454 with comp Thumper cam troubles

[woodsey]

I have a fresh rebuilt 454 with a comp thumper cam and I cannot get the engine timed properly. It will idle great but detonate under slight load. Any input would be greatly appreciated

[68is4me]

I am not a cam expert but the thumper cams are really made to sound really good at idle. I would bet your lobe separation is at 107-108. That makes them really hard to tune.

Give us you engine specs, cam specs


I am sure somebody more knowledgeable will chime in.

[woodsey]

The engine is a 454 bore 20 thou over with 9.5 to 1 pistons cast iron heads complete comp cam valve train including timing chain and gears. The cam is is a 11-600-4 thumper cam. Hope that helps. I contacted comp cams about this problem and got a very strange response so looking else where

[68Gold/white]

Really need to know the combustion chamber volume, and how far below the deck the pistons sit at top dead center.

[gmc684x4]

The thumper cam is your issue of tuning

Move your vacuum advance to mansfold pressure not on ported pressure

Here is an article i found that talks about tuning a thumper cam

https://www.hotrodders.com/forum/sbc...300.html?amp=1

[gmc684x4]

Here is another article on tuning a thumper cam and explains some of the issues

https://www.hotrod.com/articles/carb...e-big-cam-sag/

Have you confirmed the timing zero marks and timing pointer are correct with a timing tape

[woodsey]

I have but comp cams is saying to set my timing gears dot today then remove chain and turn crank 180degrees and reinstall chain and go from there. I have installed many cams and have never been told that before

[72LB]

The thumper cam is your problem, if you want any type of performance get rid of it. Those cams were made to sound good and not really perform unless you want to build your engine around it. At one point I had several laying around here in the shop from where I had pulled and replaced them with a better cam.

[gmc684x4]

I would think it will run even worse having the cam 180 degrees out

From reading up the thumper cams are very fusy to get dialed in
Are you running hei or point's distributer?
What size of carb ?
Have you went through vaccuum tuning to help improve low vacuum output of the thumper cam ?

If you cant get it tuned in i suggest switching to a non thumper cam as they are easer to setup timing

Is there any hot rod shops in your area they may have experience on setting up thumper camed engines

[woodsey]

Thank you everyone for the awesome information
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[Ironangel]

Detonating under a slight load is to much timing...Knowing what the head casting numbers are, and what your timing is set at would help. I'm curious as to what timing gear set your running, specifically what slot your cam gear is in? You may have the cam degreed in to far advanced via the wrong keyway slot. That cam wont make much vacuum at idle with that much overlap so your vacuum canister plus mechanical timing, plus the initial timing can be adding to much total timing causing the detonation. Need to know exactly how much each timing event is adding...I'm running a 108 overlap in my big block with 12.5:1 domes in 106 cc closed chambered iron heads. No Vacuum advance at all, just initial and mechanical for a total of 35 degrees @ 3000 rpm. My cams retarded 6 degrees to #1 intake. If your going to run a cam like that you have to know your timing events and how much each is adding and when...Start over before you break a ring land...Just Sayin PS. Contrary to what many say, that cam will idle and make power if its degreed in right!

[hjewell2]

Originally Posted by Ironangel

Detonating under a slight load is to much timing...Knowing what the head casting numbers are, and what your timing is set at would help. I'm curious as to what timing gear set your running, specifically what slot your cam gear is in? You may have the cam degreed in to far advanced via the wrong keyway slot. That cam wont make much vacuum at idle with that much overlap so your vacuum canister plus mechanical timing, plus the initial timing can be adding to much total timing causing the detonation. Need to know exactly how much each timing event is adding...I'm running a 108 overlap in my big block with 12.5:1 domes in 106 cc closed chambered iron heads. No Vacuum advance at all, just initial and mechanical for a total of 35 degrees @ 3000 rpm. My cams retarded 6 degrees to #1 intake. If your going to run a cam like that you have to know your timing events and how much each is adding and when...Start over before you break a ring land...Just Sayin PS. Contrary to what many say, that cam will idle and make power if its degreed in right!

These cams run even better with fixed timing , get better throttle response higher vacuum , and do not use
the vacuum advance , shoot for 34 to 36 total timing, you will notice better idle due to carb butterfly not exposing the transfer slot to get idle to about 1000 rpm.

[Ironangel]

Originally Posted by hjewell2

These cams run even better with fixed timing , get better throttle response higher vacuum , and do not use
the vacuum advance , shoot for 34 to 36 total timing, you will notice better idle due to carb butterfly not exposing the transfer slot to get idle to about 1000 rpm.

Exactly! I'm running a solid big block Clay Smith 304 cam with Howard EDM solid lifters...That idle shakes the bobby socks off the little kitty at the local hang out!

[woodsey]

The heads are iron 10101140 and the timing st is comp adjustable and it’s on the 0 setting. It has 34 degrees total timing

[AussieinNC]

Detonation under light acceleration is either too much timing or weak "gas"....

What octane fuel fo you have in this truck?

87 Should not be used

89 will detonate unless you knock the initial timing down...

92 will usually prevent detonation assuming no hot spots in the chambers etc, and your timing curve is not coming in all at once.....

Do not use ported vacuum with this cam...the cams "Thumper" sounds comes from late exhaust closing which will play havoc with vac advance..Either use full manifold vacuum or remove the vac advance totally and have the dist re-curved to use only mechanical advance.

The advice you were provided by Comp cams relates to the installation of the distributor,,,when a Chevy engine is built and timed "dot at 6 to dot at 12" the engine is set to fire cyl #6...turning the crank 360 degrees (cam moves 180 degrees), will put cyl #1 on the ignition stroke TDC.

Make absolutely sure the timing mark on the balancer is exactly correct....perform a tdc stop and set process then check the mark on the balancer perfectly at the 0 mark on the tab....if that is off even a little, you will not be able to set timing correctly....

Here is a link to a site that will show you how to find tdc with the heads in place....

https://static.summitracing.com/glob...sum-900189.pdf

NOTE !!!!! Do not turn the engine over with the starter with the stop in place...you will break something...

Always slowly hand turn the crank....

[woodsey]

Thank you for the awesome information. Hopefully I can get the 71 GMC back on the road with her brand new engine

[toolboxchev]

Originally Posted by gmc684x4

The thumper cam is your issue of tuning

Move your vacuum advance to mansfold pressure not on ported pressure

Here is an article i found that talks about tuning a thumper cam

https://www.hotrodders.com/forum/sbc...300.html?amp=1

I would try adding a Adjustable Vacuum Advance from Pertronix. It will give you more or less available timing at idle.

I have one on a 350 with 76 cc heads, I can run a lot of timing with those chambers, but I lose the top end.

[hjewell2]

Originally Posted by Ironangel

Exactly! I'm running a solid big block Clay Smith 304 cam with Howard EDM solid lifters...That idle shakes the bobby socks off the little kitty at the local hang out!

[cadillac_al]

I was just reading a thread the other day about a big block with muther thumper problems and now I can't find it for the life of me. I thought it was this one and I was checking in to see if he figured out why his valve springs broke. Maybe they are hard on valve springs or his machinist messed up the assembly or somebody didn't adjust the valves correctly. If I run across it again I will add a link here.

So woodsey, I think it would be beneficial to do some vacuum testing and compression testing to see if something is way out of spec. When I hear of tuning problems that are hard to diagnose I start suspecting the valve adjustments. Hopefully you don't have valve spring problems. The muther thumper is known for being hard to tune anyway.

I want to play with a big block and a muther thumper for the fun of it and have my eye on a cheap 454 right now. good luck.