Compression ratio

[Eleven]

I hope this is the right forum. On another thread about flat top pistons, I said that without any other changes, they were a waste of time. Another Member disagreed with good points; increasing compression increases horsepower but timing may have to be retarded to control pinging. JE Pistons says that 1 point of increased compression with a worth 3-4 percent power increase.
Assuming no changes from a stock engine, would not a horsepower increase via flat top pistons be given away with retarded ignition timing resulting in a wash?
It seems to me that changing just one item on an engine is of little value because an engine is nothing but an air pump. It has to flow as a whole or you'll end up with a net zero or a loss.
Theoretical stuff. Thank you folks for anything thoughts and helping me understand this stuff.

 

[alpine_64]

I hope this is the right forum. On another thread about flat top pistons, I said that without any other changes, they were a waste of time. Another Member disagreed with good points; increasing compression increases horsepower but timing may have to be retarded to control pinging. JE Pistons says that 1 point of increased compression with a worth 3-4 percent power increase.
Assuming no changes from a stock engine, would not a horsepower increase via flat top pistons be given away with retarded ignition timing resulting in a wash?
It seems to me that changing just one item on an engine is of little value because an engine is nothing but an air pump. It has to flow as a whole or you'll end up with a net zero or a loss.
Theoretical stuff. Thank you folks for anything thoughts and helping me understand this stuff.

You have to advance the timing with higher compression as you will be using higher octane fuel and it will be combusting at higher pressure.

When you have lower compression or octane you retard the timing.. I think ( being that member who supported the value of bumping compression) I said more compression without change will increase the performance as long as you use higher octane fuel and advance timing... I'll go check I didn't accidentally write it incorrectly.

I should also state when I rebuilt my motor I added flat tops ( as that's all I could find at .040 at the time) I got a noticeable boost in performance..but had to use the highest octane fuel to avoid pinging ... Or.. if I wanted to run the usual mid fuel I had to knock timing out ( retard it) .. so stuff that...better fuel and more timing...then when that motor lunched the head gasket ...I modified the chamber's... Skimmed the head..flowed the runners .... added a mech advance Bosch igntion ... And then really got the full advantage...

 

[alpine_64]

Just did a check... I did say advance the timing.

 

[Eleven]

You did say advance. Was driving with my son and he said the car was knocking like crazy, didn't I hear it? (because of an event I am half deaf) We put 92 octane in it (highest we could find) but it still knocked a bit so we retarded the ignition and cured the knock; that's where I was thinking of a retarded ignition. When I raced my Spitfire we had somewhere between 11 and 12 to 1, ran some crazy advance (35ish). Hard to get it out of the paddock but a screamer on the track. We also had 100 octane and sometimes 110 if you had the money and could find it. I think my thoughts come from this experience. In short, high compression is not compatible with the street without backing everything off. Sounds like I have this wrong. Thanks for your thoughts again!

 

[Bill Blue]

I agree. Years ago I read (might have come from Vizard when commenting on his head modification and the resulting lower comp ratio) "The idea is to build horsepower, not high compression ratios".
Bill

 

[Eleven]

That's a good comment BIll! I ran across a guy years ago with a 289 Mustang. He had 2 four barrel 5500 cfm or such carbs on it with a gorgeous Edelbrock cross ram manifold. Very loud. A Karmen Ghia could probably out run him. He thought carbs equaled horse power.
Anyway, what I learned here is that higher compression on the street has some value if you know how to apply it; which I don't.
Thanks for the discussion.

 

[alpine_64]

I agree. Years ago I read (might have come from Vizard when commenting on his head modification and the resulting lower comp ratio) "The idea is to build horsepower, not high compression ratios".
Bill

Vizard was/is into whatever produces power (though not all the other engine builders agree with his methods) he has a YouTube channel that's good to watch.

Re the alpine/ rootes heads, he focused on improving flow in the head (it's weak point) and this meant opening the chamber's and that inherently lowered compression. Now his aim was about improving the head flow... That said.. if you maintain the stock compression ratio and have the Vizard mods .. you gain more power than just the head mod as you get the flow and the compression.

Also to clarify... Just throwing aoad of compression in is not a great idea..and without appropriate mods you won't get the most out of it...

