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so I had my 383 lt1 brought in to get tuned . Afr looked good .. didn't miss. Somewhere or something is blocking the air flow I'm guessing .. I'm running an x pipe with 2.5 inch piping but some of the bends look like 2.25 .. also has some glass packs on it .. I'm running long tubes as well. I have a mild port job on the heads with ferra 2.02/1.60 valves.. port matched intake.. 58mm TB with a CAI..also running the 280xfi cam through an A4 with yank 2800 stall..opti is brand new as well are the plugs and wires . what could be causing it to die out after 4-5k rpms...? NEED HELP ASAP.
Most likely you are choking the **** out of your engine at higher RPM. What valve springs are you using?
comp springs that are good for like .630-.650 lift.. and I've seen cam only 280xfi cars dyno with curve that doesn't drop like mine . And I have some porting done .. so idk honestly
maybe your valve springs are not up to the XFI cam
what valve springs do you have?
around 5200 rpm the graph gets very FU, was the car misfiring?
even with stock heads and exhaust it should make more than 30x HP with that cam & tune
i got comp springs rated for this xfi camshaft . And it was not misfiring .. the thing at 5200 rpms was my torque converter locking is what I was told .
no just a 350 with stock heads. And they put down over 340 to the wheels ..
You cannot compare then. The breathing demands your engine has now are significantly higher than a stock CID engine and you need cylinder heads that can accommodate those demands. I cannot say for sure your power level will be higher because there may be other underlying issues present, however IMO, if you change out the heads for a pair of good ported ones done by a reputable shop your power and torque curves will be dramatically different and you'll be able to see actual power and torque peaks.
My cam only stock headed 350ci lt1
makes more power than that. You have serious issues somewhere. Even if your heads were ported only a tiny bit you should be making 370+ rwtq and at least 360+ rwhp with that cam and 383 cubes. And if your heads have even decent porting like 260 cfm around .500-.550 lift you should be looking at 400rwhp. Your prob losing 15-20hp tops with 2.5 inch y pipe and restrictive mufflers.
Last edited by 350 groundpounder; 08-17-2018 at 09:05 PM.
I also have a 383 with "mildly" ported heads, GM AL (2.02/1.60 valves) but with a smaller (218/224) XFI cam (need to pass CA smog and do), shorty headers, full 2 1/2" dual muffler and resonator exhaust, 52mm TB, 30 lb FI and I make 385 RWHP/397 RWTQ on a 91 octane tune. Yes I have boundaries that keep my #'s lower compared to many 383's
...so either your tune is..."not optimum", your "mild" head port job is less than "mild" or your exhaust is somehow restricted (doubt that is the issue)
Your AF suggests you are not starving (leaning out) the motor so fuel delivery seems good
We assume the cam was installed right...
that spike/rpm break-up around 5k due to TC lock up....not how I would read that
Your #'s are just flat out low and the graph is not as smooth as it should be at all
Your valve springs are "Comp spec" for the cam...than that would be the 918's I assume as that is what the cam card would show. I would find a different spring like yesterday after experiencing the 2 piece spring mod with my 918's and XFI but don't think that is the low power # issue you have. Just recommending you switch to a better spring (read one that won't break). LE advised and sold me the LT1 Lunati kit (and also said he will not use any Comp bee hive spring)
Do you have any flow #'s for your "mildly" ported heads??
Did whoever tuned the car have comments as to why the #'s are low??...or is that unknown and you just had a "dyno pull"?
I also have a 383 with "mildly" ported heads, GM AL (2.02/1.60 valves) but with a smaller (218/224) XFI cam (need to pass CA smog and do), shorty headers, full 2 1/2" dual muffler and resonator exhaust, 52mm TB, 30 lb FI and I make 385 RWHP/397 RWTQ on a 91 octane tune. Yes I have boundaries that keep my #'s lower compared to many 383's
...so either your tune is..."not optimum", your "mild" head port job is less than "mild" or your exhaust is somehow restricted (doubt that is the issue)
Your AF suggests you are not starving (leaning out) the motor so fuel delivery seems good
We assume the cam was installed right...
that spike/rpm break-up around 5k due to TC lock up....not how I would read that
Your #'s are just flat out low and the graph is not as smooth as it should be at all
Your valve springs are "Comp spec" for the cam...than that would be the 918's I assume as that is what the cam card would show. I would find a different spring like yesterday after experiencing the 2 piece spring mod with my 918's and XFI but don't think that is the low power # issue you have. Just recommending you switch to a better spring (read one that won't break). LE advised and sold me the LT1 Lunati kit (and also said he will not use any Comp bee hive spring)
Do you have any flow #'s for your "mildly" ported heads??
Did whoever tuned the car have comments as to why the #'s are low??...or is that unknown and you just had a "dyno pull"?
my tuner said that something is most likely choking out the motor to where it isn't getting enough airflow. He mentioned my exhaust but I HIGHLY doubt that's the case.. it's a free flowing exhaust set up.. I do not have any flow numbers but I have pictures of the porting on the heads .And yes im pretty sure they are 918s. I think what it could possibly be is that my valves are starting to float after 4500-5 rpms ? Should I re adjust my rockers/valves because they do sound a little loose . (I'm running 1.6 PRW rr's)
Like ****** stated. He's at 390+rw with a tiny cam.
your porting almost looks incomplete in the intake runners. Your tuner doesnt sound the greatest even thinking that your exhaust is corked up. Def check that your valves are adjusted correctly. Good port work that makes gains should look like this pic.
