Thread: Max H.p. for 2 bolt main?

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I am overhauling my engine and found out sadly that my block is a two bolt main, but what would be the safest yet highest amount of hp to put to the mains?

.....my 415 small block Chev. puts out 499 ft/lbs @4000 rpm & 438 hp @ 5250 rpm. I got an engine kit with a 4 bolt main but the machine shop found something wrong with the block. When I went back to exchange the block all they had in stock were 2 bolt main blocks. The professional engine builder who built my motor said that a 2 bolt main block would be just fine for the hp & tq that my motor would be putting out. He also said that running high RPM's was what would kill a motor that at my RPM levels I would be fine. So I just went with the 2 bolt main block.... Bill......

4-bolts are way over rated, Ive never busted a 2 bolt. theres noway to say how much HP a 2 bolt can take, or anything for that matter,RPM is critical, recipitating mass, the balance, is it normally aspirated?, high compression, these all play apart.

I have never busted a block. or had a motor fail because of the mains, fresh from a build. 500 + hp, or not.
You hired a pro, now trust them to do the job right.

....83Camaro-350, I see from reading a couple of other responces that there are other factor's to consider so; let me add that my compression ratio is 9:75 to 1, my engine was balanced & it doesn't go much over 5500 rpm's. ..... Oh, I plan on adding an 8:71 to this motor so the Jury's still out on what difference this will make!!! ... Bill....

Thank you all for your information except for streets. i dont have the kind of money to build an enigine then screw it up just to ifnd out how it will hold for the motor i want to build.

good studs in main caps/good balance work/strong rods a must
lighter pistons yes
dont run a piston with a very short skirt
8/71 blower ratio pulley size???

if its a chevy

I wouldnt worry seen alot of 2 bolts with lotsa hp & rpm

remember to get 4 bolts in the bottom of a block you gota drill out more iron
things get a little thin down there

studs are your insurance

got a buddy with a 66 chey trk blown!!!
big stuffer on it big pulley also

runs the $%#@^ out of it 4 yrs now runs 94 octane even squeezed it once or twice

I still wouldnt worry
your engine shop should of would of told you you b fine
if they know engines
2 bolts are good peices to build if u stud em

Hi zbeast, Thank you for the info ...Oh, I guess I forgot to mention that when the builder said that I would be ok he DIDN'T know of my intensions to add an 8:71!!! I plan on going by his shop in the near future and running my ideas by him. May be he'll suggest changing some 'other' items. Humm, not sure about the skirt on the pistons. Its got Flat Top Forged pistons {J & E, I believe?}, stock but shot piened rods {the 5.56 long ones}, a stock cast crank. I would imagine that I'll have to run low boost pressure. I'm thinking I'll put the 8:71 on & if it works good, well GREAT. If it doesn't then I'll built another motor with like a dart block & a bullet proof bottem end. So its not like the blower will go to waist or anything. One of the magazines did a build up on a 450 cu in small block with a dart block a while back and it caught my eye. So if the set up I have doesn't work may be I'll also let the dart block catch my wallet??? Thanks again, Bill.....

Originally posted by billlsbird
let me add that my compression ratio is 9:75 to 1, my engine was balanced & it doesn't go much over 5500 rpm's. ..... Oh, I plan on adding an 8:71 to this motor so the Jury's still out on what difference this will make!!! ... Bill....

9:75 to 1 is a bit high for a blower. If you plan to build a blower motor, you should consider how much boost? You may want to o-ring or use head gaskets made to handle the boost. Think about a steel crank, Forged 7 to 8:1 pistons. Aluminum heads? A cam ground specifically for a blower application is important. Carbs. or fuel injection, I would consider fuel injection personally- with a good supply of fuel so you DON'T ever run lean. It would be a good idea to install a programable ignition so you can taylor the timing to the boost. Your going to need a good cooling system. Free flowing exhaust. I know you said the blower is "down the road", but if that's your intention, now is the time to make the upgrade rather than buying parts twice. If your planning to go with an 8-71, I assume you are going to run some hefty boost numbers. Unless you plan to run 7 lbs. or less boost you'll should plan on beefing up the engine. Then after you've made all that new power you'll need to beef up the rest of the drive train and chassis to handle it. Or you can run low boost, but what's the fun in that? Right??

I would agree with previous comments here, the hp and torque really dont affect the engine nearly as much as rpm, yield on the bolts used to screw it all together, the balance of it all and the weight of the rotating mass. The 2 bolt will be fine for power, I happened to come by a 4 bolt for under 200 bucks so I took it. If you are gonna use a 2 bolt just do the right stuff, line bore or hone it for sure with the mains you plan to use and definately use a quality bolt like arp or studs. Rods or rodbolts are more common fails so a good set of rods and bolts are a big benefit. Pistons...well whatever you want, light is good, short skirt..not so good in the long run. Get it balanced and use good bearings. The crank, well it can be cast, that'll handle over 400 hp if its quality, just be sure they test fit it all and check all the clearances, any good shop will do this, you should be fine with a 2 bolt and lots of power, its the rpm with mismatched components that hurts...bad harmonics...short life. The hp and torque are not the numbers you should look at, but are somewhat of a loose general scale as to how much it can handle. What you don't is lots of piston force. Piston force is: part mass, part speed, and part power, this includes boost and compression (not static ratios but compression psi). I could blab on but I dont think its necessary. BUILD, BUILD!! The more power the better I say, blown is GREAT for that.

