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Discussion Starter #1
I have a good honest question here that should reguire a lot of different thoughts. What is the proper rpm's to shift? Is it at the highest torque rpm? or maybe the highest HP rpm? or is it a compromise of the two? When I run my combo on the dt dyno it shows that I max out torque at 5000 but I max out of HP at 6500. I stil am within 60 HP at 7000 but I am down 120 fp of torque at 7000. on the graph the torque and HP cross lines at 5400rpm's. In real life my motor runs into a brick wall at excactly 5400 rpm's. It just quits pullin altogether and won't rev any higher. I have the good Holley electric red fuel pump, brand new Crane springs and related stuff, New Holley 750, Trick ignition new 3/8 fuel line from inside the tank to the carb. the dt dyno shows me that the lifters are pumping up ( I dont have anti pump ups or fast bleed yet). I have some Rhodes on order for later next week. But any way,
,what determinds when you guys shift, It makes no sence to pull past the torque but maybe torque is what gets you off the line and HP is what gets you to the other line, and if that is the case then I would be better off to shift at 5400 when the torque meets the HP. Maybe not
Anyway I am goin racin for the first time in 28 years as soon as the weater clears and it ought to be fun
Bob
 

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well it has been my experience that the dt dynos are basically of no use other than when building a motor to give u a basic idea of what components will work with one another correctly...as they do not take alot of things into consideration and are based upon perfect conditions...
but in all honesty if ur car quits pulling at 5400 then that is where u need to shift bro.

just my .02 worth
 

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Sorry about that,back to your question.
Ive always used peak HP as my shift point and diff gears and/or tyre size to produce a finish line RPM 10% above peak HP.Here's why.With a 302 I had I dynoed the car and found peak HP to be at 4800RPM,I used this RPM instead of 6000RPM I had previously used and went 0.5 secs faster with no other changes to the car.This was enough to convince me I was on the right track,(no pun intended)not to mention less wear and tear on the engine etc.
Hope this helps, but it may just add fuel to someones fire.
 

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Most of the stock/super stock guys like to shift 300-400 rpm beyond the peak hp. That way the motor "falls" right into the heart of the powerband after the shift. At least that's what I have been told by a class record holder.
 

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Dyno's are good for showing you the numbers.
Actual 'assometer' (seat of the pants), and time slips are the best. My last combo made peak tq @ 5400. and peak HP @ 6400. AT the track I pulled her to 7000. It still made power/pulled good til there. AFter 7100 it would start, (again, assometer/time slips), would fall over. (not hard, but you could feel it). Lots of back to back runs, altering shift points, and 6900-7000 was optimum. (at least my combination).
Everything's gotta be right tho. (proper fuel/timing/etc. etc.). If one thing's off, it's going to throw everything else off as well). My .002
 

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Hmmm ... now there's a thought I don't hear people talking about much


Maybe, Just maybe, it's as important to know where in the powerband the next gear is going to grab hold ... as it is worrying about the power left in the gear you currently have hold of.
 

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Torque gets an object moving, horsepower keeps it moving. Easiest way to get your best shiftpoints is, as it was stated earlier, the Assometer. Go race it, short shift it, maybe around 4500, then wring it out, see which is quickest

'66 Mustang
8.89 @ 155
1.21 60'
Got 11 other cars too
 

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Alright, here's the lowdown: Horsepower is a measure of acceleration. You can increase torque with lower gears, but you can't increase horsepower with lower gears, you can just make it so you get up to peak horsepower sooner.

Since your acceleration is directly proportional to how much HP you're getting to the rear wheels, you want to have the highest average horsepower possible through the entire 1/4 mile run. If you shift right at your peak power, say 400hp @ 5000rpm, then your RPM will drop well below your peak power after you shift, like 300hp @ 3500rpm. But, if you shift a little higher, say at 5800rpm, where you're still making ~370hp, then after the shift you'll drop to about 4000rpm, where you're still making around 350hp.

So you can see that your power to the ground never drops below 350hp when you shift AFTER peak HP, while if you shift AT peak HP, it drops clear down to 300hp for a while.

Basically what you are looking to do is maximize the area under the HP curve between the RPM you shift at and the RPM it drops down to.

Also on a side note, your HP and torque curve will always cross at 5250rpm, it's all in the units (if you've done much physics). HP=(RPM*Torque)/5250
 

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Discussion Starter #10
Yes, I have noticed that on all the dyno runs that I have run on the dt dyno with my combo and others as well that the torque and HP curves cross at the same rpm, around 5250 as you stated in the last post. While I passed physics in college I guess that one got by me as to why that is. I was also wondering if that had something to do with my rev problem as well because I have tried every other thing to fix it except change to a fast bleed or anti pump up lifter....Bob
 

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Well, they didn't teach us that one in physics class, but what I meant was that all the units are related, so it makes sense that hp and torque are closely related. Lots of low-rpm torque means lots of low-rpm horsepower, which of course moves you of the line faster. But greater overall horsepower is what accelerates you down the strip faster.

