[Skip Fix]

Originally Posted by SD455DJ

To add what FJ posted, when we dyno'd 455HO's back in the days before we understood what tight quench did and the actual static compression was lower 8's (8.0 to 8.3 range) the best power was in the 42 - 44 degree range all in by 2500 rpm. But that was probably with 93 octane 100% gasoline (no ethanol). The RAIV's liked 36-38 degrees all in by 3000 rpm with a minimum of 100 octane blends of real gas.

Dennis

Interesting my RAIVs like 34.

[Formulas]

all this total timing talk assumes everyones timing marks were 100% accurate with a piston at TDC.

Most people dont check this unless they suspect a problem and just because parts are new is no guarantee of accuracy

[Drewsgto]

Thanks for the comments. My engine builder is recommending the summit 2802 or comp xe278 cams for the motor. He said the 2802 will be milder and the xe 278 will have more lope at idle but will still produce vacuum. My 7F6 heads were still factory so they need new valves and springs. Thoughts?

[Drewsgto]

Sorry I meant the comp cam xe 274h not 278.

[imposter]

im looking for something as well for a 1966 4dr bonneville, i have a 72 455 with 7m5 heads looks like a stock rebuild was done to it before i got it, im putting headers with 3" exhaust, street dominator manifold with either a 750 or 800 carb, th400 with 2.75/1.78 gear change in the transmission not sure yet about the rear end id like a solid flat tappet cam any suggestions.....

[phil400]

Xe cams don't get any love around here. You're better off with 2802 based on everything I've read.

[SD455DJ]

Originally Posted by Drewsgto

Thanks for the comments. My engine builder is recommending the summit 2802 or comp xe278 cams for the motor. He said the 2802 will be milder and the xe 278 will have more lope at idle but will still produce vacuum. My 7F6 heads were still factory so they need new valves and springs. Thoughts?

Drewsgto, You're going to need new valve springs with either cam, but my vote is for the 2802 as it's easier on the valve springs from the slower opening and closing ramps. I had an XE274 (with the default 110 LSA) in the stock '71 455HO (only 8.2 to 1 static compression ratio) and it sucked, sounded like the lifters were clacking all the time, made very little vacuum for the power brakes, no low end torque (lots of feathering the clutch from a stop), etc., etc., etc. We swapped in a Summit 2801 which ran sooo much better and it comparable to the factory 068 cam, but admittedly, a little too tame for that big 455. I think the 2802 is a good choice for a lower compression 455, but certainly no smaller....some views will vary.

Dennis

[b-man]

It’s kind of difficult if you’re wanting both good street manners and a lumpy idle.

Pick one or the other.

The 2802 is a proven performer and would be my choice.

[imposter]

im more of a lumpy but i want a solid

[imposter]

what about this one im not sure about the compression with 7M5 Heads on a 455

[glenn911]

I have the XE268 in my 461. Its very similar to the XE274 on paper. My engine idles smooth and quiet. Pulls hard and is all done at 4800. Def too small for a 455/461. If I were doing another HFT prob. just use the 2802.

[Cliff R]

"Lumpy" idle means less effficiency as you have to increase overlap to a point where you are bleeding off a good amount of cylinder pressure at idle and pulling some intake charge over into the exhaust.

Cam companies cater to that deal but for sure it does NOT make more power, improve idle quality, or engine efficiency in the "normal" driving range,. I actually LOVE the fact that folks buy into that deal because more times than not it requires advancing the timing at idle speed clear off the scale at idle speed and bringing in a lot more fuel from the carburetors idle circuit, plus additional idle bypass air to keep the throttle angle low enough to avoid "nozzle drip".

7M5 heads have HUGE combustion chambers, some of the biggest out there. On a 455 with flat top pistons you are probably going to end up around 7.5 to 7.7 to 1 compression without really tightenig things up with decking/squaring and shaving the heads down a bit. 7M5 heads are also very prone to cracking across the exhaust valve seats. I can't ever remember finding a pair where one, the other or both weren't cracked. They also have really short valves in them. Pretty much "boat anchors" or door- stops in this deal.......IMHO...........

[25stevem]

Cliff, I shake my head in vein to try a see the logic in guys like this who state that I want a real Hot Rod sound to my built/ car, but then don’t seem to mind that it can’t live up to its sound.
It’s a flat out total waste of time, money and effort to me , especially when folks like this want help from others in achieving this goal of substandard performance!

[Cliff R]

I'll put this out there as a "general rule of thumb" for a street engine build.

If you find your new build wanting, needing, liking or responding well to having to run much more than about 10-14 degrees initial timing you missed the cam choice vs the CID/compression ratio a bit.

