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  #41  
Old 02-03-2018, 11:10 AM
Randy H Randy H is offline
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Of course 2mm will make a difference. Why throw velocity and snappy acceleration away. Wether it is a good choice with this combination and desired RPM is betting researched and thought about.
Smart guys research and think.

Others should just do what smart guys tell them.

Not that I'm smart, but many over thinkers are.
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  #42  
Old 02-03-2018, 11:47 AM
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Quote:
Originally Posted by bob58o View Post
The whole point is to overthink the build and learn everything I can. Keeps me thinking instead of spending money.

Why not a big bore and stroker crank for low end? Thatís where the biggest torque gain would come from, unless Iím mistaken. Thereís a reason a 420 has double the torque of a 212.

Going from a 25mm to 27mm Valve is a bigger jump than upgrading a 27mm to a 28.5mm Valve. So does this means everybody upgrading
to a 28.5mm valve in a Hemi Head is wasting time?

I really hate when people tell me Iím overthinking things.
Sounds like a response the nuns would give me when I started raising my hand during religion classes at my Catholic grade school.

Increasing 25mm valve to 27mm valve increases Valve Curtain Area by around 8%.
Not trying to argue, but do you really think your going to feel that 8%? If this was a race engine with tight rules then it might make a difference. I totally agree that making the engine has big of displacement has you can will increase low end torque, but at what cost? Another thing, without a dyno to accurately measure changes and to do tuning you will never know if things like bigger valves are helping or hurting.

---------- Post added at 12:47 PM ---------- Previous post was at 12:42 PM ----------

Quote:
Originally Posted by Randy H View Post
Of course 2mm will make a difference. Why throw velocity and snappy acceleration away. Wether it is a good choice with this combination and desired RPM is betting researched and thought about.
Smart guys research and think.

Others should just do what smart guys tell them.

Not that I'm smart, but many over thinkers are.
I'm not an expert either, but your talking about a minibike motor for pleasure riding. You will never notice 2mm valve size difference on this engine. Now if this was a race engine being built for a certain class that has tight rules you might notice a difference. I like I said in my other post not trying to argue just stating a point.
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  #43  
Old 02-03-2018, 12:57 PM
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I agree that I may not be able to feel the difference and would need equipment like a dyno and flow bench to actually see what works best. But that is not going to stop me from trying to use the the parts I feel are optimized for what I want to do.

After-all, this is an engine BUILD thread. I enjoy picking out parts and seeing how they come together. Granted I won't have a way to really test, which is why I spend so much time researching theory and and concepts.

Use formulas to get an idea of what optimized specs might be.
Search around and see what parts are available with those specs.
Do a cost analysis of the parts I feel might work best.
Then I think about what I will later regret if I didn't modify it the way I wanted.

Sugar pills can cure many illnesses. If I THINK I put the engine together correctly, then the minibike will FEEL more powerful to me. Unless I am way off, and build a dog, and then I'll just give up trying to build an optimized engine based off math.

---------- Post added at 01:57 PM ---------- Previous post was at 01:11 PM ----------

Quote:
Originally Posted by Randy H View Post
research and think.

I like that chart.
It shows me what I want to see. LOL
Nice when you find evidence to support what you want to believe.

I want to believe that a 22mm carb isn't going to be too big.
The 19mm carb I wanted to use is 3X the price.

12HP cylinder requires 21mm - 26mm carb according to that chart.

If I figure 12HP at 4000 RPM (not sure if this is high, or reasonable), then I'd need 15.75 lb-ft torque at 4000 RPM.

On a stockish Engine (filter, exhaust, and jets only), Peak torque is around 14 lb-ft at 2300 RPM (not my dyno, not my numbers). At 4k RPM, torque is just under ~13 lb-ft.
https://www.kartpartsdepot.com/product_p/001.htm

Since RPMs are low I need every advantage in TQ get a higher HP engine. 15HP at 7500 RPM is easy, 15 HP at 5000 RPM? Probably not so easy.
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  #44  
Old 02-03-2018, 02:20 PM
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I may be wrong, but I believe that if all other parameters are dialed in. One parameter not dialed in will cause performance issues. At least, IMO, a given parameter, be it valve size, timing or whatever will shift the power curve higher or lower.

