Question re raising compression '95 f355

I think for the amount of money you would be spending building a higher compression motor you'd be better off building a well engineered twin turbo kit.

You'll miss that high pitched F1 sound though I'd imagine.

 

Or take every ounce of dead weight out of the car. This is what Ernie has been doing with his 348. Just working a different variable in the F = M*A equation.

 

Yeah, with a given ratio,but you would use the ratio that puts you closest to the hp peak as that will maximize acceleration at any speed,which is what we are after. Nobody has a race and then tells you you can't shift gears, do they? You would want to be at hp peak in 1st at 60mph,not torque peak in 2nd. That goes for any speed or output rpm. Maximum force is at hp peak. The only place that is not true is if you have no more gears available, ie 1st gear and the gear you never use on a race track. The whole rest of the time, hp is what matters. Even then, the hp is what determines your first gear ratio, so you will not use the same gearing in a low torque, high rpm motor making torque, once again, irrelevant without rpm (which is hp).

 

Williams was testing one, but it was promptly outlawed in f1 for the reasons outlined (they could operate the motor at hp peak pretty much all the time).

 

That's by far the most cost effective method, but at a certain point you run out of stuff to cut out or are unwilling to sacrifice comfort/safety and you turn to the motor. Personally, I don't want a gutted out car. I have lighter wheels, lighter exhaust, etc. but am really not looking to remove carpeting, put flimsy bumpers on etc. If I were racing, yeah, but I want a street car I can track, not the other way around. I'm not really sure what else I can do weight wise without starting to affect streetability. Maybe brakes? But that's even more expensive than some cams and an engine tune...

But to johnk's point, you don't want to sacrifice engine streetability too much either, which is where the torque comes in. It's all trade offs. I'd give up some idle quality and low end torque for 500 extra rpms and the resulting power increase, some would rather the opposite.

 

I don't mean to sound condescending, but your opening sentence says noting. You can choose a speed for each gear and find the GR that will maximize acceleration at that speed, in that gear. That will not necessarily be the max acceleration at any speed in that gear. You have 6 gears. You want to get from point A to point B the fastest. That means you have to place the shift points at specific speeds (or engine RPM) which will allow you to achieve that. To achieve the result, A to B in shortest time, you need to maximize the acceleration over the entire route, and over the entire RPM range in each gear. The acceleration will be different in each gear and in each gear the acceleration will vary over the RPM range that gear spans (unless the engine has a flat torque curve). Now, I could place the shift point at 5500 RPM for the motor I chose. That would mean that I would have G1, G2, G3, G4, G5 and G6. But how do you choose the ratios? What vehicle speeds would I use to minimize the time from A to B, or maximize the acceleration while traversing the distance A to B? What ever the ratios are, when I reached what ever speed 5500 rpm equates to in each gear (V = crank RPM x GR x circumference or tire) the acceleration of the car at that speed, at that instant, in that gear, will be the maximum that can be obtained at that speed. But one can not stay at that speed and continue to accelerate. He must shift, say from G3 to G4. So the torque now is that for G4 at the speed he was going when last in G3. So what it the torque? Since the RPM in G3 was 5500 the RPM will now be 5500 x G4/G3. And the torque, when I shift into 4th, will be G4 times the torque at a crank RPM of 5500 x G4/G3. That may be more, less or the same as the torque that will be applied when I reach the speed corresponding to 5500 RPM in 4th. The torque will vary as I accelerate from an RPM of 5500 x G4/G3 to 5500. It will depend on the engine's torque curve and in the case I sited, the torque will be greater at 5500 x G4/G3 than at 5500 RPM.

There are two issues. 1) The maximum rear wheel torque at some specified speed will occur when the gear ratio is chosen such that the engine RPM is at the value for the HP peak. 2) The variation of rear wheel torque with speed varies as Gi x the crankshaft torque at the crank RPM corresponding to the current speed, where Gi is the GR of what ever gear you are in.

Going back to my example for a minute let's say I'm in 3rd and shifting to 4th. Let's say I shift at 5500 RPM. Let the gear ratio in 3th be 3.5 (doesn't matter, it could be anything). So the rear wheel torque is 3.5 x 340 = 1190. Now, I choose G4 such that when I shift the engine RPM drops to 3500. That would put G4 at 2.22. The torque at the moment I shift into 4th gear would be 2.22 x the engine torque at 3500 or 2.22 x 410 = 913. If I rev to 5500 in 4th the torque would be 755. If you like I could change the G4 so that the RPM only drops to 4000. Then G4 = 2.45. Torque at the time of the shift would be 400 x 2.45 = 1016, torque at 5500 = 833. The car will accelerate faster in 4th now, but over a narrower RPM range and only up to lower speed. In each case, in either gear, the torque at the rear wheels at the speed when the engine reaches 5500 will be the max torque that can be obtained at that speed. I will not necessarily be the max torque applied to the rear wheels in each specific gear.

