Cylinder Pressures @ 40 LBS of Boost?

B

BlownV6

Well-Known Member
Joined
Mar 26, 2004
As some of you know I run a Stage 2 Motor in a Dragster with a 8.71 Hi-Helix Kebelco Blower with a mechanical alcohol fuel injection. I have had various growing pains with this combination but it has gone a 4.59 @ 155+ in the 1/8 mile. One of the problems has been a lack of power especially above 6500 RPM, after much reasearch and advise from other racers I am completely convinced that lack of enough valve spring pressure has been the problem and that will be corrected. Second problem is connecting rods! I have managed to bend 4 sets of rods (Corollos, Olivers and Crowers) and I am not sure why? I think its to much cylinder pressure for the rods to handle, others think its got to be hydraulicing. If it is hydraulicing I can't come up with how, I take various precautions to prevent it but who knows?

Reason for this post! Oliver is currently building me 4 sets of rods that will withstand 300 HP per cylinder. Last week I spoke with the engineer there about the issue of all the bent rods and he gave me the formula (Basic) square inches of the bore 4.030 = 12.755 x 1300 PSI cylinder pressure = 16,582# and said that Oliver rod will yeild approximately 55,000# and thus it had to be Hydraulicing! After mauling that around for a day or two the 1300 PSI just didn't sound right to me. A buddy of mine dug up an article on SCE Gaskets where dutweiller claims on a SBC making 1800 HP @ 42# Boost the cylinder pressures are 3200#! How would the compression effect this ?? More compression is more pressure?? Because I use alcohol I have used 9.6, 10.5 & 11.6 to 1 compression ratios. The higher the ratios the quicker the rods bent. If you take this square inches of the bore 12.755 x 3500 PSI = 44,642# and along with other variables could explain the problem. My real concern is that they are building my new rods with engineering based on 1300 PSI cylinder pressure and I have way more than that!?!?

Questions:
1. Does anyone know the cylinder pressures of certain combinations?
2. How does the compression ratios effect those pressures?

Thanks, Mike Booher:cool:
 
Well it's impossible to predict the cylinder pressure of a combo, because that pressure is dependent on timing, etc. I think the Duttweiler article was referring to a calculated BMEP type pressure, not an actual measured pressure. 3200 psi is probably way too high to be able to keep a lid on. An engine structure can only keep a lid on so much cyl pressure. Fortunately for us (and our engines), the real mechanism that allows us to make more and more power is not as obvious as the initial assumption of cyl pressure going out the roof.

If you want a good read on some of what is actually going on in high hp engines, start with this; it's one of the few articles out there that gets it right:

http://www.kb-silvolite.com/article.php?action=read&A_id=7

I worked at an engine research facility for several years in the late 90's and happen to have cyl pressure measurement gear that I use on my own S2/GN1's engine (and other customer's engines). I would estimate (roughly) that your engine can probably tolerate about 1600-1800 psi peak pressure max before its starts to blow gaskets. Using a 4" bore, the corresponding peak force pushing down on the piston (as well as up against the head) at 1800 psi is then about 22,600 lbs.

TurboTR
 
PS- we can pretty readily get a feel for just how close you may be to a hyd lock condition. We can estimate your airflow, estimate the corresponding a/f ratio range and thus back out the fuel flow, and then change that to volume terms. Then compare that fuel volume flow (in terms of cc/stroke) to your actual, physical chamber volume at TDC.

Paging JDEstill <tap tap Tap> :)

TurboTR
 
Thanks for the info , I will have to read through all that info several times to totally grasp all that is there. definetly very intresting ! so you have pressure reading equipment? is that done with some sort of spark plug adapter ? I dont use water in the block so I use copper head gaskets with o-rings in block and receiver grooves in the head which provides a reliable seal but eliminates the fuse link for high cylinder pressure. at what kind of HP levels are you seeing 1800 # CP and what CR and timing ??? In dutweillers article I believe it stated that the 3200# CP was estimated !! is it possible ?? is it possible that mine is higher than 3500# ??? mike
 
Yes and yes, plug adapter :)

No, I doubt seriously that you ever see anywhere close to 3500 psi, at least not for more than a few engine cycles that is ;) You have a good gasket setup I'm sure, but when excessive force is applied (upwards) the head lifts up and away, which is what allows it to blow. Even at "only" 1800 psi or so, that is like hanging 6-7 GNs off the center of the chamber; it's not hard to see that the head will deflect at that point...