A carburetor of course has is own mess of parameters, and can be tuned to function very well as long as long as it's the the ballpark. Still, I think as far as carburetors go, for an application where maximum torque and throttle response is wanted the smaller the better, but not so small that power is sacrificed at any point in the desired operating range of the engine.

Seems like common sense.

This is kind of cool.

  #45  
Old 02-03-2018, 03:03 PM
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Bob "sigh"..."The science guy".

Randy H...."the "understudy ".....
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  #46  
Old 02-04-2018, 04:45 AM
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So I searched Youtube for "Engine Math"....

This is the first thing that popped up.
So much wrong here...

There is no way this can be correct.
Says Dynamic Compression Ratio is based off three things...

Rod Length
Stroke
Installed Centerline Angle of Camshaft (not sure what this is? he uses 72 out of nowhere)

No mention of LSA or Duration. No opening or closing events. Just one number from the cam? Installed Centerline Angle? Then he shows us 106 Intake CenterLine on the cam card and uses 72 degrees out of nowhere.

And worst thing is he tells people they need a $120 calculator for sin and cos functions to figure out these obviously erroneous calculations. No way people watching youtube videos have access to a calculator with trig functions. I use the calc on my phone so much, that I often try to dial phone numbers on my calculator. I do this A LOT! The calculator is the most used program/app on my laptop toolbar.

I was slightly disappointed by the NASCAR video. This video made me angry. LOL

EDIT: I figured it out. Intake Closing, not Install Centerline Angle. This guy has 75k subscribers and has no idea what he is talking about in this video. 149 Likes and 1 dislike. I've love to hear him derive those formulas. His buddy Bob probably knows what is up.

Moral of the story... Let Bobs be Bobs. And don't quote us, especially if you are doing it wrong.

I didn't like or dislike this video, but wish I had another set of hands so I could give it four thumbs down.


---------- Post added at 05:45 AM ---------- Previous post was at 05:00 AM ----------

Tried to calculate Required Airflow to Reach HP goal.
If I use 15 HP and 12.5 AFR and BSFC of 0.55...

Air Flow = HP x AFR x (BSFC/60)
= (15 x 12.5 x 0.55) / 60
= ~1.72 lbs of air per minute

A cubic foot of air weighs 0.0807 lbs.

1.72 lbs / 0.0807 lbs per cubic foot = 21.3 cubic feet

Required airflow for 15HP at 12.5:1 Air:Fuel Ratio with Brake Specific Fuel Consumption of 0.55 =

21.3 CFM.

If 12.5 AFR, then I need 0.1376 lbs of fuel per min.
8.26 lbs of fuel per hour.
With gasoline weighing 6.25 lbs per gallon...

1.32 gallons of fuel per hour.
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  #47  
Old 02-05-2018, 01:21 PM
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It's all WOOGIE and No BOOGIE unless....
.
.
.
wait for it
.
.
.
1.3 Ratio Rockers!!!

New idea is 25/24mm clone head,
NR280-0211 Camshaft,
1.3 :1 Ratio Rockers
0.364" Lift at the Valve!
22mm flat-slide carb

Probably requires hefty springs.
Much easier/cheaper to do ratio rockers and heavy springs on heads that aren't the Hemi.

Spoiler Alert: This is probably going to go on an MB200-2. I was going to take the clone engine off and then do a Clone Build #1 for a later project, but I will probably end up using the head off the engine that comes with the bike and putting it on this Hemi, then putting the Hemi bottom end with clone head back on the MB200-2... if I go this route.

