Thumpr Junk

[Kevinb1]

I've spoken with Cliff several times on the phone and hes been a great help trying to tune my 327 that has a Mutha Thumpr cam in it. I'm at my wits end trying to tune this engine using this cam shaft .
I'm running 30 degrees initial timing and I can't stay in the garage while I'm trying to set the fuel mixture because of the gas smell coming from the exhaust.
 I've decided to remove the cam and go with something else, just not sure what.
I'd like a nice sound at idle, but lots of torque on the low end. I don't plan on driving the car on the interstate much, mostly just round the town or country roads at 60-65 MPH. I'd like to keep the stock look, so no headers or after market heads at this time.
Car/Engine Specs
1968 Camaro Conv
4-speed
3.08 12 bolt rear end (will be changed to 3.42 or 3.55)
327-10.0:1 compression (1972 Chiltons Manual)
.30 flat top pistons,
stock crank
original "double hump" 1.94/1.5 heads with Z28 springs
Mutha Thumper CAM .489/.476 lift, .235/.249 @ .050" Lift
Stock points distributor with an MSD Blaster 2 coil
Quadrajet #7028212 DH Built to the third recipe in Cliffs book.

Any advice or opinions will be greatly appreciated.

[77cruiser]

Pretty hard to have a clean smelling idle with a big cam, but if you went with something about 10 -15 deg. less duration it would idle a lot better.

[Cliff Ruggles]

Look at what cam the factory used for the 327/350hp engines and those engines had more compression.

222/222 @ .050" on a 114LSA.

The cam you are using just has too much duration and LSA is way too tight for the CID and compression ratio.

I get more complaints to the shop from folks who have chosen Comp XE cams on 110LSA's and the Thumper cams on 107LSA's than any other.

If it were my engine I would install the Speed Pro CS-179R cam instead.  It's a direct replacement for the 327/350HP camshaft, and will idle better and make more power everywhere compared to the current camshaft......Cliff

[Kevinb1]

Thanks for the response.
Cliff
I just rebuilt the quadrajet using a rebuild kit and book that I got from you. I also built it to the the third recipe in your book which was designed for large cams.
Will I need to order another rebuild kit and set the carb up for recipe#1, or #2?
Or will I just need to find another quadrajet and start from scratch?

[novadude]

It's not the LSA that is the killer.  It's the overlap that comes from tight LSA and excessive duration. 

I have a Q-jet that is built to (approximately) "Stage 1" recipe specs on a 9.6:1 355 ci running a 217/225 @ 0.050" hydraulic roller on a 108 LSA, and it idles great at 800 rpm, runs high 12s @ 108 mph, and gets very good highway MPG (~19-20 MPG with 3.36 gears and no overdrive).

Don't be afraid of the tight LSA cams - just choose duration appropriate for the combination.  David Vizard gives great SBC cam advice in his books.  I followed his "recipe" and I couldn't be happier.

[Cliff Ruggles]

Nothing is irreversible when it comes to tuning the idle system, so it's pretty easy to reduce the idle tube size, DCR's, bypass air, etc.

Been building engines here, very successfully I might for over 40 years and I am NOT fond of tight LSA, or fast ramp camshafts for street engines.

They certainly sound good when it comes to "attitude" at idle speed, but tight LSA narrows up the power curve, and pulls power down in the rpm range.  One can quickly get into trouble with detonation as well, as narrowing up the power curve and earlier increases cylinder pressure all else being equal.

I'll put up the dyno sheet of an engine we helped a customer out with.  It's a 455 Pontiac at 9.3 to 1 compression.  They originally installed a Comp 276HR cam and had trouble making the power they were looking for and it pinged on the dyno requiring rod bearing replacement. 

They called me because we build engines and it was using a Q-jet carb we supplied all the parts for.  They dyno operator was blaming the carburetor, when in fact it was the cam.  We recommended a slightly larger cam on a 114LSA, the XR276HR cam is on a 110LSA.

