Help me build a big block and teach me!

B

Bstarge

New member
Hey all,

Few years back built a 212 predator it was my intro to building karting engines and I had a load of fun doing it! Fast forward to today. I'm looking at building a big block and have my eyes on the 390 big block, but this time I'm considering buying an bare engine block instead of buying the predator or honda. I figure if I will for sure be replacing the connecting rod, cam, pushrods, springs etc... the cost might be cheaper to build from scratch and avoid the 300 bucks for a salvageable predator block, head, and maybe piston (or more for the honda). Anyone have any thoughts or experience with going from the ground up rather than using a predator as a framework?

I am not worried about cost (though obviously I would like do it as cheaply as possible). I don't plan on racing in any class and will eventually convert another kart to the same engine for some family/friend racing. I also do want to build the engine as I learn more about it and really enjoy it.


Anyone that has any experience with the big blocks l'm looking for some recommendations on the following:

rod (assuming billet standard length will be removing the governor)
flywheel (is electric start even viable with these in karting or more of a hassle? how light should I go?)
cam (I know the lift and duration matters and I want a fairly aggressive cam)
springs (thinking dual springs to go with the big high revving camshaft)
Piston (can I keep stock? I did it on the 212 and seemed to work fine)
Carb (best fit with cam and what I want to accomplish not sure what my options are)
pushrod and rockers (whats needed? thoughts on nicer rockers like champions? really make much difference?)
anything else?

Also with the recommendations what rpm should I expect to achieve if tuned properly?

Thanks Guys!
 
Start with a Vegas 460, I have 390's and wish I had the 460 for the 10 bolt sidecover and free cc's. You can buy a builders kit which deletes the bits you wont need. Don't go too greedy on revs and hp numbers and you will have good results.
 
Building a block is the same as a small block, just cost more, and the vegas 460 is the way to go like Oman said you get free cc....ie power.......If you have more per specific questions let me no..........I have build a few of them.
 
level600a said:
Building a block is the same as a small block, just cost more, and the vegas 460 is the way to go like Oman said you get free cc....ie power.......If you have more per specific questions let me no..........I have build a few of them.
Click to expand...

Looking at just buying this engine based on the Vegas Cart recommendations.

http://www.vegascarts.com/25hp-460cc-go-kart-golf-cart-racing-engine-p/vrx46-a2.htm

Its already got most of the mods I did on my first small block.

What carb options would I have for her and how does the cam on this engine compare with something like the NR racing cam below?
http://www.nrracing.com/product-p/390cam-306.htm
 
I have the stock Vegas camshaft in a GX390 with some 1.23 rockers, Mikuni 28mm carb, higher compression and some light head porting and it pulls well from mid 3000 rpm, peak power under 6000 but seems to pull ok to near 6500. In the bigger cc vegas motor all that would happen at a bit less rpm I think so going to the 306 or similar is probably a good idea unless you have a really tight technical track. You can put any carb you want on it, mikuni 33mm tm pumpers are a safe bet, I have a 28mm TM pumper I scrounged off some old dirtbike and it works pretty well.
 
Bstarge said:
Looking at just buying this engine based on the Vegas Cart recommendations.

http://www.vegascarts.com/25hp-460cc-go-kart-golf-cart-racing-engine-p/vrx46-a2.htm

Its already got most of the mods I did on my first small block.

What carb options would I have for her and how does the cam on this engine compare with something like the NR racing cam below?
http://www.nrracing.com/product-p/390cam-306.htm
Click to expand...