With about 10 degrees initial timing your engine should make at least 12-13" vacuum at 700-750rpms. Take a gander at a hand held vacuum gauge and note where the "green", "yellow" and "red" zones are at if you think I'm off base here.

Of course it doesn't mean that the engine woln't run OK, nor does it maean that you can't start "crutching" it with a LOT more timing and pig-rich carburetor. That's been going on for decades and how a LOT of us make a living with custom tuning carbs and distributors. This industry caters to "bling" and just about everyone associates "menacing" idle quality with improved engine power and performance, when in many cases nothing could be further from the truth.

Attached is an overlapping dyno sheet I've put up here many times. The 455 at 9.3 to 1 compression had a "rougher" idle with the smaller XR276HR cam than it did with the custom ground HR cam I recommended to the owner to replace it. It also made MORE power at every RPM and quite a bit more upper mid-range and top end power. Even the very experienced engine "builder"/dyno operator was amazed at how much better the big 455 ran everyplace with a much larger cam in it on a wider LSA.

Just some food for thought when it comes to choosing cams for the big 455 engines..........

[25stevem]

That’s a very huge difference even if you just want to compare both motors at 4800 rpm.

The other big difference as to how both motors would perform in a car is how much faster the motor with the custom cam would rev up!

[Cliff R]

I'd rather have a 455 iron headed "street" build that idles well, and pulls hard to well past 5000rpms making nearly 90 MORE HP at the shift point. Not so mention torque production in the upper mid-range and top end is up nearly 60-100ft lbs at every point. The naysayers usually come back with "all I want is good low end torque" and not concerned with top end power. That's complete horsechit....IMHO. Why not make more power at every RPM with better idle quality and street manners? A 455 is also going to make more "low end" power than you can handle anyhow in just about any configuration. Things done to spread that power out and push some of it to higher RPM's only results in the vehicle being a LOT faster the day you decide to take it to the track and get some numbers on it.......FWIW......

[25stevem]

You touch on a point that I have tried to get across before, and it’s a very important one that you have kindly present.

If you have for example been running a well tuned 9 to 1 compression D port iron headed 400 which should be able to roast your street tires at will in first gear, provided you have at least a 3.08 rear gear., then stepping up to a well built 455 with the same heads as the 400, but pistons used to also have a 9 to 1 comp, then we find this.

This 455 even when fitted with a lowly 068 cam will be making 13% more torque but also at a 13% lower rpm then the 400.

This then opens the door to running at least a 8% bigger cam, yet still maintaining the same drivability that the 400 cid motor had.

Both motors you would find would still be making the exact same peak hp numbers since the factory iron heads are inadequate for any 400 cid motor , no less a 455 above 5400 rpm.

Also a way to go now with the 455 is to step up to a aftermarket head with a 13% greater minimum port area .
The added minimum port area ( bigger choke point) along with even just 30 more intake cfm and a good cam pick produces the numbers as you posted in your dyno sheets .

This assumes the motor is fitted with a intake manifold that can pass the added cfm we are talking about here.

[NeighborsComplaint]

I'd put in an 041 spec cam and Rhoades Lifters. Very streetable and you get the assurance of a proven flat tappet combination and a high quality lifter. As you know, Pontiac originally intended to use the 041 cam in the 455SD with low compression heads like yours but could not meet emissions standards with it. The Rhodes Lifters will clean up the idle a touch. With a stick car, it should be a solid performer with plenty of rump in the idle and no bad habits on the street by nature of the 114 LC. It's an old combo, but a proven one if sticking with a flat tappet cam.

I run a Crane 234/242 very similar to a RAIV in a 9:1 iron headed 455 with standard flat tappet lifters a 2500 stall convertor and 355 gears and it gets around town like a normal car and idles just fine so it's easy in stop and go, etc.

I'm less concerned with how fast my car is relative to other cars of its type than I am the actual driving experience. I am in the "make it sound and perform like a muscle car and cruise it" frame of mind these days and just enjoy the torque and the rumble.

[imposter]

i understand that these heads suck on this motor, i purchased this rebuilt motor as is, im not sure what cam is in it i am putting the motor in a 66 bonneville and wanted some power and found this cam listed, just wondering if it would work, im changing the transmission gears to 2.75/1.76 with the factory stock 3:08 rear gears with headers and a 2200 converter

[NeighborsComplaint]

Originally Posted by imposter

i understand that these heads suck on this motor ...

Not sure why you would say that. Compression ratio aside, they are one of the better flowing heads Pontiac made. You can mill them and the intake .060" and get the compression back up and they will out-perform the typical 6x, 4x, 5c and 96 heads.