TurboTR
 
Don't most people with those combinations...... Blown, alcohol, lots of boost where cylinder pressure is very high, use aluminum rods??? Is this an option? What is the advantage/disadvantage for aluminum rods? I'm just asking as a possible solution, I could be way off.

Pat
 
I hear the aluminum rod question a lot. one issue is clearance with the cam, I am using a 3.750 stroke crank which moves the rod even closer to the cam and pretty much eliminates the use of an alum rod unless i remove alot of material. I went with titanium rods which i was told is somewhere in between the steel and alum rod as far as absorbing some of the shock load. titanium is made from alloys that includes alum and thus is much softer than steel. My crew chief called me yesterday with info that R & R rods which make a lot of rods for blower motors claims in their info that their rods are rated at 70,000 # ?? there must be a reason for this and he is going to call them monday morning. I have been hearing lately that blower motors are a lot harder on rods than a turbo motor because of the instaneous boost, and that this is probably happening at the hit of the throttle ??? maybe the pro-mod guy has some info ??? mike
 
Well, if I understand correctly, probably the biggest reason that turbo engines are "known" to be relatively easy on rods is because of the exh back pressure. The highest "pull" load on the rod (tensile load) occurs at high rpm as the piston is decelerated to a stop, and then changes direction at TDC. Having exh backpressure in the cyl, acting over the surface area of the piston during that process on the exh stroke significantly reduces the force required of the rod to make this happen. The tensile load will be the most likely to break the rod; a column can generally take a higher compression load than tensile load.

Let's see what sort of estimated fuel volume flow we get and check how close you might be to a hyd lock condition. Do you have a hp estimate for your combo? Or better yet, can you tell how much fuel you consume during the pass?

For grins I'll take a rough guess and say 1500 hp at 7000 rpm. If you were burning gasoline, you'd need about 900 lbs/hr of fuel flow to support that hp. Let's assume gas is about 6.2 lbs/gallon. Thus you would be burning fuel at the rate of about 145 gal/hr at that power level. Or 0.040 gal/sec. At 7000 rpm you have 116.7 revs/sec. 3 intake strokes per rev on the v6 gives 350 intake strokes per second. Each of those strokes will thus inhale 0.04gal/350 strokes = 0.000114 gal/stroke :) Knowing that 3.8L (3800cc) is about 1 US gal, we get 0.434cc of fuel per intake stroke...

I think alcohol requires roughly 2x the amount vs gasoline... lets call it roughly 1cc/stroke then. Gives us a ballpark idea at least. Your chamber volume is probably in the area of 40cc at TDC, no? Not close at all to a hyd lock condition under normal circumstances if the above is true. Not sure what might happen if say, a cyl starts misfiring. Maybe it could conceivably fill with enough liquid while the engine continues to cycle to eventually hyd lock?

Is there a chance of a sneaky leak, filling a cyl with fuel like when the engine is off perhaps? Interesting that on many late v6 F*rds for example there was a problem with an intake seal that eventually allowed a cyl(s) to fill with coolant, thus bending a rod on the next startup. Just thinking out loud. I assume yours have bent during the run...

We'll see what volume mr Estill comes up with; might be very different from my late night, paper napkin, rough "estimate" (lol). I sent him mail asking him to please chime in.

TurboTR
 
I'd find it hard to believe they are telling you that your engine is hydraulicing on alchohol as that is pretty hard to do and still make a good pass. I mean alchohol is stoich at a 6.7/1 AFR. It would be damn near impossible to hydro lock on a good clean pass as the engine just wouldn't run. I mean I have some local friend that own a top fuel dragster and Nitro has a stoich of something like 1.6/1 AFR which is pretty much 1 part fuel to 1 part air when you are trying to not break parts. They rarely have any hydro-lock issues as long as something doesn't go wrong wether it be fueling or ignition issues. Also you guys need to remember that peak cylinder pressures occur about 20 degrees after top dead center if all is well. In other words you may have a peak cyl. pressure of 1800,2000, 3000 before you fire the mixture and after the mixture is lit that figure may be 10-20 times as much. I'm thinking something else is the real problem here. Somehow you have a mechanical gremlin that is bending rods. What kind of injection are you running??? and where are the injectors in relation to the blower??? are the injectors only in the hat, or in the hat and the intake, or in the hat, intake, and ports??? Are you running a belt driven pump??? does it have a return??? Do you kill the fuel supply as you go through the traps, or are you killing the ignition? Just curious, Drew
 
Yes I agree, it certainly should be in no danger of hydro lock under normal running. Look at all the 5000 hp blown alcy engines... But otoh, what might happen if say a misfire comes up during a pass might be a different story(?)

