[taalltheway]

I don't understand the discussion…you build a 461 to save fuel? I mean with all the money you put in your car i would first think about it driving as you desire…why limit yourself with a small cam to save just a little bit of fuel?

….with the low fuel prices you have in the US i would not even think about it…even with our german fuel prices i know that fuel is the smallest part of the costs of my classic Pontiacs…i mean you will not daily drive it?

On the other hand if you wanted a torque monster a smaller cam is the way to go….but only for THAT reason!

A good compromise would be a RAIV-cam with Rhoads lifters…i have that in my GTO nice street manners but still a brutal topend.

The biggest impact will be your driving style…if you like to step on it mpg will be bad no matter how you build the engine…

With your overdrive mpg should not be bad at constant cruise no matter what cam you use...

[dmac]

For someone on a limited income, fuel mileage can be the difference between being able to drive and really enjoy your vehicle or not. I have a lot of family members between 50-100 miles away, and would love to drive my 10-11 mpg grandville to visit. But you are talking about an $80 trip versus $40 in my 20 mpg beat up old pickup. Even going from 10 mpg to 15 mpg adds up on long trips.

That $40 a couple times a month is hard to justify when it is 10%+ of my disposable income after bills are paid. For some of us, the dollars matter. At this point I personally would rather spend some money to save gas now so I can enjoy the car a lot more in the future, and if he builds it right, he'll be giving up little to be able to drive it a lot more.

I am interested in how this turns out personally. Maybe I'll be able to cruise in my convertible grandville a lot more. You will never get me to take out the 455, so I can see why he wants to stick with the 461, even if economy is important

[ronstory]

Dmac--

Well said. I want more power than the base 389 and this 400 with the cast pistons, rods and crank came back from builder as 'garbage', there is really little cost difference between building a rotating assembly for 406 vs a 461. I really was just hopping for a moderate 406 ... but those were not in the cards for engine I got off of Craigslist.

With the 461, I want enough power to justify the displacement... and still get some mileage out of it. The priority remains a balance of power and economy... with more focus on power, but not the point of getting 10-12 MPG. Been there, done that... and I'm looking for a reasonable balance.

I have a disease called Roveritis and have an old 1995 Range Rover, original body style. I pulled the 4.2L buick and dropped a 3.1L Isuzu TDI. While I will likely never recoop the cost of the conversion, my mpg when from 12mpg to 20-21mpg with mixed city/hwy driving and I got rid of all the complex engine management and fuel injection. This TDI all mechanical and was my first diesel project, so I learned a bunch... plus I drive it more since I can go 450 miles on a tank.

[dmac]

I can't do all that swapping in California on post-1975 vehicles. Otherwise I would have a 1967 pontiac 400 in my 1992 pickup, and the V-6 out of the pickup would be in the firebird, with all the engine control systems intact. I bet the firebird would get close to 30 mpg with the V6, since it gets over 20 mpg in the truck. Maybe when the rest of the truck rusts away, or falls apart, or won't pass the smog test, I'll swap the V6 anyway..

[ronstory]

You should be able to put a newer engine in an older car, provided the older car meets the emission standards for the newer car. I have a friend in CA that put a 1985 Ford Mustang 2.3L SVO turbo in a Delorean. Way better gas mileage then the Volvo V6... and better power two. He just moved over the all the emissions and the Air Qual 'referee' after digging around in the engine compartment said, "Wow... its all there... you're approved.".

[dmac]

I'll just stick with pre-1975. Still a lot of them out here. You can swap anything into a pre-1975, as long as it isn't a gross polluter with black smoke pouring out the pipe. My 1992 truck is originally an out of state vehicle from before I moved back here, so it just has to meet federal standards.I don't know what would happen if I wanted to put in a newer model engine.

My first project is the stroker original 350 two barrel back into the firebird, taking the 400 out, which for me is two years. Then converting it to two barrel throttle body injection, for another couple years. Then I'll have the 400, and I have ideas about putting it into an older one ton truck. The 400 has more torque and HP than my 8.1 liter 2006 silverado and I won't have to worry about the check engine light coming on because of a fuel cap, or malfunctioning temp sensor.

