First engine build

[Truax]

I think I'm ready to do my first engine build, was wondering if I could get a little guidance and direction along the way? I know there is all kinds of info on this forum about modifying the predator engine, and I've tried to read up as much as I can to keep the old questions limited.

I bought a hemi head predator last night after returning their new earthquake impact, that I was not happy with. The goal for the build is to get as much torque as I can out of this thing. I'm not racing or anything like that so shooting for 8000+ rpm isn't the goal for me. Has anybody built up a predator with torque being the main goal? What cams should I be looking at for this idea in mind?

 

[ezcome-ezgo]

Search for posts by bob58o. But make sure you're sitting in a comfortable chair.

 

[bob58o]

I think I'm ready to do my first engine build, was wondering if I could get a little guidance and direction along the way? I know there is all kinds of info on this forum about modifying the predator engine, and I've tried to read up as much as I can to keep the old questions limited.

I bought a hemi head predator last night after returning their new earthquake impact, that I was not happy with. The goal for the build is to get as much torque as I can out of this thing. I'm not racing or anything like that so shooting for 8000+ rpm isn't the goal for me. Has anybody built up a predator with torque being the main goal? What cams should I be looking at for this idea in mind?

If torque is your main goal, perhaps a camshaft with less than 8000+ target RPM would be more ideal. A cam designed for 8000+ RPM might not have great performance lower in the powerband. If optimized for 8000 RPM, it may not even run at 2000 RPM.

Sorry I read that wrong

My Black Mamba Jr cam was advertised as the "Ultimate Torque Cam". I had 3000 RPM clutch engagement and pulled to 7000 RPM.

My new cam is also for serious torque.
Camshaft, 285-0211: 7000 RPM. 11.0 to 1 compression or more. 26+ lb Springs Required, Small track cam for serious torque

Go to NR's site. Look at their camshafts. Their descriptions are really good. Will say Torque Cam, or Wide Power Curve, or Gets you off the corner,.... Some will say excellent mid range.. Or will say Small Track.

Most of these cams are racing cams tho. So most cams described as low RPM, High torque, will still spin faster than stock cams and may require a higher clutch stall.

Some will say for winching and marine applications. Those are for the lowest RPMs. Some will say large track. Those are highest RPMs.

It will also say what springs and Compression Ratio is needed.

I try to see how fast my gear ratio and tires will take me based off of engine RPM and pick a cam to get me to my desired speed.

For example, on my current build... I wanted 60 mph.

13" tires, 4.5:1 Gear Ratio (with TC)
7000 Engine RPM = 60 mph

So I picked a "Torque" Cam that will spin up to 7000 RPM. It should spin a bit faster. I think it should turn 7500 on this project when all said and done.

---------- Post added at 08:48 AM ---------- Previous post was at 08:43 AM ----------

An example of different cams from the 252 series from NR

252 SERIES- Approx .252 lift and 230 degrees duration (at .050).

Welded and ground cam provides significant power improvements over stock cam. Works best with mild motors using bored OEM or aftermarket carbs. Cam is BOLT IN and does not require block modifications yet keeps close to stock base circle to maintain correct valve train geometry. 26lb to 37lb springs should be used. Great with 1.3 rockers. Our number one seller.

Options:

Camshaft, 252-0207: SPECIAL HIGH TORQUE GRIND FOR SMALL TRACKS, 6000 rpm, 22 lb Springs Required, 26 recommended. 10 to 1 Compression or higher recommend, however it will work with lower compression ratios. This is one of the best choices for low end power, esp. on small dirt tracks with heavy drivers. (Oval Track)

Camshaft, 252-0211: Same as 0207, for Road Race, Wide Powerband applications (GX200, 6.5 Chinese OHVs, and Predators ONLY) Camshaft, 252-0607: All around grind for medium to large tracks. 7000 rpm, 26 lb Springs required recommended. 10 to 1 compression or lower. Great for Marine and Winch applications

Camshaft, 252-0607: Oval Track Grind, High RPM applications, 7500 rpm Max (GX200, 6.5 Chinese OHVs, and Predators ONLY)

