cam and vacuum question

[namredla]

Hi All, I don't possess the skills to decode this cam spec sheet. What I'm wondering is if I will have any issues with a brake booster. Right now my 1971 C10 is not boosted, but I am looking at buying a setup from a guy who had a radical enough cam that he suffered a lack of vacuum to assist the booster and ended up having to go with a hydraulic system. I don't think this cam is that hot, but figured someone here would know. Much appreciated. I guess I should add that the engine is not in the truck yet, I'm in the process of preparing to swap the existing 350 out, or I would just measure manifold vacuum.

1971 C10
rebuilt 327 - cam sheet attached.

[geezer#99]

I don’t think you can say Thumpr cam and enough vacuum in the same sentence!

[Grounded63]

Lift/duration numbers aren't that hot. But, the lobe separation is kinda tight. So there will be a decent amount of valve over lap. Probably mid/lower double digits.

[RustyPile]

How did you come to select this cam?? A cam this radical can open up a can of worms for the inexperienced engine builder. Low LSA cams produce lower vacuum at idle, but manifold vacuum is not the only consideration.. Carburetor, intake manifold, compression ratio, differential gear ratio, all make up the "package". You'll also need valve springs to match the lift.. Timing curve in the distributor will probably need attention also.. If you have an automatic transmission, you'll need a torque converter with higher than stock stall speed..

[namredla]

Thanks for the feedback. Grounded, I believe I read that -14 psi is what the brake booster needs (seems like it was literature from Classic), so I'm hopeful that gets me close. Maybe the best bet is to hold off on buying anything until after the engine swap and I can put a gauge on it.

RustyPile, I absolutely agree. The engine was purpose built for this truck by a good, old school builder. The cam selection was basically a result of me telling the guy that I wanted a little bit of lope at idle, he did the rest. The builder took a very systematic approach to the build and asked a lot of the questions that relate to your post: CR factored into cam, factored into dizzy recurve, factored into stall, etc. From an engine standpoint I have confidence that he got it right (and came with the dizzy he wanted on it, I'm guessing because he matched the curve to the build). We had a number of conversations about differential gearing and what type of transmission I was planning to use. Those conversations are now lost and I don't recall the specifics (and the build in general has changed a bit in my head). So I am interested in any info that talks to engine vs torque converter stall (I'm thinking 700R4 at this point to replace the stock Muncie 3 speed) I don't understand this part of the equation but google may help, and rear end gearing. I know that the engine was designed to rev quickly, he used a special short skirt piston and was very particular. Everything in the rotating assembly is light and made to get going in a hurry.

Again, I appreciate the help.

[Joyridin]

That cam has 20 degrees of overlap. That is a pretty hefty cam for a 327. This is going to be a pretty lumpy cam. It is probably going to have low vacuum especially if you are running a single plane intake manifold.

I would start thinking about a Hydroboost set-up. Pretty easy to install especially if the engine is out of the truck.

[GASoline71]

Originally Posted by geezer#99

I don’t think you can say Thumpr cam and enough vacuum in the same sentence!

Exactly. Those cams are ground to have a "lopey" idle for the car show idler/cruiser crowd. Their performance is "okay", but you have to run a crapload of initial advance (somewhere north of 25 degrees) to get them to run halfway decent. Have a buddy with one, and he has a vacuum canister and still hasn't got power brakes worth a dang.

I'd look at a different cam if I were you.

Gary

[Willshook]

Originally Posted by namredla

The engine was purpose built for this truck by a good, old school builder.

This is for some kind of serious hot rod truck or drag beast, right? Not just a daily driver? Super deep gears?

I hear custom, short-skirt pistons and I get nervous...

And all the points have been made with the Thumpr...it's always possible to get something a little "rumpty" by picking a cam with more overlap but adding that much overlap just to make noise is a waste of gas and power...

Not sure I'm really adding anything to this thread; if not, sorry...

