[minn-ta]

I am hoping to get some help here regarding my Pontiac 400 combination. I have a 76 400 that was built and put into a 1981 T/A. The idle is so rough that I can barely tune it. I really want to drive it as a daily driver.

Here is a list of parts in it as of today:
76 400 block .030 over
400 crank .030 over
Pistons are TRW2262F30
6x heads ported and intake matched 1.77/2.11
Roller rockers 1.5:1
Hydraulic lifters
RPM Performer Intake (matched to head intake)
Lunati Camshaft .534/.534
3:73 rear
Drive shaft balanced
TH-350 Trans with shift kit
27" 4 row radiator w/tranny cooler
Hooker Comp headers
Roller Timingchain
MSD Ignition Control 6A
Edelbrock 750CFM carb
Stall converter (no idea)
Mechanical Fuel pump

I have no idea what the timing is set at.
With all that said, the car is sluggish, idle is rough, it does not start very well, i usually need to pump the gas like crazy or even add gas down the carb if it gets driven once a month. When you floor the gas, there is a hesitation like the car is losing power.

There seems to be so much wrong with it that I don't know where to start.

Some friends have said that the cam is too big. Others say that the original 3.08 gears should have been left in. And some point to the carb as not being the right fit.

I am open to your suggestions.
Dave

http://seopsolutions.com/ta/daves_ta.JPG

[jonmachota78]

What are the cam duration and lsa #'s? You need to get the timing ironed out as that will be key as to the manners and performance.

[HWYSTR455]

I would suggest starting with verifying firing order, TDC, and timing. Also check to ensure there are no vacuum leaks, and check the carb base gasket, plate if it's there too.

Try to find out what cam you have too, only lift doesn't give enough info.

.

[minn-ta]

jonmachota78

Duration is 245 in 245 ex at .050 tappet lift
and
295 in 295 ex advertised duration

The part number of the cam is 07703 (Lunati)

EX Closes ATDC 10.5
EX Opens BBDC 54.5
IN Opens BTDC 18.5
IN Closes ABDC 46.5

Lobe seperation 108

[minn-ta]

HWYSTR455

Thanks. I will check everything you mentioned. I also posted the cam specs. I can tell you for sure that it has some annoying exhaust leaks! Anyone know of better headers or a way to tighten them better? These needed to be put on after the engine was in place making it very hard to get a wrench on the bolts.
Would D-Port Ram Air style factory headers work in this application?

[HWYSTR455]

That is a little big, but you should be able to get it to idle ok. I bet more initial timing would help, and it probably needs a bunch of idle mix too.

I think the headers are the way to go with that setup. And no, there's no real trick to the header bolts, suggest you use offset wrenches (pic attached), and possibly small head bolts, like from ARP.

You can use a piece of safety wire around your wrist and the wrench, so you aren't doing the gopher thing every time you drop it.

Depending on the header gasket, for the most part, once it leaks and burns the gasket, it's toast. You can trim the gaskets and notch the holes with some side cutters, and slip in indidivual pieces when replacing. Helps a little.

.

[minn-ta]

HWYSTR455

Thanks for the tips. I will take a look at the timing. By the way, the ignition is HEI in case that matter and it appears to be stock.

Yes, this cam is very big. I only get about 8 miles per gallon which sucks when I want this to be a daily driver with street performance. Any way I can improve that with a different cam that will still work with everything else I have going on?

What about the stall converter? I have no idea what it is but i guess the real question is, what should it be?

Thanks

[chiphead]

Are those 6X-4 or 6X-8 heads? I think 6X-8 was stock on standard-perf 400s. The 6X-4 were used on P350s and high-perf 400s. The -4s have smaller chambers and the compression is higher with them.

That cam is marginal for 6X-4 and way too big for 6X-8 heads. You may be able to tune it and get it to run better. What you describe is typical behavior for an engine with entirely too much cam. The late intake closing event bleeds off cylinder pressure and kills the low-RPM power. The tight 108LSA causes low idle vacuum which causes a a rough idle.

I bet your power brakes suck due to low manifold vacuum.

Like HWY says, usually you have to add a lot of fuel through the carb's idle and off-idle circuits to provide a rich enough mixture to the weak vacuum signal. You also have to increase base timing to 15-18* to increase idle vacuum and give the mixture time to burn. The fuel burns less efficiently in a low-compression engine so you've got to give more spark advance. Stock HEIs have a lazy and short advance curve. Start reading up on how to re-curve an HEI.