How about some specs on your engine components so we can figure out why Desktop Dyno is showing very different figures than your arse is feeling. If DD says your peak power is at 7000rpm, that must be a pretty big camshaft, but most hydraulic lifter cams valve float at around 6000, I think. What is the advertized RPM range on your camshaft? Maybe your other components aren't matched to such a big camshaft.

DD doesn't seem to take lifter pump-up into account on hydraulic cams, all it does is assume a less aggressive valve open/close rate when you select hydraulic.

Also, make sure you are choosing heads in DD that closely match what you are actually running. Heads make a HUGE difference in the program, and the selection is not that descriptive. I put flow numbers for my heads in to Dyno2000 and cam specs right off my cam card, and made all other selections correctly, and it's showing pretty much exactly what I figured my engine is putting out. Remember, bad data in = bad data out.
 

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On 2002-02-01 13:17, Motorhead wrote:
Alright, here's the lowdown: Horsepower is a measure of acceleration. You can increase torque with lower gears, but you can't increase horsepower with lower gears, you can just make it so you get up to peak horsepower sooner.

Since your acceleration is directly proportional to how much HP you're getting to the rear wheels, you want to have the highest average horsepower possible through the entire 1/4 mile run. If you shift right at your peak power, say 400hp @ 5000rpm, then your RPM will drop well below your peak power after you shift, like 300hp @ 3500rpm. But, if you shift a little higher, say at 5800rpm, where you're still making ~370hp, then after the shift you'll drop to about 4000rpm, where you're still making around 350hp.

So you can see that your power to the ground never drops below 350hp when you shift AFTER peak HP, while if you shift AT peak HP, it drops clear down to 300hp for a while.

Basically what you are looking to do is maximize the area under the HP curve between the RPM you shift at and the RPM it drops down to.

Also on a side note, your HP and torque curve will always cross at 5250rpm, it's all in the units (if you've done much physics). HP=(RPM*Torque)/5250
Motorhead is correct. Ideally the thing to shoot for is the highest average horsepower. However there is one more factor that has to be considered. Because the engines rotating assembly, flywheel, torque converter, and accessories, have mass, they absorb power as their RPM is increased. This power is then not available to the rear wheels. Since the motor revs up progressively quicker in relation to vehicle speed with lower gearing, the actually shift point will always be somewhere before the point where the highest average HP is achieved. To take it one step further, the ideal shift point for every gear is different. If a vehicle has a very low first gear ratio, and steep rear end gears, it will cause the engine to rev quickly. So much power is then being absorbed by the engine just getting it to rev up fast, that it would be beneficial to "short shift" first gear. In second gear the engine won't be increasing in RPM so rapidly and their will be more torque available at the rear wheels for acceleration. This is one place where Drag Strip Analyzer programs are good. They figure out in seconds what would take a human hours of math. Just as an example, an 11 second 4.56 geared, wide ratio 4 speed, 351W powered Mustangs ideal shift point, might be close to what my computer program recomeneded: 1-2 6400 RPM, 2-3 6600 RPM, 3-4 6650 RPM.
 

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Discussion Starter #13
Engine specs for my combo are as follows:
72 351C 4 Bolt
.030 flat tops with .080 recceses
stock crank and rods
510-536 lift 214-224 dur. at .50 112 LS
Crane hyd. lifters
cam is run straight up
open chamber 4V heads, 2.19 intakes 1.7 ex.
pocket ported
9.1 comp.(milled heads and block)
Wieand excelerator 4V intake (single plane)
performance ignition
750 vac. Holley
timing runs best at 16 deg.
1 3/4 long tube headers
8 qt Canton oil pan
Holley Red fuel pump
3000 stall
3.89 posi 27 inch tall tires
New Crane springs and related stuff to match camshaft.
Still quits at 5200-5400 Like runing into a wall.
I would like to sift it at about 6500 if I can get it to rev there.
Bob
 

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torque and horsepower are always the same at 5252rpm not matter what engine or combo. thats because torque is used to figure horsepower.
horsepower=torque x (rpm/5252)
thereford, when rpm = 5252, then horsepower=torque
Its explained more in the "What is Horsepower" article in the archives
 

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Bob ... Clearly there are more "seasoned" minds than mine in this forum. But (at the risk of being made a fool) here's my two cents.

