Gearing and cams on the racecar. jleutjen? TimT

[cstreit]

So a recent slew of cam threads has got me thinking about my own cams...

I have the WebCam 120/104 cam in my 3.0L racecar. I think you guys remember the specs, but FWIW...

39mm intake, 26mm exhaust ports
Webcam 120/104
46mm webers with 40mm chokes
Equal length headers and megaphones

Current setup appears to really "come on the cam" at about 4500 RPM and pulls up to my set rev limit of 7200. I'd like to drop that power band slightly I think, but just installed the new gear ratios as follows:

1 3.182
2 2.059
3 1.476
4 1.125
5 0.889
Final 3.875

According to me calculator thing, this reveals this data:

Top Gear gives 20.904 MPH/1000 RPM and a top speed of 146.330 MPH at 7000 RPM

Top Speed in 1 gear = 40.882 MPH
And changes into 2 gear at 4530 RPM dropping 2470 RPM
Top Speed in 2 gear = 63.180 MPH
And changes into 3 gear at 5018 RPM dropping 1982 RPM
Top Speed in 3 gear = 88.135 MPH
And changes into 4 gear at 5335 RPM dropping 1665 RPM
Top Speed in 4 gear = 115.633 MPH
And changes into 5 gear at 5532 RPM dropping 1468 RPM
Top Speed in 5 gear = 146.330 MPH

Which puts me in a pretty good place for this cam. Do these numbers seem about right? I only suspect them because my previous gearbox also read pretty well, but the numbers didn't quite add up for me...

What are these cams right for this gearing? Can I time them differently to get more usable power?

I think they will help some of those slower 3rd gear corners... The ones where I was at around 51-55 MPH, put me at about 3500-3900 RPM which was out of the power band, whereas this new 3rd gear jumps me up about 600 RPM more at the same speed...

[Bill Verburg]

our rear tires are another variable which also needs to be specified.

[cstreit]

Bill, rear tire diameters were part of the formula. I'm using the Quaife calculator, with a rear tire diameter of 24.5 inches * pi *25.3 to arrive at mm which the calculator uses.

[Bill Verburg]

rear tire diameter of 24.5 inches * pi *25.3 to arrive at mm which the calculator uses.

25.3 should be 25.4

using a tires revolutions per mile gives a more accurate picture of what actually happens

But anyhoo here's your box


So yes the #s you gave look correct

[jluetjen]

Chris;
Those gears look like great choices to me to according to the following criteria:

1) Whenever you shift up at redline, you don't drop below the torque peak. So your engine is always pulling from peak torque toward peak HP.

2) The drop in rev's gets smaller as you move to higher gears, this keeps the engine pulling in the peak HP zone as aerodynamic drag increases.

The only other considerations that I can't judge based on the data are...

1) You're not being forced to shift mid-corner.
2) Compare your rear-wheel torque before and after your shift points. You'll need a torque plot from dyno or data logging system to do this. You might find a small improvement by doing this.

[cstreit]

Thanks John. I'll have some data from both the old and new gearboxes after next weekend, so we can take a look at that.

I don't think I'll be shifting mid-corner. My biggest issues before were
1. Never getting to redline in 5th gear even on the longest straights
2. Exiting slower corners in 2nd meant a mid-corner shift, and doing the same corner in 3rd put the car out of the power band...

I think this box will make it up. I'll post the old curves in a few minutes...

[TimT]

I agree with John and Bill, that gearbox seems pretty well set up.
One comment caught my eye though

1. Never getting to redline in 5th gear even on the longest straights

146 is kind of a high speed, considering the power you are making, and the length of alot of our tracks. I geared my small displacement 911 for 138 at 7200. The only place that I run out of gear is Watkins Glen...

Shifting in a corner may be the onlyway. Or can you give up a bit and make this a throw away corner, and use some of your cars advantages to make up the time in othe places?

I think your pretty well set. Maybe build a spare gearbox.. youll then have a short track and long track box

[cstreit]

I'm hitting 145 at Road America now... So I won't actually lose top end with this one, my biggest beef was with 3rd gear, just too long... If I could get a longer 2nd gear though, I'd really be happy, maybe this winter...

