7 mpg and slow

[online170]

as far as i know is that a performance moter with the same setup as a stock motor would use less fuel than the stock motor since the performance motor is more efficient.

Very flawed thinking. ESPECIALLY in the V8 world.

You can get HP and throw efficiency completely out the window. Put a 350 with 70's era thinking next to a slightly warmed over, carbureted LS1. They will both make the power, but the 70's guy is gonna use 2 tanks of gas getting to the track.




Would help to know what youre combination is. Also, WHEN did you register this MPG? Isnt LPG horribly inefficient in the cold?

 

[pfcvdv]

as far as i know is that a performance moter with the same setup as a stock motor would use less fuel than the stock motor since the performance motor is more efficient.

Very flawed thinking. ESPECIALLY in the V8 world.

You can get HP and throw efficiency completely out the window. Put a 350 with 70's era thinking next to a slightly warmed over, carbureted LS1. They will both make the power, but the 70's guy is gonna use 2 tanks of gas getting to the track.




Would help to know what youre combination is. Also, WHEN did you register this MPG? Isnt LPG horribly inefficient in the cold?

as far as my combination: procomp big port heads (cast iron), performer rpm manifold, performer rpm roller cam, 1:6 alluminium roller rockers, cast pistons, electric water pump, ported the heads, polished the intake ports, got the combustion chambers polished to mirror finish, dynamax big tube headers, 2'' dual ss exhaust with x pipe and 3 mufflers,
( a way too quit exhaust. i have to mention that the mufflers are home made by my dad, in my opinion they are very restrictive because they are each 4 chamber mufflers with small tubes in them. but if i mention that he calls me an idiot)
got a completely stock th350 and probably a 2,73:1 open rear end (original motor was a 302 pontiac v8)

i registered the mpg jan 1st. it wasnt very cold outside and i run with a traditional round air filter wich sucks hot air in.
but i never tought about the season. the time i ran 7 seconds to 60mph was last summer. (the valve springs broke a few days after that)

 

[cyrus_the_great]

i am from Iran and i know how that LPG systems works ... but i don't like to install and use those on my GP

if you like to get better power so you should pay more and that 's simple rule as always , i am getting better time for 0-60 with stock 350 engine , 350 transmission and 2.41 dif and 215 tires too something about 7.5 sec

your engine tuned up to work with gasoline and different octane from LPG

LPG is so much cheaper here 11kg for 2$

 

[pfcvdv]

i am from Iran and i know how that LPG systems works ... but i don't like to install and use those on my GP

if you like to get better power so you should pay more and that 's simple rule as always , i am getting better time for 0-60 with stock 350 engine , 350 transmission and 2.41 dif and 215 tires too something about 7.5 sec

your engine tuned up to work with gasoline and different octane from LPG

LPG is so much cheaper here 11kg for 2$

LOL, i didnt no they had gbodies over there 8)
and i have no other option than to drive on lpg. i'm 18 and on college: im poor as sh*t!. it's legally not even possible to own such car for me because the insurance companies wont accept me

 

[pfcvdv]

but how can i get better mpg not thinking about the lpg system? should i get a milder cam? the performer rpm cam has an adv duration off 296 wich is big. should i do something like a 270?

 

[mrlee]

One suggestion to consider. Switch out the th350 for a properly built 200r. You'll still have power when you want it, but when cruising in OD you'll get much better mileage In lower rpm's.

 

[joesregalproject]

I would not run that on LPG ever. It isn't made for gas engines especially high output ones. That engine needs to be run on whatever the manufacturer specifies, or otherwise run it on high octane, low ethanol gasoline.

 

[bill]

I just wanna say that I think that the 2" exhaust pipes might be a bit too restrictive for your engine. Take the pipes off and see what kinda performance you get from that change. Id be running 2.5" pipe with that kinda bhp.

 

[online170]

I just wanna say that I think that the 2" exhaust pipes might be a bit too restrictive for your engine. Take the pipes off and see what kinda performance you get from that change. Id be running 2.5" pipe with that kinda bhp.

I must have missed that part!!! Holy crap! Big tube headers going into an exhaust pipe the same size as the primaries.


2" exhaust was restrictive to an anemic LG4 305 stocker. And on top of that you have THREE mufflers, with 4 chambers. No wonder. Stock was 2.5", with a Y-pipe.

 

[HurstOlds]

You have a VERY interesting build and I'm really curious to learn more about your setup.

First off, how do you know your engine is 425 hp? I apologize if I missed it, but is your engine a stroker? You listed that you have cast pistons...are they domed/dished/? What do you guess your compression ratio is for your engine? Have you checked the compression ratio? Is your engine bored out or anything like that?

It sounds like you did all the work on the heads yourself. Did you buy the engine complete, or build from scratch yourself?

I ask the above questions because I have the same problem. I have a 455 BBO that gets 8 mpg and is a dog (in my opinion). I have pretty much come to the conclusion that my engine was built for a blower. It has super low compression (on purpose). Until I can tear it open, I'm pretty much stuck with a turd...

OK, so you are saying you are getting 7 mpg. If lpg does get 20% less efficiency, let's round up and say that your engine would get about 10 mpg on gasoline.

Do you know how much less horsepower LPG produces compared to gasoline? Doing a quick search, all I could find was an article saying a new Dodge truck gets 13% less horsepower on CNG compared to gasoline. 10% less torque.
http://news.pickuptrucks.com/2012/10/fi ... 0-cng.html

This is rear wheel horsepower, not flywheel, which I'm guessing you're rating your engine at (425 hp). So let's say you get 300 hp to the rear wheels, if the article above for CNG is comparable to LGP, your car would be getting like 40 less hp to the wheels (if you were getting 300 hp normally). So you pretty much have a car getting around 260 hp to the rear wheels.

All of the above figures are based on info in regards to that new Dodge Truck. Trying to interpolate and correlate to an older engine, I'm sure you can imagine the losses are even greater with a carb system.

Now you said you are running rich, how rich I don't think you mentioned. That would kill your power and mpg.

A while back I had researched lpg systems and wanted to see how feasible it was to run. I've never actually seen a setup like yours on an older style engine, but I like it. Have you considered getting a intake adapter and running impco carbs right on top of your engine? I think one or two carbs would suffice?

What would the advantage of your system be, over having a carb right on your intake? Are you switching between gasoline and lpg on the fly or something? It looks like you're injecting lpg into a normal carb. Do you have any mods done to your carb? Or do I have this wrong?

Based on your setup, it looks like it would be a b*tch to tune, let alone try to run year round. Can you post some details on the impco system you are using? I'm curious to find out how you are mixing your gas and air.