But.. switching to flat tops in an alpine with a head that's not heavily skimmed will be worthwhile but will require you to run better fuel and adjust the timing as required.

Once above 10:1 in the Alpines they will lunch head gaskets if things are not on the money and going to a higher octane fuel and advancing the timing will be a must to avoid detonation.

Switching to a High output igjton also helps ( preferably optical elec with mech advance) and headwork will also show bigger benifits.... All ends up a slippery ( but fun) slope.

 

[Bill Blue]

Something that typically is not addressed while discussing compression ratios is the lack of good gasoline available in the states. At least good in relation to what was available "in the day." Octane in the low 90's is the best you can regularly find. I've been in many a smaller town that has only 89. When your on the road you often have to settle for what you can get. Many times that is 89 octane gasahol. The four valve engine in the DuraPine was designed to run on 89, but I try to run higher to cover any fatal missteps I may have made in my tuning of the engine.

Bill

 

[Barry]

Octane rating methods have changed since "back in the day."

There have been two basic octane test methods dating back to the late 1920's; the Research Octane Number (RON) and the Motor Octane Number (MON). Without going into a bunch of squiggly details, the RON value for a given gasoline was / is about 10 numbers higher than the MON number.

When octane ratings were first federally (U.S.) required to be shown on gas pumps in 1973, RON was the designated test method. For reference, Sunoco 190 was 87 RON and Sunoco 260 was 97.5 RON.

The current federally (U.S.) designated anti-knock index (AKI) rating system is the average of the RON and MON values ((RON+MON)/2)) and results in lower pump "octane" numbers. The 97.5 RON Sunoco 260 would be about 92.5 RON+MON / 2.

The bottom line is that current 91 octane U.S. gasoline (with or without ethanol) is approximately "AKI equal" to "old school" 96 octane (which would have been Sunoco 250).

 

[alpine_64]

Octane rating methods have changed since "back in the day."

There have been two basic octane test methods dating back to the late 1920's; the Research Octane Number (RON) and the Motor Octane Number (MON). Without going into a bunch of squiggly details, the RON value for a given gasoline was / is about 10 numbers higher than the MON number.

Australia uses RON and I know that numbers we have on the bowser are higher but the actual octane level is lower than the fuel stateside .

Our fuel is designated as ethanol content if it has E10 bring 10% ... The premium fuels are still pure and are 98ron. The Alpine with high compression runs well on that...but any lower and the pinging starts

 

[RootesRooter]

What's the advantage of the mechanical advance?

 

[Knightowl61]

Octane rating methods have changed since "back in the day."

There have been two basic octane test methods dating back to the late 1920's; the Research Octane Number (RON) and the Motor Octane Number (MON). Without going into a bunch of squiggly details, the RON value for a given gasoline was / is about 10 numbers higher than the MON number.

When octane ratings were first federally required to be shown on gas pumps in 1973, RON was the designated test method. For reference, Sunoco 190 was 87 RON and Sunoco 260 was 97.5 RON.

The current federally designated anti-knock index (AKI) rating system is the average of the RON and MON values ((RON+MON)/2)) and results in lower pump "octane" numbers. The 97.5 RON Sunoco 260 would be about 92.5 RON+MON / 2.

The bottom line is that current 91 octane gasoline (with or without ethanol) is approximately "AKI equal" to "old school" 96 octane (which would have been Sunoco 250).

Back in the early 70's in Arkansas there were always "Gas Wars" where leaded regular was 95 octane at 15 cents/gal & premium was 100+ octane at 18 cents/gal just before unleaded was getting popular and the first gas shortage. Them were the good old days.

 

[Scotty]

What's the advantage of the mechanical advance?

Following.

 

[alpine_64]

The mechanical advance allows you to set timing at a given curve that does not alter, it is tied to the RPM alone. This is good if you have consistent fuel quality and your car is dialled in nicely.

Vaccum.will advance the timing based on load to the engine, with more pedestrian tunes and usage the vacuum advance works well at least to help fuel consumption, generally at lower rpm.

The vacuum in theory provides benifits of adjusting for better burn in both low and high power situations...

The reason mech advance is used in performance engines is that the focus is on making sure you have the advanced timing at the higher rpm zones even if it costs some efficiency down low.

As a side note some motors are known to "like" vacuum.. SBF apparently being one of them.