Hacking a Hemi

[OUTLAWD]

I told myself that I was going to leave the engine stock, because the point of a $99 engine is it is cheap...

Well that didn't last long. A trip to the dragstrip on "Minibike Night" has given me the itch. One of the locals just went 7.9x in the 1/8 with a "smallblock", I cannot afford to compete, so it will just be a simple build to wake it up.

What I currently have:
Hemi Predator
Homemade step header
Stock carb - 35 main jet, 20 pilot, stock e-tube
Slipstream Flywheel ~30 deg timing
Governor spring tension increased ~4100 RPM
Knock off TAV2 with increased tension on stock spring (also have 7" driven)
12/54 gear, 15x6 tire

Parts I have collected:
Black Mamba Jr.
26lb springs
Alum retainers and suzuki locks
stock size SS valves w/ lash caps
24mm OKO Flatslide
Solid side cover dowels
Studs for side cover

I don't plan to spin it too high, 7500RPM I guess, especially with the TAV, but talking with some of the guys at the track, I am questioning the BMJr, maybe I should have gone Sr. I was originally planning a ~10:1 build, but I don't mind running race gas, but the Jr might be too short duration for more squeeze. I guess BHP rockers aren't out of the question.

My piston is 0.020" in the hole, but I like the idea of a long rod. I was leaning towards the 3.707 rod with a Wiseco, then turning a dome on the lathe to keep adequate Piston-head with a stock style gasket. That way I could bump the CR, without cutting the head. When would the stock fire ring gasket have trouble sealing?

I then saw NR's billet piston.

Piston, Billet, Domed, GX200 & 212 Predators, .490 Wristpin, 2 ring style

A billet piston is a bit overkill, but it would save me some time, and already has valve reliefs. Russell indicated the dome was ~3cc, that really isn't that much. Should I be concerned about only having 2 rings? Would I have to re-balance with he 3.707 rod and billet piston?

I am going to clean up the head, not hogging it out, just focusing on the port to bowl transition, short turn and guide area.

The easy way would just be to go with the 0.020 rod and deck the head, or leave the piston in the hole and use the thin gasket...but for some reason I feel the need to be different...

 

[OUTLAWD]

Places an order from NR, should be here tomorrow.

0.045 Head gasket
Wiseco Piston
HD wrist pin
3.707 Rod
Head studs

I took measurements of the chamber, as I plan to turn a dome on the piston instead of making it a flattop and cutting the head. I should be able to get 11:1 easily while maintaining proper clearance.

I will mock it all up and get pushrods from Studz. Not sure if I trust the stock ones even with only 26# springs.

I am pretty horrible about taking pics while I am out in the shop...would involve pausing pandora, unplugging the phone and getting it all oily...haha.

I plugged the oil sensor hole with 1/4 NPT, and tapped this 1/4 tube to 4AN adapter into the governor arm hole to provide some ventilation. Also made a small aluminum plate to reside where the oil sensor previously did.



Started cleaning up the head

 

[TheDullCarbide]

I'm likin' it! Everybody says "I'm going to keep it stock," that is until they find this site...

 

[OUTLAWD]

Thanks!

I had a laundry list of small items I wanted to do over the winter, but the timeline got shortened as the last race of the season in 10/28...

 

[danford1]

Find out what cam, compression, advance etc the guys at the strip are running. No doubt they tried many cams and found one they like.
Some guys won't tell you what they are running or will lie so you will have to talk to few others. Studz builds engines and should give you some cam advice. Also NRracing is a good place to deal with. The bigger the cam the less low end torque you'll have and the higher they will rev. If you really are going to stay around 7500 rpm, find out from experience engine places what cam they recommend. Just buying something you "think" will work doesn't always run as fast as you hoped. Once you get it running, proper gearing is going to be the next thing to figure out

On a side note, I "heard" painting the engine makes it run hotter as it keeps the heat from dissipating. The more hp you pull from an engine the hotter it will run. Heat kills engines. My suggestion is to run Amsoil 4T Synthetic Racing oil. It is formulated for high rpm air cooled engines and can handle the heat. Regular oils aren't made for a splash lubrication system and can foam up. Something to think about that when you are turning 7500 rpm heading for the finish line
https://www.amsoil.com/shop/by-prod...tton-synthetic-4t-racing-oil/?code=GBS2960-EA

Something else to remember is a built single cylinder engine with a light weight billet flywheel is going to vibrate more than a stock engine. They can buzz your hands numb if you are just casual riding...
Danford1

 

[TheDullCarbide]

