engine buildup, cam selection

[Jim Rosenthal]

I would like to hear some thoughts on cam grinds for the engine we've begun building for my GT40. Here are the basics: 302 block, (cleanup bore to 306), forged crank, rods and pistons, rod length 5.40, CR 10:1, AFR heads w/ 165 cc intake runners, and to begin with a 3x2 Weber setup that I am interested to try although it does not look original. (If that doesn't breathe well enough I will go to 4x2 Webers.) It is set up with a plenum and progressive linkage like the old Pontiac 3x2 GTO...I am using a ZF transaxle with 4.11 final drive and a .70 5th gear. I think this engine block will accept either solids or hydraulics, but would have to be retrofit for roller lifters.
I don't know very much about cam selection and I am curious to get info from all the people on the Forum who have built engines for similar cars. I'm making some assumptions, which are as follows: a car that weighs 2400 lbs or less doesn't need a cam grind with huge low-end torque, plus this final drive ratio doesn't require lots of low-end grunt; the ability to rev freely and quickly with a lot of midrange torque is a plus in a GT40; a rev ceiling of 7000 will still give me plenty of room to have fun: the car needs to be street driveable and enjoyable to drive that way.
I would appreciate anyone's thoughts on this; if it has been covered in a prior Forum topic I apologize but this is the best way I know of to get info for these cars and how to set them up. Thank you all in advance for for advice, let me just say that there are a great many things I would not have known of had it not been for this Forum and its' free exchange of ideas.

 

[Guest]

X303

I think lower torque is better than all this high rpm. You will be accerlerating more often from low speeds than high speed stuff. Doesn't sound as sexy but perhaps more useful? Also more reliable.

Malcolm

 

[Ron Earp]

Hey Jim,

There are lots of cams that would work well in your application. Since you don't have a stroker, which I am beginning to think is a good thing, then you won't have to worry so much about the huge amount of low RPM torque.

I would guess that a cam in the 0.540-0.550 lift range would be good, with an advertised duration of around 290 degrees, or around 225-230 at 0.050" would be decent. There are lots of these around, the 290H from Comp I think, then some from Isky as well. I used an X303 in mine, since the builder liked it, but I also agreed as I'd used the cam in two other cars with good results. I'm sure there are better ones out there.

I am sure opinions will vary depending on who you talk to. If you want reliable 7000 RPM operation I might would choose solid lifters (probably solid rollers, know this has been discussed) for my motor, and a different cam that favored more RPM. But then you might want different heads with a larger intake port volume, larger than the 165s. Maybe the AFR 185s or Victor Jrs.

I think there is a recipe for a 302 on AFR's website with cam choice, why not check that one out?

Ron

[ February 16, 2003: Message edited by: Ron Earp ]

 

[Jim Rosenthal]

Hi Ron: Thanks for the advice. I did look on AFRs website and they do have recs for this combination. It is amazing the power they get from only 302 cubic inches. Thanks for the reference.

 

[Ron Earp]

I checked that out too - depending on what you want that first combo cam is going to be rough on the idle, that is for sure. The second might be a more streetable choice.

Ron

 

[Bill Musarra]

I make it a habit of saving articles out of any car mag that stirs my interest. I remember an article on cams, and after searching, I fouond it. In Engines magazine the winter issue (?yr.) "Your cam Questions Answereed" It deals with everything from narrow/wide lobe displacement angle, degreeing a cam, torque vs. top end,smog vs. power, the relationship of lobe displacement angle to compression, low end torque and responsiveness, what it takes to change over form flat tappet to roller, cam vs. bore and stroke, and more. Very informative. If anyone is interested I will scan the article and email you. Don't want to offend the copyright people. It is a question and answer type article, but goes into the whys and wherefors for any engine

 

[Gary Gibbs]

Bill I would love to see that article.

Thanks
Gary

 

[Rick Muck- Mark IV]

Jim,.

I am not expert at all on this but I understand that there are cam selection issues when using Webers. Something about the intake reversion and airflow. I think there was a thread about before or possibly on ClubCobra. Be a shame to put it all together and have a problem.

Rick

 

Reversion sure can be a problem with webers, as it can also be with a crossram type manifold. Call comp or cam motion (many others as well); cam selection is not as mysterious as it used to be. You may want a healthy duration to moderate the low end torque a bit.

 

I am puzzled about the torque vs increase horse power at high RPM. When I was researching an engine for my Cobra I was told to emphasize torque as this is what gives the feeling of acceleration and makes low speed/traffic/street driving easier. When I visted with Gordon Levy, looking at a RF GT40, I was told to emphasize low torque and high RPM HP. His opinion was this was better for a mid engine car. Wouldn't the low speed/traffic/street driving still be an issue? Why does the mid engine configuration favor low torque vs high rpm HP?

 

[Ron Earp]

Hey Rolf,

Although Gordon might suggest making more high RPM HP, it isn't like the torque is low! I have driven a couple of 40s, Hershal's most recently, and that engine is fine for use anywhere. Gordon's small cube motor is quite nice for the car.

Just a guess, but I would estimate that engine is around 300-325 lb/ft of torque, fairly flat curve too, and 400+hp easily, but at 6000-7000 RPM. No problem to drive. In fact, due to the shift pattern I started in 3rd twice instead of 1st with no troubles - I didn't anticipate it, just drove off. The car only weighs around 2200lbs or so, you don't need 400 lb/ft to feel "launched".

I liked the high winding character of that motor, it is fun and easy to drive. But, you can have both too. He can build all sorts of strokers that will have high torque with the hp too.

Ron

 

Another reason for the low torque is so you don't snap the little Audi box RF uses in 1/2. Nice fit and all but really small and alot of torque probably isn't it's best friend.
Gordon would know better then I would thus the reason he's building my motor.
John

 

[Gordon Levy]

The biggest reason for the suggestions for a high hp high rev engine is it makes the car much easier to drive. In my first demo I experimented with several engines, 347 that was 500hp and 450lbft torque, an engine like what is in Hersh's car, and the Factory tried one of my 500hp/385 lbft 306's in one of their cars. The high torque engine was fun but hard to drive off of corners, kind of like the original mark II I raced a few years ago. It kept trying to break the tires loose. Unlike a front engined car, you can't hang the back of the car out with risking getting into a lot of trouble unless you are a very experienced performance driver. I like building my engines so they are smooth and predictable as most drivers are on the street and don't have a lot of high performance driving experience. There are always exceptions.
Most are very happy with 400hp in a car that will do 0-60 in 4 sec. and has a top speed of north of 150mph. Those that want the extra performance of a 500hp/8000rev engine, I am happy to built it for them. I do try to for warn them on what to expect.
By the way, we design our own cam profiles to fit each application.

[ February 22, 2003: Message edited by: Gordon Levy ]