Ponitac 455 Cruise RPM

[Widetracker]

I'm considering running a 455 in my 69 Firebird convertible with a t-56 magnum. I'm trying to figure out the ideal cruising rpm for maximum fuel mileage on the interstate. I have several Pontiac 350s but am going with the 455 with the understanding that I can turn lower rpms with its brute torque. The motor will have a Cliff's quadrajet, around 9.3 compression with 6x-4 heads, and a ram air iv camshaft with Rhodes lifters based on Jim Hands engine design. I'll also have headers. I know I'm leaving a lot of mpg on the table not going LS, but I'm ready to get this convertible back on the road and exploring options with what I already have.

I drove a stock 69 Ram Air III 400 GTO with a Muncie last summer and watched the tach at 60 in 4th and it was well below 1000 rpm (around six or seven hundred I believe) and still had plenty of torque.

[wmhjr]

If you're using a Cliff Ruggles QJet and a Ram Air IV cam, but 6X heads at around 9.5 compression, you'll have a pretty interesting low end. Unless you use 1.65 rockers, the RA IV cam is the same as the RA II. I think it's around 310/320 duration with under .500 lift. Unless you do something with the heads, you won't get RAIV power. D-port heads with small exhaust valves rather than round port. Never used those lifters - I know they're supposed to reduce the effects of a bigger cam by not pumping up the lifters at lower rpms so much. Never touched one.

Personally, I wouldn't want to be cruising around at 600-700 rpm as I think you're putting a lot of stress on the bottom end of that motor as well as probably not providing enough oiling for my tastes.

I'd try to find somebody running a similar combo and see what their tq curve looks like and then go to one of the gear calc sites to calculate your rpms at different speeds.

[Evil Stu]

The Rhoads work; you will hear them tho'.

Are you sure about the RPM of the stock RA III/Muncie combo? Under 1000 is pretty low. Most see ~2500+ with the standard 3.55 gears and a 27" tire. Even with the tallest optional gearing (2.78 I think) you should still see 1900-2200 depending on tires .

Myself I would not attempt to run it down to six or seven hundred; taking advantage of OD to lower cruise RPM can go too far as wmhjr mentions. Go for 1200, it will be smooth and comfortable without taxing the motor.

The 041 cam in my 455 will break traction easily from a 25 mph roll; behaves very differently from the same cam in a 400. There are no drive-ability or issues with street manners, hwy or stop-n-go it is tame. But it isn't the cam I'd choose for a motor that will spend all its time at 1200 RPM in sustained cruise, if only because the amount of overlap will use more fuel at that RPM than is needed. Make the car perform best in the circumstances in which you will (be honest) really use it. If you really want to loaf around at 1000 rpm build as much torque there as you can, it will hurt high RPM performance but if you never actually need it you might be happier.

[wmhjr]

Yeah, I didn't mention the question of a stock bird running a Muncie being below 1000rpm at 60. Either it wasn't, or it isn't a stock bird. Or the engine is set to a low idle while the car is falling off a cliff A Muncie 4spd in a 2.73 rear end will be at 1966rpm at 60mph. 2218rpm with 3.08s, 2557 with 3.55s and 2687 with 3.73s.

With the T56, it depends on which one. The GM with the .62 6th gear, or the LT1/LS1/Viper with the .5 6th gear. For those, 60mph will be:

2.73: 1219/983rpm
3.08: 1375/1109rpm
3.55: 1585/1279rpm
3.73: 1666/1343rpm

You're really going to need to look at the entire ratios however, as you can easily put a combo together that cruises at that "perfect" rpm you're looking for, but 1st gear is way too low or way too high, and your shift points are all messed up.

[Widetracker]

Its been a year since I drove the Judge. I'll look again next time I drive it. I was getting ready to get off the highway and was curious how much torque remained in the lower rpms at speed so I probably was going less than 60 and off on my recollection. The car is bone stock, and I know the gears are least mid 3s. I'll have to look again once cruising season starts again. I just know I was impressed with how much it still had to give when it was given so little gear (4th instead of 3rd).

