Dual plugs and compression ratio

[ratpiper71T]

If one was to build a hot-rodded but street, 2.3 liter early engine(85x66), weber 40s, what results could one expect from raising compression and dual plugs? More specifically single plug and 9.5/1 compression vs. dual plugs and 10/1? Given some durability upgrades-(shuffle pin, fully machined and prepped mag case, but standard rods)- in other words, no titanium or other really high dollar parts since I dont have that kind of a budget).
I know the 906 engine was similar in spec but higher compression and loads of expensive race parts, etc., but it wasn't built to last like a street engine. So as far as results is the bump in compression/ dual plugs worth it?- what results?

[afterburn 549]

If you have your heads off , drill em..then you have options...

[tadd]

The 906 motors are actually pretty robust. The PITA with them is the 38 mm ports which leaves a hole in the power band so you really need to run the motor out to 8.5k to shift so you dont drop out of the power band. The 'trick in the day' for the rally cars was to swap the 906 heads for the 69S heads. Identical but for 36 mm ports. Shift problem goes away.

As to your orginal question, twin plugs are useful IMHO regardless since it lets you either add more compression or get away with worse fuel. Win win. Whats even cooler is that you can have your heads twin plugged for about $400 during the rebuild, then just put the uncut lower valve covers back on and drive into the sunset and add the ignition componets later. Run premium, then switch to the cheap stuff once done.

That said, the real driver on compression and the required fuel is the cam since its really the dynamic compression you need to sweat. If you use a T cam duration, your not going to get stellar compression numbers on premium. However, a long duration (high overlap) like the 906 cam will support more static CR. My 2.8 will be running 11.5:1 and should handle premium since the 101* LC lowers the dynamic to a reasonable number.

To be honest, its about if you want a race for street (906), sporty (DC44), or tractor (solex). Personally, I really like the 906 cam . Most builders call it old school, but it has very gentle opening and closing ramps since spring tech in the mid 60s wasnt very good. This allows for high rpm operation with mild spring rates saving wear, friction, and all the goodness that brings since you can run stock strength springs (but from a good manufacturer). Toss in a set of Ti retainers and its a nice high rpm valve train. Minimizing valve bounce off the seat is also a big deal to ensure that you can get the last bit if your working for it. Thats also nice for longevity.

Last bit. Rods. You really dont need to do Ti. Most aftermarket rods are a good bit lighter than stock. Stock is like 750g a rod while most modern aftermarket steel rods are in the 550 range. Ti will get you to 450ish. Ti wrist pins are only $600 out the door and loose 20-30g where it will do the most good and not affect longevity.

t

[ratpiper71T]

Thanks for the answers guys. Afterburn549- that's what I was thinking if budget keeps me from affording the ignition pArts...
Tadd thanks- I think maybe I shouldn't have mentioned the 906- it's just that the dual plugs carbs and 2.3 liter reminds me of the pics I've seen and stories I've read. Here's what I've been kicking around in my head(so far I've only been collecting parts for my project thus the question).:
85mm cylinders
~9.5/1 or a little higher~10/1 compression pist'n
66mm CW crank
Stock rods-shotpeened, bushed and resized
Weber 40s-jetted etc. appropriately
Mod-Solex or slightly sportier cam-(must be enjoyable (read behave)on the street)
Stock or maybe some aftermarket rod bolts
Resized,case-savered, shuffle-pinned, mag case.
SSIs and stock or sport/ST type exhaust

Just don't know what results I can expect from these choices with TP vs. without or if it's even worth it- that is to say if with a sputtering, burping spitting at idle type cam and race only fuel is the only way I could really get any real HP and torque(real and/or butt dyno) out of it, maybe I should kill the TP fantasy now?

[ratpiper71T]

As a starting point reference I believe my motor is built to 2.2e spec by Tom Amon back in '84.

[tadd]

The bulk of the twin plug cost is typically in the ingnition components, not the head mods (assuming you already have the motor appart). It will just depend on how 'antique' you want to be. $400 will get you a micro squirt (pre made, twin plug, and EFI) with everything in one package. That would be $800 all in.

Again, twin pluging will see advantages at all CRs (including less advance). The higher the CR, typcially the more advantage. Setting a static CR at 9:1 with twin plugs on a mod solex would give you very safe running on the cheap stuff.

