Small Block & Big Block Chevy Specific - lack of top end power, any ideas????

As the title states, my engine seems to be lacking some top end power. It kind of stumbles and seems to lack a lot of response above 5K. My set up is a stock 69' 350 shortblock with World Sportsman II heads P&P and Lunati cam that specs out @ .507" lift and 235*/235* with a Holley 750 cfm carb and for ignition I have an MSD street fire HEI. It seemed to run best with about 36* timing without the vacuum.

I think my problem is the HEI cannot keep up above 5K because ive tried adjusting the carb and the timing every which way and the problem has never gone away. Please help with any ideas.

i could b wrong but i dont think its the HEI, it may not be the best out there but ive used it up to 5500 no problem

what kind of intake do you have and gearing?

Oops, forgot that part. Its an Edelbrock performer RPM with Mac mid length headers a y pipe and a bullet. The gears are stock 3.42's

The reason I suspect the HEI is b/c I was reading up and the MSD in cap coil produces a max of 42,000 V which when I compared to other similar coils thats usually achieved at around 5500 rpms.

hmmm well do u happen to have the part number to that cam?

my HEI is fine through 7000RPM

heres the link to lunati's site:http://www.lunatipower.com/Product.aspx?id=1950&gid=277

kenSS: What HEI do you have?

mk cool....well i can say that exuast plays a part in the rpm range some what....most of the time, although over looked often, y-pipes and narrower exhaust are for more low end tq and less top end hp this could very well be part of the problem....i would for sure try and open it up a bit more.. try a bigger exhaust tube and get rid of the y-pipe and try an x or h pipe, and if able maybe some long tube headers...do u have cats?? ditch em or get high flow ones.....it says ur cam goes to 6200 rmp and if you have an exhaust system built more for low end tq than top end hp, ur cam and exhaust could be working against each other....thus not allowing the cam its top end potential


narrow exhaust=low end
wider exhaust= top end

and y pipes are generally low end as well

does that make sense??

heres the link to lunati's site:http://www.lunatipower.com/Product.aspx?id=1950&gid=277

kenSS: What HEI do you have?

it's a stock HEI but with all MSD goodies inside. Kind of cheating on an HEI I guess.

it's a stock HEI but with all MSD goodies inside. Kind of cheating on an HEI I guess.

Maybe its not the dist. then. I was talking to a friend of mine and he said maybe too much timing and too small jetting on the carb. If I read them correctly the carb has # 72 jets on it which sounds really small.


mk cool....well i can say that exuast plays a part in the rpm range some what....most of the time, although over looked often, y-pipes and narrower exhaust are for more low end tq and less top end hp this could very well be part of the problem....i would for sure try and open it up a bit more.. try a bigger exhaust tube and get rid of the y-pipe and try an x or h pipe, and if able maybe some long tube headers...do u have cats?? ditch em or get high flow ones.....it says ur cam goes to 6200 rmp and if you have an exhaust system built more for low end tq than top end hp, ur cam and exhaust could be working against each other....thus not allowing the cam its top end potential


narrow exhaust=low end
wider exhaust= top end

and y pipes are generally low end as well

does that make sense??

I agree with everything you said but I dont think having mid lengths and a y pipe would cause the engine to stumble at high rpms. I think it would limit my top end HP potential like you said.

i see i guess the firts time i read it i didnt see the stumble part....sorry about that, i dont think that would make it stumble either lol....im not real sure where to go with it at this point or what it could b besideds mabey timing, mabey an expert will chime in

also u might make sure all your wiring is correct, if it is id just call MSD and talk to them about it, mabey others have had the same problem and they know where to guide you more

here is the MSD tech line: 915-855-7123....hope that helps good luck

thanks android, I was just looking at Holley's site and the original jetting for my carb was a #70 so I guess having #72's is ok. Something I just realized is that I was probably setting the timing wrong. How did you set the timing on yours? and should I set it while the engine is at 2000 rpms?

no problem.....and what i did was just set the initial timing with the engine off at TDC and where the distr. was pointing to the number one cylinder and then wired the plugs form the firing order....i dont have a timing light or anything so i took it to my buddy and he did it with that, although he said he didnt have to mess with it much cuz it was pretty much dead on, and i had no problems

I have a timing light but Im not too sure how to use it b/c its an adjustable one. Maybe I will start another post for that one.

ya i cant really remember either what he did exactly sorry but ya im sure theres someone on here that knows

i think if you start at 16* and have all your timing in at 3000 rpms , 36* total , would be a good place to start.make a chalk mark on your dampner so you can see it better with the timing light.