I've played with Richmond Gear's calculator a lot. My question however is what rpm I can run at cruise. I also have a GTO with 455, tri-power and a ram air iv cam and less compression. It will break loose at speed, but is an automatic so can I can't play with the rpm. I'm looking for feedback with guys loafing with big cubes and overdrive. Ultimately, I'm looking for fuel economy.

Right now, I'm just evaluating how low is too low in rpm for these big motors. I know it will be a compromise with power and really want to run an LS, but am exploring more frugal options for the time being. I have a WS6 m6 and love the balance of driveability and performance.

The 066 is probably a better factory cam for what I'm looking for, but I'm hoping the Rhodes might give me both worlds. I've read some discussion of big cubes loafing for mpg on the Buick forums, but haven't see a lot of good discussion within Pontiac circles of these motors with low gears and/or overdrive. The factory set ups had so little compression on the 455s, its a new twist to see how much efficiency can be gained.

As for D port heads there are a lot of schools of thought, but I read an article in which I believe Wanger was quoted as saying D vs. roundport alone resulted in little difference. The main reason Pontiac used roundport on the high rpm heads was so that hot rodders could make headers easier- you be the judge. What is certain is D ports are much cheaper. There is also some debate over the value of the 1.77 exhaust valves over the 1.66. I'd prefer 1.77, but again the more common heads and the ones I have are 1.66. The intake valve is still 2.11 with hardened valve seats and screw in studs. Ultimately I'd love to have a modern chamber, but again- cost.

Keep the feedback coming. Thanks!

[Josh69]

I haven't read through all this, but what I can tell you is that I get between 10 and 12mpg (on a good day) with a stroked 354 Pontiac to 383 cubes with a 4" stroke. This is with a vacuum secondary carb (Quick Fuel 735, so it has a spread bore, front half is from a 680 and back half from a 780) and Kaufmann aluminum heads which have an improved heart shape style combustion chamber, thus higher VE. My cam isn't real big, around 226/232 @ .050 with 1.65 rockers. I have a single plane intake. A TKO600 with .64 OD in a 69 bird with 3:73's in a 12 bolt. I am running electric fans. I run 1900rpm at 60mph with 25.5" tires. It starts to chug much below 1600, but I can take it down to 45mph or so in 5th before it really starts bucking. The motor sounds best around 2100rpm, which is 70mph...not that it's more efficient there, but it seems smoothest so maybe it is. Hindsight being 20/20, I would do an LS if I had to do it over.

I've heard of some guys getting mid to high teens with a Cliff's qjet, and I have no doubt they are more efficient than my Holley style carb, maybe one or two mpg, but i have a hard time believing much more than that. I do a lot of highway miles, and I commute with the car as much as possible, which was 25 miles each way, nearly all freeway. I put 5K miles on this combo last summer. I calculated nearly every tank of gas, since I drove it so much, mpg became a factor. The 455 GTO's got 8-10mpg when they were new, so I wouldn't count on much more than a few mpg better than that even with OD. But, if you already have the motor, you can buy a lot of gas for the cost of an LS conversion. I myself was contemplating EFI, such as FAST's EZ-EFI, but I've read just as many reports of guys not getting any better economy, as I have of guys getting significantly better economy...not sure if I can take the risk. I've decided that if I spend any more money, it'll go towards an LS in the future.

[68Formula]

Remember to factor in aerodynamics. It takes more torque to maintain 75mph that it does at 60mph.