These tuning methods are a crutch to get a mis-matched combination to run. A summit 2801 cam would be a much better choice. A swap to a set of milled 6X-4s with 88 CC chambers would really wake that combination up. The rest of the list of parts sounds pretty good.

[72LuxuryLeMansLa.]

Originally Posted by chiphead

Are those 6X-4 or 6X-8 heads? I think 6X-8 was stock on standard-perf 400s. The 6X-4 were used on P350s and high-perf 400s. The -4s have smaller chambers and the compression is higher with them.

That cam is marginal for 6X-4 and way too big for 6X-8 heads. You may be able to tune it and get it to run better. What you describe is typical behavior for an engine with entirely too much cam. The late intake closing event bleeds off cylinder pressure and kills the low-RPM power. The tight 108LSA causes low idle vacuum which causes a a rough idle.

I bet your power brakes suck due to low manifold vacuum.

Usually you have to add a lot of fuel through the carb's idle and off-idle circuits to provide a rich enough mixture to the weak vacuum signal. You also have to increase base timing to 15-18* to increase idle vacuum and give the mixture time to burn. The fuel burns less efficiently in a low-compression engine so you've got to give more spark advance. Stock HEIs have a lazy and short advance curve. Start reading up on how to re-curve an HEI.

These tuning methods are a crutch to get a mis-matched combination to run. A summit 2801 cam would be a much better choice. A swap to a set of milled 6X-4s with 88 CC chambers would really wake that combination up. The rest of the list of parts sounds pretty good.

I just figured a 4.15 bore, 3.75" stroke, 93cc chambers with 8cc flat tops .010" in the hole and got 8.38:1scr!!!!! Not good!!! for any cam in my world but certainly not good for that cam!!!!! It would only have 6.3:1dcr.............
Cut those heads all the way down to 87cc and get 8.80:1scr still way too low!

Karl

[minn-ta]

chiphead

I will check on the 6x type later. The car has been in my parents back yard garage all winter. I am going to get it this weekend.

I do know that the heads went through a $600 head job to get them to what they are at today. It originally was not 2.11/1.77. But I didn't not find out what the CC's were.

As far as the HEI, would it be better if I just picked up a new Mallory Distributor?

[HWYSTR455]

The guys are all on target with the suggestions etc, would be best if you knew specifics about the engine, like if the piston is below deck (how much), and head CCs.

That, along with your rear gear, and how you anticipate driving the car, will dictate your cam and stall selection.

Do you know the history of the engine, like how fresh it is? Do you know the builder?

Nothing wrong with the HEI, as long as the shaft is good. You can freshen it up if it's of unknown origin, and curve to suit the engine. It's kind of hard to beat the OE HEIs, so unless you prefer a particular distributor, or need a small cap, then I suggest leaving it for now.

.

[PunchT37]

A 245 @ 050 cam is too big for a 400 cube with either 6x head with flat top pistons. Just one`s worse than the other. All you really have is a 7.6 to 8.2 compress engine. Waaay too low for that cam. That Edlelbrock carb with that low vacumn has got to be a pain. Been there, done that.

Get a cam in the mid 2 teens, to maybe 220 or so @ 050 on intake with an 8 to 10 degree spread on exhaust, and go from there. That carb. will start to behave more with a smaller cam.

Summit 2801, Crower 60918 or Comp. XE262 cams should work much better with those low compress 6x heads. That`s if you don`t have those horrible 389/400 replacement pistons. Run a good 11 inch converter and possibly get rid of the 3.73`s and go with 3.23`s for a daily driver. The cam should go 1`st. Then tune, then worry about the converter and gears.

[minn-ta]

This is some great information you guys. Thank you so much!

As far as the engine goes, it has about 3000 miles on a fresh rebuild.
I don't know about the piston position (below/level).

Since I have the Edelbrock Performer RPM intake, should I look at the matching cam with 231° / 240° duration or is that still too much?

[72LuxuryLeMansLa.]

Originally Posted by minn-ta

This is some great information you guys. Thank you so much!

As far as the engine goes, it has about 3000 miles on a fresh rebuild.
I don't know about the piston position (below/level).

Since I have the Edelbrock Performer RPM intake, should I look at the matching cam with 231° / 240° duration or is that still too much?

