Hemi 212cc Predator Increased Compression Clearances

#1
Brand new to the forum. Thanks for having me. Been working on first engine and first project.

Anybody have experience with increasing compression on these 212 Hemi engines? It is all brand new to me.

I don't want to do any Milling.

I have a Black Mamba Jr Cam (0.275 lift) and want to use +0.020 Rod, stock piston, and perhaps a thinner head gasket, but I don't know how much room I've got to work with.

Found a build on here which is similar to mine, using same cam, 0.020 Rod, Stock gasket and milled 0.030. BUT I can't post the link.

What is a "SAFE" rod and gasket size without milling?? Is there a chance that the +0.020 Rod is too long for even the stock gasket??

I don't want to tear it apart yet because the weather just got nice. I will measure myself after some play time, but wanted to start this discussion before so I have an idea.

Thanks in advance.

Going on a buggy with 30 series TC, 7.5:1 gearing and 16"Tires. Max RPM 5000 (Cam not installed)
 
#2
Did you measure how much in the hole the stock flat top piston is. On most of mine it was .015 to .018 (4 motors) others have measured more but if it is less than .020 then you may have a slight popup. As long as you are using a un milled head you could possibly get by with a .009 predator metal head gasket as there is no squish area. You would have to clay it to make sure you had at least .030 piston to head clearance at the edge of the piston where it would be the closest. If you were using a regular non hemi head it would be a problem cause you would not have .030 piston to head clearance.
 
#3
Thanks for your response. I've been reading up on this a little, but am still a little confused - mostly due to my zero engine experience.:helpsmilie:
Did you measure how much in the hole the stock flat top piston is.
I have not measured yet. I am building both engine and buggy in stages and trying to enjoy it as much as I can in between the upgrades.:scooter: I just changed gearing and am starting to research compression so I can order a head gasket and Connecting rod (and Flywheel) to install with the Black Mamba Jr Cam and accessories.

On most of mine it was .015 to .018 (4 motors) others have measured more but if it is less than .020 then you may have a slight popup.
Would a 0.005 Pop out be OK if the gasket thickness allowed for at least 0.030 Piston to Head (?Head or Valve?) Clearance????:confused:
Or are you saying that if I'm sitting less than 0.020 In the Hole, I shouldn't use the +0.020 Rod?


As long as you are using a un milled head
I plan on using the stock piston and my un-milled stock Hemi Head (at least for now).
you could possibly get by with a .009 predator metal head gasket as there is no squish area.
Even with a slight popup?

You would have to clay it to make sure you had at least .030 piston to head clearance at the edge of the piston where it would be the closest.
Piston to head or Piston to Valve?? If it is piston to head, then I'm not sure what I'm measuring. I can figure it out if its piston to valve.

Already bought the clay.
Should I do this with stock Rod and New Cam before ordering New Rod? , or with the +0.020 rod, before I order a gasket??? Can I do this clay measurement without a gasket or should I try to reuse the stock gasket for this?

Finally, Is there a possibility that I will not be able to find a gasket (even if it is a thick 0.045 gasket) to make the Black Mamba Jr and +0.020 Rod will not work together with the stock un-milled head and stock piston? Is it just a matter of Gasket selection?
 
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#5
Slight pop out is OK if correct head gasket is used stock one is .045.

"Piston to head or Piston to Valve?? If it is piston to head, then I'm not sure what I'm measuring. I can figure it out if its piston to valve."
Piston to head. It is a little confusing with a hemi but if you think about it the closer you get to the edge of the hemisphere the closer it is to the piston at TDC. So you woyld put piston at the top with the gasket you want to use and put clay around the edges of the piston or chamber and bolt the head on. Slice the clay with a razor is a few places and measure it with a caliper. but if you are using a stock head gasket you would not have to check it unless your pop out was .015 or more which it will not be.
 
#8
Well I guess I'll try to keep my questions in a single thread...

What do you guys think about using a gx390 carb on the Hemi with these Mods? Is there a different option I should be looking at?
 
#9
Hey Bob, sounds like a great lil engine build that you're doing...and ole is giving you some great info. My stock engine that I built was a hemi head (60363) and the piston was .020 in the hole , so I bought the ARC billet rod that was .020 longer which gave me nothing down in the hole and no piston pop out. So I used the stock hemi predator .041 thick gasket to give me piston to head clearance ..plus I like the fire ring gasket. I could have left the rod size stock at .020 in the hole and used a .009 gasket and still had my clearance.
 
#10
Hey Bob, sounds like a great lil engine build that you're doing...and ole is giving you some great info. My stock engine that I built was a hemi head (60363) and the piston was .020 in the hole , so I bought the ARC billet rod that was .020 longer which gave me nothing down in the hole and no piston pop out. So I used the stock hemi predator .041 thick gasket to give me piston to head clearance ..plus I like the fire ring gasket. I could have left the rod size stock at .020 in the hole and used a .009 gasket and still had my clearance.

