Choosing a compression ratio to match the cam

[Xnke]

Ok, so it's time to get my head milled down to the chamber size I need. All the port work is done, and My N42 now has 46.5cc chambers. My pistons are 88mm diameter and have a 9.32cc dish. I have a stock L28 stroke, and L24 rods. I have a Delta .480" lift 284* duration camshaft. I do not offhand know the lobe separation angle, or lift at 0.050". My cam did NOT come with a cam card, oddly enough.

 

Now, I know from reading here that the stock N42/F54 block combinations run at about 10.3:1 compression, and ping on 93 octane. I also know that a big cam bleeds off static compression untill higher in the rev range, and needs a higher static ratio to keep the low end around.

 

How do I determine the compression ratio needed for the camshaft and piston/head configuration I have? Is there a magic constant that is used, say, for 260* duration, 9:1 is ok, for 270* duration, you need 9.5:1 or higher, for 280* duration you gotta have 10:1 or higher, etc?

[dr_hunt]

There are pleanty of static compression ratio calculators on the net, try google.

 

As for set in stone theories they typically follow 10:1 for pump gas with and aluminum head, 9:1 for an iron head respectively. That is subjective and doesn't take into consideration the cam.

 

I think you have the cart before the horse, typically you build the compression and then choose the cam and it seems that you went the other way. A larger cam bleeds off more compression as such that compression is typically referred to as dynamic. Do a little google search on dynamic compression and you'll see.

 

Being that you have a 6cyl degreeing the cam is somewhat more difficult. I think Braap can help you more there in determining your valve timing events should you have the proper tools.

 

I hope this helps you.

[1 fast z]

Do you have any quench pad on the pistons, or are they just a circle cut out for the dish? I would run 11.5-12-1. I run 12-1 on 91 octane and 36 degrees of timing on a high quench setup. I ran that same cam with this setup for awhile, but it kinda fell off at about 6600.

[Pyro]

Ok, so it's time to get my head milled down to the chamber size I need. All the port work is done, and My N42 now has 46.5cc chambers. My pistons are 88mm diameter and have a 9.32cc dish. I have a stock L28 stroke, and L24 rods. I have a Delta .480" lift 284* duration camshaft. I do not offhand know the lobe separation angle, or lift at 0.050". My cam did NOT come with a cam card, oddly enough.

 

Now, I know from reading here that the stock N42/F54 block combinations run at about 10.3:1 compression, and ping on 93 octane. I also know that a big cam bleeds off static compression untill higher in the rev range, and needs a higher static ratio to keep the low end around.

 

How do I determine the compression ratio needed for the camshaft and piston/head configuration I have? Is there a magic constant that is used, say, for 260* duration, 9:1 is ok, for 270* duration, you need 9.5:1 or higher, for 280* duration you gotta have 10:1 or higher, etc?

 

You are on the right track. You are talking about dynamic compression ratio (DCR) which considers the intake closing point along with the static compression ratio (SCR) to figure out cylinder pressure. Some people will say it is the valve overlap that bleeds off cylinder pressure but it isn't. The intake closing point on the compression stroke is what controls the amount of cylinder pressure bleed off.

 

Different engine designs have difference amount of cylinder pressure tolerance so there isn't any hard rules for static compression and cam duration tolerance with pump gas. However for a L28, a 284 degree cam runs well with 10:1 cr. And should run without any special tuning up to 10.5:1 cr with 91 to 93 octane.

[ozconnection]

You are on the right track. You are talking about dynamic compression ratio (DCR) which considers the intake closing point along with the static compression ratio (SCR) to figure out cylinder pressure. Some people will say it is the valve overlap that bleeds off cylinder pressure but it isn't. The intake closing point on the compression stroke is what controls the amount of cylinder pressure bleed off.

 

Different engine designs have difference amount of cylinder pressure tolerance so there isn't any hard rules for static compression and cam duration tolerance with pump gas. However for a L28, a 284 degree cam runs well with 10:1 cr. And should run without any special tuning up to 10.5:1 cr with 91 to 93 octane.

 

I dunno about what is said here, especially the part about valve overlap not playing a part in bleeding off cylinder pressure.

 

My understanding of this situation is when the valves are off their seats, there is a pnenomenon called valve overlap breathing. At low engine speeds, when both the inlet and exhaust valves are open (and this value increases the larger the camshaft profile is, also in reducing lobe separation) the velocity of the air/fuel mix is easily reverted because a) the intake ports are often enlarged and because they are enlarged the air/fuel mix has little momentum and is easily pushed back out of the chamber by the rising piston. Then when its time to compress the mixture at low rpm's, there is less in there than will be found at higher speeds, when intake velocity increases, momentum increases and reversion reduces somewhat. To make up for this low engine speed related deficiency, the static compression is boosted in an effort to regain some low speed torque.

