Forged 347, NITROUS... how much juice?

Lman657 · #1 (**permalink**) 07-19-2006, 12:28 AM

Howdy all,

Here's the story. 302 stroked out to 347, forged crank, forged H-beam rods, forged pistons.

The engine was built for the previous owner to drag race on nitrous, but he sold it because he was scared before the nitrous got installed.

I'm looking at the NOS Top shot that installs in the bottom air filter tray (for clearance reasons). It's adjustable from 75-150 hp.

I don't expect I'll NEED more than 150 hp (however, I've never run into anything with too much power).

Should this enging handle a 150 shot? Would it handle more? or is this pushing it?

I'll definitely be paying the 50 bucks for three dyno pulls when she's done.

Thanks a ton!
Lance

RallySnake · #2 (**permalink**) 07-19-2006, 06:38 AM

Lance,

What car are you putting the engine in? Which tires? Transmission and rear end ratio? Where will you drive/race it? How long do you want it to run before you rebuild it?

Knowing those things will give us a chance of answering your question with a fighting chance of being right.

OK?

Paul

Lman657 · #3 (**permalink**) 07-19-2006, 11:56 AM

It's an older NW exotics cobra. Ford 9" and beefed C-4 trans. Durability of the rest of the drivetrain shouldn't be much of an issue. I'm looking at a new set of american torq thrust II 17" wheels. As for tires on that.... suggestions? This will be a street car... I just want to blow the cobwebs out every once and a while. Aside from the occaisonal full throttle assault, it will be mostly cruised. I'm not sure what rear end ratio it has, but 70 mph is~3300 rpm

RallySnake · #4 (**permalink**) 07-19-2006, 02:17 PM

Lance,

I understand. You just never know when a Z06 or Ford GT will pull up next to you at a traffic light, do you? After driving my car for 19 years with a 356 ci engine and never being beat, the new cars have upped the ante. The Z06, Ford GT and Porsche Turbo are turning 0 to 60 in just under 4 seconds. My new 408 engine will put out 433 Hp and 500 Ft Lbs according to the Desktop Dyno program. The Car Test program, with those numbers inserted, predicts a 0 to 60 at 3.48 seconds. I am hoping that will be good enough. If an Enzo or McLaren F1 shows up, oh well!

The Top Shot nitrous set up looks pretty good to me. I have thought of going this way and even tried a 10 pound bottle in my trunk to make sure it will fit. It does, but takes up a LOT of space. You just have to make sure it cannot open up the solenoids unless the throttle is wide open. I would start with the smaller, 75 hp, jet and see how it runs. If you really feel the need, go with the larger jet. It mak be a really good idea to install a lower end girdle to keep the main bearings in place though.

Paul

blykins · #5 (**permalink**) 07-19-2006, 04:28 PM

I would use an ignition box that would retard the timing for you when you get into the n20.

jmarsey · #6 (**permalink**) 07-19-2006, 05:35 PM

Lance,

What kind of block is it?

Lman657 · #7 (**permalink**) 07-20-2006, 11:14 AM

Quote:

Originally Posted by jmarsey

Lance,

What kind of block is it?

block? that's one I should figure out. It's a 1974 302 block, two bolt mains. Is it safe to guess windsor or romeo?

JAM1775 · #8 (**permalink**) 07-24-2006, 09:26 PM

Pistons are good . What compression are you running?
What octane fuel is available to you ?
What heads are you using ?
I'm running 11:1 with Brodix heads 351W on 93 octane with 75HP EDL carb system. No problem with 3 degrees taken out , used in 3rd/4th gears only.
I did up it to 100HP with 4 degrees but the car got a little squirrely in 3rd gear on the street. Backed down to 75's.

ByronRACE · #9 (**permalink**) 07-25-2006, 02:03 PM

Strength limit of the engine is based largely on the tune.

If AFR and Timing aren't correct for nitrous use, everything is at risk.

I strongly recommend buying a wideband AFR system first. This will let you tune the air/fuel ratio so you don't end up running nitrous on a lean mixture. Do that one time at full throttle, and you'll have junk for an engine. It's not worth the risk when a wideband AFR meter is now in the $300 range.

Next, assuming this is a wet system that has both fuel jets and nitrous jets, start with the recommended jet sizes for both and go for something sane like a 100-shot.

Then, set the timing for about 4deg below the edge of detonation. If your engine begins to ping NA at 34deg total, set the timing to 30 total. That should be more than enough retard to keep the engine happy when on the bottle. Creep back up on the advance slowly and carefully, and only if you know what detonation sounds like and can hear it over the engine. Otherwise, leave it alone. The power difference isn't worth the damage.

Drop your plug gap down to .032-.034. If you experience any misfire, drop it more. Or, buy an ignition amp.

To test, disconnect the nitrous solenoid...go drive it...and make sure the mixture goes PIG RICH when you activate the system. That tells you the fuel control is present, and the engine should be safe for N2O. Then, reactivate the nitrous solenoid and try it again. If the recommended jets are correct, the WOT AFR should be somewhere between 10:1 and 11:1. I don't recommend you run the mixture any leaner than 11.5:1; the power gains aren't worth the potential damage.

How much power can your stock block take? Probably close to 500rwhp before it splits down the valley or rips the webs out by the mains (seen both). Rods are pretty strong. Usually the block goes first. I wouldn't recommend more than 450rwhp if you want it to live a long happy life. That means you can probably run a 150-shot on it if you so desire. Keep an eye on AFR, don't ever skimp on fuel octane, and it should live a long time.

And as suggested, if you want to "have your cake and eat it too", get a retard box that drops 4deg of spark when you arm the nitrous system...that will give you max performance in both modes without sacrificing NA performance.

Lman657 · #10 (**permalink**) 08-10-2006, 12:41 AM

Hey guys,

I'm running Trick flow heads(mildly massaged, not twisted wedge), forged internals, 11:1 compression, and MSD 6AL ignition box w/ MSD coil.

Thanks a ton for the advice guys, any more?

JAM1775 · #11 (**permalink**) 08-10-2006, 08:33 AM

I am also running 11:1, fully ported polished Brodix heads, forged pistons.
It runs great on 93 octane. Again spending the extra 200 bucks on an MSD Auto retard & Window Switch takes the worry out of the timing and shifting or gear missing on nitrous issues. Also, I've been told by 3 different race shops I was dealing with "baby slaps" of nitrous when I expressed concerns regarding 75 or 100 shot plus taking 3 or 4 degrees out. I gap my plugs at .040-042 from.045 and I did go 2 steps colder than stock. I like the NGK V tip racing plug. Plugs are honey colored.