I have built a LT1 conversion with the help of many on this forum but I seem to have come up short on the torque when compared to other similar engines and stock LT1/LT4's. The car feels noticeable lazy below 4000 rpm.
Stock Block
Dart Pro-1 with 180cc intake port, 2.02 intake valve, 1.60 exhaust
Milled heads to 61 cc and 0.039 head gasket for 9.5 comp ratio
heads flow 257 cfm at 0.500 intake and 172 cfm exhaust
CompCams 8-466-8 XtremeFI 218/224 Duration 0.534/0.530 lift @1.5
ProMagnum 1.5 roller rockers
Hooker 2149 1-3/4" long tube headers with 3" intermediate pipe
Lt1 intake, stock throttle body,
Magnaflow cat back
Dyno Tune by Kennedy Automotive
When I degreed the cam the intake centerline came out 110.5 vs 109 recommended so that might have a slight impact and I am using only 1.5 ratio rockers vs. CompCams recommended 1.6. Would these make a significant difference??
I wonder if a lower duration cam would help or something to counter the big plenum and runners with the LT1 intake.
Any thoughts out there.???
This is my street car that I run at Watkins Glen and Mosport, no serious drag racing.
thanks in advance for any help or suggestions.
__________________ Dave Porter
89 Z51 6 spd w/ ZR1 body kit
LT1 intake conversion
In my humble opinion you would benefit greatly by advancing the cam timing to where the intake LCA was 108 or even 106 degrees, especially given your compression ratio. Just advance it 4 degrees from where you are now. The stock LT1s were 10.25:1 CR if I remember correctly, so you're giving up some there. Advancing the cam timimg would get the intake valve closed earlier which would increase low end torque.
Location: So Cal- - Defend your ideas with facts. Not personal attacks.
U need the Geek-Squad.
A simple compression test will tell you if cam is too big or c.r. too low. I'm no compression expert but i would guesstimate your in the 160psi-180psi range. IMHO i don't feel 9.7c.r. is all that low but the compress test will let us know.
I like that cam but CC info does say u need to upgrade the programmer. And i kind'a suspect the 'puter too. I see u already upgraded the exh and should not be a problem though CC recommends that upgrade with the cam also.
Yea so need to find some info on the GEN II (OBD) computer upgrades - programming/software.
Now the early Opti-spark distributors have problems too but i would save this trouble-shoot for after the compression test and 'puter upgrade. I'm learning with my Opti-spark dist and have found plenty of good info for free on the net.
It may just be me, but I think the torque and HP numbers look about right for a 350 with this cam, compression, and head combo. In fact, they look pretty good.
I'd look for your lazy throttle response somewhere else than the basic build. What does "Kennedy Automotive" have to say about the issue.
I think the biggest problem is that you are used to that stump pulling, off idle, instant, torque that the stock TPI provides. Most engine modifications shift the torque curve into a higher rpm range (Tq x rpms ~ HP) to make more power. Much of the deficit, if there is one, may be in the tune. Does the advance curve provide enough low rpm spark advance, for instance? The compression ratio and the camshaft are compatible. That is a fairly mild cam and it shouldn't kill too much of the low end torque. The compression ratio could be another half, to a full point higher. It would be better if the cam timing weren't retarded, and those 1 3/4" headers aren't helping either. All in all, I don't think you are too bad off. A little attention to some details may make you happier.
By all means, "find some info on the GEN II (OBD) computer upgrades" for your Gen I, OBD I, L98 engine. Maybe if you convert your reliable HEI to an optispark, you can spend time trouble shooting IT too.
That cam ins't an LT-1 cam so I'm confused by the terminology "LT-1 Conversion". Also the :real" '70 LT-1s had a much higher compression ratio at approximately 10.25-10.5 SCR which is needed for the LT-1 cam to have any low end torque.
It seems to me you could run a DCR calc to determine whether advancing the cam 2* will have a perceptible
effect, before you tear into the engine. There is a DCR program here, if you have no other: http://www.empirenet.com/pkelley2/DynamicCR.html
As a word of advice, unless you measured the CompCams lobes yourself, I'd take their claims of 'advertised
duration' with a grain of salt. I measured two sets of lobes of a similar nature a couple years ago, and
found the actual 0.006" tappet lift points several degrees longer than what CompCams' technical specs
stated.
If it were my cam, lacking actual lobe measurements, I'd add +6 to +8 * to the seat durations before
running the DCR analysis. Note that's just MY experience with CC XHR lobes, YMMV.
FWIW: The DCR exercise should also highlight why many folks RAISE the SCR when going to a larger cam,
rather than lower it.
It seems to me you could run a DCR calc to determine whether advancing the cam 2* will have a perceptible
effect, before you tear into the engine. There is a DCR program here, if you have no other: http://www.empirenet.com/pkelley2/DynamicCR.html
As a word of advice, unless you measured the CompCams lobes yourself, I'd take their claims of 'advertised
duration' with a grain of salt. I measured two sets of lobes of a similar nature a couple years ago, and
found the actual 0.006" tappet lift points several degrees longer than what CompCams' technical specs
stated.
If it were my cam, lacking actual lobe measurements, I'd add +6 to +8 * to the seat durations before
running the DCR analysis. Note that's just MY experience with CC XHR lobes, YMMV.
FWIW: The DCR exercise should also highlight why many folks RAISE the SCR when going to a larger cam,
rather than lower it.
Have fun.
