Cam Change 400 - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 05-03-2018, 09:09 PM Thread Starter
 
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Cam Change 400

I'm running a 68 400 in my 65 goat that has been built at .40 over with 75 6X heads (94cc), block decked, heads milled and a ram air IV cam with rhoades lifters. C.R. is stated to be 9.25:1 and low end performance is pathetic. This is a four spd car with a 3:23 axle. I want to switch to a Lunati Voodoo cam and Lunati recommends their p/n 10510702 262/268 which is a hydraulic flat tappet cam. If I use this cam can I retain the heavy valve springs currently installed with the ram air IV cam and should I change the push rods? This setup has only a few hours of run time since the complete rebuild. I'm also using an 800 cfm Rochester quadrajet stage two which loads up at low end. Is this due to the cam? In fairness the car will perform well when you hit the 5000 range but this is not a good setup for a daily driver.
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post #2 of 9 (permalink) Old 05-04-2018, 06:11 AM
 
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Yes, that 262 Voodoo should really wake your 400 up, in the low & mid range.

Same springs should work. Don't use weaker springs. The Voodoo is what many call a "steep ramp" cam. This lobe profile requires more spring pressure than a cam with a lot of adv duration, having a long gradual ramp.

Paul Carter loves Voodoo cams, and a lot of spring pressure. I read on the PY forum that he recommends 130# seat pressure, for the 268 & I believe the 276 Voodoo.

But, with the 262, you can probably get by with a bit less. I say this because Comp Cams recommends their 988-16 springs for their similar XE262, but switch to the stronger 995-16 springs, for their XE268.

COMP Cams Catalog - Pontiac 265-455 8 cyl 1955 - 1981

Butler shows these springs to have 125# seat pressure, at 1.6" installed height. That is supposed to be the aprox installed height of 6X heads, with stock length valves.

https://butlerperformance.com/i-2445...tegory:1234802

Lots of Pontiac guys recommend Crower 68404-16 springs for performance HFT cams, when you have a 1.6 installed height. Butler shows 'em to have less seat pressure, but a little more open pressure.

https://butlerperformance.com/i-2445...tegory:1234802

Might not hurt to find out how much seat pressure your current springs have, if you don't already know.
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post #3 of 9 (permalink) Old 05-04-2018, 09:01 AM Thread Starter
 
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Cam Change 400

Thanks bigD for the helpful information and for confirming that I'm on the right track here. I've used a Lunati Voodoo cam in a sbc with good results and will likely use them in this application.
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post #4 of 9 (permalink) Old 05-04-2018, 12:42 PM
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First things first. You said the heads are 94 cc's but also that they've been milled. Have the chambers actually been measured since they were milled or are you just using the factory 94 cc spec? It matters. A lot. Milling the heads reduces the chamber volume and raises compression.

You also said the block has been decked - I assume you mean zero decked so that the pistons at TDC are dead even with the top of the block?
Which pistons are you using, and how much volume is in the valve reliefs and/or dishes (if present)? Lastly, what are the specs on your head gaskets (bore size and compressed thickness)? Without knowing all these measurements it is impossible to know how much compression you're actually running.

Cam - unless you've got enough compression, gear, and cylinder head air flow to support it, that Ram Air IV profile is definitely going to be a dog a low rpm. The Rhoads will help "some" but personally I'm not a big fan of them. They work by bleeding down lift and duration at low RPM, but in doing that they also introduce some 'slop' in the valve train and delay the lifter from moving until it's already getting onto the steeper part of the cam lobe. All of that adds 'hammering' to parts that can accelerate wear and cause problems. My opinion only - your mileage may vary, etc. However, if your compression is already "too high" then that big cam may actually be saving your engine by helping to keep it out of detonation. I wouldn't go changing cams to a much shorter duration (like that Voodoo) without first being sure of where you're at, compression-wise.

Valve springs. Assuming that they're 'right' for the cam you have now, switching to a cam that needs less spring pressure is always safe (mostly - it won't kill your engine but the higher than required spring pressure will tend to wear parts more quickly than springs that are "heavy enough but not too heavy). What's not safe is running springs that aren't heavy enough. If lifters aren't kept fully controlled and are allowed to be 'thrown' off the cam lobes, engine death follows very quickly.

Pushrods. Again, assuming the ones you have are the right length, a cam change won't alter that UNLESS the new cam has a significantly different base circle diameter and/or is a huge change in lift. A smaller base circle will tend to pull the rocker arm contact point on the valve stem "higher" (towards the intake ports) and a big change in lift affects how 'wide' the contact patch is - the more lift, the more rocker arm travel, and the wider the contact patch will tend to be.

