effects of gear and tire diameter change.

[rfoll]

My tires are all done, and I intend to try a larger tire with a lower gear. Currently I have a 3.73 with 10 X 26 Hoosiers. The car runs 10.90 at 120 mph. I believe I have gone as far as I can with the short tire. I tried a 4.10 gear and I launched much cleaner, but wasn't much quicker. I broke the differential housing and changed back to my other axle. Current trap rpm is a little over 6300, and with the 4.10 gear it went up to around 6900. The engine will turn the additional revs without any problem, but I'm sure I am past the useful power band in high gear. I am going to install a 4.56 gear with a 10 X 29.tire. It will be close to the 4.10 and 26" tall tire. I believe what I need is to move my rpm band up about 10 percent. The car is a 78 Skylark, about 3500 lb. with me in it. My converter stalls at 5100 rpm and the trans is a T350. The engine is a Dart 415 short block, so it will handle all the rpm I'm likely to require from it. The carb is an 800 double pumper on a Holley 300-25 Strip Dominator. I have a cold air system for the air cleaner. The heads are Brodix Spec with 2.08 intake valves, nearly the same as Track 1. The chamber is 70 cc giving me 10.3:1 cr with the flat top piston. I don't think they flow enough for the rpm I want to turn. My cam is a Howards solid roller, 259/291 int. and 261/293 ex. with a 106 lsa. I believe the current Intake centerline is about 101 or 102. Lift is about .625" net. The exhaust is 1.75 primary tube Schoenfeld headers with a full length 3" system. It has an H pipe and the mufflers are Ravens. I don't have a pile of money to throw into this this year, the new slicks will pretty much eat the budget. The things I have on hand to try are as follows. Retard the cam to straight up. I have an 850 double pumper. I have a set of 1.6 rockers if anyone thinks it would help. I have another Solid roller cam, A comp 296 BR4. The Comp cam is a few degrees larger, but I wonder if the narrower lsa will cause the rpm to fall off too quick. Would better flowing heads later on negate the need for more cam duration? A 64 cc chamber would raise the CR about 1/2 point. Any discussion about this will be helpful because I believe most of the low hangin fruit has been consumed. Thanks in advance Rick.

[frnkeore]

The 29 x 4.56, will give you 123 @ 6.5k.

The rest of the low fruit is your 850 and 1.6 rockers. .042 more lift if, you still have enough spring travel.

I'd try those 3 things first, before taking a dive into the engine.

[rfoll]

The 29 x 4.56, will give you 123 @ 6.5k.

The rest of the low fruit is your 850 and 1.6 rockers. .042 more lift if, you still have enough spring travel.

I'd try those 3 things first, before taking a dive into the engine.

The converter slip changes the rpm at the end. I have an expensive recording tach and a recording AFR meter and both showed a 6900 rpm through the lights with the 26" tire and the 4.10 gear. My calculations say the 29" tire and 4.56 gear will be 1 percent more than the 6900. Tire growth on the taller tire may help.

[David Redszus]

Considering only rear end and tire diameter:

3.73..26"....20.74 mph/1000rpm
3.73..29"....23.13
4.10..26"....18.82
4.10..29"....20.99
4.56..26"....16.96
4.56..29....18.92

[6.50camaro]

You can always tighten the lash for a few runs to see if the motor want more duration with the extra lift of the 1.6 rockers .
I calculate that you have about 7% converter slippage at the finish .That's not to bad the 4.56 might raise the rpm's enough to drop the slippage some and not see little if any change in rpm's at the stripe . Dan

[rfoll]

Considering only rear end and tire diameter:

3.73..26"....20.74 mph/1000rpm
3.73..29"....23.13
4.10..26"....18.82
4.10..29"....20.99
4.56..26"....16.96
4.56..29....18.92

Your calculations agree with the math I did a year ago. The 26"/4.10 is roughly the equivalent to the 29"/4.56. The 26"/4.10 crossed the stripe at 6900 rpm, roughly 10 percent higher than the 26"/3.73 setup I have run for nearly 20 years. The question remains, how to best move the rpm power band up another 10 percent. The 1.6 rocker seems easy but it will require checking piston to valve clearance. It might be necessary to change pushrod length. The head flow from the Stan Weiss database indicates an intake flow of 261 at .600, and at .700 it's 269. How much difference will another 5 cfm and a faster opening rate make? Is the 800 cfm carb too small for the rpm? My records show the intake centerline at 101. I had a converter with much less stall speed years ago. What can I expect from installing the cam straight up? I have always run cams at 4 degrees or so advanced.