Doctor J - thanks for the input on the dynamic compression ratio, I had heard of this but honestly did not understand, the site you pointed me to was very easy to follow and helped a lot.
It looks like I am leaving some potential power on the table. Although I still need to degree the actual duration measurements, here is what I found based on catalog data, and guessing at the stock 0.006 durations:
Note: I started with a 89 L98 as my starting point.....
I learned something new today - I thought the '89 had a stock 10:1 SCR, like the later models, but
you're right - the spec book shows them at 9.5:1 nominal. Now I see where you got the idea of
using new heads with a larger CC. It would have been better to up the SCR when the new heads
went on. Changing head gaskets again, and advancing the cam, are going to be a major PITA for
modest gains.
BTW, did you also check the heads mechanically to verify they can take a 0.570" valve lift?
Re the parametric analysis of DCR, my point above was that the 'advertised' specs are (IMHO)
at least suspect, if not downright misleading. Until you measure that cam, or something similar,
the cost/benefit of mechanical changes will be similarly suspect. FWIW, I've found that making
a cam mistake to the 'small' side is easier to live with than an error on the 'too big' side for a daily-
driver.
Also:
The idea above of playing with the spark advance at low throttle settings is probably a good one.
I'm not very familiar with the 165 ECM, but with the 727 adjusting the cruise spark advance is a
major benefit to drivability with a stouter cam. There is some discussion of computer tweaks here,
though it applies to the later ECMs. (Part-throttle drivability is not part of most 'dyno-tunes'.): http://www.thirdgen.org/techboard/di...ve3-patch.html
Location: So Cal- - Defend your ideas with facts. Not personal attacks.
Not sure what part is LT1 myself?
Quote:
Originally Posted by Scott Marzahl
That cam ins't an LT-1 cam so I'm confused by the terminology "LT-1 Conversion". Also the :real" '70 LT-1s had a much higher compression ratio at approximately 10.25-10.5 SCR which is needed for the LT-1 cam to have any low end torque.
Yea Scott, i was reading the post as he had upgraded to the GEN II reverse cooling eng with MFI/SFI, OBD I (or OBD II) and Opti-Spark dist. But since Mr. gearhead won't answer your question it ain't worth the effort to persue helping. I don't think Mr. gearhead is trying to be rude - just takes it for granted what a/his LT1 converson is. And it would have been helpful to know what has been done to upgrade the computer - as the "Doctor" covered this need and provided some great advice.
MHO is playing with cam timing is a lot of work and difficult to optimize without several attemps.
Oh-well.
cardo0
__________________
74 T-top, L-48, 4spd, A/C, narly blue. Goodwrench 350 underwent new heads (SR Torquers), Schneider solid cam and Thorley headers due to domino effect of broken motor mount >broken fan shroud>remove radiator & A/C package. Now runs a Performer Air Gap with a lars majic chokeless Qjet on top. Also Street dampner, MSD dist, 6AL and Baster II. http://members.cardomain.com/cardo0
to clarify what I meant by "LT-1 conversion" is I installed an LT1 intake manifold on a Gen I block, thereby reusing all the 89 computer sensors and OBDI computer gear. I guess that would of helped clarify in hindsight..
The computer chip was upgraded twice by a forum member and finally by a local dynotune specialist who indicated that he tuned it as best it could be. He indicated that the rest of the hesitation was likely due to the cam.
DoctorJ was right on with his advice, I just need to follow through on it, after this cold wave passes and degree this cam to see what I really have.
Location: So Cal- - Defend your ideas with facts. Not personal attacks.
Nice upgrade.
A general thumb rule is for every 2 degrees of cam advance the power band shifts 'bout 200 rpm lower. Now i'm guessing there is a limit to this and i can recall a couple of Pontiac street racers that would just advance the cam an entire tooth of the cam sprocket, which i remember = to 8.3 degrees - while they did break motors frequently.
Well your LT1 conversion is very impressive Dave and a very modern hotrodding trick. I have heard of this before as the LT1 intake needs to be drilled for a dist and water passages. Well my '94 Z28 LT1 has some good power but it does have hessitation at low rpm. The LT1 intake short runners have a reputation for lower torque - but breathe much better at higher rpm than the previous L98 intakes. I'm just learning on my LT1 but from what i've read not to expect much at low rpm - better to install steeper gearing for compensation.
And really getting it all out here on the forum helps all of us so i encourage u to post your results here.
Corvette Store
Looking for missing low end torque LT1
IMHO i don't feel 9.7c.r. is all that low but the compress test will let us know.
I see u already upgraded the exh and should not be a problem though CC recommends that upgrade with the cam also.
I'm learning with my Opti-spark dist and have found plenty of good info for free on the net.
Yea Scott, i was reading the post as he had upgraded to the GEN II reverse cooling eng with MFI/SFI, OBD I (or OBD II) and Opti-Spark dist. But since Mr. gearhead won't answer your question it ain't worth the effort to persue helping. I don't think Mr. gearhead is trying to be rude - just takes it for granted what a/his LT1 converson is. And it would have been helpful to know what has been done to upgrade the computer - as the "Doctor" covered this need and provided some great advice.
I have heard of this before as the LT1 intake needs to be drilled for a dist and water passages. Well my '94 Z28 LT1 has some good power but it does have hessitation at low rpm. The LT1 intake short runners have a reputation for lower torque - but breathe much better at higher rpm than the previous L98 intakes. I'm just learning on my LT1 but from what i've read not to expect much at low rpm - better to install steeper gearing for compensation.