Find out what cam, compression, advance etc the guys at the strip are running. No doubt they tried many cams and found one they like.
Some guys won't tell you what they are running or will lie so you will have to talk to few others. Studz builds engines and should give you some cam advice. Also NRracing is a good place to deal with. The bigger the cam the less low end torque you'll have and the higher they will rev. If you really are going to stay around 7500 rpm, find out from experience engine places what cam they recommend. Just buying something you "think" will work doesn't always run as fast as you hoped. Once you get it running, proper gearing is going to be the next thing to figure out

On a side note, I "heard" painting the engine makes it run hotter as it keeps the heat from dissipating. The more hp you pull from an engine the hotter it will run. Heat kills engines. My suggestion is to run Amsoil 4T Synthetic Racing oil. It is formulated for high rpm air cooled engines and can handle the heat.
https://www.amsoil.com/shop/by-prod...tton-synthetic-4t-racing-oil/?code=GBS2960-EA
Danford1

When you say "big" cam, I hope you mean duration. NrRacing told me the powerband of a cam is usually determined by the duration. Lift gives you more flow and therefore HP across the board. They used the 274 series and 280 series as examples. The 274 has less lift than the 280, yet it's powerband is higher because it has 40* more duration. Also, at higher rpm, you might want more lift just because air is flowing faster. The 280 cam might hit good at 5000rpm, but a 320 cam with the same duration won't perform much better because the 280 already flows enough air. Now the difference between a 280 and 320 lift at 9000 rpm is more pronounced because everything is flowing faster.

And as for painting the engine, I would think the engine gets hot enough without paint, but I've seen people who swear heat makes the engine more powerful. Some cut fins off the cylinder and blades off the flywheel to make the engine hotter. Also, the concept of the HEMI head is to reduce heat dissipation for more power. I don't quite understand how making the engine hot gives you power. IMO I'd just keep it at the temp the rings like.:shrug:

 

[ole4]

Valve lift
The formula is:
Valve lift / valve diameter = L/D ratio
According to Comp Cams a minimum ratio of 25% for hot street engines. A ratio of 28-30% for hardcore performance. And 35% or better for all out race engines
Once a valve is open more than 25% of its diameter the curtain area is now greater than the port and any increase in flow is negligible unless there are other issues such as shrouding or port turbulence.

 

[OUTLAWD]

Thanks for the comments. What about the build looks like something I "think" will work? The only deviation from a basic cam swap is the long rod...

This isn't going to be a race bike, just a bike that is raced. I have no intentions of building a M5 sipping, 10000 rpm screamer. Maybe someday I will, but not with this build.

Eric at Studz and Russell at NR have been helpful, as well as some of the members here.

 

[danford1]

The "think" comment was made because of the mentioned Jr. vs Sr. cams in post #1. It wasn't supposed to be negative in any way. Sorry if it sounded that way it wasn't my intention. It was meant to be a generalization.

Let us know what cam you decide to go with.
Thanks
Danford1

 

[OUTLAWD]

NR ordered arrived a couple days later than expected, so I finished up the head while waiting on parts. The 26# springs proved to be a little too strong for me to successfully use my drum brake spring installer, so I brought the head into work to install the springs.



My NR order arrived, I spent Friday night mocking up and machining the piston before I was leaving town for the weekend.

With the 3.707" rod and the standard Wiseco, the piston was 0.115" out of the hole. Making a ~10 degree cut, I made a pop-up that gives adequate P to V clearance, and bumps me up to ~10.7 static CR.







I verified the bearing and ring tolerances, and checked the sweep on the rockers with the stock pushrods. Just need to get a new head gasket, CM pushrods and a petcock to use the flatslide.

I expected the new piston/rod to be heavier, but came in at 26 grams lighter.

 

[ole4]

I like the way you did the piston! Gives it somewhat of a squish area. Did you check valve spring coil clearance at max lift? The Suzuki locks give a smaller installed height but you should be OK with the .275 lift of the mamba jr.

 

[TheDullCarbide]

That piston is awesome!:clap: It's exactly like a domed piston now! Even better that you don't have to cut valve reliefs in like the Bullfrog piston.

 

[danford1]

I like what you did to the piston. When I saw the picture I thought Uh Oh, then I saw the turned piston and smiled. Good job.
Danford1

 

[OUTLAWD]

I like the way you did the piston! Gives it somewhat of a squish area. Did you check valve spring coil clearance at max lift? The Suzuki locks give a smaller installed height but you should be OK with the .275 lift of the mamba jr.

