Hemi Predator Build #2

[bob58o]

Just for some data on cylinder head temps. Back in the early 90s I raced box stock flat head Briggs 5hp and we would shoot for 385 degree head temp. If I saw the temp start with the number 4 while racing ( indicating the head temps were in the 400 plus range) then I would reach back and adjust the mixture screw on the carb. Most of the time it would only vary about 10 degrees either way of 385 degrees once dialed in. Spark plug reading is quite a science. Plus there is a lot of things that can lead to inaccuracy. Cylinder head temp and Exhaust gas temperature is a lot more accurate for tuning.

How long did it take the engine to get up to temp? A few minutes at idle? A few warm up laps? My engine temp hasn't shown anything near 400 degrees using the spark plug ring gauge.

Guys that race seem concerned with cylinder head temps. I'm guessing either by means of dyno or track time, they know that their engine performs best at 385 or 400 or whatever. I know people like to tape up the blower housing, to increase head temp and also reduce drag from the flywheel fan.

I haven't ran this engine hard yet, nor have I had a chance to do much tuning. It seems to run pretty well with stock carb jetting at low throttle positions. I did raise the needle one notch. Maybe I'm rich at 1/2 throttle causing my low temps. I'm not even sure I have had it wide open yet. And I built this engine for wide open, 7000 rpm-ish power. Big valves, big ports, big carb, 250 duration cam. The torque converter/gearing/compression (along with cams 111 LSA and 102 ICL) still gives me enough low end to pull the front wheel up easily. I think the slide helps low end on bigger carbs (compared to a butterfly). Can't wait to see what it can do on the top end. Hope she screams. She should. I have my steering dampers worked out, so hopefully I can avoid a death wobble at 65 mph.

So how do I use cylinder head temp?
Say I tape up the blower housing and abuse it for 30 minutes and am only showing 325 degrees...
Does this mean I can add more timing?
Does this mean I can lean out the jetting?
Does it mean I can use lower octane fuel?
I should use a hotter spark plug?

I guess I should use cylinder head temp, along with plug reading, and road testing to dial it in.
Or I'll box up me along with my engine and ship it KF dyno tuning shop. LOL

 

[vpd66]

Cylinder head temps aren't going to help you much at idle or part throttle. We only used it to tune the motor at wide open throttle as we were racing. Back in the day when we raced flathead Briggs engines we didn't let off the throttle much. The head temps at idle seemed to be around 250 degrees when the engine was warm at idle. I remember some times when I was racing and watching the head temp gauge ,that has the motor/kart accelerated out of the corner the head temp would rise and then level off at a certain temp. This was the temperature we were concerned about. Most of the time I never even looked at the gauge. We ran Digitron gauges that were cylinder head temp/rpm. They had max recall features and I'd check the head temp and rpm after the race. Like I said, once the head temp was dailed in it didn't change much maybe plus of minus 10 degrees. So cylinder head temp is only going to really help you with wide open throttle tuning.

 

[Whitetrashrocker]

For what it's worth. On small aircraft, they use cht and egt. There are controls for throttle and mixture. On take off and landing it's full rich mixture. But one you get to cruise speed. You lean it and watch the temp rise. When it reaches max limit you richen it just a little. The temps will drop. This is where it's the best fuel mix.

Now you don't have a mixture control so your just gonna have to collect data.
But your good at that. Lol.

 

[bob58o]

For what it's worth. On small aircraft, they use cht and egt. There are controls for throttle and mixture. On take off and landing it's full rich mixture. But one you get to cruise speed. You lean it and watch the temp rise. When it reaches max limit you richen it just a little. The temps will drop. This is where it's the best fuel mix.

Now you don't have a mixture control so your just gonna have to collect data.
But your good at that. Lol.

So basically you are saying If I keep octane, timing, spark plug, etc. constant and am not reaching around 375 degrees at WOT, then I can probably lean out the main circuit and make more power???

Makes sense to me, but that is how I took it because that is what I wanted to hear.

 

[Whitetrashrocker]

In theory yes. I think the power gain would be nominal. But when your looking to conserve fuel. Every little bit helps.

 

[Noseoil]

I don't think you'll see high temps until it's wound out pretty tight & pulling for some amount of time.

