First of all this thread is not meant to anger anyone on either side of the the V8 vs Rotary argument, I have to interest into which engine is better in this thread. Here I intend to open a discussion on the physics of weight distribution, handling, and engine PLACEMENT. I will not try to convice you one way or the other but I would like those of you with some knowldege of physics to politely step forward and discuss theory in a polite manner.

So get started we have the following assumptions and hypotheses.

1) V8 weigh much more than a rotary and will the weight distribution.

-Not true the weight difference is not significant if we are referring to a turbo rotary setup, as the front to rear bias changes only marginally. This has already been proven by a number of members.

2) my theory is that engine position is much more important the overall weight and is what truly changes handling dynamics at the limit of adhesion.


Take the following:

1) ----O---O---- This would represent the typical weight across the rx7

2) --O-----O--- The difference is subtle. Both ends of the car technically weight the same but the car would be victim to a slight pedulum effect.


Porsche engineers have always faced a similar problem with the 911, back in the day they attempted to even out the distribution by putting lead in the front bumper, this result in a large swing of the car as the MASS if the vehicle was at the extremes of the chassis. In the RX7 the effect is not nearly as marked but it's there, via the engine and axel location ratio (I made this term up but it describes things well imho). Obviously this is more marked without turbo components.

The underlying argument here is that the majority of the mass should be centered, even both ends weigh the same. Anyone care to jump in? This is meant to be purely a discussion of handling and physics. The thread title is just to illustrate the theory.

 

You are right, it is about placement of the engine. Which is why you mount it back as close to the firewall as possible, to try and retain the factory 50/50 balance. Then, before the swap is complete, you set it up on jackstands (at the middle of the frame of the car) You could also use two jacks, and you see how much it wants to tilt one way or the other. Then you relocate things, so that you get it back to 50/50.

The V8 only weighs about 150-250lbs more than the standard 13b. (this of course depends on the V8 that you ultimately use. If you were to use a 305 block or a 289, youll be on the low end, if you use yourself a 350, a 383, a 402, youd be at the very high end.)

As with any vehicle, you want the weight to transfer to the wheels, and hold traction instead of roll, or slide. (That is if we are talking about holding traction in a circut application, or some such.)

The optimal position of the engine (I would think) would be low and back, trying to keep weight as low to the ground as possible, while moving it toward the rear to offset the increase in weight. Something that would help to ballance this out, would be the T56 transmission, or whatever transmission that you decided to use for the swap as well. As would the diff. If you were to source a C4 or C5 diff, and suspend that in the rear of your car, the weight there would change (slightly higher) as well as the strength of the component. Not to mention there are far more parts available for those differentials, for track, and strip applications.

You want the back end to have enough weight on it to hold traction, since your drive wheels are back there, but you also dont want it to be grossly out of balance with the front, because then you will understeer.

It has been done so many times though, that someone has already figured all of these things out pretty well. Interesting thought though.

 

Ah excellent insight, thank you Rotary Noob. I haven't seen a v8 swap in person just vicariously through other people's pictures, which illustrate the engine placement much father forward than the 13b (which is natural for a longer engine).

However, as you have pointed out people have already possibly addressed the issue. Perhaps another V8 owner could chime in as well. It just seems to me that when talking about weight distribution people place too much importance on weight but forget about actual engine placement, when compared to the original setup.

 

IIRC the term you are looking for is the polar moment of inertia. The farther from the center of rotation a given mass is the more it resists being moved back and forth. Pick up a sledge hammer by the end of the handle and try to swing it back and forth. Now move your hand up to just under the big hunk of steel and try swinging it. That is what you are talking about. Also, IIRC, when figuring how the value the "distance" number is squared so the difference from say 2" to 8" is 16x's more not 4.

And yes it has a huge affect on how a car feels. Take a car like a Lotus Elise with everything close to the center of the car and compare how it feels to a 944 Porsche with the engine in front and the tranny in back. Both will go fast and the Porsche may be a little easier to drive because it steps out slow and comes back slow. The Lotus will respond to twitches to the steering wheel faster.

 

I believe your correct, it's been a while since I took freshman physics, so the terms are a little fuzzy. In event this feels like a great discussion thus far.

