Help with 60ft vs MPH vs ET

wheelsup · Post by **wheelsup** » Sat Dec 17, 2005 6:05 pm

I'm trying to figure out if my combo has any more left in it. It has a slow 60ft at 1.57, a mph of 132, and an ET of 10.50 - with no tire slip. (track is at 5800 ft)

I have a 3000 lb car, roller cammed 454, 5700 stall, 1:76 glide, 4:56 spool, 32x14.5 tires, ladder bars, 6600 rpm at the trap.

I'm wondering if I change the rear gears to 4:71 (to keep it under 7000 rpm) if it would imrove the 60ft, lower the mph and leave the ET unchanged? Or do I just have a mismatch somewhere?

It just seems that with 132 mph the car should be quicker. Any ideas?

Fatman · Post by **Fatman** » Sat Dec 17, 2005 6:56 pm

Wheelsup

Do you know the rpm at which it is making peak power? This might help with the replies.

wheelsup · Post by **wheelsup** » Sat Dec 17, 2005 8:20 pm

Horsepower approx 7000 rpm
Torque approx 5400 rpm

MadBill · Post by **MadBill** » Sat Dec 17, 2005 11:27 pm

Since you are having no trouble hooking up, if the engine internals are good for say 7,500 RPM, going to ~ a 5.11 would get more HP area 'under the curve' and increase the tire thrust by 25% all the way down the track.
Also, depending on cam and fuel, at that altitude you can probably get away with at least 15:1 or more CR..

Speedy Goomba · Post by **Speedy Goomba** » Sun Dec 18, 2005 12:47 am

wheelsup wrote:Horsepower approx 7000 rpm
Torque approx 5400 rpm

Optimal gearing puts the engine 5% over peak HP crossing the stripe. You should cross the line at 7300-7400rpms.

I'd say you need a 5.13 gear.

MadBill · Post by **MadBill** » Sun Dec 18, 2005 11:30 am

I assumed a MPH increase from the change, thus my suggested ratio is slightly lower than Speedy's.
(That's my story, and I'm sticking to it!)
PS: Such a big gear might overwhelm the tires at launch and require a little less converter for best results. (which in turn will have less slippage at the top end, pulling the revs down and demanding a re-think of the axle ratio...)

flashback · Post by **flashback** » Sun Dec 18, 2005 12:34 pm

For comparison, we run right at 130 on a good day (127-128 normally) and our best et was a 10.300 and averaged in the 10.40-10.50 range in the heat of the summer...our 60ft times are consistently between the 1.40 and 1.44 range. I thought that my 60 ft's were a little weak for our combo, but maybe they're where they ought to be???
We run a 383" SBF low .600 lift roller cam...12.5comp, C4 trans w/brake and an 8" converter, 4.56 gears w/29.5x10.5x15W tire...leave the line at 5000 and shift at 7400. On a good weather day we'll go through the lights right at 7400 but in the heat of the summer the rpm is a couple hundred lower.
Any suggestions on this combo for us??

Thanks.

hotrod · Post by **hotrod** » Sun Dec 18, 2005 4:01 pm

From my experience up at Bandimere (in my car turbo AWD) an ideal run is when my mph and et multiplied together comes out to approx 1352.

If you look at NHRA national records for your class, you can get a very good idea of what an ideal run for typical chassis setups is in your class should be. Most come out very close to 1320 - 1350 depending on the class.

Working that formula backwards on an ideal run with a trap speed of 132, you would have an ET of about 10.24 as a limit for your chassis and power level (based on my cars numbers).

If you have a log book for runs, crank the ET X MPH numbers and you will see a pattern for best possible combos.

If the ET mph product is high your probably giving away ET, if it is low,your trap speed is weak (engine may be laying down on top).

Each car and driver /strip is a bit different so my numbers will not directly apply to you but they should be close.

I use these derived numbers as a "run quality index" It gives me a quick check on the quality of a run after making changes, even though the ET and MPh both changed.

