Compression too high?

[burntz]

Hi guys. NewB here. I did some searching but couldn't find anything relevant. I hope someone can help me.

 

I'm building a L28 with the flat top pistions, the pistons protrude above the block deck by .022. The head I'm using is a N42 that has been shaved a bit. Combustion chamber CC's average 38.6. Now if I use a Felpro gasket and a .020 head shim my compression comes to about 11:1. All the engine builders here in my area are V-8 guys and don't have a clue when I say Nissan. Am I asking for trouble with this compression ratio? The car won't be a daily driver, more of a taking her out on the weekend weather permitting or going to a car show. I do intend on using a MSD ignition and either a stage II or stage III cam. The engine will be carbed by SU's.

 

Thanks in advance.

[v80z]

My ex race car ran 13.5:1 with venolia forged pistons.

[Guest iskone]

Your fine.

Why even bother with a stage 2 if it's only for the weekend? My daily(well sunny days) has a .580/278.

 

Isk

[burntz]

Isk, if you don't mind me asking what engine set up do you have?What octane should I expect to use with this set up?

[Mack]

11:1 is fine. Ive been running it on my daily driver for about 9 months now. Its a stock short block with flat tops and Ive got a Maxima N47 on it, by my calculations its good for about 11:1. It has 40cc combustion chambers and a felpro gasket. I run premium gas on it and have no problems, except with the stock injection system. I get ping ONLY after the AFM maxes out at 3.5K RPM, otherwise, I couls stick it in 5th at 20mph, floor it and not get ping. It makes for a lively motor!

[Nismo280zEd]

I thought 10.5:1 Is the highest you could run on pump gas?

-Ed

[JMortensen]

A bigger cam will let you run more compression, as will a better chamber design. F54 with flat tops and an N42 is about 10.5:1 and several people have complained about pinging. My L28/E31 with about 11:1 requires 95 octane. I'm sure Isk's HUGE cam helps him run pump gas, but IIRC he was having some pinging issues too. I think as a general rule Ed is right. 10.5:1 seems to be about the limit of pump gas, and is really pushing the limit in places like CA where 91 octane crap is the best we can get from the pump.

[Nismo280zEd]

I didn't realize the larger cam duration and lift could allow you to run a little more compression, I'll have to remember that.

Makes sense now that I think about it.

 

-Ed

[burntz]

O.K. so let me see if I've got all this correctly.

 

With 11:1 compression I will need to:

 

1. Run 95 octane or better

2. Use a 160 degree thermostat

3. Use a big cam (270-280 lift)

4. Run minimal timing advance

5. Upgrade to a MSD ignition or similar

6. Run the carbs on the rich side.

7. Stay away from city stop & go driving

 

What if I ran a second head gasket? It would be two Felpro head gaskets with the head shim in the middle. That would increase my chamber volume by 7.37cc's reducing my compression ratio to 9.65:1.

Do you think this would hold up? Or would there me too much friction between the block and the head causing the gaskets to fail?

 

Thanks

[JMortensen]

There really are no absolute answers on this one. My engine runs 11:1 with a .490/280 cam and needs 95 octane. I also have the chambers polished and there are no sharp edges. Backing off the timing really hurts power, you'd be better off IMO to run less compression and more timing. I don't think MSD is going to help. We're talking about preventing pre-ignition, which is ignition that happens before the spark plugs fire. Carbs on the rich side does help somewhat, but also loses power. Stop and go didn't seem to affect my motor too much.

 

A standard thickness head gasket will drop the compression a tiny bit. A Nissan gasket is already .050" instead of Falure-Prone's .040" thick. If that's not enough I'd suggest the HKS gasket and you can peel off layers until you hit the thickness you want. I've heard of people stacking headgaskets before, so that might work, but you could actually get the thickness you want within .010 or .020 with the HKS.

 

You might have to shim the cam towers and play with the lash pads too, depending on what has been done to the head. Just making sure you were aware of that situation.

[Guest iskone]

O.K. so let me see if I've got all this correctly.

 

With 11:1 compression I will need to:

 

1. Run 95 octane or better

2. Use a 160 degree thermostat

3. Use a big cam (270-280 lift)

4. Run minimal timing advance

5. Upgrade to a MSD ignition or similar

6. Run the carbs on the rich side.

7. Stay away from city stop & go driving

 

What if I ran a second head gasket? It would be two Felpro head gaskets with the head shim in the middle. That would increase my chamber volume by 7.37cc's reducing my compression ratio to 9.65:1.

Do you think this would hold up? Or would there me too much friction between the block and the head causing the gaskets to fail?

 

Thanks

 

First My setup:

 

N42 1mm over

E88 Ported/Polished/Milled I don't have the chamber cc's yet.

Forged domed pistons Don't have specific dome yet. I was told by the the PO the "pistons" are 11.5-1. I think the whole combo is 11.5-1 more like 11.7-1.

.580/278

35* total advanced timing

I will have the motor back together soon and much more acurate specs at that time' date=' I can pass them along.

 

Your to do list for 11-1

 

1. I don't know how good the gas is there but I think 92 would work.

2. Cooler the better (but not too cool)

3. Big cam 270-280 DURATION

4. Run as much timing as possible

5. Good plan but not nessicery for high CR

6. It's better to run rich then lean if you don't want to break anthing but get them tuned right in the first place and you won't have to run on the safe side.

7. Drive hard drive fast every minute of your life!!!!

 

Yes a big came bleeds off compression and vacum.

 

When it comes to aluminum heads you can generally run 1 pont more CR than with iron. Combustion chamber design is a key factor as with cooling.