 

Here is what I think, IF you wanna do anything other than drift or drag then keep the rotary (ex autox, circuit, rallyx, twisty driving). I personally think the rotary is a lot smoother engine and I love battling twistys with it.

I think the ls1 would be very cheap and very good for a drift and a drag car.
Lots of power for not a lot of money.

I don't believe that one is better than the other, they both have their ups and downs.

 

Well said, but the argument could be had both ways. I wonder if differences are noticible only at the limit or before it as well.

 

if you look at it in weight distribution the weight of a v8 is mostly at the top of the block especally with a overhead cam setup and having such a limber car the weight distribution of the v8 being top heavy would put a larger moment arm on the engine when cornering thus making the not so much heaver engine even heaver because of the moment of inertia like iegrew said.

the most benifital part of the rotary engine is its size and geometry not really its weight. now if there was just as much research that went into rotary engines as there is in piston engines we would have a lighter more efficent engine. like the old 350's there was low power high fuel consumtion and lots of weight just like the rotary's now. now sence i have digressed soo far off topic the v8 seems like a better option but it is 30yrs more advanced so take that into consideration also.
just my 2 cents
eric

 

Are you suggesting that weight increases with more ci. in a sbc? If similiar block and head materials are used a 455 ci sbc should weigh about the same as a 350. If all emissions equipment , a/c and ps is installed the ls1 swap is proven to gain less than 60 lbs. up front in the fd.

As far as the weight placement the ls1 is not an ohc engine. I could see all the weight placement engineering in this thread come into play with say one of those ford dohc monstrosity's. The low cam position combined with a lightweight valvetrain and composite intake manifold makes the ls1 very light up top.

Bottom line is that an lsx powered 2nd or 3rd gen remains a well balanced machine.

 

PLUS if you look at the difference in a SBF and a SBC the ford motor with a dual sump mustang (fox chassis) oil pan will sit lower and further back in the chassis than a similar small block chevy, I am talking 302 ford vs. 305 or 350 chevy not ls1 or lt1 motors.

plus I understand the weight thing and how the weight of a v8 sits higher than a rotary but you can off set that some with aluminum heads and intake and when I had my 302 out on the ground next to my 12a the size wasnt that much more, and I agree with the 150 lbs difference, that isnt a whole lot, plus there are things that you can remove from the engine bay and move to the back or mid rear of car for better balance.

rotarys are a cool motor and I am currently getting on with a rotary build up also and I will have two cars one v-8 and one rotary. so I will compare them after.

 

If I do a swap it's either a 26B Quad turbo or something absolutley rediculios or a Viper motor even if I have to extend the wheel base. I'll go ***** to the wall with a swap other wise it's just not worth it to me.

 

One of my favorite pics. Corner weighted FD

 

Sorry guys, but this thread honestly sounds like its coming from the perspective of "I like the rotary better and here is how I justify it".

Dont forget that the T56 transmission is a good bit heavier than the stock tranny. This is weight as low as possible, and right in the middle of the car.

The reality is... that an LS1 converted car remains a very confident machine.


This is laughable, sorry. An LS1 converted car feels close enough to the SAME that 99.9% of this forum wouldnt notice other than the fact its not electric engine smooth feeling like wobbly triangles are. I autox my converted car, and rarely drag race it. I assure you that on a circuit or twisty's an equal car w/ the different engine would stomp a rotary powered car. Why? Because the handling is nearly the same, (ie: you wouldnt notice it, nor would anyone you know) but the power is all over the RPM band and isnt peaky.

Lastly, every rotary person I have taken a ride in my car has liked it.. alot. Even the ones that swore it was a horrible thing to do before the ride.




That being said... the basis of this thread, which is debating the physics of it remains true. A lower polar moment of inertia is better. However... its simply not a closed vaccuum system. There are many more factors than that, and as porsche has proven with the 911, and GM has proven with the C5 corvette, (note: engine up front, tranny out rear) you can make a car with crappy polar moment be a very capable, if not downright monsterous car.

 

Far from it, I honestly wanted the perspective of V8 owners, I want to know what really matters, just the 50-50 or WHERE the weight is?