.And yes im pretty sure they are 918s. I think what it could possibly be is that my valves are starting to float after 4500-5 rpms ? Should I re adjust my rockers/valves because they do sound a little loose . (I'm running 1.6 PRW rr's)
Can't see or hear what you do but...
Valve float, for me, is pretty clear to hear when it hits and the motor just noses over, makes no power and misfires. Your dyno graph has some example of valve float usually when it starts there is a continued dropping jagged edge graph whereas yours has a big spike then more of a sputter roll downward . Valve float is ypically due to valve springs not up to the cams needs or poorly adjusted valves. 918 spring should allow that cam to rev 6500 no problem....until they break. 20k mi is max life expectancy for them with the XFI grind. 918 has a installed height spec of 1.800". Stock heads need the spring pockets machined down some to allow that. Otherwise the installed spring height is around 1.75". "Probably" not a problem...but may be regarding coil bind or more so "close to it" which not only effects performance of its spring rates it fatigues much quicker. If you measured the installed height of the spring then did the math of what ever that is vs 1.800" and then knew the new "lower" coil bind spec using that XFI w/1:6RR you would knwo it coil bind was a issue. Rule of thumb is you want at least .030 Cush between max lift and real coil bind.
I would bail on the 918's even if they were not the problem. At very least re-lash valves if there is a concern some or all may be off
Valve train needs to be set up right with quality parts to have a well working long life.
Valve float, for me, is pretty clear to hear when it hits and the motor just noses over, makes no power and misfires. Your dyno graph has some example of valve float usually when it starts there is a continued dropping jagged edge graph whereas yours has a big spike then more of a sputter roll downward . Valve float is ypically due to valve springs not up to the cams needs or poorly adjusted valves. 918 spring should allow that cam to rev 6500 no problem....until they break. 20k mi is max life expectancy for them with the XFI grind. 918 has a installed height spec of 1.800". Stock heads need the spring pockets machined down some to allow that. Otherwise the installed spring height is around 1.75". "Probably" not a problem...but may be regarding coil bind or more so "close to it" which not only effects performance of its spring rates it fatigues much quicker. If you measured the installed height of the spring then did the math of what ever that is vs 1.800" and then knew the new "lower" coil bind spec using that XFI w/1:6RR you would knwo it coil bind was a issue. Rule of thumb is you want at least .030 Cush between max lift and real coil bind.
I would bail on the 918's even if they were not the problem. At very least re-lash valves if there is a concern some or all may be off
Valve train needs to be set up right with quality parts to have a well working long life.
Do you think its possible that i didn't adjust my valve lash correctly and that is what could be causing the issue ?
poor valve lash will cause the motor to run like ***....which should be pretty evident to you or tuner. if would run like crap all through the rpm range
we just see a few pics and read your description but can't tell if you installed parts right.
your exhaust would need to be very restricted (read just not because you have non mandrel bent exhaust pipe) to cause a loss of power like you have. CATS could be restricted severly but that usually shows up as very high STFT as exhaust fuel loads up pre-CAT. You could take a IR temp gun and measure temp at front of CAT and rear of it. If there is a 200 (read a lot higher than rear) degree difference you have a clogged CAT
poor valve lash will cause the motor to run like ***....which should be pretty evident to you or tuner. if would run like crap all through the rpm range
we just see a few pics and read your description but can't tell if you installed parts right.
your exhaust would need to be very restricted (read just not because you have non mandrel bent exhaust pipe) to cause a loss of power like you have. CATS could be restricted severly but that usually shows up as very high STFT as exhaust fuel loads up pre-CAT. You could take a IR temp gun and measure temp at front of CAT and rear of it. If there is a 200 (read a lot higher than rear) degree difference you have a clogged CAT
okay. Well im running no CATs and i took the mufflers off anyways .. the exhaust is free flowing juts like it was before. I wonder if my pushrod length could be the issue?
no...it would cause quicker valve guide wear if they were to far off but not so much of a performance issue.
maybe your cam install is off, or tune is not right, or quench is so off it affects power
if you and your tuner are so convinced that there is exhaust restriction, do a dyno with open headers...I doubt it will reveal a miraculous 50 hp gain
if valves were just lashed wrong it would run like crap throughout the rpm range
it's definitely not the exhaust or my valves . My tuner said exhaust and I know it's not that .. I lashed my valves correctly .. I've done that many times and previous motors haven't had this issue .. I talked to Lloyd and we talked about what could be the issue . I'm changing my lifters due to possible lifter collapse because the might be restricting oil from going through.. my bottom end and heads were done by a respectable machine shop around where I live .. my quench should be good. My motor is running at 11.1 compression .. my camshaft was installed by the shop and my tuner has worked with a ton of LT1s.. so I'm pretty stumped on what it could be but I am going to change my lifters.
08-17-2018, 03:34 PM
Weird Dyno numbers on 383 lt1 
