lack of top end power, any ideas????
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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 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.
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From: San Antonio, TX
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.
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.
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From: San Antonio, TX
heres the link to lunati's site:http://www.lunatipower.com/Product.aspx?id=1950&gid=277
kenSS: What HEI do you have?
kenSS: What HEI do you have?
Teching In
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From: Odessa, TX
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??
narrow exhaust=low end
wider exhaust= top end
and y pipes are generally low end as well
does that make sense??
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heres the link to lunati's site:http://www.lunatipower.com/Product.aspx?id=1950&gid=277
kenSS: What HEI do you have?
kenSS: What HEI do you have?
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From: San Antonio, TX
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??
narrow exhaust=low end
wider exhaust= top end
and y pipes are generally low end as well
does that make sense??
Teching In
Joined: Jun 2009
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From: Odessa, TX
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
Teching In
Joined: Jun 2009
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From: Odessa, TX
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
here is the MSD tech line: 915-855-7123....hope that helps good luck
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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?
Teching In
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From: Odessa, TX
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 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.
Launching!
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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 *** 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.
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 *** 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.
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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.
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.
Launching!
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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).
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).
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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.
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.