Thread: Give me your opinion on this engine combination....

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Thanks to all of you who helped me out on my 408 combination. I have all the parts I need to assemble the shortblock but I'm not ready to shell out the funds for the camshaft and valvetrain components at this time. So, I think I'll repair the 350 that's in the car now while I'm saving up some money so I can ride until the 408 is done. The story on the 350 is some idiot(me) was detailing the engine getting ready for a car show, and got a little too happy with the water hose. One of the connecting rods got all bent up about it. It has been in the car for around 12 years but I've only put a little under 10,000 miles on it. The inside is spotless and there is almost no wear on the cylinders or the bearings. All it needs is one rod and piston so it won't be that expensive to fix it. When the 408 is done I can save this engine for a truck I'm building. It's a 4 bolt bored .020, balanced, speed pro h631p pistons(flat top 2 valve relief), 64cc camel hump heads, Comp xe268 cam, 1.5 rockers, Perf. RPM intake and a 600 carb. and 1 5/8 shorty headers. I've never been happy with this engine. I've always had detonation problems even with 93 octane in the hot summer and it seems like this pile of parts should make more power. It's a dog. I degreed the cam when I built it (106 I believe) and I rechecked it a while back and It's right. When I removed the head to pull the bent rod I did a little measuring and found the pistons are .028-.029 down in the hole. I've got a .039 head gasket so that gives me .068 quench. I'm thinking about getting some .015 head gaskets to get the quench around .044 and using my 72cc sportsman II heads on it which I believe would drop the dynamic compression to around 8:1. Give me your thoughts on this combination. Maybe some of you can dyno sim this for me and see if a change in cam timing or rocker ratio might be of benifit. Thanks for your help guys....oh yeah here's a bigger shot of the car. The bottom of the hood is painted now

724 in the cylinder @ 0.020" over
72 in the heads
6 in the piston deck height @ 4.020" x 0.028"
6 in the flat-top eyebrows (swag)
3.2 in the gasket @ 4.100" x 0.015"

811 total filled
87 total compressed

9.3:1 static compression ratio

CompCams XE268 closes the intake @ 42 ABDC installed straight up.
Add 15 for KB calculator, enter 57.
7.86:1 dynamic compression ratio.

RPM HP TQ
2000 127 333
2500 164 344
3000 218 382
3500 269 403
4000 310 407
4500 345 403
5000 375 394
5500 380 363
6000 374 327

With a 7.86 DCR and 0.043" squish, it should run well on pump gas in my opinion. Makes a nice torque curve from 2500 to 5500. Bolt on a set of 1 5/8" equal-length, long-tube, tuned headers and make 417 hp @ 5500 and 440 ft/lbs @4000 through the mufflers. Then, bolt on a 750 vacuum secondaries carb and make 426 @ 5500 and 446 @ 4500. It just gets better and better. Nice combination of parts.

I just figured out I've been entering in the piston head volume wrong in the KB calculator and that's been throwing me off. Thanks for the help. You're the man

Originally Posted by 35chevy

Tech, where did you get the figure for the intake valve closing point? the cam card shows 61 abdc @ .006 and the camquest software shows 38 @ .050. I just figured out I've been entering in the piston head volume wrong in the KB calculator and that's been throwing me off. Thanks for the help. You're the man

42 ABDC came off the CompCams site. It won't give you the 0.050" figure directly, you have to use the intake centerline and intake duration @ 0.050" to figure it for yourself. This time, I cheated though and clicked the cam part number into the DynoSim where the IC @ 0.050" pops right up.

One more question...The recommended valve springs for the cam install @ 1.75 and coil bind @ 1.060 which means they coil bind @ .660 lift. This cam has a .477/.480 lift and if I use 1.6 rockers it should be .509/.512. Looks to me with everything set up properly I should have .148 to spare before coil bind with the 1.6 rockers. A tech at comp cams told me a while back that won't work. Why?

Originally Posted by 35chevy

One more question...The recommended valve springs for the cam install @ 1.75 and coil bind @ 1.060 which means they coil bind @ .660 lift. This cam has a .477/.480 lift and if I use 1.6 rockers it should be .509/.512. Looks to me with everything set up properly I should have .148 to spare before coil bind with the 1.6 rockers. A tech at comp cams told me a while back that won't work. Why?

I always figure it like you did, then put 'em in the bench vise and squeeze 'em down to max lift height. If you can slide a 0.010" feeler gauge in the 5 spaces between the coils for a total clearance of 0.050", you're good to go.

The Comp tech guy might have been advising you not to run the 1.6 rockers with an Extreme Energy grind. The profile is already ground right on the ragged edge of allowable geometry. Visualize the tappet edge digging into the cam lobe. With pressures reaching or exceeding 250,000 lbs per square inch at the tappet/lobe interface (with a nominal 300 lbs over the nose spring), you don't need the 1.6 rockers feeding the fire. You don't need 'em on this 350 anyway. The dyno I did above was with 1.5 rockers. You can sort of figure this for yourself by deducting the 0.050 duration from the advertised (0.006") duration. A difference of 60-70 degrees, as used back in the early days, will be very easy on everything. Latter day cams will generally be on the order of 50-60 degrees difference. Most all Crane Cams used a 56 degree difference. The Comp XE268H uses a 44 degree difference on the intake and 50 degrees on the exhaust. 44 degrees means 22 degrees on the opening ramp and 22 degrees on the closing ramp and that's in crankshaft degrees. It's only 11 degrees at the cam. You're picking up from 0.006" tappet lift to 0.050" tappet lift in only 11 cam degrees. That's why they call it Extreme Energy.

The Comp tech guy might have been telling you not to run the 1.6 rockers with an Extreme Energy grind. The profile is already ground right on the ragged edge of allowable geometry, so any further acceleration with faster rockers might......not work well.

That's probably it.....I didn't look at it that way. Thank you Sir