Why does a tighter lsa need more compression?

[6.50camaro]

IMHO, Cranes advising higher copmression is not a result of tighter centerline put of overall intake duration and later closing off intake . The tighter centerline is to keep the exhaust from opening to early because they always think you need more exhaust duration than intake . The degree of overlap is just a result of that ,thats all . Dan

[Belgian1979]

I tend to agree with Mike. I don't think that a tighter LSA needs a higher CR. I do believe that a tighter lsa creates a higher compression pressure since, all other variables kept the same, the intake valve would close at less degrees ABDC.

[CamKing]

In circle track racing, there are a bunch of classes, where the engine rules are almost identical, but some are limited to lower compression.
These classes run the same size engines, heads, manifold, Carb, and turn the same RPM. The lower the compression the tighter the LSA and ICL needs to be.

[n2omike]

As was said... All else being equal, the tighter LSA closes the intake valve sooner, which traps more air... providing a higher dynamic compression ratio.

Saying a tight LSA requires more compression is a statement from out in left field, and totally backwards.

My theory on why the catalog says that, is that you're looking at cams for two completely different applications. A STREET cam (wider LSA) needs to be run on pump gas, so it will recommend a lower CR. A more serious cam made for racing... where straight pump gas may not be required... will bend that recommendation towards a higher compression ratio for max power.

Regardless of whether or not you're comparing street and race cams, catalogs are not perfect. They get parts added, they get edited... and are not 100% consistent. They're not exactly a valid scientific resource.

Good Luck

[Belgian1979]

I always read that the higher CR recommended for some cams is due to the reduced compression pressure and therefore lost power when using cams with longer duration. So the longer the duration, the higher the CR is the recover some of the lost power which is caused by less trapped air at the end of the compression stroke. This in itself doesn't explain what octane fuel you need imo although it might be an indication imo.

[n2omike]

I always read that the higher CR recommended for some cams is due to the reduced compression pressure and therefore lost power when using cams with longer duration. So the longer the duration, the higher the CR is the recover some of the lost power which is caused by less trapped air at the end of the compression stroke. This in itself doesn't explain what octane fuel you need imo although it might be an indication imo.

Yes... Hence the 'all else being equal' statement at the beginning of my reply above.

In response to your query... all else being equal... The longer the intake duration, the later the valve closes during the compression stroke, and the less air it traps to be compressed. To make up for this, the longer the intake duration, the higher the compression ratio needs to be in order to maintain good cylinder pressure.

Longer Duration = Higher Compression Ratio Needed
Wider lobe separation = Higher Compression Ratio Needed

[Lizardracing]

I'm just here to complain about the word "dynamic compression" when IVC is actually a mechanical thing that CANNOT change therefor isn't dynamic at all but instead STATIC!

Climbing down now.

Carry on using the wrong terminology if you wish.

[lc-gtr-1969]

I'm just here to complain about the word "dynamic compression" when IVC is actually a mechanical thing that CANNOT change therefor isn't dynamic at all but instead STATIC!

Climbing down now.

Carry on using the wrong terminology if you wish.

Hmmm... No, cannot say I agree here. Static compression is the mathematical caluclation of the compression ratio, compressed volume... Dynamic is in reference to cylinder pressure within the moving engine. The two are different... The intake valve closing angle bleeds off some of the air column (in a running engine), hence 2 engines with the same mathematically derived compression ratio (static) will have different operating cylinder pressures at fixed rpm if they have different cam specs (ie one with 106lsa installed at 104 will have higher cylinder pressure than a 112lsa installed at 110, presuming the same duration and static compression). This is why the answer to the OP question is that tighter LSA does not need more compression. When duration is the same, if you widen the LSA you should raise the comrpession ratio accordingly to match the later intake closing angle and subsequent intake bleed off.

Happy to be proven wrong here, maybe i've been getting it wrong all these years...

[GARY C]

I'm just here to complain about the word "dynamic compression" when IVC is actually a mechanical thing that CANNOT change therefor isn't dynamic at all but instead STATIC!

Climbing down now.

Carry on using the wrong terminology if you wish.

Hmmm... No, cannot say I agree here. Static compression is the mathematical caluclation of the compression ratio, compressed volume... Dynamic is in reference to cylinder pressure within the moving engine. The two are different... The intake valve closing angle bleeds off some of the air column (in a running engine), hence 2 engines with the same mathematically derived compression ratio (static) will have different operating cylinder pressures at fixed rpm if they have different cam specs (ie one with 106lsa installed at 104 will have higher cylinder pressure than a 112lsa installed at 110, presuming the same duration and static compression). This is why the answer to the OP question is that tighter LSA does not need more compression. When duration is the same, if you widen the LSA you should raise the comrpession ratio accordingly to match the later intake closing angle and subsequent intake bleed off.

Happy to be proven wrong here, maybe i've been getting it wrong all these years...

That and it can be adjusted via installed ICL plus it changes with RPM where compression ratio is fixed.

[plovett]

Dynamic is in reference to cylinder pressure within the moving engine.

This is not correct. Dynamic compression ratio does not refer to the actual cylinder pressure in a running engine. What you are talking about is "actual cylinder pressure in a running engine." DCR is just static compression ratio based on the stroke when the intake valve closes.

paulie

[77cruiser]

Maybe the term just needs to be changed from DCR to IVCCR.

[lc-gtr-1969]

Dynamic is in reference to cylinder pressure within the moving engine.

This is not correct. Dynamic compression ratio does not refer to the actual cylinder pressure in a running engine. What you are talking about is "actual cylinder pressure in a running engine." DCR is just static compression ratio based on the stroke when the intake valve closes.

paulie

I think my choice of words and phrasing could have, actually, should have been clearer but notice I said 'reference to', not measurement of? Whilst you can (and we do) calculate dynamic compression theoretically, it is in an attempt to predict what happens regarding cylinder pressure in a moving engine, ie to give a reference point to show how different IVC events impact upon the 'effective' compression ratio compared to true static compression ratio.

Nonetheless, it's a helpful prediction of the effective compression an engine will see as it is attempting to account for the bleed-off that occurs from the IVCA. The word dynamic may be misleading but it is the accepted industry term, even though it is not measuring 'actual' compression but rather its a theoretical reference, still something I use.

[Steve.k]

The tighter lsa is more often used where theres a lack of oxygen or higher Da where you need to build cylinder pressure quickly. This would also apply to smaller cube higher output engines. In our low oxygen area i typically run 102-106 lobe sep on 351-400 cubes. On the bigger cube engines that do not have as much tq defeciency I’ll go wider.

[plovett]

Maybe the term just needs to be changed from DCR to IVCCR.

Yes, the name DCR is unfortunate as it is misleading. I was thinking CACR, Cam Adjusted Compression Ratio.

[77cruiser]

Maybe the term just needs to be changed from DCR to IVCCR.

Yes, the name DCR is unfortunate as it is misleading. I was thinking CACR, Cam Adjusted Compression Ratio.

It works.