I don't think you have the cam for what you are looking to do. Those 4V heads want High RPMs ... The intake want's high RPMs ... but the cam really doesn't.

I'm building a 351C Australian engine built for low end torque ... The cam I have selected has the same specs as yours except for a higher lift (it's a hydraulic roller) it produces (DT Dyno est.) 414 HP @ 5,500 RPM ... But on the torque end it produces over 435 Ft Lb all the way up to 5,000 RPM with a peak of 472 @ 3,500 RPM.

If my estimates are any reflection on what your cam does ... It should start falling off at 5,500 RPM ...

And on the flip side, where the cam is trying to do it's best work (at lower RPMs) ... The heads and intake are a handicap.

OK ... I'll bend over and grab my ankles ... you guys can take turns kicking my A$$ now




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<font size=-1>[ This Message was edited by: monstermach on 2/2/02 8:35am ]</font>

<font size=-1>[ This Message was edited by: monstermach on 2/2/02 10:29pm ]</font>
 

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Well, my turn to show how its done when trying to look like a fool.

For a 4v cleveland, that cam is way too small, the convertor is too tight, and the gear is too tall. And the non pump-up lifters are killing it too. Those heads like rpm, as does that intake, and yet the gear is designed for a more "torquey" motor, as is the convertor. That wide LSA on that camshaft is also a big hinderence, it doesnt allow for enuff port velocity to properly fill the chamber. The last hydr cam I ran had 225* @ .050 duration on the intake, but it had a 108 LSA. It pulled real good to 6400 with stock 4v heads, weiand intake, and a 750 vac secondary carb.
 

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Discussion Starter #17
WHOA, CALM DOWN FELLOW FORDIANS, Now we are getting to the point of the post. BUT the problem is that I have gone back and forths between the Wieand and the good ole Edelbrock F351 and the rpm's didn't change one bit, still closes off at the same rpm's. However, with that said, you would not believe the amount of torque that this thing put's out on the bottom end. I run 265-50-15 radials on the rear and off idle it goes up in smoke. Not what I have heard about 4V heads. What I am asking is this: Run my combo on your dt dynos, mine say's that with this cam I should put out 401 HP at 6000 and 403 at 6500. I know that the power is there but at a lower rpm than you would expect. I intentionaly built this thing as a low mainenence street car to begine with. That is why I picked the cam that I did. BUT now it is time to go a racin and I needed your input. I am sure that I will lose a lot of bottom end torque so I will have to rev it higher with a bigger cam. I don't want to tear it back down ( it only has about 1000 miles on it)so I have to decide to go bigger and risk hitting the pistons, or stay the course with the cam that I have in it and shift it at 5200. I don't believe that I am doing the heads justice if it remains as it is. But like I said, this bitch is a beast off the line. SO run it on your dt dyno and help me dicide
Bob
 

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Bob P. did you ever try decreasing your lifter preload to 1/8 turn, as I had reccomended before? I got about 500 more rpm from my 4V cleveland by going from 1/2 turn to 1/8 turn lifter preload. Let me know if the rhoads help too. Good luck, Tom
 

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Bob ... Is the cam you refer to on the "Cam ... Flat Top" post the one you are considering for this build?

If so ... I think you should see some real improvement ... a more high RPM oriented cam will put you where you seem to want to be ...

Jeff Given "MightyMach" has a good Mechanical Flat Tappet cam in his 351C 4V and I have a good Mechanical Roller in mine ... We could get you part numbers if you would like to compare.

The real brute strength of the 351C 4V is in "top end" power
 

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Now for my off the wall, off the mark, off the cuff remarks. I don't think the cam is stopping it at 5400 rpm. It is also not a 6500 rpm type cam either. My 'intuition' tells me that those specs should peak around 5500 and give you useable power to 6000. So, that leaves the question: "What is 'stopping' the engine at 5400 rpm?" My first thought was ignition. What type of ignition system are you running? If it's a 'box', does it have enough current supply capability on the input wire? Spark plug gaps might be too big. Even with a high powered box, most engines like .040 to .045" gap. My next thought is fuel supply. Do you have 6 psi at the carb wide open at 5 grand? Another thought, Is the cam degreed in straight up? (I know you said it is.) It could be substantially advanced according to your description of huge torque but lack of rpm. Degreed means checked against true top dead center, not just thrown in according to the keyways provided. Another thought is that the springs could have a 'harmonic' which let's the valves float enough around 5400 to 'stop' the engine there. If it is a single spring without damper, this is possible. If you can 'drive through' this flat spot after another 300 rpm and it picks up again, it is spring harmonics biting you. Just a few thoughts. Tell me if I'm way off...
 
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