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So which way are you going to go to fix the problem? The bob weight is known so swapping to a 3.250 stroke crank would be the easiest route. Or you could get another set of pistons and then rebalance the 3.400 crank and build a 347 inch engine. Either way would work. If the pistons you have now were ground on to match weigh them they won't take them back but if not it shouldn't be a problem.
The engine balancer said he ground a little off the crank, I definitely can not return it. Plus, the cost of another crank is less than 1/2 of another set of pistons, and either change will require re-balancing, I'm going with the new crank.
If they can just re-use the bob weights, it would obviously be less work, and hopefully he won't have to charge me as much. But if he had to cut the crank, wold he have had to cut the pistons as well? Is it a regular practice to cut the pistons? I know some rods come with extra materials for this.
Given the correct crank is a shorter stroke (by .075"), I expect the throws to be lighter. So which part will need to be cut to balance this? I'm thinking counterweight.
The way it works is the person doing the balancing will weight all of the parts, match weigh them, and then calculate a bob weight. Since you're going to be using all of the same parts except for the crank they can just make up a set of bob weights to what they used previously and balance the new crank. This should be easy and take a lot less time than the first time.
The stroke will be shorter but on most stroker cranks that also means that the counterweights are cut down less for clearance at bottom dead center. This should make the crank heavier and have more to take out for the same amount of bob weight.
I told SCAT that I got the wrong crank with my kit, and they referred to Summit for return/replacement. Summit was really good about it; they sent me a new, correct, crank from their Nevada warehouse, and even included a UPS shipping label. It arrived at my door Wednesday, and I brought it over to the shop today (Thursday), and sent the incorrect one back. The shop checked the new crank to make sure it did not stick out of the deck like the other one with a couple of pistons.
Next it's back to the balancer, who says he needs to repeat everything. I'm hoping he won't charge me full price.
Here's a mystery to me: Every example installation I've seen of these stroker kits showed the bottom part of the cylinders needed to be carved out a little to provide clearance for the rod bolts. If you do not, since the stroke is a little longer than stock, they will most likely hit the bottom of the cylinders. When he had the longer stroke crank in there, with all the pistons and rods installed, he was able to turn the rotating assembly with no interference. We looked at the closest approach between both ends of the rod bolts to the base of the cylinders in each bank, and saw almost 1/4" of clearance at the closest. With this correct crank having a .15" shorter stroke, or .075"shorter throw, the clearance was even more.
So, did I somehow get a block with really short cylinders? It is a 1987 block. I thought they were all the same? Should I have the cylinders carved anyway?
One little detail: The SCAT 1-94055 kit comes with ARP 12-point cap screws on the connecting rods. Maybe they stick out less than standard rod bolts?
When you build a stroker engine the amount of interference with the block that you find is really up in the air. It mostly depends on your choice of stroke obviously but your choice of connecting rods and just the block casting itself can be big variables. Most of the time the Scat capscrew rods are about the best available when it comes to block clearance from what I've seen. I've seen blocks where a particular combination will clear on every cylinder on one side with plenty of room yet hit every one on the other side. You just have to check it and if it clears it clears.
Also sometimes I'll take 8 of the rod bolts and cut down the height of the heads in the lathe. Just taking off .080 or so isn't enough to make them difficult to tighten but it can make a tremendous difference in how they clear the block.
Got the engine back today, and the first thing I checked was the clearance between the bottoms of the cylinders and the rod bolts. They all look about like this:
at least 1/8" between the bolt head and the nearest cylinder at closest approach. Is this enough? I hope even if I rev the engine past 6000 rpm, the crank throw won't stretch that much.
The other end seems to have even more clearance:
I'll install the cam next week and try to map out a couple of the lobes.
Yesterday evening I ground a cam just like your cam for a customer who brought in a stock "TE" cam. I used a master for an E303 and brought the lobe separation in 4 degrees. It worked out nicely. The overall size of the intake lobe ends up being 1.570 vs 1.640 for the exhaust. This is because the intake lobe has to change more(gain more duration) to get to 220 duration than the exhaust does. You can think of it sort of like your squeezing down on the lobe, base circle to nose to get the duration to increase. I could have probably made them come out a little closer on overall lobe size by moving the intake lobe only 1 degree and the exhaust side 3 degrees to still get a lobe separation that's 4 degree tighter. In this case I was going from 116 to a 112.