Then I can do a clone bottom end with a hemi head or just add more parts to my boxes and bins of parts.
Carbs, exhausts, blocks, heads, gaskets, cams, valves, springs ... I feel I shouldn't have as many parts as I do. I've only messed around with 5 engines total. 2 Hemis, 1 Clone, 1 Tecumseh, and the Powerland 420.

And the funny thing is... It is still cheaper to buy a new predator than to use what I have and fill in the remainder. By the time I buy a side cover, crankshaft, Piston, gaskets, seals, valve cover,.... It's already over $100. I'll have to build a stroker engine just to use some of my parts. Guess I should start selling. But hoarding spare parts seems much more logical.

Thinking clone bottom end with hemi head might be good for the supercharger. Lower static CR = more available boost. More boost = more air. More air = more power.
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  #48  
Old 02-05-2018, 01:39 PM
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http://www.nrracing.com/product-p/nrrock.htm
Rocker Arms, 4140 Billet Steel, 1.2 or 1.3 ratio, Use with 26 lb and larger springs. Use with 252 and larger cams. . (If using SS valves, lash caps are required),
Price is per one set (pair).
THESE ARE THE SIMPLEST TO INSTALL, STRONGEST, AND BEST PERFORMING ROCKER ARM WE SELL


http://www.nrracing.com/product-p/nrrock-rt.htm
Rocker Arms, Roller tip
NR Racing Rockers machined from chrome moly steel
Hardened roller tips and shafts
Dyno tested to 10,000 rpm and proven to make more horsepower
Uses stock shaft & Pivot Nut
Fits under stock valve cover.
These Rocker Arms require shorter Pushrods, typically 5.160 or 5.110 depending on cam and head milling.
Choice of 1.2 or 1.3 ratio
Price is per one set (pair
Attached Images
  
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  #49  
Old 02-05-2018, 02:23 PM
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The GAB is so deep here, I'm lost.
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  #50  
Old 02-05-2018, 04:27 PM
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Quote:
Originally Posted by ezcome-ezgo View Post
The GAB is so deep here, I'm lost.
That is alright, because now I know what I want to do so I'll recap a bit.

-Hemi Bottom End
-Flat Top Piston
-Clone Cylinder Head
-Milled from 22cc until I run out of Piston 2 Valve Clearance or reach ~10 :1 Static Compression Ratio
-25/24mm SS Valves
-3 Angle Valve Job
-Aluminum Retainers
-5.5mm Middle Groove Keepers
-Hardened Lash Caps
-NR280-0211 Camshaft
-1.3 :1 Ratio Rocker Arms
-26 lb Valve Springs (recommended by Russell at NR for this cam with 1.3 Rockers)
-CrMo Pushrods (need to measure for length) Maybe 5.160" (-0.110") or 5.110" (-0.150")
-Billet Lifters
-Guide Plate for 1.3 Rocker Arms
-22mm Flat-Slide Carb


After all my research, this is what I think will give me the most area under the TQ vs RPM curve from 2000 - 5500 RPM.

If you have any questions as to why I chose a particular part, I'll do my best to explain my thought process. But that might get scary.
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  #51  
Old 02-05-2018, 05:37 PM
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$100 for the cam, $76 for the valve package, $90 for the ratio rockers, $24 for the pushrods, and $28 for the lifters. $11 shipping.

Cam
https://www.nrracing.com/product-p/200cam-280.htm
280-0211

Valve Package
http://www.nrracing.com/product-p/nrvalvepack1.htm
25/24mm SS
Lash Caps
26 lb Valve Springs
Aluminum Retainers
5.5mm Middle Groove Split Keepers
Valve Spring Shims 0.030"
Billet Guide Plate for 1.3 Rockers

Ratio Rocker Arms
http://www.nrracing.com/product-p/nrrock.htm
1.3 :1 Ratio
4140 Billet Steel
Use with 26 lb or larger springs
Use with 252 or larger cams
Lash caps required with SS valves
THESE ARE THE SIMPLEST TO INSTALL, STRONGEST, AND BEST PERFORMING ROCKER ARM WE SELL

