Thumper
I first thought the thumper style cams were a fix for low compression motors
but now I think just for those that want a lopey idle and are not aware of the torque they leave behind.
I agree;
If you install 279TH7 Thumper at 102.5 per the spec you will get the following;
279/297/107/118comp/100power/
62* Ica/74* overlap/ 74 effective.
If you take an XE274H, and install it at 104, you will get
276/286/110/118comp/101power/
62*Ica/61*overlap/ 54Effective
Since the Ica's are the same, installed in the same engine, the cylinder pressure will be exactly the same.
Since compression plus power is 218 versus 219, you can roll these cams around to the same Ica and see that all the differences between these two is in the overlap
And that is where the idle-lope is generated.
Ok but between these two;
the Thumper is 227* @.050
versus 230* on the XE,
And the thumpR has .486/.473 lift
versus .488/.491 on the XE
So, the XE has the potentail to make a bit more power, with a slightly broader powerband, all other things being equal
So, it is all about the sound.
But between these two, it's pretty close in power.
However, I don't think anyone would install this XE at 104, because it knocks the Effective overlap down to 54*.. The proper install might be 108. Then you would get the following;
276/286/110/114comp/105power,Ica of 66*/61* overlap/60*effective
That 4* later Ica will cost you 6 or 7 psi cylinder pressure but the 5 extra degrees of power extraction is gonna show up as better mpgs. And of course, the true potential of the overlap can now be realized.
As for the loss of cylinder pressure, that's only a problem if you didn't have much to start with........
But hey; the XE started out with 3* less advertised intake duration......
When I look at cams, I look at what is left over for compression plus power. In these two we see 218 for the Thumper and 219 for the XE. So basically the same. These numbers are getting pretty low for a streeter. You can trade away a few power degrees to get a few compression degrees,but mostly that's a bad idea, because at low rpm, a lot of energy is going straight into the exhaust system, instead of to the crank ...... and streeters spend an awful lot of time at low-rpm, including hiway cruising. So I like to see numbers closer to 227.
But sometimes, to get it, you gotta close up the LSA, which,IMO, is not a bad thing in a streeter, if you put second gear where it needs to be.
Take this XE274H for example;
If you close it up from 110 to 106, you get the following
276/286/106/118comp/109power/
Ica of 62*/69 overlap/68Effective
Waitaminuit, where have we seen 62* Ica before? Look up, waaay up, Hyup on the Thumpr, but now we have 109* of extraction, and 68Effective degrees of overlap, with compression plus power =227, a nice street number. All you gotta do is cruise at a high enough rpm to ensure the highest vacuum, lean her out and mid 20s mpg is on the horizon.
So why aren't streetcams all 106* LSAs?
IDK
The narrow 106s are purported to have a narrower powerband. So when the trans gets into second, the Rs drop to 59% and if the engine falls off the cam, you lose.
But I say Com'on, the tires are spinning all thru first gear, so put second gear where you need it to be and who cares.
So this XE274 which is
[email protected] power-peaks at about 5200, and say it has a short plateau of 300 rpm so you shift it at 5500. And the Rs drop to 3245; for a powerband requirement of 2255 with a 727/904.
So put some 4.30s on that, which will put 5500 at 40 mph in first, and 60 will be 4900 in second, 5500 will be 67 mph, and the power peak of 5200 at 64mph.
What's wrong with that picture?
Oh.......... no overdrive,lol.
Well ok, put a 110Lsa in there with 3.23 gears and be slow then.
Or get a bigger engine.
Or a manual trans;
I have a 4-speed with a 1.92 second gear When the 4-speed shifts at 5500 the Rs fall to just 4015 for a powerband requirement of just 1500. So it will have no problem with a 106LSA. You auto guys just need 4000TCs to keep up,lol.
Ok so once more, apples to apples, 279ThumpR7;
to XE274H with a 106LSa. Thumpr first:
279/297/107/118comp/100pwr/62*Ica/74E-overlap
276/286/106/118comp/109pwr/62*Ica/68E-overlap
I know which one I'd buy;
and it doesn't rhyme with bumpR.
Now; to answer why the ThumpR has 297* of exhaust;
To get the 74* of overlap on a 107 Lsa, it has to be that much.
It is strictly a function of the math.
[{(Lsa x2)+overlap}x2] less the intake duration = the exhaust duration
say it was a 110cam, and keeping 74 overlap; then the exhaust would have to be 309
Say it was a 105cam, and keeping 74 overlap; then the exhaust would have to be 289
Say it was the XE274H
(110x2 plus 61)x2 less 276=286
Lets say you ordered the thumpR with the Lsa opened up to 112, you would get
279/297/112/113comp/95pwr/67*Ica/64overlap/64E-overlap.
Notice that com+pwr is now down to 208 and that mother will suck gas big time.
Let's check it;
(112 x2plus64) x2 less 279=297; Hyup it works,lol.