With the 460 or 390 the best carb for Karts are the 33mm pumper carbs from Mikuni, instant throttle response and great idling....as for cams, I have run most all of Nr cams and they work just fine, the 306 has a very strong midrange in the 390 3K to 6k, there 307 cam is a good mid cam with a strong upper end, 3.5k to 7k, but you will need a carb at least 30mm to see the upper range of this cam, ported heads, and bigger valves.... Dyno Cams makes a great set of cams also,..there 308 is a great all round cam, I am running there Mod1 right now in a drag mini bike and it's pulling beast from 4k to 7k..will rev to 8k with 40mm carb, but my heads have been ported and welded, heart shape cambers and 40mm in valves......But as I write this I have to tell you the most fun I ever had was with the NR 280 cam, it pulling hard from idle, stock head and valves, it's a torque monster in a kart engine
 
I too have a VC460 i race at New Castle. The VC460 doesn't put out he 25hp they say it does i don't think. it struggles to pull the same speed a similar hp 2 stroke pulls.... in a modified state.

I took the VC460 and upgraded to:
306-0411 cam
1.1:1 rockers
some mild port work (not a lot. worked on the short radius and manifold matching)
tm33 pumper carb
small diameter aluminum flywheel


You're biggest problem is going to be keeping chains on it. that motor will snap chains so quickly. regular black RLV chain will snap in a lap. Black on gold? garbage. gold on gold RLV chain last me about 40-50 laps on my jackshaft and maybe double that from the clutch to the axle. I think its due to the small diameter flywheel. i would go with the large diameter flywheel if i was you. it will help idle quality and hopefully make your chain last longer (it should dampen the power pulses).

This winter i am upgrading to belts instead of chains. less mess and stronger. I am also bumping up my port work. Cutting the valve guide down, unshrouding the valves, going to a 40mm intake valve, and a longer duration cam. Looking to build around 35hp. finally getting my dyno up and going.
 
manyfunnies said:
I too have a VC460 i race at New Castle. The VC460 doesn't put out he 25hp they say it does i don't think. it struggles to pull the same speed a similar hp 2 stroke pulls.... in a modified state.

I took the VC460 and upgraded to:
306-0411 cam
1.1:1 rockers
some mild port work (not a lot. worked on the short radius and manifold matching)
tm33 pumper carb
small diameter aluminum flywheel


You're biggest problem is going to be keeping chains on it. that motor will snap chains so quickly. regular black RLV chain will snap in a lap. Black on gold? garbage. gold on gold RLV chain last me about 40-50 laps on my jackshaft and maybe double that from the clutch to the axle. I think its due to the small diameter flywheel. i would go with the large diameter flywheel if i was you. it will help idle quality and hopefully make your chain last longer (it should dampen the power pulses).

This winter i am upgrading to belts instead of chains. less mess and stronger. I am also bumping up my port work. Cutting the valve guide down, unshrouding the valves, going to a 40mm intake valve, and a longer duration cam. Looking to build around 35hp. finally getting my dyno up and going.
Click to expand...

It sounds like your living my life on the other side of the world, upgrading to 35hp, belt drive and finishing a dyno off!
 
Oman said:
It sounds like your living my life on the other side of the world, upgrading to 35hp, belt drive and finishing a dyno off!
Click to expand...

yeah, theres less viable options for these engines as they make more power. Belts were inevitable for me i just had to get there first. i struggled so much keeping sprockets on the thing. it wasn't till i went to tapered bushing sprockets that i stopped damaging keyways and destroying sprockets. at that point i was ready to fix the next problem.

Post your build. im interested to see what others are doing to there engines.

Additional advice:

1. go with bushing style sprockets if you chose to use a jackshaft. if you don't and plan on just putting a 1" clutch on the engine, make sure you get a very low stall speed clutch and dont get close to it why riding.
https://www.mcmaster.com/#62375k1/=1a89767
2. get a mikuni. preferably from a builder who knows this engine. I bought my mikuni from Carlson motorsports and have never regretted it. all i have to do is occassionally change the main jet which is super easy. it starts in <1s of hitting the flywheel with an electric starter. i originally bought a tillotson 191 as i liked being able to adjust the needles on the track. it was nothing but problems.
 
Rebalanced crank, Parsons forged rod and flat top piston, .356 cam on 1.3 gage rockers, ARC finned flywheel with PVL ignition, 40x31 non welded head with my port work. Have a 38SBN to try on it for starters running on premium pump gas. 35mm carbon 8MGT belt drive to SMC axle clutch. I want to finish the dyno first before I assemble the new motor, stuska xs19 with dynertia3 electronics, just need to weld up the frame and it should be running.
 
