RJ,

Lee Willis is of course going to be the expert on this topic since not only does he own both the Carrera and the ‘Vette (lucky b******d), he also has significant experience in automotive engineering.

I did a Google search for horsepower vs. torque and found a few good primers. Here’s one of many: http://vettenet.org/torquehp.html

One thing that I did see missing in some of these discussions is what affects the torque curve of an engine. I thought I read (a long time ago) that torque was affected by volumetric efficiency. That is, how much air/fuel mixture an engine can take in vs. the volume of the cylinder. Having more air/fuel means a bigger explosion and more downward pressure on the piston (torque). So a V8 with 346 cubic inches would have 346/8 = 43 cubic inches per cylinder. If the cylinder could take in 43 cubic inches of air/fuel per intake cycle then the volumetric efficiency would be considered 1 or 100%. Then the torque of the engine would be at it’s maximum. This is for naturally aspirated engines. For forced induction it is possible to get a volumetric efficiency greater than 1. But either way the torque of an engine is at it’s peak at the same time that the volumetric efficiency is at its peak. This is assuming that the other factors that affect torque production such as air/fuel ratio, spark timing, etc are all correct.

Lee’s explanation of camshaft placement (head or block) and profile makes perfect sense. The camshaft and the other valve train components make up a large factor in volumetric efficiency since they control the airflow into and out of the cylinder. Intake port and manifold shapes also affect this.

I hope this helps.

 

Volumetric efficiency is the key, absolutely. And cams have an effect and interaction with it many folks don't understand: it is really simple.

This is a slight simplification, but any basic engine (bore, stroke, and heads) has a certain inherent volumetric efficiency you can get from it. This means it has a basic potential for a certain amount of torque. Here, for example's sake, let's say 400 ft lbs.

Believe it or not, you can effect this basic inherent capability of the engine to breath only slightly by varying the cam profile (unless you are truly bone-headed in your cam design): what you really do by playing with (lift, ramp rate, duration, spring tension, etc.) is control the RPM where the engine will be able to make its maximum torque. Air is fluid and has certain properties that you have to live with (intertia, compressibility), so:

You can put in a short duration cam and make the engine produce its 400 ft lbs orso down very low (1600 RPM) and it will produces less and less torque as RPM builds: this is a tractor egnine, no good for drags but very desireable for some applications (if the tractor is plowing along at 2500 RPM and encounters more resistance, as it slows down the engine's RPM drops, but its torque increases, so it pulls harder automatically and is unlikely to stall).

If you now increase duration and lift in the cam you basically move the RPM range of maximum efficiency up to higher RPM, but this means efficiency (and torque) at very low RPM are less. Still, you get more HP. Remember, HP = torque times RPM/5250. That tractor engine produced 400 ft lbs at 1600 RPM, but probably produced only 250 ft lbs at 5250 RPM (250 HP at high RPM).

Now, put the right long duration big lift cam in (around 260+deg and .65 inch life in a LSX) and it will produce those maxmum 400 ft lbs at 5250 RPM and give you 400 HP at that RPM, but probably only give 250 ft lbs at 1600 (75 HP) and it will feel weak at low RPM, particularly compared to that tractor engine, and it will have just a slightly ragged idle, air flow being so non optimum there that it can't quite run smoothly.

Go even more radical and you can move the torque peak up to 6550 RPM, where 400 ft lbs equals 500 HP. But torque at 1600 is now evev lower, and idle is rough.

DOHC engines have some advantages when you need to push the torque peak higher: lower mechanical intertia (no pushrods, lifters, or big rockers to push back and forth 3000 times per minute) but really, only when you need to do that. they make no sense for a tractor engine or anything up to about 5000 RPM. So you generally see people use them only when they want max HP from a certain displacement, as in racing engines (classes are defined by displacement limits, not weight or engine size limits) or in Europe where cars are taxed based on displacement (3 liters costs much more than 2 liters).

Finally, it is worth noting that note all DOHC engines are equal. Consider the 3.6 engine in my Porsche:320 HP, and in a '99 Cobra, 4.6 liters and 320 HP, too. Alot of what you get depends on how much you are willing to spend on the details (the same is true of pushrod engines, look at what you get, but what it costs, to go to 7000 RPM in the LS7)

 

Major thanks to Lee, Riley & Navy Flyboy!
You guys sure know a lot about cars and are helping me better understand my vette. But with that said, are you all saying I can't even beat out a Boxster?? []
Me oh my, what's a girl to do.

Thanks again for taking the time to reply and share your intellectual property. You make this Forum a great place for all.

Take care, stay safe.
RJ

 

Man, alive, youse guys know your engineering. Wow.

Just as an afterthought: here's a story. Had a 5.0L in our last GT Mustang. It was built all wrong. Low comp pistons (I believe intended to be S/C later), high lift cam, headers.... some other insignificant things in this discussion. We dyno'd that thing one day; it put out LESS horsepower than a stock 5.0L due to mismatched parts. The power curve was totally messed up, and where the cam came alive, the heads and other intake issues presented themselves. The cam was wrong for a S/C, and the pistons were wrong for N/A. It was a mess internally.

I put it up for sale the very next day; didn't like the car enough to tackle the mess.


Um, not sure about that!!

