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Author Topic: Diesel power  (Read 2792 times)

Jjemc

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Diesel power
« on: May 10, 2018, 06:08:24 PM »
In general, is a 400hp Cummings diesel better or more effective than a 350hp Cummings diesel? These will be in a 40ft unit.


Back2PA

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Re: Diesel power
« Reply #1 on: May 10, 2018, 06:21:18 PM »
In general, is a 400hp Cummings diesel better or more effective than a 350hp Cummings diesel? These will be in a 40ft unit.
I've got a Cat not a Cummins, but it's not really a Cummins question. When I was towing a Cherokee my 38 footer did fine with 350hp. Now, with a very heavy toad I'd say I'm a little under-powered. So yes, 50 more hp will be "more effective" at climbing hills but you'll burn more fuel - there's no free lunch. The extra HP won't really make any difference when you're at speed on flat terrain since you're not using it all anyway.
Scott
Fulltiming in a 2005 Newmar Dutch Star 3810, Cat 350
Eezrv TPMS, 970W Solar, Tri-Metric Battery monitor
2002 Dodge RAM 1500 Quad Cab toad
Stowmaster towbar & Brakemaster toad braking system

Jjemc

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Re: Diesel power
« Reply #2 on: May 10, 2018, 06:48:29 PM »
Your reply is about what I thought. I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline. Mind is still in a fog.

Utclmjmpr

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Re: Diesel power
« Reply #3 on: May 10, 2018, 07:38:34 PM »
 That is what a 6 speed transmission is for.>>>Dan
Vary rare American Tradition 38TT/330 turbo Cummins
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Back2PA

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Re: Diesel power
« Reply #4 on: May 10, 2018, 08:30:10 PM »
I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline.
It's really about expectations. If you never want to be in the slow lane climbing a hill in a 40 footer (probably towing) you're going to need more like 450hp, maybe more.
Mine was fine until I decided to drag a 6500# truck, obviously that makes a difference (I need the truck). If all I did was hill climbing I guess I'd be more bothered, but the reality is that I might have a slow climb or two in a couple thousand mile trip. So I get to the top five minutes later, who cares? Back when I had an underpowered 460 Ford 40' gas rig, climbing long hills wide open meant worrying about overheating, red hot manifolds, etc. (plus listening to the noise). Cat & Cummins are perfectly happy wide open for an hour while you enjoy the scenery. And the noise is all "way back there..."
Scott
Fulltiming in a 2005 Newmar Dutch Star 3810, Cat 350
Eezrv TPMS, 970W Solar, Tri-Metric Battery monitor
2002 Dodge RAM 1500 Quad Cab toad
Stowmaster towbar & Brakemaster toad braking system

Gary RV_Wizard

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Re: Diesel power
« Reply #5 on: May 10, 2018, 08:42:55 PM »
Is 400 hp more than 350?  Of course!  The Cummins ISL engine has more hp, more torque and more liters of displacement than the ISC 350, plus it produces the hp at a lower rpm.

But what you really want to compare is horsepower to weight ratio. For a given weight coach, more horsepower will always accelerate it more quickly.  In general, RV makers shoot for around 1 hp per 100 lbs of coach weight.  That gives acceptable if not blazing performance (your car probably has more than 1 hp per 20 lbs).
Gary
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Gary Brinck
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Home: Ocala National Forest, FL

RedandSilver

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Re: Diesel power
« Reply #6 on: May 11, 2018, 09:33:18 AM »
I drove a 40ft unit with a 350hp and it felt like  it was a struggle to get up a slight incline. Mind is still in a fog.

I have a 40ft with a Cummins 350 and mine doesn't struggle to get up a slight incline at all even towing my trailer.
But on the highway I usually drive 55-58mph and going up big hills I might slow to around 50mph or so.

Sure the 400hp will carry more.  However like said no free lunch.
It will cost more and probably get a little lower mpg and use bigger oil and fuel filters etc.
But if you feel the need to drive at 70+ then it would be the better choice for you IMO.
2002 Rexhall Rose Air  Cummins 8.3  350hp
West MI Summer   Central FL Winter

Gary RV_Wizard

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Re: Diesel power
« Reply #7 on: May 11, 2018, 07:54:07 PM »
You can't generalize by coach length - gotta talk weight.
Gary
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rls7201

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Re: Diesel power
« Reply #8 on: May 11, 2018, 10:43:15 PM »
8.3l ISC is no longer in production. Cummins has a 6.7l ISBX and a ISL 9l for most applications.
The 9l is a redesigned 8.9 that is delivered in different HP ranges. So a 350 HP engine is the same block as the 400 HP engine.