[68Formula]

I haven't read through all this, but what I can tell you is that I get between 10 and 12mpg (on a good day) with a stroked 354 Pontiac to 383 cubes with a 4" stroke. This is with a vacuum secondary carb (Quick Fuel 735, so it has a spread bore, front half is from a 680 and back half from a 780) and Kaufmann aluminum heads which have an improved heart shape style combustion chamber, thus higher VE. My cam isn't real big, around 226/232 @ .050 with 1.65 rockers. I have a single plane intake. A TKO600 with .64 OD in a 69 bird with 3:73's in a 12 bolt. I am running electric fans. I run 1900rpm at 60mph with 25.5" tires. It starts to chug much below 1600, but I can take it down to 45mph or so in 5th before it really starts bucking. Hindsight being 20/20, I would do an LS if I had to do it over.



I've heard of some guys getting mid to high teens with a Cliff's qjet, and I have no doubt they are more efficient than my Holley style carb, maybe one or two mpg, but not 6. I do a lot of highway miles, and I commute with the car as much as possible, which was 25 miles each way, nearly all freeway. I put 5K miles on this combo last summer. I calculated nearly every tank of gas, since I drove it so much, mpg became a factor. The 455 GTO's got 8-10mpg when they were new, so I wouldn't count on much more than a few mpg better than that even with OD. But, if you already have the motor, you can buy a lot of gas for the cost of an LS conversion.

Sounds like there's some tuning to do. My low compression iron-headed 455 get's more than that with a 3310, non-overdrive automatic and 3.55 gears. Are you running vacuum advance?

[Evil Stu]

I haven't read through all this, but what I can tell you is that I get between 10 and 12mpg (on a good day) with a stroked 354 Pontiac to 383 cubes with a 4" stroke. This is with a vacuum secondary carb (Quick Fuel 735, so it has a spread bore, front half is from a 680 and back half from a 780) and Kaufmann aluminum heads which have an improved heart shape style combustion chamber, thus higher VE. My cam isn't real big, around 226/232 @ .050 with 1.65 rockers. I have a single plane intake. A TKO600 with .64 OD in a 69 bird with 3:73's in a 12 bolt. I am running electric fans. I run 1900rpm at 60mph with 25.5" tires. It starts to chug much below 1600, but I can take it down to 45mph or so in 5th before it really starts bucking. The motor sounds best around 2100rpm, which is 70mph...not that it's more efficient there, but it seems smoothest so maybe it is. Hindsight being 20/20, I would do an LS if I had to do it over.

I've heard of some guys getting mid to high teens with a Cliff's qjet, and I have no doubt they are more efficient than my Holley style carb, maybe one or two mpg, but i have a hard time believing much more than that. I do a lot of highway miles, and I commute with the car as much as possible, which was 25 miles each way, nearly all freeway. I put 5K miles on this combo last summer. I calculated nearly every tank of gas, since I drove it so much, mpg became a factor. The 455 GTO's got 8-10mpg when they were new, so I wouldn't count on much more than a few mpg better than that even with OD. But, if you already have the motor, you can buy a lot of gas for the cost of an LS conversion. I myself was contemplating EFI, such as FAST's EZ-EFI, but I've read just as many reports of guys not getting any better economy, as I have of guys getting significantly better economy...not sure if I can take the risk. I've decided that if I spend any more money, it'll go towards an LS in the future.

Sounds like there's some tuning to do. My low compression iron-headed 455 get's more than that with a 3310, non-overdrive automatic and 3.55 gears. Are you running vacuum advance?

Josh69,

I think you can do MUCH better. I see 13 mpg with a 455, thm400 and 3.55s in mixed driving, .

My combo is:

1970 GTO, 455+.034", 0 deck, Wiseco's, Eagle rods, 60919, Rhoads, E-Heads, 1.65 Scorpions, RPM, Custom Qjet by Cliff, Doug's 1 7/8", THM 400, 10" Continental "Jim Hand special", 3.55s

At 10.5:1 compression I drive 7500 miles a year on 89 octane. I've had the car since 1984, with the original iron headed 455 I got 10-12. I haven't bothered doing an OD swap, maybe this fall.