Now, the Porsche Boxster S has a few more ponies, but nothing to rob a bank for, IMHO.

I am hopeful Lee will pipe in and correct me where I am wrong here, but I believe the Boxster isn't really designed to be an all out car like the 911's and Carerra's etc. I believe it's intended to be an introductory car into the world of Porsche, and they made it a little "tamer" to appeal to a broader market. Still a fast car, no doubt, but against a Vette like yours, I suspect it'll lose.

I *believe* it's not anywhere near the 100% volumetric efficiency riley and Lee made note of above. This is where my knowledge falls out, though, so I'm gonna now step aside for the guys who know their stuff......

 

Thanks, Topspeed!
When I bought my vette it wasn't with the intention of having the fastest car on the road, although that would be nice, but I love learning about all this stuff. Can't say I 100% understand it all, but it is amazing.

You are right - these guys are extremely knowledgeable.

Thanks for your input.
Take care, stay safe.

 

Yeh, I ain't no expert either, like the guys here are! But, I do have a bit of an ol' school background in buildin' hot rods, so, some of that experience still matters, thank God. I am a softare engineer, and about 70% of what was said made perfect sense to me, the rest of it was Black Voodoo Magic....

Trust me, you don't want the fastest car in the world, Rita, it would be a huge handful to ya, in your day-to-day drivin'. We built a few radical cars back in the ol' days that were just so bloody tempermental and tough to drive... sure, they could beat the pants off most anything on the road at the time, but they were an absolute handful to pilot. And they needed constant tinkering, tweaking, and tuning. It was fun at the time, but as I got older I find that I wanted 70% of what they offered, without all the headaches... enter the Corvette. It's a perfect balance, in my books.

But, yeh, anyways; yep, you ain't gonna pull any 9 second passes in the quarter mile in your car, unless you drop a bazillion bucks into it... but, it handles better than 99.9% of the cars on the road, launches harder'n 99.9% of the cars on the road... and it looks like a million bucks just sittin' there. The perfect balance, in my humble opinion. Great thread, and I sure learned a lot from the guys, too!

 

"here here" TopSpeed
I couldn't agree more. I don't think the majority of us bought our Vettes because they would blow every car away, But I tell ya what, Nothin Like AMERICAN muscle, and these Vettes are the best bang for the buck.
IMO. don't like the imports, and never will! nor will I own one!

 

I'm totally with ya, dude, totally. I ain't proud of the fact that I am "closed minded" about 'em, but that's the way I am; no use in denyin' it, I don't think. We have a tuner shop next door; see tons of Civic's and Acura's in there on the dyno gettin' tweaked... some of 'em have some pretty serious stuff into the car, they can outrun my Vette hands-down... true enthusiasts impress me, so I am interested in some of it... pop by for a visit frequently, just to see what's what.

But at the end of the day, you just wouldn't catch me dead in one.

 

First of all, a Boxster is not a Carrera. The fastest Boxster is a bit slower than the slowest Carrera.

I don't think of the Boxster (or the new coupe version of it, the Cayman) as being an entry level Porsche -- it is lower cost (I think it runs mid 50s usually versus mid 80s and up for the Carreras). Instead, I think it is Porsche's answer to the Miata and Z4: a true two seat convertible sports car. Power is not its thing, but handling is. Mid-engined, light, perfect balance, it is superb to drive and it goes through tight corners very, very fast. Around something like the Monaco Gran Prix circuit, I doubt a 'vette could beat it. Around something like LeMans, no contest.

Bosters have somewhere betwee 2.5 and 3.4 liters depending on model, etc., and the figures quoted above are for an older, probably slowest made, model. New S models are about as fast as early C5s in a straight line.

No recent vette (including the LT1 C4s) has to worry about losing a straight line race with a standard Boxster (i.e., and non-S model, even brand new).

 

TopSpeed, Lee, Rich, all of you guys - you are so right.

Rich, you are right about the Vette being the best bang for your buck, and I do love the fact the Corvette is a bit of Americana.

Topspeed, you're right, I couldn't even handle the kind of power found in some of the fastest cars, but I grew up with brothers and tend to be somewhat competitive. I am still trying to adjust to walking away from challenges. Here's my little confession: I CAN GET SO IMMATURE WHEN DRIVING MY VETTE. It is just such a rush to feel the power and I must admit, I love when people admire my car. Although, I know some guys get mad and think "That @#$%* has a nicer car than I do." All I can say is, "Sorry, boys, I worked real hard to earn the money that bought this baby, so I have earned the right to own it."

And big thanks to Lee - for being so generous to share his time and knowledge with all of us in this Forum. My degree is in CS, so I love figuring out how things work. But if the truth be told, I have to read Lee's explanations a few time to get a good grasp and sometimes I have to look technical terms up that everyone else probably knows. But like TopSpeed said, at this stage of my life, I don't want to spend too much time fixing things, so I won't be adding too many mod.


Thanks again, all.
Take care, stay safe.
Rita
p.s. If you click on the attachment line, you'll find a pic of my silver letters. Not a very good picture, but it is the best I have so far.

[IMG]local://upfiles/2390/975D5DBE77154026B699171B58ABCD98.jpg[/IMG]

[IMG]local://upfiles/2390/B9353F184C324A58B0764755CCDE9DF6.jpg[/IMG]