Dumb Dyno Question

[ACHiPo]

I'm ok with the dyno numbers for my mild 470, but they start at 3000 RPM and I'd like to know what to expect from idle up to 3000 since that's where it'll likely spend most of the time. 

Why do dyno pulls start so high?  How, other than "butt dyno" do I know how the motor will behave on the street?

The motor is a BBM cast iron 427 sideoiler with a SCAT 4.15 stroker rotating assembly, stock Edelbrock heads with stock rockers, flat hydraulic lifters, and a Blue Thunder intake.

It's going into a street 427 Cobra replica.

Thanks in advance.

[blykins]

It's not wise to pull an engine down that low with a full load on it.  It's a good way to torch holes in the pistons.

A dyno is generally used for telling you how much horsepower you make at WOT at a given rpm.  You don't cruise at WOT, and if you go WOT, you're either gonna accelerate or lose traction. 

Post the cam specs and I'll tell you how it will behave on the street. 

[ACHiPo]

It's not wise to pull an engine down that low with a full load on it.  It's a good way to torch holes in the pistons.

A dyno is generally used for telling you how much horsepower you make at WOT at a given rpm.  You don't cruise at WOT, and if you go WOT, you're either gonna accelerate or lose traction. 

Post the cam specs and I'll tell you how it will behave on the street.

Brent,
Thanks.  I’m currently traveling and don’t have access to the cam card.  I’ll post it when I can access it.
Evan

[blykins]

Evan, was it my camshaft?  I can't remember.  If so, I can look up the specs. 

If you have access to the dyno sheet, you can extrapolate the data and see what the trend would be below 3000 rpm.  All-in-all, on a stroked engine with a small-ish cam (read more street than strip), the street manners are going to be just fine. 

[jayb]

The way a dyno works is the engine is trying to pump water through an orifice.  The size of the orifice is controlled by the computer.  It can neck down to a small size, so that when the engine is trying to pump water through that orifice, the load on the engine increases, and eventually the engine can't accelerate.  That's why the dyno is called a water brake; the force the engine exerts trying to pump the water is resisted by the orifice, so that the engine can't increase in speed.  For your dyno results, the minimum speed, that the engine was braked to at the start of the pull, was 3000 RPM.  So, the computer reduced the size of the orifice to the point where the engine could only get to 3000 RPM, despite being at full throttle.

When you go lower in engine speed, obviously the engine will pump less water.  The orifice can only neck down to some minimum size, so if you are running at a low engine speed, say 1500 RPM, despite the size of the orifice the engine can still pump all its water through the orifice, and so it will accelerate past that 1500 RPM point.  Therefore, not enough engine braking is available from the dyno to hold the engine at 1500 RPM.

The engine needs to be braked in order to get valid torque data, so in the preceding example there is no way that you can get data from the engine at 1500 RPM.  I've run quite a few engines down as low as 2500 RPM, and one down to 2200 RPM, and lower is probably possible, but it depends on the engine and the dyno.  Also, there is a bit of a paradox there, because if you build an engine with a whole bunch of low end torque, the dyno will be less likely to be able to brake it at the lower engine speeds.  If you build an engine with less low end torque and more top end horsepower, it may be able to be braked to a lower speed, but who cares, since the engine is designed to make power at the top end.

Brent, I have no idea why running an engine at a lower speed on the dyno would cause it to burn pistons.  Can you explain your rationale with that comment? 

[blykins]

Jay, I've been asked this question multiple times on different forums.

Most guys want to know why they can't get a dyno sheet that shows hp/torque from off-idle throughout the range.  *If* the dyno were able to pull an engine down that far, most street/strip engines are not designed to be lugged at lower rpms.  It would be akin to hooking up to a 10000 lb trailer, heading up hill, setting it on 1500 rpm and socking it to the floor.  Depending on the engine, the ignition curve, compression ratio, etc., you could get detonation in those conditions.

In earlier EMC's, it was often a "gripe" by some of the contestants that the dyno operator was pulling the engines down too far and it was very audible in the dyno room. 

Most of the "hot rod" engines we build won't even be efficient at lower rpms, much less be able to support a full load.

Your reason was more probably more valid than mine as a lot of dynos can’t function that low, but I set a lot of my engines up with quick curves, or even locked out timing, and I doubt they would survive on a full load at 1500-2000 rpm.  A car will either accelerate or smoke the tires at that rpm.  A dyno will hold the engine back.

[ACHiPo]

Brent,
I did not get the cam from you.  The engine builder provided it.

Also I was mistaken--the dyno pull started at 3400 RPM, not 3000.