Oman said:
Rebalanced crank, Parsons forged rod and flat top piston, .356 cam on 1.3 gage rockers, ARC finned flywheel with PVL ignition, 40x31 non welded head with my port work. Have a 38SBN to try on it for starters running on premium pump gas. 35mm carbon 8MGT belt drive to SMC axle clutch. I want to finish the dyno first before I assemble the new motor, stuska xs19 with dynertia3 electronics, just need to weld up the frame and it should be running.
Click to expand...

you dont think that makes 35hp? is your head/carb setup that restrictive?

How does your belt setup work for you? are you taking the belt all the way to the axle clutch?
 
No that is the motor that will make an easy 35hp, It wont have huge compression or lots of timing and an exhaust tuned for low-mid range. I forget the exact numbers but the head flows over 150cfm @ 28". would probably make near 45hp if you wanted to push it on good fuel.
Current motor is a 390 with stock valves, mild port cleanup (108cfm), vegas cam and 1.23 rockers, 10:1 compression, 28mm mikuni. I estimate that one about 23hp when I had my engine sim software, which I lost in the last computer crash :(
 
One thing a lot of people don't consider when talking potential power from motors is the operating band, I need the engine to pull from a low rpm clutch lockup to keep the clutch heat under control. That means the engine has to be "on" and pulling strongly for about 50% of its total rpm on my track, about 3200-6400. If I could slip the clutch at 4500 rpm I could cam it up to take advantage of that, but keeping clutches cool becomes harder. If the engine can be peaky, ie a gearbox motor or a single pass drag setup etc you can really push the power up at the expense of low end torque.
 
Belt setup - I don't run a jackshaft, it comes straight off the crank pulley to the axle clutch, just like a chain. Having some teething issues with the axle clutch but otherwise happy. I would do a jack shaft in future though with belts both end.
 
Yes I have a flowbench, the cam came from Parsons/Bullfrog, probably a dyno grind. It's been a while since I purchased all the bits I've forgotten a lot of the details. The last figures I can find on the head were 150cfm and 145 through a mikuni manifold. The port hasn't been enlarged very much, no epoxy or welding. Ive done some more grinding since those measurements so I'll have to get it back on the bench, I'd like to see 150cfm through the manifold, I may try taper out the manifold a bit on the lathe to get there.
 
manyfunnies said:
yeah, theres less viable options for these engines as they make more power. Belts were inevitable for me i just had to get there first. i struggled so much keeping sprockets on the thing. it wasn't till i went to tapered bushing sprockets that i stopped damaging keyways and destroying sprockets. at that point i was ready to fix the next problem.

Post your build. im interested to see what others are doing to there engines.

Additional advice:

1. go with bushing style sprockets if you chose to use a jackshaft. if you don't and plan on just putting a 1" clutch on the engine, make sure you get a very low stall speed clutch and dont get close to it why riding.
https://www.mcmaster.com/#62375k1/=1a89767
2. get a mikuni. preferably from a builder who knows this engine. I bought my mikuni from Carlson motorsports and have never regretted it. all i have to do is occassionally change the main jet which is super easy. it starts in <1s of hitting the flywheel with an electric starter. i originally bought a tillotson 191 as i liked being able to adjust the needles on the track. it was nothing but problems.
Click to expand...

Vega 460 block, Billet piston and rod, welded heart shape chamber and ported head 12.5 :1 comp. with 40/32 valves on gas, Dyno Mod 1 cam, welded 1.5" intake tube and 40mm carb........upgrading to billet crankshaft and side cover this winter.............It in a drag race mini bike.

Kart motor is a Gx390 clone block, ported head and milled head with 38/31 valves 13:1 comp, billet piston and rod, Honda crankshaft,33mm Mikuni pumper carb on meth and a NR 307 cam.
 
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