After a couple of false starts, I finally got some measurements off this cam. Having never done this before, I'm sure my technique probably did not yield the most accurate results. I installed the cam, lined up the dot on the cam sprocket with the 0 degree point on the crank sprocket, and found TDC using a fixed reading on the dial gauge by approaching it going forward and backward, noting the angle on the degree wheel, and split the difference and set that as the 0 reference. I installed a pair of lifters onto the lobes for cylinder 1, placed the tip of the dial gauge onto the top edge of each lifter, zeroed the dial indicator, and turned the crank, reading the degree wheel at different lifter heights. Since the advertised lifts were for 1.6:1 rockers, the first reading I took was at 0.03125" lift for the 0.05" valve lift reading. Then I took degree readings at every 0.05" lift increment/decrement.
In this list, the 0-degree mark is where everything is referenced, at the intersection of the tail of the exhaust ramp and beginning of the intake ramp.
The max lifts are very close to advertised (.499/.496 measured vs .498/.496)
I'm seeing intake duration of 243 and exhaust duration of 246 at .05" valve lift; much longer than advertised.
If I use what I believe are the centerlines of the intake (116 ATDC) and exhaust (115 BTDC), I calculate a lobe separation of 115.5, which is wider than the 112 advertised.
These differences may be due to measurement errors on my part, and the somewhat stickiness of the dial gauge I was using (Harbor Freight). Maybe I'll try again with an old cam I know about.
This took a surprising amount of time, for just cylinder 1. If I was paranoid, I would measure all the cylinders.
I won't be able to accurately measure the actual base circles, just their differences after I install heads and rockers, and checking with the checker pushrod. That's going to determine the actual pushrods I'll be ordering. If the differences aren't too great, I figure I can order the a length for the smallest BC, and shim the pedestals on those with bigger BC.
The durations are always listed at cam not valve lifts so that's why your .050 duration numbers seem so large. This is the reason why even stock cams from flathead engines with no rocker arms tend to seem pretty big. From what I've seen checking the durations at lower valve lifts can be difficult to do in the engine. Getting the centerlines correct is very easy but as you go farther and farther down either side of the lobe small errors in lift measuring start to make much larger variations in duration.
That is a subtle bit of information I did not know about duration measurements. So based on my measurements, the durations from .05 lift at opening to .05 at closing shows 223 intake and 224 exhaust. But those number may be longer than actual due to the way I measured them in the engine.
What do you think of the shapes of the lobes? Aggressive, Mild?
And does the 116 ICA seem reasonable? My crank sprocket has keyways for +/- 4 degrees, so I can change that if it can help street driving.
The LSA seems to be similar to stock TE cams, so not the 112 advertised. Maybe that will be more computer friendly.
Your duration ended up being about 4 degrees larger at .050 than the cam that I ground the other day using the E303 lobe on the TE core. This means that you've got about 2 degrees more duration on each side of the lobe. This would give you 4 degrees more overlap if the lobe separation was the same but since yours is 4 degrees wider on the LSA it will end up having less overlap than the cam I ground even though the duration is higher.
If it is on 116 I would try to put it in with some advance 3 or 4 degrees would probably be enough.
After some rather annoying delays at work, I finally got back to the engine.
I installed the cam with a 4 degree advance as you recommended.
I then tried to take measurements for the pushrods using the Comp adjustable checker. I measured a known 6.250" rod with my digital caliper for reference, and adjusted the measurements I got for my engine. I bolted the heads down to 70 lb-ft torque on the ARP studs using a used set of Fel-Pro gaskets that I measured to be .042". I bolted the rocker pedestals to 20 lb-ft, and set the lifter preload at 1/2 turn after contact, which works out to about 0.018" based on the 1/4-28 threads on the adjuster. I'm not sure what the tolerances SHOULD be, but I found quite a range among the 16 lobes:
It looks like the lobes with the largest base circle are 2E, 7E, and 8E with their PR lengths measured at around 6.258, closest to the stock 6.250 spec.
The longest length is 1I at 6.289, about .031" longer than the shortest.
Shopping for pushrods, I see Comp shows 6.300" as their next size up from 6.25". It looks like Summit has a couple of choices in between 6.25 and 6.30.
I measured the full plunger depth on the lifters to be about 0.140". Assume I use 6.300" rods, that would leave about 0.098" of plunger travel for 2E, 7E, and 8E. Would this be enough, or should I order different lengths, including some of those odd lengths available from Summit?
I think that the 6.300 should work fine. The lifter has plenty of room to make up for the differences and it won't effect performance. These small variances in the overall size of the lobe make no real difference. .100 change in base circle only changes the .050 duration by about 2 degrees on a roller cam and on a flat tappet it doesn't change the duration at all.
