Twin-plugging, C/R and boost levels on 930?
 

Hi,

'78 RoW 930 motor, 8:1 compression ratio, SC cams, stock wastegate.

Motor is apart right now, with some upgrades (headers, K27, I/C, adjustable WUR) "while we're in there".

I have a fairly good idea of how much it'll cost me to twin-plug it, and I'm trying to decide if it's worth it - i.e. if there's better "bang-for-the-buck" elsewhere (like brakes, suspension, paint etc).

With adequate and correctly-tuned fueling as a given (as well as correctly timed ignition), what is likely to be a "safe" boost limit for 8:1 C/R, and how much would that be likely to increase when twin-plugged? 5%? 10%? Suck it and see?

Given a "safe" boost level, what sort of variance is there between that and a "higher, but still sane" boost level?

Just trying to get a feel for what the envelope looks like - not even necessarily going to run higher boost, but would like to know where the ragged edge actually is so I can factor some margin in for hot days/bad gas etc...

Thanks!

Andy

Sorry about ignoring your question, but where are you going to source your adjustable WUR?
Pat

LOL. No problem Pat.

Quote:

but where are you going to source your adjustable WUR? Pat

I obtained a remanufactured Bosch 054 - i.e. the correct 930 WUR - and shipped it to Brian Leask, who pronounced it good, converted it to be adjustable as per his web site http://members.cox.net/930wur/, pre-set it for the intended application (just as a starting point, obviously) and shipped it together with his RPM boost solenoid kit very promptly - should be here Any Day Now... :D

I considered having my stock WUR converted, but had reasons to prefer the route I took.

Would also have considered the digital WUR - except that the 930 version seems not to be available just yet; the models available are for N/A cars and have no boost enrichment.

Cheers,

Andy

Twin plugging on a 930 isn't as much of a benefit because the piston doesn't have the dome that blocks the flame travel. It's my understanding that 98mm and larger pistons on a 930 benefit from twin plugging because the combustion chamber is so large.

I had mine twinplugged and wonder if I should have. On turbo's there is real doubt if it is worth it or not. There are some downsides, more plug cables to buy, more plug cables to come loose, more plugs to buy, more plugs to install in awkard angles (like behind the turbo), custom drilled valve covers, ...

If you do your own work I would be tempted to skip it for now, put in a good AFR guage and see if you think it is necessary. The downside is of course engine out again if it is necessary, unless you do all this testing with engine out. I would guess (because I am not that tecchnical) that twinplugs might be more justified on a CIS car than on an EFI car. With EFI there are more ways you can tune to deal with the issue.

Either way, this is just an owners opinion NOT an expert opinion. :)

The big gain in twin-plugging is it reduces the advance requirements for complete combustion. Other gains are; allows you to raise the theoretical compression ratio by increasing the boost without preignition or detonation, slight increase in HP(2%), mileage.

100 octane. Don't run 93 or lower fuel in it if you are gonna run 8.0:1 static compression + 1bar of boost.

It has been tried, and its not gonna work.

Thanks for the clarification! It had just seemed a little counter-intuitive to me, taking a simplistic view, that a 97mm wouldn't get much of a benefit, but the larger sizes would - because, in percentage terms, there's obviously only a small delta between a 100mm cylinder and a 97mm cylinder...

But in Real Life, things often aren't that simple..

Heh. I had the old-style 930 valve covers on the motor. I noticed some 964 covers for sale, guy selling them told me they were "different studs, won't fit". Bought his late-style 930 covers instead.

Mentioned this to my wrench when I dropped them off. He looked at me like I was trying to wind him up, and said "yes, the 964 covers are M6 instead of M8, you would need to drill them out slightly".

I had thought it was a different stud pattern, rather than merely a smaller diameter hole...

Ah, now that's almost precisely what I was looking for, thanks!

This is a street-driven car, and I don't want to tow a bowser of octane booster on road trips, casual adjustment of boost levels on a CIS car (i.e. on race gas) having been covered elsewhere.

If I'm understanding what I'm hearing correctly, although there's some benefit, twin-plugging still probably won't make enough difference to let me run 8.0:1 compression with higher boost levels and pump gas on CIS.

In which case, twin-plugging isn't cost-effective for me at this time, and the money will be better spent elsewhere...


Thanks everyone.

Andy, I'd say so far you have a good mind set for what these pups need.

I'd add: don't be swamped by "boost increase fever", remember that each .10 bar is only 15-22 hp and honestly...you for all intents and purposes can't feel that difference, but your motor will.

I'd stop at .9 bar with your setup, 8.0:1, and optimized CIS (as it looks like you are doing).
I'd always run a blend of pump and race gas even on the street. FWIW, I do with my car.

