Yikes! Compression!

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L31MaxExpress

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Yeah, I built a 383 with 210 PSI cranking pressure on 93 octane. Thing can tractor-chug up a hill off the IAC at 450 RPM without stalling. It's comical.
Sound about like the 355 I built with a small rv cam and 52cc 601 305 heads. It would idle around town at 800-1000 rpm in overdrive with a locked torque converter.
 

CrustyJunker

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Yeah, mine was a miscalculation on my part. I already had all my build parts before I found out that my pistons were at .000" deck on assembly. Between that and an emissions-friendly cam, I was stuck with it. I expected lots of knock and high NOx on testing, but the results were much better than expected. It's efficient with a very broad torque curve, but I'm sure I'm leaving power on the table with a better matched cam. Because of the cubic inches alone, I'd think it would rip with some extra duration.
 

L31MaxExpress

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Yeah, mine was a miscalculation on my part. I already had all my build parts before I found out that my pistons were at .000" deck on assembly. Between that and an emissions-friendly cam, I was stuck with it. I expected lots of knock and high NOx on testing, but the results were much better than expected. It's efficient with a very broad torque curve, but I'm sure I'm leaving power on the table with a better matched cam. Because of the cubic inches alone, I'd think it would rip with some extra duration.
With great quench you can get away with more compression than otherwise possible. My pistons have a broad flat surface that matches the same surface on the head. The quench distance is 0.041". Excellent quench as the heads and pistons were designed. I have had a bigger cam in the same longblock. Just went smaller to work within the rpm range of the truck manifold.
 

L31MaxExpress

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Fired it up on the stand with a dual plane EPS intake, Q-Jet and HEI. Timing is 14° initial, 36° total at 4,000 rpm. Vacuum advance canister is a L82 with 10° @ 10 in/hg. Idle mixture screws adjusted for highest vacuum. At 750 rpm it pulls nearly 20 in/hg vacuum. At 625 rpm it pulls 17 in/hg. Throttle response is instant. At 3,000 rpm and no load it is pulling about 23 in/hg vacuum and likely would have pulled a bit more had I blocked the ported vacuum ports off on the carb. With the 5.3 fixed orifice PCV "valve" it pulls about 4 in/hg crankcase vacuum at idle when I blocked the vent and measured the vacuum at the dipstick tube. Rings seem to be sealing great.

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thinger2

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High cranking compression and no EGR? You better watch the knock sensors for activity!

"I" would leave the EGR functional.
[EDIT]Did the lifters have oil in them? Pumped-up properly? If the lifters were full of air, you'd get high cranking compression readings due to the shortened duration.[/EDIT]

I've got a few of those compression testers--it would be worth connecting yours to "shop air" and seeing if the compression tester gauge reads the same as the gauge on your air pressure regulator. And--I learned this the hard way--the spark-plug threaded ends and crimps on those hoses is suspect. I threaded the hose into the head, and when I tried to un-screw it, the hose just turned on the metal piece. Took an hour and lots of harsh language to get the adapter back out of the head. I took the adapter and hose to a local hydraulic shop so they could re-crimp the assembly after cutting off 3/4 inch of the end of the hose.
Yessir! check it with as many different compression testers as you can.
That is make it or break it time.
Never let your test equipment tell you a story.
See if the backup equipment has the same story.
If not,
time to stop and do some backtrack to see what went wrong.
 

L31MaxExpress

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Yessir! check it with as many different compression testers as you can.
That is make it or break it time.
Never let your test equipment tell you a story.
See if the backup equipment has the same story.
If not,
time to stop and do some backtrack to see what went wrong.
Yes sir...Checked it with two different compression testers. Also used a DCR Calculator for a sanity check. 11:1 6" rod 383 with a 198 intake duration on a 108 LSA and 104 ICL aligns almost perfectly with the results. Rhoads V-Max lifters bleed about 20° @ 0.050 off the duration at low speeds. If the Pat Kelly DCR calculator is accurate, I am at 9.6:1 DCR.
 
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thinger2

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Yes sir...Checked it with two different compression testers. Also used a DCR Calculator for a sanity check. 11:1 6" rod 383 with a 198 intake duration on a 108 LSA and 104 ICL aligns almost perfectly with the results. Rhoads V-Max lifters bleed about 20° @ 0.050 off the duration at low speeds. If the Pat Kelly DCR calculator is accurate, I am at 9.6:1 DCR.
Awesome. I brought it up because so many people will drop a few grand on an engine and then measure it with 30 bucks worth of harbor frieght crap.
 

Dropped88

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Also feel like sharing this. Robbed a stock intake manifold of all the hardware. Had been running that intake a while back on the Tahoe, had put a solid 75K on that 305. Still had the EGR enabled in that tune. I had been using a bottle of GM Top Engine Clean prior to every 7,500 mile Mobil One oil change. Basically pulled the intake hat off, unplugged the MAF, cycled the key on, off, on, then started so that the PCM would fail the MAF. With the engine idling I slowly poured the top clean down the IAC opening in the throttle body. At approximatly 1/2 can, I would increase the pouring speed until the engine stalled out. Then waited a few hours. Reinstalled the TB hat, plugged the MAF in, cranked it up and took it on a spirited drive. What the intake looked like with the regular top clean. Plan to top clean the 383 the same way moving forward in the future.

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What gm top engine clean do you use? The one im thinking of they quit making
 

Erik the Awful

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Awesome. I brought it up because so many people will drop a few grand on an engine and then measure it with 30 bucks worth of harbor frieght crap.
Dang, I feel attacked.

Nice work! Now I'm really wanting to build a run stand. I'm not brave enough to run my engine on the build stand.
 
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