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Old 02-02-2005, 11:17 AM
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what's up with pontiac engines (or BOPs)

just wondering why a very mild almost nothing of a cam for a sbc, will rock a pontiac engine. for example the summit 2800 cam has only 204/214 at .050. this is almost stock for a sbc, but it is spozed to be fairly good for a pontiac. same for a 214/224 cam like my xe262. it's a nice streetable cam in my camaro, but those specs are spozed to make a pontiac 400 really hairy.

is the pontiac engine design just taking better advantage of things somehow? what things? i have always read how crappy the pontiac heads are. my chevy has about the best stock heads out there, 487 castings, what am i missing here? why do sbc's have to have more of a cam to hop them up compared to pont. and as far as i know, are all bop's about the same?

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Old 02-02-2005, 03:09 PM
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I dont know much on the cam part of your question, but as far as I know the pontiac heads are some of the best factory heads made, ram air IV (or so I have been told). I have a 69 gto with a bone stock 400, not judge , and she still spins the tires, I would love to do a cam swap and head work someday
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Old 02-02-2005, 04:02 PM
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Different factors

You have to remember cam choice is not really a function of engine displacement as it is compression ratio.

Pontiac engines, being that the vast majority of them were made after '71, are severely detuned smog motors and later in the '70's had an abysmal compression somewhere around 8:1 or 7.5:1. So, cam choice has to be conservative in contrast. The heads flow similar to the average Chevy iron head.

I would say the average mildly built chevy has around 9.5:1 compression, so adjust the duration figures accordingly. Compression ratio and cam are fundementally related, as cylinder pressure is what actual makes power.

Andy
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Old 02-02-2005, 04:35 PM
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An Interesting Pontiac Site

http://www.pontiacpower.com/
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Old 02-03-2005, 01:21 PM
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well, they're not shy, that's for sure.
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Old 02-03-2005, 02:02 PM
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Bore vs Stroke

Good question.
Pontiacs have a longer stroke. On a 350 the stroke is 3.75 vs 3.48 for the chevy. Wilder cam would be multiplied by the difference in stroke. eg, filling the cylinder more.
Pontiacs are notorious for good torque but not as high revving compared to chevy.
6200-6500 RPM on a street Pontiac is screaming.
That's my theory on your question.
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Old 03-16-2006, 06:01 PM
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if anyone is still reading, info is info

Short and sweet . You will blow that succor. At 3.75 stroke verse rod length is the magic. The connecting rod bolts break. Even if I doesn't happen right then, there is something called crack propagation. once it starts it's just a matter of when. The 3.48 is long for a sbc to some extent. Its not the stroke or rod length that is singly importent. It's a ratio called rod to stroke. R/S -Your engine can't breathe at 2.00 r/s, at 1.75 it's slow or blow. I will always take sbc at 327.The last one I read, this is a peak at the reason. It's no opion 428 ho. Putting together engines is more complicated than this, but you got that right. Let's go racing. mark
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Old 03-16-2006, 06:23 PM
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Quote:
Originally Posted by fearsomefairmont
You have to remember cam choice is not really a function of engine displacement as it is compression ratio.

Pontiac engines, being that the vast majority of them were made after '71, are severely detuned smog motors and later in the '70's had an abysmal compression somewhere around 8:1 or 7.5:1. So, cam choice has to be conservative in contrast. The heads flow similar to the average Chevy iron head.

I would say the average mildly built chevy has around 9.5:1 compression, so adjust the duration figures accordingly. Compression ratio and cam are fundementally related, as cylinder pressure is what actual makes power.

Andy

Even though this is an old post, I think its pretty much completely false. All V8 engines were underpowered and had low compression in the late 70's not just the pontiac motors. Most chevy V8's from 305 to 454 had a compression ratio of 8:1 and in some rare cases, even less.

