matching cam/ stall/ and rear gears

[Forces]

this is a side question to this link http://www.chevytalk.org/fusionbb/sh...hp?tid/131229/ which was recently posted in a 383 thread.

 

1. How do you 'match' your cam, stall speed, and rear gears? What calculations do you use?

 

2. What does your rear end gear ratio have to do with cam selection? Is it related to the amount of time you will spend in your ideal RPM range?

 

3. Case study:

Lets say you have an engine built primarily to obtain a maximum 'peak hp' vs. a 'street' engine built to perform over 1,500 - 6,500. How would that effect what stall and rear gears would be the best fit?

[grumpyvette]

this is a side question to this link http://www.chevytalk.org/fusionbb/sh...hp?tid/131229/ which was recently posted in a 383 thread.

 

1. How do you 'match' your cam, stall speed, and rear gears? What calculations do you use?

 

 

In most cases the your cam manufacturer will list a cruise RPM your stall should ideally be just a few hundred rpm lower,for steet use,as to the cam sellection, your trying to have the cams power range fall between the stall and max SUSTAINABLE rpm,(think valve float, volumetric efficiency and trans shift points) for hydrolic cams thats almost always at 6000-6300rpm

on solids it al;most always EITHER at 4500rpm of piston speed or when your getting just under valve control issues (pick the lower rpm value)

 

2. What does your rear end gear ratio have to do with cam selection? Is it related to the amount of time you will spend in your ideal RPM range?

 

its so you maximize the volumetric efficiency (cylinder fill/scavaging)in your torque curve, and maximize the time you spend in the most productive power range your engine produces

 

3. Case study:

Lets say you have an engine built primarily to obtain a maximum 'peak hp' vs. a 'street' engine built to perform over 1,500 - 6,500. How would that effect what stall and rear gears would be the best fit?

 

race engines that produce a higher and narrower power curve need to be geared so youll spend a great deal more time closer to max rpms (between the torque and power peaks

 

 

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

 

OLD POST

 

we have all heard it, " you need massive low rpm tq" "you need a screaming high rpm hp peak" well heres some info,

More in-depth description:

http://www.revsearch.com/dynamometer/torque_vs_horsepower.html

http://www.dynacam.com/Product/Torque_vs__Horsepower/torque_vs__horsepower.html

http://vettenet.org/torquehp.html

http://auto.howstuffworks.com/horsepower4.htm

http://homepage.mac.com/dgiessel/engine/hpvstq.html

 

first thing to keep in mind is that theres no such "thing" as horsepower, horsepower is a mathmatical formula explaining the RATE at which TORQUE (rational force ) can be applied increasing the torque at a given RPM or increasing the RPMS with a given torque increases THE RATE(HORSEPOWER AVAILABLE TO DO WORK)GEARING ALLOWS YOU TO APPLIE THAT FORCE at a given force level but not at a RATE APPLICATION LEVEL that exceeds the engines peak power.

put a differant way, if we ignore drive line friction loss,if an engine puts out 400 ft lbs at 6000rpm (400 x 6000/5252equals (457 hp)we can gear it to apply 1600 ft lbs at the wheels by running the gearing at 4:1 but that also cuts the application rate by 4:1 so the HP applied stays constant at the 1500rpm at the rear wheels 1600 x 1500/5252 equals (457 hp)

 

the formula for hp is (tq x rpm/5252=hp

example

450 ft lbs of torque at 3000rpm=257hp

450 ft lbs of torque at 6000rpm=514hp

because the torque is available at that higher RPM RATE and at the higher rpm useing gearing the rotational force the engine supplied can be applied faster or slower to the rear tires

 

[color:"red"] here read these ALL CAREFULLY [/color]

http://www.69mustang.com/hp_torque.htm

 

http://www.ubermensch.org/Cars/Technical/hp-tq/

 

http://vette.ohioracing.com/hp.html

 

http://auto.howstuffworks.com/engine1.htm

 

http://auto.howstuffworks.com/horsepower.htm

 

http://auto.howstuffworks.com/question622.htm

where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.

example

in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)

in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,

you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.

a 3.08 rear gear and 700r4 trans matches the first example well but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.

