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Hi everyone. I ended up putting a 1968 Pontiac GTO 400 engine in my 1968 Pontiac Firebird. I rebuilt the entire engine and reused the original cam and heads. All new bearings and piston rings. It's got a 4 barrel Carter carburetor, Hooker headers. Rebuilt the Saginaw 4 speed transmission and has a new clutch. The rear end is original as well.

That being said the car and motor run really well. The only problem I am having is I think I should have more acceleration then I do. From the stop position I punch on the gas and it doesn't have a real fast acceleration. It sounds mean and tough but no get up and go. The car shifts fine and runs well at high speeds.

I need to know what the steps are in finding out why I am not getting a lot of initial acceleration in first gear. The initial punch is good but dies off quickly and the rpm's go really high so I have to shift into 2nd and then 3rd. The engine has been running for about 2 months driving on the weekends. I'm trying to figure if this is just the nature of the car with low acceleration or if I am missing something. I don't even know what information you need so let me know. I am hoping it is a simple fix or if I need to change/upgrade equipment?

Thanks in advance for your help.
 

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What gear is in the back and what is the first gear ratio?As I remember sags have a very low first gear.My 69 bird has a 3.27 first and a 3.31 rear and does exactly the same thing.Tom
 

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There could be a wide range of causes.

" It's got a 4 barrel Carter carburetor...initial punch is good but dies off quickly..."

From your description, I'd guess that the Carter carb may be at least part of the problem. A GOOD, properly tuned Q-jet might fix it.

Then, if you have a 2.54 1st gear, and 3.08 or less rear gear, that could be a big part of the problem.

A good dist timing curve, with all the advance in by no more than 3000rpm, will probably help.

"...reused the original cam..."

Does that mean that it is the original 1968 GTO cam ? Or just the cam that was in the engine when you bought it ? If it's a cam that produces very little low rpm torque, that could be a big part of the problem.

"... How do I figure those ratios?..."

I suppose you could get a ruff idea, by shifting to 1st gear, then having someone turn the engine over 1 revolution, while you count the driveshaft revolutions. About 2 1/2 revs would be a 2.54 1st gear--a little past 2 3/4 revs would be a 2.84 ratio--just over 3 revs would be a 3.11--and 3 1/2 revs would be a 3.50 ratio. The charts below say you can tell by the number of rings on the input shaft.

Saginaw 4 speed

1st 2nd 3rd 4th

No lines 2.84 2.01 1.35 1.00

1 line 2.54 1.80 1.44 1.00

2 lines 3.11 2.20 1.47 1.00

3 lines 3.50 2.47 1.65 1.00

http://www.gitdirt.com/gm4speedsaginawratios.htm

http://www.hotrodreference.com/605/gm-four-speed-transmission-gear-ratios/
 

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Discussion Starter · #5 ·
I have tuned the carb based upon what I could find about setting it with a vacuum guage. At idle it runs about 700 rpm. I didn't rebuild it just cleaned everything internal.
I will have to figure out what the rear gear is. I'm trying to get that info now and will post once I get that.
I have the timing set at factory 9 degrees BTDC
The cam is what was in the engine when I got it. I am assuming it was original to the engine as all signs of it look original. I'll also do some research on that based upon the numbers that are on it.
 

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"...I have the timing set at factory 9 degrees BTDC..."

The most important number is the TOTAL advance, at higher rpm. Many, if not most iron head Pontiac engines seem to run better at somewhere between 32 & 36 degrees of TOTAL advance, all in by no more than 3000rpm(with the vac advance unhooked and vac source plugged). To check the total advance, you need a GOOD dial back timing light, or have the balancer marked, in the 30-38 degree range.

Our bracket engines seemed to have a bit more off idle power with at least 12-13 degrees of initial advance, at idle.

All our 455 bracket engines had a Q-jet carb.

"Here is a picture of the Saginaw spline, I think that is a one line?"

If so, then that, coupled with 3.08 or worse rear gears, explains at least part of your lack of 1st gear acceleration. :yes
 

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Discussion Starter · #8 ·
I found the cam numbers stamped on the front is a P. This is how that breaks out:
Int. Dur. - 273, CL - 112, 1.50:1 - .447", 1.65:1 - .447", O.L. - 54,
Ext. Dur - 289, CL - 113, 1.50:1 - .447", 1.65:1 - .447"
This is the website I found that http://www.wallaceracing.com/camcode1.htm
"If so, then that, coupled with 3.08 or worse rear gears, explains at least part of your lack of 1st gear acceleration"

That being said, would it be better to change the Saginaw or the rear end?
 

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While the timing light is hooked up and the vacuum line going to the distributor disconnected, give the engine a good rev and see if the timing mark moves way up off the scale. Look at the distance from 0° to 12° on your timing scale, and figure about twice that distance is 24°. If the initial is set at 12°, then another 24° when you rev the engine should get you into the mid 30°s at higher RPM. This is just a quick visual test to confirm the mechanical advance is working, it does not determine exactly how much total you have or at what RPM is is all in at, only that the weights are moving out and make sure they come back in and the timing returns to the initial setting. Also, when you re-hook up the vacuum advance hose, be sure you see with the timing light that the timing mark jumps up off the scale.
 

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"...would it be better to change the Saginaw or the rear end?"