Rocker arms - you didn't mention which rocker arms and ratio you're running now, and are you planning to change them?

Lifters. Are you planning to keep the Rhoads or will you be changing those also?

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post #5 of 9 (permalink) Old 05-04-2018, 07:57 PM Thread Starter
 
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Cam Change 400

I don't have any exact specs for this engine other than what I have stated. It is supposedly set up at 9.5 to 1 cr. I do know that it is running a ram air IV cam that leaves me with poor low end performance with this low cr. I was also planning on replacing the rhoades lifters since it idles (sounds)like solid lifters and I don't care for that. I understand the purpose of the bleed down lifters is to calm the idle on a radical cam but I won't need them if I go with a milder cam. I will definitely try to find out more about this setup before I make the change and I don't have a problem with purchasing new valve springs to erase any doubt about issues there. If I go with Lunati I will use their cam and lifter kit, pushrods and springs which should cover it. Thanks for the input Bear.
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post #6 of 9 (permalink) Old 05-04-2018, 08:07 PM Thread Starter
 
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Cam Change 400

Correction, the cr is stated to be 9.25 to 1. If it turned out to be higher and detonate with a milder cam, I would mix 112 with 93 to raise the octane high enough to eliminate the detonation. I won't be driving it enough for the cost to be a factor.
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post #7 of 9 (permalink) Old 05-04-2018, 10:21 PM
 
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Originally Posted by big iron View Post
Correction, the cr is stated to be 9.25 to 1. If it turned out to be higher and detonate with a milder cam, I would mix 112 with 93 to raise the octane high enough to eliminate the detonation. I won't be driving it enough for the cost to be a factor.

9.25 to 1 is a good number for a street engine. You did not note if you knew if the heads had been worked to flow better than stock or not, but will assume they are stock. The stock heads are said to flow up to .450" of lift as their max limit. Of course heads which have been ported/cleaned up or have bowl work will flow much more.

With a 9.25 compression, my opinion would be to select a cam with a 112 Lobe Separation Angle (LSA) as mentioned earlier. Factory cams typically used a 113-115 LSA. I would not go too high on lift if stock heads, maybe .470" max.. The RA IV cam, if you are using the 1.5 rocker arm ratio and not the 1.65 ratio, is .470". The duration is too large for a street engine and indeed suffers on the low end and picks up at higher RPM's. I would not go any more than 280 degree of duration max with a stick and less with an automatic - just like factory did.

In a nut shell, my opinion, dual pattern cam (more duration/lift on the exhaust), 112 LSA, 270-280 advertised duration, .450-.470" lift w/1.5 rocker arms.
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post #8 of 9 (permalink) Old 05-05-2018, 03:23 PM
 
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In a nut shell, my opinion, dual pattern cam (more duration/lift on the exhaust), 112 LSA, 270-280 advertised duration, .450-.470" lift w/1.5 rocker arms.
If you decide against the 262 Voodoo, and would like more adv dur, as mentioned above, the Lunati 10510312 looks to fit that description.

Street Master Hydraulic Flat Tappet Cam - Pontiac V8 276/286 - Lunati Power

The Crower 60241 & 60242 also appear to qualify. But I don't like the price of Crower cams. You can just about buy a custom grind as cheap as some Crower cams.

https://www.crower.com/camshafts/pon...m-280-hdp.html

The Crower 60916 is a bit cheaper.

https://www.jegs.com/i/Crower/258/60...8aArelEALw_wcB

https://www.summitracing.com/parts/c...caArSsEALw_wcB

Last edited by bigD; 05-05-2018 at 03:40 PM.
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post #9 of 9 (permalink) Old 05-06-2018, 05:55 AM
 
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Just want to share this. Over the winter, I installed a Voodoo 268 cam in my original, stock 400 engine; '71 GTO. Removed one that someone put in which I felt was much too big for it. To my knowledge, the #96 heads are stock. I'm going to assume the CR is also stock at 8.2:1. Stock intake and carb. Ram Air manifolds. 3.55 gears behind a TH400 trans. The motor runs well. Gives me about 18" of vacuum for my power brakes. As for power and performance, not really sure. I really don't know how it is suppose to respond and feel. It's a nice steady pull to 5000 rpm's. I guess it's pretty good. It's a nice cam. Absolutely no regrets. Sometimes, I get caught up with comparing it to my other cars, and that's not a fair thing to do. They are different creatures all together. I'm hoping to take it to a local drag strip soon and try it out in the 1/4 mile. I know my '67 GTO ran consistent 14.2's. I don't think I can expect that from this one. We'll see.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

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