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Old 01-24-2006, 05:02 AM
KM2 KM2 is offline
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Straight eight and 7 cylinder engines?

Straight eight engines have slowly vanished from the automotive scene in the 50s. Now I understand that with the then current material technology and bigger displacement engines the demands made on a relatively long crankshaft and the overall length of the engine made designers switch to a V configuration. As I am not an engineer (and had no luck in searching on the net for clues), I'd like to know whether there are other reasons for not choosing such a configuration. Specifically, what I'd find very intertesting would be a straight eight made as a combination of two motorcycle 4 cylinder engines, which are reasonably small in displacement and short lengthwise, meaning that the mentioned two problems could be overcome.

Additionally, I came across 7 cylinder engines being used to propell underwater torpedoes (Saab Underwater T2000). They do not seem to be radial (from my understanding) and use something called axial valves. Anyone know more about it?

Additionally, I am interested in some decent literature on the subject. Browsing through Amazon does yield results, but most books are in the 200 US$ plus bracket, and that is used. Any suggestions welcome
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Old 01-24-2006, 05:47 AM
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There are several reasons why Straight eights have gone and are being replaced by V engines. First the occupied space is important, even if you add two motorcylcle engines together it will still be a long engine. A second disavantage is that it becomes impossible to put a straight eight in a transverse position, it will simply become woo wide. (The Miura V12 was the best they could ). That rules out any option to use it for a FWD car if so desired.

A third issue might be of more technical nature, and relates to the length of the crankshaft. The larger the number of inline cylinders, the longer the shaft has to become, and the more difficult it becomes to reduces vibration, certainly at the high revs modern engines are now achieving.

I'm not familiar with the 7 cylinder torpedo engine that you mention, but in marine diesel propulsion you can get almost any configuration you want, just by putting together more cylinders. 7 cylinders engines are rather common, also 9,10,11,12,13 and even 14 cylinder versions have been produced. In terms of vibrations the single crankshaft poses the same problem as described above, but these engines barely go over 100 revs (a minute ), while producing over 100,000 BHP.

And welcome to UCP, hope you are going to like it here.
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Old 01-24-2006, 06:56 AM
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Straight eight and 7 cylinder engines?

The application would clearly be RWD

It would still mean a shorter engine than most inline 6s or even fives, so that could be handled. As for crankshaft vibration, given that it too should be shorter (and that the inline sixes of say BMW are very smooth), I guess it's possible?
There is also a question of balance (which partially influences vibration) IIRC, with a straight or boxer 6 being perfectly balanced, with the next great configuration being a V12. Correct me on this one, it's been a long time How does an inline 8 do here compared to a V8?

Coming back to the maritime 7 cylinder. What makes various odd cylinder numbers impractical for automotive use, then, vibration? Is it not really possible to build them in a way to rev to 6 or 7000 rpm in that configuration (V7 for instance)?

Thanks for the prompt answer and help
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Old 01-24-2006, 07:26 AM
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I am not convinced that putting together two four cylinders would result in a shorter engine than one 6 cylinder with the same displacement, or even making a single I8 block. Here is pic of an Alfa 8C engine, where the power is taken from between the 4th and 5th cylinder, probably because they built together two 4 pots.
I also added a picture of the Peugeot 907 V12 engine, where they put together 2 V6 engine, resulting in a larger space between cylinders 3 and 4.

I am not enough of an engineer to tell you if an I8 has a natural balance, the I6 as you rightly stated has, and BMW is the perfect proof of that. A V6 ideally is fitted with additional balance shafts, as there is no natural balance.

In principle there should be no reason why not to build an inline 7 or even a VR7 (like VW's VR5, with 5 cylinders under a narrow angle, covered by one valve cover. I guess the figure 8 gives more prestige than 7.
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File Type: jpg alfaromeo8CMengine.jpg (299.4 KB, 55 views)
File Type: jpg Peugeot907-1s.jpg (251.2 KB, 49 views)
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Old 01-24-2006, 08:23 AM
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Straight eight and 7 cylinder engines?

I do agree that with the same displacement (especially if your bore would be similar) the I8 would be longer than the I6. On the other hand, a 1.2-2.0 litre I8 will probably be shorter than an automotiv I6 of say 2.5 - 3.0 litres. It's not a totally fair comparison, but on the other hand a motorcycle derived engine will rev significantly higher and allow you a comparable if not superior power output even with such a power discrepancy. Of the two V8 motorcycle derived engines that I know off, the Powertec unit (a combination of 2 1300ccm Hayabusa units) gives off 361hp from 2.6 litres, only because it is limited to "only" 10500rpm (the engine could take more but it is not necessary), the Drysdale unit (own block with Yamaha FZR 400 heads) gives in the region of 160hp from 750ccm. Both comfortably more than per litre than the 3.0 litre from BMW, which gives up to 265 hp or the 3.2 in the M3 with 343hp.

