69 Rear Wheel HP

[Don W.]

What should the ballpark number be for rear wheel HP for a 427/390HP engine and what gain might be expected with some head work? thanks...Don

[Duke W.]

Re: 69 Rear Wheel HP

I don't have any data for that engine. So how about if you get some for us?

Have it tested on a DYNOJET, not a Mustang or any other type of dyno. Make sure the shop can email the test files to you or transfer them to a diskette or CD so you can load them on to your computer and view them with the WinPrep software from Dynojet.

START THE PULLS AT 1000 REVS! Top gear is preferred, but consider the speed rating of your tires. Pump up the rear tires, cold, to the maximum pressure on the sidewall placard.

Two pulls within a couple percent means the data is probably okay, but since you usually have to rent an hour unless you're part of a "group day" type event, bring some different dist. springs, timing light, dwell meter, and do some "dyno tuning".

Do a basic tuneup first - check dwell, timing, cap, rotor, plug wire resistance, etc. There's no point running on a dyno if the car is not in tip-top tune. Make sure it will pull smoothly from 1000 revs or less in third and fourth gear. If it won't, something is wrong.

If you do some pretesting on the road and at least get a SOTP feeling for how each configuration works, you'll get more for your money. Have a WRITTEN TEST PLAN and make sure you keep track of each test's configuration, and co-ordinate what you want to do with the operator before you set up the appointment.

I'm amazed how many guys spend good money on dyno testing and come away with a single graph that starts at 3500 and ends at 5000 - a complete waste of money. If you plan properly you can get a lot of valuable data including the full torque/power curves from off-idle to redline and determine an optimum spark advance curve.

Do the testing in cool weather. Make sure the shop has plenty of external fans. If the fan clutch engages it will cost 15 lb-ft peak torque and 10 peak HP. Using a garden sprayer to spray down the radiator and fan clutch thermostat will help.

Proper head massaging is worth in the range of 7-10 percent more top end power and will extend the useable power bandwidth at least another 500 revs without affecting idle quality or low end torque.

Duke

[Don W.]

Re: 69 Rear Wheel HP

Hi Duke,

I did have it dyno 3 years ago when the car was in Phoenix area. Joe Reno of Reno Racing in Mesa, AZ performed the dyno tests at a shop in Chandler, AZ. The car is a 69 427/390HP THD400 with 3:08 rear axle; no AC. Joe did 5 or 6 dyno runs tuning the carb jetting, mixture and timing and took the car up to full RPM to go thru the whole torque and HP curves. He has the charts; I may have 1 summary chart but I'd have to dig to find it after 3 house moves. The summary data is below. I think the rear wheel HP is low for car via power down re-build by the prior owner. Which is why I'm asking the question. If I have it rebuilt up the standard spec and then can get some add via some head work; I may see a significant change in rear wheel HP.

Thanks,

Don

2 sets of dyno runs; 1st run to check baseline status, air/fuel mixture, etc. Carb primary jets and larger needle and seat changed; float level corrected by 1/4"

Results:
dyno used to reset idle mixture
dyno used to reset timing from 4 degrees to 16 degrees BTDC; vette mechanic reports '80's engine rebuild likely had pistons changed to ~ 8 - 8.5:1 (due to 16 BTDC)
+ 16 HP from 1st run to 221 rear wheel HP
+ 17 ft-lb of torque to 310 ft lbs torque 2 rear wheels
car run up to 135 mph on dyno
later highway fuel economy test showed 14.43 mpg highway

[Duke W.]

Re: 69 Rear Wheel HP

Without understanding the type of dyno used, correction factor type, if any, and the details of the engine configuration, the data is meaningless.

Without knowing the full context of the data, it's just garbage in, garbage out.

When dyno testing an auto trans, at least two pulls should be made from idle in LOW to get the low end torque. Pulls in drive won't get the low end torque because the trans will downshift when you hit WOT.

Duke

[Michael W.]

Re: 69 Rear Wheel HP

Just for fun and probably a coincidence, but I took some old rule of thumb numbers for guessing RWP when only OEM gross HP is known. I got 219.37 HP so maybe your 221 is not far off.

My (acknowledged) crude method:

convert gross HP to net: deduct 25%

390 -25%= 292.5

convert net to RWP: deduct 25%

292.5 -25%= 219.37

[Duke W.]

Re: 69 Rear Wheel HP

A '69 big block will have more loss to net HP because of the two inch pipes I'd call it at about 18 percent versus 11 percent for a SHP SB with 2.5" pipes, no accessories other than the water pump and alternator, and the fan clutch does not tighten, but then the OE gross ratings are typically 10-15 percent high.

Driveline tire loss is typically 15 percent for manuals in direct drive and 20 percent for autos.

So: 390(.85 to .90)(.82)(.80) = 220-230 SAE corrected on a Dynojet.

A decent Flint-built or equivalent L79 should make about 220, but the L36 has more average power over the range due to the greater average torque.

Duke

[Dennis S.]

Re: 69 Rear Wheel HP

Duke
So if my rebuilt '69 L36 pulled 408.7 hp @ 5439 rpm on the engine dyno (no tuning performed) and I have 2.5" side exhausts and an M21, my RWHP should by 408 x .89 x .85 = 308 SAE corrected on a Dynojet. BTW Don, the heads have not been massaged.

How about torque? Does the same math apply? Pulled 500.5 lbs-ft @ 3123 rpm on the engine dyno so 500 x .89 x .85 = 378. Assuming good traction, that L79 will only grow smaller in the rear view mirror.

If I decide to go find a Dynojet and spend some more money, I'll let you know the results before additional tuning is done.

[Duke W.]

Re: 69 Rear Wheel HP

Your SAE gross data indicates that your engine is something beyond an OE L-36. The torque definitiely indicates greater than 427 CID and/or it was tested with headers rather than manifolds. With a medium to high overlap cam, decent headers and open exhaust generally yield 5-10 more peak torque than manifolds and open exhaust, but the increase in peak power is usually less than this range.

I have no idea how much back pressure your side pipes develop, so I can't say with any accuracy what the net/gross factor might be, but if your engine breaks 300 SAE corrected RWHP on a Dynojet, it's had some significant massaging.

The conversion factors I quoted are generally applicable (for manifolds and OE under-the-car exhaust) to the engine configuration classes I specified from the torque peak and beyond. Different engine configurations in terms of displacement and specific output along with exhaust systems configuration will have their own conversion factors, and it's a matter of getting enough test date for similar configurations to see if there is a correlating conversion factor.

For the OE exhaust system, a big block will usually have a lower net/gross factor than a small block because it's pumping more exhaust volume, and back pressure increases with the square of exhaust flow.

Duke

[Clem Z.]

Re: 69 Rear Wheel HP

make sure the oil temp is at least 200 degrees because cold oil can cause 15+ HP loss. a standard for dyno tuners to do is test with cold oil and after they perform their magic tuneup they test with hot oil.