increasing compression, running on regular gas?

[Southcross]

higher octane yes.... the timing curve is actually the same as the normal 1.6L, now it very well may be advanced a few degrees and its true European form it ran without a CAT or O2 sensors (so no exhaust restrictions and it always ran somewhat rich). The Golf I GTI wasn't made for fuel economy

[metromizer]

All things being equal, I think you will see a noticable gain from higher compression ratio at mile-high Denver.

But I've never tuned up there with pump gas, my experience has been with canned race fuel, which is real consistant. I'm not sure how much increase your engine will tolerate before it pings and you need to retard timing and/or increase octane fuel. I have a buddy who builds race engines (in Colorado Springs) who has some Pike's Peak experience (racing from 5000ft to over 10,000ft presents some tuning challenges). He would be an excellent resource.

You don't say weather of not your Rabbit has carbs or EFI (I don't know those cars very well) but another thing to play with, before I personally would swap head gaskets, is to jet the carburator(s) or tweak the EFI for high altitude on a chassis dyno. It is uncanny the amount of fuel you take away during tuning (reduced jet sizes), to get a carburated normally aspirated engine to run right when going from sea level up to 5,000ft.

Only after some investment in dyno tuning (couple hundred bucks), would I think about increasing compression ratio.

The increase in power I'm betting you'll realize from and increase in compression ratio comes from two sources:

1) Increased cylinder filling efficiency
2) Slightly more oxygen and fuel packed into a smaller space.

There are plenty of combustion theory experts on this forum that will do three times better a job explianing why, but most people miss #1. My experience is more practicle in nature, and have first hand experience doing nothing but raising compression ratio by 2 points and realizing almost 20% increase in peak power output at high rpm (race engine). I recently attended a talk that statused progress on raising compression to unheard of levels through a better understanding of the combusion event (with the goal to eliminate pre-ignition). Just when you thought the internal combustion engine was understood, some computer modlers come up with new diagnostic tools to lead the way 9hopwfully) to cleaner and better combution.

When I've raced at Denver with my car that is tuned for sea level, raising compression ratio or using a fuel with slightly less octane rating, advancing the timing 2-3 degrees, always helps get a little of the power back. Weather or not you would reduce fuel consumption is not known, but a quicker race engine tune usually means it's operating more efficiently.

Anyhow, it sounds to me like you are on the right path.

[Southcross]

the car is Mechanical Fuel Injection Fuel is metered by air flow through a meter, which has the benefit that if you tune the engine lean or rich... its always "proportionally" lean or rich... It runs best, IMO, when either stoich or just slightly lean. In rich form, its SUPER rich at idle, but produces a ton of top end power, which is great for my racing project cars

The car has been tuned and retuned, including dyno time, over the last several years. To the point I couldn't get anything else out of it. The only things that were having an effect on fuel economy at that point were how I drove it. The car has very predicable fuel economy... when I was driving 80% rushhour highway miles I was getting a straight 30mpg, now that I'm driving 100% city miles down now to 25-26 (I want to bring that back up to 30), and mountain drives (@ 5000-6000 rpms in 2nd and 3rd gear yielded a low of 24mpg). Since I don't spend a lot of time on the highway, I have an unconfirmed return of 40-45mpg driving from Denver to CoSprings and back

[getnpsi]

alumimum headed cars can usually run into the 9.0-9.8:1 area and still denote 87 octane in the manual. Iron head cars, like the stock 5.0 mustang i have. just look at it wrong and they are already knocking. Since you will have the head off i suggest getting some simple cleanup work done to eliminate "hot spots" where preignition can happen even with mid grade and premium fuel.

the 10% ethanol statement I do not agree with. They add their ethanol, and when tested chances are its still freakin 87 octane. You'll never get a free bonus from an oil company!

Again as stated in your post, driving style will still be the deciding factor. More power from a dead stop when driven correctly can help your city mpgs. Also more TQ will allow you to short shift the car more than before, as well as staying in a higher gear longer on an incline.

[Shawn D.]

Originally Posted by getnpsi

the 10% ethanol statement I do not agree with. They add their ethanol, and when tested chances are its still freakin 87 octane. You'll never get a free bonus from an oil company!

Ethanol itself has a very high octane rating. There's nothing suspicious about gas containing it still being rated at 87 octane.

[getnpsi]

Originally Posted by jesse.rizzo

The different "grades" of gasoline are misleading anyway. Now that up to 10% ethanol is required nation wide (the pump doesn't have to say if it does contain ethanol), and since ethanol raises the octane of gas, the "regular" is often the same octane as the "premium". Some places have started charging the same price for all grades.

But, unfortunately, as you said, the only way to know for sure is testing.

I will NOT put 87 octane 10% ethanol into my SRT-4 and boost away. i WILL put 91 in. I know ethanol is >100 octane, but that is ethanol, not a blend. We do not have that in California, 87 out here is 87. null point. Don't use it in your performance car blindly. "E-85 regular" sold in places ive never even visited is a different argument.

Octane number is a rating of the detonation properties of the fuel. Gasoline has many types of fluids in it, all with slightly different octane numbers.

[metromizer]

Originally Posted by getnpsi

Octane number is a rating of the detonation properties of the fuel...

Agreed. Another otane attribute is burn rate. The higher the octane number, the slower the burn rate. The slower the burn rate, the more stabil it is, and less likely to detonate. But... this also works to hurt engine performance. Much like really low compression ratios, too high of octane fuel used with too slow a burn rate in a given engine makes it a lazy pig, down on power, not effeicient, and likely (I've never tested this) a fuel waster.

[metromizer]

Southcross, speaking of raising compression ratio;

Since you have some building experience, tuning and race experience, I suggest talking with guys (there are several) who race that Rabbit engine you have. Rather than just thinning the head gasket and hoping for the best, there might be a combustion chamber modification that will break up charge statification and improve burn (after all, that is an older engine you are working with). Some head porters call this modification "fast burn" chambers. At the same time, having the same guy touch up the bowl area under each valve, and of course matching the intake ports to the intake manifold, 60-80 grit cartrige roll to roughen up the port to improve laminar flow to improve intra-port fuel atomization may of may not help. The idea with fast burn chambers is to better mix the charge through promoting swirl in the chamber just before combustion, to reduce detonation and promoting a more complete burn. If you can do that, you can raise the compression ratio and use the same octane fuel. The engine becomes more efficient through better cylinder filling. That's the theory anyway... much more difficult to get it all correct and get it to be an improvement without a bunch of trial and error.

Oh, I suggest you take an hour or so and check out and read about "singh grooves in cylinder head" by doing a google search. This DIY mod seems to help out older engines and enable relatively more CR with the same octane fuel. I don't know anyone personally who has tried it, but it sounds plausable and quite a few DIYers have blogged about their personal results.

I hope this helps

[some_other_dave]

Originally Posted by metromizer

The higher the octane number, the slower the burn rate.

That is not universally true. In fact, a friend of mine who worked at a refinery insisted that the octane rating had nothing to do with how quickly the mixture burned. I didn't buy that as a universal either, as I know of cases where high-octane fuel did indeed burn slower than low-octane fuel.

-soD

[Southcross]

Originally Posted by metromizer

Oh, I suggest you take an hour or so and check out and read about "singh grooves in cylinder head" by doing a google search.

interesting... I've seen them before on, oddly enough, on a ricer forum/thread... I passed the idea of as pure ludicrous.