[namredla]

I appreciate the feedback but you guys make me a little bit nervous! I'm a decent mechanic, but I'm not an engine builder (wish I was). The guy who built the engine for this truck was well respected among all of the regional shops, and he built this one based on a fairly basic understanding of what it was going in, what I wanted, and some parameters. No, this isn't for a drag beast, or for anything other than a cruiser. I wanted a nice sounding lope. The engine build itself was a several year long event, and by the time I picked it up the guy was retiring (in his nineties), and some of what he had done had been forgotten (basically all I know I have said above in bits and pieces). So it will just be a surprise I guess. I think this engine was more of a fun project for him just because I went in looking for something retro and kinda cool, not the typical 1000 hp beast that he was used to building. He had a good time and it was one of the last engines out of his shop. All that said, I am at this point planning on a hydroboost..... LOL

[GASoline71]

Any engine builder that would recommend a Thumpr cam, I might question his sanity.

There are a lot of good camshafts out there with a "lope" that make good power and torque that are way more efficient than the Thumpr series camshafts. For example, A good cam that has a good "sound" to it and makes some decent power but not a drag cam is the Comp XE268H. There are a lot of cats on here using this exact cam and are extremely happy with it. I had one in a 350 that was in a '79 GMC K25 and it worked fantastic with the setup I had in that rig. I believe it is vacuum friendly as well.

Gary

[Willshook]

Originally Posted by namredla

No, this isn't for a drag beast, or for anything other than a cruiser. I wanted a nice sounding lope.

It's hard to come into these situations and give feedback - don't want to make you nervous, but a lot of these old skoole engine builders make a racing engine...that is very unhappy on the street. I have "detuned" a fair number of them in my engine building career. It kinda sounds like that's what this guy built...

Can you share whatever you have for specs on this thing overall (receipts, etc) and maybe we can make some adjustments not in the core assembly, but maybe the rest. The lift numbers aren't crazy, which makes me a bit less nervous, but yeah...a Thumpr isn't really a choice most builders would choose unless it's a rat rod or a hot rod where it's more about show than go. Again, that kind of engine is FINE if that's what you want - but it's not a fun daily driver when it won't even idle decently, you've got an auto that's surging at stoplights, the fuel economy sucks, and you have lousy low- and mid-range throttle response.

I don't think you're going to be happy with a high-revving 327 in a daily driver - they're amazingly fun engines in a light, manual-transmission 'Vette or Nova that's geared for them - and the cam is probably the easiest way to 'tame' that type of engine to go behind an automatic and street gearing.

I don't wanna be 'that guy' taking shots from the cheap seats - but it's a lot easier to swap a cam on the stand than in the chassis.

You can tell me to shut the heck up any time - seriously

[Willshook]

I guess it's also time to be sure that you're super clear on how break-in works on a flat-tappet cam and that you're fully prepared for that critical first start. I can get into it if you need it, but it tends to start big arguments some places so would rather do via email.

[Tom]

Thumper cams bleed low speed pressure on purpose to make the lope. That reduces vaccum. You can always plumb in a vaccum reserve canister, in a 327 its iffy.

[Willshook]

The "rumpty" sound everyone loves is a cam designed for high RPM power (6K+). To make power at that RPM, where valve events are happening VERY fast, more "lead time" is needed where valves are opened and closed earlier and earlier. This "lead time" results in overlap - times in the cam cycle when both valves are open. For a high-RPM cam, this means a portion of the intake charge is going right out the exhaust at lower RPM and especially at idle.

A Thumpr cam is intentionally designed with excessive low RPM overlap, without changing all the other cam timing that would be associated with a high-RPM cam so you end up with a somewhat less crappy mid-range than if you were just using a high-RPM cam. But it's still an exceptionally crappy cam.

you can get a kinda lopey idle from even a good street cam just by retarding it 4 degrees or so during installation using a keyed timing chain set. this moves up the power peaks, so there's some other negative impacts - but with a mild cam it's still less crappy than a Thumpr IMHO.

For me, I'd rather have something that sounds awesome with a nice idle and a good snort than something that stumbles and barks, but that's just my preference.

Gonna kinda bow out on this one I think - always available via PM.