Stu

[wmhjr]

As for D port heads there are a lot of schools of thought, but I read an article in which I believe Wanger was quoted as saying D vs. roundport alone resulted in little difference. The main reason Pontiac used roundport on the high rpm heads was so that hot rodders could make headers easier- you be the judge. What is certain is D ports are much cheaper. There is also some debate over the value of the 1.77 exhaust valves over the 1.66. I'd prefer 1.77, but again the more common heads and the ones I have are 1.66. The intake valve is still 2.11 with hardened valve seats and screw in studs. Ultimately I'd love to have a modern chamber, but again- cost.

Keep the feedback coming. Thanks!

I have nothing against D-Ports. I'm running SDs first set of true 320cfm D-Port KREs. My point was that the power stats of the RAII and RAIV were using the round port heads. There is quite a bit of difference between D-ports and round ports, and rest assured the reason Pontiac used them was NOT so that hod rodders could make heads easier. In fact, they were a huge performance advantage at the time, and all other things being equal they are truly superior. Just consider the largest primary tube sizes available for D-ports vs round ports. There's a reason for that. Also remember that Wanger doesn't know crap about engines. He's a marketing guy. Not an engineer, builder, designer - a marketing guy. There are 2 reasons why I didn't use round port heads. First, because the superior heart shaped chambers of the SD KREs allows for less timing at given compressions. At least at the time (don't know if Edelbrock has actually released their new poncho heads to production) only the KRE heads had the heart shaped chambers. That equals more pump gas power safely - up to a point. The second reason is that the D-Port headers are a real PITA on a 1st gen poncho A-body. Round ports are even worse.

Like I said, not talking down about D-Ports. Just trying to paint a realistic picture. If you've got non-ported 6x heads running that RAII/IV cam at 9.5 compression the Rhodes lifters might help soften the effects of that cams normal low rpm behavior but you're still going to be having a bunch of unburned fuel.

I would not plan on any cruise rpm of less than 1400 at a minimum. With a TKO, I think it's going to be pretty uncomfortable (bucking, etc) at much lower. I absolutely would not apply any throttle under load at lower rpms. The pontiac is a long rod motor where the stroke gives it great torque. However, it also makes for some vulnerability in the bottom end and in the lifter bores. Heavy car, low rpm lugging for extended periods of time equals poor oiling and too much stress on the bottom end.

[SD455]

I think camshaft selection is the critical piece to this. I would call dave at SD and see what he would recommend for a hydraulic roller. The 744/041 with Rhodes lifters is working with old technology and may not give the best results. The 455 will definitely build good low end torque, but like some of the other posters have mentioned, running at too low of an rpm does create some other issues. I have heard of a customer of Dave's getting close to twenty on the HWY with a tko, and a hydraulic roller.

[online170]

I think below 1500rpm youre getting into oiling issues. Below 1000 for a long period of time is a bad idea, especially with nice big journals like that on your crank, youre gonna starve the rods for oil. A quick fix could be a remote oiling system that maintains pressure electrically rather than mechanically.

Heres a video of my SBC 1987 firebird with a mild 355 (250hp, maybe 300tq @ wheels) and an LT1 T56 (0.5 6th). 3.42 rear gear, and 275/40/17 tires. EVeryone said i couldnt use 6th, but i had a 4 corner idle 750DP that made it possible. A 2 corner idle wouldnt shift into 6th.

I cruised at highway speeds (100km/h) all the time without issue at like 1450rpm. With the 700R4 it was 2200rpm. I drove to montreal from ottawa once (~2hrs), the first time with the 700 itused 1+ tank of gas. The second time with the T56 it used just over 1/2 a tank.

I calculated what the mileage was with google distance travelled and the amount of fuel put in, and it came out to 33.67 USmpg. Lets say i made a mistake and the tank wasnt really empty, and you apply a fudge factor, thats still 30MPG! I couldnt beleive it, my dad was tagging along in a 2005 toyota camry, he had to fill up and i didnt on the trip.

http://s111.photobucket.com/albums/n...t=Pix11983.mp4