New Comp Cams Thumpr cams; No Sir. I don't like it.

jerry S · Apr 2, 2007

THis is a new hydraulic roller to be retrofitted into non-roller 351W blocks.

Here are the specs.

ADV DUR: Intake: 299 / Exhaust: 319
Dur @ .050: Intake: 243 / Exhaust: 257
Lift: Intake: 552 / Exhaust: 538

Things were looking good until I saw the 107 LSA, which is not optimal for a car with an automatic transmission. I don't like these cams that make peak HP instead of making a lot of power under the curve. While peak sells magazines, it doesn't win races.

It seems like their whole marketing of this cam is how it sounds rather than how it performs. Are people really that lame in that they give priority to sound over performance?

BlackGMC · Apr 2, 2007

Since it is a comp cam, couldn't you order the same exact cam except on a higher LSA? Maybe a 112 or 114?

jerry S · Apr 2, 2007

BlackGMC said:
Since it is a comp cam, couldn't you order the same exact cam except on a higher LSA? Maybe a 112 or 114?

I could if I wanted to to pay the cost of a custom grind and if I also had plenty of favorable dyno simulation results for that cam but with a different LSA. I would be hesitant to order a cam without running the profile on the computer. You just don't know what you would end up with. And since I don't have the software, I am going to be conservative and take a pass.

BlackGMC · Apr 2, 2007

The cost is the same from comp cams. From my understanding a off the shelf cam costs as much as a custom cam at least from Comp Cams

jerry S · Apr 2, 2007

BlackGMC said:
The cost is the same from comp cams. From my understanding a off the shelf cam costs as much as a custom cam at least from Comp Cams

I did not know that. Still, without a dyno sim or real world results, I would be hesitant to order that cam but with a different LSA without having some empirical evidence as to how it would run.

brianj5600 · Apr 2, 2007

jerry S said:
Things were looking good until I saw the 107 LSA, which is not optimal for a car with an automatic transmission. I don't like these cams that make peak HP instead of making a lot of power under the curve. While peak sells magazines, it doesn't win races.

The Engine Masters entry by School of Automotive Machining used a cam with only 104* lsa. That competition is based on average HP and TQ. It might depend on more than just the lsa. Lsa and duration in combination are what determine overlap, which is what shapes and places the curve.

Helmantel · Apr 2, 2007

I believe they charge about $20-30 extra for a custom ground cam, so it's not very expensive in the overall scheme of things. Make sure that your instructions are clear though. In my case, it took 3 cams before they got it right.

I read an ad for these Thumper cams the other day. To me it sounded like they proudly anounced a new line of cams that combine the poor idle qualities of a race cam with the limitations of a street cam....

FYI, David Vizard wrote a nice article for Popular Hotrodding (about half a year ago, it's probably on their website) on how to pick an LSA. He started with picking the valve overlap and then continued with the LSA, which was based on intake valve diameter and engine displacement. The larger the valve size for a given displacement, the larger the LSA. For a 302 with 1.94" valves, the optimum was around 110 degrees. The opening duration was then whatever 2x (half the overlap +LSA) resulted in. I'm not sure how that applies for a cam that is advanced or retarded though. I don't know if that is THE way of figuring out what cam you need, but David Vizard is a knowledgable person, so it can't be all wrong.

Many people seem to look at duration and LSA as two seperate things, but a 280 deg cam with a 106 deg LSA actually has the same intake valve closing timing as a 272 deg cam with a 114 deg LSA. The 280 cam will have more overlap and thus a rougher idle, while maximum power will probably not differ all that much (due to identical valve closing timing). The 280 cam will be more affected by the effect that the exhaust pulses have on the start of the induction, which can depending on the engine speed be both good or bad.

302 coupe · Apr 2, 2007

it sounds like those are small base circle cams, ground on non billet cores. I'd be scared to run one, the valvetrain would be all over the place because of the weak core. Get a cam that makes the power you want, the sound "is what it is."

ashford · Apr 2, 2007

jerry S said:
I could if I wanted to to pay the cost of a custom grind and if I also had plenty of favorable dyno simulation results for that cam but with a different LSA. I would be hesitant to order a cam without running the profile on the computer. You just don't know what you would end up with. And since I don't have the software, I am going to be conservative and take a pass.

i have desktop dyno if your curious send me your setup

S-Car-Go · Apr 2, 2007

+1 to everything 302Coupe said.

