Can I run the KB107’s and those heads on 91 octane?
yes
but at 62cc, you are gonna have to co-ordinate your parts pretty carefully for 91 gas, and to your elevation.
For closed chambers and tight Quench, your pressure limit for 91 gas is about 165psi.
For best results, the subsequent Scr needs to be co-ordinated with the EFFECTIVE DCR, and the cams ICA (Intake Closing Angle).
So if the engine is operating at sealevel, it will be able to operate with LESS Scr than one at some higher elevation.
The target is always the same; namely an Effective Dcr of about 8.0..
So;you sortof have to know
1) at least about, where you want the power to be.OR
2) how much power you want, OR
3) definitively, the application.
But, for best results,nothing can be calculated until the operating elevation is known.
You already gave us #3, and sortof #1. and we know it has to cruise at 70/75 with 28.8 tires, with an A518 overdrive, and I assume with
a LockUp TC. Since you are budgeting for a rear end, lets look at that first, because it sets the stage for everything else.
Ok, so a 5.9 bruiser in a 3800# truck is gonna need some OFF-THE-LINE JUMP.
Ima thinking an Effective rear gear of 11/1 with 27s so that would be 11.7 for the 28.8s. And that, with a 2.74 low gear, works out to 4.30s or maybe 4.10s, depending on the stall. 4.30 x 2.74=11.78, and 4.10 x 2.74=11.23
Lets try 4.10s. These will get you 75=2476 in LU. 70=2311@70. And that will allow almost any cam. So, lets see what the other gears are doing;
32 mph with 4.10 x 2.74 will be 4200 in first, and 2260 in second, so that's pretty good. For passing 55 will be about 4500 @10% slip,in Second, so that will work. and 60 will be 4900(10% slip) in Second gear, which IMO is a lil low in the rpm band..
4.30s would make it 5140, and 4.30s will cruise at 75= 2600/ swell for a dual purpose machine. But not the best, obviously, for fuel economy.
My vote is the 4.30s because, hitting 60 at 5140 allows a power-peak of 4840, and that spit out the cam as about 218@.050, which could be a 262 advertised in a flat-tappet, or maybe a 268 in a roller. That's already a big cam for cruising with, so lets sacrifice some power and go back to the 4.10s.
4.10 hit 60@4900, allowing a power peak at say 4700 and that spits out the next smaller cam at 212@.050 which could be a 264 advertised. Yeah lets see if we can make that work with your Scr.
Are you following my reasoning?
to recap; I'm looking at 4.10s and a 212@.050 roller cam, so far. And 4.10s will cruise 75=2480 in LU.
The starter gear is 2.74 x4.10 x24/28.8= 9.36 corrected to a 24" tire, perhaps a tiny bit low, but the 212* cam, I think, has that covered if we can get the cylinder pressure up.
So lets go work on that.
the 212 as a roller could be 266 advertised on the intake and say 272 on the exhaust. We are gonna need to keep the pressure up so I would normally choose a 110 LSA. But in your case, with the fairly aggressive 4.10s, lets try a 108. So that recaps to 266/272/108. And that will get you 120* of compression so an Ica of 60*. And an extraction of 115* so good on gas too. The Effective overlap comes to 52* so that will have a nice lil hump on the power curve; I like it. Lets see what it looks like installed at 500ft elevation.
You won't believe it!
At 9.7Scr this makes 161psi and a VP of 141 Both of which are perfect. At 161 with closed chambers and tight-Q, you can run 89 gas. Remember I assumed 500 ft elevation.
At 900ft, I can up the Scr to 10.0, which will raise the pressure to 165 for 91 gas and the VP climbs to 145, which will make a real tire fryer.
Now; I like post #10, that was my thinking when I built my combo.
So now the question remains , how to get to 10.0 or at least close to but not over it.
So assuming an overbore of .020
the swept volume becomes 744.6cc, and to get to 10/1 Scr we need a total chamber volume of 744.5/(10.0 less 1.0) = 82.73cc
IIRC the heads are 63 and the .039 FelPro is 8.8, so we are short 10.9cc. the KB107s have a 5cc eyebro so we need a deck clearance of 5.9cc = .028 Badboom! lol.