 

Also at low speeds, the throttle may be partially closed, increasing the vacuum in the manifold. This will also reduce the available 'fill' of air/fuel mixture the engine can ingest and alter the volumetric efficiency of the engine. This will effect your dynamic compression ratio to something less than your calculated compression value.

 

I had a friend of mine who built up a nice engine with 11.5 to one compression. The initial camshaft he chose was way too mild and it pinged like crazy regardless of what fuel he used. He began to back off ignition timing and saw huge losses in his power band so he was advised to go and try another bigger camshaft by an engine builder. He did that and found that the engine responded well to the new shaft and he was able to move his timing back to almost stock initial and total settings and the power was right there. He did have to use premium all the time but he was very happy with the end result.

 

I think this is what you might end up doing too. Don't get too bogged down with the 'figures' (duration this, lift that) because its what works on your engine at the end of the day that matters, your 'combination' like it did for my mate and his combo. The rules suggested by others may get you in the ball park and you may be lucky and get it right straight away. Then again, you might not. Like I said, be prepared to try another cam if the first one ain't right. Good luck mate.

 

Cheers.

[Xnke]

Great information guys, this was exactly what i was looking for. I was thinking 9.7:1 compression as a target, as I'll get this with a 40cc chamber and a Fel-pro gasket. Will 10:1 be controllable since I have zero quench in my piston/head configuration?

 

 

I like to run 87 or 89 around here, because the corvette crowd (made over on the other side of town from me, so there are LOTS here) runs the price for 93 octane waaaaay up in the summer. it's 60-80 cents more a gallon than 89, and they run out of 93 here a lot too in the summer.

 

I have an EDIS setup going in with Megajolt controlling it, and will be running a pair of modified SU carbs. (8mm flatted throttle shafts, instead of 10mm rounds, and eventually custom needles.) I figure that should get me good fueling and ignition timing control, so hopefully the PING with my NO QUENCH setup can be handled. I was wanting to go with an MN47 head with custom dished pistons, but just could not find a set of cast pistons with the right pin configureation, diameter, and a flat top to start out with.

[ozconnection]

Great information guys, this was exactly what i was looking for. I was thinking 9.7:1 compression as a target, as I'll get this with a 40cc chamber and a Fel-pro gasket. Will 10:1 be controllable since I have zero quench in my piston/head configuration?

 

 

I like to run 87 or 89 around here, because the corvette crowd (made over on the other side of town from me, so there are LOTS here) runs the price for 93 octane waaaaay up in the summer. it's 60-80 cents more a gallon than 89, and they run out of 93 here a lot too in the summer.

 

I have an EDIS setup going in with Megajolt controlling it, and will be running a pair of modified SU carbs. (8mm flatted throttle shafts, instead of 10mm rounds, and eventually custom needles.) I figure that should get me good fueling and ignition timing control, so hopefully the PING with my NO QUENCH setup can be handled. I was wanting to go with an MN47 head with custom dished pistons, but just could not find a set of cast pistons with the right pin configureation, diameter, and a flat top to start out with.

 

You guys seem to have a low octane rating compared to what we get down under. It makes me wonder if we're using the same values to measure our fuels??! For example, our regular unleaded fuel is 91-93 and our premium fuel is rated at 97-98 octane.

 

Is this engine your going to build used as a daily driver or just a weekend blaster? That fact alone would influence my decision as to how much compression the engine will have. If you build yourself a high comp. motor and can't find the right fuel easily to run it, what a pain and you'll have to carry additives with you just in case. That, of course is not such a big deal if you just take it out on weekends and stuff.

 

I touched on this point in my last post, that timing is often pulled out to stove off pinging. The real problem is too high a compression ratio. Yes, a bigger cam can and often does help this situation. However, the torque you lose at the lower half of the rev range using a bigger camshaft profile may become unacceptable to you. Just something to think about.

 

I'm about to add a head to my L28 which is currently stock with an N42/N42 combination. When the new head goes on, I'll have a verified 9.94 to one compression motor. Even with our high octane fuels, I plan to run a fairly conservative camshaft so I'll need to be aware of potential ping. I don't really want to run a bigger cam since my efforts in this case are to boost low down torque anyway. I want/need to be able to run full ignition advance, hence my fairly 'low' 10:1 compression.

