Stage 2 GX390/Predator 420 Specs?

SquidBonez

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I know most people here have experience with small blocks, but I've seen a few people who run big blocks here too. Looking to run a Stage 2 Predator 420 (that is billet rod/flywheel, .308 cam, 65lb valve springs, chromoly pushrods, stock carb with larger jet, high flow air filter, and a header pipe) on a project and I'm looking to get around 25-30 horsepower and 6000-6500 RPM. No porting, polishing, or milling. I managed to come across a dyno sheet from a 420 race engine that put out 35 horsepower, 38ft-lbs, & a little over 7000 RPM (see below). Problem is that I have no idea if there was any porting/polishing/milling, and it was also running an aftermarket carb, so my output will probably be a bit lower. That being said, does anybody have any dyno numbers or estimates for a Stage 2 Predator 420 or any other GX390 clone?

420 Dyno Sheet:
 

vpd66

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Unless I'm seeing things it looks like it makes 34.83 hp at 5750 rpm and 37.76ft-lbs of torque at 3750 rpm. They tested it to 7250 rpm but that is not where it made the most HP and torque. I have GX390 clone sitting in my backroom I'd like to build someday when I can find something to put it in. If I were you I'd contact NR racing. The dyno sheet you show is from them. They have all the parts to build big blocks and they are very good to deal with.
 

SquidBonez

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I'll be sure to contact them. And yeah these big blocks make most power down low, I just specified RPM because even though I'd be out of the power band I'm aiming for a specific top speed range which is in part determined by RPM.
 

vpd66

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What I don't like about that Dyno pull is it has a very "peaky" torque curve. The torque goes up quick at low rpm but seems to peak and then drops. I'd like to see a more flatter torque curve.
 

KartFab

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this is about right but yours will make less because the carb is more restrictive than the stock appearing 24mm
 

65ShelbyClone

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Why the stipulation that there be no head work? That's where ~30%+ of the power gains come from.

Problem is that I have no idea if there was any porting/polishing/milling,

420 Dyno Sheet:

Yes it's ported, yes it's milled.

What I don't like about that Dyno pull is it has a very "peaky" torque curve. The torque goes up quick at low rpm but seems to peak and then drops. I'd like to see a more flatter torque curve.

It makes 85% of peak power over a 2500rpm spread and 85% of peak torque over a 3000+rpm range so it's pretty representative IMO of any engine built to that level, even automotive.
 
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