They were nicely rewarded with more power at every rpm, decent idle quality, plenty of vacuum at idle, improved throttle response, and street manners........Cliff

[novadude]

Been building engines here, very successfully I might for over 40 years and I am NOT fond of tight LSA, or fast ramp camshafts for street engines.

They certainly sound good when it comes to "attitude" at idle speed, but tight LSA narrows up the power curve, and pulls power down in the rpm range.  One can quickly get into trouble with detonation as well, as narrowing up the power curve and earlier increases cylinder pressure all else being equal.

You can't paint it with a broad brush and say "tight LSA is bad" for all engine combinations.  My car weighs 3300 lbs, runs 108 mph in the 1/4, gets 19-20 mpg with no OD, and idles smooth enough for Grandma to drive it to church.  This is a 9.6:1 355 SBC.  With a manual transmission, I can pull it down to ~1300 rpm with no lugging, etc.  It runs way better than it would with the old outdated '151' L79 cam (which I've used before), and still pulls hard above 5k rpm with only 217 @ 0.050.

There's more than one way to get it done. 

[novadude]

You can't paint it with a broad brush and say "tight LSA is bad" for all engine combinations.  My car weighs 3300 lbs, runs 108 mph in the 1/4, gets 19-20 mpg with no OD, and idles smooth enough for Grandma to drive it to church.  This is a 9.6:1 355 SBC.  With a manual transmission, I can pull it down to ~1300 rpm with no lugging, etc.  It runs way better than it would with the old outdated '151' L79 cam (which I've used before), and still pulls hard above 5k rpm with only 217 @ 0.050.

There's more than one way to get it done. 

A Pontiac with 30 deg valve seats is likely a whole different story....

[77cruiser]

Probably adds to the O/L factor with more low lift flow.
BTW my 421 seems to idle pretty good with 107 lsa & gets 18 mpg highway.

[Cliff Ruggles]

I'm not painting any picture at all, just stating facts based on DIRECT testing over many decades.

A 355 with 9.6 to 1 compression would NOT like an L-79 camshaft, you need closer to 10.5-11 to 1, then it would tear your head slam off and make well over 1hp/cid.  We built a 355 for a very good friend some years back and it ran into the 11's in his 68 Nova with nothing more than 4.10 gears and a mild stall converter.

We've dyno'd that cam in very well prepared 350cid street engines and depending on cylinder heads selected, quench distance and true static compression ratio will make 380-420hp.

The factory LT4-"Hot" roller cam is another very good choice for those engines, built quite a few of those as well.

We've also played around with a few of the modern "fast ramp" profiles, and tighter LSA and they fair better when the compression is a bit lower as they tend to build more cylinder pressure early in the rpm range.

Compression is your friend with cam choice, so basically you have to pic the cam based on CID and compression ratio, and also the drivetrain parameters, intended use of the vehicle, etc.......Cliff

[novadude]

you have to pic the cam based on CID and compression ratio, and also the drivetrain parameters, intended use of the vehicle, etc.......Cliff

Can't argue with that.  The higher the CR, the better a wide LSA cam will work.  For a 9 to 10:1 SBC, tighter is usually better though.  You might want to check out some of the many dyno tests David Vizard has performed. 

My cam is a hyd roller with 270/278, 217/225 @ 0.050", .495"/.500" on a 108 LSA.  Not very far off of LT4 HOT cam specs except for LSA.  In my 9.6:1 engine, I can't think of any good reason to spread the lobe centers.  It has great manners as-is, and it is not the least bit "peaky" compared to other similar engines I've built on wider LSAs. 

Vizard's got the dyno tests to support the tighter LSA choice on a ~10:1 SBC.  I was skeptical based on all the wives tales about "bad" behavior with tight LSA cams, but I figured I'd give it a shot.  Glad I did, as I believe it pulls much harder in the mid-range as compared to a wide LSA cam. With a Muncie, my car spends a lot of time in the 3800-5200 band in a typical 1/4 mile pass.

Wide LSA isn't always the right choice, but tight LSA isn't always the right choice either.  No way would I have picked a small 108 LSA, 217@0.050 deg cam if I had an 11:1 engine. 