Gary RV_Wizard

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Re: Diesel power
« Reply #9 on: May 12, 2018, 06:57:12 AM »
rls7201 is right about the ISC being discontinued, but in a used coach the odds are strong that a 350 hp diesel is an ISC Cummins.   If talking about a newer coach, the ISBX is usually employed if the hp need is under 360 and it comes in 300, 340 and 360 hp versions (in recent years).    The ISL is available (for motorhomes) in 330, 350, 380, 400 and 450 hp versions, but in practice only the 380, 400 and 450 hp versions are offered by the major RV chassis builders.
Gary
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Gary Brinck
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Oscar Mike

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Re: Diesel power
« Reply #10 on: May 12, 2018, 07:26:48 AM »
My Dynamax comes with the Cummins® 8.9L ISL Turbo Diesel Engine (350hp/1,000 lb.-ft. of Torque), with a Allison® 3200 TRV Six-Speed Automatic Transmission. I've not weighed the coach yet, the GVWR is 33,000#.

I cannot comment on its performance yet either, as our maiden voyage is coming up on the 17th, which includes towing my Jeep wrangler Unlimited and so it will be my first opportunity to actually experience what the coach will do. 

Gary RV_Wizard

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Re: Diesel power
« Reply #11 on: May 12, 2018, 07:47:01 AM »
John,
Thanks for that info, John. I should have been more specific!   Since the big "super C" motorhomes are built on a medium or heavy truck chassis rather than a Class A motorhome type, they employ different spec engines and trannys.   The ISL9's used for the medium truck application are available in configurations as low as 260 hp and 720 lb-ft of torque.
« Last Edit: May 12, 2018, 07:50:54 AM by Gary RV_Wizard »
Gary
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Gary Brinck
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Oscar Mike

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Re: Diesel power
« Reply #12 on: May 12, 2018, 10:21:56 AM »
As usual Gary you are spot on.

ISL9 for Medium-Duty Truck: Specifications Advertised Horsepower: 260-380 hp, Peak Torque: 720-1250 lb-ft, Governed Speed   2100-2200 rpm, Number of Cylinders 6.

irishtom29

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Re: Diesel power
« Reply #13 on: May 12, 2018, 04:05:23 PM »
Gary is easily the most perspicacious person on this forum, I can’t recall his ever being in error. He might well be the most well informed person on a certain subject of anyone I’ve encountered on internet forums.

docj

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Re: Diesel power
« Reply #14 on: May 12, 2018, 04:20:48 PM »

But what you really want to compare is horsepower to weight ratio. For a given weight coach, more horsepower will always accelerate it more quickly.  In general, RV makers shoot for around 1 hp per 100 lbs of coach weight.  That gives acceptable if not blazing performance (your car probably has more than 1 hp per 20 lbs).

With all due respect, horsepower (which is a measure of energy per unit time) defines the speed at which you can climb any particular hill, but it is torque that determines your acceleration.  Torque is the rotational analog to force.  Most of us are aware that Force= mass x acceleration.  For rotational physics, torque governs the rate of change of angular velocity.  In simple terms, it's how quickly the spinning driveshaft will speed up (that is, how quickly the vehicle will accelerate.)  To provide a specific example, if you lose momentum on a long hill climb, your ability to recover any of that lost speed is a function of how much torque you have available.

MH manufacturers are reluctant to talk about torque because diesel engine torque is pretty much a direct function of engine displacement.  No amount of tuning can change that.  Many of today's "affordable" MH's are based on the Cummins ISB engine with its ~800 ft-lbs of torque.  It gets pretty pricey to jump to an ISL with ~1,200 ft-lbs or to one of the really big block ISX's with >1,600 ft lbs.  I love my CAT C-12 with its 1,550 ft-lbs of torque but there's no way I could afford a new MH with an engine of that size.
« Last Edit: May 12, 2018, 04:24:20 PM by docj »
Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/braking system
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Gary RV_Wizard

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Re: Diesel power
« Reply #15 on: May 13, 2018, 09:33:19 AM »
docj is correct, but I will add that simply quoting peak torque numbers doesn't help all that much. Torque is not the same over the RPM operating range, and peak torque is almost always at a different rpm than peak horsepower.   The transmission will shift to maintain peak horsepower under load, not peak torque, and for good reason.