[geeteeohguy]

The combo in my '67 GTO convertible is not a ball of fire, but it's fun to drive and will still burn posi rubber. It's the born with 400, bored .030 (406) with 1970 #15 small valve 455 heads (85cc now) with the original Q-jet, original points dizzy, and an 068 cam. Stock TH400. I swapped out the 3.36 peg leg rear (was getting around 15.5mpg) for a freebie 2.56 posi, re-geared the speedo, and my MPG went up to over 20. I regularly get 20.5-21 mpg on long trips, at speeds of 75mph plus. I have no idea what I'd get if I went only 65mph or so....everybody out here drives 80-ish. My enjoyment of the car went way up, as it's now as economical to drive as my '94 gutless Toyota truck with a v6. With moderate compression, a mild cam, and tall gearing, these cars can surprise you. The guy who gave me the rear end said he used to get 22 mpg in his '67 GTO hardtop with it with a stock engine (670 heads) at 65mph or so. With the torque of a 461, a lazy gear or OD trans to keep the RPM's down, coupled with a decent advance curve and a miserly Q-jet, good numbers are possible.

[b-man]

I've been watching this thread with interest, I'm wanting the same things from my street-only Pontiac 421 HO engine (bored +.030" for 426.6 cubes) - Reasonable power with some decent fuel economy to go along with it.

I have always admired how well the GM LS-series engine can run with their short-duration high-lift cams with very wide lobe separation. Fantastic performance with very admirable fuel economy.

They do of course have a very well scienced-out modern combustion chamber design and fuel injection setup on their side that our traditional Pontiac V8s don't have. These engines also have high flowing intake ports, about 260 CFM for the LS1 and 325 CFM for the LS3. Most of the common Pontiac D-port heads flow around 200-210 on the intake.

However I just want to take a look at the factory camshafts used in some of the more common performance engines.

1998-2000 LS1 310/320 HP 5.7 liter (346 cubic inches)
Duration @ .050 - 198.86/209.25
Lift - .498"/.497"
LSA - 119.45

2001-2002 LS1 315/325 HP 5.7 liter (346 cubic inches)
Duration @ .050 - 196.37/208.72
Lift - .464/.479"
LSA - 115.92


2008 LS3 430 HP 6.2 liter (376 cubic inches)
Duration @.050 - 204/211
Lift - .551"/.525"
LSA - 117

What most would consider to be tiny cams as far as duration numbers are concerned make excellent power. The LS1 engines are actually a little under rated, making right at 1 HP per cube. Granted these are all smaller-cube engines compared with most of the big-inch Pontiac engines many of us build, most of us consider a 389 or 400 to be the smallest displacements worth considering when shelling out the dough for an engine build.

Perhaps a custom grind with modest duration and higher valve lift would be the ticket here. Maybe some moderate intake port work to enhance the flow but keeping the velocity high, something in the 230 to 240 CFM range. No Rhoads lifters.

That 461 would surely respond to the better airflow from the modified intake ports and high valve lift, but keeping the cam duration on the shorter side may help reap the fuel economy reward. Maybe even swapping to a set of small-valve heads (1.96"/1.66") would be wise to keep velocity high at lower engine speeds (in overdrive at 1800 to 2200 RPM) with little to no loss in actual port flow.

Off the shelf cam? Perhaps the Summit 2801 with Harland Sharp 1.65:1 (1.72:1 actual ratio ) roller rockers would be the ticket, you'd have .509" intake lift and .534" on the exhaust side. Enough cam to reap 375 HP out of that 461 I would guess, and keep the low-RPM manners for overdrive cruising intact.

A friend had a '66 GTO with a 440-cube roller-cammed engine with 72cc E-heads and a beautiful home-built fuel injection setup exhausting through H-O Racing Tri-Y headers. Was also running a 200-4R overdrive with a 3.73 gear (not exactly sure of the rear gear). The cam had about 250 degress of duration @ .050. and it didn't get good mileage on the highway, about 9 MPG from what I remember him telling me. Big cam = poor fuel economy I suppose, no matter how slow you try to make the engine turn at highway speeds.

For myself, I decided to use the factory camshaft profile that the Pontiac engineers spec'd for my '64 421 HO. I went to the 9779068 cam which is the same exact profile as the original 9770543 421 HO cam. The 421 HO 9770716 72cc heads have small 1.92"/1.66" valves, compression will be close to 9.3:1 due to the use of forged 23cc dished pistons. I figure this 421 should make 350 HP through the factory log exhaust manifolds. Will be running the 200-4R overdrive with a 3.36 rear gear, hoping for 20 MPG on the highway at close to legal speeds in my high-optioned '64 LeMans convertible.

