importance of and improving power curve

[NormS]

A 421 should have significantly more torque than a 355. If you feel that going from 27.06 to 24.05 is too big a change, then try something in between.

[Keith Morganstein]

If the engine is making peak HP at 7500, running it 1500 rpm past that is like pissing into the wind. The engine probably makes more Hp at 8000 than it does at 9000, so I would say to gear the truck to run about 8000-8200 early in the run,and that should improve sled speed in that part of the pull.

We currently run a 27.06 final drive, to get the same wheel speed at 8000 rpm that it currently has at 9000 rpm it would need a 24.05 final. That would give it very similar axle tq to last years 355, and that set up gutted out at the end at every pull.

You think the 421 would stay on line at the end with a 24.05 final ?

Tire speed/sled speed comes much easier with rpm than gear.

Randy

I would want more wheel speed. It appears that the engine has enough power to do it. Maybe not, but I’d want to try it.

[randy331]

I would want more wheel speed. It appears that the engine has enough power to do it. Maybe not, but I’d want to try it.

Here's my thinking on it.

The 355 made 500 Lbs and we turned a 28.8 final. That set up would have had 14400 Pounds axle tq at peak tq rpm. That set-up went to 7400-8400 rpm mid track and gutted completly out at the end every time. 100% of the time. That set up would have 11344 pounds axle tq at 8000 rpm.

The 421 is making 610 tq on q16 and and we have a 27.06 final. That makes 16506 axle tq at peak tq rpm. It pulls to over 9000 rpm most of the time and has been brought back down to peak tq rpm at the end but sometimes spins out as high as 7000 rpm. At 8000 rpm it has 12934 lbs axle tq at 8000 rpm.

So a 24.05 final with the 421 would have 14670 pounds axle tq at peak tq rpm and 11496 axle tq at 8000 rpm. So at both points it'd have just very little more than the 355 had. So with that final the 421 would gut at the end and I don't think it'd get much past 8000 most of the time.

I agree with you guys that it needs some more gear but not 24.05. I think 26.3 is our next higher option. Got a guy with a 434 that has that gear and he can't pull it. Won't get on line good and wipes out at the end. He uses his 28.8 final. I'd like to see it on Creason's dyno so I'd have an idea if we could pull it. He'd trade us for our 27.06 gear. The 26.3 final would drop our 9000 rpm tire speed to 8657 rpm, but with less axle tq.
I need more power vs gear it will pull info.


We ended up with 1 5/8 x 1 3/4 home made step header on it cause nothing but custom stuff is available for that truck. Anyone got a guess what a bigger head would be worth on the 421 ?


Randy

[groberts101]

We ended up with 1 5/8 x 1 3/4 home made step header on it cause nothing but custom stuff is available for that truck. Anyone got a guess what a bigger head would be worth on the 421 ?


Randy

Holy chit! Not sure what your peak horsepower number is on this deal(725'ish?) but you must be pushing some SERIOUS velocity though those smaller primaries!!! Does that primary pipe cover up the exhaust port exit at all.. or you swaged/tapered it at the flange?

Just hypothesizing here, but I'd think your powerband would fatten up and nose over a little later with a straight 1.75" or stepped 1.75"/1.875" combo.

[nitro2]

If the engine is making peak HP at 7500, running it 1500 rpm past that is like pissing into the wind. The engine probably makes more Hp at 8000 than it does at 9000, so I would say to gear the truck to run about 8000-8200 early in the run,and that should improve sled speed in that part of the pull.

We currently run a 27.06 final drive, to get the same wheel speed at 8000 rpm that it currently has at 9000 rpm it would need a 24.05 final. That would give it very similar axle tq to last years 355, and that set up gutted out at the end at every pull.

You think the 421 would stay on line at the end with a 24.05 final ?

Tire speed/sled speed comes much easier with rpm than gear.

Randy

For the part of the pull where you normally run 9000 rpm (assuming peak power at 7500 rpm or so ) the gear change from 27.06 to 24.05 will generate MORE axle torque because the engine will make more power at 8000 rpm than at 9000 rpm (i.e. the engine torque increase going from 9000 to 8000 rpm more than offsets the decrease in torque from the gear change). But anytime you aren't spun up high like that (whether it be at the start of the pull or the finish) this won't be true and the gear change will try to bog you down more.

[steve316]

by the rpm on the tack during the in cab video I wouldn't change the gear at all. By the tack the pull didn't go past 8200.

