Howdy

I’m running my first big block and really enjoying it. I can drive in the rain no problem if I take it easy and it lays black stripes any time I ask it to.

It’s a 512 motor but really mild- forged source kit, ootb stealth heads & valve springs, comp hydraulic flat tappet 240°/246° & .507”/.510”.

The cam card says power from 2300-6500 but I assume that’s in a 383 motor. I have a 2400 stall.

I built this thing trying to be smart and keep things cheap and mild for a fun durable street motor. I tuned it on the chassis dyno and stopped at 5500rpm- the torque is over 525 and slowly tapers down to more like 450 at 5500rpm but the rear wheel hp was still climbing there, hitting 362 at 5500.

Would I be beating the crap out of this thing if I raised the rev limiters to 6500? I know from stroking harley motors that when you go over 4” stroke piston speeds get kinda high but not crazy until more like 5” so I’m thinking that valve float with the hydraulic flat tappet will be my limiting factor?

How high would you twist it? After playing around with it some I started wondering how much power I’m leaving on the table by keeping a low redline, especially since rwhp was definitely still climbing at 5500.

Thanks!

Radar

Go back to the chassis dyno and make a few pulls, adding a few hundred rpm to the top of each pull......... up to the point where you encounter valve float, or reach an rpm limit you’re comfortable with.

I think you’ll reach the valve float stage well before 6500.

Whatever rpm it occurs at....... the “redline” should be a few hundred rpm lower than that point.



Look your sheets over more carefully.
The hp is just a calculation of tq and rpm. If you have 450tq@5500 rpm...... that’s 472hp.

Conversely....... 362hp@5500 is 344ft/lbs.

HP is always higher than TQ at rpms above 5252, TQ is always higher than HP at rpms below 5252.

I wouldn't shift it past 5800-5900 tops.

I have a similar set up. Best et has been shifting at 5500. 7.02 in the 1/8 and 11.08 in the 1/4. Thats at around 4100 lbs. Runs out of steam above that. No reason to beat the snot out. Of it if it performs better shifting lower.

That combo is probably all done by 6000 rpm. You can push it past that if you want, but I doubt the car will go any faster. Try it and see what happens. My guess is that you'll want to shift it at 5500 to 5800 with a rev limiter of 6100 or 6200.

BINGO I agree

Thanks fellas

The dyno results were a little funny because his sparkplug wire clip to find rpm was on the fritz. So he did it by wheelspeed instead which skewed things a little when the tires grew at speed- it made the tq/hp lines not cross where they were supposed to. I didn’t really care because I was chasing the tune not a number but I will say my old combo mph’d exactly where the calculators said it should based on my weight and the hp number off his dyno.

When the truck hit the rev limiter at the top of a pull the first pull it scared the crap out of me! Thought something let go in the valvetrain for a second. Here’s the sheet from the first and last pulls. X axis scale is mph in 2nd gear, 3.21 ratio.

People do and spend a lot to pick up a few hp. I figured since my hp curve hadn’t nosed over yet at 5500 redline maybe running it to 6000 would be free power. Unless I eat the cam or a valve. At that point a tiny nitrous kit would be cheaper!

Also I think this setup might run a low 12 if I sorted out the launch some but it’s not going to see much track time.

I think I got spooked talking to a new coyote mustang owner that had 11 sec timeslips with a stock car & drag radials.

For reference the first pull (bumpy line) we stopped at 5000, the last pull ended at 5500.

I’m not sure how it works exactly but I suspect the HP might be low because as the tires grew my gear ratio was constantly dropping. The lines should cross at 5252 iirc? Which would be 1/2 way between the endpoints of the two pulls.

I don’t know if the hp is low or the tq is high but they are not crossing where they should. What I do know is the tq curve in my operating range smoothed and flattened out in a very good way after some timing changes.

You’d have to know exactly what the speed was at the point the motor was at 5252rpm.

That chart only goes to 75mph........ no way the wheels only going 75mph at 5500rpm in high gear.


If you’re not in high gear, then I’m sure the added gear reduction is throwing a wrench in the calculations.

Oh........ and don’t even try and correlate the wheel hp number to the crank hp.
An excersize in futility.

Spitballing here, but I bet the 55/5600 shift point is going to be the sweet spot!

Here’s something I just thought of......

How does that Mustang dyno determine tq/hp?
Is there a load cell or is it an inertia dyno.

With a load cell...... I guess I don’t really know what they use for a formula to come up with “wheel hp” based on the “wheel tq”.

On an engine dyno with a load cell, you get a reading off the load cell, and apply the math to arrive at the hp figure. Without knowing the rpm for the reading/measurement taken by the load cell...... there’s not really a way to know what the “hp” is.

I never do dyno pulls in second gear it screws the results. Go back and do it in high gear and keep going up on rpm until it noises over

Found this in the Mustang dyno operators manual:



If it’s not engine crankshaft relative, then it won’t necessarily cross at 5252, since it’s also not rpm related.

Good info here- thanks!

The mustang can load the roller it’s not just inertia. I’ve heard it called ‘the heartbreaker’ because it doesn’t give crazy inflated numbers.

Like I said- I was trying to be smart and use my buddy’s dyno to put a good street tune on my sniper timing and afr control. We definitely succeeded- the torque curve is way way flatter and it gained 20 hp/tq.

I will probably get it down the track sooner or later and try different shift points. For now I’ll probably raise the limiters to 6k and shift a little north of 5250. Stuff happens fast up there and traction is limited on the street anyway

Also- we tuned in 2nd so it wouldn’t kick down. I’m not sure how we could avoid that?

Just disconnect the kickdown lever from the throttle linkage for the pulls.

For what you were doing, the tests were fine...... as long as you used the same procedure for all.
Then you’re just looking for the numbers to be better than they were...... and then you know you’re making progress.

I'm going to assume your tranny is full automatic, correct?
If so you would have to rev it up slowly on the chassis dyno at part throttle until it is above 4500 RPM in high gear probably to make it not down shift and then slowly floor it and have them start to record the pull once your at WOT
I spent some time on a chassis dyno with my race car several weeks ago, I would rev it up to around 5100 RPM at part throttle in high gear (Powerglide) and the dyno operator would give me a thumbs up when he was ready to start the recording and I would floor it
I had a 7200 RPM chip in the three step and didn't think about that until after the car was off the dyno
have fun, learn some more

I was super happy with the power improvements down in street driving range. I always get nervous about beating on my equipment loading it up with resistance on the dyno- the chassis and driveline will never see that much stress on the street because the tires unload first. Since my slip yoke let go later that day and took out my transmission doing a holeshot I’m guessing we put plenty of beating on her while strapped down. It’s probably a good thing we didn’t do any more testing on the rollers but I am a little regretful we didn’t keep spinning it till it nosed over.


Now the truck has been back in action a couple weeks with 1350 yokes and a fresh 727. It probably won’t go back on the dyno unless I do bigger headers or change something major but if/when that happens I’d like to be able to run it in high gear. So pin the throttle pressure lever it all the way back? I’d hate to burn up my clutches with low atf pressure. Unfortunately I want results from 2500 or 3000 up- starting at 4500 sounds more like race engine stuff.

Thanks for the lively discussion fellas. I’m always learning