Hi everybody, I have a doubt so I think this is a good place to discuss it

My question is: High compression ratio really increases mpg?

This question arises because a friend is rebuilding a big block 400 that came out from a truck. He said to me that is going to raise up the compression to 15:1 with iron ported heads, forged rods an racing valve job to use the engine on CNG and it expects to have an increment of 25 and 40% on MPG.

Can this be true or is only a speculation?

 

CNG or propane can use higher compression. It does not have the power of gas. I remember my father in law had a ford pu with a propane setup, rebuilt to run on propane
talk to someone who knows more 15 to one sounds too high. Especially true if he plans to run a dual fuel setup

 

if I'm not mistaken he wants to run mainly on CNG and is going to install a water/methanol injection kit to use with premium gas only in case that runs out of CNG

 

The theoretical efficiency of the engine is strong function of compression ratio. BUT: Improvements from 8 to 10:1 have pay good dividends. But above that, you are dealing with the law of diminishing returns (and 15:1 is a bit too much, 12 may be more reasonable). Second, the major inefficiency of the big engines comes from what is cold "pumping losses". This is essentially your vacuum at part throttle (the piston fights this vacuum at the intake stroke). No matter what your compression ratio is, you still pay the price through that, and the only way around it is a 318 or a /6. But ... having said that. The gaseous fuel displaces some of the air in the intake, so you may get some relief in pumping losses from this (not due to the compression ratio), but you will also pay the price in volumetric efficiency at full throttle.

Probably the main benefit of the CNG is the cheaper fuel, but you cannot fill it anywhere, and the fuel you can carry around is limited.

 

Maybe this is a problem in USA but here almost all of the gas stations sells CNG. I don't understand so well why is better a compression of 12:1 than 15:1 but I think is because in big engines the air/fuel mixture is not so good like in a small block? is this the reason of why some people put large intake manifold on their big blocks?

EDIT: I forgot to mention that my friend is going to use the engine in a 1976 Dodge dart sedan, will be amazing to feel how this car perform with a big block

 

The limit to compression ratio is detonation. 15:1 will probably be too much for detonation at WOT, unless you retard the timing a bunch. Also, the clearance volume in the heads may not allow such large compression ratio anyway. Even if you could fabricate special pistons to cover up that volume, you may have valves hitting the pistons. Even if you resolved that, your peak cylinder pressures may be too high for the bearings. Even if you resolved that, the benefit would be marginal over a 12:1.

 

2 points over stock gasoline pistons is all you need to run CNG or propane. And that is only necessary to regain the lost HP. Oh and hardened exhaust valve seats are a MUST.

 

I think that will be better to discuss the bolded text because my personal experience is that if somebody has the correct A/F ratio with CNG in an unleaded engine the valve seat will not suffer any premature wear.

This project sometimes makes me want to get a big block, put in the car and run on CNG but for the moment I can be happy with my 360

 

Regardless of fuel used the valves NEED to be lubricated In the olden days it was leaded fuel... Unleaded fuel they went to harden valve seats to compensate for the lack of lead in the fuel. YES YOU NEED hardened valve seats even with CNG It has no valve lubricating additives With a 400 you are already starting with a motor set up with hardened seats the bigger the cyl the more it would be subject to detonation

15/1 would require C116 octane racing fuel, Not street friendly

 

So in this case the valve seats must be hardened or it should be fine with the stock valve seats set up?

What kind of seats will be fine?

How a hardened valve seat will affect the gas performance?

 

Our Local Ferry Corporation (one of the largest ferry fleets in the world, see www.BCFerries) is involved in converting to CNG. Back as far as 1986 we ran the first ever passenger ferry in the world on CNG. These were stock production Caterpillar Diesels with carbureted CNG and 5% diesel injection for ignition. Probably 18:1 compression ratio.

Matter of fact, the worlds shipping fleets are at the crossroads of fully converting to CNG, and it has become a major employment opportunity for shipyards that can do the work. Oil as fuel, is just too darn expensive now.

I see no issues with 15:1 as CNG is a soft hit compared to propane or gasoline.

As one posted noted, it takes a lot more CNG to get the same bang, and storage volume is the only consideration that may be a limiting factor. And I believe this has been addressed with fiber wrapped aluminum high pressure tanks.

 

Fact: Propane will increase mileage. Done properly with the right matching setup one can increase mileage 40%+ over gas

Fact: Propane decreases engine performance when compared to gas.

 

Precisely why you need to start out at 10:1 cr to get gaseous fuels to perform better/equally.

A freind had a 86 d250 ish 4x4 with a 360 stick, mild cam 4.10:1 gearset and homemade headers. You wouldn't know it ran on propoane. A true rocket that lasted well over 400, 000 miles before burning a valve.


eg. CNG creats approximately 6 times less BTU's than diesel fuel. Thus it takes 6 times the volume to get the same/equal performance (althought the CNG is rated in giga joules for thermal units)

On a side note with running cng and very high compressin, you must have very well scrubbed cng as a propane spike has a tendency to blow cylinder heads off.

Here is the world leader in CNG and Hydrogen fuel development for automotive and industial applications. http://www.westport.com/

 

Here's a lot of good information about CNG and propane

I'm not 100% sure but I think CNG has a bit more calories than gas, something like 2300 more than gas. Is this true?