Thanks! I don't have the numbers off the top of my head, but the coil clearance was fine with the mild lift. To have a true squish area it would need to have ~15:1 CR. I mapped out the chamber with a depth mic then found my tightest tolerance and went from there. I see guys mill the head to accept the piston above deck, but turning the piston was easier for me. Plus I am not sure how well I would sleep at night thinking about those crevice volumes...

That piston is awesome!:clap: It's exactly like a domed piston now! Even better that you don't have to cut valve reliefs in like the Bullfrog piston.

Thanks! I got in touch with Bullfrog, though never got a response, and $200 for a piston was a tough pill to swallow. I kind of had my heart set on the long rod. The billet domed piston is an option for the 3.707 rod as well, but the only feedback I found wasn't encouraging, and it only gives ~3cc. I could easily get 12:1+ if I left the dome higher and added valve and spark plug reliefs, but this was an easy ~10 minute job on the lathe. Well, 10 minutes X2 because my first cut only had 0.015 to the intake valve.

I like what you did to the piston. When I saw the picture I thought Uh Oh, then I saw the turned piston and smiled. Good job.
Danford1

Thanks! I would have liked to protect for a slightly larger cam, but these things come apart so easily, I'm not too worried. besides the winters here are long enough, I might need a few projects.



I picked up an 0.045 head gasket, pushrods and a petcock from Studz on the way home from work. Sometimes I have the tendency to rush things near the end...which I almost avoided this time. I had studs to cut down for the side cover, but didn't like the nuts that came with the studs, so I re-used the bolts for now, with solid dowels I picked up from Tim Isky. Torqued the head studs with the new gasket in place, dropped the pushrods and lash caps in and set lash. Flywheel torqued and coil gap set, I dropped the engine in the frame. Once I added the CHT ring to the plug, it only took 1 index washer to align the plug. I should have ~ 0.100" to the ground strap.

These are projected tip plugs, and piston clearance is a common issue from what I understand. Has anyone seen a performance difference between a projected tip plugs that are spaced out and a non-projected tip plug? I just happen to have a small stock of non projected tip NGKs in various heat ranges left over from my small block Mopar.

The original plan was to install the original re-jetted carb for first fire and break in, then move on to tuning the flat slide. But with the governor removed, I didn't have a good throttle linkage solution, and I didn't want to fab up something that I would just take off in a day or two. So on went the OKO 24mm flatslide. I topped it off with break-in oil, rigged up a temporary fuel line, and it roared to life on the third pull.

I am still ironing out some carb issues, as it does not want to start unless I crack the throttle. By the time I leaned out the pilot it was getting late, and I felt my neighbors shouldn't have to hear it anymore. Even with the hot-dog muffler on my header, this thing is quite a bit louder than stock, it really pops. The revs are quick and responsive. I took what ended up being a quick video of my feet on the second fire, but will try to get something post worthy tonight.

 

[TheDullCarbide]

How much meat is left in the piston crown after you machined it?

 

[OUTLAWD]

This is the Wiseco they list for the long rods, saying you have to face ~0.115 off to bring it to zero deck with the 3.707 rod. The edge of the dome is close to deck height, the rest thicker, tapering to the original crown height in the center. I didn't think to measure the crown thickness. When it comes apart I can take some measurements. Or if I could get the CAD, I could figure it out.

 

[OUTLAWD]

I went down to the smallest pilot I had, ~0.012". Fires up on the first pull, cracking the throttle a hair. According to the mixture screw adjustment, I could go leaner still. And that it starts cold without the choke...

Rode it around the block, it is very responsive, or you might say touchy. I will drop the oil and recheck lash after one more heat cycle. I will likely have to up the engagement speed of the driven. I have weights and spring to bring it up to ~3500rpm.

 

[TheDullCarbide]

I completely glanced over your TC setup. That TAV2 with the 7" driven is going to treat you well. Curious are you going to ride the TAV2 mostly in the "shifting" range or are you going to rev the engine at top-end like a clutch? (Dang I have way too many questions!)

 

[OUTLAWD]

I will have to see how it behaves. To be honest, this thing will likely spend most of its time putting around the pits when I'm racing my car, and a limited amount of time racing the bike itself.
Even stock the thing could loop on the launch, and it hit 38mph with the governor. Making it leave more aggressively may help at the track, but make it a bear to ride casually. I will play around with different combos and see what works best. Even if I have to swap out TAV parts at the track, that is a 5 minute job.

 

[OUTLAWD]

It has been raining the last couple days, so I have not had any chances to ride it and get a decent video. but here is a short clip from my phone.

https://youtu.be/7j7xXxXmDMg