[steve25]

It’s a good thing that you re-jetted that carb because the air to fuel ratio on the previous owners dyno sheet shows it being very much on the lean side for full throttle.

[HWYSTR455]

Just to give calculations as to what to expect w R&P changes:


Tire height = 27

MPH = 60

3.23 RPM = 2476

3.73 RPM = 2860

RPM difference = 384

Also will comment that HP calculations changed in the 70s from gross to net.

A 5 speed certainly would wake it up, as well as other suggestions made.


.

[Tandyman]

I'd like to understand how this 400 rated at 250 net flywheel HP can put down 247.9 at the rear wheels.
Doesn't seem right to me.
Glenn

[HWYSTR455]

Modern Musclecar Technologies Inc appears to be using a RW dyno:

https://modern-muscle.com/products-services/tuning/

.

[HWYSTR455]

Originally Posted by Tandyman

I'd like to understand how this 400 rated at 250 net flywheel HP can put down 247.9 at the rear wheels.
Doesn't seem right to me.
Glenn

OEMs under rated HP numbers to side-step insurance rates, which impacted car sales.



.

[Tandyman]

HWYSTR455, how much HP do you think the 72 400 actually made as compared to its 250 net rating ?
Glenn

[steve25]

In 71 I believe the same motor was rated at 300 hp but I need to check , but anyway that would explain the hp at the rear wheels.

[Tandyman]

Steve ,The 1971 GTO 400 was rated at 300 HP Gross which then became rated at 250 Net in 72 . Regardless , are you saying that
250 HP Net at the flywheel should show pretty well 250 HP at the rear wheels?
Glenn

[JLMounce]

Originally Posted by Tandyman

Steve ,The 1971 GTO 400 was rated at 300 HP Gross which then became rated at 250 Net in 72 . Regardless , are you saying that
250 HP Net at the flywheel should show pretty well 250 HP at the rear wheels?
Glenn

As HWY pointed out earlier, manufacturers around that time did intentionally underrate engine power for insurance purposes. It's not a stretch as a result for that to happen.

This engine has a suspected cam change that may have also added a slight bit of power.

[chiphead]

Originally Posted by HWYSTR455

OEMs under rated HP numbers to side-step insurance rates, which impacted car sales.



.

I thought the reason they went to SAE NET test method was to reduce the funny business everybody was doing up through 1970? I'd assume the 1972 test results would reflect performance of real production vehicles.

As for this car making the flywheel HP at the wheels, that's possible once you change the cam and the factory tune.

What heads are on this engine? I'm trying to work out how the 150 PSI test results could indicate the cam grind installed.

[Formulajones]

Glenn, the engine that was gross rated in 71 is virtually the same engine in 72, just the way the factory rated the engine changed to a net rating.

If you took two 400 gto's to the dyno that were completely stock, one being a 71 model and the other being a 72 model, they would dyno nearly identical given possible small variances that occur. They are the same engine.

Dynos vary quite a bit depending on where you go but I would expect an engine that makes 300 flywheel to possibly make 220 to maybe as much as 250 at the tire depending on a lot of variables.

There is nothing that guarantees that 400 is making 300 gross hp at the flywheel either. Even the gross ratings were conservative in many cases. It could be making 310 or 320, never know.
250 at the tire today from a 55 year old stock smog motor isn't all that bad no matter how you cut it.

[Cuttyman9]

Definitely check your clutch.
By no means is a Pontiac weak under 4k rpm. I have a 75 trans am, its rated 185/320. It is an absolute monster despite low compression and no camshaft essentially.

The major difference in your setup is that its manual vs mine being auto.

Your clutch linkage is adjustable and it could be as easy as that, but it's possible the clutch is smoked if you've been trying this over and over.

[Tandyman]

FJ, I am aware of the 71/72 gross/net rating change and agree that they are pretty well the same spec motors. What I did not expect was that they could show the same rear wheel HP as the Net rating. I have had personel experience with a 71 GTO 400 (auto in my case) just off the showroom floor back in the day and it's tire smoking ablities were impressive.
The Op's experience is not measuring up to my expectations for whatever reason.
.
Glenn

[steve25]

Chip he says his 72 is stock head wise so that means the 7K3 98 CC casting.

[unruhjonny]

Originally Posted by 67Lemons

Just confirmed from the previous owner that the dyno sheets are at the rear wheels with 100k on the odometer, he even comments that the 250hp was the original factory rating and they got that with 100k on the clock.

My knee-jerk reaction was that the dyno operator had his correction factor set intentionally to hit expectations - I read 1.04
Maybe I need to put my cynicism aside, and this was purely for the dyno being used(?) - because there is drivetrain loss and loss from the dyno.

But lets pause for a second;
As mentioned earlier, 1971 & 1972 were essentially the same engines, and made the same power;
The only significant change during those two production years was that 1972 model year engines received hardened valve seats as a part of the cylinder head manufacturing process.
The 1971 power figures (going off memory):
SAE: 300hp
Net: 250hp
That 50hp difference was essentially supposed to be from the drivetrain.

