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Discussion Starter · #1 ·
I am curious what compression ratios people are getting away with in 'L-6' engines w/o destroying parts. I am on my last chore engine build before starting my 3.1L project. I am ready to choose pistons and head.

Some new vehicles and motorcycles are getting away with impressive compression ratios but these have more modern (high turbulence) combustion chambers and in the case of the cars electronic ignition control with nock sensors so I don't think these are valid indicators of what is possible with in an L series engine.

I have E31, N47 and P79 heads but may look at purhasing a P90. Don't want to spend $ to get big valves in E31, don't like liners in N47 and P79.

CR from 9 to one all the way up to 12 to one are avail. w/ 1mm or 2mm head gaskets, flat or dished pistons and various cumbustion chamber volumes...........

Thanks for the input.

Tomh
 

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Discussion Starter · #2 ·
Re: I've seen some up to 12:1...

Hey Tom,

I have seen a couple of successful aprox. 12:1 build ups that proved to be torque enhanced beyond expectations. keep in mind that this CR is less forgiving to tuning error.

have fun!

ZDAD
 

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> I am curious what compression ratios people
> are getting away with in 'L-6' engines w/o
> destroying parts. I am on my last chore
> engine build before starting my 3.1L
> project. I am ready to choose pistons and
> head.

> Some new vehicles and motorcycles are
> getting away with impressive compression
> ratios but these have more modern (high
> turbulence) combustion chambers and in the
> case of the cars electronic ignition control
> with nock sensors so I don't think these are
> valid indicators of what is possible with in
> an L series engine.

> I have E31, N47 and P79 heads but may look
> at purhasing a P90. Don't want to spend $ to
> get big valves in E31, don't like liners in
> N47 and P79.

> CR from 9 to one all the way up to 12 to one
> are avail. w/ 1mm or 2mm head gaskets, flat
> or dished pistons and various cumbustion
> chamber volumes...........

> Thanks for the input.

> Tomh
tomh,
assuming you're running carbs with the setup, i would find it very difficult to believe that you can run 12:1 and consistantly avoid detonation on pump fuel. with a great ignition system and carbs, i would say 10-10.5:1 is the max for all practical purposes, especially if you're using the car as a daily or frequent driver. yes, keeping the car in tune is important, but if you have a daily driver and carbs, any higher ratio would likely require daily tuning to avoid any problems.
andy [email protected]
 

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Discussion Starter · #4 ·
> I am curious what compression ratios people
> are getting away with in 'L-6' engines w/o
> destroying parts. I am on my last chore
> engine build before starting my 3.1L
> project. I am ready to choose pistons and
> head.

> Some new vehicles and motorcycles are
> getting away with impressive compression
> ratios but these have more modern (high
> turbulence) combustion chambers and in the
> case of the cars electronic ignition control
> with nock sensors so I don't think these are
> valid indicators of what is possible with in
> an L series engine.
First stroking the motor makes little power but changes your power curve Second 10.5:1 should max your motor (you wont always have perfect pump gas) Third an N42 head (no liner N47) has biggest stock valves and when bolted onto L28 flattop block gives 9.5:1 comp mill it and shim the towers to get easy 10-10.5:1
> I have E31, N47 and P79 heads but may look
> at purhasing a P90. Don't want to spend $ to
> get big valves in E31, don't like liners in
> N47 and P79.

> CR from 9 to one all the way up to 12 to one
> are avail. w/ 1mm or 2mm head gaskets, flat
> or dished pistons and various cumbustion
> chamber volumes...........

> Thanks for the input.

> Tomh
 

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Discussion Starter · #5 ·
here's what you need to do!!!!!!

> I am curious what compression ratios people
> are getting away with in 'L-6' engines w/o
> destroying parts. I am on my last chore
> engine build before starting my 3.1L
> project. I am ready to choose pistons and
> head.

personally if you are building a monster of a motor, and want the best performance
go with the p-90 head because it has the best combustion chambers to prevent knocking, and to up the compression buy pistons that are domed instead of dished or flat. that way you get your good combustion chamber and the compression exactly where you want it.

