Recently did a cam/head swap on my 350 chevy. Originally it had "487" casting heads, a 204/212 duration cam, and a performer intake with a quadrajet carb. The motor ran fine before I did the swap. The motor has flattop pistons. I put on vortec heads that have a z/28 type spring good for .550 lift. The cam I used is a comp extreme energy 218/224 duration .462/.469 lift. I used a cloyes true roller timing set that had 3 different ways to install the cam. We installed the cam "straight up" because I was told the cam already has 4 degrees advance ground into it. I used a .041 head gasket. I also put on a performer rpm intake. I am really dissappointed in the performance I am getting. In fact I think it ran better with my old setup. Currently running 12* initial advance, 32* all in by 2800 rpm. About 48* total with vacuum advance plugged in. It just seems the motor is really unresponsive for being almost 10:1 compression. Any ideas of where to start looking for where I went wrong? Thanks.
07-06-2009 08:44 AM
Have you tried it running with 91 octane fuel yet? Close to 10.1 compression ratios require it.
07-06-2009 09:32 AM
I would not use Comp Cams if they were free, UNLESS is is a custom grind, then they seem to work fine. I have have had the exact same experience as yours and ultimately it was the cam. In Fords, Chevs, and most recently a AMC. The 401 ran OK with the Comp Cam, about the same as the stock cam that it replaced, but after swapping it to a Isky, it ran like it should.
07-06-2009 09:35 AM
i've had trouble with comp too. that brand of cams has officially been scrapped
07-06-2009 09:51 AM
the gasket is probably too thick, but that is not a big enoguh issue to cause a major power loss- your timing loosk a hair too low as well. And as noted that cam may not be optimal- but shoudl still be decent. How does it do over 3,000 RPM- that is where you should notice the biggest difference.
07-06-2009 12:19 PM
Well, I'm definetely going to have to do some more investigating because I know something is wrong. The motor is in an 86 chevy 1/2 ton 4x4. I know it is heavy but it wont even think about breaking the tires loose. It has a th350 with a 3.42 rear end and stock converter. The vortec's are 906 casting #. This particular cam is spose to run in the 1300-5600 rpm range. I can't get it to climb over 4500 rpm. We did verify true TDC and marked the balancer according to that, but yes it is an aftermarket timing pointer. However, we played with the timing quite a bit with no real change in power. When hooked up to a vacuum gauge it was pulling 14 hg. The valve springs I used are spose to be good to .550 lift. Could these springs be too heavy for the cam I am using?
07-06-2009 01:09 PM
Originally Posted by Chevy21
Could these springs be too heavy for the cam I am using?
The spring rate has nada to do w/the current problem.
If you are absolutely positive the ignition timing is fine, I'd suspect the cam/timing chain and gears has been installed a tooth off.
07-06-2009 01:11 PM
I`m curious, did you use the same Quadrajet carb when you got the performer RPM? if the performer RPM is a square bore pattern the Q-jet will bolt up but the secondary`s will not work due to it being a spread bore carb. I sure hope that`s not happened. Next is, where is the vac advance connected to? a ported or a manifold vacuum source?
07-06-2009 01:49 PM
What is the casting number of your vortec heads? (under the valve cover)
New, recent GM Parts heads or the original '96 to 2000 GM OEM L-31 truck heads, or after market copy?
Apparently the new/recent GM service heads are from Mexico and do not flow the same as the old OEM L-31 heads or make as much power. (typically down 20hp according to some engine builders dyno results)
Hmmm I did not know this about the mexico castings, my 062 castings were from scoggin dickys and there was "assembled in mexico" in the casting? 20hp is a huge deal. I researched the hell out these heads and that info never came up before now, makes me wonder...I would like to know more about this :confused:
I have recently done the same vortec swap and have a comp XE 12-235-2 grind which is a 210/218 dur @ 0.050 with 447/462 lift and 111 lsa 4x4 cam. Also used 41 thou gasket, performer EPS with edy 1405 carb, 1 inch spacer, headers. Cam is straight up and run 12 deg initial adv, 45 total with the vac adv can, 3 spd tranny with 3.73 in a 72 C10. I am getting very good performance compared to my old 76cc heads, other than some valve train noise which I can't make go away. Also I made the mistake of not measuring the piston to deck height so I don't know the squench or the true CR. It will smoke the tires without dumping the clutch and break the tires loose when shifting hard into 2nd. There is something a-miss with your setup for sure.
07-06-2009 02:43 PM
Originally Posted by F-BIRD'88
You have to join the forum to read in the "advanced" section this is not the first mention of the difference in the new Mex cast heads.
Joe Sherman who comments in this forum thread is a internationally recognized custom racing engine Guru (among others that hang around this forum) that hase worked quite a bit on Vortec head combos and knows his stuff.
I have not personally flow tested the new Mex heads myself but have worked with the older OEM 062's that were made here in Canada and installed on the 96 to 2000 L-31 truck engines.
They flow tested at 228cfm in stock form with stock valves.
Seems that every time I am on this forum I gain some more knowledge. I would have thought that any 062 casting from 96 on would be the same. So when we are taking L-31 it means from 96-2000 only. After that (2000) a 062 casting would be the mexico head and not really a L31? Thanks F-BIRD
07-06-2009 05:29 PM
Well shoot, I may have to go back and double check that it didnt get installed a tooth off. Although it shouldnt have because we turned the engine over to make sure that the dots on the cam and crank lined up with each other after making a revolution. Quesition: There were 3 different ways to install the cam; retarded, straight up (which I used), and advanced. Installing it advanced increases low speed torque while retarding it increases high end torque. Would it have been better to install it advanced for low speed torque in a heavy vehicle like mine?
Anyway, yeah I used the same quadrajet carb. I had to use an adapter for it though going from the spreadbore carb to squarebore intake. The adapter is the open center style. Well, I am going to do a compression test on it tonight and see what I find.
07-06-2009 05:37 PM
I would get a holley carb and throw the adapter in the trash. This is part of the problem itself. Open adapters disrupt flow, vacuum signal plus hurt low end torque.
If the vacuum advance isn`t connected to a manifold source then do so, see if there is a major difference. If ping is detected you`ll need a adjustable vacuum advance unit. The combo were running is similiar to yours and it runs like gangbusters.
07-06-2009 05:53 PM
Go get a 3310 holley 750 vac sec
07-06-2009 10:24 PM
Well, I didnt get a chance to do the compression test on it tonight. The vacuum advance unit is hooked up to manifold vacuum. Before the swap, I did buy an advance kit for it. I think I ended up using the two medium springs. The exhaust is Hedman 1 5/8 headers into 2 1/2 in. pipes with bullet mufflers. Does anyone know where to buy jets and metering rods for q-jets? I guess I wouldnt know where to start with those. Well, in the meantime, I will look for an adapter with a split plenum. If worse comes to worse I may have to tear it back down to see where the heck I went wrong.
07-06-2009 10:40 PM
Originally Posted by Chevy21
Does anyone know where to buy jets and metering rods for q-jets?
HERE is a place that has a LOT of tuning parts for the Q-Jet, some that are hard to find like the power piston springs in sets that you don't have to buy a ton of other tuning parts to get, like w/Edelbrock.