Way too much! Look at the cam numbers PunchT37 supplied. He is a mechanic with over 20yrs experience.

Karl

[WARPed]

I am with Punch on the carb, my experience with those Edelbrock carbs is they never perform like one would expect - just an all around turd in my book. But first off get your timing resolved and check for vacuum leaks before you do anything else then tackle the header leak issue and you may be surprised - Another consideration is how are the valve adjusted - takes just a few tight ones and it can kill the performance.

[jonmachota78]

Yes what they all said.
If you must drive it the way it is diaal in the timing . Start with 12 initial, all in by 3000, total of 35 with the vac can unhooked. Then hook the vac can back up and I'd use ported vac with that combo. Low compression will like more timing than that but I would start there...

[HWYSTR455]

You really can't choose a cam until you find out more about the engine. Do you know the previous owner, or can you get in contact with them or whoever built the engine?

Worse case, you may have to remove a head, but if you're planning on doing a cam, that isn't too bad. If you discover a large chamber, you can rectify (to some degree) and then you can properly choose a cam for the engine (with the correct CR).

.

[PunchT37]

Since he has the 2262F piston, he has low compression no matter which 6x head he has. I have a 246/253 at 050 cam in one of my 455`s on 108. It`s a rough idling SOB on 462 cubes with 6x#4`s ported to mid 260`s. Pulls 6 to 7 pounds of vac. Runs like a bitch when you mash it though.

I had that cam when I had 62`s on it with 11 to 1 compress. Not a bad cam at that level. Got tired of buying 100 octane gas. Roughly 225 psi cranking pressure with the 62`s and 170 with the 6x.

That 400, with that cam and those heads is gonna be even worse than mine.

[chiphead]

Yeah, that cam is WAY too much for the displacement and compression.

What you do with the engine's setup depends on how much money you want to dump into it. The cheapest way to address this is to use a summit 2801, or a similar cam with 205-218 intake duration and 112 LSA. Maybe even drop back to the 2800 since compression is so low. The earlier intake closing will build some vacuum and the wider LSA will smooth the idle some and broaden the powerband. The power output will be low because of the low compression. I figure 300-325 HP at 4800 RPM is doable if you leave the heads alone.

The proper way to "fix" this is to pull the heads and figure out what is really going on. CC the heads, figure out the deck height and verify piston valve reliefs. The either mill the heads to reduce chamber size or sell them and buy a set of heads that will boost compression properly. Ideal compression for a motor like this would be 9-9.5:1. Combine that with a properly set up HEI and a custom built Quadrajet carb. That'll give 375-400 horses but not be so tune-sensitive that it blows up at the first tuning mistake or batch of bad gas. It'll have a stable idle, run cool, not load up and die in traffic, make power right above idle and make enough vacuum for power brakes. That's what we call "good street manners".

Its really sad that so many guys come here on PY with engines that are mismatched like this. Poorly running, weak engines with no throttle response and bad street manners. And then we've got to be the bearer of bad news and tell them to tear their new engine apart to fix it. I'm not bashing the OP, I feel bad for him. He's got this engine that should be pretty strong, but whoever chose that top-end package really screwed the pooch.

This happens way too often, and I'm sure the regulars here have seen their share of it in person. Where are all of these mis-matched engines coming from? And it's not just pontiacs. I can't tell you how many "muscle car" engines I've seen where somebody used forged pistons, good internals but used low compression smogger heads, a cam that is WAAAAYYY too big, and then top it with that AFB-style Edelbrock carb that doesn't work for crap. I guess I shouldn't be surprised but I am. *rant off*

[Ben M.]

Originally Posted by PunchT37

Summit 2801, Crower 60918 or Comp. XE262 cams should work much better with those low compress 6x heads. That`s if you don`t have those horrible 389/400 replacement pistons. Run a good 11 inch converter and possibly get rid of the 3.73`s and go with 3.23`s for a daily driver. The cam should go 1`st. Then tune, then worry about the converter and gears.

100% agree here, have the same engine and it's a great driver with no problems or idle troubles.

I will disagree that the extra eyebrow relief pistons being absolutely terrible for a low compression driver. Definitely not optimal and no good reason to go with them but if they are installed, no reason to tear the engine apart to replace them on a driver like this (I have a similar engine with them and no troubles there either).