I posted a reply on this yesterday. It was there for a few secondes and then disappeared...

Anyway, if you have an adjustable light, the procedure for setting the timing is pretty straight forward. First, make sure the TDC mark on the damper is visible (white paint or chalk works). Start the car and let it warm up and reach normal idle (900 rpm or so). Unhook vacuum advance if you have it (can't tell from the photo). Turn the nob on your shift light until the pointer and TDC mark line up. Read the gun...this is your base timing (12 to 16* should be good). Now, have someone take the engine up to about 3000rpm. Repeat the procedure (vacuum advance still unhooked). Read the gun. This is your mechanical advance. By 3000RPM, your mechanical advance should be all in. You should be somewhere around 36*. If you are a lot less than that, your mechanical advance needs adjusted (lighter springs to allow the advance timing to come in quicker).

Remember, these timing numbers are general numbers. You will need to find out what works best for you. Also, your problem doesn't sound like a timing issue. Did you get the right valve springs with that cam? It sounds like valve float to me.

The springs I am using are the ones that came with the heads.

Heres a link to compare the ones I have vs the ones that lunati recommends for it.
http://www.summitracing.com/compare.aspx

if it doesnt show up, its model # 830110 vs 73943

The lunati tech I talked to before ordering the cam said it would work ok with the cam but he could be wrong....

Let me know what you think and thanks for the help with the timing.

I checked out the springs. The World springs (830110) seem more suited for a hydtraulic roller cam...not a flat tappet. They are rated at 130lbs on the seat at 1.850" installed height. You are probably closer to a 1.750" installed height. I don't know the spring rate, but I bet your seat pressure is 150lbs or more and your open pressure is 350lbs or more. That's what I run on my hydraulic roller that is turning 6800RPM. The Lunati spring is 108lbs on the seat and 340lbs open. The springs MAY be causing lifter collapse, not valve float. How did you set the lifter preload when you installed the cam (1/2 turn, full turn, etc)?

Mess with the timing first. If that doesn't get you anywhere, you may want to try setting the valves again. Find the zero lash point and do 1/8 turn preload. See if it makes any difference (good or bad).

I messed with the timing a little and it is still doing the same thing but now that my timing was set the right way my fuel pressure went up to over 12 psi so I have to readjust it.

When I set the rocker arms I tightened them untill the pushrod had some resistance and then I tightened it about 1/2 turn more.


Unfortunately I was test driving it after setting the timing and my transmission pooped out on me. So now tuning the engine is on hold till I can afoord to fix it.

sorry to here about your tranny. if it's not one thing it's another.

sorry to here about your tranny. if it's not one thing it's another.

Yup, talk about being pissed off...:mad: I was even considering selling it. But that wouldnt make much sense after putting so much money into it. I just want it to run right for once!

I'd agree with too much spring, and restricted exhaust flow. A single 3" isn't going to be enough.

I know people who swear by hei's and make a lot of power, could it be leaning out on the big end. Try putting it on a dyno and get an air fuel ratio

I'd agree with too much spring, and restricted exhaust flow. A single 3" isn't going to be enough.

Well, if nothing else fixes it It will only be $100 to buy the recommended springs and the retainers. Tomorrow I will reduce the fuel pressure and see how it reacts, I will also reset the timing.

I had to drive the car down the street even with it being hard to shift, on my way back home I pulled it out of third (hard) and whatever was holding it back let go. So for now it seems to be temporarily fixed. I know it probably wont stay fixed but hopefully it will last enough for me to tune the engine.

Sorry to hear about the trans problems. Maybe it is just the linkage binding?

12psi is a lot of fuel pressure. I would expect it to cause problems at low speed, and not so much at higher RPM. Have you pulled a spark plug to see what they look like? If you get a chance, take one out and look at it. Let us know what you see.

how does messing with the timing change your fuel pressure?

fwiw i would first start with the fundamentals, can you check your fuel pressure at w.o.t. ? If the engine isn't getting fuel that will cause your stumble and lack of top end performance for sure. How are your spark plugs, are they a tan color? If they're black and sooty then they are dry fouled from being too cold a heat range. Although expensive, i swear by iridium spark plugs, they fire easier than conventional electrode plugs and would help at high rpm if that's where the problem is. What you can also do first is simply narrow the gap on your plugs 0.005" if they're currently gapped > 0.045".