Thanks for taking the time to put that thread together so nicely.:thumbsup:

I think I'm going to shoot for the stock size gasket and +0.020 Rod for now (My mind might change when I actually measure, but having to much fun to tear it apart just yet).


I never did an intro but here goes:

Before this project, my experience was limited to a week in 9th grade when we took apart and rebuilt a lawnmower engine. I've had to learn everything from videos and forums. This is extremely helpful! My mechanical and fabrication skills are about as limited as my engine knowledge. BUT because of threads like yours on sites like this, I'm fairly confident I can get SOME things done that I would have never thought possible. Even bought the cheap HF 90 Amp 120V AC Flux Core Welder that I'm going to try to convert to DC Electrode-negative with bridge rectifier and capacitor. Maybe I'll eventually work up the Manhood to try to port and polish myself. I try to research everything first, because it's all brand new and I want as few surprises as possible.

I have a BS in general physics, but partied more than studied in college... And general undergrad physics just scrapes surface of most concepts of physics without getting into engineering or design. So I have to ability to learn and have a basic understanding of some concepts, but some times need to talk (or be talked) through stuff to satisfy my selfish need to understand how stuff works at a fundamental level.

I am currently a bartender, but previously worked on configuring and calibrating X-ray spectrometers, and small nuclear energy sources (college research).

Any Whooo, Thanks a bunch!!! Glad to be here!
 
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#11
Very cool Bob, and this is one great forum with some awesome people that's always willing to answer most questions. Glad you're here and remember to do a build thread on that engine. Good Luck !
 
#13
OK, I got the +0.020 ConRod, SS valves, 26lb springs, Black Mamba Jr Cam, stock rockers, stock pushrods, and ported the intake a little bit. I have been running the stage 1 exhaust pipe without the included screw on muffler.

After installing the new Rod, I was 0.003" in the hole. I got the 0.036" head gasket (should of went thinner, should of been a doctor).

I also ordered the 390 carb (Big Dint - Japan) and adapter (ARC) along with a vac pulse fuel pump.

I've been reading its not easy to tune these carbs to run on gas (and the carb may be too big), but that is my plan cause I have no clue ablut alky.
I've read 0.029 Low and 0.042 Main with stock 390 E Tube might be a good place to start.

Any info would be helpful. Anybody know the stock jet sizes in these carbs?
 
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#14
OK, I got the +0.020 ConRod, SS valves, 26lb springs, Black Mamba Jr Cam, stock rockers, stock pushrods, and ported the intake a little bit. I have been running the stage 1 exhaust pipe without the included screw on muffler.

After installing the new Rod, I was 0.003" in the hole. I got the 0.036" head gasket (should of went thinner, should of been a doctor).

I also ordered the 390 carb (Big Dint - Japan) and adapter (ARC) along with a vac pulse fuel pump.

I've been reading its not easy to tune these carbs to run on gas (and the carb may be too big), but that is my plan cause I have no clue ablut alky.
I've read 0.029 Low and 0.042 Main with stock 390 E Tube might be a good place to start.

Any info would be helpful. Anybody know the stock jet sizes in these carbs?
You may also buy a copper head gasket to your specs on ebay. This guy is great, Super fast service! also may want to consider cutting "eye brows" in the piston for valve clearance ( you may want to go with more cam later and may need the clearance) I have learned you can NOT have to much valve clearance for the hemi! You can do the with the stock valve and stick on sand paper easily. Good luck!
 
#15
Some results may vary.black mamba jr, .20 longer rod, .20 off head surface, and steel .oo9 head gasket
No ptv clearance problems on 3 hemi duplicate engines checked with clay. I would say without any doubt a stock head would clear with a metal gasket and this combo.
 

fhpe77

Active Member
#16
Bob,

My build is very similar to yours. Predator Hemi, Black Mamba Jr. cam, 26 lb springs, +0.020" Arc rod, piston 0.005" in the hole, 0.027" MLS head gasket, & head milled 0.035". Piston to head clearance is 0.027" + 0.005" = 0.032". I used stainless steel valves which may have a slightly thicker margin and will reduce P to V clearance. I clayed it and found that I had 0.090" clearance on the intake valve and 0.105" clearance on the exhaust valve. A little tight but so far no P to V contact. I cc'd the head (19 cc) and static compression ratio worked out to 10.56:1. It runs well like this and pulls easily to 6700 rpm (around 50 mph with 5:1 gears on a centrifugal clutch) before I chicken out and throttle back.

-Ray
 
#17
You have plenty of p to v. Exhaust is the critical one as piston is chasing the closing exhaust during overlap. .060 is a good safe ex clearance so you have a big safety margin.
 
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