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Old 04-03-2011, 06:12 AM
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HELP! Opinions needed. 305 vs. 350 for MPG's.

Here's the situation: 5,000 Lb. truck with a brand new 700R4 and 1200 stall lock up converter with 2.73 gears, that is not going to change. I drive about 80% highway and 20% city on rolling hills in the upper Mississippi River Valley. I don't tow with the truck and only do some occasional light hauling. It is an older Silverado with full power, cruise and AC and has a 4/6 lowering kit. It currently has a roller cam TBI 305 out of a Camaro that has started using a LOT of oil recently and I'm ready to build a new engine. I'm not looking for HP at all, I'm actually satisfied with the overall performance of the truck. If I gain some power that's a definite plus but NOT a requirement. I will be building an engine very soon and have most of the hardware. I'm keeping the TBI and will be using the stock intake and a set of '187 Swirl Port heads off another 305 TBI. That part is locked in, not open for discussion.

I have 2 cam choices a Vortec roller or the Ramjet 350 roller and already have the cams. They have similar duration and lift specs with the Ramjet having about .020" more lift. The Vortec has 111* LSA vs. 109* for the Ramjet which might make the Vortec easier to tune the ECM for. Actually, I also have the 305 "Peanut" roller cam but I lied and I really would like just a "tiny" bit more oomph than it provides but otherwise I'm all ears on this part. I also have a set of new 1-1/2" full length headers and set of new 1-5/8" full length, both ceramic coated. I'm leaning toward the 1-5/8" mainly because the 1-1/2's are so crappy I have to rework them just to make them work. The 1-1/2" are from a name brand US company, cost over twice as much as the 1-5/8" generic eBay headers but if you lay them side by side you'd swear it was the other way around. I'd send them back but Summit won't reimburse my shipping in EITHER direction! But, I digress and that's a story for another day........

Here's my dilemma, the current high mileage 305 has averaged 18.3 MPG on 87 Octane. I'd like to equal or better that and I'm about 99.99% certain with the right shortblock and tuning I can get in the low and maybe even mid 20's. I have two 305 roller cam cores I can build or I have a 350 roller cam core. For the 350 I already have a set of 21.7cc D-cup pistons which would put quench at .045" with a .021" head gasket and compression at 9.13 assuming the heads CC at 58. I can get 12cc D-cups for the 305 which would put it at 9.16 to one with .010" off the deck and a Victor 305 specific head gasket that's .027" thick, quench would be .042". Budget is not a big issue so I'm willing to pop for the correct pistons for the 305. Whichever I build will be bored .030".

If it were a lighter vehicle it would be the 310 without question but in a 5,000 Lb, brick with Bonneville gearing I think it could go either way. Using DD2000 the 355 makes 40-50 more FtLbs. FWTQ and Peak FWHP is nearly identical except it comes at 4,500 in the 355 vs. 5,000 in the 310. I know I will need a custom tune for the ECM no matter which one. Given my type of driving and usage which shortblock would you build with fuel economy in mind, a 310 CID or a 355 CID? I promise I'll just sit back and take it in with no defending of one over the other because I'm honestly on the fence and deep in the throws of "analysis paralysis". I need to pull the trigger on this build very, very soon! HELP!

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Old 04-03-2011, 06:26 AM
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fuel economy is nothing more than a math question.

the three biggest factors are... weight,aerodynamics,and engine efficiency.

a 150hp 4 cly would have to work twice as hard as a 300hp v8 actually making the v8 more fuel efficient than the 4 cly in a 5000lb truck assuming that both engines are in proper tune.


i would only assume that the extra 50cubes will make that engine work more efficiently to move that 5000lb beast.
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Old 04-03-2011, 11:38 AM
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I would use the 350. If you arre worried about the computer, remove the glove box and get the numbers off the computer. You can buy computers all day long for less than $50 on ebay.
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Old 04-03-2011, 11:52 AM
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The 350 should have more low end torque,so it should pull a taller gear.
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Old 04-03-2011, 12:00 PM
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The computers just plug in, it can be changed in a few minutes. They can also be tweaked. http://www.moates.net
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Old 04-03-2011, 12:15 PM
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The TBI computer is 20 years old. It's like using a new computer verses a 20 year old computer. You can change the computer to flash and then you can tune it with a laptop.
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Old 04-03-2011, 12:59 PM
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Gosh! Does anybody on here think he can just get a new computer? Could anybody cut and paste a pic of that computer he can get?
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Old 04-03-2011, 01:02 PM
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310 vs 355

