Silly stock 400 question
 

I know this is a silly subjective question but how strong was the 1972 400 in the GTO? Considering a good state of tune, no obvious mechanical issues etc.

I ask because the stock 400 in my 72 GTO can’t spin the tires from a dead stop and needs to be revved to about 3k before it will spin the tires. Basically stock as far as I know with 117k, I see evidence of a cam swap at some time but stock idle, compression is 140-150 across the board, timing optimized with a curve that comes in around 2800 and total at 34 degrees, proper tune on the factory Quadrajet with jetting and rods suggested by Cliff. Car has a factory M20 and 3:23 gears, it feels strong through the gears but wow it feels like a turd off the line.

I always heard that these stock 400’s were pretty strong with a lot of torque so I don’t know if it’s just me or this engine is tired or something else. Again, I know it’s a silly question but figured it will bring some discussion if nothing else.

If there is evidence of a cam swap, it may have reduced your low-end torque slightly. Coupled with the weight of your GTO and your gearing, I could understand the car being a little sluggish off the line, but moving nicely through the gears.

Your M20 should have a 2.52 first gear. Multiply your 3.23 rear gear and you get an effective starting line ratio of 8.14:1. Most performance street cars with lower power, would prefer a 9:1 to 11:1 starting line ratio for maximum acceleration out of the whole. If you've got very tall tires on the car, that makes the effective ratio even worse. The factory tire would have been between 26 to 26.5" in diameter. A modern 27" tire is usually used.

The easiest way to skin that cat would be a gear change. A 3.55 gear gets you just under 9:1 starting line, while a 3.73 gets you solidly to 9.4. You'll notice much better first gear performance by doing this, at the expense of highway cruising.

For a best of both worlds approach, you can keep the 3.23 rear gear and go to a wide ratio TKX. You get even better highway cruise ability and the 3.27 first gear gets you a 10.56:1 starting line ratio. That would completely transform how the car behaves.

Harmonic balancer timing mark verified?

In other words, slipped outer ring?

Yep, verified the mark prior to timing the engine.

Interesting take on it and a TKX is planned for the distant future so I need to keep that in mind.

Cam ICL could be too late. Kind of a pain to check that.

I bet that the new cam that was put in was not degreed during install and is at least 2 degrees retarted currently and maybe more depending on how the timing chain is.
You can check the chain’s condition easily in 25 minutes.

Pull all the plugs out.

Kill power to the coil.

Next remove the distributor cap such that you can see the rotor.
Next grab the balancer and move it back & forth about 1.5 inches and you should see only a slight lag in the rotor responding to the balancer movement if the chain is good.

Do you at least know how much lift that cam has?

The OP didn't really indicate that the engine was running poorly. It just lacks performance from what he is/was expecting.

A cam not degreed correctly, or a loose timing chain is always a possibility, but if we're being honest, unless the engine is running really poorly, it's probably not something that's going to be checked or dealt with.

If we take the engine for what it is, a stock 1972 400 with the possibility of a mild cam, I would posit that this car is behaving in a way it always would have and the expectation that it would have great performance isn't necessarily accurate. Especially not against modern vehicles.

It's a car that with a driver will weigt somewhere between 4000-4200 lbs. It has an engine that from the factory made 250 net hp, the calculation of which started in 1972. It would have been rated around 325 ft lbs of torque. There's nothing here to indicate that the installed cam is making a punch more power, or that it's install would be robbing a bunch. The engine has to carry somewhere between 15.25 to 16.25 lbs per hp. That's basically the HP to weight ratio of a 2000 Honda Accord v6.

The car in question wasn't fast when it was new and it's certainly not fast now, especially when compared to modern vehicles.

So, if you take the engine for what it is now, and the desire is to increase performance of the car as a whole, it's all in the torque multiplication. The car needs more gear to help motivate it's weight with the power it has. Yes you may gain back a little power and torque by optimizing the cam timing, but it's not going to do what going from low 8:1 starting line ratio to a 10:1 starting line ratio is going to do.

A 2.52 and 3.23's isnt going to set the tires on fire at the stop light drags.

It'll be poochy by design till you get going, pretty much exactly what you described.