One of the advantages of a diesel is that it delivers torque at much lower rpms than a gas engine, so you get more horsepower in the rpm range used for accelerating from a standing stop. Further, any big displacement engine delivers more torque than a smaller one, all across its rpm range, so it always has more hp available at rpms below the hp peak.
Gary
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Home: Ocala National Forest, FL

docj

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Re: Diesel power
« Reply #16 on: May 13, 2018, 03:29:52 PM »

One of the advantages of a diesel is that it delivers torque at much lower rpms than a gas engine, so you get more horsepower in the rpm range used for accelerating from a standing stop.

For those who have never examined the torque vs rpm curves for a diesel, they are quite a bit different from what we are used to for gasoline engines.  A gas engine develops its max torque in some "power band" usually found at mid-RPM's.  The exact location and width of the torque band depends on the particular engine, but suffice it to say that peak torque will probably be in the 3,000-5,000 rpm range and it will fall off significantly below ~2,000 rpm.  At an engine idle speed of ~500-900 rpm it has rather low torque which is why it is so easy to stall a gas engine with a manual transmission if you're new at it!  ;D

In comparison, a big block diesel will have a fairly flat torque curve which may even increase with falling rpm until only slightly above the idle speed.  This gives diesels their reputation for being able to slog through most anything.  As long as the engine manages to turn it is developing close to its maximum torque.  For my CDL I learned on a straight truck powered by an International diesel with a 6-on the floor manual transmission.  Learning to handle that transmission seemed, at first, to be the most difficult part of the course.  But I can say that in several weeks of driving it, I only stalled it once.  It was nearly impossible to stall!  ;D

I've attached the torque and hp curves for my C-12 engine.  Notice how torque increases with decreasing engine speed. Coupled with a 34,000 pound MH this engine produces fantastic off-the-line acceleration.
Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/braking system
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Back2PA

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Re: Diesel power
« Reply #17 on: May 13, 2018, 03:55:33 PM »
That's a beast you've got there Joel. You're probably using a little C7 like mine as a starter motor  :-\
Scott
Fulltiming in a 2005 Newmar Dutch Star 3810, Cat 350
Eezrv TPMS, 970W Solar, Tri-Metric Battery monitor
2002 Dodge RAM 1500 Quad Cab toad
Stowmaster towbar & Brakemaster toad braking system

Gary RV_Wizard

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Re: Diesel power
« Reply #18 on: May 13, 2018, 05:03:58 PM »
The Cummins ISL (8.9L) is nowhere near that flat, so that gigando C12 (and the Cummins ISX15) have an advantage there.  I've attached the 2017  Cummins ISL9 torque/hp graph for comparison.  This is the 450 hp version.  It's worthwhile noting that the very same engine tuned for  370 hp instead of 450 produces its max horsepower at 1650 rpms instead of 2100.  In modern electronic engines, the engineers can adjust performance fairly radically.
« Last Edit: May 13, 2018, 05:08:14 PM by Gary RV_Wizard »
Gary
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docj

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Re: Diesel power
« Reply #19 on: May 13, 2018, 06:42:39 PM »
That's a beast you've got there Joel. You're probably using a little C7 like mine as a starter motor  :-\

What I find awesome is watching it start in cold weather.  It doesn't have any glow-plugs nor does it need an engine heater unless temps go below ~15F.  Just crank and it fires up.  Rough for a moment then it settles in.  Quite a monster!  :)
Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/braking system
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billwild

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Re: Diesel power
« Reply #20 on: May 14, 2018, 12:30:49 PM »
I had the 400 hp Cummins in my coach and loved it. It gave good acceleration right from a dead stop and has lots of power to tow, and it was very dependable.  If you travelled around 60-65 miles per hour the fuel mileage was not that bad.  I would buy another if we were in the market to do so.