Not so fast. The 107s into your Magnum block are gonna be about .012 down which is only 2.5cc. So the total chamber becomes (2.5 +5 +8.8 +63)=79.3 and so the Scr becomes (744.5 +79.3)/79.3=10.39 a lil much. Furthermore the Q is up to .039 +.012=.051 borderline too loose for detonation control. So we have a double whammy to solve.
First thing is this;The cheapest easiest best way to solve this is with an .028 gasket and a slightly later closing intake. I am very confident of the ability of the FelPros to handle even 11/1 Scr at 180 psi; so I would like to keep them. But your combo is wanting .040 less .012= .028 gasket. Quite a few guys have had luck with the .028s so lets give them a try. So this solves the Q problem. But it changes the total chamber volume to 2.5 +5 +6.8 +63=77.3 and so the Scr becomes 10.63. Whoops.
To get the pressure back to 165, the Ica needs to be bumped out to 68*, which is a friggin big street cam so I don't like it. This combo want alloy heads with which you can jam the pressure up to 180 psi still on 87E10.
So no, this won't work.
We need some more chamber volume, but we need to maintain the Tight-Q or jump the Q outtasight past .080, which will require an open-chamber design. I'm not fond of that idea. So then, the next idea is a Dish in the pistons, but the piston needs a step on it to bring the Q back into line.
The KB 232 is such a piston, but the dish is 18CC ! bringing the total chamber to 88.8 and the Scr to 9.38 with the .039 gasket and to 9.58 with the .028 gasket. Ok 9.58 is in the ballpark. The Wallace says a pressure of 156psi, with a VP of 137. Ok that'd doable with 89 gas for sure and maybe even 87. Not he best but pretty close.
Let me advance that cam to the max and see what happens; Badaboom, I get 160psi and Vp of 144 , carumba back in action!
Ok to recap;
the KB 232s drop in whereever. you have to machine the Quench step off to ballpark the Q into the zone of .022 to .050, generally .032 to .040, while simultaneously selecting whatever gasket to get the total chamber to about 86.8cc or less, which will ballpark you into an Scr of 9.58 or better. Then install the 266/272/108 roller-cam from 1 degree retarded to 4 degrees advanced, to get the pressure up around 160psi for 89 gas.
If this all sounds a lil vague, it is because I have no idea exactly at what deck-clearance, the 232 pistons will drop in at. If I knew that, I could be more specific.
However, I suppose it would be possible to find flat-tops that come in at more than .012 deck clearance........ but that would kill the tight-Q.
Ima thinking with those small chamber iron heads, the Q-step and dished 232 piston is gonna be the answer, for this size of cam..
And there is no other type of piston that will work between this size of cam, and one with a much later closing intake.
However, the KB107 has enough meat in the crown, that you could easily machine a D-cup into it, to satisfy the Scr requirement of your engine.
Actually, the KB 107 is probably the blank from which the 232 was designed, I'm guessing, to satisfy this exact situation.
What if we just pumped the power up, and never mind about "best gears"
Well lets go back to the KB107s in at .012 below deck . With the 8.8cc gasket, the 5cc eyebrows and the 63cc chambers, this totals ~79.3 and at .020 oversize the swept is 744.6, as before and the Scr is then 10.4..
The absolute smallest Ica you could probably get away with is 65* for a pressure of 166psi@ VP of 140 very nice numbers but will need best gas at WOT. Ok so on what roller cam would you find an Ica of 65*?
Hey check it out;
I get
276/284/108, in at 107. That is big as far as flat tappets go, but in a roller could be just 223@.050 I like it! The 223 cam will power peak at about 5000, and the 108LSa will want to be shifted at perhaps 5300, so that is working out with the 4.10s.
So because this just went from very expensive to about as cheap as it gets, lets whack those KB107s in there with whatever gasket gets you to a Quench of .022 to .042. Then calculate the actual for real Scr. And then select a cam to drop the pressure below 165psi,which we now know is possible, so you can run 91 gas or less; I like it