I believe that the only sale lit that showed the 1972 L78-400 hp was for the Canadian market GTO brochure - otherwise, it's absent from all 1972 literature.

Both years used the same style intake and exhaust manifolds, true dual exhaust, 8.4:1 compression, 067 cam, and no "smog" (eg: EGR ect) devices;

They were essentially a carry over from 1970 with a full point drop in compression.

Back to the correction factor;
I have understood that often dynos will show much more than a 4% correction - and that will very quickly yield different numbers.

That being said, the dyno is really best when comparing before and after dynos, or for tuning your setup.

As mentioned earlier, I would double verify that the gear set hadn't been changed, because I am wondering if you have less gear than expected.

2.52 (M20) x 3.23 = 8.14 effective 1st gear ratio
2.52 (M20) x 2.73 = 6.88 effective 1st gear ratio

If my memory serves me correct, you want to be somewhere between 8 & 9 with your effective first gear ratio.

Up till I removed my car from the road, I was not running an ideal combination, and I still had plenty of useable torque - then again, I also probably had more than a full point of compression on you...
2.2 (M22) x 3.55 = 7.81 effective 1st gear ratio

I did also lose a little low end torque when I changed from a 068 cam to a 744 cam....

I hope this helps some

[SD455DJ]

The 250 net horsepower rating for the ’71 & ’72 400 4-bbl is fairly accurate as installed in the vehicle after taking into account losses from internal frictions of the transmission and rearend, along with log exhaust manifolds and exhaust system, then include (deduct) the parasitic losses from the alternator and water pump (not the optional power steering pump and AC compressor, which would be grater losses). The ‘78/’79 W72 400 is essentially the same engine with a 220 hp net rating due to slightly lower compression (8.0 vs. 8.2) and more exhaust restriction from the single exhaust catalytic convertor (higher flow for W72). The W72 cam specs are very close to the 067 cam specs for ‘71/’72. A stock W72 should be close to 300 gross horsepower as well.

I believe Cars magazine had a road test of a ’72 lemans GT with the L78 400, M-20 4-speed trans and a 3.55 Safety Track rear end and they got it down to 14.40’s at 99 mph in quarter mile tests. Now surely they could be exaggerated times, or a super-tuned example, who knows.

Dennis

[Formulajones]

Originally Posted by Tandyman

FJ, I am aware of the 71/72 gross/net rating change and agree that they are pretty well the same spec motors. What I did not expect was that they could show the same rear wheel HP as the Net rating. I have had personel experience with a 71 GTO 400 (auto in my case) just off the showroom floor back in the day and it's tire smoking ablities were impressive.
The Op's experience is not measuring up to my expectations for whatever reason.
.
Glenn

Most ratings are likely a bit soft from the factory. You can couple that with any particular dyno that also could be a smidge "happy"
With that said, I would expect a healthy 71-72 stock 400 to put down 250-ish to the tire, especially with a stick, and most cars have exhaust improvements these days that trump the stock crush bent 2 1/4" garbage from the factory. I would expect by now it also has a better tune in it than the lazy junk the factory puts in it.
My completely stock 70RAIII only put down 301hp on a chassis dyno and it's nothing but a higher compression 400 with a pinch more camshaft than a 71-72 model, and that was through a 400 turbo and 12 bolt.
That same day another early model 400 bird only did 270 and it supposedly had work done to it. So this stuff can vary quite a bit.

[steve25]

Sounds normal .
Don’t forget that all birds carbs openings where intentionally limited to make the GM rule hold.

For example the 366 hp motor in the A body was only 345 in the F body.

[chiphead]

I'm pretty sure the Gross and Net testing were both done on an engine dyno.

SAE standards J1349 and J2723 describe the Net test methods. The difference is the Net test procedure required the production configuration. Factory air cleaner, cooling fan, all factory accessories, production carb and ignition tune, production exhaust system, production intent fuel type and octane, all that sort of stuff.

If you look at the engine masters type testing thats been done in recent years, then you know the stock fan can knock off 15-20 HP. Install the production airfilter, all belt-driven accessories (fully loaded alternator, AC compressor, PS pump, etc), production tune and exhaust to the engine dyno mule and 50 HP disappears very quickly.

[JLMounce]

Originally Posted by chiphead

I'm pretty sure the Gross and Net testing were both done on an engine dyno.

SAE standards J1349 and J2723 describe the Net test methods. The difference is the Net test procedure required the production configuration. Factory air cleaner, cooling fan, all factory accessories, production carb and ignition tune, production exhaust system, production intent fuel type and octane, all that sort of stuff.

If you look at the engine masters type testing thats been done in recent years, then you know the stock fan can knock off 15-20 HP. Install the production airfilter, accessories, tune and exhaust to the engine dyno mule and 50 HP disappears very quickly.

This is correct. SAE net does not take into account any rotational or friction losses from the drivetrain.

That said, because the engine is still being run on a stand and not physically in the car, there can be times when even the SAE net figure could be decently off actual. Typically because of hot under hood air, the power figures go down further, but there are documented cases where because of the design of the car and it's engine compartment, power may actually increase.