Core quality and machining come into play here. If you don't want a custom cam, Jay Allen can choose one for you for $50. That's based on your car (weight, gears, tranny, heads, use, yada,yada).

302 coupe · Apr 2, 2007

S-Car-Go said:
+1 to everything 302Coupe said.

Core quality and machining come into play here. If you don't want a custom cam, Jay Allen can choose one for you for $50. That's based on your car (weight, gears, tranny, heads, use, yada,yada).

I got it from Jay, just relaying the info.

Helmantel · Apr 3, 2007

302 coupe said:
it sounds like those are small base circle cams, ground on non billet cores. I'd be scared to run one, the valvetrain would be all over the place because of the weak core. Get a cam that makes the power you want, the sound "is what it is."

Do you have experience with them?

I'm using a CompCams hydraulic roller cam with a reduced base circle. The cam has Xtreme energy lobes with .533 lift and just 208 deg duration @.050, so the opening rates are pretty steep. IIRC, the push rods you use with reduced base circle cams are .150" longer then the ones used with the regular roller cams, which means that the base circle is 0.300" smaller, which is actually quite a lot. I can imagine that would cause some flexing.

It runs fine, but the valve train is a bit noisy. I suspect that it could be the rocker arms, but I'm going to replace them soon, so I guess I'll find out then.

Using those link bar lifters and a regular roller cam would be the best solution, but they're so darn expensive (~$450).

302 coupe · Apr 3, 2007

the benefits of a roller cam are wasted when a reduced base circle is used.

I have personally never used a reduced base circle cam, I know better and have learned from others who have dealt with them, its just a compromised design. They really aren't a problem until you get up in the rpms, if you're just cruising around you're ok. But IMO, people that just cruise around have wasted effort swapping to a roller cam in the first place. If you're using the engine for performance, and you want to use a roller cam, definately use a full size billet core and link bar lifters. Do it once, do it right, never look back.

jerry S · Apr 3, 2007

302 coupe said:
the benefits of a roller cam are wasted when a reduced base circle is used.

I have personally never used a reduced base circle cam, I know better and have learned from others who have dealt with them, its just a compromised design. They really aren't a problem until you get up in the rpms, if you're just cruising around you're ok. But IMO, people that just cruise around have wasted effort swapping to a roller cam in the first place. If you're using the engine for performance, and you want to use a roller cam, definately use a full size billet core and link bar lifters. Do it once, do it right, never look back.

I have some questions and hope that you will jump in here.

I was under the impression that in order to use a roller cam in a non-original roller block that you had to use a reduced base circle cam and either the dog bone/spider or the expensive ($500+) retro fit lifters. Am I misinformed? Are all retro-fit hydraulic rollers reduced base circle cams? How do you tell one from the other?

302 coupe · Apr 3, 2007

retro-fit cams are reduced base circle, they have to be to accomodate OEM type hyd. roller lifters, due to the location of the oil hole. Non roller blocks have shorter lifter bores, and the OEM lifter oiling system just doesn't match up, unless the lifter can somehow be lowered in the bore. To do this, the cam is made with a smaller base circle, which lets the lifter sit lower. The problem is that the cam core is now too small and weak to support high rpm, aggressive ramps, or adequate spring pressure (which are the main benefits of a roller cam). The cam will deflect, which makes the valves do crazy things, similar to valve float. Sometimes when this happens, people assume its the springs, so they go to a stronger spring, which makes the problem worse. As an analogy, this is equivalent to building a stroker motor, and using a little dinky rod to accomodate a big crankshaft.

standard base circle cams require the use of link bar lifters, which do away with the spider/dogbones, and have the oil holes lowered on the body of the lifter, to work in conjunction with the short lifter bores on non-roller blocks.

You do not have to use a reduced base circle cam in a non-roller block, I have never said that. The BEST way to swap to a roller cam, is to use a standard (full-size) base circle cam on a billet core with link bar lifters. There are no special parts, and no compromised designs. It just works.

jerry S · Apr 3, 2007

302 coupe said:
You do not have to use a reduced base circle cam in a non-roller block, I have never said that. The BEST way to swap to a roller cam, is to use a standard (full-size) base circle cam on a billet core with link bar lifters. There are no special parts, and no compromised designs. It just works.

