Suggestions for advancing timing?

[Cinnabar325is]

So I'm working on optimizing the power from my recently built 2.8 stroker (M52 crank). The camshaft is a Cat Cams 298, which I know is huge, but I haven't found it to be as bad as I would have expected on the street. The car idles well (though nice and lumpy!) and doesn't stall ever. Either way, the cam came with the head, so I figured I'd give it a whirl.

Now, the rockers and valve springs are OEM parts, not upgraded. I'm certain the cam would make good power beyond 7k RPM, but I'm afraid to rev the valvetrain beyond 6500, at which point the cam is making boatloads of power.

So, I figured it might be worth a shot to install an adjustable cam gear and see if advancing the timing a few degrees would move the powerband down in the rev range a bit, thus allowing me to access more of the power band. Again, I think I'm leaving a lot on the table above 6500rpm.

Does anyone have any suggestion on how many degrees advanced I should start out with? I know this is best done on a dyno and I do plan to take that step with the WAR chip, but I'm just curious for any preliminary ideas.

Thanks.

[digger]

i'd be worried about the valves smashing into the pistons and that is first thing to check, and don't adjust anything without advise from the dyno otherwise how would you know if you did something positive

[Cinnabar325is]

Dyno time is in the works, that's for certain. In the meantime, I would try to advance a few degrees and see how the butt dyno feels about it.

Am I correct in my belief that advancing the timing will shift the power band down in the rev range?

I would check to ensure there's no valve/piston interference by turning the engine a few times by hand. That should do the trick I would think, unless I over-revved it and floated a valve.

[MR 325]

That engine should easily rev and make power to 7000. Why didn't you at least upgrade the rockers when you built it though?

I personally don't like adjustable cam gears. digger is right though, with that big of a cam you've got to be careful with changing the timing.

[petrolhead]

So I'm working on optimizing the power from my recently built 2.8 stroker (M52 crank). The camshaft is a Cat Cams 298, which I know is huge, but I haven't found it to be as bad as I would have expected on the street. The car idles well (though nice and lumpy!) and doesn't stall ever. Either way, the cam came with the head, so I figured I'd give it a whirl.

Now, the rockers and valve springs are OEM parts, not upgraded. I'm certain the cam would make good power beyond 7k RPM, but I'm afraid to rev the valvetrain beyond 6500, at which point the cam is making boatloads of power.

So, I figured it might be worth a shot to install an adjustable cam gear and see if advancing the timing a few degrees would move the powerband down in the rev range a bit, thus allowing me to access more of the power band. Again, I think I'm leaving a lot on the table above 6500rpm.

Does anyone have any suggestion on how many degrees advanced I should start out with? I know this is best done on a dyno and I do plan to take that step with the WAR chip, but I'm just curious for any preliminary ideas.

Thanks.

You would be effectively down-tuning the engine.. What's stopping you from replacing the valve springs? Does the cam manufacturer recommend a set? This is not the first time someone writes that they have a hot cam with good power just where they run out of rev range. Do you KNOW that the rockers give up at 7000 rpm? 500 rpm is 9% more , anyways..
There's very few things a NA engine makes it's power from, revs being the second most important thing..

I personally don't like adjustable cam gears.

It takes some tools and understanding about timing, but I'd say it's a very important component. Without one , you won't be able to even get the timing to "zero" so to speak. Let alone be able to use timing changes as a tuning tool/method.

Sorry if the above is gibberish , my technical english isn't all that great.

[digger]

I use the catcam 298 on inlet and 285 on exhaust on my 3.1L but use VAC springs and MM peened and inspected rockers. On a smaller engine and bigger exhaust lobe will make power band more biased to the topend. On the otherhand the inlet and exhaust plumbing may counter this and i suspect the engine would be over cammed.

If all you are going to do is rotate by hand as a means to check then do this after you have advanced the cam by a larger amount that you will actually run and do it with zero lash to give some more tolerance. Then wehne you set the clearnace proper then you have some margin. It is no guarantee against doing the proper checks though and there are risks involved....you can muster up a tool (needs to give leverage) that can depress the valve down against spring at various points around TDC to give some indication of clearnace. If advancing the cam the inlet valve will get closer to the piston.

i went with the 298/285 because i was afraid of the piston to valve clearance which is compromise that i never would like to make.

It is hard to say what advancing will do, most likely it will make more in the low and middle but fall over a bit quicker on the topend but that is just general talk. it might make no difference or just give more everywhere

[Cinnabar325is]

That engine should easily rev and make power to 7000. Why didn't you at least upgrade the rockers when you built it though?

I personally don't like adjustable cam gears. digger is right though, with that big of a cam you've got to be careful with changing the timing.

When I built it back in April/May, I was completely new to E30s and M20s in general and really had no idea how the cam or the motor would run and I didn't yet know how much the rockers were a weak point. In hindsight, I should have upgraded the rockers and springs and I will in the future. Hand in hand with replacing/modifying the head, which was supposedly resurfaced by the PO only 8k miles ago, but is not quite flat judging by the seeping headgasket. Actually, your cylinder heads from Myster-e are probably where I'll look.

You would be effectively down-tuning the engine.. What's stopping you from replacing the valve springs? Does the cam manufacturer recommend a set? This is not the first time someone writes that they have a hot cam with good power just where they run out of rev range. Do you KNOW that the rockers give up at 7000 rpm? 500 rpm is 9% more , anyways..
There's very few things a NA engine makes it's power from, revs being the second most important thing..

