731 cid Outlaw 4.840 Bore Space Nitrous Engine - Engine Builder Magazine

731 cid Outlaw 4.840 Bore Space Nitrous Engine

When it comes to building a 4.840" bore space engine for drag racing, the class is limited on deck height and limited on bore space, but anything else pretty much goes. Pat Musi of Pat Musi Racing Engines recently finished development on his state-of-the-art 4.840 engine platform, and this 731 cid nitrous combo is freaking awesome!

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Over the last few years, we’ve had the privilege of speaking with Pat Musi of Pat Musi Racing Engines on a number of occasions. He’s a legend in drag racing and engine building circles, and when we get any chance to spend some time with him, whether on the phone or in person, it’s always well worth it.

Earlier this summer, we took a trip to Mooresville, NC to check out a few shops in the area, and of course, Musi Racing Engines was high on the list. Pat Musi has a busy schedule between PDRA, Street Outlaws, building engines and his many other obligations, but he was able to take some time to tour us around the shop and show us a few builds in progress, such as this 731 cubic inch Outlaw 4.840” bore space nitrous engine.

“What we mean by 4.840” is the bore space, meaning from the center of this cylinder to the center of the next,” Musi explained. “A stock big block from the OEM is 4.840” from center to center, which only allows you an aluminum block to go 4.600” bore. The way we get to the 5.000” bore is we spread the bore centers out. Then, you can put a bigger cylinder in it. But, this is a rule they came up with for an Outlaw class. You can put as much crank in it as you want. You’re limited to the deck height. This is actually an 11.700” deck. They limit you on a deck and they limit you on a bore space, but anything else pretty much goes. It can be a billet block. I’ve been working on this for a year.”

One of the biggest accomplishments Pat had when developing his version of a 4.840” engine was getting a larger camshaft in it.

“A major accomplishment was to get a 70mm camshaft in here. We have that same cam in our 5.300” motor, but that engine is longer. Because of the lifter layout, we literally only have .060” that the cam can go forward or back without contacting lifters. It’s really a tight package, but we achieved it.

“For example, a Pro Stock engine is a 4.900” bore space, which gives you the extra .060”. On this engine, you get very tight in the lifter. From lobe to lobe, we literally have .060” that the cam can go forward or back. That’s how close it was. We went back and forth with block blueprints and the layout had to be right. Bullet built the steel camshaft for us from scratch. It took a lot to do.

“The benefit will be there – a 70mm cam is a pretty stiff cam in our world – we are looking for big things to come out of this engine. Again, you’re limited to the 4.600 bore, so you have to work with that, but everything else is pretty state-of-the-art. They make rules and guys stretch them as far as they can – you know how it goes. Racers will figure out a way, but we’ve given them a good foundation here and we’re pretty excited about these coming out.”

The 70mm camshaft links up to a Jesel belt drive, just like Musi’s 959 engine combos. This particular build was just waiting on a set of heads to arrive before getting completed.

“We’ve got three of these we’re doing right now,” Musi says. “These engines start with a CN billet aluminum block and the heads are coming from Dart, but this engine is all my baby. I’ve designed the whole deal and I’m pretty proud of it. It should be really state-of-the-art in the 4.840 world – that Outlaw world.”

Aside from the CN block, the Dart heads, the Bullet cam and Jesel belt drive, this 4.840 build also includes Jesel keyway lifters, GRP rods, a Dailey dry sump “overdriven” oiling system with a Moroso dry sump pan, Victory valves, retainers and keepers, PSI springs, and Jesel rocker arms – steel exhaust rockers and aluminum intake rockers.

“Danny Jesel and I found that aluminum rockers cushion the whole deal and are actually a better deal for our application,” he says. “We shy away from the steel even though everybody thinks you’ll get less deflection. Well, you can calculate the deflection, and in fact, all of it will deflect, but we like the aluminum intake and steel exhaust setup.”

Additionally, like all Musi engines, this 4.840 features a Musi Racing Engines intake manifold that gets built in-house.

“They’re all fuel injected,” Musi notes. “I’m known for fuel injection. We kind of made everybody go to it now. I mean if you want to run a big engine, it’s pretty much come to that. I did it in 1999, and they laughed at us at first, until we started wearing them out a little bit. Now, it’s the normal.

“A nitrous engine, on that subject, is a very hard engine to tune on fuel injection or carburetors. It’s not as forgiving as a supercharged deal or alcohol deal. Those have their own case of problems, but nitrous comes on in stages and we work with EFI Technology exclusively. I’ve worked with Graham Western for over 20 years now and we are really happy with his support with his product. EFI has come a long way and I’m lucky that I got onboard a long time ago when I did.”

As you can assume, the 731 cid 4.840 Outlaw build will have three stages of fuel injected nitrous at the ready.

“Nitrous is based on displacement, meaning as you go up in engine size, you could put more nitrous to it,” Musi says. “It all hinges on that intake valve. At the end of the day, it’s a normally aspirated engine, so we get beat up from the supercharged guys saying, ‘You’re 959 inches and we’re 521.’ Yes, but you’re increasing your displacement four or five times with a supercharger. We can’t. It’s normally aspirated.

“Whatever that intake charge can pull on displacement, that’s how much you can spray. If you try to put more than that in it, it’s going to blow up because it gets suspended in the plenum. I’ve done a lot of talks and people are starting to understand that. This motor will have three stages versus six on a 959. Again, it’s really reliant on displacement in terms of how much you can put in it. This engine is 731 cubic inches.”

With a lower compression of 13:1 because of the nitrous, Pat says he’d be happy to make about 1,580 to 1,600 horsepower with this build normally aspirated. We’re all excited, including Pat Musi, to see and hear how this engine combo does on the track.

Engine of the Week is sponsored by PennGrade1Elring – Das Original and NPW Companies. If you have an engine you’d like to highlight in this series, please email Engine Builder Editor Greg Jones at [email protected].

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