benefit of twin plugs
 

Guys,
I am just completing a 3.5 Turbo with 7.5:1 compression with all of the upgrades for performance to rev to 8K once in a while. However, I am running a CIS system with all of the intake and exhaust ports bored out to 38M. -- 935 valves, titaium retainers, performance springs, C2 Cams, entire engine and crank balanced including clutch. Palter rods, Andial 3.5 cylinders and pistons. Andial fuel enrichment sytstem. Custom 964 wide open exhaust -modified. Very big custom configured turbo

I have heard that it is a waste of money to upgrade this engine to twin plugs since the compression is not that high? Plus I would not like to put more money into the engine since I may change it to EFI some day.

Is there and advantage to adding twin plugs to this engine. Is there some other system that I can add meanwhile which will increase the spark over the stock CD unit which can just be bolted on. Or is there someone who builds a special hot spark system for a CIS engine like this that can be just be bolted on?SmileWavy SmileWavy SmileWavy

Hmm well your static c/r is low but your boost is ??- there for for the long mythical equation = cool factor-lots/.more burn, less chance of detonation. If you were to gamble the safe bet the twinn plugs are the hand to play as they open the door to a lot more options in the end.
If your heads are off and you are undecided have them drilled anyway and if you change you mind later..............

Twin plugging will give you more safety margin from detonation. Plus if your heads are off the car now, there can't be a better time to do something like this :)

Both comments are on the money. While you "normal" compression ratio is 7.5:1, the dynamic compression (under boost) varies and can be exceed 12:1 easily on a turbo.

Twin plugging the heads is relatively inexpensive, it's the ignition that will cost you. If your builder even thinks this could be an issue better do they heads now and put a blank in them for later. It's better than tearing down the engine later if you do need it IMO.

I'd spend the time to do the math do see if you're gonna need it. What boost levels are you running? What octane are you going to run?

The only thing that is "waste of money" on that spec is CIS. I strongly recommend you to do the twin-plugging. It will give you bigger margins when adjusting ignition timing and allow you to get better mean cylinder pressure thus more power.

Forget reving to 8k...there is nothing there with C2 cams, certainly not when using CIS. It will choke the engine long before.

The stock CDI system on your car is hard to beat. With everything you are doing, you at least need to change the ignition system. The ability to map the spark is the biggest weak link you have on your CIS car right now (yes, EFI is better, but CIS is still very respectable).

Goran is right about the C2 cams to 8k, unless you retard the hell out of the cam timing. If so, don't look for boost anytime soon... By the way, what turbo are you going to use?

As for twin plugging, your displacement is at a point to seriously consider it - 3.5L. Anything above that is required due to the speed of the flame front for this displacement and effective compression ratio (higher CR = harder to light the flame). At your point in this build, the adivce to go ahead and put the additional holes in your heads is good. Your stock CDI system will be ample to drive a nice hot spark in a single plug configuration if you choose. Again, the main problem is mapping the timing to the MAP and RPM.

To do this right (looks like that's what you are doing - building the worlds most powerful CIS car ;) ), you are going to need to spend more money on a good iginition system.

well for some more analytical advice....as you know bolting a turbo on is ez!....the rest is spendy.....for a turbo to do its job ,get the most from it,and not turn your pistons to peanut butter-consider a aftermarket management sys.

Please read chapter four of my Engine Rebuild book - it's all explained in there...

-Wayne

Pressurized intake manifolds make up for cam limitations in a very fun way.
My engine with stock SC cams pulls to 7000 rpm with no noticeable fall off, I'd be surprized if he doesn't have useable power to 8k.

Sammyg2 - your cam selection is probably one of the best for all-around drivability if you have a 3.3 or 3.4L turbo. You can see on my dynos that the SC cams did a great job of applying power early and holding on to 6500 rpm. Now that I have super C2's, I've lost a fair amount of low end, but gained a bit on top. And the SC's are hard to beat for low end torque.

All good points. One more to throw in there.

If you are going with EFI down the road that would be the best time to activate the twin plugs since it will be a cheaper at that point. So if your pretty sure that EFI is in the future then machining the heads now would be a good idea.


In the meantime just be conservative with the boost on pump gas. If you have some track days putting in race gas will allow you to run higher boost. Maybe one of the gurus on the board can suggest the best timing settings for the different octane/ boost levels.