 

My came is tiny at a meare .580/278 when I drop in the .511/296 or maybe the .620/306 then we can talk [b']HUGE!!!! [/b]MUUUHAHAHAHA :twisted:

 

BTW. I might be selling an E88 that flows 200cfm has the 296 cam installed and has a fresh rebuild, got any interested parties??? I want big brakes.

 

Isk

[Mack]

O.K. so let me see if I've got all this correctly.

 

With 11:1 compression I will need to:

 

1. Run 95 octane or better

2. Use a 160 degree thermostat

3. Use a big cam (270-280 lift)

4. Run minimal timing advance

5. Upgrade to a MSD ignition or similar

6. Run the carbs on the rich side.

7. Stay away from city stop & go driving

 

What if I ran a second head gasket? It would be two Felpro head gaskets with the head shim in the middle. That would increase my chamber volume by 7.37cc's reducing my compression ratio to 9.65:1.

Do you think this would hold up? Or would there me too much friction between the block and the head causing the gaskets to fail?

 

Thanks

 

1. I run on 92 octane and only get ping when my EFI system decides the AFM is maxed out and goes on its little preset curve thingy. I can tune it out by plumbing a pot inline with the coolant temp sensor and making the engine run richer.

 

2. I use whatever NAPA is selling, I think it was a 170*

 

3. Ive got a stock 260Z cam. same lift as all the rest, but 256 degrees duration on both intake and exhaust.

 

4. I have my advance set to factory specs.

 

5. Im running factory 280ZX ignition.

 

6. I run factory EFI, unless Im racing or otherwise, then I can always put the POT in.

 

7. Are you kidding? I deliver pizzas with my car! it runs JUST fine!

 

and I used the felpro gasket and I have never had any problems with it. Ive probably put about 10K on my car. I really need megasquirt, that would help a lot the "ping" problems. the stock EFI REALLY sucks.

[Dan Baldwin]

11:1 is pushing it without a huge cam. I'm running 11:1 on 93 pump, but with a ~310deg/.550" cam. If you're really at 11:1 with a Felpro + .020 shim, then a 2mm HKS gasket would put you at 10.5:1, a bit more realistic. I dunno of two Felpro's is a good idea. Didn't Norm try that once?

[Dan Baldwin]

Regarding timing, I always seem to hear the "more is better"/"run as much as you can" approaches being preached. In my *limited* experience, you can run the timing too advanced and lose a lot of power even with no pinging. On more than one occasion I've gone to the dyno with too much advance and made more power by backing it off. My philosophy is to back it off until it starts to hurt power. I've found my engine makes the same power from ~34 -38deg total advance, and loses power on either side of that range. So I keep it set to 34/35 total.

[JMortensen]

Good point Dan, but I think the issue that was being discussed is running lower than optimal timing to prevent pinging. I'd rather get the octane needed along with the optimal timing to prevent the pinging than run less timing than optimal to prevent the pinging. There is quite a bit of power in the timing. When I first got my engine together I tried to run it on 91 octane and it pinged pretty bad. I was able to cut down the timing to about 25* total and it would **almost** not ping, but it was really down on power compared to upping the octane and advancing the timing.

[clarkspeed]

As stated earlier, polishing the chambers and breaking sharp edges helps. Some octane boost also, if your right on the edge of pinging.

[Guest iskone]

11:1 is pushing it without a huge cam. I'm running 11:1 on 93 pump, but with a ~310deg/.550" cam. If you're really at 11:1 with a Felpro + .020 shim, then a 2mm HKS gasket would put you at 10.5:1, a bit more realistic. I dunno of two Felpro's is a good idea. Didn't Norm try that once?

 

Damn!!! That cam is crazy!!! Is that a track car?

 

Isk

[Dan Baldwin]

JMort,

"Backing off the timing really hurts power,..."

This isn't always true, and for a lot of Z's I bet the OPPOSITE is true!

I know YOU know what you mean here (backing off timing from OPTIMAL hurts power), but a lot of folks will read this and assume more advanced timing is always better. I once ran a stock 240Z distributor at 18 deg initial advance because I thought it was "better". It may have been better down low, but above 3000 rpm I was LOSING power because the max advance was up around 43deg! Anyway, I just like to pipe in to let people know that the idea is to set timing for maximum power, and not to just set it as high as they dare, or advance it 'til they get ping and then back it off a smidge.

 

Anyway, sometimes backing off timing HELPS power, and advancing it HURTS (which you know, I know!).

 

 

Iskone,

Come to think of it, the cam may be 305 rather than 310 duration. And that's at zero lift, not .050". Anyway, yes it has become something of a dedicated track car. About the only street driving I do with it is to and from the track. I do go to tracks hundreds of miles away, though.

[Guest iskone]

Well as far as my timing goes 35* is the intial & total all in one. basicly I set it for 35 and it stays there. I might be taking it apart today so keep an eye out for my up coming post.

 

Here is a sneak peak.

 

 

[Mack]

Daaaaaaaaaamn! what ignition system is that?!? lol trick set-up!

 

as far as how I got my compression number ..... Maxima N47 = 40cc ombustion chamber....

 

so, stock L28......43mm x 43mm x 3.14159 x 79mm = 458.9cc's.

 

compression volume... (44mm x 44mm x 3.14159 x .8mm(felpro gasket)) + 40cc = 44.865cc's

 

so, (458.9cc's + 44.865cc's) / 44.865cc's = 11.22:1 compression! damn, higher than I thought, HA ha!

 

 

is my math off anywhere? I don't think so. The chamber shape really helps with the P79/P90/Maxima N47 its that really nice heart shape. I did not do any chamber massaging, I did do a very mild port job on it tho. If I had a better FI system, Im sure I could get rid of the ping all together. it just sucks that the stock AFM maxes out at 3500RPM. time for a Z31 or MAP based system.