And IMO and ME, a normal, heads bolted on engine will never stand 3000 psi in the cyl, much less 10x that amount. Even a good, stout diesel with say 18:1 CR and at full power and 25 psi boost will likely be limited to less than 2000 psi max in the cyl.

TurboTR
 
Sorry for the delayed response, I have to work to pay for all these parts. turboTR your math is better than mine right now, but I did do some math for you. cam driven waterman 1100 super sprint pump at 4000 =5.36gpm@ 30 psi , at 6000 = 7.96 gpm @ 60 psi and at 8000 = 10.65 gpm @ 100 psi. I use 6 nozzles in the hat , 2 in the rear of the blower and one in each port . the total square inches of these is .0178466 and I use a .050 main return which is .0019635 so for basic figuring 89% of the fuel being pumped is going through the motor and 11% is returning to the tank. if it is hydraulicing it has to be at start up or shut down! I do use the fuel shut off to let the motor suck all the fuel out of the system before turning off the ignition. IN doing plug checks I never find excessive amounts of fuel in the cylinders and thats why i cant believe that its hydraulicing and think it must be to much cylinder pressure.

called several people today about cyl pressure including Jim Oddy but got no real answer , so I called Oliver and talked to the engineer on my rods . seems he was recently working on a pro-stock project where 1300 to 1400# cylinder pressure is the norm and so that just came out. turns out that my rods are designed to yield 165,000 #. now I dont how much cylinder pressure that equates to but they based it off several pro-mod combinations that make over 400 HP per cylinder !!! that works for me. this is an issue that I would still love to learn more about. mike
 
Yes it is an interesting topic, one of the most interesting on these boards. Afraid you will find ~ no real info on it though, but alot of misconception- especially in the popular press :) Not many race people have measured cyl pressure either. At the research inst we measured and watched it on every project engine in real time; each dyno cell had a dedicated analog scope for readout. Your source sounds about right to me on the 1400 psi number; maybe a bit more, but probably not alot more. If you ever want to see just how much yours makes, well hey let's measure it- we have the technology :)

Is hard to equate that into a rod yield number though, whatever they mean by that. I would think though, if you had enough cyl pressure to bend rods by sheer brute force, that your head seal would probably go away long before that...

Wow at 6000 rpm you're pumping 8 gpm; that is pretty darn healthy, could fill a 50 gal drum in 6.5 min at that rate :)

TurboTR
 
From my conversation with the engineer at Oliver , yield means the point at which any material , steel, alum,titanium loses is shape and does not return to its original shape. example all connecting rods , pushrods , pistons ,head bolts ,etc. flex , twist , distort , stretch , compress, etc. but as long as they return to there original shape they did not yield !! the yield of any given material is measured by the square inches of its cross section. if the material in steel rods is 110,000# per sq inch and the cross section in the middle of the rod is .5 sq in , then the rod has a yield strength of 55,000 # when we see rod bolts advertised at 300,000# this is the material that it is made from and will have a yield equivilent to its cross section. yields generally are engineered into connecting rods so that they do not stretch apart and is directly effected by the weight of the piston, pin , stroke and RPM, normally a yield that will keep the rod from pulling apart is more than adequate on the compression side !! sometimes its not !!!! back to the fuel issue some of you guys are using 160# injectors about how many gallions per min is that ?
 
How sure are you that you're not running into detonation on your runs? Do you have EGT probes for all six holes, so you can see the relative fuel change over the course of a run?

You mentioned that you have a .050" return from the pump...is it just one stage of return, or do you have a 2-or-3 staged return with an air timer? I'm assuming an Enderle-style barrel valve? Cam-driven monster pumps are great at delivering lots of volume progressively with engine speed, but with a blown/injected setup you've GOT to make sure you're leaning it out right over the entire run. How many gears in your trans?

Floating the valves could definitely cause you a world of headaches, as you mentioned in your first post. How often do you check them, and with what equipment? What is your tolerance for spring fatigue before you replace them? If you've got the power for mid-4's at 7,500+RPM, they may not last as long as you think.

