[78P-T/A]

Ok, maybe a stupid question and there may be many threads about this from before..... but, in my 78 Trans Am I have the Pontiac 400 (l78 not w72) - but with 6x4 heads, edelbrock performer intake, edlebrock 750 carburettor, long tube headers and 2.5" Pypes dual exhaust. Th 350 auto and 3.23 rear end. Otherwise stock. Now I'm wondering if I have anything to gain from upgrading the camshaft? And recommendations for this?

[steve25]

Those motors do not have much compression ( low speed cylinder pressure) to trade off for more top end power.
That's why Pontiac engineers went thru the trouble to make a Cam just for the W72 option.

What I might do is change out the rocker studs to full 7/16" as this will be needed for a rocker arm swap.

Put on a set of 1.65 roller rockers and get the motor tuned on a chassis dyno.
You will likely find a increase of 12 hp from the dyno tune which is well worth it.

The higher ratio rockers will add .040" more lift and some 3 more degrees of duration all without effecting your idle or more importantly you current cylinder pressure.

Your 6X heads will respond well to the added valve lift and the stock valve springs will handle the new .440" valve lift, although if they have more then 40K miles on them I would suggest installing new ones along with new valve stem O ring seals.

With your stock Cam added 1.65 rockers will not cause a push rod clearance issue.

[78P-T/A]

Originally Posted by steve25

Those motors do not have much compression ( low speed cylinder pressure) to trade off for more top end power.
That's why Pontiac engineers went thru the trouble to make a Cam just for the W72 option.

What I might do is change out the rocker studs to full 7/16" as this will be needed for a rocker arm swap.

Put on a set of 1.65 roller rockers and get the motor tuned on a chassis dyno.
You will likely find a increase of 12 hp from the dyno tune which is well worth it.

The higher ratio rockers will add .040" more lift and some 3 more degrees of duration all without effecting your idle or more importantly you current cylinder pressure.

Your 6X heads will respond well to the added valve lift and the stock valve springs will handle the new .440" valve lift, although if they have more then 40K miles on them I would suggest installing new ones along with new valve stem O ring seals.

With your stock Cam added 1.65 rockers will not cause a push rod clearance issue.

Hello and thank you for your reply. So I have more to gain from this than a new camshaft? I don't know much about this, so I'm looking for opinions. I have no idea about the age or how many miles the springs have. The car has 92000 miles... but according to the previous owner the heads have been overhauled but don't know what was replaced..
What would the shopping list consist of if I want to do this?

[grivera]

Also work in a performance timing curve for your distributor and check your timing chain slack.

[78P-T/A]

Originally Posted by grivera

Also work in a performance timing curve for your distributor and check your timing chain slack.

Yes, I adjusted the timing this summer, it was a bit off, noticed a big difference afterwards

[78w72]

With the 6x-4 heads you can have them milled ~.030 to increase compression some, that will help selecting a new cam that IMO will add more power than just using 1.65 rockers and you can keep the stock 1.5 rockers or upgrade to roller rockers or roller tip type but the stock stamped 1.5 rockers would be fine. Stock rocker arms can be made adjustable by using SBC crimped lock nuts or poly locks & adjust like an SBC, 7/16 studs are best but for a lower lift cam the stock studs are usually fine.

You will get a lot more power/performance from a good mild cam than just using 1.65 rocker arms. All depends on what you want from the car and how much work you want to do.

You already have headers and good exhaust system that will take advantage of a good cam. I have a 78 400 W72, but since you already have the 6x-4 heads your motor is the same compression as a w72 just not the same cam... and the block is weaker than the W72 XX block, but for what youre doing that is not a concern, 500557 blocks are fine for street use under 400ish HP.

Some members on here dont like the comp XE cams but they are designed for lower comp engines and if tuned right they work very well in these lower compression engines, i have the xe268 which is a mild cam with ~.480 lift, can use common valve springs from crane or comp, i installed it "straight up" and so far only real tuning ive done is a Q-jet carb built by cliff, but stock timing curve, stock intake with headers and 2.5 exhaust... car runs excellent in all street conditions and runs mid 13's at 103+ mph with old hard BFG street tires & taking it easy on a new at the time clutch, plus its a lower mile #'s matching car so wasnt going for max times, just a fun test & tune day at the dragstrip.