Camshaft, 252-0611: Same as 0607, for Road Race, Wide Powerband applications (GX200, 6.5 Chinese OHVs, and Predators ONLY). Also recommended for Mini-Bikes

Camshaft, 252-1007 (120 ONLY): High RPM Grind for Large tracks. 8500+ rpm, 26 lb Springs required 11 to 1 Compression or higher*

Marine and winch board applications work best with 252 0211
Mini-Bikes work best with 252 0611

---------- Post added at 08:55 AM ---------- Previous post was at 08:48 AM ----------

Search for posts by bob58o. But make sure you're sitting in a comfortable chair.

What you thought I wouldn't be all over this one? LOL
These are the posts I come here for. I get to play engine builder with somebody else's money. Even if it only gets built in my head.

---------- Post added at 09:00 AM ---------- Previous post was at 08:55 AM ----------

And I read too quickly. I see you are NOT trying to turn 8000 RPM, so sorry about that.

 

[ezcome-ezgo]

I didn't want to presume you are available. But as usual, you did not fail to be informative.

 

[bob58o]

1.) Increasing compression is usually good for Torque. So head milling is good. Thinner Head Gasket is good.

2.) Smaller Diameter Carbs work well to keep the intake charge velocity up. A 28mm carb might scream at 9000 RPM, but not move air fast enough at 4000 RPM. Similar with the exhaust diameter and exhaust gasses Large carbs and large pipes move more air at high rpms, but slow it down. You want fast flowing air at low RPM.

3.) High Duration Cams, cams with more overlap, and cams with a later intake center line tend to move the powerband towards higher RPMs.
Cams with lots of overlap may require more static compression to regain some of the combustion pressure at lower RPMs.

Hard to look at just one of the numbers on the cam profile to determine the powerband. Have to consider the how each of the specs affects the others and consider the whole package.


LOL I have to stop myself. I could go on for days.

 

[Truax]

Thank you for the input, I was hesitant to post with how much information there is already available. Read over most of the options at NR, think I will be going with that 252-0207. If smaller diameter carb are what I'll be after for torque, maybe I'll just play with the jetting and bore on the stock carb. That being said with the direction I'm thinking here will I need a fuel pump to keep up with this still or can I get away with gravity? Again sorry for repetition of old topics, I really do appreciate the guidance.

 

[bob58o]

Thank you for the input, I was hesitant to post with how much information there is already available. Read over most of the options at NR, think I will be going with that 252-0207. If smaller diameter carb are what I'll be after for torque, maybe I'll just play with the jetting and bore on the stock carb. That being said with the direction I'm thinking here will I need a fuel pump to keep up with this still or can I get away with gravity? Again sorry for repetition of old topics, I really do appreciate the guidance.

Gravity should be fine.
You should try the stock carb first. And go from there. I think the stock carb has about a 15-16mm ventruri. Some common upgrades are a gx390 carb, or 22mm mikuni (or knock off). I believe the stock carbs reach maximum suckage around 5500 RPMs, so if you are looking at a 6k RPM cam, the stock carb is a good starting point. If you want a little more top end later on, then maybe try something slightly bigger.

 

[OzFab]

We could throw facts, figures, old wives tales, etc, at you & argue back & forth all day about the best source of torque but, at the end of the day, it comes down to how much torque you want & how much you're willing to pay for it...

Quite literally, IMO, the best/simplest source of torque is in gearing; using a predator, you can gear for masses of torque but, the trade-off is top speed will be very low...

There are very few NA 4 stroke engines that still produce torque above 6000rpm (unless you start talking about purpose built racing engines) so, shooting for anything above that would, IMO, be a waste of time & money...

Personally, my engine has had very little work & it's extremely torquey...

 

[bob58o]

Quite literally, IMO, the best/simplest source of torque is in gearing; using a predator, you can gear for masses of torque but, the trade-off is top speed will be very low...

All I want for Christmas is a Honda / Clone / Predator bolt on 3 three speed manual transmission with 1.5:1, 1:1, 0.67:1 internal gearing capable of 10k RPM input and 25HP for under $300.

