OT: Real Engine Physics

Just a few real world questions I've had knocking about my head. Why is the V configuration generally better for race engines? In F1 if it's better to have a wider V angle for lower center of gravity lower profile, etc. (ie. the Renault 110 degree engine) why doesn't somebody make a horizontally opposed boxer? If that isn't good why does it work well for Porsche and Subaru? What are the advantages with a pushrod? I understand they have a bit of a lower center of gravity but have more moving parts and are heavier. Why are the still around? I understand the NASCAR it's probably mandated and maybe in Trans-Am but I think the C5-R and the Panoz also choose to run a push rod, why? TIA

I heard you can get more low-end torque from a pushrod. Not sure why. Valves open - valves close. Why would the mechanism for opening them affect low-end torque? Oh sure, I answer with a question :P Euan Gilmour wrote:

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> > Just a few real world questions I've had knocking about my head. > > What are the advantages with a pushrod? I understand they have a bit of > a lower center of gravity but have more moving parts and are heavier. Why > are the still around? I understand the NASCAR it's probably mandated and > maybe in Trans-Am but I think the C5-R and the Panoz also choose to run a > push rod, why? > > TIA

-- Jason Monds http://www.proracingclub.com - Pro Racing Club! (GPL) http://www.scprc.com - Stock Car Division of the PRC. (N4, Fun Run Sims) (Please remove 'no extra spork' when replying)

Euan Gilmour <egilmour@*NOSPAM*home.com> wrote in message news:mlvl7.140720$[email hidden]... > Just a few real world questions I've had knocking about my head. > > Why is the V configuration generally better for race engines?In F1 if Packaging. A straight-10 would be horrible to fit in! > it's better to have a wider V angle for lower center of gravitylower > profile, etc. (ie. the Renault 110 degree engine) why doesn'tsomebody make The Renault engine isn't working too well... I've heard mutterings about airflow and inlets, given there's a central airbox. > a horizontally opposed boxer? If that isn't good why does it workwell for > Porsche and Subaru? Intakes, I suspect, and the fact that they're not quite so critical on a road car. Flat-12s have been done by Ferrari among others; the BRM H-16 is a double flat-8, too. > What are the advantages with a pushrod? I understand they havea bit of > a lower center of gravity but have more moving parts and areheavier. Why I believe the major *dis*advantage is actually the compliance in the valvetrain - pushrods and rockers bend and flex more than directly-operating OHC systems. The main advantage I know of is that you can run both banks of cylinders from just one camshaft, running down the centre of the 'V'; the camshaft drive is also shorter and neater. > are the still around? I understand the NASCAR it's probablymandated and > maybe in Trans-Am but I think the C5-R and the Panoz also choose torun a > push rod, why? Marketing, I suspect... they want to base the race engines on the ones in their roadgoing cars. There may be other reasons though - I'll await further comments! Jonny

>Why is the V configuration generally better for race engines? In F1 if >it's better to have a wider V angle for lower center of gravity lower >profile, etc. (ie. the Renault 110 degree engine) why doesn't somebody make >a horizontally opposed boxer? If that isn't good why does it work well for >Porsche and Subaru? > A V configuration is probably the most compact, for one. I think one important reason the V type is most common is because it allows for a good intake manifold design. With an inline four cylinder, if you've only got one air intake/carburetor, it's going to be closer to the middle two cylinders than the outside two. Therefore, the intake runners will be longer for two of the cylinders than the other two. For optimum cylinder charging however, you want to tune the intake runner lengths (the pipes that go from the carburetor to the cylinder head intake port) to a specific length for maximum cylinder charging at a certain rpm, and you want them to all be as close to the same length as possible. If the lengths are different, you end up with two pairs of cylinders that make peak torque at two different speeds. That might make a flatter torque curve, but in high rpm racing it's generally more desirable to have one really high torque peak, as long as the rules allow enough transmission gears. (Some engines offset this somewhat by using wierd camshafts that have different valve timing for different cylinders.) Of course, a single carburetor V-8 is little more than two inline four cylinder engines joined together, so I don't know exactly what point I was just trying to make there :-) Anyway, horizontally opposed engines require more than one carburetor/intake unless it doesn't need to spin very fast. From a book I'm looking at now: "In multi-cylinder engines the cylinders and their disposition are arranged to eliminate as many of the primary and secondary forces and moments as possible. Complete elimination is possible for: in-line 6 cylinder or 8 cylinder engines, horizontally opposed 8-cylinder or 12-cylinder engines, and 12-cylinder or 16-cylinder V engines." Those engines will be the smoothest and can probably be run to the highest rpm before something breaks. Of course, it doesn't say that a V-8 is one of the good ones, so..... ? A couple years back I wrote some engine simulation stuff, but didn't get into multi-cylinder models for a bunch of reasons I won't bore you with :-) I do recall realizing that the cylinder bank angle would have an impact on a couple of things. The cylinder firing angles was one. Since you'd want cylinders to fire at the same angle relative to each other, one firing at 0 degrees, the next firing at 90 degrees (compared to the first one), the third at 180, etc., all the way around, you probably will get a crazy wobble with a bank angle of 10 degrees instead of perfectly flat, 60, or 90 degrees. This book talks briefly about a twin cylinder horizontally opposed engine that fires at 180 degrees, 540, 180, 540, etc., rather than 180,360,180,360, (or is that 180-0-180-0 ?) which would cancel out the forces and run nice and smooth, as a bad example. It's sort of like having the pedals on a bicycle at 90 degrees to each other instead of 180, it makes things a bit wobbly and the engine won't spin very fast without breaking something. However, if it's cheap to build it this way, it fits where you want it, and it doesn't need to spin at 100,000 rpm, it might be just fine. One other thing that bank angle and V/inline/etc. arrangment may have an impact on is the timing of the opening/closing of the valves and the pressure pulses that move through the intake and exhaust runners. It's desirable to have them in tune (by selecting appropriate lengths), and I think changing the bank angle can effect both this and balance at the same time. Anyhow, these all interelate along with how the throws on the crankshaft itself are arranged. That doesn't really answer your question, but it's the best I can do right now! >What are the advantages with a pushrod? I understand they have a bit of >a lower center of gravity but have more moving parts and are heavier. Why >are the still around? Pushrod engines are cheaper to build, that's probably the only benefit. The pushrods are relatively flexible compared to the camshaft or tappet, so the valve lift doesn't follow the cam profile as well as an overhead system might. There's also more mass to move around, so you need stiffer springs, which wears out the camshaft faster and also causes power loss from the added friction. Perhaps NASCAR and the others might have mandated them to keep costs for the teams down. Otherwise, they all would spend millions more designing four/five valve overhead cam systems, only to get restricted somehow to keep the speeds down. Just my opinion though, I don't really know for sure. Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