Best of luck with the 461.

Bart

[paul s.]

Originally Posted by b-man

I've been watching this thread with interest, I'm wanting the same things from my street-only Pontiac 421 HO engine (bored +.030" for 426.6 cubes) - Reasonable power with some decent fuel economy to go along with it.

I have always admired how well the GM LS-series engine can run with their short-duration high-lift cams with very wide lobe separation. Fantastic performance with very admirable fuel economy.

They do of course have a very well scienced-out modern combustion chamber design and fuel injection setup on their side that our traditional Pontiac V8s don't have. These engines also have high flowing intake ports, about 260 CFM for the LS1 and 325 CFM for the LS3. Most of the common Pontiac D-port heads flow around 200-210 on the intake.

However I just want to take a look at the factory camshafts used in some of the more common performance engines.

1998-2000 LS1 310/320 HP 5.7 liter (346 cubic inches)
Duration @ .050 - 198.86/209.25
Lift - .498"/.497"
LSA - 119.45

2001-2002 LS1 315/325 HP 5.7 liter (346 cubic inches)
Duration @ .050 - 196.37/208.72
Lift - .464/.479"
LSA - 115.92


2008 LS3 430 HP 6.2 liter (376 cubic inches)
Duration @.050 - 204/211
Lift - .551"/.525"
LSA - 117

What most would consider to be tiny cams as far as duration numbers are concerned make excellent power. The LS1 engines are actually a little under rated, making right at 1 HP per cube. Granted these are all smaller-cube engines compared with most of the big-inch Pontiac engines many of us build, most of us consider a 389 or 400 to be the smallest displacements worth considering when shelling out the dough for an engine build.

Perhaps a custom grind with modest duration and higher valve lift would be the ticket here. Maybe some moderate intake port work to enhance the flow but keeping the velocity high, something in the 230 to 240 CFM range. No Rhoads lifters.

That 461 would surely respond to the better airflow from the modified intake ports and high valve lift, but keeping the cam duration on the shorter side may help reap the fuel economy reward. Maybe even swapping to a set of small-valve heads (1.96"/1.66") would be wise to keep velocity high at lower engine speeds (in overdrive at 1800 to 2200 RPM) with little to no loss in actual port flow.

Off the shelf cam? Perhaps the Summit 2801 with Harland Sharp 1.65:1 (1.72:1 actual ratio ) roller rockers would be the ticket, you'd have .509" intake lift and .534" on the exhaust side. Enough cam to reap 375 HP out of that 461 I would guess, and keep the low-RPM manners for overdrive cruising intact.

A friend had a '66 GTO with a 440-cube roller-cammed engine with 72cc E-heads and a beautiful home-built fuel injection setup exhausting through H-O Racing Tri-Y headers. Was also running a 200-4R overdrive with a 3.73 gear (not exactly sure of the rear gear). The cam had about 250 degress of duration @ .050. and it didn't get good mileage on the highway, about 9 MPG from what I remember him telling me. Big cam = poor fuel economy I suppose, no matter how slow you try to make the engine turn at highway speeds.

For myself, I decided to use the factory camshaft profile that the Pontiac engineers spec'd for my '64 421 HO. I went to the 9779068 cam which is the same exact profile as the original 9770543 421 HO cam. The 421 HO 9770716 72cc heads have small 1.92"/1.66" valves, compression will be close to 9.3:1 due to the use of forged 23cc dished pistons. I figure this 421 should make 350 HP through the factory log exhaust manifolds. Will be running the 200-4R overdrive with a 3.36 rear gear, hoping for 20 MPG on the highway at close to legal speeds in my high-optioned '64 LeMans convertible.

Best of luck with the 461.

Bart

Very good post, Bart. I will differ somewhat about cam selection though. Since it is cylinder pressure that makes the crank turn, the cam should be aggressive. In a low rpm engine, it won't hurt a bit... it can only help.

I believe what the engineers did with the LS engines should be emulated. Agreed, fuel injection and the combustion chambers help, but there is something we can do.

It is well known small cams can work with good flowing heads. GM does it... it works. The ENTIRE vehicle combination must be taken into account, not just cylinder head flow. X2 on the Rhoads lifters. Ram Air IV cam and rhoads lifters in a vehicle using a 3.23 gear just doesn't make sense (rant LOL).

