LT4 Hot Cam dyno results and points to ponder
Thread Starter
Registered User
Joined: Apr 2003
Posts: 74
From: Balaton,MN
LT4 Hot Cam dyno results and points to ponder
I just had my 95 Formula Convertible M6 on the dyno, and after some fine tuning by HiTech it put down 356.7 ft-lbs and 362.2 hp 
Dynoruns 1 and 4 graph.
The Dyno run video
Point 1:
Looking at the graph there is a dip in the torque curve from about 2200 to 3200rpm, HiTech diagnosed this as possibly too much exhaust flow at lower rpm, causing reversion and resulting in loss of torque in that rpm range.
The suggested solution was to try 1.5 ratio rocker arms for the exhaust valves.
Point 2:
I am getting valve float starting around 6000rpm.
Diagnosis, not enough valve open spring pressure.
Possible solution, add .060 - .090 shims under the springs to increase the valve open spring pressure.
Unfortunately, at this time, I do not have any flow numbers for the heads from LS1speed, nor do I know which valve springs they used (Comp Cams 986 or 987?). I will have to check with them on Monday.
If I could get any cylinder head experts to cogitate these two points and give me their thoughts, it would be greatly appreciated
Thanks,
Kevin
Dynoruns 1 and 4 graph.
The Dyno run video
Point 1:
Looking at the graph there is a dip in the torque curve from about 2200 to 3200rpm, HiTech diagnosed this as possibly too much exhaust flow at lower rpm, causing reversion and resulting in loss of torque in that rpm range.
The suggested solution was to try 1.5 ratio rocker arms for the exhaust valves.
Point 2:
I am getting valve float starting around 6000rpm.
Diagnosis, not enough valve open spring pressure.
Possible solution, add .060 - .090 shims under the springs to increase the valve open spring pressure.
Unfortunately, at this time, I do not have any flow numbers for the heads from LS1speed, nor do I know which valve springs they used (Comp Cams 986 or 987?). I will have to check with them on Monday.
If I could get any cylinder head experts to cogitate these two points and give me their thoughts, it would be greatly appreciated
Thanks,
Kevin
Last edited by KOSletten; Jun 30, 2003 at 11:40 AM.
Registered User
Joined: Jun 2001
Posts: 872
From: "Shoulda Gotton An LS1 Racin"
Originally posted by treyZ28
you shouldn't be floating with that mild a cam and an 6000rpms. people are doing fine with LT4 springs. I suggest you find out whats up with that
you shouldn't be floating with that mild a cam and an 6000rpms. people are doing fine with LT4 springs. I suggest you find out whats up with that
Just so you know...
Registered User
Joined: Nov 2002
Posts: 588
From: Indiana
If you suspect a spring problem, pull a spring and have it measured at installed height and at installed height minus full lift. You will then know your seat and open pressure. CC lists the 986 and 987 spring seat pressures at 117 and 121 lbs. respectively at 1.800 installed height. Springs also tend to lose a little bit after initial use. If you want to see the effect shims will have, then add some additional readings to the test, with the shimmed height thrown in. Find out what the proper seat and open pressure for that cam is and install accordingly.
I just did this, and diagnosed a serious spring problem. A local machine shop did the testing for me for a very nominal charge. Well worth it.
I just did this, and diagnosed a serious spring problem. A local machine shop did the testing for me for a very nominal charge. Well worth it.
Thread Starter
Registered User
Joined: Apr 2003
Posts: 74
From: Balaton,MN
Re: LT4 Hot Cam dyno results and points to ponder
Originally posted by KOSletten
Point 1:
Looking at the graph there is a dip in the torque curve from about 2200 to 3200rpm, HiTech diagnosed this as possibly too much exhaust flow at lower rpm, causing reversion and resulting in loss of torque in that rpm range.
The suggested solution was to try 1.5 ratio rocker arms for the exhaust valves.
Point 1:
Looking at the graph there is a dip in the torque curve from about 2200 to 3200rpm, HiTech diagnosed this as possibly too much exhaust flow at lower rpm, causing reversion and resulting in loss of torque in that rpm range.
The suggested solution was to try 1.5 ratio rocker arms for the exhaust valves.
Anyone know if changing to 1.5 ratio rockers for the exhaust valves would/could get rid of that dip in the torque curve?
Registered User
Joined: Feb 2002
Posts: 5,645
From: Bridgewater, MA
Re: Re: LT4 Hot Cam dyno results and points to ponder
Originally posted by KOSletten
Anyone know if changing to 1.5 ratio rockers for the exhaust valves would/could get rid of that dip in the torque curve?
Anyone know if changing to 1.5 ratio rockers for the exhaust valves would/could get rid of that dip in the torque curve?
Registered User
Joined: Apr 2001
Posts: 3
From: Columbus, IN
Pretty damn close to the numbers I got with the hotcam and homeported heads. I'm using the valvesprings from IRS and have bad valve float above 5900 rpm. With a good set of springs and some more tweaking, I hope to break 370rwhp.
95 Z28, M6, Hotcam, ported LT1 heads, Edelbrock headers and catback, 30lb svo inj, 58mm TB, 4.11 gears 12.77@112
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