I see. I found a killer retro-fit hydraulic roller cam which is also a reduce base circle: comp cams 35-426-8. When used in a non-stroker with my heads, intake, and a 10:1 CR, this engine produces 435 hp at 5800 rpm and 442 ft lbs torque (engine factory.com sells this crate). Except for the cam, I have the exact combo only my CR is 1 point higher, which should net me 3% more power. And, and, and, I was gonna send my heads in for porting while the engine is out.

My flow numbers go from this:

Intake/Exhaust .100" 70/57
Intake/Exhaust .200" 128/104
Intake/Exhaust .300" 183/142
Intake/Exhaust .400" 219/163
Intake/Exhaust .500" 251/170
Intake/Exhaust .600" 255/174

to this after the CNC program

Intake/Exhaust .100" 79/61
Intake/Exhaust .200" 145/121
Intake/Exhaust .300" 198/168
Intake/Exhaust .400" 241/178
Intake/Exhaust .500" 279/196
Intake/Exhaust .600" 284/205

given the extra flow, I should make even better numbers than the proven combo from engine factory but I was concerned about the cam. But if I can grind the same cam as the comp cams 35-426-8 but on a standard sized base circle while using the link bars, new push rods, and beehives, then I should be in like Flynn. I honestly believe I can make 475 peak hp and close to 490 ft lbs of torque, which would be killer from a non-stroke, NA 351W.

302 coupe · Apr 3, 2007

ditch the beehives, and spend that extra $$$ on a custom cam. www.camshaftinnovations.com. He will hook you up with a cam, springs, and the best advice.

68RCodeConv · Apr 5, 2007

I had a 347 stroker built. I specified that I wanted to drive the car on the street and that I needed 12" of vacuum at idle (for the power brakes).The engine builder got a custom cam ground by Bullet. There was a big difference between my idea of "streetable" and the cam grinder's opinion. He ground me a 240/249 solid roller cam with .590 lift (.570 with .020 lash) and 108 LSA. At 1000 rpm it had 4" of vacuum and didn't smooth out until 2000 rpm. Pulled hard above 3000 but it was not what I wanted. So I pulled the cam and sent it back to Bullet. Talked to Bullet and they said that I was the first person to send a cam back because it was too radical. Seems that most people want a really lumpy cam. I told them that I was actually going to drive my car on the street. So they reground the cam to 236/236 with the same lift and 109 LSA.

Now it idles at 750 with just a little lope, pulls 5th gear at 1500 rpm and pulls hard all the way to 6500. Just perfect for me.

By the way, I've got solid roller lifters with link bars.

jerry S · Apr 11, 2007

302 coupe said:
retro-fit cams are reduced base circle, they have to be to accomodate OEM type hyd. roller lifters, due to the location of the oil hole. Non roller blocks have shorter lifter bores, and the OEM lifter oiling system just doesn't match up, unless the lifter can somehow be lowered in the bore. To do this, the cam is made with a smaller base circle, which lets the lifter sit lower.

I called to Comp to inquire whether this retrofit hydraulic roller (XR288RF-HR / 35-426-8) is a standard or a reduced base circle cam. The tech with whom I spoke said that any reduced base circle cam would have to be special ordered and that the cam I was asking him about is, like all off the shelf retrofit hydraulic rollers for a 351W, is indeed a standard base circle cam. Hurray!!

jerry S · Apr 20, 2007

302 coupe said:
ditch the beehives, and spend that extra $$$ on a custom cam. www.camshaftinnovations.com. He will hook you up with a cam, springs, and the best advice.

You are right about the behives. The place porting my heads (Ford Performance Solutions / Avenger Cylinder Heads) talked me out of them. They have had bad results with them on pushrod engines. They tell me that on modular motors, beehives can give you up to 6 extra hp on an engine dyno but that these are prone to breaking with alarming frequency on pushrod engines with aggressive roller cams. And you don't want that.

New Comp Cams Thumpr cams; No Sir. I don't like it.

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