the head spec's and engine info and told hm I wanted max trq at approx 5,800 and Max HP at 1,000 rpm's later but it should have some legg's after max power. We would turn the AA at 5,800 ( pk tq) off the nitrous at 6,800 on the bottle. So how did it do on the dyno: Max trq of 711 at 5,800 Max power 855 at 6,600 rpm's and 850 at 6,800.

    As far as the heads being to big; I think not. The old engine pulled a 113% VE with a BSFC of .48. This engine was dyno'd on a DTS dyno that doesn't calc VE's but the BSFC was .366 avg. It is doing something right. I would have no reason to believe the VE's would be any less if not higher. edit( and did you see that trq cuve?) I'm now wondering what I could do to get better BSFC number's ? Not.

    I have had several bad ass engines built and have been around 3 of Dan Crower's engines. As a bench mark for a single carb engine we strive to achieve a minimum of 2.0 HP per CFM of head flow. This engine comes in at 2.08 HP per cfm. If it had a bigger carb on it and if I had Mobil One in the pan when it was dyno'd it could/should have put out more than 960 hp. If it did that would be 2.13 hp/cfm which tell's me they are not to big, but looking at the trq curve it pulled tells me they aren't to big. Three Engine Master's Engine I was around did 2.10 HP/CFM. Also when the 300 hp nitrous system is on, the large ports are good to have.


    That's it for now. But there is always room for improvement. And if I had the money to put on a vac pump and a kick out Steph's pan along with a bigger carb I could see this engine close to 1,000 hp. None of those would be internal changes and it would be right at 1.77 HP cu.in. and 2.22 hp/cfm with a single carb......Hummmmmmmmm

    Sleeper CP
    Big Inch Ford Lover
    Last edited by Sleeper CP; 01-20-2008 at 09:28 AM.

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    Steel(Scott),

    I had some time to kill last night so I started to try to run some numbers on this BMEP thing again.

    After I ran the number's on 11 different engine's I did what I should have done FIRST. I ran your known number of a BMEP of 200 psi at 679 lbs. ft at 5,500 rpms. The equation is actually real easy so the 11 calculations didn't take to long: BMEP = trq x 150.8/ cu.in.

    So your engine is 679 x150.8/467= 219 psi (per the calculation) This number is 19 psi over your number off the dyno. It is 10% high from your dyno sheet. I have to trust your sheet and I would guess the calc is approx 10% high.

    I then ran the posted number from Danherc: 672x150.8/528=191 psi. His dyno BMEP is 175 psi as printed on the sheet. The calc is 9% high for his dyno sheet. ( If I had Dan's uncorrected number's I could compair the two)

    This lead me to ask what your uncorrected dyno number's are? If your uncorrected trq. number at 5,500 rpm's is 620 lbs. ft or your uncorrected hp number is 649 HP @ 5,500 it all will make since. If those aren't your number's I just waisted a bunch of time and I don't know how we would ever compare apples to apples.

    Also, I have no idea if shaving 10% off the number's I have already calculated will correct them or not.(?)

    Sleeper CP
    Big Inch Ford Lover
    Last edited by Sleeper CP; 01-22-2008 at 06:52 PM.

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    Mexican food gives me good bmep numbers.

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    619.3/648.5 observed @ 5500. I'm not sure I understand the question. It was my understnading that the dyno is making the calculation for BMEP using the uncorrected torque number. I think that's what I posted earlier. 619.3 X 150.8/466.86 = 200.039
    Quote Originally Posted by Sleeper CP View Post
    Steel(Scott),

    I had some time to kill last night so I started to try to run some numbers on this BMEP thing again.

    After I ran the number's on 11 different engine's I did what I should have done FIRST. I ran your known number of a BMEP of 200 psi at 679 lbs. ft at 5,500 rpms. The equation is actually real easy so the 11 calculations didn't take to long: BMEP = trq x 150.8/ cu.in.

    So your engine is 679 x150.8/467= 219 psi (per the calculation) This number is 19 psi over your number off the dyno. It is 10% high from your dyno sheet. I have to trust your sheet and I would guess the calc is approx 10% high.