That depends on exactly what kind of driving you'll be doing. If you'll be doing a lot of high speed freeway cruising, it would be better to have the high speed rear, and a trans with a lower 1st gear(higher numerical ratio), to increase low speed acceleration. But, if you don't do the high speed cruising, some rear gears, in the 3.23 to 3.73 range, would increase acceleration without changing the trans. The 3.73's would give the biggest boost in acceleration, but would also increase high gear rpm, at any speed, and would use more fuel.

If it's not a high speed daily driver, I'd go with the 3.73's. They'll be more fun. IMO :yes

I had a GTO with a 2.20 1st gear Muncie & 3.90 rear gears, and loved it. But gas was less than $.50 a gal. :smile22:
 

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Discussion Starter · #14 ·
Thanks a lot ponyakr for all of your help. I am still trying to figure out what type of rear end gear I have in the car at the moment. Then I can move forward with tranny and rear end decisions. The car is just going to be a weekend driver to shows and not so much highway driving, so the 2.20 Muncie and 3.73rear end sounds like a plan. I always planned with going to a Muncie once I got a little more money. The Saginaw was cheap to build and already had it laying around. Except after I bought the hurst shifter and linkage I realized you can't use it on a Muncie. So hopefully I can recoup some of that money after selling it and put it towards a Muncie and shifter.
I will be doing the timing adjustment first to make sure that is correctly set this coming weekend and order a carb rebuild kit to make sure that is up to par as well.

At least I am heading in the right direction with your suggestions
Thanks again.
 

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"...2.20 Muncie and 3.73rear end sounds like a plan..."

I'd go with a 2.52 Muncie. I think those 2.20 Muncies were meant to be used with either the 3.90 or 4.33 rear gears. The 2.52 1st gear will help, when using less than 3.90 rear gear. Will also probably be easier to find a 2.52. :yes
 

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Pull rear diff, and count teeth in gears...

If I had the 68 I would have looked at Pistons and carbon edge along top rim of cylinder...if clean and walls appear to be smooth---well I did same thing I changed out all the bearings mains and connecting...

Then I had the heads done twice...triple angle cut valves...

The Cam I would have replaced as I did mine with oil pump and filter screen. New lifters

Thats the min I would do in the engine....

The rear gears are important to know and probably wouldnt hurt to change the gear oil...

I think the engine is a 68 an needs Cam.

Gearing is probably .308 or 2:73 that may also be the reason its doggy....But Hey I run 2:73 and I like mine...you guys would say its a doggy, but after most people wind out at 110 WOT, I still keep pulling away...and away, and away...lol

I also believe the Carb could be dumping too much fuel on lower rpms and then coming into its zone in later RPMs....your dumping too much initially and it bogs the engine, then it will rip right up to RPMs...

The problem there is the mistake I made a few times...

Giant Carb dumping gallons. Bog engine and then...

Put it like this. I could be sitting at a red light. when it turned green I could stomp the pedal to the floor running a 750 Holley Mech secondaries and the car would simply die. Flood that puppy right out...

When I dropped down to a 600 and did the same thing the car launched off the line...I need friends who can set up a carb professionally...not back yardy using the metering rods to tweak to the set up of the car...

The reason this is very bad...Gasoline slides along the walls of the cylinders, washing away the oil, get to thin in the oil and start spinning bearings...from Gas wash out, only ruined about three engines just learning that part...

I know I dont sound like Ponyakr, with ratios and degrees...I just try to get across what I have encountered....seems like it is just like one of mine...

There are a couple of things to check out...

Best Luck and hope it all comes together...
 

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Discussion Starter · #17 ·
Gang I figured out the problem after talking with other Pontiac sites and this one. I just took everything apart on the carb. I was suggested to look to make sure the secondaries were opening. I had no idea what the hell that meant as I am a newbie at all of this???? I started with the throttle cable and found that it was not pulling the throttle totally open. The one I bought from summit had extra aluminum on it and I had to shorten the length. Once shortened it would open the throttle and secondary valves all the way manually. Then I noticed on the carter carburetor there was a part of the choke that was getting in the way of the secondaries opening as well so I had to shave some of the metal so that it wouldn't catch. After doing all of that (a whole hour) after spending two weeks trying other things. The bird flies out of the stop position and squeals the tires like I just had stole something. I wish it could be all the time I spent figuring this out. I want to thank everyone who posted for your help. I'll post the throttle cable bought for reference if anyone else has any problems like me. Sometimes it's something just simple, but if you don't know you don't know!!!!!
 

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I am right now busy with other stuff on my car and these issues are far ahead for me but while this subject is here I wanted to chime in:

I have a similar issue, the car is slow, it goes through its 3 gears very quick (TH400), engine revs high but car is super slow. So slow that forget even getting on a freeway. My speedo still not fixed and there is no tach so I cant tell exactly but it feels like 3000 rpm at something like 40mph.. Maybe I have tranny with weird ratios? Or its rear end or combination of both? Where do I look for the numbers?

Thanks,

Neal
 

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Please help me to understand what rear end I got my bird. I have 10 bolts, looked for marks and numbers and this is all I could find (the pic is upside down). But it does not line up with codes as far as I have searched, maybe this is not even a Pontiac rear end...

IMG_4085.jpg


Thanks,

Neal
 
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