V8's vs I6's - Page 4 - PeachParts Mercedes-Benz Forum

Shoomakan · #1 09-03-2010, 04:44 AM

Somebody on page 1 said I6's are more compact. That's the farthest thing from the truth, man. V8's are essentially as long as inline 4's, but they're alot wider. And width is not something usually lacking under any car's hood. The I6 is one of the longest engine configurations around, not to mention the heaviest.

I dropped an old 5.0 V8 from a Mustang into my old 240SX, and the front end of the car rose a few inches. That V8 was significantly lighter than the 2.4 litere KA24 that was in the car. V8's are light, too. New all aluminum units are even lighter.

V8's are also extremely revvy when they're breathing properly, more so than a inline 6. I think it's because they have double the displacement, but not double the weight. Less relative intertia, I guess.

So, since this page is about V8's vs V6's, here is what I think:

V8:

Lighter
Easier to package
Lots more power usually (unless they're the same displacement, which is rare) (ex: TVR 4.0/4.5 inline 6, I think those came on the Cerbera)

I6's:

Usually revvier, but again, depends on the V8 (E92 V8 comes to mind)
Heavier
Less vibration
More linear powerband

And I'd like to correct the assumption that V8's spend more fuel. Highway mileage, V8's are almost always more economical than I6's, usually due to way longer ratios.

V6's I find quite useless, to be honest. V8's are just a TAD bit bigger and have better characteristics.

Any other opinions? I'd really like to hear 'em.

MAG58 · #2 09-03-2010, 11:11 AM

Hmm....
V8's are still not light engines.

KA24DE = 368lbs (Source: Drifting.com)
Ford 5.0L Weight = just under 460lbs (source: gomog.com)

I'm not sure where you got that a 302 ford is anywhere near the weight of an I4.

V8's require a substantial amount of work to be a 'revvy' engine because by the nature of the design, they require heavy first and last counterweights. Unless it's a flat plane motor. Then they're very displacement limited and still vibrate. I've built both, and if they're both setup right, the I6 will still out turn a V8 unless it's a small displacement motor vs. a larger displacement under square I6. Again, perfect balance means a bunch of weight can be taken out of I6 bob weights. Unless you do like BMW and put heavy metals in the counterweights ($$$) you're dealing with big, ungangly counter weights lest you like vibrations.

And what are your statistics for highway mileage here? The LSx is the only engine that comes to mind that is close to the modern I6's in mileage relative to body weight. The Atlas I6 did very well against the 5.3 LSx despite having over a liter of displacement disadvantage. I'm not sure what ratio you're talking about here, but there are also surprisingly few V8's that have decent rod/stroke ratios and even fewer that have bore/stroke ratios near/above 1:1 any more. They keep trying to be more compact which means bore goes away in leiu of additional stroke, though unfortunately rod/stroke ratio suffers.

Yes, Iron I6's are heavy because they require a substantial amount of bracing within the engine to dampen NVH of a crank that long. But the aluminum I6's are not heavy at all. Have you weighed the current BMW N series Aluminum/Magnesium I6's? They compare VERY well to any OHC V8 save maybe the featherweight UZ engines.

Packaging is a relative issue. I can tell you right now that an I6 is much more friendly to work on in the long, narrow hood of my 240Z whilst a V8 accommodates much better to the short, wide engine bays of the current trends.

RBYCC · #3 09-03-2010, 03:21 PM

Quote:

Originally Posted by MAG58

Hmm....
V8's are still not light engines.

KA24DE = 368lbs (Source: Drifting.com)
Ford 5.0L Weight = just under 460lbs (source: gomog.com)

I'm not sure where you got that a 302 ford is anywhere near the weight of an I4.