In my experience the theoretical best possible 60' time for my car is near 178.5 / mph, if that applies to your set up, you should be able to get near a very high 1.39 if the chassis and track prep are perfect.

For example using that 178.5/mph, that would indicate a theoretical limit of about 1.733 for my power level. My best all time 60' on this car is a 1.727 on street tires. That is approx 0.1316348 of my ET.

I use my mph x 60' time to get a "launch quality index" , on a perfect run it would be near 178.5, but as you compute these two run quality indexes you can see things like a gradual decline in launch quality ( tires getting hard etc.) even though your actual times are continually improving due to other changes.

Applying that same fraction to your car would result in a best possible 60' near 1.382

If you have a good collection of run slips and or a log book, you can learn all sorts of things by running some numbers on all the runs and see how things fall out. I find these derived numbers very valuable as a sanity check against what I think is happening. You can also run these numbers on your competitors and get a feel of where they are running in a run quality sense even if their times are better or worse than you.

You will see some drivers that consistantly run very high qualitiy runs even though their equipment is not up to par for the class in power, while others make up for poor quality with raw power.

Larry

T RICK · Post by **T RICK** » Thu Dec 22, 2005 5:38 pm

I used to run a SG car at Bandemire similar to your combo except it was 2300 lbs with driver. 460 bbc flat tappet, small port, small valves, glide 1.76 5.13 rear. With the throttle stop off. The best I ran was 10.04 @130mph never cracked the 9s. I would go the 5.13 rear. Shed some weight if you can. If max hp and tq is know (was it dynoed?) stall to max rpm shoud be around 1500-1700 rpm. We uesd Quater and Quater junior softwear to model changes and found at the time it was pretty close.

Rick

hbps1978 · Post by **hbps1978** » Sat Dec 24, 2005 1:19 pm

I personally would say go with a 4.88 gear from my own experience with a heavy big block car.

T RICK · Post by **T RICK** » Sat Dec 24, 2005 3:43 pm

The problem with racing on the mountian in denver is not only the elevation it is the extreme changes in air density. A good air day is 7000ft corrected. Most summer racing is 8500 to over 10,000ft corrected. The 4.88 would work at sea level but I would prefer the 5.13. The reduction in HP is the problem, more gear is needed to carry the 60ft or it will fall of like a rock. A lot more info is really needed to be real specific. Our comp car we ran a PG with a 1.90 to a1.98 low gear until we built more power then we ran a 1.82 and could mantian the same 60 foot readings. . Call Mark or Bob at Baldwin auto (303) 371-7176 Comp racers we racer together from super street on up for years.

Rick

Arthur · Post by **Arthur** » Sun Dec 25, 2005 4:53 am

Merry Christmas one and all,

The best solution l see is to use a 3-speed G/box, Transbrake at 4800rpm,your induction system is....?, your combo should 60ft at 1.35 when sorted, this would take you where you should be,rear gear to be changed as a final tweak,

All the best,

hbps1978 · Post by **hbps1978** » Sun Dec 25, 2005 8:19 am

T Rick, yes I would agree with you on the 5.13. I forgot that wheelsup stated that he was at 5800 ft.

wheelsup · Post by **wheelsup** » Sun Dec 25, 2005 2:48 pm

T RICK - I had a 1.96 low until I broke it last July. Thats when I went to the standard 1.76 gear. My 60 ft was better by .060 but mph at the top was down to 130. The ET was the same with either gear.

Arther, I did run a T-400. Same story, quicker 60 ft but slower top end charge - ET was within .020. I think the additional weight and lower efficiency of the 400 slows it down on the top end.

Induction is provided by a 1050 dominator with a Victor 454R manifold. It pulls like a bottle rocket at the top end.

T RICK · Post by **T RICK** » Sun Dec 25, 2005 4:56 pm

When talking gears and convertors and the desire to not wind the engine makes it hard. First where is the powerband and how wide is it. Second where do you shift out of first gear. Do you know your convertor efficencey? As far as wanting to limit the RPM what is your reason? is this a class car?

Rick

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