To the O.P.: I don't quite get the large displacement choked down to deliver fuel efficiency... not bashing AT ALL... just believe a 350 built with efficiency in mind; small chamber e-heads, short roller with a LOT of lift, and of course, overdrive. Either a small carb or fi will do quite well. In fact, I was thinking of offering something like this! Large displacement engines have shown enormous pumping losses according to the engineers... of course, I have no way of checking that specifically... so I would trust them LOL.

[dmac]

Originally Posted by paul s.

Very good post, Bart. I will differ somewhat about cam selection though. Since it is cylinder pressure that makes the crank turn, the cam should be aggressive. In a low rpm engine, it won't hurt a bit... it can only help.

I believe what the engineers did with the LS engines should be emulated. Agreed, fuel injection and the combustion chambers help, but there is something we can do.

It is well known small cams can work with good flowing heads. GM does it... it works. The ENTIRE vehicle combination must be taken into account, not just cylinder head flow. X2 on the Rhoads lifters. Ram Air IV cam and rhoads lifters in a vehicle using a 3.23 gear just doesn't make sense (rant LOL).

To the O.P.: I don't quite get the large displacement choked down to deliver fuel efficiency... not bashing AT ALL... just believe a 350 built with efficiency in mind; small chamber e-heads, short roller with a LOT of lift, and of course, overdrive. Either a small carb or fi will do quite well. In fact, I was thinking of offering something like this! Large displacement engines have shown enormous pumping losses according to the engineers... of course, I have no way of checking that specifically... so I would trust them LOL.

Curious if you could guess at what it would cost you to build and sell a pontiac 350 built for mileage with efi, thinking 2 barrel throttle body. Figure it with not needing to exceed about 5000 rpm, to save on costs. I think there is a market for those of us who want to get out and drive, but can't pay for gas at 6-9 mpg.

[ronstory]

OK, lots of great input everyone, THANK-YOU!

I'm now waffling between the a 068 style cam and a 068 "plus", with a bit more duration like the 2802. I been doing searches on the summit cam variants and 455, but just did a search on the 068 and 455 and hit one of Jim Hand's old articles.

http://www.bafb.org/1968firebird.org...harticles5.htm

Towards the bottom he recommended a 068 but cautioned that using Rhoads lifter may build too much cylinder pressure and cause detonation. Now Jim's engines are typically higher compression than my combo, but I don't know the compression were I would run into problems.

Time to do more searches and look for wider LSA cams.

[77 TRASHCAN]

The end compression ratio can be a big determining factor in your final cam selection. If you get over 9.5-9.75 you probably need the 60243, to keep octane sensitivity down.

Is you block zero decked??? Zero decking helps in combustion effiency AND MPG wise.

I used the calculator on SD performance's sight. W/ pistons .020 down, ratio is 8.85, and at zero deck 9.15:1 (I guessed at valve relief volumes @ 6 cc's).

Either of these ratios I would think to be good for your combo (I'm not the expert, mind you!!! others???), to use the 068 cam. That cam is such a nice grind for a lot of combo's. Stock valve train parts are only needed. Ported or unported heads will generally see improved performance, depending on combo.

I'd just run stock exhaust manifolds. The RPM's you'll be running the engine at won't be pushing the need for any extreme exhaust flow, and it'll still make great power when needed!!!

Keep in mind that picking the proper cam for the end combo, will make everything work out, IF properly setup. What ever end MPG you get will be favorable, and fun to drive,(if you stay on this current path!!!

[ronstory]

Yes, the block will be zero deck with likely about 0.005 in the hole. I like a bit of margin and the calculations are below for a roughly 9.1 CR assuming the head chambers end up at 102cc.

Engine 461+0.040

Bore (in) 4.16
Stroke (in) 4.25
Swept Volume (cu in) 57.76522752
Engine Displacement 462.1218202
Deck Height (in) 0.005
Deck Volume (cu in) 0.067959091
Gasket Bore (in) 4.3
Gasket Thickness (in) 0.039
Gasket Volume (cu in) 0.566359794
Piston relief (dish +/dome -) (cc) 4.3
Chamber Volume (cc) 102
Total Volume 64.88636572
Compression Ratio static 9.11179701

I could go Cometic gaskets at 0.027 or 0.030" but my builder pointed out it would be about $170 extra over a standard gasket set... and that I would never be able to tell the difference.