I'm sure that Cat recommends HD springs for this cam. They are on my to-do list. I can't guarantee a rocker arm might break at 7k rpm, but I don't want to take that chance, so I've been limiting myself to 6500.

I use the catcam 298 on inlet and 285 on exhaust on my 3.1L but use VAC springs and MM peened and inspected rockers. On a smaller engine and bigger exhaust lobe will make power band more biased to the topend. On the otherhand the inlet and exhaust plumbing may counter this and i suspect the engine would be over cammed.

If all you are going to do is rotate by hand as a means to check then do this after you have advanced the cam by a larger amount that you will actually run and do it with zero lash to give some more tolerance. Then wehne you set the clearnace proper then you have some margin. It is no guarantee against doing the proper checks though and there are risks involved....you can muster up a tool (needs to give leverage) that can depress the valve down against spring at various points around TDC to give some indication of clearnace. If advancing the cam the inlet valve will get closer to the piston.

i went with the 298/285 because i was afraid of the piston to valve clearance which is compromise that i never would like to make.

It is hard to say what advancing will do, most likely it will make more in the low and middle but fall over a bit quicker on the topend but that is just general talk. it might make no difference or just give more everywhere

Good information, thank you. Intake plumbing is a Miller MAF kit, with standard M20 manifold and exhaust is a long tube header to full 2.5" mandrel system.

I think I might test it as you said at 5 degrees advanced and see what happens.

[jlevie]

On a stock M20B25 engine you can be about two teeth off in either direction before the valves start kissing the pistons. With a stock cam and a head that has been milled the maximum amount (to raise CR) valves will kiss the pistons at about 1 & 1/2 teeth off. A big cam is going to reduce this margin.

There is a way to find out how much you can advance or retard cam timing before you get into the danger zone. That is done by removing the head and putting modeling clay over the valve cuts in the pistons. Then set set the cam timing and install the head. Lightly oil the valve faces, so they won't stick to the clay. Use an old gasket (already crushed) and old head bolts at 30-40ft-lb. Turn the engine over through two revolutions, remove the head and measure the thickness of the depressions in the clay.

My preference over an adjustable cam gear is an offset bushing in the cam sprocket. Use an adjustable gear to find the sweet spot on a dyno and then fab up an offset bushing to go into an enlarged index hole in the cam sprocket that results in that cam timing desired. That makes for a fixed timing that can't change.

[Cinnabar325is]

More good information, thank you. I do gain a little ground back from the installation of the cam because my stack height is reduced by a 0.5 mm from a standard B25. I gained 9mm stroke with the crank (half being 4.5mm) but I'm using 5mm shorter eta rods.

I have to go look at the cam gear and see how many degrees is equal to one tooth.

[clarkson]

I am using weak test springs on #1 to check how much clearance I have between valve and piston. These will allow me to see how far I can adjust the cam sprocket from tdc.

I am building the same motor, 2.8 with 130mm rods. I am using a Dbilas 276 cam. Hopefully I can ad some pics of the test springs today.

[Cinnabar325is]

That's a good idea, I'd like to hear what you come up with and see those pics.

Yesterday, I was running short on time, but I quick threw the new timing belt, tensioner, and cam gear on and adjusted the gear to 5 degrees advanced. I rotated the motor over by hand a few times with no interference. I'm going to play with it some more as suggested above before feeling safe, but that was a good first step.

[jlevie]

That's a good idea, I'd like to hear what you come up with and see those pics.

Yesterday, I was running short on time, but I quick threw the new timing belt, tensioner, and cam gear on and adjusted the gear to 5 degrees advanced. I rotated the motor over by hand a few times with no interference. I'm going to play with it some more as suggested above before feeling safe, but that was a good first step.

Rotating the engine over by hand and not feeling interference does not mean that valves won't be kissing pistons at higher engine speed. The valves will float.

[Cinnabar325is]

Okay, the adjustable gear I'm using allows about 6 degrees advance and 10 degrees retarded. I moved it to 5 degrees, removed the plugs (for ease of rotation), and checked for tight spots/contact. There was one spot in the rotation that was just a bit tight, I'm thinking that a valve was just touching a piston there? I could rotate through it, however, with little effort.

I then adjusted to 10 degrees retarded and tried again. There were more tight spots this time and one in particular that I was afraid to force through, so I did not.

Back at 0 degrees, there were no tight spots of any kind. I moved to 3 degrees advanced (which is less than half a tooth difference: one tooth is ~7 degrees) and felt no tight spots.

I then cranked the motor over a few times with no plugs in. Again, no collisions.

I'm thinking I'll give it a shot a 3 degrees advanced and see what happens. Again, this is less than half a tooth advanced over normal.

[whodwho]

This all sounds a bit dangerous for the longevity of this project but since you are in the middle of it and about to run on it...

Since you are wanting advanced, advance back to full to where you found that tight spot, note which valve is open retard back to 3* and try to manually open the valve till it touches again and see how much it can move. A dial indicator is probably really needed here to know.

Just remember it doesn't take much to bend a valve when leverage ;)

[jlevie]

With a stock cam 3deg would certainly be safe. But with the cam you have I know of no way to judge the safety that doesn't involve direct measurement of the clearance.

You could try it and see, but understand that if the clearance isn't sufficient to account for valve float the first sign of trouble could result in bent valves (or even swallowing a valve).