PushRods
http://www.nrracing.com/product-p/cmpr-55.htm
CrMo
3/16"
GX200, BSP "Clones", 6.5 Chinese OHVs
5.260 Stock Length; Use with stamped Rocker Arms
5.160 (-.100); Use with Billet & Roller Tip Rocker Arms
5.110 (-.150); Use with Billet/Roller Tip Rockers along with extensive milling (>.065")

5.440 (+.180); Allows use of 160 Lifters in 200 blocks to reduce valve train weight. Reduces combined lifter/Pushrod weight by approx 7%. Use with stamped Rocker Arms
5.340 (+.080); Allows use of 160 Lifters in 200 blocks with Billet or Roller Rockers. Also use with some Aluminum Rockers

Lifters
http://www.nrracing.com/product-p/billetlifter200.htm
Our billet lifters are intended to be used in modified Honda, Chinese OHV and predator engines with aggressive camshafts. The billet lifters are precision CNC machined from superior grade alloy steel and heat treated to resist breakage and ensure engine durability. Standard length can be used with most pushrods. Fits GX200 and 6.5 Chinese OHVs. Sold as a Pair
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  #52  
Old 02-05-2018, 06:48 PM
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Looks like u got a plan man I was looking into the same cam, looks like I'll be following another similar path with ya, only I'm just gonna go with standard length cm pushrods and billet rod with the 26lb spring and ss valve set but I'm gonna keep the stock rockers. Since I'm gonna take off the governor I think stock size parts with upgraded alloy will be the safest bet with that kohler electric start flywheel lol.
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Old 02-05-2018, 07:34 PM
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Quote:
Originally Posted by Made View Post
Looks like u got a plan man I was looking into the same cam, looks like I'll be following another similar path with ya, only I'm just gonna go with standard length cm pushrods and billet rod with the 26lb spring and ss valve set but I'm gonna keep the stock rockers. Since I'm gonna take off the governor I think stock size parts with upgraded alloy will be the safest bet with that kohler electric start flywheel lol.
I'm not sure you need SS valves and CrMo Pushrods with stock rockers and 26lb springs.

The cam says it works well in STOCK and modified engines.
You might be able to just swap the cam and springs. Stock retainers, stock pushrods, stock valves. Might be able to use the stock rod since RPMs will be low, but I will probably upgrade mine as well. I didn't get that far because I want to check P2V clearance before thinking about milling, or gasket thickness, or rod length.

26 lb springs work with stock retainers, stock valves
37 lb springs require CrMo pushrods.
37 lb springs with Aluminum Retainers which are for SS Valves

You might want to upgrade the valve train anyway, but it MAY not be necessary.

I've run a BMJr with stock valves, lifters, pushrods, rockers, and 26lb springs.
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  #54  
Old 02-05-2018, 07:50 PM
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Ok, so looking at what 'works' out there in the engine scene, just to add to whatever everyone else is saying:

a 20mm round slide works good on a stock valve briggs flathead: (tried it out this weekend haha- looks like a briggs L206 knock off
https://www.ebay.com/itm/Carburetor-...53.m2749.l2649

Briggs valve size is 21.5mm

A 22mm roundslide works good on the clone/predator 212's with mild modifications.

Predator gen 1 25mm intake
predator gen 2 (hemi) 27mm intake
predator gen 3 (non-hemi) 25mm intake
Clone/Gx200's 25-27mm intake valves

Open (highly modified) predator/clone/gx200 engines can take a 24mm flatslide

predator 301, gx240/gx270 can take the 28mm flatslide

predator 240/270 has a 29mm intake valve size

gx340, 390, predator 420, 10 bolt sidecover 440, and 460's etc can take anywhere from a 32mm flatslide, or 34mm round/flatslide
honda gx390 has a 36mm intake valve size,

Highly modified 340's, 390's, 420,s 440, 460's etc can take a 33mm pumper, or a 38mm carb. These typically have 38 or 40mm big valves put in.