Thanks! I got almost all of it from right here on this board.

I also think I know about enough to be dangerous, which is why I appreciate the sanity checks...

Quote:

I'd add: don't be swamped by "boost increase fever", remember that each .10 bar is only 15-22 hp and honestly...you for all intents and purposes can't feel that difference, but your motor will.

Yeh, I was somewhat tempted by a suggestion to run a programmable EBC and use higher boost in the mid-range, tapering off towards the top.

But if twin-plugging makes little or no discernible difference to a 3.0 or 3.3 motor that isn't on-the-edge, then why bother?

Quote:

I'd stop at .9 bar with your setup, 8.0:1, and optimized CIS (as it looks like you are doing). I'd always run a blend of pump and race gas even on the street. FWIW, I do with my car.

I'll leave it at .8 bar, keep some margin. The whole question of twin-plugging was really prompted by "does it make sense to do the (relatively cheap) machining while the heads are sitting on the bench anyway".

Some time ago, I found this discussion of toluene as an octane-booster very interesting:

http://members.rennlist.com/951_racerx/RocketFuelFAQ.html

(links to more octane booster tests/information on the level above).

Cheers,

Andy

P.S. Think of flame ringing the heads and cylinders "while you are in there"
Steve Weiner recommends it highly.

P.P.S. SC heads are nice too

F.R. will cost about $600, the SC heads flow quite well and work great
but you probably really don't need them.....or do ya? ;-)

The whole question of twin-plugging was really prompted by "does it make sense to do the (relatively cheap) machining while the heads are sitting on the bench anyway".
Andy [/B][/QUOTE]

Andy- do you plan on ever going somewhat nuts with the car? If you think that you will ever want to EFI the car I would go and have them machined, plug them and use them if and when the time comes. I had a whole bunch of stuff done to my heads last year (3.2 ports, ni-resist ringed, ported, new valves, etc..) and now wish I was flexible enough to kick my own a$$. I didn't want to spend the money after machine work such as coil packs or 964 distributer. But now Im pissed that I didn't do it :mad: because I need it to run the boost that I want.

But unless your going to do something nuts some day, its not worth it.

LOL. You swine... :)

The guy who would do the twin-plug work also recommended flame-ringing.

The machining prices are double the US going rate over here, it seems, but it's not frequently done so not that much choice if you want someone who's done it before.

However; there's no tell-tales of any blow-by on the heads/cylinders at all - in fact, the top-end was in great shape and had good leakdown figures very recently. So this didn't really seem necessary - will just use a Supertec stud kit this time around and call it good.

SC heads? No, they'd just make far too much power, I don't need 'em...

Sigh. :D

Sorry to hear that - that must really suck after all the other work! Maybe with water/alcohol injection you could get that boost level anyway without tearing it down?

I'm trying quite hard to just do things that "make sense" whilst it's apart; you'd have to spend far more than I intend to on this rebuild to be considered "somewhat nuts" in this company anyway. :D

The car is a hybrid - mid-year S chassis, 3.2 brakes (currently), 915 trans, 21 & 30 t/bars. It's lighter than a 930, which changes the equations somewhat. Way I figure it, there's a whole bunch of stuff (brakes, suspension, transmission, driver) that should be addressed if the power output were dramatically raised; not on the road map for this time around.

If twin-plugging wouldn't offset, say, 0.5:1 on the compression ratio, then it doesn't seem worth doing at all in my application. I was considering laying the groundwork for running up to (say) 1.0 bar, but if I can't make that with pump gas and my C/R, then well, hey, never mind.

It's very drivable off-boost with 8.0:1 and SC cams, so this does seem like a pretty good compromise for a mostly street-driven car which does duty as my daily driver. It's not like it's slow, exactly...

As the car makes road trips - and thus runs on pump gas - the boost will stay stock and the priority becomes "get my car assembled, I need my boost fix!"

Thanks everyone, your input really helped.

Andy

"The car is a hybrid - mid-year S chassis, 3.2 brakes (currently), 915 trans, 21 & 30 t/bars."

You have a show stopper in the mix.
Your 915 tranny will not endure much more than 350ft/lb of torque over the long haul. Unless you plan a tranny upgrade you should forget about twin plugs and 1.0bar of boost. Given the light weight and gearing advantage you will have an incredibly fast car without all that.

Yep. My car has 413 lb ft of torque - at the wheels.
So that puts it at about 488 lb at the flywheel if you go forward with the tricks we've discussed/you've considered.

That is a bit higher than 350 lb ft ;-) , so treat your 915 gently.