Tim, can you review the cam-timing procedure for me? Any thoughts with my RPM drops as to how I should "tune it?"

[snowman]

WHOOO, what about the TRACK?????

The gearing and so forth is a function of the TRACK. Nothing else.

You need to be hitting redline at the fastest portion of the track, and have gears that put you in the corners where you need to be. It all depends on the TRACK that you are on.

Your cam determines where the power band is. High or mid or low, again depending on the TRACK and its corners. Given a fixed cam, the power band is limited to the max rpm that the engine can stand without breaking. More power is better. Unless it breaks. Given that, gear it to match the track you are on. Doing so is a real art, and makes the difference between winners and losers.

NOw thats not real helpful, but again its an art. You need an artist to win.

[jluetjen]

Hi Snowman;
You're right, the track does have a lot to do with the gearing. In a perfect world (or at least in professional racing series such as NASCAR, IMSA, Super Touring Cars, etc) the gears are changed for every track. Especially in series where the engines are very tightly wound and as a result have a comparatively narrow torque spread.

Chris (as well as most of the rest of us) lives in the real world of the amateur sports car racer and the situation is a little different. First and foremost, most of us don't have the time to rebuild gearboxes after every event, nor carry 2 or 3 spare boxes. Secondly in general our engines are tuned to last a season or two, so we're turning fewer rev's and hopefully generate a wider torque spread. Thirdly, rather then racing on 12 tracks a season, most of us race on 3 or 4 at most. I can't speak to Chris's situation (but he does make reference to some above), but of the tracks that I'd race on (NHIS, LRP, Pocono, Watkins Glen), I plan on using 1 box. I might have a few extra rev's at LRP and NHIS, but that won't hurt the motor and will most likely not cost me more then 1 or 2 tenths per lap. Keep in mind that we're talking about a full sized "sedan" with a comparatively fixed aero package, not a formula car where the adjustable wings and aero package define the top speed. As far as the lower gears are concerned, as long as I'm staying within the desired rev range and I'm not shifting in the corners, I doubt that it will make a measureable difference. Chances are that traffic or individual driving skill will have a larger impact on lap times.

So in an ideal world, I'd agree with you. But in the real world of limited budgets, limited time, sub-professional drivers, etc. I'm just not convinced that track specific gearing is the biggest bar on the pareto of things which will impact lap times or more importantly finishing positions at the amateur level.

[jluetjen]

Hi Snowman;
You're right, the track does have a lot to do with the gearing. In a perfect world (or at least in professional racing series such as NASCAR, IMSA, Super Touring Cars, etc) the gears are changed for every track. Especially in series where the engines are very tightly wound and as a result have a comparatively narrow torque spread.

Chris (as well as most of the rest of us) lives in the real world of the amateur sports car racer and the situation is a little different. First and foremost, most of us don't have the time to rebuild gearboxes after every event, nor carry 2 or 3 spare boxes. Secondly in general our engines are tuned to last a season or two, so we're turning fewer rev's and hopefully generate a wider torque spread. Thirdly, rather then racing on 12 tracks a season, most of us race on 3 or 4 at most. I can't speak to Chris's situation (but he does make reference to some above), but of the tracks that I'd race on (NHIS, LRP, Pocono, Watkins Glen), I plan on using 1 box. I might have a few extra rev's at LRP and NHIS, but that won't hurt the motor and will most likely not cost me more then 1 or 2 tenths per lap. Keep in mind that we're talking about a full sized "sedan" with a comparatively fixed aero package, not a formula car where the adjustable wings and aero package define the top speed. As far as the lower gears are concerned, as long as I'm staying within the desired rev range and I'm not shifting in the corners, I doubt that it will make a measureable difference. Chances are that traffic or individual driving skill will have a larger impact on lap times.

So in an ideal world, I'd agree with you. But in the real world of limited budgets, limited time, sub-professional drivers, etc. I'm just not convinced that track specific gearing is the biggest bar on the pareto of things which will impact lap times or more importantly finishing positions at the amateur level.

