One more chance for the big block

It's been over two years getting the GX390 and VC460 engine to work on the road courses. Three engines grenaded in the last two races. Max rpm was 6700, two cranks were balanced, one was not. Billet rod failures seemed to be the problem. I'm thinking these engines aren't the most suitable for this application, or at least the rods aren't. Probably the reason why we were the only ones racing them.

Anyhoo, going to re-block, modify and rebuild to give it one more shot. I did pick up two Honda CRF450R engines that are on the bench and ready to come into the game. It should be a fun fall season.
 
It was a mid shaft displaced comminuted fracture of the con rod the first time. I will open the other two sometime this week.
 
I'd like to know what all got destroyed.
What cam and head mods were done?
I spin mine to 6500 rpms but just for under 10 seconds for drag racing
 

paulkish

Premium User
What other brand rods can you get for the engine?

Are there brands out there some will say "we never have a problem with 'xyz' rods"?
 
Generally the rods we use are fine for short tracks and sprint racing. There aren't too many people that use them in road race applications. Only one other racer in the midwest, if my memory fails me correctly. So as far as a rod brand not having problems that might be for me to discover. One rebuild will feature the Vegascart rod and a drop in compression ratio. Fingers crossed.

On a good note the jackshafted 4 disc Bully, 428 chain and Chautauqua gears and sprockets show almost zero wear.
 

flattop1

Dawg 89
Might be time for something like a Horstman I beam or forged steel / Carillo type . Mid Shaft fracture vs Beam is two different scenarios.
Custom parts =boat loads of cash which negates the options .
 

95 shaw

Premium User
What is the rod length and pin diameters you are using?

Have a suzuki gsxr1000 rod in hand to compare. Its a beefy looking steel piece with insert bearings. I know they spin these to the stratosphere. Lol
Titanium versions available.
Rod length = 111.5 mm (4.390)
wrist pin = 16mm (.630)
crankshaft rod journal = 34.982 to 35 mm (1.377 to 1.378 inches.
 
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flattop1

Dawg 89
Thats darn close .
Bullfrog racing says 4.44 c/c. With 1.416-1.417 rod journal
No wrist pin size . Regrind the crank , then thats next broken crank .
 
I was told, in another group, the Vegas rod may be weaker vs an arc rod or bull frog rod.
If I want to make bigger power, to toss the Vegas billet rod.
So honestly, what do you all think?
 

95 shaw

Premium User
Thats darn close .
Bullfrog racing says 4.44 c/c. With 1.416-1.417 rod journal
No wrist pin size . Regrind the crank , then thats next broken crank .
What about the other way?
Bore/hone the rod for Arc bearing inserts.

Just a thought.

Downside, these are pressure oiled engines. No oil hole in rod.
But, no hill for a climber.
 
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Smooth_1

Member
Generally the rods we use are fine for short tracks and sprint racing. There aren't too many people that use them in road race applications. Only one other racer in the midwest, if my memory fails me correctly. So as far as a rod brand not having problems that might be for me to discover. One rebuild will feature the Vegascart rod and a drop in compression ratio. Fingers crossed.

On a good note the jackshafted 4 disc Bully, 428 chain and Chautauqua gears and sprockets show almost zero wear.
I tried road racing a GX390 for a couple of years with very little luck. I was running Stock Animals before the GX390. One of my first Animals had a ARC rod and it failed with less than 2 hours on engine. After that expensive lesson, we went to the World Formula rod and have had NO trouble since. Two of my engines had ten plus hours of use with no signs of real wear. I run a ARC rod in my GX390. So far, no failures.(knock on wood) Somebody needs to make a World Formula style rod for the GX390. I also had chassis issues with that heavy engine. Too much chassis flex(1.25 inch tubing). I was almost parking it in the corners to keep it on the track. Also struggled with gas carbs(jetting problems). My engine has BIG lift cam with roller valve train. Big valve head ported and milled. Billit rod, side cover and valve cover. Crank is balanced. Last time I ran it, 38mm Mikuni round slide on gas. With my jetting wows, the engine would fall on its face around 6100. You are motivating me to try mine again and not sell it to a dirt tracker who could run it on alky.
 

Smooth_1

Member
It's been over two years getting the GX390 and VC460 engine to work on the road courses. Three engines grenaded in the last two races. Max rpm was 6700, two cranks were balanced, one was not. Billet rod failures seemed to be the problem. I'm thinking these engines aren't the most suitable for this application, or at least the rods aren't. Probably the reason why we were the only ones racing them.

Anyhoo, going to re-block, modify and rebuild to give it one more shot. I did pick up two Honda CRF450R engines that are on the bench and ready to come into the game. It should be a fun fall season.
I saw a shirt video of a CRF 450 with tranny taked off running a 4 disc Bully inboard.(No juackshaft) Can't seem to find video again.
 

flattop1

Dawg 89
It was a mid shaft displaced comminuted fracture of the con rod the first time. I will open the other two sometime this week.
Big end ?
Im not a programer but would it be hard to change a program too add material in that area ? Or even make it say 10 % thicker overall ?
 
Smooth One:
You are lucky with that ARC con rod. The one in my gas VC460 lasted less than 10 minutes. BTW: I was having the same rpm issues as you are. I switched the 1.3 rocker ratio to 1.2 and switched the Mikuni 38 to a 34 and it worked great right up until it blew up. The GX390 runs, I mean RAN, on alcohol and it was great out the gate. No chassis issues, its mounted on a Tonykart, no bottoming out.
 
I'm rebuilding two crf450r engines. One has no trans, a 4 disc Bully and a Hegar jackshaft. Mounting it on an Intrepid shifter chassis. The other has the trans and a four disc. I'm undecided about jackshafting it. Anyhoo I'm looking forward to getting them out on the track and seeing what breaks next.
 
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