Just a few words about MPG, I have the same truck and get about 22-23 mpg with a similar commute. My engine is low milage, and I used an L98 (1991) TPI Z28 engine with stock iron heads, and I am still running the original roller cam, however I have 3.08:1 rear gear. I did replace the torque converter with a towing unit from ATS that has 200% more friction material for better lock up.
I burn 87 in the winter and a 15% mix in summer to avoid pre-det.
As for the 350 vs 305, eithier will work out, however the 350 has advantages long term if you have replacement issues.
Good luck with your choice.
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Old 04-03-2011, 01:22 PM
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The computer that is in the truck might work with a 350. Without the numbers you will never know. To change it to flash you modify the computer not change it. All the plug ins to the sensors and the computer does not change. It can be switched from a factory setting to one using a laptop.
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Old 04-03-2011, 01:44 PM
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Quote:
Originally Posted by Hondo78
Just a few words about MPG, I have the same truck and get about 22-23 mpg with a similar commute. My engine is low milage, and I used an L98 (1991) TPI Z28 engine with stock iron heads, and I am still running the original roller cam, however I have 3.08:1 rear gear.

Are you running the TPI or did you put the TBI from the truck on it? I have a new set of 3.08 GM gears that I had considered putting in it if I add a posi later on. I also have a 3.23 posi from an older F-Body, my truck is an '88 so it still has 28 spline axles. I know neither one is a big change but if it could improve performance a smidge without hurting mileage and I'm in there anyway............. But for now I'm sticking with the 2.73.

To answer all the computer posts.......... I'll be using an EBL Flash from DynamicEFI to "tune" the ECM. The 305 and 350 use the same ECM just different chips.
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Old 04-04-2011, 06:19 PM
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I am running the stock L98 1991 5.7 engine, 700R and computer with a Vat's mod. The Truck set up is much like the CHP Caprice TBI, and with the stock CHP tune, my TPI is comparable to the TBI performance wise. An FYI the TPI only has two fuses! Thus the pink wire left bank and orange wire right bank must shoot all at once, hence the long runners on the intake. Crude system by today's standards but works well. My current project is a 5.3 from a 1500 complete from fan to drive shaft, and pan to computer, in a 1954 Chevy 1/2 ton and 3.25:1 gears, plan on 25 MPG+ if not the fan will be replaced with electric ones like my TPI system has.

Quote:
Originally Posted by Hippie
Are you running the TPI or did you put the TBI from the truck on it? I have a new set of 3.08 GM gears that I had considered putting in it if I add a posi later on. I also have a 3.23 posi from an older F-Body, my truck is an '88 so it still has 28 spline axles. I know neither one is a big change but if it could improve performance a smidge without hurting mileage and I'm in there anyway............. But for now I'm sticking with the 2.73.

To answer all the computer posts.......... I'll be using an EBL Flash from DynamicEFI to "tune" the ECM. The 305 and 350 use the same ECM just different chips.
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Old 04-04-2011, 07:06 PM
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Thanks Hondo.
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Old 04-04-2011, 07:28 PM
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Quote:
Originally Posted by Hippie
Here's the situation: 5,000 Lb. truck with a brand new 700R4 and 1200 stall lock up converter with 2.73 gears, that is not going to change. I drive about 80% highway and 20% city on rolling hills in the upper Mississippi River Valley. I don't tow with the truck and only do some occasional light hauling. It is an older Silverado with full power, cruise and AC and has a 4/6 lowering kit. It currently has a roller cam TBI 305 out of a Camaro that has started using a LOT of oil recently and I'm ready to build a new engine. I'm not looking for HP at all, I'm actually satisfied with the overall performance of the truck. If I gain some power that's a definite plus but NOT a requirement. I will be building an engine very soon and have most of the hardware. I'm keeping the TBI and will be using the stock intake and a set of '187 Swirl Port heads off another 305 TBI. That part is locked in, not open for discussion.

I have 2 cam choices a Vortec roller or the Ramjet 350 roller and already have the cams. They have similar duration and lift specs with the Ramjet having about .020" more lift. The Vortec has 111* LSA vs. 109* for the Ramjet which might make the Vortec easier to tune the ECM for. Actually, I also have the 305 "Peanut" roller cam but I lied and I really would like just a "tiny" bit more oomph than it provides but otherwise I'm all ears on this part. I also have a set of new 1-1/2" full length headers and set of new 1-5/8" full length, both ceramic coated. I'm leaning toward the 1-5/8" mainly because the 1-1/2's are so crappy I have to rework them just to make them work. The 1-1/2" are from a name brand US company, cost over twice as much as the 1-5/8" generic eBay headers but if you lay them side by side you'd swear it was the other way around. I'd send them back but Summit won't reimburse my shipping in EITHER direction! But, I digress and that's a story for another day........

Here's my dilemma, the current high mileage 305 has averaged 18.3 MPG on 87 Octane. I'd like to equal or better that and I'm about 99.99% certain with the right shortblock and tuning I can get in the low and maybe even mid 20's. I have two 305 roller cam cores I can build or I have a 350 roller cam core. For the 350 I already have a set of 21.7cc D-cup pistons which would put quench at .045" with a .021" head gasket and compression at 9.13 assuming the heads CC at 58. I can get 12cc D-cups for the 305 which would put it at 9.16 to one with .010" off the deck and a Victor 305 specific head gasket that's .027" thick, quench would be .042". Budget is not a big issue so I'm willing to pop for the correct pistons for the 305. Whichever I build will be bored .030".