V8's require a substantial amount of work to be a 'revvy' engine because by the nature of the design, they require heavy first and last counterweights. Unless it's a flat plane motor. Then they're very displacement limited and still vibrate. I've built both, and if they're both setup right, the I6 will still out turn a V8 unless it's a small displacement motor vs. a larger displacement under square I6. Again, perfect balance means a bunch of weight can be taken out of I6 bob weights. Unless you do like BMW and put heavy metals in the counterweights ($$$) you're dealing with big, ungangly counter weights lest you like vibrations.

And what are your statistics for highway mileage here? The LSx is the only engine that comes to mind that is close to the modern I6's in mileage relative to body weight. The Atlas I6 did very well against the 5.3 LSx despite having over a liter of displacement disadvantage. I'm not sure what ratio you're talking about here, but there are also surprisingly few V8's that have decent rod/stroke ratios and even fewer that have bore/stroke ratios near/above 1:1 any more. They keep trying to be more compact which means bore goes away in leiu of additional stroke, though unfortunately rod/stroke ratio suffers.

Yes, Iron I6's are heavy because they require a substantial amount of bracing within the engine to dampen NVH of a crank that long. But the aluminum I6's are not heavy at all. Have you weighed the current BMW N series Aluminum/Magnesium I6's? They compare VERY well to any OHC V8 save maybe the featherweight UZ engines.

Packaging is a relative issue. I can tell you right now that an I6 is much more friendly to work on in the long, narrow hood of my 240Z whilst a V8 accommodates much better to the short, wide engine bays of the current trends.

You ever build a push rod elephant motor ?

I built a 426 five over 12.5:1 CR Hemi in the late sixties that redlined around 9000 RPM with just a cam and valve train work.
Crank was stock and balanced and block was lined bored.

Bad pic..it's over forty years old !!!

Needed the RPM to trap over 135MPH with a 5.12 rear gear.

The Chevy Rat and Porcupine motors of the era also had the ability to rev without a roller.

The latest model Dodge Motorsports 500CI with 15.1:1 compression and a roller cam redlines at 9500 RPM.

Shoomakan · #4 09-04-2010, 02:44 AM

I don't actually know the weights of both engines, Mag. But the 240SX front end lifted up a full 2 inches at least when I dropped the 302 in there. I assumed it was lighter!

I wasn't talking about bore and stroke and stuff, man. I meant tranny/diff ratios. My old 5.0 would do ~200km/20L with the manual box and 2.73 rear end (highway mileage).

You're right about the packaging thing, but we were talking about 2 different eras of modifying cars, I guess. My dad had an old 240Z, still tells me till today that it and the old Mini Cooper S were his favorite cars of all time.

MAG58 · #5 09-03-2010, 04:16 PM

I'm fully aware any configuration can reach a certain rpm. I was referring to 'revvy' as the ability to change rpm's rapidly and gain rpm's rapidly. If you were running the stock crank there was a ton of rotating mass there. Yes I've seen hemi's and built BBC's and BBF's, none of the cranks are light in the counter weights unless they've had a bunch of work done to them.

RBYCC · #6 09-03-2010, 11:05 PM

Quote:

Originally Posted by MAG58

I'm fully aware any configuration can reach a certain rpm. I was referring to 'revvy' as the ability to change rpm's rapidly and gain rpm's rapidly. If you were running the stock crank there was a ton of rotating mass there. Yes I've seen hemi's and built BBC's and BBF's, none of the cranks are light in the counter weights unless they've had a bunch of work done to them.

Tell me more about the big blocks you have built...
I'm curious...!!!
Maybe I can learn something...

I always was under the impression that there were many more factors then the "crank" when it came to "revving"
You talk a great deal about "rotating mass' but yet to mention the importance and difference of assembly clearances.
Do you build your race motors with wide main clearance ?

How important is line boring and dynamic crank balancing?

Isn't the conversion of cylinder pressure into crankshaft torque another major factor?

What about windage, how do you avoid adding to dynamic weight and inertial resistance in your builds ?

Selection of vibration damper make any difference?