[CamKing]

Power above 8,000 is meaningless, because the sled weight is much less when you leave. As the weight goes up on the sled, it drops the RPM below 7,000, before it finally gives out. You want to focus on increasing the efficiency in the 7,000-8,000 rpm range, so it won't pull the RPM's down so far. Move peak torque up towards 6,500rpm. Design intake runner length for 7,000rpm. Design exhaust runner length for 8,000rpm. You can give up power below 6,000, and above 8,000, to increase the VE in the 7,000-8,000 range. Above 8,000, you just want the valvetrain to be stable. You can be down 100hp at 9,000rpm, and the sled will pull the engine back into the power curve.

[randy331]

I'd say if I did something that lost 100 hp at 9000 rpm I'd no longer need to worry about valve train stability at 9000 rpm cause it'd never seen 9000 rpm again while hooked to a sled. The truck/sled combo weight is about 35000 - 40000 LBs it takes some power to accelerate that much weight even before the weight comes forward and starts draggin the pan. Last years 355 would never see 9000 rpm even running a lower gear than this year, so it takes some power to get to 9000 rpm.

But,.. I know power ain't 100% of this either cause we were second Sat. to an engine I did and I know it's less power than ours.
I did the engines in 3 of the trucks Sat. so I'm starting see some power/power curve vs gearing and sled set ups that helps with future decisions.

The variables shuffle the leader board around. 2 weeks ago we were second place to a truck we have been beating regularly and we put 18 other trucks behind us. The next pull that will likely shuffle around again.

Randy

[randy331]

Holy chit! Not sure what your peak horsepower number is on this deal(725'ish?) but you must be pushing some SERIOUS velocity though those smaller primaries!!! Does that primary pipe cover up the exhaust port exit at all.. or you swaged/tapered it at the flange?

Just hypothesizing here, but I'd think your powerband would fatten up and nose over a little later with a straight 1.75" or stepped 1.75"/1.875" combo.

The current headers fit the ex ports pretty nice, but I still think there is power in a bigger pipe. That's one of the plans for next year.
Trying a bigger header and bigger carb both are on the list.

Randy

[77cruiser]

Is it going back to the dyno after the season is done? What header was it dynoed with?

[randy331]

A 421 should have significantly more torque than a 355. If you feel that going from 27.06 to 24.05 is too big a change, then try something in between.

The 421 does have more tq, but we pulled a 28.8 gear with the 355 and it gutted it at every pull. We run a 27.06 currently with the 421 and it has yet to gut, but has been pulled back down to 5500 ish rpm. So I think some more gear would help, at least at some tracks, but not 24.05. 24.05 is pro stock truck gearing.

26.2 is our next option I think.
I could buy a 3 speed kit for it and have 3 options, but then I'd never know which one to run. LOL

Randy

[randy331]

Is it going back to the dyno after the season is done? What header was it dynoed with?

That's the plan at some point over the winter. We dynoed with a 1 3/4 x 1 7/8 header cause the pulling truck headers were still in use at the time.
But, I need to make or have a header made first.

Randy

[77cruiser]

I'd be curious to see how much the 1 5/8 step headers were hurting you.

[randy331]

I'd be curious to see how much the 1 5/8 step headers were hurting you.

It may not be as much as we think.
I may be disappointed when I buy/build a nice straight 1 3/4 x 1 7/8 step header and gain 7 HP but loose 5 Lbs tq. But you never know,.. could be 25 HP.

Randy

[CamKing]

I'd say if I did something that lost 100 hp at 9000 rpm I'd no longer need to worry about valve train stability at 9000 rpm cause it'd never seen 9000 rpm again while hooked to a sled. The truck/sled combo weight is about 35000 - 40000 LBs it takes some power to accelerate that much weight even before the weight comes forward and starts draggin the pan. Last years 355 would never see 9000 rpm even running a lower gear than this year, so it takes some power to get to 9000 rpm.

But,.. I know power ain't 100% of this either cause we were second Sat. to an engine I did and I know it's less power than ours.
I did the engines in 3 of the trucks Sat. so I'm starting see some power/power curve vs gearing and sled set ups that helps with future decisions.

The variables shuffle the leader board around. 2 weeks ago we were second place to a truck we have been beating regularly and we put 18 other trucks behind us. The next pull that will likely shuffle around again.

I'm just telling you what all my winning customers are doing. They're using gear to turn 2,000 rpm past peak HP, and the sled pulls them back towards peak hp. Maybe they have different type sleds. In your video, you were at 8,000rpm for a while, and then the sled pulled the RPM all the way down into the 6,000 range.