 

[ezcome-ezgo]

It should have a reverse gear too. Why not?

 

[mckutzy]

I guess if the Chinese managed to come up with a cheap trans for a big twin type motors , that actually had Hobbed gears and not cast pot metal.... Then we could see a trans for that kinda money...

 

[ezcome-ezgo]

It would need to be engineered and an example built in the western hemisphere (or Japan) first, then they could reverse engineer it, give it a few years and finally we would have what we need.

 

[Truax]

A guy can dream. So I got that 252-0207 cam from NR, billet con rod and flywheel from ARC, and 22 lbs valve springs. From what I've gathered it sounds like I may be able to get away with stock valves and retainers, or am I mistaken there?

 

[bob58o]

Stock valves, retainers, rockers, and pushrods should be just fine.
Here's where I get a little confused. I've never used 22lb springs or 18 lb springs. One of them doesn't work with the Hemi. They are much taller than stock. I can't remember which (it may be the 18lb ones). But the 26lb springs will also work with stock valves, retainer, rockers, and pushrods and I know they fit the Hemi.

Let me look. Watch this video. As a matter of fact, watch the entire series. They aren't short but packed full of good info.

https://www.youtube.com/watch?v=-jWR2078D5I

---------- Post added at 09:30 PM ---------- Previous post was at 09:22 PM ----------

BTW, the 252 cam is probably fine for you, but I hope you looked at all of them. I wasn't suggesting it, just using it for an example. It should work great for you, but I hope you realize that I wasn't trying to steer you in any particular direction.

 

[OzFab]

...billet con rod and flywheel from ARC, and 22 lbs valve springs.

Sorry to say but, those parts won't do much to increase torque; pretty much all they will do is allow for higher RPM...

Put simply, to increase torque, you need to increase air flow; in simple terms, an engine is merely an air pump & the more air that can flow through, the more torque it will produce so, forget the billet rod, flywheel & valve springs & look more towards compression ratio, valve size, head capacity, port work, high flow air intake & exhaust...

The last two stock items are very restrictive... intentionally; replacing them alone will give you a torque increase...

Apart from those, the most important mod I performed was replacing the 18cc head for a 14cc head; smaller capacity = higher compression ratio...

P.S. The only bottom end mod I performed (as in, below the head gasket) was the governor removal...

 

[bob58o]

It was the 18 lb spring that will coil bind with a stock cam in a hemi head. The 22lb ones work!

 

[Truax]

I did check out most of the cams on the NR site, this one seems like the best option. OzFab I am doing the intake, jetting + e-tube, and exhaust. I also ordered thinner gaskets to slightly increase compression hopefully. I know the said mods to go beyond 5200 rpm are primarily for hp gains and whatnot. I have just wanted to do an engine build for awhile and have yet to do it. I'm not even sure what I am putting this thing into when it's all said and done. Thanks again everyone for the input and guidance it is much appreciated.

 

[OzFab]

I know the said mods to go beyond 5200 rpm are primarily for hp gains and whatnot.

Umm, not exactly; any mods to go beyond 5200rpm (billet conrod, lightened flywheel & heavy valve springs) are only useful IF the engine is still producing torque at that point which, in most cases, is NOT the case...

If you look at the torque curve of a stock predator, it starts to drop at around 2600-3000rpm so, only removing the governor & fitting the aforementioned parts is all but pointless because the engine has stopped producing torque by that stage...

On the flip side, you can leave the bottom end intact & only perform top end mods (hi flow air intake & exhaust & rejetting the carb if necessary) & the engine will be more powerful; why? Because it's producing more torque...

Hope that helps...

 

[Truax]

I hear what you're saying OzFab, that being said I'm still going to build myself an engine. Just something I've been wanting to do for some time. Not disagreeing with you by any means, what you're saying is completely valid. Maybe I'll regret it down the road, but for nothing else I want the experience of it.

 

[ezcome-ezgo]

Go forward and conquer, Land of 10,000 Lakes Man!