First thing that's misleading about the above cam specs is that they really tell you very little about a cam. Just because a chevy cam and a pontiac cam have 214/224 duration @.050 really tells you very little. This tells you nothing about the ramp speed or overlap. That said, however, they probably are relatively similar cams, and the two motors will respond differently. Here's why:

A cam will make its most power at a certain RPM because of the timing of the intake pulses coming into the chamber. A bigger cam runs choppy at a lower idle because the intake pulses are not moving fast enough during the overlap part of the cam timing (when both the intake and exhaust valve are open) for the rushing exhaust flow to actually evacuate itself and create a low pressure area behind the exhaust valve to actually suck more itake charge in. Because the velocity is too slow, the engine is reburning a charge that already has exhaust gasses in it, and thus gives you the choppy idle of the "big cam."

THe reason a chevy motor and a pontiac will respond to a similar cam differently is because each motor's intake pulses are different at different RPM's. This has to do with the bore to stroke ratio and the head design, but also the length of the total runner. Each pulse constitutes a "runner's full" of intake charge. Given that the distance from the carb to the actual combustion chamber is different in either motor (most notably because a pontiac has a much wider intake manifold) the pulses are actually different amounts of air and fuel and are gotten at different RPM levels.

K
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Old 03-19-2006, 03:25 PM
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tru pontiac

Amen Andy, I don't reallyconsider anything after74 a pontiac. Last pontiac v8 rolled off the line november 78. Even the unbeliaveable 301 turbo. In the big noise made back them some consumers raised cain about getting other engines inPONTIACS. ETC GM took the point of view that all were GM engines and used where ever needed. I was working on something a while back, but the manual listed several pollution systems, and said there could be other options depending on parts on hand. What I was working on wasn't like anything in the book. So much for real iron. mark
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Old 03-19-2006, 03:38 PM
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tru iron

I am goind to add another little comment. A engine is indivual everthing counts, Personally, because I like low end torque I want the rod to stroke ratio first. but it is only the beginning. Its all down hill after 1968. WANT to learn about air pulse, look ar the mopar crossram. mark
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Old 03-19-2006, 04:07 PM
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Quote:
Originally Posted by madamstn
I am goind to add another little comment. A engine is indivual everthing counts, Personally, because I like low end torque I want the rod to stroke ratio first. but it is only the beginning. Its all down hill after 1968. WANT to learn about air pulse, look ar the mopar crossram. mark

Yep, the 413 crossram intake ran dual quads on it and put the engine up over 100% volumetric efficiency. Mopar tuned the pulses so that the best VE (and thus torque) came at what they determined to be the car's cruising RPM at highway spees (which was in the mid 2000's of RPM range). The tradeoff, however, was that the manifold was twitchy to tune and had flat spots at other RPM ranges. Also because of the large runner length, pressure drops (dropping the throttle from idle to WOT) were laggy to respond to.

Rod to stroke ratio has very little to do with an engine's torque rating. Its really only an issue to think about in high RPM motors.

K
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Old 03-20-2006, 06:26 AM
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cross ram

I was curious about who was, old enough, or savy enough to know which set up I was talking about, You are correct of course, break the connecting rod bolts at high rpm's. I will still take BIG PONTIACS. The only big upperend that comes to mind is Ford oversquare. m
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Old 03-20-2006, 06:30 AM
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curious

K How did you happen to know, they didn't exactly grow on trees, very very rare. mark
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Old 03-22-2006, 03:42 AM
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air on the back side

I know this is way out in left field, but Iwas asked a question recently I was't postive about the anwer. If you understand air pulse this will make sense. Anybody know about scavageing on a gas engine, obviously a turbo is related but not the same thing. Headers are a passive example, But I could not think of a application. It might be significant on the big long stroke engines. I haven't tried the math on anything in a long time, K, Andy have you ever crossed paths with any thing along that line. I am not sure , but some of the interesting stuff was more talk than truth, the RAIV wasn't that good, or at least some of those major diversion heads, and Pontiac's venture into OHC v8's. Worth more in historical terms than on the street. thx mark
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Old 09-11-2009, 09:45 PM
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pontiac was the only one out of all the gm companys that always had fully machined combustion chambers... my stock 6xs have 2.11 and 1.77 valves. Now isnt that bigger than any stock sbc? besides maybe an LT or LS engine(i dunno about those)

w/ a little porting and screw in studs, these heads have awesome power potential...

these heads are on almost every pontiac engine i've ever seen that's 76 and up ...at least till 79
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