Which is exactly why I wanted all possible input. burtoncr posted an excellent illustration of a properly installed V8, I would like to know however if the more ideal front to rear distribution achieved isn't offset by the top heaviness? I agree highly with your comment just look at the current 911 Turbo/GT3 those cars are monsters. Suspension tuning is awesome on the Z06 as well. I see where your coming from, believe me this is meant to be a friendly discussion

 

 

You missed my old sig ~ Tri-turbo T51 Spl 20B

 

The majority of the weight of an V-8 is concentrated at the crankshaft, which is at a lower point in the car than the E-shaft of a rotary.. Much less a cast iron turbo manifold hanging off to one side. When guys run high boost for these 400+RWHP 13Bs, usually a FMIC is installed, which is more weight at the very nose of the car. Not to be a *****, but this subject has be beaten to death, and you could go on reading for hours with a simple search. The fact is, a properly done LS1 conversion, even if keeping all accessories, will affect the weight balance negligably, and one done removing A/C, PS, and relocating the battery to the bins actually centers the cars weight slightly. Does the swap have an effect at all, yes of course, but so does me throwing a bag of groceries in the hatch, but that doesnt mean my car doesnt handle anymore.

 

I didnt mean you in particular. In all honestly I had people that replied in mind much more so than yourself

 

ho gives a hoot if it does .. if u really want to kno go ask the v8 rx7 forums.

 

This one never fails to crack me up.

OK, I'll grant that the LS1's composite intake cuts weight and resists heat soak better than aluminum, and casting and machining technology has improved over the years, not to mention electronic fuel injection... but other than that, you're still dealing with the same basic design that Chevrolet introduced in 1955. So how is that 30 years more advanced than the rotary, introduced in 1959 and culminating in the Renesis a few years ago?

 

Center of gravity is far more important than weight distribution. Low and centered is ideal.

In fact, 50/50 distribution only means the car will tend to handle neutrally, and not only does the weight balance change the minute you add passengers or put the car into motion, but you can unbalance a car with "perfect" weight distribution simply by using the wrong shock valving, spring rates, sway bars, or even tires.

Bottom line? 50/50 looks good on sales brochures, but it doesn't mean you won't get your *** kicked by a car without even distribution. A rear bias is actually desired because it reduces dive on braking and increases traction by loading the rear wheels. Formula One cars have a rear bias, for example.

 

I can agree with Jimlab. Although weight distribution is important there are other issues that may arise when swapping engine. For instance, with a 50/50 setup one might have completly different dynamics when considering the roll axis and roll rates. And with the new CG, one could consider the effects that the location will have. The roll moment might change enough that the suspension natural frequency is not ideal.

The nice thing about weight balance is that it's easy to measure.

This is an interesting discussion.

 

If thats the case the tranny is upside down to get the input shaft in line with crank. The eshaft and crankshaft are at the same height in the two engines unless you lower the tranny with the engine. The V8 has its crank almost at the bottom right about the level of the pan gasket. Some have it tucked up inside the block for strength but even then its only an 1 1/2" or so. So pretty much the whole engine is above the crank.

 

Really? Have you measured?

I hate to break it to you, but V8s sit lower (at least in an FD) than the rotary. They require a K-member engine cradle open at the back to make this possible. The engine has to move down not only because it's taller, but because the T56 case and bellhousing are significantly larger in girth than the OEM 5-speed.





The steering rack has to move down to clear the balancer and oil pan which, as you might have guessed, is usually about half full of crankshaft counter weights and the rotating assembly at any given time. There's plenty of weight below the centerline of the crankshaft, in other words.

Depends on the engine, but you're right; the crankshaft (the single heaviest component in the engine, besides the block) is at the bottom of the block, and it sits lower in the car than the rotary's eccentric shaft. You can tell just by looking at the drive pulley location of both engines. The camshaft ends up at about the same height as the eccentric shaft.





The crankshaft doesn't sit any higher in the block. The block just has a skirt around the base which helps make the main caps and bottom end more rigid.

Well, except for the main caps, the thickest portion of the block, half of the crankshaft and about half of the rotating assembly, the oil pump, oil pan, and starter, etc.

The center of gravity of the average V8 is about midway between the crankshaft and the camshaft centerlines.

 

Agreed
Well stated this discussion is going great
Keep it civilized, if you don't have a educated statement that positively contributes to the conversation, why participate? This discussion is not about the engines power characteristics , but their dimensions how they sit in the chassis, and center of gravity of each. And how these affect handling and how any issues might be addressed.