Is this car for street use only?

It sounds like you got a hell of a car on the way, must see pix when car is done. I dont know, but I used to run 18 psi on my stock 930 block with a K-27 through a kokeln intercooler and euro fuel sys. The only things to go wrong was one of the compression rings broke and on that same cylinder the walls were scored. It turned out that the injector on that cylinder was bad. But I beat the crap out of that motor (180 mph + runs, 6000 rpm stutter box launches etc..) and it went like that for 40,000 miles on top of the 30k it had. I dont recomend this for everyone, but with what you are having done you will have a bunch of fun when you hit the gas. I could have been lucky, but I think these cars are capeble of more than people think or care to explore.

Try that 1.2+ bar on a track day and watch it blow up ;-)

I'm missing it like hell, because it was already not slow. I just lost the Star Trek warp speed effect and was wondering if I could regain that, and maybe dilute the on/off lightswitch effect a tad. Which I enjoyed like hell personally, but it was quite an anti-social trait...

Actually, the car looks nice in photos but doesn't look like much up close - which I preferred for a driver; my 944 looked quite nice at 5 feet or so when I bought it, and I died a little bit inside every time some lazy jackass at the grocery store put another ding or scratch on it when I was buying milk.

A car with oldish paint that already got keyed suits me just fine. And it looking like an SC with a turbo wing on it just adds to the fun... :D

When it's running again, it'll look almost exactly like it did before, which is the way it looked when I bought it from another Pelican Nick (air-cool-me).

Might change the wheels/brakes, but that'll be the extent of any visual changes.

Nick pretty much built the car I wanted - and I wasn't even looking to buy a car two weeks before leaving the country.

Hahahaha.

Originally, I thought I'd only use it for street. Until I bought it, that is.

Now I think I know why people have track habits, and I'm looking forward to acquiring one myself pretty soon...

I just don't dare explore the limits of the handling on the road. Car feels like it's on rails, and I know it's just laughing at me.

Other than not treating the throttle with huge respect in wet/slimy conditions, or any of the standard 911 no-no's, I can't imagine what the hell I would have to do to unstick it - and I certainly don't want to play there on the street....

Hey Rarly - your car (and the others on the board with 930 motors and 915s that survived) was very reassuring when I was trying to decide if the combination of 930/915 was viable.

I'll be sticking with the K27, Kokeln and GHL headers and 0.8 bar. I was thinking twin-plugging would give better throttle-response, help clean up the mid-range and maybe give more options for the future (I mean, hey, there could have been a short bell-housing 930 tranny in the future, too, for all I know). But if it doesn't help much on 930's, OK - that's why I asked :)

The 915 had already done many, many miles and was probably tired to start with. The odo reads 229,000, didn't work for some time (odo exempt in WA), and for all I know the 915 was in the car for every single one of those miles - including the ones it did with the 3.0 and the infamous, much-travelled BAE turbo kit Nick used to have in the car, which many of you probably recall...

It will be refreshed and fitted with the WEVO bearing plate retainer and the stiffer 915 side cover, which I'm hoping will help keep everything where it should be. I came this >< close to fitting an LSD, but I'm currently considering a spare 915 to build up/swap in Just In Case...

The traffic light Grand Prix isn't really where this car lives; 2nd gear 40MPH to *cough* - how bloody fast? better slow down now... is one thing it does really well.

Merging really is such fun with the right tool for the job....

1st is for pulling away in traffic, no point taking it above 3500-4000 in that gear, certainly don't want boost because it'll rev into the redline so fast you can't react in time...

I never dump the clutch - at least, not from a standstill, because I never pull away with more than 2200 on the tach.

Rolling the throttle on in 2nd is good practice - and even that, on a greasy road, can bite pretty hard/fast when the boost hits if you're careless or sloppy, even if the wheels are pointing straight ahead...

Just soooo Not Interested in chirping the tires on a shift to 2nd...

I learnt not to hurry or force any shifts; some shifts are faster than others, but there's no point trying to get to any of them faster than the box will let you.

I can live with more-frequent-than-usual rebuilds on the 915, so long as I don't break it.

Anything I missed for keeping the 915 alive?

Here's the sleeper, anyway... Thank god it's not red...

http://forums.pelicanparts.com/uploa...1173903932.jpg

http://forums.pelicanparts.com/uploa...1173903987.jpg

Good looking car.
You should not have any issues with the 915 if you keep the torque below 350 ft/lb and stay out of wheel hop. No amount of tranny beefing will protect you from the hammering damage of wheel hop. Narrow tires are a big plus - burning rubber is cheaper than exploding parts.

Nice looking car.