 

I noticed that you've changed the rods in your engine. Obviously that incurs ar rod/stroke ratio variance. That has an effect on the way the piston travels through tdc and bdc. Based on that change, I'm assuming that you'll be running higher revs. Nice one. Another friend of mine built his L28 with even longer rare L14 rods and said the way the engine ran to the redline was very noticable compared to his stock L28 bottom end, with everything else being the same. You should have a nice engine when you're done!

 

Cheers.

[jt1]

Kevin Shasteen has posted some great info on this in the past.

 

jt

[ozconnection]

Kevin Shasteen has posted some great info on this in the past.

 

jt

 

Links?

[Xnke]

Yeah, premium fuel here is 93 octane, and even that is not always available. The car is to be a daily driver, at least for a while. The relatively large camshaft is due to it already being on hand, although I have several A stamp cams and one F stamp stock cam available as well.

 

I'm wondering if 9.7:1 should be a decent stopping point, I can always shave the head again later but it's much harder to put metal back on...doable, and I have done it, but it's much harder...

[ozconnection]

Yeah, premium fuel here is 93 octane, and even that is not always available. The car is to be a daily driver, at least for a while. The relatively large camshaft is due to it already being on hand, although I have several A stamp cams and one F stamp stock cam available as well.

 

I'm wondering if 9.7:1 should be a decent stopping point, I can always shave the head again later but it's much harder to put metal back on...doable, and I have done it, but it's much harder...

 

For a street engine at this point in time....9.7:1 is just perfect IMO!

[rsicard]

Ok, so it's time to get my head milled down to the chamber size I need. All the port work is done, and My N42 now has 46.5cc chambers. My pistons are 88mm diameter and have a 9.32cc dish. I have a stock L28 stroke, and L24 rods. I have a Delta .480" lift 284* duration camshaft. I do not offhand know the lobe separation angle, or lift at 0.050". My cam did NOT come with a cam card, oddly enough.

 

Now, I know from reading here that the stock N42/F54 block combinations run at about 10.3:1 compression, and ping on 93 octane. I also know that a big cam bleeds off static compression untill higher in the rev range, and needs a higher static ratio to keep the low end around.

 

How do I determine the compression ratio needed for the camshaft and piston/head configuration I have? Is there a magic constant that is used, say, for 260* duration, 9:1 is ok, for 270* duration, you need 9.5:1 or higher, for 280* duration you gotta have 10:1 or higher, etc?

Xnke: The first thing to consider for compression ratio and reducing detonation is the configuration of the piston and cylinder head combustion chamber. The best way to reduce detonation and increase power is to have a piston with a quench pad or flat top. The combustion chamber should match up with the piston to promote adequate quench/squish. Then the proximity of the quench pad or top of piston to the quench pad of the cylinder head is VERY important to reduce detonation. A space of .030" between piston quench pad and flat portion of the cylinder head yields a great detonation reducer. A bare aluminum cylinder head can be run one point of compression over a cast iron head. The aluminum head absords heat faster than cast iron.

 

After these two configurations and compression ratio are set, then select the appropriate camshaft.

[ozconnection]

Xnke: The first thing to consider for compression ratio and reducing detonation is the configuration of the piston and cylinder head combustion chamber. The best way to reduce detonation and increase power is to have a piston with a quench pad or flat top. The combustion chamber should match up with the piston to promote adequate quench/squish. Then the proximity of the quench pad or top of piston to the quench pad of the cylinder head is VERY important to reduce detonation. A space of .030" between piston quench pad and flat portion of the cylinder head yields a great detonation reducer. A bare aluminum cylinder head can be run one point of compression over a cast iron head. The aluminum head absords heat faster than cast iron.

 

After these two configurations and compression ratio are set, then select the appropriate camshaft.

 

This is of course the theoretical optimal configuration we would all lust after but Xnke doesn't have these parts to play with if that's my understanding of what I read in his first post (F54 block with dished pistons and a 'soon to be milled' N42 cylinder head.)

[kiwi303]

You guys seem to have a low octane rating compared to what we get down under. It makes me wonder if we're using the same values to measure our fuels??! For example, our regular unleaded fuel is 91-93 and our premium fuel is rated at 97-98 octane.

 

Cheers.

 

Yep, NZ and Aust use RON, and the US uses SON or something I think. the ON stand for Octane Number, but the bit at the front means different things.

 

NZ 91 equates to a US 88 I think (from what I have read in an AA article about taking your car travelling on a carnet du passage) so their 87 octane is still slightly under NZ base fuel octane. We have 91, 95 and 98 here in NZ.

[Zmanco]

Xnke, I have that same regrind. I didn't receive a cam card either, but over the phone they said it was 234 duration at 0.050" lift. I'm pretty sure the lobe separation is 108 degrees. FYI, they assume a rocker ratio of 1.5:1. The actual lift I measured on mine was closer to 0.400".