I am running a L28 with flat tops pistons(zx motor) with a E-88 head pushing right at 10.1:1 compression and running fine on premium pump gas.

also something else to consider is that the E-31 head is a good head in the fact that it has good sworl(sp?) patterns like the p79 and p90 heads but lack in the valve sizes, but it ups the compression real quick with just flattops and a thin head gasket. 10.5-11:1 in the area

hope this helps

jesse75z
 

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You need to look at the cam your going to use. A hot cam with a lot of duration 300% or more will allow a lot higher compression ratio than a mild cam with 270% or less, of duration. This is because a hot cam with lots of duration (and overlap) lowers the static cylinder pressures. Example: you have a Z with stock cam, compression 170psi across the board. Get that bonus at work and buy that hot cam set up. Install it and take a compression test. Now with the hot cam your only getting around 130psi. Its kind of like lower the compression ratio, but not really. Ignition timing is another thing to look at. You won't be running 22% of static ignition timing, a mild cam and 11:1 compression ratio on pump gas. Plus if you run a high compression motor lean it will run hot, burn holes in pistons or detonate (ping).
You can't just go by what some people have for a compression ratio unless you build a copy of they're motor or go a step or two past what they did. I would have to say mid 10:1's is about the limit for a streetable Z motor (any gas motor in fact). You could get a little more if the ignition was retarded but then you'll lose more power than having a little less compression ratio and more advance on the timing.
I hope this will help.

> I am curious what compression ratios people
> are getting away with in 'L-6' engines w/o
> destroying parts. I am on my last chore
> engine build before starting my 3.1L
> project. I am ready to choose pistons and
> head.

> Some new vehicles and motorcycles are
> getting away with impressive compression
> ratios but these have more modern (high
> turbulence) combustion chambers and in the
> case of the cars electronic ignition control
> with nock sensors so I don't think these are
> valid indicators of what is possible with in
> an L series engine.
I have E31, N47 and P79 heads but may look
> at purhasing a P90. Don't want to spend $ to
> get big valves in E31, don't like liners in
> N47 and P79.
CR from 9 to one all the way up to 12 to one
> are avail. w/ 1mm or 2mm head gaskets, flat
> or dished pistons and various cumbustion
> chamber volumes...........
Thanks for the input.
Tomh
 

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107,597 Posts
Discussion Starter · #7 ·
> I am curious what compression ratios people
> are getting away with in 'L-6' engines w/o
> destroying parts.

snip

> Thanks for the input.

> Tomh

Thanks to one and all for the responce. I was hoping this would create a lively thread. I know this is a very complicated recipe - even the same set of parts will run differently depending on tune, advance/retard of the cam, ignition...Glossing over all of the specifics, I hoped for enough inputs to get some sort of feeling for what people might be having some success with.

Also - what I am building here is not logical. I have a driver & w/ 3 '71's and a '70 parts car (hit from rear, unibody bent at weak spot just in front of luggage tray, can't open doors, real bummer!) I figure I can go wild w/ the ruffest car.

I am going to probably design for a CR just around 10.5 to 1 w/ the 1mm HKS gasket. Though I do not expect to, I figure I can always lift the head and stick in the 2mm if I have to (as Bob H says - It's only money!).

I am currently thinking of a cam that gives around .5 lift and 275 to 285 degrees of duration. I know this is a higher RPM selection, but I am hoping that the 3.1L grunt and the 3.90 gears will get me up to 3000 or 3500 where I hope the engine will come alive.

I have small DCOE's (40's) that I will try with 32mm venturis. I also have a set of Mikuni 50 PHH (**** THOSE HONKERS ARE BIGGG) that I got years ago from Arizona Z-car that are missing some parts. I'm working w/ ET at AEM to try to put them back together. Plan to use the Webers for now & try to get a manifold welded up to fit the Mikuni's for future reference. Ignition will be jacob's.

I'll keep the group posted. Don't expect an engine birth party soon though, money comes slowly for this project, unless I win power Ball!
 
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