Here are specs on my motor

bottom end is stock l48 motor circa 1975
cam = ***** thmpr (running straight up)
heads = 2.02 valves 64cc chambers 180cc intake runners
intake = edelbrock performer
carb = 600cfm edelbrock performer
roller rockers and flat tappet hydraulic lifters
2800 stall

Here is my issue. I put the new heads/cam in the motor and it doesn't have the power at all that I think it should. Here is what I have done recently:

I just checked compression on all cylinders. Low cylinder was 145 and high cylinder was 160. I don't know what it should actually be, but I thought they all seemed fairly close so I ruled out valve adjustment problems. Rockers are dancing pretty well, so I think I can rule wiped lobes on cam.

I also rebuilt the carb the other day and it seemed to help slightly. Still doesn't idle that well.

I resealed the intake too, to try and make sure I didn't have any vacuum leaks. I have also plugged off all vac lines to try and limit the amount of vacuum leak I might have.

Timing:

I have vacuum advance unhooked and have set all in timing at about 35 degrees. I just read a thread on here and it stated that total timing (centrifigal + vacuum) should be like 48-52 degrees. It just seems to me that when you stomp it and are at WOT you would have no vacuum so basically would be running at the 38 degrees from centrifugal advance????

I really thought these heads would wake this motor up as it had the old smog heads with tiny valves. So far, I would say that it runs maybe as good as it did before I pulled the motor I understand it may not be a monster, but seems like it should be significantly more powerful than it was in its prior state.

I am running out of ideas and patience. Any help would be greatly appreciated.

 

That cam (***** Thumpr flat tappet) has 235/249@.050" lift. That is a very big cam for an L-48 even with 64 cc heads. You are basically at about 9:1 compression depending on what head gasket you used. That cam will run with 9:1 but probably likes 10:1 or 10.5:1 to really work. That is why your compression check is so low.

You basically have a very mismatched combo. The cam will start to make power at 3000 rpm and be great at 6500 rpm. The Performer manifold runs from idle to 5500 rpm. The Edelbrock "non-Performer" carb (it is anything but a Performer despite what advertising says) is probably good up to about 6000 rpm. The stock L-48 cast pistons and crank are only good to 5500 rpm.

Swap the cam down to something like a Comp HE268 or XE262 and watch the car come alive. If you want to keep the "all show, no go" Thumpr, you need to step up to at least a cast steel crank and hypertuic pistons (or go all forged) so you can rev to 6500 rpm. Then swap the intake for a Performer RPM at least and throw a 750 Holley on it.

I run a lot smaller cam (216/228 @.050 hyd roller) with 10:1 compression and my car pulls hard from 2000 to 6000 rpm.

 

235* @ 0.050" is too much cam w/ ~ 9:1 street 350

 

Which heads specifically? Need more head detail before any thoughts can be shared about the cam.

The Thumpr line of cams are made to have extreme overlap to give a "rumpty" sound. They give up a certain level of fuel economy, low-speed driveability and power to do so. So it's important not to look too hard at the duration numbers as for the Thumprs they basically don't make sense

From a post I wrote on another board:

If you have the cam card with seat-to-seat events just add Intake Valve Opening (IVO) and Exhaust Valve Closing (EVC). That's what "overlap" is - the duration that both valves are open.

Let's look at a common street cam: CompCams XE262H Cam Card

Take the valve timing at .0006" (may also be stated as .001 on some cards, but NOT .050):

Valve Timing @ 0.006" Lift:
IVO: 25
IVC: 57
EVO: 69
EVC: 21

25+21 = 46 degrees overlap

Let's compare this to a Thumpr cam: Comp Cams 279TH7 Cam Card

Valve Timing @ 0.006" Lift:
IVO: 36
IVC: 63
EVO: 76
EVC: 41

36+41 = 78 degrees overlap (!) Yeah, this one's going to rumble :metal:

Understanding cam timing (as well as us mere mortals can ) really helps make some informed choices on cams. It's not a "black art"; the basics are easy to get your head around if you spend a little time reading and thinking about it. Once we get into high RPM, high HP and mondo heads it gets far more complex - but up to about 1.4 HP/CID it's pretty straightforward.

CompCams Cam Timing Tutorial

David Vizard Cam Basics Overlap Estimator

 

The carb [600 cfm] and Chevy iron heads aren't 'bad' but they aren't the best, either. However, I think your problem may be that the carb linkage isn't allowing the secondary plates to open all the way.