DSCN1352.JPG

DSCN1354.JPG

[rfoll]

I forgot to mention that the lash is already tightened to .015"

[David Redszus]

Two simple questions if you please.

Where are the torque curves for your various engine configurations?

Why, in a drag race, the pursuit of speed instead of acceleration G forces?

[rfoll]

The only tool I have available for power curves is the cd version of the Comp cams software. Using a cam spec very close to what I have show a torque peak of 5000, and HP peak of 6500. But that info is based on an intake centerline of 106. Stepping up to the 850 carb shows a power increase, assuming I can get it dialed in properly. Curiously, the 296 BR4 cam shows a loss of 10 ftlb. and 4 HP. I'm not sure of what you mean by your second question. I'm not doing any kind of competition with this car. My only goal is reduction of elapsed time. This car started out running 14 flat at 100 mph. I managed mid 11s with a total investment of $2800 including the new slicks and wheels on it at the time. The car isn't pretty. People ask about when I will paint it, but my respose is paint won't make it faster. It gets some attention when the hood is up to cool it off, and they all ask "how much HP?" I really don't know what it is, but I do know what I can do to go faster. My goal here is to find another useful 500 rpm and maybe shave another tenth from the et without spending another $2000. I'm pretty sure the cylinder heads are a restriction, and a compression boost from a smaller chambe would be welcome, but my house needs a new furnace and I can't spare the cash.

[Stan Weiss]

Have a look at this thread.

viewtopic.php?f=1&t=62601&sid=4628841cf ... fc77ba4adf

What I think David was asking was why care about MPH? Sometimes you can improve et and actually run less MPH.

Stan

[rfoll]

I don't care about mph. With the current setup, 3.73/26", the car pulls hard right to the finish line. If I over rev the engine much over 63- 6400 in 1st or 2nd gears, I lose et. I can feel it lay down most obviously in second gear. Fuel pressure doesn't drop so the problem isn't there. It just feels like it's running out of something, be it carb, cam, or cylinder head flow. Normal gear changes drop rpm about 1000. The previous setup with the 4.10 gear/26" tire had me in high gear around the 1/8 mile marker. It's a long pull in high gear to the end and a significant amount of that is beyond that 6400 rpm. When I made the 4.10 gear change, I expected an improvement in et, but at the most maybe 1/2 tenth. The car launches much cleaner with that setup, 60' is more consistent. It seems to lift the tires smoother instead of jerking them off the ground. But it just doesn't have that vicious charge for the last half of the pull in high gear. If the engine stops pulling hard, i believe I am missing out on some available et. I've been reading about running the engine past the power peak for years, but it sees to me this is happening too early in high gear. The bigger carb seems to be an obvious first step If I can get the jetting correct.

[rustbucket79]

For the price of a timing cover gasket you could move the cam to the 4 degree retarded position, that will shift things higher. (Assuming appropriate cam and valve clearance) Rocker ratio might help but I would be more tempted to put the money towards a larger duration cam.

[1980RS]

Last year ran some 30X9 radial slicks for the 1st time ever and they made my car quicker and faster that any 28" tire I had run in the past. Even spinning a little the 30"s were still better than the shorter tires. I have always found the bigger tq. engines for me seem to work better with a taller tire but my high revving 358 likes the shorter tire or more gear. Will need to get a 28" radial slick some day to test it out.

[David Redszus]

In a drag race application, the most important parameter is linear acceleration.

A recording accelerometer (via GPS or sensor) will provide much more useful information
than time slips or ass based accelerometers. These can be had for about $450, which is very cheap.

If we know the acceleration G forces and mph, we then know the road power and rpm.
We now have an on-track dyno; and we can stop guessing.

[econo racer]

I wonder what the ET would be if you pulled a degree or two. Also I think an 850 carb might get you a better time. I like the 410 gears better. High rpm might not be better with your combo. Your still fast for your weight.