Gear changes will fix it. I'd suggest a 5 speed swap with more 1st gear, a rear gear swap as well is typically what I do, and then enjoy the benefits of low rpm cruising too. Best of both worlds.

Are you getting full throttle when you hit the pedal to the floor?

You did not mention if you are running headers or exh manifolds, but if your running the latter you need to check if the heat riser flap in the passenger side exh manifold is rusted in place and is partially closed .

Tons of folks miss this power strangling problem.

I just went through a bunch of headaches with my bone stock 71 400/400. TCS was the culprit in my case. Your 72 came with that garbage as well. The factory carb also does not have a provision for ported vacuum. The stock can on my car added 20 degrees of timing off manifold vacuum and the stock "hood hinge" springs wouldn't allow full mechanical advance until 4200 rpm! After putting in lighter springs, limiting the advance can, and bumping initial timing it's like a new car.

Your total mechanical timing does not come in until 2800 and yet your complaining that the car can’t spin the tires until 3000 rpm sounds to me like you can bring in your total mechanical timing a lot sooner.

Have you investigated how much timing you can bring in before you hear the motor ping in 4th gear?

That stock cam is a 067 in that year and by 3000 rpm it’s well into it peak torque range and maybe even on the out bound side of the peak torque band, so by 2500 rpm with the needed proper timing it should be starting to pull hard.

All fantastic suggestions and to answer a few questions, the only reason I can assume that the cam was replaced is because there was a box of old factory GM lifters in a tattered Erson Cams lifter box in a pile of spare parts when I received the car, the box looks to be from the 80’s or 90’s based on what I can see online.

I have not checked the timing chain yet but when I replaced the HEI unit (due to sticking weights) and installed the new distributor I didn’t notice any slack when I turned the engine over by hand to align the timing marks. Still want to check for chain slack anyway just in case, great suggestion.

The previous owner owned the car for 25 years and at some point must have switched mechanics because some of the most recent work looked a little shoddy to me which is why I went through the tune up with new plugs, wires, distributor, carb tuning with jets and rods.

Exhaust manifold was leaking when I purchased the car last year due to crack so I replaced the factory log on the passenger side without the flapper valve. I also installed a new 2.5 Pypes system since I was doing exhaust work.

I did verify full throttle opening with the pedal to the floor.

The TCS system appears to be disabled but I should dig in a little deeper to be sure. When I installed the new distributor I used an adjustable vacuum advance can and verified my total timing at 34 degrees so that’s good to go.

All of this work was basic tune up stuff IMO and I did all of it in the winter before I really drove the car last year to make sure that it was safe and sound for the road so I can’t say if it improved vs the way I purchased it or not. It just seems that while the car doesn’t have any major issues the performance is pretty lackluster and maybe that’s the way it’s supposed to be or shall I say the way it was when the car was new? I know a few of you guys had these cars when new so I was hoping for some opinions on that.

I think Jason hit the nail on the head with his assessment, I guess these cars just weren’t “fast” back in the day and all the stories I’ve heard over the years about the impressive Pontiac torque doesn’t apply to these cars once the compression dropped and gas crunch started?

Good point, I have NOT tested for ping in 4th gear and definitely went with a safe curve and tune, maybe there’s a little more to be found by lowering the when I bring in total timing.

Clutch slipping? Absolutely sure it’s a 3.23 rear gear?

Also if the vehicle has AC from the factory it’s very unlikely that it has 3.23 gears out back , the norm I believe by 1972 was 2.73 and that would explain a lot .

I know for a fact that 1970 that with AC and a wide ratio 4 speed that 3.23 was the norm.

With a 14” G70 tire on the car and with 3.23 gears the engine rpm in high gear should be 2510.

The build sheet lists the 3.23 and I verified it during the rear end service.

I forgot that I had dyno sheets in the paperwork when I got the car, not sure if it was an engine dyno indicating that maybe it was rebuilt or chassis dyno and the numbers are at the rear wheels?

It certainly sounds like for all intents and purposes, this is a well running stock or near stock configured car.

If the engine accelerates smoothly without any hiccups, coughs, bangs or dead spots, it's likely that it just doesn't make the power you were hoping for.