Have someone sit in the car while you are looking into the carb (engine OFF, of course). Have them put the pedal on the floor and you can check to see if the secondaries are opening all the way....or not. {Using your hand to exercise the carb throttle is NOT the same as depressing the accel pedal.}

 

BIG CAM! for your combination, who picked that out? I would have stayed in the mid 220@0.050 range if I were choosing cam for your motor.

 

Probably the most common cause of lower power for sure. I can't think of any street GEN I that I've seen on the dyno that had the butterflies 100% vertical...

 

Have some mercy on me guys!! I picked the cam. This is my first build and picked it because hot rod/car craft/ some magazine did a combination similar except with vortec heads and made great power with that cam. I know it is referred to as the show cam, but I thought it would be ok since it made such good power in that article.

My heads are aluminum. They are called flo-teks and are sold in southern Indiana at a place close to my house. Supposed to be descent heads for the money.

I have a question about the overlap. You mention that the cam is why my compression pressure is low. How would the overlap of the cam cause this. Unless I am missing something, the overlap doesn't occur during the compression stroke. Please don't think I am trying to argue...just trying to understand.

Also, if the cam is, in fact, bleeding off some of my compression: What compression numbers should I be seeing with my L48 bottom end and 64cc chambers??

Thanks f

 

I don't think anyone's beating you up - but you asked

There are two factors here - duration and overlap.

Long duration cams are made to produce peak power at high RPM, sacrificing low RPM torque. Folks like to brag about horsepower, but torque is what you feel in your "butt dyno". I expect your perception of lack of power is exactly that - you're feeling the car being sluggish at low- to mid-range RPM. If you wind it out, around 5500 or so it'll wake up...but this isn't really a street cam.

High overlap is another high-RPM cam design. As RPM increases, there's less time for valve events - so we start overlapping the events more and more so we can make max power at high RPM...sacrificing low- and mid-range economy. You'll NEVER get a smooth idle out of a Thumpr cam, and if you read their web site the cam is specifically for people that want a "rumpty" sound at idle. That's coming from a significant amount of the intake charge exiting the exhaust. There's always some, as this is one of the ways the exhaust valve is cooled, but the Thumpr has a bunch. Thumprs are reasonable cams designed with unreasonable overlap...just for sound. Forgive me, but I generally hear them referred to as the "Ricer Fart Can of Cams" - they make more noise than power.

So, in the end - the cam is doing what it's designed to do, not what you want it to do. It's a high-RPM cam that's specifically designed to give a racy idle. What you appear to want is a cam with good "grunt" off the line and not so much high RPM horsepower.

I've not personally worked with those heads, and without some specs I can't offer any guidance on the right cam...but I can guarantee it won't be a Thumpr.

Finally, be careful trying to replicate magazine builds. Keep in mind that sometimes they're trying to make a specific point (bore/stroke ratio, etc.) rather than being a good build overall. They're also being run on an engine dyno with an electric water pump, no accessories and dyno headers...so the power numbers are far from what you'll see in the car. Finally, if you don't duplicate the build exactly, don't expect anywhere near the same results.

So, the GOOD news is that you picked the wrong cam, but it's a VERY easy and relatively inexpensive swap to fix it. Post a pointer to the head flow specs and you'll get a good bit of feedback on alternatives.

 

Your disappointment sounds very similiar to mine when I finished my top-end rebuild. Motor was marginally better at best. I was very disappointed until I thought to check the carb linkage (as other have pointed out already). What a difference having secondaries makes.

 

Thanks for the input guys. I have checked the primary butterfly and it does open. I have not done a check to see if the secondaries are opening. If I shove the throttle quickly from idle, the secondaries will open a little. But, driving down the highway under load I haven't checked.

 

Do not confuse the secondary air valve with the secondary throttle plates. The air valve is at the top of the carb and the plates are in the base. "Have them put the pedal on the floor and you can check to see if the secondaries are opening all the way". You need to push the air valve open and look down inside the carb to see if the throttle plates are opening fully (verticle). Don't be surprised if the air valve doesn't go fully verticle. The car must be at operating temperature, but not running.

 

I did some looking at the cam card and I now understand what you mean about the big cam killing my compression. I noticed that the card states that the intake valve doesn't close until around 67 deg ABDC. That is a lot of the compression stroke that is unusable because of the intake valve hanging open. This makes sense now why you need pretty high compression to make a cam with lots of duration run well.

Well, I guess I get to take her back apart. I will just look at this as a lesson learned type thing. Not really excited about tearing it back apart, but I would also like to drive her again!