Bill

gravesdiesel

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Re: Diesel power
« Reply #21 on: May 14, 2018, 02:20:20 PM »
The Cummins ISL (8.9L) is nowhere near that flat, so that gigando C12 (and the Cummins ISX15) have an advantage there.  I've attached the 2017  Cummins ISL9 torque/hp graph for comparison.  This is the 450 hp version.  It's worthwhile noting that the very same engine tuned for  370 hp instead of 450 produces its max horsepower at 1650 rpms instead of 2100.  In modern electronic engines, the engineers can adjust performance fairly radically.
And then you can add tuners to them that adjust it more to your liking.  I have had a Bullydog "Triple Dog" tuner on my 2003 Cummins in my Dodge Ram 3500 nearly since I bought it new.  I added 4" exhaust and an open air intake at the same time.  It made a beast of an engine even more wild.  With almost 200,000 miles on it, I have had zero problems, except for the factory clutch was not strong enough to handle the added power.  I upgraded it to a South Bend Clutch several years ago and have had no problems, even when towing my 16,000 gvw gooseneck trailer.  I am considering a new truck and the Cummins 6.7 engine is even stronger than my 5.9 with the tuner.
2016 KZ Spree 262 RKS
2003 Dodge 3500 4 door flatbed 4x4 diesel, 6 speed
1996 Dodge 3500 extra cab flatbed 4x4 diesel 5 speed
2006 Arctic Cat TRV diesel 4x4 ATV
(2) 1981 Yamaha G1 2 cycle golf carts
Many other diesels on the farm!

KandT

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Re: Diesel power
« Reply #22 on: May 14, 2018, 03:51:31 PM »
Winnie is looking at making an electric RV.     https://www.usatoday.com/story/money/cars/2018/05/09/winnebago-drives-toward-electric-powered-motorhome/592446002/

It won't be happening tomorrow but sure will be nice to have all that torque when it does!
2005 Winnebago Vectra 36RD
American Car Dolly
2009 Accord Toad
It's not a problem.  It's a project!

rls7201

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Re: Diesel power
« Reply #23 on: May 14, 2018, 07:36:58 PM »
For those who have never examined the torque vs rpm curves for a diesel, they are quite a bit different from what we are used to for gasoline engines.  A gas engine develops its max torque in some "power band" usually found at mid-RPM's.  The exact location and width of the torque band depends on the particular engine, but suffice it to say that peak torque will probably be in the 3,000-5,000 rpm range and it will fall off significantly below ~2,000 rpm.  At an engine idle speed of ~500-900 rpm it has rather low torque which is why it is so easy to stall a gas engine with a manual transmission if you're new at it!  ;D

In comparison, a big block diesel will have a fairly flat torque curve which may even increase with falling rpm until only slightly above the idle speed.  This gives diesels their reputation for being able to slog through most anything.  As long as the engine manages to turn it is developing close to its maximum torque.  For my CDL I learned on a straight truck powered by an International diesel with a 6-on the floor manual transmission.  Learning to handle that transmission seemed, at first, to be the most difficult part of the course.  But I can say that in several weeks of driving it, I only stalled it once.  It was nearly impossible to stall!  ;D

I've attached the torque and hp curves for my C-12 engine.  Notice how torque increases with decreasing engine speed. Coupled with a 34,000 pound MH this engine produces fantastic off-the-line accelerati
on.

Doc, just so you know the old Ford EFI 460 developed its maxinum torque of 400 ft lbs @ 2000 RPM. Some gassers do grunt down low.


docj

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Re: Diesel power
« Reply #24 on: May 15, 2018, 03:22:29 PM »
Doc, just so you know the old Ford EFI 460 developed its maxinum torque of 400 ft lbs @ 2000 RPM. Some gassers do grunt down low.

Heck, I was raised on a Galaxy 500 with the old 351 V-8 and a two-barrel carb.  That was a very different engine from the 352 W or C that most of today's young'uns are used to.  I had been told that it was a converted truck engine used as an entry level automobile V-8 before the 289/302 was available.  It was always overshadowed in the lineup by the various configurations of the Thunderbird 390 V-8.  The 351 was a gas-guzzling monster, but it did develop a lot of low-end torque. I don't know where it's torque peak was, but it was a lot of fun to take off the line!
Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/braking system
WiFiRanger Ambassador/RVParkReviews administrator
Follow our adventures on Facebook at www.facebook.com/weisstravels.net

Oscar Mike

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Re: Diesel power
« Reply #25 on: May 15, 2018, 03:41:34 PM »
We have a two to three mile 6% grade close to where I live. I just took the motorhome and Jeep out and did a test run. I took a long enough run to cruise at 63 mph (Cruise Control) before the grade started and it cruised up the grade at about 61-62 mph towing the JKU Rubicon. My previous coach would drop to around 43 mph going up that grade at about 3,800 RPMs.