[Cliff Ruggles]

270 degrees seat timing is smaller than a stock replacement 350 camshaft, so it will enjoy tighter LSA in a lower compression build. 

Tight LSA narrows up the power curve and increases VE at lower rpms.  Combined with the roller lobe profiles that cam would have excellent cylinder filling abilities to offset the lower compression and make good low and mid-range power.

I build engines for a living, and dyno all of them, so we have pretty extensive experience with camshafts.  Cylinder head flow is also a BIG player with camshaft selection, and the better the ports flow the less duration you can get away with and still make respectable power.

Roller camshafts also can get away with less seat timing as they supply a much larger "window" for air flow once the valves are off seat.  A flat cam that small would not delivery the same results you are seeing with the roller cam, it would need to be 10 if not 15 degrees bigger to accomplish similar results.

Folks don't take any of that into consideration, and the current trend is to lower compression down around 9.5 to 1 for pump gas, then install smaller cams on tighter LSA's to bring the power back.

Here I build SBC engines with very tight quench, usually around .025-.035", excellent flowing heads, and shoot for 10.5 or a little higher SCR (iron heads).  Combined with a longer duration cam like the CS-179R or something similar you will enjoy decent idle, strong/broad/flat power curve, and excellent upper mid-range and top end power.  This will also happen on pump gas without running hot, overheating, detonation, etc.......Cliff

[Cliff Ruggles]

(Continued from the last response had too many characters)

Kind of interesting but when folks call here, and this happens dozens of times each week, they absolutely HATE Thumper, Mutha-Thumper, Comp XE, and similar cams on 110 or tighter LSA.  The complaints are ALWAYS the same, shootty idle quality, stinky exhaust, poor throttle response, "reversion" at low rpm's, and just not very user friendly anyplace.  When I start asking questions about "quench distance", heads used, head flow, true static compression ratio, camshaft installed ICL, etc, most don't know any of the answers.  Right to start with IF they don't know anything abut the heads other than they were 1.94 intake valve size, they don't have a CLUE as to how well that engine is going to work with the total combination of parts used.  Chevrolet made some excellent factory heads, OK factory heads, and complete GARBAGE when it comes to performance potential.  Many are nothing more than "door stops" but I continue to see them used in many "performance" engine builds despite the many and readily available EXCELLENT heads out there these days.  This tells me that the machinist, engine builder, or most of the folks involved in the engine build really don't know what they are doing, aside from poor camshaft selection for the engine based on the parts involved and intended use, etc. 

In a lot of cases when I ask about why a certain cam was used, I get nothing more than "my engine builder said bubba used one in his engine and absolutely loves it".  Problem is, Bubba's engine may have completely DIFFERENT parameters as the one in question, hence very poor results from the next poor soul who tried to use the same cam.

In this deal I consider compression my friend, as it opens up choices for larger cams and a LOT more engine power over a broader rpm range.  Most of the Internet jargon you read in that regard will tell you to avoid high compression and use "modern" camshaft to bring back all the lost power, so for sure PLENTY of ways to skin that cat/opinions on the subject......Cliff

[429bbf]

excellent discussion for a carburetor forum . I've seen more people blame the carb for there problem than anything else . they take a 8.5 c.r. engine stuff in a big cam , put on a set of headers. and say that quadrates are junk .my advice to them is give me the quadrajet and install an 850 holly that way you truly have a gas guzzling pos.fwiw

[novadude]

so for sure PLENTY of ways to skin that cat/opinions on the subject......Cliff

Agree with this. 

My 9.6:1 355 with Vortec heads and tight LSA runs 108 mph in a 3300 lb car, gets ~19 mpg with no OD (cruising at 3200 rpm), and has a great idle quality and drivability with a Q-jet and only 217 @ 0,050.  Lots of bigger engines aren't running that kind of mph.  I don't regret not going for more compression and wider LSA. 

My quench distance is 0.044-0.046 (depending on which hole I measure), and I don't believe there is much to gain by running it .010" tighter.  I did pay attention to all of the details on this engine. 

Everybody's got their own opinions, but I am very happy with the performance of the combo I've designed.