I did a series of runs to dial in the main jet on a stretch of road at WOT (it's a rural airport, a paved entry road from an interstate). I was running too rich at first (bogging at below max rpm), so I had to drop down a jet size, run & drop down again. I found out where it went lean & felt a little starved, then went up a jet size & called it good.

Basically, I watched the rpms for fall-off & could feel where it started running right doing the same pass each time. It took a little while since I hadn't run a Mikuni before & my first guess was so far off, but once it's properly jetted you will see it come together. Better to run rich first than lean out at high rpms with too much heat.

The temp gauge will be good for "fine tuning" but you need to get pretty close for that to come into play. Until you can run at WOT for a longer sustained pull, it just won't come up to temperature. Short runs won't let the head warm up enough, so you'll be "flying low" at WOT on the pavement when the gauge does its job.

Let us know what you find. I changed gearing to back down from 60 mph a bit, as it was getting too scary for an old guy, but that's just me.

 

[bob58o]

Let us know what you find. I changed gearing to back down from 60 mph a bit, as it was getting too scary for an old guy, but that's just me.

I am not geared for high speeds yet. I first wanted to see what kind of RPMs I could pull with lower gearing, which I still haven't had a chance to do.

13" Tires
54T Rear Sprocket
10T TC Sprocket
7" Pulley

6.05:1 in high gear
7000 RPM is around 45mph
7500 RPM is 48mph
8000 RPM is 51mph

It easily lifts the front wheel if I am not careful. I tried a few drag racing launches where I was sitting on the very front of my 18" seat and cracked it wide open. Still lifts the front. I could easily do with less torque for daily driving.

Once I'm ready to test higher speeds, I'll put a 14T on instead of the 10T.

4.32:1 in high gear
7000 RPM is 63mph
7500 RPM is 67mph
8000 RPM is over 70 mph

If I don't get at least 60 mph once, this build will be a huge failure.

After I see how many RPMs I can get with a 10T,
And I see how fast I can go with a 14T,
I will probably settle with a 12T.

5.04:1 in high gear
7000 RPM 54 MPH
7500 RPM 57 MPH
8000 RPM 61 MPH

 

[bob58o]

I like finding prices for machine shop services!
$75 for 5 dyno pulls!!!
http://stoutracingengines.com/services.html

Machine Shop Services
Deck Block $ 15.00
Port & Polish $ 45.00
Machine & Flow Stock Carb (w/offset shaft) F/H $ 70.00
Machine & Flow Stock Carb Animal (plus parts) $ 45.00
Machine & Flow Stock Carb Clone (plus parts) $ 45.00
Mill Head (stock) $ 15.00
Mill Head and Port (mod) $ 65.00
Sleeve Block (includes sleeve, line bored & honed to size) $ 90.00
Line Bore & Hone w/torque plate $ 40.00
Hone w/torque plate $ 20.00
Cut & Lap Valve Seats $ 12.00
Install Valve Seat (includes CV seat) $ 22.00
Install Valve Guide (includes CV guide & cut seat) $ 16.00
Clearance Stock Rod $ 5.00
Remove Bearing From Crank & Clearance $ 12.00
Install Crank Bushing
(includes bushing $25.00 with machining) $ 18.00
Backface Valve Chamber (intake & exhaust) $ 12.00
Chamfer Lifter Bore (intake & exhaust) $ 5.00
Weld Ex. Lifter Bore $ 15.00
Repair hole from broken rod $ 35.00
Deck Block $ 15.00
Dyno Test Engine
(additional tests on same engine $10.00) $ 35.00
Valve Job ~ Raptor $ 35.00
+ parts
Valve Job ~ Animal / Mod Animal $ 45.00
+ parts
Engine Rebuild ~ Raptor $ 175.00
+ parts
Engine Rebuild ~ Animal $ 225.00
+ parts
Labor Rate per hr.
(welding, fabrication, kart set-up, etc...) $ 30.00
Clutch rebuild (complete blueprinting) $ 45.00
+ parts

 

[Kentucky Boy]

I think I just made the 13,000th view of your build thread.

 

[bob58o]

I think I just made the 13,000th view of your build thread.

I'm pretty sure I made the 12,999 before that, but thank you.

 

[Nosandwich]

I think I just made the 13,000th view of your build thread.

What? You only read Half?