Coming down the grade I set the Jake Brake on high and it slowed us down considerably.\

I am pretty satisfied.

rls7201

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Re: Diesel power
« Reply #26 on: May 15, 2018, 10:33:45 PM »
Heck, I was raised on a Galaxy 500 with the old 351 V-8 and a two-barrel carb.  That was a very different engine from the 352 W or C that most of today's young'uns are used to.  I had been told that it was a converted truck engine used as an entry level automobile V-8 before the 289/302 was available.  It was always overshadowed in the lineup by the various configurations of the Thunderbird 390 V-8.  The 351 was a gas-guzzling monster, but it did develop a lot of low-end torque. I don't know where it's torque peak was, but it was a lot of fun to take off the line!

Now Doc your showing your age. That old Galaxy was a 352 V-8 which started life in 1958 as a 332, then it graduated to 360,361,390,391,406,410,427 & 428. All FE motors. Then Ford developed the small blocks, 221,260,289,302,351 C-W-M & 400. I had the privilege(?) of working on all of them when I was playing auto mechanic in my younger days. We could step back to the beginning of the "Y" blocks that started life as a 239 in 1954, then went on to 272,292 & 312. Yeah, I'm a Ford gear head. GRIN But alas the ol gassers just won't torque with the oil burners. I try with my 460/528 stroker. I should be good for over 500 lbs of torque at about 2000 RPM and 375-400 HP at 3500 RPM. Have to find a dyno one of these days.
« Last Edit: May 15, 2018, 10:40:19 PM by rls7201 »

Spring Creek

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Re: Diesel power
« Reply #27 on: May 16, 2018, 06:01:21 AM »
Had a friend who had a 1974 F250 with a 360.  He said he could pull a plow but couldn't pass a gas station!
Kurt
2018 Winnebago Minnie Winnie 31K - 2011 Equinox

Gary RV_Wizard

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Re: Diesel power
« Reply #28 on: May 16, 2018, 08:13:55 AM »
LOL! Lots of nostalgia here. I owned a few big honking V8's in my day as well, e.g. a Dodge 440, a Chrysler 413 Hemi, and a Ford 460. Hot stuff in their day, but none of them produced more useful go-power than the 3.5L (213 cu in) V6 I have today in my daily driver.

More to the point, none of them, nor their more modern siblings, produce more than about 450 lb-ft of torque at any RPM, wheres the diesels we are comparing to produce 2x, 3x or even 4x that at RPMs in the range 1000-1400.
Gary
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Summers: Black Mountain, NC
Home: Ocala National Forest, FL

docj

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Re: Diesel power
« Reply #29 on: May 16, 2018, 09:04:16 AM »
Now Doc your showing your age. That old Galaxy was a 352 V-8 which started life in 1958 as a 332, then it graduated to 360,361,390,391,406,410,427 & 428. All FE motors. Then Ford developed the small blocks, 221,260,289,302,351 C-W-M & 400. I had the privilege(?) of working on all of them when I was playing auto mechanic in my younger days. We could step back to the beginning of the "Y" blocks that started life as a 239 in 1954, then went on to 272,292 & 312. Yeah, I'm a Ford gear head. GRIN But alas the ol gassers just won't torque with the oil burners. I try with my 460/528 stroker. I should be good for over 500 lbs of torque at about 2000 RPM and 375-400 HP at 3500 RPM. Have to find a dyno one of these days.

Thanks for the walk down memory lane!   8)
Sandie & Joel

2000 40' Beaver Patriot Thunder Princeton--425 HP/1550 ft-lbs CAT C-12
2014 Honda CR-V AWD EX-L with ReadyBrute tow bar/braking system
WiFiRanger Ambassador/RVParkReviews administrator
Follow our adventures on Facebook at www.facebook.com/weisstravels.net