The motor had a Crane RV cam in it, so I think I will just slide that back in it for now. I would expect that I could have quite a difference with that setup just having the higher compression heads with better flow characterstics. If not, I will just roast marshmallows on it Update: Crane cam is p/n HMV-278-2

Thanks for all the input. This newb appreciates the help very much.

 

I have nearly the same cam and set-up, the main difference is that I have 10-1 compression and my car launches like a freakin rocket FROM AN IDLE when I mash the pedal. I cant believe 1 point in compression is going to make that much of a difference between running great and running like crap so I think you have other issues..
What is you vaccum at when idleing? I'm running 8hg so I would assume your even lower than that with less compression so if you have a 6.5pv in the carb, it can also be causing a rich idle issue..
What type of ignition system are you running? With that cam and the low cylinder pressure it produces, especially at the low end, your going to need some good spark to burn the fuel (your probably already fouling plugs if you dont). If you dont already have it, I would highly recommend a high output coil and some type of multi spark system like a Mallory Hyfire or MSD 6A.. These cams like alot of intial timing too. I run mine locked out at 38 degrees (no vacuum advance and my weights are tied off) and I use an ignition cut-off switch for starting (you could run a start/retard if you wanted). I also run on pump gas and dont get any spark knock.. That alone made a world of difference on the low end and smoothed out my idle too.. Your idle will probably be best around 1000rpm.. As soon as you mention the word "***** thumper" cam, you will get flamed know matter how it runs.. Trust me, I have one and it runs awesome and sounds like a top fuel drag car.. I have 295/50-16 tires on it and I cant even get traction from a roll if I stomp on it..

Do you get any hesitation off idle? IF so,you will need to open the secondary buttlerflies slightly.. If the idle increases, back downthe primary curb idle. I personally would switch to a different carb like one of the new Summit carbs (they are cheap and run great).. I had one and it was a VS and it ran as good as my new Mighty Demon DP. At that time I had other issues and thought it was the carb so I switched but that wasnt the issue.

Edit: Actually my ***** Thumper cam is one size bigger than yours.. Mine is 257 @ 050 so there is no reason it shouldnt run decent if the tuning is correct..

You just have to tune these cams like its an all out drag car, not a street car and thats why so many people cant get them to run right.

 

I would like to see you check the secondaries on the highway
But in the driveway that sounds good that they only open alittle. I'm not sure where your complaint is??? Is it low end torque or is it the way it revs out at the topend?

 

If the secondary throttle plates "only open a little" when you fully depress the accelerator [with the car sitting], that's exactly where they will open on the highway. The air valve (upper plate) needs to be opened manually in the driveway and, if the air valve is properly released by the choke mechanism, the air valve will open by itself during normal operation. There is a spring loaded air valve adjustment that regulates the rate of opening (angle of opening vs. the airflow into the secondaries) but that is just the 'fine tuning' last step to optimize the secondary action. You MUST get the throttle plates to stick straight up when the pedal is on the floor if you expect to get full power out of your engine. If that pedal action is off by as little as 1/4", you can lose a bunch of power.

(I would hazard a guess that at least 50% of C3 owners that have Q-Jets do not have secondaries that are fully open at WOT.)

 

hate to say this, But I did a search on those heads ..floo-teks they come from TSCH which is the Pro-Comp people, they seem to be China Pro-comp heads... if they are they are not any better than a stock pair of heads. they cost about $550 each...

the power comes from the heads.... imho, junk the heads and get some good quality ones.

Cam, the one you chose is great for "rat-rods" they want to sound bad ***, but they are not. that cam is more for show "sound" than go.

I agree, may be a carb problem...secondaries


and you are defiantly mismatched all over the place..


good luck...

 

No dice on the secondary plates. They are 100% open.

Funny thing. I had my wife opening them and I got very excited because they were not opening but about 1/8th. I then realized why that the choke was not opening and the linkage was keeping it from opening all the way. Once I release the throttle linkage, they opened right up. Darn!

I also just did a vacuum check. I was hoping that it was going to be low. To my surprise it shows about 6 inH20 (11 mmHg) at idle. I think that is pretty respectable vacuum.

 

6" is low vacuum. About right for that cam. Stock cams run 15-18" and most people like no lower than 10-12" for performance cam.

Try that Crane cam and get back to us. I think you will be pleasantly surprised.

 

11mmHg is about 4"Hg and would be very low for any engine. Something is not right with that set-up.