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Recommend a Cam after Head Change

  
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Recommend a Cam after Head Change

 
bradshdj bradshdj
New User | Posts: 45 | Joined: 02/13
Posted: 07/21/14
08:57 AM

I currently have a WT block Pontiac 400, with 62,000 original miles.  The car has 4X heads and a 268 Comp Cam.  Next spring I am looking to install 85 cc Aluminum Kauffman heads.

What camshaft do you recommend for this Pontiac 400 for street performance, with a 4 speed, 3.42 gears, in a 74 Trans Am.  I am currently using a Eddy RPM intake which I plan on keeping but can a bigger carb.  Currently the car has a 750 HP Holley.  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 07/21/14
02:51 PM

hello;


hey good to hear from you again. 70 bird here. i lost my screen name when the site changed over.

ok here's one of my famous interrogations.

What are you wanting out of it?

what did you like about the 268?

what did you not like about the 268?

was that the 268h or XE268h cam?

what octane gas are you going to run?

do you want more bottom, mid or top end power? No, more everywhere is NOT an option, lol.

the next step up is a lunati voodo 276 cam. it would work well with your combo but you are pressing your luck with stock pistons and i would worry about valve clearance if the valves in the new heads are bigger.

http://www.lunatipower.com/Product.aspx?id=1777&gid=287


CARB - I would stick with the 750. its quite easy to over carb an engine.
.  

bradshdj bradshdj
New User | Posts: 45 | Joined: 02/13
Posted: 07/22/14
08:47 PM

For the Head and Cam change, street performance and an occasional trip to the track.

In it right now is a Hi-Energy 268H with matching springs, retainers, seals, locks, Comp Cam 1.52 Roller Tips.

I really liked the more responsive engine with the 110 LSA.  I would like to keep that.  MY midrange is great but I think its a little lacking in the higher RPMs.  I want a bit more top end power.

Gas is 93 Octane  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 07/22/14
09:01 PM

hello;

if you have at least 9.5 static compression, this one rips. if you have less than 9.0, i would run the next size down from this or stick with what you have

this will wake it up but is not obnoxious.

http://www.lunatipower.com/Product.aspx?id=1777&gid=287  

bigD bigD
User | Posts: 155 | Joined: 05/14
Posted: 07/23/14
02:19 AM

Icon Quotebradshdj:
I currently have a WT block Pontiac 400, with 62,000 original miles.  The car has 4X heads and a 268 Comp Cam.  Next spring I am looking to install 85 cc Aluminum Kauffman heads.

What camshaft do you recommend for this Pontiac 400 for street performance, with a 4 speed, 3.42 gears, in a 74 Trans Am.  I am currently using a Eddy RPM intake which I plan on keeping but can a bigger carb.  Currently the car has a 750 HP Holley.


The ID charts show a WT with 4x heads came in '74 with a 4-speed and had only 8:1 CR. So is it numbers matching ?

Anyhow, at 8:1 compression you need one of the small steep ramp cams like the XE series and the Lunati Voodoo series. These cams are designed to increase power in low comp engines by increasing the cylinder pressure.

You could also increase your CR by using Cometic .027 thick head gaskets with a 4.160 bore size.

http://www.jegs.com/i/Cometic-Gaskets/245/C5711-027/10002/-1?parentProductId=1288713

But since you are switching to 85cc heads, that changes things. Let's use the CR calculator and figure out a good combo.

http://www.wallaceracing.com/cr_test2.php

Let's plug in these numbers and and see what we get.

400 std bore, 85cc heads, deck height .020, valve reliefs 6.6cc, gasket .027 x 4.160   CR = 9.03

That's OK for an iron head motor. But with alum heads you want at least 10:1 CR.
Let's plug in some 74cc KRE heads and see what we get.

Changing nothing but the head cc makes an even 10:1 CR. I'd say that's just about perfect for a pump gas motor. BUT...

If the bottom end is original, then the cast rods and pistons/rings are 40 years old.Increasing the CR could cause the motor to start smoking.   Frown

Therefore, if the bottom end is all original except for the cam and lifters, I would not put the $2000+ heads on it. At the very least, I'd bore .030 and buy Sealed Power forged pistons and moly rings for it.

If you don't wanna rebuild the short block, I'd just leave it as is for now.

So you have some decisions to make.