A set of quality aftermarket rods really isnt that much of a stretch from redoing stock rods by the time your all done and you get to dump a lot of mass which does wonders for longevity. It all comes down to how you want to spend. Rpm is going to give you the most bang for your buck short of a turbo. The key is to decide if you want to spin over 7.3k or not. Stay under that number and stock parts are just fine. Over that and you really should do aftermarket rods, Ti retainers, quality springs, cross drill the crank, and shuffle pin.

To be honest, there really isnt anything wrong with the stock S cam or one of the high lift variants. Just sporty enough.

For a 2.3 the stock 36 mm head will flow more than enough regardless of what cam you choose.

t

[Eagledriver]

Quote: "That said, the real driver on compression and the required fuel is the cam since its really the dynamic compression you need to sweat. If you use a T cam duration, your not going to get stellar compression numbers on premium. However, a long duration (high overlap) like the 906 cam will support more static CR. My 2.8 will be running 11.5:1 and should handle premium since the 101* LC lowers the dynamic to a reasonable number." End Quote>


It's amazing that this idea keeps cropping up. While the idea that "dynamic" CR seems plausible at first blush, it's not reality. If air were incompressible it would have some merit. For a dynamic CR to be less for a given cam, the volumetric efficiency would have to be less for that cam. The actual VE can be found by looking at the torque vs displacement for an engine. The higher the torque the higher the VE. A 906 has a max torque of 152 ft/lbs. A 2 litre 911T makes 116 ft/lbs.

At high RPM (whatever the max torque RPM is), The engine is filling it's cylinders all the way and compressing that air. At a VE of 100 percent your "dynamic CR" and your static CR are the same.

-Andy

[Flieger]

Below a Mach number of about 0.3 air is for engineering calculations considered incompressible. Ergo, at low rpm a large overlap cam has a lower dynamic compression ratio but at higher engine speeds that changes. That high rpm power is why they made the long overlaps in the first place. If air were super-compressible there would not be such a power loss at low rpm.

[tadd]

Andy:
I happy to oblige that the term 'dynamic CR' is a bit of a misnomer since it is really a collection of variables for a figure of merit. The term is already in place and IMHO posseses a convient encapsulation thus I tend to use it for a quick and dirty discussion.

I will point to Taylor that has a graph of several PV loops at various RPM and the RMS peak pressure does increase as the motor approaches the optimum rpm for the cam design. Although this is VE driven as you stated, as pressure within the cylinder it is a direct observable directly connected to BMEP. Having more fuel/air in the same fixed volume will yield a larger pressure reading. Gotta love PVnRT.

Although what adjusting VE does is modify the combustion dynamics of the burn process. As im sure your quite aware since our beloved air cooled porsches are simple two valve hemispherical chambers are strongly negative meniscus shaped they are quite lazy (minimal turbulence) at low rpm. At higher air speeds significant tumble is induced. Significant 'grain boundaries' from turbulence improve detonation resistance by breaking and distorting up the improper burn front thus allowing for more static CR to be tolerated. this is useful in both deflagration conditions as well as specialty use for mass detonation conditions.

So although the term is 'fluffy', I'm not sure it's worth throwing the baby out with the bath water.

I hope the OP will accept my apologies for dirtying his thread with semantic detail.

t

[Eagledriver]

Tadd,

You are clearly more educated on this subject than I am. Does all this mean that at higher RPM the flow in the combustion chamber is so chaotic that it is less likely to support detanation?

My argument is only about how much air gets into the cylinder. The most air gets in at Max torque, and the amount of air that gets in determines how much that torque is. Using this data we can say that the 906 motor at peak torque gets more air in the cylinder than a 911T does at it's max torque RPM. Therefore the 906 has a higher effective compression ratio than a 911T even though it has way more overlap.

-Andy

[Henry Schmidt]

The last two street engine I built for myself were twin plug relatively low compression (9.5:1) 2.45 liter engines running Mod "S: cam and 40mm Webers.
These low compression twin plug engines allow for some very cool timing options and no issues with street gas.
As I've said before, fuel is getting worse not better and building an engine with lower compression guarantees peace of mind in the future.
The dyno numbers on the 2.45 were 180 RWHP @ 6400 rpm. Pulls from the ground and doesn't stop until over 7000.

