just installed my hotcam, how high are you racer taking the rpm's on this cam. 2800 stall will add 3:45 's this fall thanks.

 

12,300rpm.
Be sure and video your first run for us.....

Larry
code5coupe
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Did you change out the LT1 springs? If not don't bump up the rev limiter to 12,300 yet. I believe that the LT1 is limited by the valve springs so if you just popped in the Hotcam I would not increase the rev limit. Valve float sucks!!!
However, I have been wrong before.

 

Mine made peak HP on the dyno at 6200 RPM. That's also with fully ported heads and more compression than stock. I still think that's around the number most people have stated though. I hope you installed good valve springs with it.

I think you're right this time though.... Valve float still sucks, last time I checked!

 

With really good valve springs you can take an LT1 to 6500 RPM's safely for short bursts. Above that I don't think the bottom end can handle it.

Set the rev limiter to 6500 and try to set your shift points in the 6300 range.

 

I'm sure the mechanicals can take 6500rpm, the question is: how much power will you be making at that engine speed?
Going to 6500rpm may be counterproductive.
Once you go much past the power peak, revving higher is pointless.

Larry
code5coupe

 

The stock timing chain, wether new or not, is also a weak link when it comes to high rpms.

 

Mine seems to run best shifting at 5800, (factory setting) I have tried holding it longer in each gear but I usually run a little slower. Alvin told me my injectors are maxed out and basically I am running out of fuel on top end. I am going to eventually get the 30lb injectors and get him to retune it.
By the way, where is south boston?

 

Not always true. As long as you're still putting more power to the ground than you would be by shifting into the next gear, you want to stay in the current gear as long as possible.

A good example would be to compare an LT1 to an L98. Even though the LT1 makes peak power somewhat before the redline, gear multiplication still means that power to the ground is greater in first gear than it will be in second gear all the way to the rev limiter. Now take a TPI car that peaks at 4500 RPM or less. In such a setup power falls off so rapidly that there actually becomes a point that the drop in power actually outweighs the gear multiplication, making it necessary to shift even though the engine can continue to spin higher RPM's.

Most modified LT1's although they peak before redline will continue to make more power to the ground in the initial gear because torque doesn't fall off too quickly, meaning you want to wind them as tight as possible before shifting.

 

gear multiplication still means that power to the ground is greater in first gear than it will be in second gear all the way to the rev limiter.

I would like to see something more difinitive to support this theory, such as 60,' 330', 660' et's with raised shift points each time, say, 500rpm increments. I don't see how gear multiplication can overcome the rapid fall off of torque (and therefore HP) much past peak power for overral acceleration. I have always thought, as a rule of thumb, shifting about 500 rpm's past peak HP generally results in the best overrall et's.

I do have some experience in my altered roadster (years ago: 505" BBC with powerglide, injected alcohol). Whether I shifted at 7500 or 6800 made no difference in ET or mph. All I did was stress the engine at high rpms. So I eventually put a 7000 chip in the shifter and just leaned back and enjoyed the ride.

 

 

The definiative proof is quite simply in the math. IE think about this for a 6-spd car you have a first gear ratio of 2.68 and a second gear ratio of 1.80. You have a rear gear ratio of 3.45

Say your engine makes 300 ft-lbs of torque at 4000 RPM's.

In first gear you are effectively putting down 2774 ft-lbs as opposed to 1863 in second gear at the same RPM.

A six speed car looses 2100 RPM's from first to second gear, so look at the following example.

Imagine a car that makes 300 pounds of torque at 4000 RPM's but only 250 pounds of torque at 6100. In first gear at 6100 RPM's it will effectively put down 2312 ft-lbs as opposed to the 1863 it would be putting down if it were to go to second gear and land at 4000 RPM's. If you work it backwards you can see that in order to match perfectly the 1863 ft lbs in second gear at 4000 RPM's you would only need to make some 201 ft lbs of engine torque in first gear at 6100.

As such you have to have a VERY rapidly falling torque curve for it to make sense to shift any earlier.

Clear as mud?

 

I think the best way of going about this is at the track. you'll be anywhere from 6k to 6.4k but won't know for sure until you try YOUR car out.

 

Right, because the track will take into account EVERYTHING.

