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1968 gto 400 build questions

9K views 49 replies 10 participants last post by  actmobmar 
#1 ·
hello, i'm new on here and looking to build up a 1968 gto convertible that i inherited when a good friend of mine passed away too soon. the car has a ys code block(400) in it that appears to match the car, along w/ #48 ram air 3 heads. the car has a 4 speed in it, and i am looking at taking the tremec 600 that is currently in my '69 camaro, and has a .6 overdrive, so i am considering 3.73 gears. i'd like the car to run mid 12's as most production cars these days do, and i want to keep the factory block and heads so it appears original. the car currently has headers, performer intake, demon 650, and hei ignition. any help to guide me in the rite direction in this build would be great, as i am not familiar w/ pontiacs! thanks!!
 
#2 ·
You are on the right track with your "looking factory" appearance theme. A car that nice with apparent numbers matching should stay that way.
What a fantastic sounding car.
You should get the PHS docs for it too.
To run the #48 heasd you will need a dish piston to lower the CR a tad.It will be too high for pump gas. Pistons with a "D" shape would be the best option .
Your build is totally within the bounds of reality.:thumbup:
Also with the 4 speed you have, It would be a plus to keep it. You should ID which one it is. If it is the close ratio trans, that would be best. Pontiacs make lots of torque. A low first gear will probably not benefit you in that respect.
 
#3 ·
i would like to build a car that gets decent mileage, thats why i was thinking about using the 5 speed, we go to the daytona turkey run every year and typically put 5-600 miles on in a weekend, and i'd like to try to go to more car shows. do i need to do a stroker kit to get to my goals or what would be required? i was going to get rid of the performer intake and go w/ the performer rpm. i don't mind the simple bolt on stuff(i'd probably grind the edelbrock off the intake and paint it). kinda my goal here is to put the car together as my friend would have done if he was still here. the engine in the car runs, but i don't trust the person who had done the machinework so i'd like to go thru the engine and improve it.
 
#4 ·
I would perform a simple engine health test before ripping into it.
Do a compression test
check the oil pressures, cold and fully warmed up.
Check engine vacuum at Idle .
Check the crankshaft end play to be sure nothing going on there.
You may want to degree the cam to determine what it is also.
Be sure to read up on how to do the tests. Crankshaft Coalition would be a good place to explore for a tutorial.
I would opt for a stroker if you need to rebuild it, just dont forget what i said about the compression ratio, as when stroking it, you will increase the cylinder volume and that will raise the CR higher.
LA Tech
 
#8 ·
A couple of the better Pontiac guys are right here, no need to go further. Between Bill (WDCreech), Lynn (LATECh) and myself, we gotcha "covered".

400 HP from a pump-gas 400 Pontiac is no problem at all. It takes a little port work, good induction/exhaust and a strong valve train. The factory Q-Jet intake will make more power than Performer (this is not a Chevy, and doesn't "act" like one).

48s are not "Ram Air" heads. This is a common area of confusion. 48s are '69 "large valve, small chamber" heads. Good ones, no doubt, but only a relative "handful" ended up on Ram Air engines (366 HP). Most were on manual trans 350HP 400s. Some were on 350HO engines, as well. The term "Ram Air head" should be reserved for "round port" (exhaust) heads with small (72 CC) chambers. This is to avoid confusion.

The Tremac is a decent transmission, but the Pontiac makes a WHOLE LOT more low-end torque than the more modern engines. We've seen success and failure using them. The Richmond 5-speed is KNOWN to be able to take the power. The TK-900 is pretty good, too, having driven a '79 T/A with one of our 461s in it.

You don't need to "stroke" the engine to make power. A 400 Pontiac is no punk. In fact, that's the engine that made GTO's "reputation" (which is NOT getting beat). With a good set of connecting rods,the Pontiac is tough and powerful. However, stroking it makes for the performance of a 455 without the weakness of the 455 (block). 400 blocks are physically stronger than 455s simply because the "hole" (main tunnel) down the middle is smaller, leaving more material in the bulkheads where it's needed.

Butler Performance is a good shop. But Pontiac "stroker kits" are available form every parts warehouse in the country where Eagle products are sold. Nothing 'rare" about Pontiac performance parts, and for the most part, fall in between BBC and SBC for "price" (like the CID and power levels).

Have fun! Cool car, a GTO ragtop!

Jim
 
#9 ·
would this package get me to my goals- the car currently has a crane solid lifter cam, not sure of specs(never got them) but i know it is .500 lift at the valve,harland sharp 1.5 roller rockers, crane pushrods, clean up the bowls and port match the intake to the heads, performer rpm intake, aftermarket forged rods, 9.5:1 compression, the stock crank(400) or do i need to stroke it? hei ignition, headers and 2.5" exhaust, holley 650. do i need to go further w/ the heads?
 