Im sure there are better cams and maybe some others will suggest some, but for a very basic mild cam change, the car runs very good & does respectable times for a low compression 400.

[steve25]

Don’t even bother milling the heads only .030” since doing such will only hand you a measly 1/4 point gain in compression.
Go .045” or better yet .060” or put that money that would have gone into a .030” cut to better use elsewhere!

The only thing is with a .045” or .060” cut you will have to cut the intake flange at least .030” to get everything to bolt up happy.

Like anything else these days milling beyond just a clean up cut has gotten expensive.

Also milling beyond.030” will call for shorter length push rods.

Yup, everything starts to snow ball at some point.

[track73]

I had a Summit 2800 cam in a low compression 400. I loved it. It was very "torquie" at low rpms and it was great with 87 octane. When I put 1.65 rockers on the emgine I broke valve spring after a few miles , so I put 1.5's back on with no problem.

[grivera]

If you decide to go through the trouble of pulling the heads for milling may as well get a better set of heads like 1969 large valve 62’s which typically measure at 75cc - that’ll be a game changer for cam selection

[78w72]

The suggestion to mill the heads .030 was to avoid snowballing with pushrod length changes or needing to mill the intake or intake surface of the heads, I have gone .040-.045 without needing to do either on 6x-4 heads on 400 & 455 blocks, at most had to elongate the intake bolt holes a tad.

According to wallace racing calculator, milling 6x heads .030 reduces them 6cc, using that number on their compression calculator and doing such, shows a change of more than a measly 1/4 point, closer to 1/2 a point (.4) using the common auto populated numbers.

Milling the heads isnt needed to benefit from a cam change vs 1.65 rockers, but the OP did say he was told the heads were rebuilt at some point so most likely they have already been milled some unknown amount, using stock numbers on the high side of 6x-4 CCs (94cc) shows 8.38 compression, -6cc shows 8.80... there are cams that will work pretty good at an estimated 8.5 compression.

Just a suggestion based on what the OP asked about & will have better results than just doing 1.65 rockers.

[Skip Fix]

I think a head mill will be minimal improvement then you have to deal with valley pan and intake fitment. The 400 flat top piston 6X-4 Summit 2801 cam Perfumer HO manifolds runs well. The 6X-4s cc'd 93.5 then in clean up I smoothed the ridge down in the chamber milled them 0.050 and they still cc'd 93cc. I did have issues with teh valley pan as well as the bottom of the Performer hitting it after the milling.

I did add 5.103 "regular" D port length valves to get a better IH valve spring vs the 4.98 6X length. I have a brand new Manley stock replacement 4.98 set I'll give away for shipping!

[78w72]

Originally Posted by Skip Fix

I think a head mill will be minimal improvement then you have to deal with valley pan and intake fitment. The 400 flat top piston 6X-4 Summit 2801 cam Perfumer HO manifolds runs well. The 6X-4s cc'd 93.5 then in clean up I smoothed the ridge down in the chamber milled them 0.050 and they still cc'd 93cc. I did have issues with teh valley pan as well as the bottom of the Performer hitting it after the milling.

I did add 5.103 "regular" D port length valves to get a better IH valve spring vs the 4.98 6X length. I have a brand new Manley stock replacement 4.98 set I'll give away for shipping!

.050 milling and only .5cc reduction?? something seems off on that, unless smoothing the ridge in the chamber was a big reduction in volume to make a .050 milling only reduce .5cc.

The last set of 6x-4 on my 400 CCd at 92cc and .030 was right about what wallace head milling chart shows, ~5-6cc. Same results from most of what Ive read over the years too. Everyones results will vary based on what is actually done & the combo of other parts used.

If the OP doesnt want to pull the heads to mill, there are still cams that will work good at the estimated 8.5 compression.

['ol Pinion head]

To get the OP's topic back on track, his 78 T/A's 400 engine is the base 400 engine, that means 6X-8 heads & memory serves right the beveled top 400 pistons which also lower static CR. Not the same as a W72 engine.