[tadd]

Andy:
To your question, the simple answer is yes. Deflagration (burning) starts as a point, either be ionization from the spark plug or pressure/heat induced (detonation). As that kernel expands a burn front is generated. The farther (longer) that front travels un impeded the more oomph (technical term) it has. So turbulence basically creates shear fronts which impede the detonation front not allowing it to build enough oomph to break things. It's one of the reasons why multi valve motors have an advantage by adding tumble and swirl along with squish bands and the like utilized in the combustion chamber design. Put another way, your extracting the same work since 'all' the fuel burns, it's just happining in lots of little burns rather than one big one so breakage can be avoided.

I agree with all you've said. What I was trying to convey is that more overlap is useful because it shifts peak VE to more favorable conditions (higher rpm, better det resistance) for combustion utilizing a higher static CR. In other words we can take advantage of more static CR with a more aggressive cam because it softens a higher static CR (by lowering part throttle VE). Thus 'dynamic CR'. Again, a bit of a misnomer but I didn't invent the term .

t

[ratpiper71T]

Tadd, no need to apologize. Although some of the discussion surpasses my practical experience, I absorb things like a sponge and I enjoy every bit of info- be it anecdotal or esoteric, whatever- I'm a hopeless nerd at heart!!
Henry- thanks for the input, I wish I could afford to wave my hand and abra cadabra one of your builds to my doorstep, though I'll have to settle for some of your parts and machine services when I finally get this ball rolling!
Im a little ashamed to admit, I am hoping to get this over 200 HP, thoug I know it's not a wise muse to chase- measuring your manhood in numbers and all--
So your opinion is lower compression is a wiser choice- do to gas quality? That makes me think a bit...

[tadd]

ratpiper:
To be honest, if it were me, id just build the motor with the microsquirt/twin plugs and spend a thousand on a short box. Mucho exciting. IMHO no one needs to be traveling over 130 anyways on the street .

If you have the cash to spend, build the 8k+ bottom end (x-drill, rods, ect). That way if you want more in a year or so, all you need do is change pistons and cams over a winter break.

A 'slow' car running is more enjoyable than one sitting on blocks...

t

[Henry Schmidt]

Originally Posted by ratpiper71T

....clip....
Im a little ashamed to admit, I am hoping to get this over 200 HP, thoug I know it's not a wise muse to chase- measuring your manhood in numbers and all--
So your opinion is lower compression is a wiser choice- do to gas quality? That makes me think a bit...

The 2.45 (70.4 x 86) w/180 RWHP equates to over 200 crank hp. Truly a fun motor.
It's cost effective, easy to drive and with the exception of twin plug it relatively cost effective.
I would second the idea of short gears.

[ratpiper71T]

I know it's tough to beat displacement, and the 70.4 crank wins hands down- though I was leaning toward the 66mm for the earlycar cool factor-- downside being the only way to get to 2.5 or around it is to bore out the spigots-- what's your take on short stroke options, Henry? This is the original #s matching 2.2 case.

Quote de ratpiper71T

....clip....

Im a little ashamed to admit, I am hoping to get this over 200 HP, thoug I know it's not a wise muse to chase- measuring your manhood in numbers and all--

So your opinion is lower compression is a wiser choice- do to gas quality? That makes me think a bit...

The 2.45 (70.4 x 86) w/180 RWHP equates to over 200 crank hp. Truly a fun motor.

It's cost effective, easy to drive and with the exception of twin plug it relatively cost effective.

I would second the idea of short gears.

[Turbo_pro]

Originally Posted by ratpiper71T

I know it's tough to beat displacement, and the 70.4 crank wins hands down- though I was leaning toward the 66mm for the earlycar cool factor-- downside being the only way to get to 2.5 or around it is to bore out the spigots-- what's your take on short stroke options, Henry? This is the original #s matching 2.2 case.

I love short stroke engines. What could be cooler than 8000+ rpm on a street motor.
That said, even on a race track, it hard to maintain those numbers.
The reason for the 2.45 is that it's about as large as you can build without stressing the case. 86 mm slip in maintaining material in the spigot. The 2.45 also makes the horse power at a lower rpm greatly increasing the longevity on a street engine.
In one hand you can rev to 8000 on a long straight and on the other hand your can pull through a corner like a freight train.
The older I get.......... well that's another thread.

[Hcarraro]

Thanks for all the inputs. Very intersting and informative thread.

Henry