#10 ·
Just my thoughts...as Mr. P-Body mentioned, there are several guys on this forum that have you covered but, they can correct me if I'm wrong. I love the Demon Carbs but wouldn't you get more out of your engine if you upped the CFM to 750 or even a bit larger? I built a 428 Pontiac a few years ago and ran a Demon Carb even larger with great results...If I remember correctly it was an 850 CFM.

Like I said...just my thoughts.

Ray
 
#11 ·
what widetrack said,plus



the HEI needs to be upgraded,the stock HEI needs a better external coil.Im not sure what the 400 heads flow so get them tested and make sure the cam matches the heads.It sounds to me like a 13 second combination not 12.
Still not enough information.Are you using slicks or drag radials?
 
#13 ·
To add to my last post. back in the mid 70's, I had a 326 Firebird...I put on a 650 CFM Holley and it worked great...Well as great as a 326 can work...LOL...I dropped in a 400 with a bit of head work, cam, lifters, Offenhauser duel port intake and a set of Appliance headers (has anyone else ever heard of Appliance Headers?), a few other minor mods and kept the 650 Holley...When I walked on it, I think I had more power with my little 326. I went to a 850 Holley double pumper and it woke everything up.

That's why I thought he may be under carburated.

Ray
 
#14 ·
"Demon" carbs are no longer available. They were genuine imitation Holleys, anyway. An AED Holley will make more power than an equal sized Demon.

Rule of thumb, a 400 CID engine needs at LEAST a 750 CFM carb. With the 650, it will run out of "breath" right around 5,000. In olden times, the 800 CFM "spread bore" Holley (list # 6213) was the hot lick on a GTO.

What exactly ARE your goals for power? BTW, it was the Offenhauser Dual Port that kept your 400 from making good power. It's a door stop.

Jim
 
#15 ·
I did not know that Demon carb weren't available anymore...thanks for that. The Offenhauser may be a door stop...and I'm in no position to argue but, all I was saying was that going up in CFM on the Carb woke the 400 up...and in 1976, 77 we didn't have all of the offerings in regards to speed parts for Pontiacs that they do now.

Thanks for the information Jim

Ray
 
#16 ·
i want it to be a fun street car that will go to the track a couple of times a year, i will put drag radials on the back, just in case i run into a new mustang or camaro on the street, the new cars are pretty quick, and i want this to at least be able to run w/ some of them. i know goals need to be kept realistic, though. incidentally, the tremec resides behind this engine in my '69 camaro convertible, i built everything rite down to the headers, all the turbo parts are based off an ebay kit, did it 3 years ago, car runs low 10's fairly easily, i am putting a 4l80e in the camaro and putting the tremec in the gto. it is nice to know that there are people out there willing to help w/ this project, as i am a lil lost w/ the pontiac- i've worked on chevys for a long time, so thank you all. i think i am going to keep the stock crank, put a good set of rods in it, and figure out the pistons/compression/heads with help from you guys. is there any info available on porting #48 heads? here is a pic of the camaro engine
 
#17 ·
is there any info available on porting #48 heads? here is a pic of the camaro engine
Craig Hendrickson and Kern Osterstock published a couple of books that were available through H-O Racing Specialties, one of which was "Pontiac High Performance Engine Design & Blueprint Assembly". It had a section, with illustrations, on porting Pontiac heads. Very informative reading. I had to replace my origional copies and found both this and their "Pontiac Heavy Duty Parts & Specs" at one of the book clubs, but if all else fails, I see Craig post on the PY forums website, once in a while.

Bill

 
#18 ·
I ran mid 12's using a Holley 4777 and a Performer intake on a 455 w/6X heads in an all steel '81 Camaro, and went quicker than that using a 428 in a '68 LeMans (also all steel), so I see no reason you can't get into the 12's w/your 400 using a decent cam and the '69 #48 heads you have. I'd clean the bowls and give them a good valve job. No harm in port matching the intake and heads (not gasket matching, BTW), but full-on porting isn't necessary to run high 12's.

Whether your idea of stock appearing includes headers I don't know, but using long tube headers will make running 12's a lot easier unless you use the good manifolds- the low-po manifolds hurt power too much. The 400 spins higher than a 455 all else being equal, so using a bigger carb certainly won't hurt.

I'm guessing the pistons aren't flat tops, else the compression would be higher than 9.5:1.

Some reading if you're in the mood:

Engine info
Info/sites
 
#21 ·
Personally I like that cam. It has a powerband that compliments a stick shift 400 Pontiac, especially w/freer flowing heads. Not to say your 48's won't be a good match, but if ported the cam can make more power.