IE on my car the math tells me I want to shift at 6500 RPM's. Unfortunately though if I wait for 6500 to come up I hit the rev limiter, etc. Thus I have my shift light set lower in an attempt to compensate for my reaction time, shift time, etc, etc.

 

Actually you are absolutely correct........ and your explanation is a good one

I don't understand why people continue to have such a hard time with this concept......another clear as mud explanation is....

.......again saying the same thing in another way is that at any MPH you want to be in the gear that puts the engine at an rpm that puts the most torque at the REAR WHEELS..........NOT MAXIMUM ENGINE TORQUE...

or my favorite way is at any mph which gear puts the engine at the highest hp as determine by a dyno test..........note rwhp equals "net" crank hp...so again this is saying the same thing in another way....

 

Clear as mud?

I understand the example you have given. (I am a numbers kind of guy). What I do not understand is how a streetable engine, say one that hits peak HP at around 5200-5500, would accelerate faster if rev'd all the way to 6500 instead of around 6000. I do not think we are in disagreement on the multiplication factor of gears, I simply do not see many streetable engines making power at 6500. At that level no amount of rear gear ratio will make up for the drop off in power. The key here is at what level the engine HP peaks.

I think you have quantified a good example, but I'd hate to see anyone floating their valves thinking they will do better shifting at 6500. I believe what you are saying in your example is if there is enough of the torque curve left at some rpm past maximum HP, there is a benefit to revving to that level before shifting. I agree.

My roadster example was pretty clear evidence for once the engine exceeds its power curve, further revving is a wasted. Another pretty good example was a local RED ran a 671 blown 454 on alcohal. This guy reved that poor engine close to 9,000 before the 1-2 shift(powerglide). Eventually the engine grenaded bigtime. Meanwhile he was simply running 7.90's like the other cars in his class. Of course, I did not see him shift at a more sane 7500 as a direct comparison for the point I would like to make. I believe he would have turned about the same or probably even better ET's if he had.

Nate, I'd be curious if you would be willing to do a few back to back passes the next time you are at the drags, some shifting at, say 6000 and some at where you shift now, around 6400 and share the time differentials with us?

 

The dyno is always off by a couple hundred RPM's for some reason. My rev limiter is set ot 6500 RPM's. As you can see it continues to make power all the way to 6500 RPM's. The torque does drop off but not enough to justify shifting any earlier.

As for the drag racing I have done so on my car many times. Before I got my shift light dialed in exactly I had plenty of experience on this although it was quite accidental. The first time I set my shift light too late and I hit my rev limiter, of course my times suffered. Then I had the shift light set too soon and my times suffered. Since my shift light is mounted in the dash it can be hard to adjust. I finally just said screw it and drove it the old fashioned way and ignored the shift light. I immediately picked up 2 tenths. I later split the difference on the shift light and I'm more consistent.

As far as the next time I'm at the track, unfortunately that won't be any time soon. The local track owner is a complete jackass and I refuse to give him any more of my money

BTW my the dyno graph above is from my car, with hot cam and good flowing heads. So even a fairly mild cam can make good upper end power. Allow me to justify it though by saying that I have killer valve springs along with titanium retainers and 10 degree locks. You do have to have a stout valvetrain to pull it off, but it is perfectly streetable, I drive it every single day.

 

I think this is the key to your argument, being the power band of the engine. It is possible to have a very low peak and still have the capability to turn a high RPM. Imagaine a fairly small cam and / or an overly restrictive exhaust. Power peak and RPM potential are two completely different objects which are mostly goverened by different things. I would completely agree that on an engine that peaks in the 5200-5500 region it is very UNLIKELY that it is built to handle 6500 RPM's but it could be. You just have to be very cautious and know what you're dealing with.

A good example of this is the L98, it'll spin 6000 RPM's all day long. It might not make any usuable power, but it will spin it. As such the optimal shift point will likely be somewhere in between that horsepower peak and the actual physical redline of the engine.

We're not in disagreement, we were just kinda talking about slightly different things. I'm with ya now. Being said though the LTx engines like RPM.

 

Nathan's right about effects of gearing an example would be "top speed is usually not obtained in sixth gear!"

 

30 MILES NORTH OF DANSVILLE VA