If it were a lighter vehicle it would be the 310 without question but in a 5,000 Lb, brick with Bonneville gearing I think it could go either way. Using DD2000 the 355 makes 40-50 more FtLbs. FWTQ and Peak FWHP is nearly identical except it comes at 4,500 in the 355 vs. 5,000 in the 310. I know I will need a custom tune for the ECM no matter which one. Given my type of driving and usage which shortblock would you build with fuel economy in mind, a 310 CID or a 355 CID? I promise I'll just sit back and take it in with no defending of one over the other because I'm honestly on the fence and deep in the throws of "analysis paralysis". I need to pull the trigger on this build very, very soon! HELP!
With the proper chip, I rather doubt there would be much if any mileage difference. The drag of this truck pretty much calls for how much power the engine, any engine will have to supply to move it at x miles an hour over x terrain. That won't change much since these engines weigh the same, the weight component of all the drags won't change either. The extra displacement is pretty minor more than anything it will provide a little extra torque on the bottom end allowing this to run comparable to the 305 on a little less throttle.

The computer chip is different between the 305 and 350 and possibly the injector rating, you need to know this data going forward. A new chip has to be programmed these old OBD 1 computers do not respond to flash programming, that's for OBD 2 systems. See these people http://www.tbichips.com/ The TBI system is very sensitive to manifold vacuum, camshafts that change that with additional duration, lift or LSA changes need to be provided for in the reprogram. Since TBI likes a lot of vacuum the longer LSA cam that you have would be the better choice which is the reverse of your thinking. Getting up to about 200 degrees on the intake and 210 on the exhaust is about as far as TBI can be pushed without getting into costly restructuring of sensors and chips, this gets into the stuff that TPIS had done with these things back in Manifold Absolute Pressure's heyday. It would really like an LSA more like 114 degrees but the roller Vortec can be made to work acceptably, the Ram Jet cam will be less flexible. Keep in mind that changes to rocker ratio from OEM 1.5 will look the same to the MAP sensors as more duration and less LSA which means your driving the manifold vacuum in the downward direction that TBI doesn't like.

Bogie
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Old 04-04-2011, 07:55 PM
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The early 90s Caprice 350 TBI used the L98 cam.
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Old 04-04-2011, 07:57 PM
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Quote:
Originally Posted by oldbogie
With the proper chip, I rather doubt there would be much if any mileage difference. The drag of this truck pretty much calls for how much power the engine, any engine will have to supply to move it at x miles an hour over x terrain. That won't change much since these engines weigh the same, the weight component of all the drags won't change either. The extra displacement is pretty minor more than anything it will provide a little extra torque on the bottom end allowing this to run comparable to the 305 on a little less throttle.

The computer chip is different between the 305 and 350 and possibly the injector rating, you need to know this data going forward. A new chip has to be programmed these old OBD 1 computers do not respond to flash programming, that's for OBD 2 systems. See these people http://www.tbichips.com/ The TBI system is very sensitive to manifold vacuum, camshafts that change that with additional duration, lift or LSA changes need to be provided for in the reprogram. Since TBI likes a lot of vacuum the longer LSA cam that you have would be the better choice which is the reverse of your thinking. Getting up to about 200 degrees on the intake and 210 on the exhaust is about as far as TBI can be pushed without getting into costly restructuring of sensors and chips, this gets into the stuff that TPIS had done with these things back in Manifold Absolute Pressure's heyday. It would really like an LSA more like 114 degrees but the roller Vortec can be made to work acceptably, the Ram Jet cam will be less flexible. Keep in mind that changes to rocker ratio from OEM 1.5 will look the same to the MAP sensors as more duration and less LSA which means your driving the manifold vacuum in the downward direction that TBI doesn't like.

Bogie


Thanks for the input, I'm thinking the difference may be too small to measure as well. That's why I'm asking for opinions.

My thinking was that the Vortec cam with it's wider LSA would work better with the ECM. I think you misunderstood me there.
The EBL Flash is a piggyback system for the OBD1 ECM's like mine which allows flash tuning. In fact, if I understood correctly my '747 ECM is the primary one it was initially designed for. I've been in contact with TBIChips and he kept trying to steer me toward higher flowing heads, intake, etc. Not at all in tune with what I'm doing. I want TQ and throttle response BELOW 2,500. My cruise RPM at 55 MPH is about 1350, cruise at 70 is under 1800 and I like it there. Makes for a more relaxing commute. I have no plans to deviate from a 1.5 rocker ratio.
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