Life after Holley Sniper - back to Carb
I know. Wanted to be sure to have a smooth idle back then before tuning. Didn’t change the cam since.
What specific cam would you recommend?
I guess I have to take a closer look to the engine anyway, because I still have some bad vibrations going faster then 60.
when I put it in drive both died immediately
As to the carb(s)
I can almost guarantee you that your throttles were too far closed, and the transfer-slots had gone to sleep. This almost always happens when guys try to run too much Idle-Timing.
The cure is embarrassingly simple; just increase the curb idle screw adjustment to open the transfers, set the idle-speed with less timing, then reset the mixture-screws as may be required..
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As to what cam to choose,
well they tell me I'm lousy at that game, so, keep this in mind while I rant.
Here are some things that will help you get started down this road.
1) Budget
2) Application; what do you expect out of this combo?
a) low-speed torque
b) hi-speed Power
c) midrange pull
d) fuel economy
e) long cam-life
3) the Chassis
a) the transmission
b) the stall
c) the rear gear ratio
d) overdrive
e) power to weight ratio
Because you already have a generous compression ratio, alloy heads, and an AG intake, you are way ahead of the game, and cam selection will make or break the smile factor.
Your cam selection will depend on how you answer the big three questions above.
If yur anything at all like me, you want everything! and that's gonna require;
1) a generous budget
2) an overdrive;
3) a race-type rear gear, cuz that will keep your small engine on the pipe
4) a modest but generous stall, to get you launched
5)
as for #1;
cuz there's no sense dreaming if yur broke; save up
As for #2,3,and 4;
the OD is not just for hiway economy, but more; so that you can run appropriate street gears for the modest sized engine.
For a small-engine-streeter, having the right gear at the right time is probably more important than anything. This allows you to choose alternates among the other parameters. For instance; if you can use 4-series gears, you won't need a big stall.
But this goes back to budget. If you need or want to hit the hiway for lotsa miles, but you cannot afford an overdrive, then you have to sacrifice something else, be it gears or stall or both.
I'll tell ya, installing an overdrive is the best move I ever made on my combo. I saved up 4 years to get it, and during that time, there was a lot of stinking parts swapping going on.
So then, say you have no budget for an overdrive, nor for a higher than current factory stall, and are stuck with say 3.23 gears. This is gonna require a hi-torque cam to get off the line, and when she gets into Second, she'll be done early.
But if you can at least afford a higher stall convertor, say going from 2000 to 2400, now you can up-cam maybe two sizes. This will push your powerpeak up a similar 400rpm, which causes a power increase at the top.
But, with 3.23s, now you find out the the powerpeak is at 75 mph, where you almost never drive or don't want to drive. So now you need say at least 3.91s, to bring the powerpeak into a more useable mph range. And thus your long-distance hiway cruising rpm becomes, lets say, "harsh".
So, as you can see, cam selection, with existing chassis limitations, becomes very important.
Now
lets talk about your already existing engine;
With a tight quench design this alloy-headed dream-318 can run 190 psi still on 87E10. As compared to 155psi with iron open chamber heads, the entire low-rpm torque is increased, which is like bolting on a turbo; and this amount of pressure makes a stunning streeter. And it doesn't stop there, that increased torque translates directly to top end power. If you keep the pressure up in that 190 plus range, you can run nearly any modest rear gears, and your engine will feel much bigger than it actually is.
The sad news is that in a given engine, cam selection, pressure, and operating elevation, go hand-in-hand; the later the Intake valve closes, the lower the pressure will be. and the same at higher elevation. So then, to keep the pressure up, with each bigger cam, the Static compression Ratio has to keep up.
Now, in post #1, you say the Scr is 10.5. to be sure, this is a very good number. To achieve that in a 3.94 bore, requires a TOTAL combustion chamber size of 69.6cc. That's pretty small. and that's gonna be hard to get down to with 63cc heads. So I'm assuming your builder actually knew what he was doing, and that 10.5 is real and True. If it is, then any cam with an
Ica of 54* is gonna hit the 190 psi mark....... at sealevel.
In Austria, at 3000 ft, the pressure is predicted to drop to 173psi........ So you see where I am going right?
Now, a cam with an Ica of 54* is generally considered a pretty small street cam, like about a stock 360 2bbl cam. Your Lunati at 262/268/114 cam, 220/226@050, in at 110 has an Ica of 61*, which, at 3000 ft is predicted to make just 163 psi. The Scr would have to be pumped up to 11.8, to get to 190 psi.
Here's the part where I tell you to go do a compression test, to see where you currently stand; as others have already suggested. Here's the deal, at 163 psi you could have just run iron heads at 10.5Scr and made more power on them with the same cam. How is that possible? Because the alloy heads suck so much heat out of the chambers that at your power level, you need about an extra point of compression, just to break even.
So, Like I think I said in a previous post, I would throw that heat-sucking, pressure bleeding, power not making 114LSA cam away........ lol.
Of course you see what I did there right?
I assumed that you have a true 10.5 engine and
I assumed the cam was in at 4* advanced, and
I assumed that Jakuzzie lives in Austria, and
that Wiki is correct when it says the average elevation in Austria is about 3000 ft.
if you're at Neusieidler Lake, elevation 377ft, that's gonna be a different calculation.
Ok now lets talk a lil bit about cams. I have a rudimentary understanding of cams, and experience with only a few of them , so here's what I have found;
1) for gas mileage
you need about 108 or more degrees of Power Extraction, the more the better. Going down to 100 degrees the cam will suck dry every gas pump in your neighborhood. By 112*, it is steeling degrees from somewhere else. Is that bad? Not every time.
2) as for compression, you need about 120*s. This will vary with the desired cylinder pressure
3) as for over the nose power, you need a bunch of overlap, 60* is good, 66 is better 72 or more is getting to the limit of streetability. A reasonable compromise would be 62*.
4) ok lets add up what we got so far. This is an exercise.
120+108+say 62=290. We only have two full revolutions plus the overlap in the which we can make this work, so, in this case; 720+62=782 degrees. Subtracting the 290 leaves 492 for intake plus exhaust. Now adding back the overlap of 62= 554 total, and dividing by 2=277 degrees to be allocated to each of intake and exhaust durations.
However, for flexibility in setting the installed center line, and maintaining a decent overlap, I'm gonna give 4* from the intake, to the exhaust, making it 273 intake/281 to the exhaust. Next I'm gonna take these numbers and calculate the Lsa, like this
(intake plus exhaust) less overlap, divided by two, and I get 107.5.
Next, I'm gonna gather up all those numbers and get;
273 intake/120 comp/108 Power/281 exhaust/on a 107.5/ in at 103.5
Next I'm gonna see what might be commonly available. And I see;
272/280/108 which, installed at 104, at 3000 ft is predicted to make just 163 psi
272 intake/120 comp/108 Power/280 exhaust/on a 108/ installed at 104.
Compare that to your Lunati cam
262 intake/119 comp/108 power/268 exhaust/on a 114/installed at 110
Easy peasy. Is this (the one in Blue) the cam you should get?
Answer ; NO!,
the Ica is too late! with only 120 degrees of compression, the cylinder pressure is predicted to be just 164@
[email protected] Scr. This would be fine for an iron-headed 318, great actually, but with alloy heads, it will be down on power/torque, as compared to what it could be, cuz the alloys with a tight Q can easily support 190 psi, which I previously mentioned, would take an Ica of 54 degrees@3000ft.
So I gotta stop talking here, cuz all this talk is based on assumptions of elevation and true Scrs, and the total lack of hard data, lol. At this point, it is all just parlor talk, designed to get you thinking.
Happy HotRodding