    I then ran the posted number from Danherc: 672x150.8/528=191 psi. His dyno BMEP is 175 psi as printed on the sheet. The calc is 9% high for his dyno sheet. ( If I had Dan's uncorrected number's I could compair the two)

    This lead me to ask what your uncorrected dyno number's are? If your uncorrected trq. number at 5,500 rpm's is 620 lbs. ft or your uncorrected hp number is 649 HP @ 5,500 it all will make since. If those aren't your number's I just waisted a bunch of time and I don't know how we would ever compare apples to apples.

    Also, I have no idea if shaving 10% off the number's I have already calculated will correct them or not.(?)

    Sleeper CP
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    Last edited by scott foxwell; 01-22-2008 at 11:34 PM.
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    Quote Originally Posted by steelcomp View Post
    619.3/648.5 observed @ 5500
    Well I'll be damned, my calculator worked correctly. Well that part makes since, but as you and I have said " if it is calculated off of observed number's how do you ever compare one engine to another? Or even the same engine tuned dyno'ed on a cool Winter day one year and a Summer afternoon another year". Hummmmmm.

    Thanks though, at least that part of the puzzle is answered. And within in
    1 point on each.

    Sleeper CP

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    Quote Originally Posted by steelcomp View Post
    619.3/648.5 observed @ 5500. I'm not sure I understand the question. It was my understanding that the dyno is making the calculation for BMEP using the uncorrected torque number. I think that's what I posted earlier. 619.3 X 150.8/466.86 = 200.039
    Yes you may have on the other thread. But then can anyone ever compare apples to apples ? Well I guess for a comparisons sake you can use corrected trq #'s just so you have a base line?

    Sleeper CP

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    Quote Originally Posted by Sleeper CP View Post
    Yes you may have on the other thread. But then can anyone ever compare apples to apples ? Well I guess for a comparisons sake you can use corrected trq #'s just so you have a base line?

    Sleeper CP
    I think we also already agreed (again, on the other thread) that comparing corrected numbers was the only way to compare apples to apples...like you say, if that can really be done.

    So for comparison's sake, my 467 had a "corrected" BMEP of 679.8 X 150.8 / 466.86 = 219.581.
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    Quote Originally Posted by steelcomp View Post
    I think we also already agreed (again, on the other thread) that comparing corrected numbers was the only way to compare apples to apples...like you say, if that can really be done.

    So for comparison's sake, my 467 had a "corrected" BMEP of 679.8 X 150.8 / 466.86 = 219.581.
    The 565 observed number was 706x150.8/565= 188.4 psi

    The correct number (trq) was 765x150.8/565= 204 psi

    Both engines must have been tested under similar condition with both correction factor's at 60 ft. lbs. edit : My dyno had a 8.5% correction and yours had a 9.5% correction. close enough.

    Sleeper CP
    Last edited by Sleeper CP; 01-23-2008 at 08:54 AM.

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    BMEP Review:

    For what’s it’s worth on the subject, I find this interesting about the BMEP equation:

    I dyno’ed the same engine in ’93 and ’95 every thing was the same except we retarded the cam 4* to gain a few 100 rpm’s on the topside. The results were interesting:

    Engines spec’s: 565” 10.2:1 comp., 1050 carb, A-460 heads 408 cfm at .700 lift.
    Corrected Hp 783 at 6,250 & 728 lbs. ft at 5,000.(774 hp @ 6500)

    Observed trq. Was 690 @ 5,000 690x150.8/565= 184 psi BMEP

    The same engine was dyno’ed in ‘95 and the corrected numbers were:

    HP 780 @ 6,500 & 678lbs. ft. @ 5,250 (756 [email protected] 6,750)

    Observed trq. Was 644 @ 5,250 644x150.8/565= 172 psi BMEP.

    Both set up’s produced 1.39 HP per cu in.

    I would guess that retarding the cam 4* didn’t allow the intake port to trap as much intake charge ? (opens later/ closes later) Maybe some of the charge was released on the compression stroke? Just a guess.

    Now fast forward; new engine (still 565") 11.8:1 Comp. And 152 more hp and a HP/cu in of 1.65

    The current engines BMEP is (706x150.8/565) =188 psi BMEP

    The BMEP is only 2% more, the hp/cu in. is 19% more and the engine make’s 152 more HP. So after all this, I now know why Jon Kaase has never looked at a BMEP number on a engine and he build's 1,700 HP 813" IHRA engine's.

    Back to square one...

    Sleeper CP
    Big Inch Ford Lover
    Last edited by Sleeper CP; 01-23-2008 at 06:32 PM.

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