[RAIV55]

My opinion is, the biggest difference between two similar engines is the tune up, especially timing and pump shot. Rotating weight plays a huge role. For a graphic example unbolt the torque converter from the flexplate and try.

[tempest455]

Good point! An 8" converter added to the throttle response of an old motor I had even in park.

[uneasyrider]

Originally Posted by tempest455

Good point! An 8" converter added to the throttle response of an old motor I had even in park.

Lower moment of inertia, less energy to rotate, faster acceleration..

[goquick]

Z-Code, Was that a pcture of your Shotgun? On how quick an engine rev's, I've had had both slow and quick revver's. I really don't care how it rev's when it's not making the vehicle go down the track. On the track or on the road,That's where the real test is.
For sake of topic, the rotating assembly is lighter in the 350 as compared to the 400. There is larger holes drilled into the 350 cranks counterweights because of the smaller and lighter piston.

[mzbk2l]

Thanks, goquick. That's the only Pontiac 350 I've had, so I don't have a basis of comparison to know if ALL Pontiac 350s (in general) rev faster than most Pontiac 400s (in general).

Guess I'll have to go buy more junk!

[Z Code 400]

Goquick,

That was a photo of my first 'Blue Crescent' 429. It was in a Mustang and I drove it for a while, fixed it up and eventually sold it, trading up to a very rusty Talledega in the process, which I also fixed up and sold.

I have owned a few 'Shotguns' over the years, the best was the one I installed in my 1972 Ford Torino Fastback...Robert

[Will]

Originally Posted by mzbk2l

The 400 was my engine for about six years. I tried various carbs on it, changed timing, and usually replaced the cap, rotor, plug wires, and air filter before they even got dirty. (I was very enthusiastic back then.)

None of those things ever gave that engine the "snap" that the 350 had. They were both stock-appearing Pontiacs, although I never got deeper than valve covers or oilpan off of either one so I don't know how stock they were internally.

Did you actually change the rate of timing advance, or just the initial timing setting?

What about the exhaust system? Both of those cars would have had catalytic converters originally. If they still had them it's possible the one behind the 400 could have been partially plugged.

The only difference between a 350 and a 400 is the bore size, and theoretically the 400 with it's larger bore and better breathing should actually rev quicker...

[mzbk2l]

Interesting, Will; thanks. Both cars had stock manifolds and dual exhausts. Nothing fancy, just some pipes bent at a local shop; 2-1/4", I believe.

I did not change the ignition timing curve on my 400, only the initial.

That 350 is still on a shelf in the garage; maybe one of these years I'll tear into it and see if I can identify anything special in it.

[uneasyrider]

An example:

Theoretical cfm (total for engine) = (rpm x cid)/3456

Q (flow rate in cfm) = V (velocity in ft/min) x A (area of the inlet/port/carb in sqft.)

So theoretically (not taking in factors like ram tuning, friction etc.) for a 467cid engine with a 4.25in stroke will need at 6,000rpm a flow rate of,

Q = (6,000rpm x 467ci)/3456 = 811cfm total (carb size)

Now divide that by 8 and you need 101.4cfm(ft^3/min) per cyl.

Q=VA, 101.4 = VA.

At 6,000 rpm your piston velocity for a 4.25in stroke is equal to:

piston velocity(V) = (stroke(in) x rpm)/6

V = (4.25in x 6,000rpm)/6

V = 4250ft/min

Q = VA

101.4ft^3/min = 4250ft/min x A

A = 0.0238ft^2 = 3.42in^2

So you need an intake port 3.42 square inches to feed each cyl. at 6,000rpm. That's an intake port 1.848in x 1.848in.

That is just an example of the port size you need to get the right velocity to match the velocity needed to fill the void left by the piston at 6,000rpm for the above engine in theory. That doesn't say a thing about how well the port will work at lower or higher rpm or about friction, ram tuning etc, it's just straight flow rate. Also assuming that the there is enough pressure drop and turbulence to keep the fuel suspened in the air and mixed properly. I'm sure there would be in the above example but what about lower rpm? That's what makes a quick responding engine is the proper velocity over the range. That's why they make the multi runner intakes they have today. NOW add in the fact that you have a low moment of inertia (light weight bottom end) and that will give the complete package. Light weight/low moment parts are the most improtant because that is what has to be moved by the explosion. Heavy parts/higher moment take more energy to rotate thus the mass can't accelerate as fast for the given energy input/bang.

Now take into account friction, ram tuning, cam design, port design, manifold design, etc., etc. BUT all of that is just to optimize the above.

P.S. That doesn't inclued spark timeing and flame front travel issues that will effect the above.

[BAD455TA]

Great topic MZ!!! I have been wondering the same thing myself...only because I have seen this guy online who builds some 455's that rev fast as hell! And I have no clue what he's doing different. I assumed much of it was in air flow. I figured that he was just moving air very quickly through the engine, therefore the quicker you can move air through your engine the faster it revs and the more horsepower your motor contains. So thinking this I was just thinking you would have to have some amazing head work. I don't think it's so much the cam you have, as long as it's not stock, especially in the later years. Mainly because there are guys with massive cams that rev fast and guys with mild cams that can still rev fast so I pretty much ruled that option out.

Some of you guys mentioned wires along with some tuning currently I am running some cheapo wires to get my car running until I can reach deep into my pocket to get some. I know this will make a world of a difference, but will it really make it rev faster???

I don't really have another TA to compare it to so I don't exactly know how fast it revs but I'm sure it could do a bit better with a better carb and some tuning.

Here is the video I am talking about...again great topic!

http://video.google.com/videoplay?do...83334739652163

[Will]

The reason I asked about rate of advance vs. just changing the static timing is because the advance curve can have a big effect. Quicken up that advance curve, richen up the carb, and as long as the exhaust isn't too restrictive you should notice a difference.

[mzbk2l]

Will, now that I think about it, when I bought this 1976 350, the guy had a points distributor in it. I went to the junkyard and just grabbed whatever Pontiac HEI I could find to drop in it, so I really have no idea about the ignition curve.

I'm really curious now to get it in a car again and play with some parts to see if I can tell what made it act like that.

[glhs#116]

Originally Posted by BAD455TA

Here is the video I am talking about...again great topic!

http://video.google.com/videoplay?do...83334739652163

Well, to start with that engine has no clutch or torque converter attached... Sure wouldn't mind a bit of that on my car though.

Sam