 

links youll need to figure out correct rear gear ratios

 

 

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

 

http://www.miata.net/garage/tirecalc.html

 

http://www.wallaceracing.com/reargear.htm

 

http://users.erols.com/srweiss/calcmph.htm

 

http://users.erols.com/srweiss/calcrpm.htm

 

http://users.erols.com/srweiss/calcrgr.htm

 

http://www.prestage.com/Car+Math/Ge...io/default.aspx

 

http://www.geocities.com/z28esser/speed.html

 

http://server3003.freeyellow.com/gparts/speedo.htm

 

http://www.pontiacracing.net/trannyratios.htm

 

http://www.tciauto.com/tech_info/gear_ratios.htm

[dr_hunt]

this is a side question to this link http://www.chevytalk.org/fusionbb/sh...hp?tid/131229/ which was recently posted in a 383 thread.

 

1. How do you 'match' your cam, stall speed, and rear gears? What calculations do you use?

 

2. What does your rear end gear ratio have to do with cam selection? Is it related to the amount of time you will spend in your ideal RPM range?

 

3. Case study:

Lets say you have an engine built primarily to obtain a maximum 'peak hp' vs. a 'street' engine built to perform over 1,500 - 6,500. How would that effect what stall and rear gears would be the best fit?

 

Well, IMO it depends on what your trying to do. Assuming 1/4 mile, then the converter stall speed is about 500 rpm below peak torque rpm. Gearing should allow you to trap at peak HP. (ie. 6500 rpm max HP, peak torque at 4200, run a 3800 stall converter, with a 26 inch tall tire, probably about a 4.30 gear)

 

Street driving, converter selection is the same, but typically for the street you don't run over a 3500 stall, and gears are whatever gets you your happiest cruise rpm which is about 40 to 50 percent of your redline rpm, which puts you into the engines efficiency range. (ie. 5000 rpm redline would give you a cruise rpm of about 2000 rpm at 75mph, perhaps 2500 if the application involves a heavier vehicle or towing. Peak torque around 2700 rpm so you would select a 2200 rpm stall converter)

[A. G. Olphart]

Forces, your #3 (case study) seems to indicate that you are planning a track car. This is for others reading the thread whose cars will see road time...

 

High stall (loose) converters can develop a lot of heat on the road, shortening transmission life. The 1/4 mile setup Dr. Hunt has described would also be a bear to live with on the highway. These problems may be avoided/ minimized, by running an overdrive and a lock up converter.

 

(Sorry if this seems to be stating the obvious:oops: , but we have a very broad range of knowledge and experience in our membership).

 

 

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[Forces]

Forces, your #3 (case study) seems to indicate that you are planning a track car. This is for others reading the thread whose cars will see road time...

 

High stall (loose) converters can develop a lot of heat on the road, shortening transmission life. The 1/4 mile setup Dr. Hunt has described would also be a bear to live with on the highway. These problems may be avoided/ minimized, by running an overdrive and a lock up converter.

 

(Sorry if this seems to be stating the obvious:oops: , but we have a very broad range of knowledge and experience in our membership).

 

 

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Thanks for the info. I am planning on making some trips to the track this summer, but MY car is gonna be 75% street and 25% 1/4 mile, so I would rather aim towards a street build. My engine is made to perform best at ~1000 rpm - 5000 rpm, got 3.90s in the back and a TH 350 stock lock up for now. I am going to use an over drive as soon as I get some of the other issues out of the way that take priority over the trans right now.

[A. G. Olphart]

Thanks for the info. I am planning on making some trips to the track this summer, but MY car is gonna be 75% street and 25% 1/4 mile, so I would rather aim towards a street build. My engine is made to perform best at ~1000 rpm - 5000 rpm, got 3.90s in the back and a TH 350 stock lock up for now. I am going to use an over drive as soon as I get some of the other issues out of the way that take priority over the trans right now.

 

You are very welcome.

 

Boy, do I speak 'Priorities'.

 

 

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