And imagine being the only manufacturer offering a 7 or 9 cylinder car
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Old 01-24-2006, 08:27 AM
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Quote:
Originally Posted by KM2
And imagine being the only manufacturer offering a 7 or 9 cylinder car
yeah being different worked for the Reliant Robin three-wheeler

Different isn't better

Finding a firing order that avoided huge stresses and vibrations would be VERY difficult !!!!
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Old 01-24-2006, 08:38 AM
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Quote:
Originally Posted by KM2
I do agree that with the same displacement (especially if your bore would be similar) the I8 would be longer than the I6. On the other hand, a 1.2-2.0 litre I8 will probably be shorter than an automotiv I6 of say 2.5 - 3.0 litres. It's not a totally fair comparison, but on the other hand a motorcycle derived engine will rev significantly higher and allow you a comparable if not superior power output even with such a power discrepancy. Of the two V8 motorcycle derived engines that I know off, the Powertec unit (a combination of 2 1300ccm Hayabusa units) gives off 361hp from 2.6 litres, only because it is limited to "only" 10500rpm (the engine could take more but it is not necessary), the Drysdale unit (own block with Yamaha FZR 400 heads) gives in the region of 160hp from 750ccm. Both comfortably more than per litre than the 3.0 litre from BMW, which gives up to 265 hp or the 3.2 in the M3 with 343hp.

And imagine being the only manufacturer offering a 7 or 9 cylinder car
with the risk of slightly running in circles, why would the Powertec unit use the V8 set up when putting two bike engines together? It may also have to do with the location of the race engine, which is behind the driver, but before the rear axle. To achieve that with a similar displacement I8you will need a significantly longer wheelbase, with all the handling consequences thereof.
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Old 01-24-2006, 09:04 AM
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Straight eight and 7 cylinder engines?

Firing order probably does have something to do with there not being 7 or 9 cylinder engines out but as there are some for marine applications, it shouldn't be impossible

I imagine that the compactness is of vital importance for racing, as it gives you more possibilities to get the weight distribution and aerodynamics right. On the other hand, if you want to have a retro looking roadster or in fact a replica of some 30s machinery, the length of the I8 would be less of a problem (on the other hand it is narrow, not exactly a disadvantage either).

If you want to be different, you should count on occupying a very small niche and stick to it, without grandiose plans in the back of your head. Plus different, in order to be accepted, should not be worse and quite possibly needs to be better. And Robin Reliant cannot exactly boast of uniquely practical or other appealing characteristics?
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Old 01-24-2006, 09:25 AM
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Quote:
Originally Posted by KM2
On the other hand, if you want to have a retro looking roadster or in fact a replica of some 30s machinery, the length of the I8 would be less of a problem (on the other hand it is narrow, not exactly a disadvantage either).
interesting that you should say that, here is a replica of the famous Auburn Speedster, where the original I8 has been replaced by a Ford V8
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Old 01-24-2006, 10:06 AM
KM2 KM2 is offline
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Works for the Auburn but a Bugatti 35B replica is just plain wrong with an I4 engine

As for the other replica builders, they usually do not develop engines themselves, and as there are no I8s on the market, they work with what is easily available. From a maintenance point of view that makes a lot of sense. That engines can profitably be developed for small production runs has been demonstrated by TVR and if not much goes wrong Connaught (2.1 litre V10).

Last edited by KM2; 01-24-2006 at 10:08 AM.
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Old 01-24-2006, 10:31 AM
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NSXType-R NSXType-R is offline
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It's off topic but Acura has an I-5 engine in the first generation 2.5 TL. I was always wondering how the engine would be balanced.
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Old 01-24-2006, 10:42 AM
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With a balancer shaft the same as the Audi Quattro 20V no doubt

Was sent a link to a good site with the basics --- http://www.autozine.org/technical_sc...ne/smooth1.htm
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Old 01-24-2006, 10:57 AM
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first (modern) five cylinder was the Mercdes 240D 3.0, from the mid seventies... It ran quite OK.
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Old 01-25-2006, 03:47 AM
KM2 KM2 is offline
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Drive a Volvo 850T-5R with a 5 cylinder engine and the engine is wonderful. Even after the car has run 200000 miles It has a very distinctive sound, more characterful than a six IMHO.

Excellent site, btw. explaining it in a language I can understand. No exotics, though. For an I8 I imagine it's close to a flat plane V8, meaning two balancing shafts?

Would rotary engines (with an even cylinder number) work like a boxer? How about with odd numbers?

Last edited by KM2; 01-25-2006 at 04:08 AM.
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Old 01-26-2006, 11:35 AM
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Quote:
Originally Posted by KM2
Would rotary engines (with an even cylinder number) work like a boxer? How about with odd numbers?
Dude...u better be joking Rotarys dont have cylinders lol... needless to say they they rotors. And they can work on ANY number of rotors due to not needing balancing like a piston engine (one piston balancing its corosponding piston) as there is only ever one direction of motion, not like the thrashing back and forward of a piston engine.
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