So how do you minimize the time from A to B, with 6 gears and a given HP/T curve?


If you like, you can write:

Tr = HP(RPMr x GR)/RPMr

where the "r" indicates the rear wheel and RPMr x GR is the crank RPM. You can see that the only way to keep Tr at a maximum for all speeds (speed is a constant times RPMr) would be to keep the engine at the crank RPM where the HP peaks, 5500 in my example. Now, since RPMr varies linearly with speed,

RPMr = Speed x C,

the crank RPM, RPMc, is

Speed x C x GR =RPMc

and if you want the crank rpm to remain at 5500 for any speed,

5500 = Speed x C x GR,

we see that the GR mist go like 1/speed.

And this is the basically the CVT being talked about. Having driven a car with CVT it is exactly how it works. If you floor the car for max acceleration the RPM quickly rises to a specific value and stay there while the car accelerates. But, at the same time, with a CVT acceleration goes like 1/V because the rear wheel torque go like HP(max) / V since V is linearly dependent on rear wheel RPM.

 

The point is, more HP gives you more force at the rear wheels, regardless of whether the torque increases or not. More average HP gives you more overall acceleration even if you don't change the gearing (unless you have completely the wrong gearing), regardless what the torque does. Even if you just increase rpm and not torque, you will accelerate faster overall as you will shift later and you will be making more rear wheel torque hanging in the lower gear than you would have in the old motor where you would have to have shifted already. That was the original point, I'm not sure what you're trying to do mathematically but you just illustrated my point that you need to keep the engine at HP peak to make maximum rear wheel torque at any given speed (output rpm). Increasing the HP, whether by increasing torque at the same rpm or increasing rpm at the same torque (or even increasing one and decreasing the other, as long as the HP increases) will generate more acceleration. The absolute torque value does not matter, it's torque at rpm that matters, and that's HP.

 

Pretty much puts the lie to the idea it is all about technical advancement and innovation.

 

[ame]http://www.youtube.com/watch?v=x3UpBKXMRto[/ame]

Of course it's not. There's a ton of politics, cost blah blah that goes into it, but they sell it as something else.

 

You 355 guys are limited in HP gains from compression. Not enough headroom in the stock specs for it to matter or even justify a portion of the actual cost. Then there's the detonation issue, even though you have knock sensors, insufficient octane is just going to retard timing and output in favor of engine survival.

Reduced reciprocating mass is a good way to glean some extra power at rpm. Won't give you jack at lower revs. Larger valves probably not something you can do when all the real estate in the head is taken by those 3 intake and 2 exhaust valves. Spin the motor higher? Sure, go ahead, but have a back up for when that grenade goes off. You probably have about 200 rpm overage without incurring a disaster.

This is where the 348 has much more potential. We do have more room in terms of compression and real estate for larger valves. Our engines conservatively turn around 7750 at max, 8000 with chips. We can probably turn higher, but then its time for performance cams and who wants a car you can't drive at low revs on the street? Still, we could probably benefit from lighter valve train assemblies too. Our math shows that the greatest potential for overall performance is developing the mid-range.

All in all, it takes a comprehensive approach to gain horsepower from both motors. Ferrari wasn't leaving too many spare ponies lying around the factory floor even for the cause of enhanced engine life and mileage. Tuning for your mods is probably the best thing we can do right now, what with the variety of modifications we have in this section - intake, exhaust, bypasses, throttle bodies, cats, etc. A couple of HP or torques (as they say on Top Gear) per mod and the next thing you know, you've got a pretty nice package.

 

Ok, let me see if a picture helps. In the picture below the black line represents the torque or acceleration as a function of vehicle speed or rear wheel RPM. It represents where the engine is running at max HP regardless of vehicle speed. For an F355 this would be approximately 8000 engine RPM. The dark green lines represent the torque curve for each F355 gear ratio, 1st to 6th. Thus they are the enging torque multiplied but the respective gear ratio. The red dot indicate where the engine RPM would be 8000. The blue dots where the engine RPM would be 1000. The dark green curves are representative of an engine with torque rising with RPM. Max rear wheel torque for each gear occurs at the red dots (max HP) and then drops off as the rpm increase. The light green curve is similar but shows the torque behavior for an engine which produces greater torque at low engine RPM. Its is shown only for 1st gear. You can imagine what it would look for other gears. What I have been trying to get across, apparently very poorly, is that 1) in any gear the it is the engine torque curve the dictates the acceleration at any point below the max torque curve. 2) with a transmission with fixed gear ratios it is not possible to keep the engine RPM constant, therefore on the max torque line (I know, no brainer). 3) Acceleration in any gear will always be less then the max acceleration except at the point of max HP. 4) However, in any gear the actual acceleration for speeds lower than the speed at max HP may be less than or greater than the acceleration at the max. It depends on the shape of the torque curve. If the engine had a flat torque curve, acceleration, while different in each gear, would be constant for the give gear selection. Additionally, above the point of max HP the torque must decrease. If the engine could rev without damage, as Pete said, remaining is the lower gear would improve over all acceleration, at least up until the torque for that gear dropped below the torque level for the next higher gear. For example, between 1st and 2nd in my plots this would occur when the rear wheel speed exceeded 3.5.