Back to his asking about a camshaft change, the only camshaft change that will have any benefit with that bone stock YU 400 longblock is one that increases cyl pressure. Im not a fan of the Comp XE line of cams & have no use for them in higher compression builds, but at 7.6-1 compression a smaller Comp XE with its increased ramp speed & fuel charge cyl filling are about it.

Desiring to have the bottom end professionally rebuilt with an over bore & forged pistons, swapping heads to a pair of done up 6X-4's one couldnt go wrong milling them .030-.035 & zero decking the block. Sourcing '70 & earlier D-port big valve 400 heads just to raise C/R will mean going with a much larger cam. Swapping to those "performance" heads will bring up not only the missing accessory bolt hole issue for the long alternator bolt (issue I've solved a few times), but also the Need for a gear swap in the rear, as well as convertor swap for best performance. Is all of this expense what the OP originally asked about???

[78w72]

Originally Posted by 'ol Pinion head

To get the OP's topic back on track, his 78 T/A's 400 engine is the base 400 engine, that means 6X-8 heads & memory serves right the beveled top 400 pistons which also lower static CR. Not the same as a W72 engine.

Back to his asking about a camshaft change, the only camshaft change that will have any benefit with that bone stock YU 400 longblock is one that increases cyl pressure. Im not a fan of the Comp XE line of cams & have no use for them in higher compression builds, but at 7.6-1 compression a smaller Comp XE with its increased ramp speed & fuel charge cyl filling are about it.

Desiring to have the bottom end professionally rebuilt with an over bore & forged pistons, swapping heads to a pair of done up 6X-4's one couldnt go wrong milling them .030-.035 & zero decking the block. Sourcing '70 & earlier D-port big valve 400 heads just to raise C/R will mean going with a much larger cam. Swapping to those "performance" heads will bring up not only the missing accessory bolt hole issue for the long alternator bolt (issue I've solved a few times), but also the Need for a gear swap in the rear, as well as convertor swap for best performance. Is all of this expense what the OP originally asked about???

OP stated in the 1st post his engine already has 6x-4 heads...

[MarkS57]

FWIW, I bought a used 350 from a 1975 Firebird to replace a trashed 301. I had the heads rebuilt (5C-4's) and cut .030". The car, 79 Bonneville ran very well with that engine; although I have no pre / post comparison to offer. No issues with the valley cover, manifold, pushrods etc, all bolted up fine.

[napster]

All I'm Going to say is XE274. Read the rest in the link. The sound of the video will do my talking.

https://forums.maxperformanceinc.com...21#post6373821

https://www.youtube.com/watch?v=oP91rZkA84Y&t=9s

[74Grandville]

another vote for the Summit 2800, The stock L78 cam is tinny.

2801 might work too, but with the bevel stock piston, compression will still be pretty low.

[78P-T/A]

I have to say that I am overwhelmed by the commitment you show guys! Very fun for someone like me who doesn't have a very good understanding of the mechanics involved here. I don't have much money, but I'm trying my best to give my car what it deserves. It was my dad's car, when he died of cancer at an early age I took over the car and it means a lot to me! That's why I also ask questions about what I can do to upgrade, which also doesn't cost me too much, to get a little more power in the car Simply because I think the car deserves it

[Stan Weiss]

Originally Posted by 78w72

.050 milling and only .5cc reduction?? something seems off on that, unless smoothing the ridge in the chamber was a big reduction in volume to make a .050 milling only reduce .5cc.

The last set of 6x-4 on my 400 CCd at 92cc and .030 was right about what wallace head milling chart shows, ~5-6cc. Same results from most of what Ive read over the years too. Everyones results will vary based on what is actually done & the combo of other parts used.

If the OP doesnt want to pull the heads to mill, there are still cams that will work good at the estimated 8.5 compression.

I didn't know what the combustion chamber on those heads look like. If I deck a 4.12" bore block 0.030" that is 6.55 cc's.

Stan

[Stan Weiss]

What is the duration and lift of the present cam.

Have you done a cranking compression test? If so what psi have you seen.

Stan