Do I raise my compression ratio?

[ozconnection]

Just curious about this scenario and would like your input please.

 

I did a hot compression test yesterday on the L28 and the average value was 179.6 psi. There is only a 5% variance amongst all the cylinders. This is a stock N42/N42 combination with a stock Nissan headgasket and camshaft.

 

I recently ordered a Kameari 0.8mm headgasket to bump my compression up from 8.3 to 8.7 to one.

 

My question is whether this mild increase in compression will throw my engine into severe detonation? At 38 degrees total advance at the moment, I can still get some flash detonation with rapid acceleration (but this is usually dependant on how hot the day is).

 

I'm hoping to gain some more torque without having to drop the timing down heaps (below 30 degrees for example), which would defeat the whole purpose of the upgrade.

 

What do you think?

[Drax240z]

You should be able to manage that modest compression ratio increase by running higher octane fuel.

[ozconnection]

You should be able to manage that modest compression ratio increase by running higher octane fuel.

 

LOL, I already am!

 

But thanks mate.

[Drax240z]

What octane fuel do you have available to you?

[ozconnection]

What octane fuel do you have available to you?

 

Premium unleaded pump gas in Oz is supposed to be 98 octane. That's what the label on the bowser says!

[Drax240z]

I'm surprised to hear you are having issues then, though I realize it's pretty darn hot there in the summer. It might be the speed in which your advance is coming in, rather than just the total amount of advance. Dropping it to 36° total advance might give you the little space you need.

 

A few other questions:

1) What sparkplugs are you running?

2) Is your cooling system in good shape?

3) What is your coolant mix?

 

The setup you are moving to has been done successfully numerous times without detonation on lower octane fuel than you are running.

[ozconnection]

My plugs are one heat range colder, BP7ES NGK's. 280zx dizzy with MSD 6 ignition.

 

My cooling system has been a topic of discussion lately. To sum up, I'm running a modified 3 core radiator, new 180 degree thermostat, water with water wetter, thermostatically controlled electric fan with factory shroud, LD28 water pump and a 20 psi cap. Sorted as far as I can tell.

 

I was pushing the timing issue and wanted to see how far I could go with some minor upgrades to the cooling system etc. I do believe I've engineered enough headroom to accomodate the slight increase in compression, but as we all know, I like to proceed with some caution, especially since I'm already seeing excellent cylinder pressures with an 8.3 compression (stock) motor which BTW has seen years of abuse (over 120 dyno pulls) and multiple interstate hauls and the usual weekend thrashing.

 

What would I expect to see cylinder pressure wise after changing the HG? 190psi? Is this too much for the old L28 and am I just asking for trouble?

[z-ya]

What is your max advance? If you are having detonation problems, you are probably just running too much advance.

 

The problem with the Kameari 0.8mm head gasket is that if it detonates with that gasket, it won't blow. You will probably break ring lands instead. If you want a stock-like thinner head gasket, use a Felpro (1mm). In general, bumping the CR up by 0.4 isn't going to make much difference in power. If you can get 98 octane at the pump, I would swap in flat top pistons to bump the CR to 10:1. This will get you at least 50WHP.

Edited April 5, 2010 by z-ya

[Drax240z]

Except he's having detonation problems at 8.3:1... 10:1 isn't going to solve those issues.

 

Cam specs? Is it installed "straight up" or advanced/retarded slightly?

[letitsnow]

Remember that 98 in Oz isn't the same as 98 here.

[Drax240z]

Even 98 RON shouldn't pose an issue, that's roughly 93 octane (R+M)/2... 8.8:1 CR should be fine on that with everything else in line.

[hoov100]

Whats wrong with taking a few degree's of spark out of it? for the thermostatically controlled fan, you might want to toss it on a switch, as your allowing the engine to get hot and heat soak, before the fan turns on.

[ozconnection]

Just to recap, because a few of you guys are asking questions about stuff I've already mentioned.

 

38 degrees total timing all in by 3500rpm. (mechanical advance) 18 degrees static timing with 16 degrees in the vacuum advance mechanism. This is connected directly to manifold vacuum.

 

The cam is a stock grind. 240 degrees intake with 248 degrees exhaust duration. Very mild. Camshaft is advanced to 8 crankshaft degrees (4 camshaft degrees).

 

So many times guys have posted their compression pressures and they're around 130-140 psi. The comment made is that that's alright. So, is my stock cam causing such a high reading (average reading in my L28 is 180 psi) or the fact that the stock cam is so far advanced?

 

So this lends itself to the next question, does the cranking compression vary when the cam is advanced or retarded? Is this the reason that advancing the cam helps with low end torque by improving the compression pressures?