Yes, Pro Mod rods are aluminum. And although they hold up well, we replace them after 16-18 runs in the BAE Hemis. Some of the other don't even last that long. Have you ever actually asked your rod mfg. how many cycles they should last? If I were you, I'd make sure you're pulling the pan after every other run or so, and running through a re-torque until you figure out what's up.
 
Another note of intrest engineer said that metals are tested and rated in some way ( over my head ) for strength using some test in the millions of pounds. I asked him about how titanium compared with alum and steel as far as abrorbing shock and these are the examples he gave steel 33 M # , titanium 17 M #, alum 13 M # so as he said the titanium is very close to alum. I thought that is a plus.

Quick Wrench, I do need help !! been racing 30+ years but this is the first experience with any type of power adder!! since you will understand most of this , here is the scoop. carbon fiber TF hat , 8.71 kobelco hi-helix with front discharge, blower is set back with the discharge basically centered in the center of the engine, tunnel ram intake, enderly nitro barrel valve @ 72 %,one hi-speed poppet set at 150#, powerglide with 6250 stall, use throttle controller on trans brake at 5800. v-300 race-pak with 6 egt, manifold pressure and temp , fuel pressure at hat dist block, oil pres , crankcase vacuum , and ring style head temp sensor. that being said , all that info is great but knowing what to do with it -- I might be lacking !! HELP !!!
 
What is your maximum ignition advance ? Is it locked down at full advance or does it have a curve? How quick does it reach 40lbs of boost ? I'm guessing less than one second here.
Is the fuel curve able to keep up with the boost response?

As far as power loss on the top end....what do your heads flow like? Really hard running alcohol motors have special needs on the exhaust side.
Dale
 
Originaly srarted with 25degrees and started retarding .5 degree every 100 rpm at 6000 rpm. in other words initial timing would remain until 6000 and every 1000 rpm it would lose 5 degrees. this is done with a MSD digital programable ignition and HVC coil. started raising initial timing and 60 ft times ( along with other changes ) went from 1.13 down to .999 . motor never has pulled after about the 60 ft mark so started taking some and eventually all of the retard out and it made very minimal gains in performance down track. best speed at 660 is 155 and this seems off by at least 10 mph. I have made one 1/4 mile pass to get my advanced ET license and it went 7.12 at 186 and I was relieved that it made it to the finish line , it sounded and felt terrible !!! this could all be from the lack of valve spring pressure. Noal Manton a valve spring engineer came up with figures that I need 423 # of seat pressure on the intake to control 40# of boost with alcohol, I had 285# !! he told me that it wouldnt run above 6700 and thats exactly what I feel in the car and see on the race-pac. he claims that with gas this would be reduced by about 30% or 300 # and thats what Ive been hearing since. boost is instaneous ! on the computer you can tell when the throttle opens by the boost. it has 40 # before it leaves the starting beams and thus the fuel does lag behind in in pressure until about say one second. did not flow these heads , i have flow many others and the norm is usally 320 on intake and 240-260 on exhaust . Back to the rod issue , detonation is a real possibility especially during the burnout , we never had the computer on for burnout and i have been informed that i sould look at that closely !!! and that is how i will begin testing with my new motors by getting the burnout right !!
 
Hmm, well just some very general comment here. The timing should decrease with load (MAP), not with rpm. Might be why it feels less powerful on top. Vs rpm it should advance to some max advance number at some rpm, say full advance by around 3k or so, then level off or perhaps bump up slightly as you rev towards red line. If you are really pulling about 10 deg as the engine goes from launch to red line, might easily help explain why it is "down" on power some up top. Vs load it should be a pretty constant downward slope as MAP increases. Just some thoughts. You may not have the equipment to achieve this sort of curve.

The way yours runs with 6000 stall and all, I assume that once it clears the line it never sees less than about 7k rpm. Maybe stays in a narrow band from around 7500 to 8500 or so? If that's the case, would think the timing should be some known constant, fixed, safe but still powerful value in that narrow range, not dropping radically as it advances in rpm. Hope that makes sense.

Plus, if the fuel lags as you say, of course it could be doing something nasty in the cyls for many cycles until it catches up :)

TurboTR
 
What I was saying is I used to take timing out as the RPM increased , but over time because it would not run above say 6500 i started removing the timing curve , ended up with a locked curve , meaning what ever the initial timing was was what it had through out the run. this never really showed much difference in the ET or speed !! it just doesnt want to accelerate with or with out timing !! thats what has me puzzled 40 # boost and 850 average HP just doest add up !!! hope the valve springs bring it to life.
 
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