I think the reason for this is that a pushrod valve train has an extra lever in each mechanism: the rocker arm. This can provide mechanical gain so that the valves can be opened farther with a milder cam grind. Thus the cam runs smoother with less float, etc., at least until high revs where the extra mass, spring pressure, and flex degrades the operation. -- Scott Willingham Jason Monds <[email hidden]> wrote in message news:[email hidden]... > I heard you can get more low-end torque from a pushrod. Not sure why. > Valves open - valves close. Why would the mechanism for opening them > affect low-end torque?

From what I've learned by watching TV... the biggest problem with a wide angle V engine is chassis rigidity. The engine mounts must be lower down because of the wide V and this increases the chance the chassis will flex. Which...I guess is a bad thing. dave henrie "Euan Gilmour" <egilmour@*NOSPAM*home.com> wrote in message news:mlvl7.140720$[email hidden]... > Just a few real world questions I've had knocking about my head. > > Why is the V configuration generally better for race engines? In F1if > it's better to have a wider V angle for lower center of gravity lower > profile, etc. (ie. the Renault 110 degree engine) why doesn't somebodymake > a horizontally opposed boxer? If that isn't good why does it work wellfor > Porsche and Subaru? > > What are the advantages with a pushrod? I understand they have a bitof

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> a lower center of gravity but have more moving parts and are heavier. Why > are the still around? I understand the NASCAR it's probably mandated and > maybe in Trans-Am but I think the C5-R and the Panoz also choose to run a > push rod, why? > > TIA > >

If you want a great book on these topics and similar ones, I highly recommend "The Design and Tuning of Competition Engines" by Philip H. Smith (ISBN 0-3876-0140-1). A link directly to the publisher's site is below. http://www.bentleypublishers.com/product.htm?code=G140 -Bill

"Euan Gilmour" <egilmour@*NOSPAM*home.com> writes:

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> Just a few real world questions I've had knocking about my head. > > Why is the V configuration generally better for race engines? In F1 if > it's better to have a wider V angle for lower center of gravity lower > profile, etc. (ie. the Renault 110 degree engine) why doesn't somebody make > a horizontally opposed boxer? If that isn't good why does it work well for > Porsche and Subaru?

A boxer probably is heavier than at least a narrow V engine, but it also is much wider which may be the real problem. But boxers have been used even in F1, so it all ends up in weighting advantages against disadvantages of different designs.

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> > What are the advantages with a pushrod? I understand they have a bit of > a lower center of gravity but have more moving parts and are heavier. Why > are the still around? I understand the NASCAR it's probably mandated and > maybe in Trans-Am but I think the C5-R and the Panoz also choose to run a > push rod, why? >

I'd say the overhead cam engine is both heavier and have more moving parts, especially if it is a V or boxer engine. And a lot more complicated on top of that (e.g. having the camshaft(s) far from the crank complicates the camshaft drive a lot). _ Mats Lofkvist [email hidden]

Sounds reasonable. Thanks :) Scott Willingham wrote:

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> > I think the reason for this is that a pushrod > valve train has an extra lever in each > mechanism: the rocker arm. This can > provide mechanical gain so that the valves > can be opened farther with a milder cam > grind. Thus the cam runs smoother with > less float, etc., at least until high revs where > the extra mass, spring pressure, and flex > degrades the operation. > > -- Scott Willingham > > Jason Monds <[email hidden]> wrote in message > news:[email hidden]... > > I heard you can get more low-end torque from a pushrod. Not sure why. > > Valves open - valves close. Why would the mechanism for opening them > > affect low-end torque?

-- Jason Monds http://www.proracingclub.com - Pro Racing Club! (GPL) http://www.scprc.com - Stock Car Division of the PRC. (N4, Fun Run Sims) (Please remove 'no extra spork' when replying)

"Mats Lofkvist" <[email hidden]> skrev i meddelandet news:[email hidden]... > > "Euan Gilmour" <egilmour@*NOSPAM*home.com> writes: > > > Just a few real world questions I've had knocking about my head. > > > > Why is the V configuration generally better for race engines? In F1if > > it's better to have a wider V angle for lower center of gravity lower > > profile, etc. (ie. the Renault 110 degree engine) why doesn't somebodymake > > a horizontally opposed boxer? If that isn't good why does it work wellfor > > Porsche and Subaru? > > A boxer probably is heavier than at least a narrow V engine, but > it also is much wider which may be the real problem. But boxers have > been used even in F1, so it all ends up in weighting advantages > against disadvantages of different designs. There is also the issue of routing exhaust in boxers, requiring that the engine aswell as the gearbox to be mounted higher. -Bj�rn