This cam in a 400 should make just enough vacuum (11-13 in.) for power brakes, but if it was a problem I'd expect you'd have said so already. Do you know if it was installed at 108 degree ICL? You might try advancing it a little but that's something for the fine tuning stage.

BTW, do you know what pistons (description or part number) are in your engine? At some stage it would be good to CC the heads and measure the piston deck clearance to get the exact compression ratio. Your heads (assuming they're 400 heads and not the 350 heads, which I seem to remember had smaller chambers but you'd need to verify that) are listed as 72cc in most places, but they usually measure somewhat higher than that if they're unmilled.

In any event, running the numbers through a calculator using 72cc chamber volume, a 0.041" quench distance, a 5cc valve relieve volume and a 0.030" over bore, I get a little more than 10.5:1 CR.

Using a wider 0.061" quench (unmilled block and 0.041" head gasket thickness), it's a little more than 10.1:1.

It would take a dish of about 15cc to get you to 9.5:1, using the tighter 0.041" quench, or a 12cc dish using the wider 0.061" quench. Note- All these figures use an arbitrary gasket bore diameter of 4.2", and a flat top piston valve relief of 5cc. Adjust this as needed for the parts being used.
 
#22 ·
i've never driven it so i don't know how much vacuum it has, and the car itself hasn't been driven since 1988. when i broke the cam in i ran it on an external fuel source, and the car wouldn't idle-the carb was dirty from all the years sitting, and i didn't want to rebuild it at the time because i still have alot of work to do on the car-suspension, brakes,etc. i'm stiil on the fence about stroking it, it sounds as though i can meet my goals without doing it. i plan on pulling the heads this weekend, but at a minimum i will replace the rods w/ eagles and port the heads.
 
#24 ·
There are guys who are already experts and others who need a little help, so it's hard to know where everyone fits at first. So if this is old news, just disregard it.

i've never driven it so i don't know how much vacuum it has, and the car itself hasn't been driven since 1988. when i broke the cam in i ran it on an external fuel source, and the car wouldn't idle-the carb was dirty from all the years sitting, and i didn't want to rebuild it at the time because i still have alot of work to do on the car-suspension, brakes,etc. i'm stiil on the fence about stroking it, it sounds as though i can meet my goals without doing it.
You can run 12's at 400 ci providing you can get enough traction, the gear ratio is aggressive enough to get the car off the line good, and you are good at shifting the manual box. The first 60' mean a lot when talking about drag racing ET, so you're right on target w/a 3.73 rear gear combined w/the Tremec 600 transmission.

Stroking it would be great if it's in the budget.

i plan on pulling the heads this weekend, but at a minimum i will replace the rods w/ eagles and port the heads.
Good deal. This will give you the opportunity to measure how far down the cylinder the pistons are at TDC (piston deck clearance), as well as to see what the piston type (forged or cast), and design (flat top/dished) is as well as the number of valve reliefs (some rebuilder-type pistons have 8 valve reliefs to accommodate both early and later valve angle, this adds volume to the piston "dish" over what a 4 valve relief piston has). Obviously this will be the time to CC the combustion chambers as well.

The reason for all this measuring is to know what the static and dynamic compression ratio is, as well as the quench distance. A page w/links to dynamic CR calculators is here. Other areas of the valve train that need to be verified can be found at the links below.

The following may be putting the cart before the horse, but if you're considering swapping to 1.6:1 or 1.65:1 ratio rocker arms, check the fit/relieve the pushrod holes to allow the pushrods to fit correctly while you have the heads off. Not necessarily that I think you need more lift, but just in case.

Valve train points to check
Valve train geometry
Valve spring installed height
Adjusting solid lifters
 
#25 ·
would the additional lift be of any benefit? also, what is an easy way to check how far the pistons are down in the bores(deck height), as i've never checked that, w/ the chevies the machinist always checked. i have dial indicators and calipers, i was thinking a straight edge over the deck w/ a depth caliper over it, get the number, subtract the thickness of the straightedge and that would be my measurement?
 
#26 ·
I have found that unported D-port Pontiac heads (like your #48's) work well w/around 0.480"-0.500" lift. If the heads were ported you would see more benefit from more lift.

What rockers are on it now? The stock Pontiac rockers will spec slightly less ratio than advertised, but aren't as bad as most stock Chevy rockers. If you have stock rockers you'll pick up a little lift just having an accurate 1.5:1 ratio.

Using higher ratio rockers will require the springs be checked for coil bind, along w/the other clearances mentioned in the previous links like piston to valve clearance, rocker to retainer, retainer to seal/valve guide boss, etc. Good pushrods should be used.

Even w/stock rocker arms/ratio you should use straight wall studs (as opposed to the stock bottleneck studs). ARP p/n 190-4003 or BBC studs will work.