If you want to substitute HP for torque it's a wash. You start with torque at the crank, convert it to HP at the crank, then convert that back to torque at the rear wheel. It comes out the same, torque at the rear wheel is just crankshaft torque time the gear ratio.
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Pete and John, at this point are you guys even in disagreement about anything? From my perspective it seems like you're pretty much on the same page.

 

dunno, let's hear more about the cams.

 

If you want power and torque you MUST increase displacement. Just look at the displacements of the 3 series engines. From 308 to 360. Each increase in displacement, gave more power and torque. Now the 3 series is gone, and we are into the 4 series of displacement. 4.3, 4.5 and now 488.

The 355 is maxed out as it is. It was the first production engine in the world to produce over 100hp per liter. That is/was quite the feat. Old school hot rod ring tricks won't work with this power plant.

If you want your 355 to go faster, maybe transplanting a 360 or 430 engine would be a better option. That would be a cool project!!

 

You can fit a 360 crank

 

Yes you can, and the 360 has variable cam timing which helps tremendously. You can feel a 360 coming "on cam".

You would have to bump a 355 to 4.0l, to get "modern power" out of it. And I'm not even sure if that's possable.

 

I wouldn't agree with -must- but it certainly makes it easier.

I like the 360 crank idea.

You are right that specific output for a stock mid 90's V8 is pretty bad ass

348- 91hp/l
355- 107hp/l
360- 111hp/l
430- 112hp/l

 

LOL!!!

 

My modded 348 is currently - 97HP/L

I'll crack 100HP/L eventually.

 

With most all engines, to get any significant gain in power, displacement is increased. Unless you are going with forced induction, which is a whole other can of worms.

Ferrari did a magnificent job with the 355 engine. It runs on the razors edge. Durability is the price we pay.

 

Cams/compression/cyl head porting are going to do more than displacement.

Change displacement and none of the above and specific output will likely drop slightly due to shifting power band to the left.

Again, I agree that additional displacement does make it easier to make more power within the same rpm range, but not necessary.

 

So what is the stock 355 good for rpm wise. Though we hear often enough about valve guides, we almost never hear about bottom end or top end failures other than the odd timing belt tensioner, and there are plenty of 355 challenges out there that were run hard for extended periods. Is it 9krpm? 10krpm (wasn't schumacher's 355 modified to run at 10k)? We know that Ferraris big issue was getting the thing to pass emissions preventing more aggressive cams, so where would it top out with stock valve train and bottom end but with whatever cams we could put in it? I know Toda goes to solid lifters and shim under for their top spec cams,but could we run some more duration and safely spin the thing to 9500 or 10k?I'm thinking the ti rod bottom end wouldn't care, but not sure what the valve springs and lifters can stand up to.

let's face it, that's a lot easier to do than redoing the bottom end with a 360 crank given the difference in mating to the trans/clutch and the need for custom pistons and/or rods. Plus, well, 10krpms!

Let's say you had an earlier'95 with the higher flowing heads, lighter pistons and larger injectors and you were no longer concerned with emissions...And the engine was coming out anyway...

 

Just to keep it simple, I would have no problem with optimizing timing with the stock cams and having a 9k rev limit.

I suppose you could do slightly upgraded cams, keep the same IVC point but slightly increase lift/overlap/duration but it would really have to be something with proven results unless you want to test it yourself.

Personally, over 9k rpm I would want to go through the bottom end as well (just my opinion, stock may very well survive no problem) but if it didn't that's an expensive problem!

In the end, I think the biggest issue with significantly increasing rpm is what Brian mentioned in post #9, timing belt life.

 

At this point, not really, just whether you consider torque to HP. To my mind HP is sort of the long way around. You start with crankshaft torque, compute HP from it, then to calculate acceleration you have to convert it back to torque. Pete says you need more HP. I say you need more torque. 6 of one, 1/2 dozen of the other.

However, I think the plot should be interesting because the max HP curve shows just how much potential acceleration is lost with a fixed gear ratio transmission. That max HP line would be the potential acceleration that you could have with a CVT.

Oh, I will add one thing. You can not just say you need more torque or more HP. It is necessary to consider the sharp of the torque curve over the useful RPM range of the engine. I presume that's well understood, but I'll just say it for completeness. For example, if changing cam timing results in more midrange torque but a loss at the top end, does than make the car quicker or slower over all? It's not just increases and decreases, it's also where and by how much.

 

Just drop a 360 engine in complete. While you are at it, add a 360 F1 transaxial too.