 

I ran that cam in an N42 head on an L28 with flat top pistons bored 0.030" over and it was a really fun combination for both the street and track. It idles well, handles low rpm traffic, yet makes good power past 6k rpm. I did experience detonation in the 4-5k rpm range and had to reduce timing significantly to cure it. This was on premium pump gas and at elevation (6k ft). My point is that some people are able to run this combo with full advance, and some are not.

 

I use MS to control timing, so can adjust it for different rpm ranges. I was surprised by how much torque goes away when the timing is retarded say 10 degrees. As the revs climbed and hit the range where I had backed it off, my seat of the pants could feel the torque fall off. There was a thread a few years ago I started on this subject and I think it was BRAAP who said he'd prefer a slightly lower CR in order to run full advance vs. the opposite, and I now completely agree.

[Pyro]

the reason that overlap doesn't effect cylinder pressure is the 4 stroke engine isn't trying to build cylinder pressure during that part of the cycle (over lap period). It is during the compression stroke that cylinder pressure is made and the exhaust valve is long closed by then. However, the intake valve can be still open during the compression stroke, even as the piston is rising in the bore. This is especially true in long duration cams. so the amount of stroke the piston actual gets to compressed is reduce if the intake valve is still open.

 

overlap does effect the idle quality (vacuum) and does help the intake fill the cylinders by way of exhaust scavaging.

[johnc]

Isn't this a little backwards? Wouldn't you pick a comparison ratio/horsepower target and the choose the cam to match?

[Xnke]

Yes, ideally you would. At the time, I was dead set on a shaved P79 with 1mm over flat top pistons, until my P79 head was declared trash by inspection and my block needed a 2mm bore to clean out the rust that the PO had allowed to collect in the bores. (I bought it understanding that it had been recently bored and honed to 87mm, but never had been used. I didn't know it was bored probably two years prior, and never oiled.)

 

After that, finding a piston that wasn't 600$ for a set and a head that didn't have the water jackets so badly eaten up that the jacket leaked into the ports or didn't seal to the block was so hard I said screw it, put the cam back on the shelf, bought a set of ITM 1mm oversize Z22S pistons, used a set of L24 rods and a lightened and balanced L28 crank, and bought a running condition N42 head off a local to me member here. I'm just trying to get this engine back together with a minimum of problems and a maximum of fun-daily-driver.

 

Zmanco, is it really that over-sold? I bought it because of the .480" lift! I figured the extra lift would help it breath while keeping the duration short enough that I wouldn't have too many idle problems. On the other hand, will it work with the stock springs and retainers? I have the Fel-pro Ford valvle seals, so really I'm asking about coil bind issues. I suppose I could set up the head with the cam in it and check, but if it doesn't work with the stock springs, and it's oversold by .080", I'll send it back and they will either refund my money or regrind it to a true .480" lift.

 

Looks like I'll be setting up for a 9.7:1 compression ratio, and since I'm using Megajolt and EDIS6 for timing control, I'll just deal with any detonation as I get there.

[Zmanco]

Please disregard the numbers I gave you earlier. I went back and found my notes and here are the numbers:

 

Max height across lobe peak and base circle: 1.550"

Width across "sides" of lobe (caliper rotated 90 degrees): 1.250"

 

Cam lift = 1.550 - 1.250 = 0.300".

 

Assuming a rocker ratio of 1.43 (based on stock Nissan cam numbers), valve lift is 0.300 x 1.43 = 0.429"

 

I suspect they are using a rocker ratio of 1.6 which would then predict a lift of 0.480".

 

Again, sorry for the mistake in my numbers before. I shouldn't have gone from memory.

[Xnke]

A stock "A grind" cam lift is .397". I wish that cam companies would at least measure the things something close to standard, not picking some exotic rocker arm ratio, or custom lifter, etc.

 

Makes me wonder how the MSA cams are measured...if they are at least using stock nissan rocker arms, they would be better than this.

 

 

EDIT:

 

I went to Schneider Racing Cams website and apparently, the MSA camshafts are designed for a 1.48 rocker arm ratio. The closest Schneider cam grind is the 274F, with a 0.480" lift and 274 duration (215 duration at 0.050"), on a 107 degree lobe center.

 

I emailed Delta Camshaft to confirm the lift and duration and get the right information as to rocker ratio that they use. If they are really designing for a 1.6 ratio rocker, then they need to say so, and we as possible customers need to know such. I'll probably call all the other common cam regrinders that get talked up on HybridZ and get that information as well, this kind of stuff can make or break a purchase desicion.