Seeing the pattern? Take the intake valve size and get a carb with a venturi about the same size, or a tiny bit smaller, and you have yourself a carb that gives you the best performance. Bigger isnt always better, just get one that fits the intake valve size.

If you get a carb that has a venturi larger than the intake valve size, your low end will suffer, but you will probably get a boost in performance only in the top end. If your carb flows more than your head allows, then its really not the weak link. It should flow the right amount. Too big=sucky performance at a driveable rpm range.

Also, after reading about what head/valve size. I can tell you from experience that a stock hemi head is better than a stock clone head just because it flows better and has bigger valves.

If you are getting bigger valves, you better have bigger INNER DIAMETER seats. or you just killed more flow. If you take the same seat, and put a 27mm valve on it and a 28.5mm valve on it, the 27mm will flow better because the 28.5 has an extra 1.5mm to flow around.

What I think russel does at nr racing is take a seat that is already like 25 or 26mm, and cuts it so the inner diameter is 1 or 2 mm bigger and puts in a slightly bigger valve, but then keeps the exhaust valve the same, then does his multi angle valve job, porting etc. Just what i think he does, not sure about reality.

Annnd thinking about what nr racing has said about cams, it seems like the 1.3 ratio rockers, or any ratio rockers really dont give you much improvement on a short duration, low lift cam anyway (which is the one you are looking at). I think i read somewhere that the best combinations are 1.3 on a 275ish cam, 1.2 on a .308, and 1.1 or 1.0 on a 356.

From a seat of the pants feel. I really like the 280 2011 grind, but then sometimes i feel like im missing out on rpms too.
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  #55  
Old 02-05-2018, 08:37 PM
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I was wondering if the extra lift of the ratio rockers was actually going to help and I came across this from Ole4. It gives some valve dimensions, port dimensions, and flow rates at various lifts up to 0.400".

18cc Hemi Head
28.5mm Intake
25mm Exhasut
Before and After Porting

And some dyno numbers on a AKRA legal stocker (Barry Young)
On this engine, porting hurt power below 5,050 RPM (destroying it at 4k RPM).
Attached Thumbnails
flow.jpg   Before porting.jpg  

After Porting.jpg  
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Old 02-05-2018, 09:48 PM
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Hey Bob with that cam and the 26lb springs do u think that the stock lift would be enough or do u think long term it would wear the valves faster, main reason I mentioned the cm push rods and ss valves is I want to build for some longevity, also I found arc sells cm pushrods for same price but the billet rod is $60 and the ss valve set on there is also $60 so if ur shopping around that could save you some money, not advertising for anyone just shopping for basically same parts so figured I'd share prices I found
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Old 02-05-2018, 10:29 PM
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Quote:
Originally Posted by Made View Post
Hey Bob with that cam and the 26lb springs do u think that the stock lift would be enough or do u think long term it would wear the valves faster, main reason I mentioned the cm push rods and ss valves is I want to build for some longevity
Sure those parts should increase longevity of the engine.
Still the valve guides might wear out and need replacing.

Not sure what you mean "would stock lift be enough"?

Thanks for the price alert! I love ARC RACING!!!
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Old 02-05-2018, 10:41 PM
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Would longer pushrods and maybe the +.020 billet rod give the springs more depression and cleaner movement of the valves, I'm just worried at stock the springs will be too stiff, cause like I said I was planning on getting the cm pushrods ss valves and billet rod for more durability but if it's at stock compression those springs may be too stiff
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Old 02-05-2018, 11:58 PM
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Quote:
Originally Posted by Made View Post
Would longer pushrods and maybe the +.020 billet rod give the springs more depression and cleaner movement of the valves, I'm just worried at stock the springs will be too stiff, cause like I said I was planning on getting the cm pushrods ss valves and billet rod for more durability but if it's at stock compression those springs may be too stiff
Don't worry about the springs being too stiff. If you use springs stiffer than needed, then you are throwing away power. BUT that camshaft requires 26 lb springs. At least I think. The 0611 needs 26+ lb springs. The 0211 says needs HD springs, but doesn't specify. The general description of the 280 series says requires 26 lb springs.