I'm sure you know that the same engine in different chassis with different drive trains "rev' differently?
That's when transient torque comes into play...

dka-66 · #7 12-29-2010, 01:59 PM

Quote:

Originally Posted by RBYCC

I'm sure you know that the same engine in different chassis with different drive trains "rev' differently?
That's when transient torque comes into play...

Quote:

Originally Posted by RBYCC

The most important thing to consider is not the number of cylinders but the engine design and the chassis it's installed in.

The real determinant of how freely an engine, regardless of cylinders, bore stroke, valve train design is the "transient" loss created by the chassis.

I cut and pasted to make this easier.

Can someone want to school me on this? If not can you recommend a decent site or better yet some books on this?

RBYCC · #8 12-30-2010, 04:10 PM

Quote:

Originally Posted by dka-66

I cut and pasted to make this easier.

Can someone want to school me on this? If not can you recommend a decent site or better yet some books on this?

Try to give you a very basic concept...

Let's use the AMG 5.4L Kompressor engine.
I have this engine in a 2005 G55K.
It drives thorugh an automatic transmission and three ( front, center, rear ) differentials.
The same engine in a 2005 E55 drives through an automatic transmission and the rear differential.

The additional power transmission items in the G55K drive train create more loss then the lesser number of items in the E55.

Simply put the same engine with more losses will not rev as freely as the engine with less drive train/chassis losses.

400Eric · #9 09-04-2010, 04:14 AM

And let's not forget how the 308ci Flathead straight six Hudson Hornets tore up bigger, overhead valve V8s on the NASCAR tracks in the 50s!

Seriously, it is really hard to do a proper A-B comparison between I6s and V8s because the I6s for some reason tend to have poorer rod length to stroke ratios and poorer bore to stroke ratios than the smaller V8s do. And yes, we do have to use smaller displacement V8s so we can have similar displacement engines in this comparison. For this comparison to be valid there can't be any other variables.

I'm no BMW guy but their little 3.0 V8 might work for such a comparison but the problem is we would still run into the problem of their sixes having poorer rod length to stroke ratios and poorer bore to stroke ratios than that little V8 does.

You guys are gonna hate this but I think one valid comparison is the Big Three's pick-up diesel engines. They are all similar sized, four valve turbo engines and they are all comparable in performance in both stock and modified form the only difference being that one of them is an inline 6 and the other 2 are V8s. Maybe it winds up being a moot point. (At least as far as trucks go anyway.)

I dunno, like I said a few pages back, I kike em both but Roncallo has me fantasizing about V12s too.

Hey Shoomakan, do you know board member Jay Rash? You guys should hang out!
Regards, Eric

RBYCC · #10 09-04-2010, 12:06 PM

Quote:

Originally Posted by 400Eric

And let's not forget how the 308ci Flathead straight six Hudson Hornets tore up bigger, overhead valve V8s on the NASCAR tracks in the 50s!

Seriously, it is really hard to do a proper A-B comparison between I6s and V8s because the I6s for some reason tend to have poorer rod length to stroke ratios and poorer bore to stroke ratios than the smaller V8s do. And yes, we do have to use smaller displacement V8s so we can have similar displacement engines in this comparison. For this comparison to be valid there can't be any other variables.

I'm no BMW guy but their little 3.0 V8 might work for such a comparison but the problem is we would still run into the problem of their sixes having poorer rod length to stroke ratios and poorer bore to stroke ratios than that little V8 does.

You guys are gonna hate this but I think one valid comparison is the Big Three's pick-up diesel engines. They are all similar sized, four valve turbo engines and they are all comparable in performance in both stock and modified form the only difference being that one of them is an inline 6 and the other 2 are V8s. Maybe it winds up being a moot point. (At least as far as trucks go anyway.)

I dunno, like I said a few pages back, I kike em both but Roncallo has me fantasizing about V12s too.