 

Increasing the compression ratio by changing HG's may send the compression pressures to over, say, 190 psi. Perhaps by reducing the advance on the camshaft, peak pressures may be lower but the overall spread of compression pressures may be wider/over a greater rpm range. What do you guys think??

 

We know what happens when we put a bigger cam into an engine but can we do the same with an existing cam? Have any of you guys with pinging engines retarded the camshaft a little to reduce detonation? Does it work?

[bjhines]

I'm just going to throw this out there. Try a different distributor... I had this problem with a mild compression L24 and found that the distributor advance mech was trashed. I swaped to a different distributor and the problems ceased. I believe that someone had "fixed" the advance mech before I got it. It was randomly jumping timing around 3000 RPM.

 

You could try stiffer springs in your dist if the advance mech checks out.

[kolonelklink87]

I'm just going to throw this out there. Try a different distributor... I had this problem with a mild compression L24 and found that the distributor advance mech was trashed. I swaped to a different distributor and the problems ceased. I believe that someone had "fixed" the advance mech before I got it. It was randomly jumping timing around 3000 RPM.

 

You could try stiffer springs in your dist if the advance mech checks out.

 

If you go in this direction I still have the electric dizzy from the l24 we picked up. It should be yours anyway... (?)

 

-pete

[Drax240z]

I think you nailed it on the head with the cam timing. Any plans for a cam with a bit more overlap in your future?

[z-ya]

I have seen turbo motors (7.5:1) make 180psi across all cylinders, so I'm not convinced it is cam timing.

 

bjhines might be on to something. I had a advance spring in the distributor break, and it caused similar symptoms. Remove the breaker plate and look at the two advance springs. Make sure they are intact.

 

Did you measure the 38 degrees of advance with a timing light, or are you just estimating using the idle timing?

 

Pete

[Leon]

I think you nailed it on the head with the cam timing. Any plans for a cam with a bit more overlap in your future?

 

I also agree with oz's assertion that it's the cam. The 280Z's cam is already designed for low end torque, and by advancing it you are shooting those cylinder pressures even higher. I think retarding the cam a bit, or going with a longer duration cam should bleed off the pressure enough to prevent knock while holding the timing where it is.

Edited April 7, 2010 by Leon

[olie05]

My engine (10:1cr) was doing about 215psi with the stock camshaft and flat tops.

 

After I installed the cam it came down to about 200-205.

 

Also, 38 degrees total advance seems a bit much. I run on the conservative side. 28 degrees at full throttle, 37 degrees at cruise/light load.

 

I also run 93 (R+M)/2 octane. With no detonation issues.

 

I hope some of those figures help you decide whats up with your engine.

 

What I would do:

1) verify ignition timing

2) check camshaft timing.

 

#2 is much harder to do once the cam sprocket has been in place for a while, but possible to do with a degree wheel. Might be easier to just do the timing chain+tensioner+sprockets at this time to get everything right.

 

As others have mentioned: it sounds like the last thing you need to do right now is to raise your compression ratio.

[ozconnection]

Thank you gentlemen for all your inputs so far.

 

My timing is verified by an Innova timing light, one of those fancy jobbies that tell rpms, dwell and volts along with the actual number of degrees advanced. Lovely bit of gear!

 

With this timing light I could easily build up a timing map of degrees advanced versus engine rpms. I could then test another dizzy (thanks Pete, I'll take that L24 unit ) and do the same with that one. I can look at the character of the curve and test both in car to decide which one responds better.

 

As for the cam, I could try retarding it a bit, and test that. I have so many cams here it's silly, all bought for projects I haven't built yet. I really need to get a wriggle on and build an engine with a decent static compression ratio so I can try some of 'em.

 

I can't believe that 10 to 1 compression with a stock cam gave you 215 psi cylinder pressures at 28 degrees advance without ping. Was that a flat top, stock piston with with a stock N42 head? (All bets are off if the head was modified though).

 

As for my camshaft drive, it was all replaced when I first bought the car some 40K km's ago so I'm happy it's still all in good, serviceable condition.

 

As for L28ET's running 180 psi cylinder pressures, their camshaft is so mild, isn't it just like the one I'm currently using? Just how do you get a decompressed 7.5:1 engine like that to run 180 psi though?

 

I just stopped to think about this situation again and came up with "the" question I guess I'm trying to ask. What is the highest static compression ratio I can go with a stock profiled camshaft? Caveats are pump gas, no turbo or super charging and no nitrous. (have I just spoilt the party for any of you? )

 

And not more than 30 mins ago I got my Kameari 0.8mm HG. Oh, its beautiful

 

Thanks guys!

Edited April 7, 2010 by ozconnection