The cam is a Comp Cams custom grind hydraulic flat tappet: 230 degrees intake, 236 degrees exhaust, 0.560" lift on both intake and exhaust, 110 degrees separation. It looks pretty similar to the Xtreme Energy cam 33-248-4: http://www.compcams.com/Company/CC/c...?csid=939&sb=2

Hydraulic flat tappets and stock rockers for reliability. RPM will be limited to ~6k.

While this is not much more aggressive than a "grocery getter", I'm hoping it will be a reliable and fun motor for my Cobra.

Torque is pretty flat at 490 ft-lbs from 3400 to 4500, then rolls gently down to 370 ft-lbs at 6k.  HP starts at 310@3400, peaking at 455@5300, then rolling gently to 420@6k.

Thanks,
Evan

[blykins]

That's a pretty mild bumpstick for a 470 inch motor.  It will be both reliable and fun in a Cobra, and it won't be fussy with you.  Plenty of low end grunt. 

[andyf]

I'm ok with the dyno numbers for my mild 470, but they start at 3000 RPM and I'd like to know what to expect from idle up to 3000 since that's where it'll likely spend most of the time. 

Why do dyno pulls start so high?  How, other than "butt dyno" do I know how the motor will behave on the street?

The motor is a BBM cast iron 427 sideoiler with a SCAT 4.15 stroker rotating assembly, stock Edelbrock heads with stock rockers, flat hydraulic lifters, and a Blue Thunder intake.

It's going into a street 427 Cobra replica.

Thanks in advance.

If you want to do street driving testing on a dyno then you'll need to find someone who has a eddy current dyno rather than a water brake. With a eddy current dyno you can do idle testing and part throttle low speed testing. Most water brake dynos are designed for WOT testing and do not have enough control features to test an engine at idle or off idle. Water brake dynos are basically an "all or nothing" design. They would cost a lot more if they had more control features and most people only use them for WOT testing so there isn't any reason for the extra expense.

It would cost a bunch of money and a lot of time but if you really had to do part throttle or low speed testing you could call around and find someone who does truck engine testing. But I can almost guarantee you that when you tell them you want to run a FE on their test stand they'll just tell you to forget it. They aren't going to spend several days trying to figure out how to hook a FE engine up to a big rig test stand.

So it is what it is unless you have a ton of money and time to go chase the unicorn.

[Falcon67]

>It would be akin to hooking up to a 10000 lb trailer, heading up hill, setting it on 1500 rpm and socking it to the floor.

Make it 11,500 and that's pretty much what the F-350 does every time I get on the freeway going to the track on the long up slope out of town.  :lol:

Also for your consideration - when most of us build a hot rod type engine, it'll be backed - for an auto anyway - with a performance converter that will flash well past any "low end" type RPMs.  It can be driven at those RPMs but will most past 2500+ with any sort of throttle application.  That also creates much less reliance on timing curves.  The street/strip type stuff I run all used locked timing. 

[ACHiPo]

That's a pretty mild bumpstick for a 470 inch motor.  It will be both reliable and fun in a Cobra, and it won't be fussy with you.  Plenty of low end grunt.

Brent,
Thanks.  That’s what I’m shooting for.

Evan

[ACHiPo]

If you want to do street driving testing on a dyno then you'll need to find someone who has a eddy current dyno rather than a water brake. With a eddy current dyno you can do idle testing and part throttle low speed testing. Most water brake dynos are designed for WOT testing and do not have enough control features to test an engine at idle or off idle. Water brake dynos are basically an "all or nothing" design. They would cost a lot more if they had more control features and most people only use them for WOT testing so there isn't any reason for the extra expense.

Andy,
Thanks.  I was more curious than anything.  I may try out a chassis dyno once things are broken in, although I’m not sure if it will tell me any more about the low RPM range.

Evan

[andyf]

A chassis dyno with a brake on it will tell you what you want to know. You need to be able to apply a small load to the engine in order to simulate part throttle driving and cruise conditions. So the chassis dyno can't be just an inertia drum, it also needs to have some sort of brake to apply a steady state load. Call around and ask the chassis dyno shops what their setup is.

[mbrunson427]

Andyf, I PM'ed you. About something totally different

[ACHiPo]

A chassis dyno with a brake on it will tell you what you want to know. You need to be able to apply a small load to the engine in order to simulate part throttle driving and cruise conditions. So the chassis dyno can't be just an inertia drum, it also needs to have some sort of brake to apply a steady state load. Call around and ask the chassis dyno shops what their setup is.

Thanks!