You're correct that smog era engines can't really compare to the pre-smog era stuff. There's a big difference in a 1972 400 with a 7k3 head and a 1969 400 with a 48 head. Remember, it's not just Pontiac that suffered. Everything from that era just sucked.

Your car could absolutely benefit from some extra ponies, and double checking everything discussed here could free up some, but it's not going to radically change anything.

For ****s and giggles I looked up the published 1/4 mile times for a 72 GTO with the 400 engine option. Between 15.5 to 16.5 seconds. If you get the car to a strip, or have other means to run a 1/4 mile, if you end up in that range, you know you're probably where the car should be.

But it does sound like you've got a decently well-sorted car. You get to decide if you want to do anything to it. There's options abound to make power from what you have now all the way to 4 digits.

I think I'd find out for sure what rear gears are in the car, and if possible I'd try to get a TKX in there quickly. The wide ratio trans with the deep 1st gear will absolutely transform how the car behaves without sacrificing on it's ability to cruise. The only real drawback is a lighter wallet.

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.

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.

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.


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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

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

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

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



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

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.

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.

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.

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.

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.

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.
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Glenn

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

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 :)

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

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.

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.

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.

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.

That’s not the same company, the one that dyno’d the car was in Texas and doesn’t seem to exist anymore. The previous owner said that his club rented the place for the day and they all brought their cars and the facility tested them but he doesn’t know the dyno used.

Really great conversation, I appreciate it if for nothing else to get a better idea of what some folks actually experienced with these cars back in the day.

I think the car runs good for what it is and appreciate the additional suggestions such as the clutch. While I don’t feel that the clutch is slipping, especially since I can’t get it to slip, anything is possible and I’ve already purchased a new clutch but haven’t had time to replace it yet.

Yes, still has the original 7K3 heads (numbers matching) and I am pretty certain that the used lifters that came with the car were from the car so I’m only assuming a cam was changed for whatever reason. It certainly doesn’t sound like a it has a camshaft more than stock IMO. I the link below is from the day I installed the new exhaust.

https://youtu.be/qRysRd31pJk?feature=shared

It's the same engine. There is no carb limiting on mine LOL. I think people, at least anyone performance minded, caught onto that deal pretty early on back in the day.

FJ , the Net horsepower rating is flywheel HP. There are going to be significant HP loss through the the transmision and drivetrain. That means Rear Wheel HP has to be less than the Net HP . Thats what I expect to see.
Glenn

That's true in a perfect world but this stuff isnt exact. The factory ratings were fudged in a lot of cases. People with different types of dynos and depending on who's running it that day. You have correction factors based on location and weather that day. It just varys.

What I do see is a stockish 72 Pontiac 400 making 250 rwhp. That's nothing out of the ordinary from my experience tuning and dynoing this stuff.

A point brought up to me elsewhere is what size tires I’m running. With the at least 10 year old 235/60/15’s there was no issue smoking the tires, once I installed the new Cooper Cobra 255/60/15’s (27” diameter) that went away unless the clutch is dumped at 3k.

Well that then is flat out proof that with the Cam you have your gearing is right on the edge .

With 98CC heads, I come up with 8:1 compression.

What is the base timing and total timing? 16* base and 34-36 by 2500 should add some pep. What octane do you use? Try 87, it burns faster so its like advancing the timing.

If this engine is pumping 150 PSI, it's gotta be a small cam. Can't be be larger than a 067 grind. Maybe advance the cam 2* or use a Summit 2800.

Long term, adding the TKX would be best performance improvement overall.

When these cars were new they came with a tiny tire most of the time barely measuring 26" tall. That helps tremendously getting away with a horrible effective gear ratio. People today, me included, install taller larger tires.
The problem with a stick shift is if you're already dealing with a lazy effective gear ratio like a 2.54 with 3.23's, it only gets worse with a taller tire.

Hence the suggestion to go 5 speed and even up the rear gear a smidge. With a 2.87 1st gear your low end problems go away and with a .68 overdrive you gain highway cruisability better than youve ever had before

Quote: "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."

Here is the article from the SS&DI March 1972 issue...14.37 at 98.04 mph!

Dennis