for the adjustable timing light, what you first do is set the dial to whatever base timing you want. Let's use 10 degree BTDC, so set the dial on 10. With engine at idle, no vacuum advance hooked up, you should read 0 on the timing mark on the crank pulley. As you turn the timing light dial back to 0, the timing illumination on the crank pulley will move towards 10 dBTDC. With base timing set rev engine up to 3000 rpm and illuminate the timing mark on the crank pulley, turn the timing light dial upwards past 20 until you being to see the timing marks illuminated on the pulley. Adjust the dial to illuminate 0 on the pulley, then whatever the dial is at on the timing light is your timing.

i kinda disagree with the exhaust being the problem, a 3" is not too small. Also, if the exhaust is choking the engine then that manifests itself as the engine hitting a brick wall at a certain rpm & load, not a stumble. If the other stuff checks out, then i would look into valve train next, specifically the springs and your lifter preload and seat pressures like was said. At that rpm, springs and valve train as a whole plays a very big part. You mentioned you used the springs that came with the head, the springs should also be matched with the cam & rocker arms.
http://www.chevyhiperformance.com/techarticles/0508ch_valvesprings_guide/index.html
http://www.compcams.com/technical/FAQ/FAQValveSprings.asp

OK, so I adjusted the timing to 15* @ idle and 35* total. I will leave it like this for now until I get some more time to mess with the fuel press. reg.

While I was adjusting the timing I checked out the FP gauge thats in line with the carb and it was now reading 15psi!!! I couldnt get it to go any lower than 12psi by adjusting the regulator.(causing fuel to come out of the fuel bowl adjusting screws) So I wanted to see how it would react if I gave it some throttle. I held the throtttle steady @ around 2K (hwy RPM's) and the FP went down slowly to 2 psi. I tried revving the engine some and the press. went down really fast to almost 0.

So my thoughts are that my fuel pump is just not able to keep up or my regulator is not doing its job. I bought the pump off another 4th gen but I thing it might have been a 6 cyl.

if you are revving the engine to 2000-3000 rpm in neutral, then there is no load on the engine and it is using very little fuel... certainly no where near what it would need in gear under load. So you definitely have a fuel delivery problem. In neutral you should be able to bounce it off the rev limiter (or float valves) and your fuel pressure should not change.
But under load at w.o.t. it's not uncommon to only have 1-2 psi of fuel pressure at the carb when there had been 6-10psi at idle.

what kind of fuel pump are you running? Do you have an electrical one in the tank or on the frame rail, or is it a mechanical fuel pump mounted on the engine?

Its an in tank pump off another fourth gen. I have one off of a 94 z28 also but the sender doesnt work on it. So bought gas tank with good sender in it and I just swapped tanks. But the thing is that it might have come out of a 6 cyl camaro.

Heres a link to the regulator I have and how it is hooked up.
http://www.holley.com/data/Products/Technical/199R7914-3.pdf
I have it hooked up like "Figure 1" It used to be hooked up like "option A" but then my FP gauge didnt even register any pressure even while the engine was runnning.

Found a new walbro 255 lph fuel pump for $50 so I got it today. Now I just gotta find some time to install it this weekend.

So I got around to installing the fuel pump and I now have a steady fuel pressure. The regulator was set @ 3psi and I was too tired to mess with it after swapping the pump out by myself(major PITA btw) Good news is that it holds the pressure even at high rpms even though its in neutral. It does surge up to about 6 psi sometimes though.

I took it out for a test drive and theres no more popping at the top end but it does loose some power at about 5K but it picks back up if I stay in it. Im assuming that now my issues are adjusting the pressure and then maybe playing with the timing some more.

Thanks for your help guys...It has come in very handy.

Finally for some good news. I was driving around today in the 100* weather and the car was running a little hotter than normal. Usually it runs at about 190* but because of traffic it was at a hair over 210* Ive always been uncomfortable driving it hard when it is at this temp nut there were some idiots messin with me at the light so I sped off at the light. When I did this I noticed the sputtering completely disappeared:confused: I found this a little odd because just a few hours before It was sputtering a lot. Anyway, I took it up to 6K in 2nd(close to 75mph) with no hiccups at all. I thought it was just by chance but I tried it several times after that and it ean great.

Could it be that its been running too cold this whole time? or maybe I need better spark plugs. Ive been running it w/out a thermostat b/c my last one seized up on me while driving and I just havent gotten another one.