I would take the head off and check how far down in the cylinder the piston sits at TDC.
Then put the head back on and install the camshaft and side-cover to check piston to valve clearance.
Once you know how close your valves come to your piston and how far in the hole the piston sits @TDC, then you can decide if you can get a longer connecting rod.
Since you have the head off, you might want to replace the head gasket.
If you get a new head gasket, you might get a thinner one.
Maximize CR by selecting a connecting rod length and gasket thickness that gives you 0.030" Piston to Head Clearance and ~0.065" Piston to Valve Clearance.
Use CR calculator to figure out if the rod and gasket gives a CR that works with your octane range. Since using an NR cam, check the calculators output vs the recommended CR on the Cam description.

With the stock rockers, stock lifters, and without milling the head, stock length pushrods are probably what you need - even with a thinner head gasket. Longer pushrods will mess up your geometry and probably cost you power and decrease valve train life.
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Old 02-06-2018, 02:31 AM
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Quote:
Originally Posted by KartFab View Post
Also, after reading about what head/valve size. I can tell you from experience that a stock hemi head is better than a stock clone head just because it flows better and has bigger valves.
You think bigger valves and ports will make more power, even at lower RPMs?
If I wanted to use 1.3 Rockers on a head with 27/25mm valves, I would use this...
http://www.arcracing.com/dj-0214-ohv...cylinder-head/
Clone Head 27/25mm valves. (fits Hemi block)
Hemi heads are supposedly complicated with big rockers.

Quote:
Originally Posted by KartFab View Post
A 22mm roundslide works good on the clone/predator 212's with mild modifications.

Predator gen 1 25mm intake
predator gen 2 (hemi) 27mm intake
predator gen 3 (non-hemi) 25mm intake
Clone/Gx200's 25-27mm intake valves

Open (highly modified) predator/clone/gx200 engines can take a 24mm flatslide
The latest Predators (non-hemi) come with 27/25mm valves, nice ports, 5.0mm valve stems, upgraded retainers with split keepers, and flow better than any of the other Predator heads. These have RTC 6, RTC 8, RTC 9,.. heads. The 5.0 mm stem helps improve flow, but is weaker. No SS valves available with 5.0mm stems. So you need to install valve guides for 5.5mm stems or bore out the stock guides if you want to use SS valves.

JTC heads and RTC2, maybe RTC4 Predator heads have the smaller 25/24mm valves

Quote:
Originally Posted by KartFab View Post
Annnd thinking about what nr racing has said about cams, it seems like the 1.3 ratio rockers, or any ratio rockers really dont give you much improvement on a short duration, low lift cam anyway (which is the one you are looking at). I think i read somewhere that the best combinations are 1.3 on a 275ish cam, 1.2 on a .308, and 1.1 or 1.0 on a 356.
Was it ARC or NR? and What did they say about short duration cams?

Those 275ish cams...
Black Mamba Jr has 230 duration
Dynocams 275 has 240 duration
Black Mamba Sr has 246 duration
NR 274 has 260 duration

The 308 has 245 duration.

The 356 has 255 duration

Isky has a 0.330" lift with 230 duration.
Interesting. But the LSA always seems backwards to me. 104 LSA should have more top end than 108 LSA. 104 Intake CenterLine would have more bottom end than 108 Intake CenterLine, but that is not what it says.
104 LSA=MORE BOTTOM END, 108 LSA=BEST ALL AROUND, 110 LSA=MORE TOP END
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