Originally posted by pro70z28
9:75 to 1 is a bit high for a blower. If you plan to build a blower motor, you should consider how much boost? You may want to o-ring or use head gaskets made to handle the boost. Think about a steel crank, Forged 7 to 8:1 pistons. Aluminum heads? A cam ground specifically for a blower application is important. Carbs. or fuel injection, I would consider fuel injection personally- with a good supply of fuel so you DON'T ever run lean. It would be a good idea to install a programable ignition so you can taylor the timing to the boost. Your going to need a good cooling system. Free flowing exhaust. I know you said the blower is "down the road", but if that's your intention, now is the time to make the upgrade rather than buying parts twice. If your planning to go with an 8-71, I assume you are going to run some hefty boost numbers. Unless you plan to run 7 lbs. or less boost you'll should plan on beefing up the engine. Then after you've made all that new power you'll need to beef up the rest of the drive train and chassis to handle it. Or you can run low boost, but what's the fun in that? Right??

Pro70, Thank you for the advice ... Yes, I'll be limited to low boost with my compression. But the motor is already built. I think I'll just hook up the 8:71 at low boost until I get a stronger motor built. Oh you mentioned aluminum heads? It has AFR 195's on it. So that should help fight detonation. I like your thoughts on using injection and a programable ignition. I was planning on using carb's but the injection & pro ignition are a better idea...... I don't want to go 'into' the motor and put in a forged crank, forged rods, blower cam, dish pistons & such because by the time I would do all this I would have thrown out all but the block and heads! I'd be better off just starting a new and using this motor for something else. ....Thanks again, Bill.....

ps, I wish you the best of luck on your front end situation....

Ok billlsbird, I might have something for you, since you are limited by your compression and dont want to tear down the whole engine to change the rotating mass I have an idea for you. Since you are already putting on a blower no matter what it seems there may be a way for you to lower your compression without changing over expensive parts. The new Cometic gaskets can be had at up to .125 inch compressed thickness, this I believe is the maximum thickness made. The smallest that is made by this company is the .039 compressed gasket. If you have a similar small gasket, which I believe you do you can take off your heads and add this gasket to effectively lower your compression ratio without causing head gasket issues. NOW I have read that this has been done to a big block that puts out 950 hp and there are no issues with the gaskets. You, of course dont have to jump to the biggest one but its an idea. However, here is the rest of the story, the mating surface between the head and the block must be VERY true, so you may or may not have to disassemble the block to make sure that it is good enough for these gaskets...which should be done with a blower motor anyhow.. Next, you would likely require longer pushrods to compensate for the extra thickness, and keep in mind that the quench may not be so great either, BUT since you have aluminum heads and a blower, lower psi, maybe the fuel injection and pro injection would completely keep you from this. The gaskets are like steel shims that sandwich a billet piece and I have heard good to great results with them so far. Just an option you may want to look into. www.cometic.com is where you can get more info on them. You can choose to use em or not to, either way you will make some nice power going blown. Good luck!

Originally posted by 1stGenCamaro
Ok billlsbird, I might have something for you, since you are limited by your compression and dont want to tear down the whole engine to change the rotating mass I have an idea for you. Since you are already putting on a blower no matter what it seems there may be a way for you to lower your compression without changing over expensive parts. The new Cometic gaskets can be had at up to .125 inch compressed thickness, this I believe is the maximum thickness made. The smallest that is made by this company is the .039 compressed gasket. If you have a similar small gasket, which I believe you do you can take off your heads and add this gasket to effectively lower your compression ratio without causing head gasket issues. NOW I have read that this has been done to a big block that puts out 950 hp and there are no issues with the gaskets. You, of course dont have to jump to the biggest one but its an idea. However, here is the rest of the story, the mating surface between the head and the block must be VERY true, so you may or may not have to disassemble the block to make sure that it is good enough for these gaskets...which should be done with a blower motor anyhow.. Next, you would likely require longer pushrods to compensate for the extra thickness, and keep in mind that the quench may not be so great either, BUT since you have aluminum heads and a blower, lower psi, maybe the fuel injection and pro injection would completely keep you from this. The gaskets are like steel shims that sandwich a billet piece and I have heard good to great results with them so far. Just an option you may want to look into. www.cometic.com is where you can get more info on them. You can choose to use em or not to, either way you will make some nice power going blown. Good luck!

1st. Gen, Thank You ... I went on their web site & I'll call them tomorrow. That could be the perfect solution to my compression situation. I need to pull at least one head anyway as I have a striped out spark plug hole & I believe tapping it out from the inside is the only way to go. And yes, you are correct, my gasgets are not thick ones...... Bill....