[77 TRASHCAN]

Stock Fel Pro's are .039, crushed. With your calculation, .027+.005= .032. That number could bite you.

If you zero deck it,and even end up with the pistons .001-.002 out of hole, you'd, still be good. .035 seems to the the reasonable quench limit. Never heard of anyone having a problem w/ that quench #. Stock head gaskets are much cheaper...they have a bigger bore, but that's not a worry here, my opinion...

[KEN CROCIE]

Carefull on you calculations. A Crane is measured @ .004" lift. A Comp Is measured at.006" lift. This small difference will change the adv.dur. 8 DEG. On Iskys, I have had to measure @.009" to meet thier adv. claims, which if measured like a Crane, The Isky 272 would measure at 290-292 duration.
My choice for your car is the Crane HMV 278. This cam in your car will make more torque from 1800 rpm up and idle with a slight lope. A recently dyno'd 428+030 with similar specs made 385HP and 445 torque. Yours will make a little more,maybe 390-410 hp with 460-470 tq.

[ronstory]

Yes, I tried to point out that there was likely some specmanship in the numbers since they are factory propaganda derived. That said, I'm just using the advertised duration as a general comparison for lobe ramp speed. IMO, I trust the duration@0.050 for a fairer apples to apples comparison.

As I slowly go insane trying to pick the "right" cam... I started to wonder if there is a relationship between dynamic compression ratio and efficiency. So I rewrote the spreadsheet to have add DCR based on the cam specs, stroke, rod length and calculated SCR. The results are below for listed from highest DCR down and I stopped when I got to the trusty Pontiac 068 cam. With the highest being only ~7.5 DRC, that should be in the ballpark for pump gas.

Would it be reasonable to think that a higher DCR would be better for MPG provided that I stayed off the secondaries? Since I want performance without gas gluttony, should I look for the one of the higher DCR cams with the largest duration at 0.050"?

