I'm considering building up a USDM H22A1 engine, keeping in mind I live in California where it's all 91 octane. The H22 is 10:1CR, and I'm wondering how high I can go. The big unknown is all the combined effects of everything... I'm well aware cam overlap, pistons, head, fuel, spark, load, etc, all effect at what rpm and load it'll start pinging at.

My question is, how do you know, before building up an engine, what dynamic compression to aim for, how close to the "edge" of a giving octane rating, can you get to?


Modified by kb58 at 2:07 AM 12/26/2004

 

Are you wondering how much higher than 10.1:1 you can go? That would all depend on how high you plan on reving the engine. First consider that your engine has three compression ratios (well, four because it has vtec, but for sake of argument that can be ignored).

Static compression ratio: chamber volume + swept volume / chamer volume = in this case 10.1:1

Effective compression ratio (this is the important one): The compression ratio that is measure begining at the point when the intake valve closes. If you think about it you can't begin building compression in the cylinder until you close the intake valve, otherwise it'll just blow back into the intake. On a modern 4-valve engine the effective compression ratio can be somewhere around 8 to 8.5:1.

Dynamic compression ratio: Effective compression ratio x volumetric efficiency. There is 1 point in the rpm band (<U>peak torque</U>) where volumetric efficiency is highest. At lower rpm the incoming air/fuel charge does not have enough inertia to overcome the force of the piston coming back up on the compression stroke and some is forced back out the intake valve, but as rpm builds and the velocity of the A/F charge increases it is able to resist this tendancy and can almost fill the cylinder to 100% capacity (but it rarely does except on extremely well prepared engines, contrary to popular belief). VTEC is able to change the point peak torque is reached because it can change cam profiles.

So to answer you question, you don't actually aim for a dynamic compression ratio because that is a variable. You build the engine around the effective compression ratio. You set the effective compression ratio by taking your static compression ratio and determining where you need to close the intake valve to get it in that 8 to 8.5:1 ratio. To do that you will need the help of a knowledgable cam grinder. Picking a cam out of a catalog on an engine you have put together with various off the shelf components is a crap shoot unless you know what you are doing. One of the most knowledgeable cam grinders in the business is Dema Elgin in Redwood City, Ca. (www.elgincams.com). If he does not have a grind to fit your needs he will point you to someone who can - he is way cool.

To get your true compression ratio you can't just buy '10:1 pistons' because your engine will not turn out to truly be 10:1. You need to have the engine 'CCed' as some people call it, basically it means that you need to have the combustion chamber volume measured by precisely filling it with a fluid using a burret so you can get an exact reading. You also have to take into account compressed head gasket thickness, piston crown volume, etc. to get an exact reading. That is a whole separate post in itself.

As to whether or not you can go higher than 10.1:1 static compression ratio, you can if you plan on reving the engine higher. The reason is because to be able to run a higher static ratio you need to close the intake valve later to keep the effective ratio in the 8 to 8.5:1 range. Closing the intake valve later will require that the A/F charge be moving at a higher velocity to overcome the upward motion of the piston, thus pushing the point in the rpm band that peak torque is reached up. Assuming the engine can handle spinning faster, moving peak torque up in the rpm band = more horsepower and also means you can stay in a given gear for a longer period of time which means you have the advantage of torque multiplication of a lower gear for a given speed which = faster acceleration which = faster 0 - 60 or 1/4 mile or 0 - 150 or whatever you feel like doing.

Let me know if that helped or just confused the hell out of you and I'll answer any question you have





Modified by AutoEng2002Si at 9:48 AM 12/27/2004

 

That was excellent.

Very well explained.

-s

 

My 12.2:1 LSVTEC ran fine in AZ with 91... Do a search on Toulene, it bumps up octane.

 

toulene works well, but watch the ratios as it will foul up your O2 sensor and the cat if you have it.
stan

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by AutoEng2002Si &raquo;</TD></TR><TR><TD CLASS="quote">Are you wondering how much higher than 10.1:1 you can go?</TD></TR></TABLE>

Exactly... and a very good explanation, thank you. Say we have a H22A1 which will never be reved higher then stock (7500 or so), running on 91 octane only. The question is, how does the engine builder know where to start, as far as piston and cam choices. So based on your explanation, as long as the effective compression ratio is left at 8-8.5, the engine can have whatever cams and pistons. So it can be worked out mathamatically, given a particular cam, what compression piston go with it, to stay within the above limitations.

My confusion was all the other variables... but maybe I'm making it too complicated. That is, if everything else stays the same (fuel, ignition, etc) then it's only the cams and pistons to deal with? I'm assuming of course that the fuel system is well tuned.

 

If you have the timing cards that came with the cam you can figure out what the effective compression ratio is assuming that it quotes valve duration in degrees from seat to seat and not duration @ .050". The duration at .050" is irrelavent since, as I said before, you can't have any compression when the intake valve is still open. Unfortunately, somewhere in history cam companies started quoting durations @ .050" which is useless information unless they also give you the seat to seat duration (from the time the valve lifts off the seat to the time it sits back down). Even the man who first started using this measurement, Harvey Crane, admits its useless unless you are also given seat to seat duration and I think that is what he originally intended it to be used as. If two cams have identical seat to seat duration but one has more duration @ .050" it means that the valve is being accelerated open faster and therefore the valve gets opened more, earlier. This is great unless you go too far and accelerate the cam follower right off the nose of the cam (called valve float).

So if you know how many degrees ABDC (after bottom dead center) that the intake valve closes you can use pretty basic trigonometry to figure out what effective compression ratio will result from a given static compression ratio and with a little more calculating you can figure out what static compression ratio you would need to use with a certain cam to keep your engine in that 8-8.5:1 effective ratio range. Unfortunately, its been a long time since I've done that math so I leave it up to the cam grinders.

In reality the cam is one of the last things you should buy anyway. It is a tuning item. Valve timing events are changed around the rest of the engine the same way ignition timing and fuel mixture are.

I'm not sure if you are making it too complicated (I know my explanation is but they always will be). You could basically go as high on the compression ratio as you like as long as the intake valve is cloased later to compensate and keep the effective compression around 8-8.5:1. Unless you are planning on moving your redline higher you wouldn't want to do this though because as you close the intake valve later the point where peak torque is reached moves higher up in the rpm band and why would you want to shift when you just got to peak torque? 10.1:1 for an engine reving to 7500 running exclusevly on 91 octane is just about right so you don't need to change it.

You said you are confused about other variables but I am not sure exactly what variables you are talking about. You state, 'if everything else stays the same (fuel, ignition, etc) ...' , does this mean fuel and ignition components or fuel and ignition maps (meaning the tuning)?

As far as depending on toluene as an octane enhancer I wouldn't even go there because who wants to store an explosive carcinogen in their garage and then have to go through the hassle of mixing it for each tank. If you're going to go through that hassle why not just buy race gas? Of course it is an explosive carcinogen too but at least it's pre-blended and made to be burnt in an engine.

 

^^^^^^wow.

on a second note...can i order race gas in barrels to my house and store it in a garage, and if so, where can i do that?

if i can buy 100 octane then i can tune my car to run better on the pcts then with <U>JUST</U> 94 for my high 12:1 comp ls/vtec.

 

Keith Black pistons website http://www.kb-silvolite.com has some good stuff about comp. ratios.They have some good calculators.I've backed in some street engines I've build and they fit the model nicely.Since seeing this I have build a couple engines using there calculator spec.s with good results.
Glenn

 

That's pretty cool, thanks, link bookmarked.