Art is good. I like art. But the successful management of resources first, and secondly the management of physics will determine the winners on the track. No matter what the series.

[cstreit]

A spare gearbox would be nice, possibly even affordable, it's really as much of an issue of the time to install it as anything else. I get back from the track late Sunday, travel and work during the week often, then basically have the following weekend to do the "once over" on the car. That means going over every nut/bolt/fastener, looking for fatigue and cracks, bleeding brakes, making adjustments, rotating wheels, changing oil, planning the next trip (supplies, food, spares), etc...

When I actually have to FIX something time is often very pressed, meaning late nights. These are occasionally okay, but more than once in a while makes it not very enjoyable when I still have to do my paying job.

With most tracks I run, save road america, a really short gearbox would be perfect, Road America with 3 straights of almost a mile would require a different one, but I just don't have the time to yank the motor every other week...

But technically you are correct snowman, but with my schedule, very difficult. I'm hoping my motor lasts 3-4 seasons!

[thethrill]

Try 38 mm venturies. This should give you more bottom end with a negligible (if any) loss on the top end. You will gain significant driveability! Do you have the tall secondary atomizers? They are a must for race carbs.

FWIW, my stock 3.2 w/46 mm webers has 36 mm venturies and pulls very hard from idle to redline, 6,500. The torque curve is very flat!

Good luck!

[cstreit]

I do have the tall secondary venturis if that's what you're referring to. I thought about playing with the primary venturi size. I previously had 36mm venturis, but opened the heads up a bit to early SC spec, so PMO recommended these.

[TimT]

Chris

I was able to interpolate cam timing by doing some dyno pulls with my cams advanced or retarded.. The info I found only applies to my cam and engine.. You can move your power around by advancing or retarding the cams....

Im lucky enough to get to a dyno every month or so ( my friend rents one for a day to fine tune his customers cars) so I tow my car along to get on the dyno sometimes..

Id just leave the cam timng the way it is....People drum into my head make only one change at a time..... I find that hard to do

[cstreit]

Good point and well taken Tim. Thanks.

[dd74]

The one thing I'd add are tire size. Could going to a shorter sidewall help, or is that out of the question for your class? Or are shorter sidewalls even a valid consideration?

Another note: I've never thoroughly checked my gearbox, as far as to what RPM it descends after a shift, but yours resembles mine, which is a 915 with 7.31 r/p hooked up to a 3.0.

Hmmm...

[snowman]

Sorry, I didn't mean to come off like a snob. There are a few, maybe even dozens of things that can be done that are low(er) budget. The first and easiest, if your rules permit, is to have a tall set of wheels/tires and a short set. The tall set if for the fast tracks and the short is for the short ones. I would vote this one to have the most bang for the buck of anything you could do.

If the cars on the street at all I would go for a torque type cam, IE low and broad torque band, eg 2000 to 6000 RPM. Next I would look at all my favorite tracks and prioritize them. Next comes the difficult part, pick the corners that are most important on each track. That usually means the ones that have a long straight or whatever following it. Gear the car for these corners and give up the corners that don't matter much. See if it is possible to obtain a set of gears the makes these improtant corners work for you. My guess it that you can find a solution for 2 or3 similar tracks. Next use the wheel size, if possible to adjust for fast and slow tracks.

In my view it really takes a good solution to do this and still win. Any pin head with unlimited money can buy all the gears he needs, not nearly the problem it is to make it without the money. Since there seems to be at least a couple of people with to much money at any one race, and they will have the right gears for the track, the trick is to beat them at one or two tracks and let them have the rest (or make more money).

Given the numbers you gave and the cam you have I suspect your gears are way to tall for most any track, except MIS or CA speedway. I would gear it to top out at about 130 for a faster track, eg Willow Springs, or 115 for the more typical slower track or parking lot setup. On the other hand if you want to drive it on the street, I would change to a differen't cam,( you are running 11.5 or higher CR with your cam arn't you?) one that lets you get by with taller gears so you can still drive it on the road. In your case I would suggest something about 222 degrees at 0.050" lift and 0.45"lift or maybe even a little milder. Again thats just my opinion, nothing more.