Hey Shoomakan, do you know board member Jay Rash? You guys should hang out!
Regards, Eric

The most important thing to consider is not the number of cylinders but the engine design and the chassis it's installed in.

Overhead valve will rev different then side valve.
Pushrod versus SOHC or DOHC
Chain driven, belt driven, gear driven cam..
Compression ratio...very important because it's what drives the crank.

The real determinant of how freely an engine, regardless of cylinders, bore stroke, valve train design is the "transient" loss created by the chassis.

An engine will spin freely under no load...it's the load ( transient not constant ) that will alter based on condition how fast and engine can rev.

Put a 15K rpm F1 engine in a G55 chassis...heavy with three locking differentials..
Will it spin up as quickly...???

Common sense....

Reality check Mr. Crank Man...it's the transient load that determines how fast the engine RPM delta will be...

All else about crank design is more text book then real world...

Shoomakan · #11 09-04-2010, 05:24 AM

Yep! Coincidentally, Jayrash and I have been friends for a while. I had no idea he was this popular, though! He's already given me invaluable advice.

400Eric · #12 09-04-2010, 05:42 AM

Yeah, you've gotta love that Jay! He's the man!
Don't ride in a car if he's driving it though!
Regards, Eric

400Eric · #13 09-04-2010, 09:20 PM

"Overhead valve will rev different then side valve.
Pushrod versus SOHC or DOHC
Chain driven, belt driven, gear driven cam..
Compression ratio...very important because it's what drives the crank."

Yes, All of these and more.....

For this comparison to be valid, both engines need to have the same head/combustion chamber design, the same induction/breathing/fuel delivery, the same bore to stroke ratio, the same rod length to stroke ratio, be the same displacement, be installed in the same chassis, preferably be from the same manufacturer. I don't think we can possibly meet all these criteria so I don't think this question will ever be properly answered.

In the meantime I'm working hard to spank that C36 on our next track day 9/18/10. Looking for a couple of tenths!

With all of this "I6 vs. V8" talk, this race is the only case of two guys actually putting it on the track to find out! Yes, it's less than ideal in that it's just straight line racing and doesn't meet hardly any of the above criteria but it's all we've got at this point unless some of the rest of you attempt to do better! Who's gonna come watch?
Regards, Eric

RBYCC · #14 09-05-2010, 12:25 AM

Quote:

Originally Posted by 400Eric

"Overhead valve will rev different then side valve.
Pushrod versus SOHC or DOHC
Chain driven, belt driven, gear driven cam..
Compression ratio...very important because it's what drives the crank."

Yes, All of these and more.....

For this comparison to be valid, both engines need to have the same head/combustion chamber design, the same induction/breathing/fuel delivery, the same bore to stroke ratio, the same rod length to stroke ratio, be the same displacement, be installed in the same chassis, preferably be from the same manufacturer. I don't think we can possibly meet all these criteria so I don't think this question will ever be properly answered.

In the meantime I'm working hard to spank that C36 on our next track day 9/18/10. Looking for a couple of tenths!

With all of this "I6 vs. V8" talk, this race is the only case of two guys actually putting it on the track to find out! Yes, it's less than ideal in that it's just straight line racing and doesn't meet hardly any of the above criteria but it's all we've got at this point unless some of the rest of you attempt to do better! Who's gonna come watch?
Regards, Eric

You got it...

Keep in mind that you may be modding your car but never engineering it...
You have to live with the basic design that the manufacturer provides.
All else is esoteric textbook BS...

The most important part of the equation is the chassis and its ability to get the power to the road surface.

For instance two identical chassis setups, tires etc with one using a 400HP engine and the other a 1000HP engine....
My money is on the 400HP....

Uncontollable power is like having no power...numbers only count if they make contact with the asphalt...

JayRash · #15 09-05-2010, 05:44 AM

calculate the volume increase and u will know. i say its alot. i guess it drops comp to abt 8.5.

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