You can use a straightedge like you described. Measure to the piston's 'quench band' (top image) if the piston has a dish or dome. Do not measure to the bottom of the dish/valve relief or the top of the dome, in other words. Take the measurement directly over the wrist pin (red arrows bottom image). Measuring over the wrist pin will keep the piston rocking in the bore from skewing the measurement.



 
#28 ·
Good rockers. If you think there's any chance you might want to run the 1.65's, go ahead and open the pushrod holes up now, while the heads are off. I'd go ahead and check all the clearances, too just to be sure. That way all you'd need to do is swap rockers and lash the valves.

In my experience changing only the rocker ratio hasn't required a different p-rod length, but it never hurts to check to be positive. You can check for the correct p-rod length during the mock-up phase or even when the 1.65 rockers are installed by using an adjustable pushrod.

Click on image for details on making this tool:


Other homemade pushrod length checking tools at HomemadeTools.net
 
#29 ·
i pulled the head off the pass side, piston to deck is .006-.008, so block has been decked. there are no numbers on the pistons, i think they are kb hypereutectics? they have a dish w/ 2 valve reliefs. i am thinking about porting the heads and putting the engine back together- everything inside looks perfect. what do you guys think? i just have alot of money to spend on other parts of the car
 
#31 · (Edited)
i pulled the head off the pass side, piston to deck is .006-.008, so block has been decked. there are no numbers on the pistons, i think they are kb hypereutectics? they have a dish w/ 2 valve reliefs. i am thinking about porting the heads and putting the engine back together- everything inside looks perfect. what do you guys think? i just have alot of money to spend on other parts of the car
Someone put some thought into the engine. The D-shaped dished pistons are the best type of dish to have- they optimize the quench effect.

The KB 346 piston has a D-shaped dish, 17cc volume. That piston volume, combined w/a 0.040" quench will get you the 9.5:1 CR you were told the engine had. Unfortunately, the KB piston has the ID on the side of the piston (shown below), so you'd need to pull a piston and rod to see.

The forged Icon KB p/n 891 is 14cc IIRC. It has an intake valve relief of:
.199” deep x 2.140” diameter. Exhaust relief is .201” x 1.830"

You can still use the same procedure used to CC the combustion chambers to see what the dish volume is, by using some grease around the outside diameter of the piston to keep the measuring fluid from seeping out. Then using a clear cover made from scrap plexiglass w/a couple holes in it (one to add fluid, the other hole to release the trapped air) or use an old CD w/the plating removed (the center hole is big enough so you don't need another hole) sealed to the block deck w/grease, you can add fluid using a graduated burette or a large syringe (get them from a pharmacy or vet.

It'll take a little math to subtract the volume above the piston at TDC from the dish volume, but it's not that hard to do. If you choose to go this route and want more info just ask.

 
#30 ·
Get a positive ID on those pistons so you can determine the CCs of the reliefs. That way you can figure your CR.
Porting ....yes...mild.
Clean up inside the bowls (behind the valves) and the runners. Clear any flashing from the port openings and gasket match the openings.
Dont go whole hog in the runners. Hogging them out will not improve street performance.Stock runners perform well with just a smoothing and some short turn radius work . Lower the port floor a little at the short turn radius ...a little. Take a little of the bend out of it.
Any bumps in the runners should be removed. Myheads had some wierd little bumps in a few of the ports. I suspect they were ID marks on the original casting mold to ID them, and the marks just transfered to the part when it was cast. look in there, you will see em.
Keep it simple when porting and dont over do it. When porting , imagine water flowing through the heads and how it would flow better. Air will act similar. Not sure if you can picture that but it was put to me that way, I think I kinda get it.:D
 
#32 ·
Seeing as how this engine seems so far to be assembled correctly (no small deal!), check the ports real close to see if they've already been worked over.

One fairly quick/easy way to tell is to measure the port throat diameter right below the seat's last angle. See how close to 80% of the intake valve's diameter this area is (around 1.7" would be about 80%). That would indicate serious effort had been made, not just a brush w/a sanding roll to smooth things a little. This does require a valve be removed, obviously.

You can also look for evidence of the chamber being relieved to remove shrouding of the valve. Sometimes you will be able to see some scribing left from where the ports were gasket matched, as well as seeing how the gasket port opening compares to the head/intake port opening.
 
#33 ·
one thing that bothers me about the whole thing after studying it for awhile is the lack of a crosshatch in the cylinders, the motor maybe has an hour or 2 of run time, but yet no crosshatch? something doesn't make sense. i need to post pics of the combustion chamber, it is pretty thin(where the head gasket fire rings are) between the cylinders. i pulled an intake valve and see no evidence of porting there, but wonder if the chambers have been played with
 
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