Brand Model AID AED ID050 ED050 IdegTo50 XdegTo50 IL1.5 EL1.5 IL1.65 EL1.65 LSA DST SCR DCR
Ultradyne - 254 262 199 207 27.5 27.5 0.4 0.43 0.44 0.473 112 3.468 9.10 7.43
Crane H-260-2 260 272 204 216 28 28 0.43 0.45 0.473 0.495 112 3.387 9.10 7.25
Crower 60239 260 266 203 213 28.5 26.5 0.42 0.42 0.462 0.462 112 3.387 9.10 7.25
Summit K2800 262 272 204 214 29 29 0.42 0.44 0.462 0.484 112 3.359 9.10 7.19
Ultradyne - 262 272 207 217 27.5 27.5 0.43 0.45 0.473 0.495 112 3.359 9.10 7.19
Comp. Cams XE268H 268 280 224 230 22 25 0.48 0.48 0.528 0.528 110 3.330 9.10 7.13
Lunati 6702 270 280 204 214 33 33 0.42 0.44 0.462 0.484 110 3.301 9.10 7.07
Lunati 30706 270 280 220 230 25 25 0.47 0.49 0.517 0.539 110 3.301 9.10 7.07
Ultradyne 60550 270 288 214 224 28 32 0.44 0.45 0.484 0.495 110 3.301 9.10 7.07
Comp. Cams XE274H 274 286 230 236 22 25 0.49 0.49 0.539 0.539 110 3.243 9.10 6.94
Crower 60240 270 276 210 221 30 27.5 0.42 0.45 0.462 0.495 112 3.243 9.10 6.94
Crower 60210 278 288 229 239 24.5 24.5 0.48 0.5 0.528 0.55 108 3.243 9.10 6.94
Pontiac 9779066 273 282 200 210 36.5 36 0.41 0.41 0.451 0.451 111 3.228 9.10 6.91
Comp. Cams 276AH-10 276 284 228 236 24 24 0.47 0.47 0.517 0.517 110 3.213 9.10 6.88
Crane H-272-2 272 284 216 228 28 28 0.45 0.48 0.495 0.528 112 3.213 9.10 6.88
Crower 60915 272 279 211 220 30.5 29.5 0.42 0.44 0.462 0.484 112 3.213 9.10 6.88
Summit K2801 272 282 214 224 29 29 0.44 0.47 0.484 0.517 112 3.213 9.10 6.88
Pontiac 10003402 273 289 200 213 36.5 38 0.41 0.41 0.451 0.451 113 3.167 9.10 6.78
Crower 60241 276 281 215 221 30.5 30 0.46 0.46 0.506 0.506 112 3.152 9.10 6.75
Ultradyne - 276 286 221 230 27.5 28 0.45 0.45 0.495 0.495 112 3.152 9.10 6.75
Ultradyne - 280 288 223 231 28.5 28.5 0.46 0.49 0.506 0.539 110 3.152 9.10 6.75
Pontiac 9779067 273 289 200 213 36.5 38 0.41 0.41 0.451 0.451 114 3.136 9.10 6.71
Crower 60916 278 289 221 229 28.5 30 0.46 0.47 0.506 0.517 112 3.120 9.10 6.68
Comp. Cams XE284H 284 296 240 246 22 25 0.51 0.51 0.561 0.561 110 3.089 9.10 6.61
Crower 60242 280 286 221 229 29.5 28.5 0.46 0.47 0.506 0.517 112 3.089 9.10 6.61
Lunati 30702 285 300 228 235 28.5 32.5 0.47 0.47 0.517 0.517 110 3.073 9.10 6.58
Crane H-278-2 278 290 222 234 28 28 0.47 0.49 0.517 0.539 114 3.057 9.10 6.55
Crower 60243 284 290 228 235 28 27.5 0.48 0.49 0.528 0.539 112 3.025 9.10 6.48
Ultradyne - 288 296 231 239 28.5 28.5 0.49 0.51 0.539 0.561 110 3.025 9.10 6.48
Lunati 30707 289 310 238 242 25.5 34 0.48 0.49 0.528 0.539 110 3.009 9.10 6.44
Summit K2802 282 292 224 234 29 29 0.47 0.49 0.517 0.539 114 2.992 9.10 6.41
Crower 60918 288 298 214 224 37 37 0.44 0.47 0.484 0.517 112 2.959 9.10 6.34
Crower 60211 296 308 236 242 30 33 0.51 0.52 0.561 0.572 108 2.959 9.10 6.34
Comp. Cams XE294H 294 306 250 256 22 25 0.52 0.52 0.572 0.572 110 2.926 9.10 6.26
Nunzi 2041 288 302 222 232 33 35 0.45 0.46 0.495 0.506 113 2.926 9.10 6.26
Crane H-288-2 288 296 226 234 31 31 0.46 0.47 0.506 0.517 114 2.892 9.10 6.19
Ultradyne - 296 301 239 247 28.5 27 0.51 0.53 0.561 0.583 110 2.892 9.10 6.19
Pontiac 9779068 288 302 212 225 38 38.5 0.41 0.41 0.451 0.451 116 2.824 9.10 6.05

[Blued and Painted]

Mr. Ken, the crane cam below has a 114 LSA. Where would you want the intake center line on ronstory's combo.

[KEN CROCIE]

B&P: if you mean the hmv278, it is ground 5 DEG. advanced, which is where we usually run them. GOTO Crane website and look up 283801 for timing card.I hope Ronstory took int. closeing in calculating dynamic C/R.

[rohrt]

Seems like the 068 is proven, however I wonder why more people don't recommend the 066 or 067 since the OP wants to cruise at 1800rpm. Their shorter duration brings the peak torq down quite a bit. I would still vote for the 068. Seems like a nice compromise between performance and economy.

I couldn't find any more data on Jim H using the 067 cam.

http://www.dapa.org/building-a-stron...iac-camshafts/

http://forums.maxperformanceinc.com/...hlight=068+455

[dmac]

Ron- you seem to want a very wide power band. You can start by eliminating those cams with the narrowest LSA. I found that it is easier to eliminate as many as possible from the list. Then maybe eliminate those with super fast ramps to reduce the chance of cam/lifter problems by a small degree.

If you decide to use 1.65 rockers, you can eliminate the highest lift cams, or vice versa if you decide on 1.5s. Several of those cams look like they would need taller/stiffer springs if you use 1.65 rockers, and the stiffer springs don't help economy. And if you pick a cam that is slightly too big duration wise and doesn't give the mpg you think is possible, you can always add rhoads lifter later to 'shrink' it

My thinking is that dynamic CR is 'theoretical' in the sense that it assumes certain things about the motor breathing capabilities, unless the calculator knows your head flow at different lift, carb flow, air cleaner, exhaust.