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1975yellowC3 10-24-2012 05:10 PM

383 cam change internal clearance questions
Here are my specs

75 Corvette
Floo-Tek AL Heads (let's assume they flow same as vortecs) 2.02 valves, 180cc intake runners
670 Street Avenger
Timing = ~24 initial / 36 total by ~2500
CR = 9.5:1
Performer RPM Intake
383 Scat rotating assy
2500 stall
gears = 2.73's
Current Cam - Muth thumpr cam flat tappet
@ .050 = 236 / 245 intake /exhaust

I posted many questions a while ago about my combo and you all were helpful. I have it running pretty good, but I know my gears are killing me and it fights the converter at idle even though this is the recommended stall speed from Comp. This was my first engine build, and I copied a hot rod engine build. Obviously, severly mismatched for my gears/stall.

1. I am thiking of going with maybe an XE268...maybe a Hydraulic roller this time. Will that get me back some bottom end I am missing with the thumpr? Is it still way to much cam for my gears? I am a little scared of too much DCR if I go down to something like the 262. I would think even the 268 would have to be much better than what I currently have?

2. I am a little scared about my internal clearances if I change cams. I made sure when I installed the thumpr that I had at least .050 everywhere. I thought maybe with the smaller lift cam I would be OK, but not sure about the larger LSA of the xe268 and how that will affect things?

3. Do I just need to suck it up and change gears!

Any thoughts would be appreciated.

ssmonty 10-24-2012 05:46 PM

Personally, If I were you, I'd do both the cam and gears, as well as an overdrive tranny.
I don't think 9.5:1 w/alum heads would make for too high of a dynamic CR with a 268 cam and keep some of the lopey idle sound. 342 to 373 gears would really wake things up, but the rpm at hwy cruise would get old fast without an OD tranny.(not to mention drinking some gas)
I spoke too soon. What kind of tranny do you have?

techinspector1 10-24-2012 06:17 PM

I don't find a Mutha Thumpr with 236/245 duration @0.050". I find a 12-601-4 flat tappet Mutha Thumpr with 235/249.
If that's the cam you have, the intake valve closes at 39.5 degrees after bottom dead center and will yield a dynamic compression ratio of 8.19:1 on the KB calculator when used with a static compression ratio of 9.50:1.
I have looked through a plethora of cam grinds to find one that will give a good thumpy idle (I assume that's why you installed the Thumpr in the first place) and still give an acceptable DCR. Here is the cam I'd shove into the motor....
Crane Cams 10017 - Crane Hydraulic Flat Tappet Camshafts - Overview -
It has nice, long ramps that will be easy on the valvetrain, closes the intake valve at 31 degrees after bottom dead center and will generate an 8.57:1 DCR when coupled with a 9.50:1 SCR.
See the specs on page 60 here....
You'll still need more gear.

1975yellowC3 10-24-2012 06:22 PM

I have the original 3spd auto in there now. I believe it is a TH400. I think even if it is a TH350 they both have 1:1 final ratio.

ssmonty - I have toyed with the idea of OD tranny. I have really been reluctant to change gears because of the higher highway rpm. Not sure I want to do that, but I realize it's a tradeoff and without it I have what I have!

The motor sounds really really good right now. I honestly don't really care about the sound, just want some good off the line power.

So...I have read many a thread on here that gears are by far best bang for the buck. I am just curious how much it would really help. It will turn the tires from a stop as is (P295/50R15s) have to floor it and it will spin probably 20 feet. I started out on this journey just wanting some neck snapping take off. It takes off pretty well, but I was expecting much more. I am assuming it has to be my gears.

Do you think it is worth changing cams? Will I really notice that much seat of the pants feel between what I have and the 268 off the line?

1975yellowC3 10-24-2012 06:31 PM

Thanks techinspector1! You helped me out a whole bunch a while ago on this setup...appreciate it. As usual, you are right on your specs at .050! I had them wrong.

I wasn't looking for the sound when I installed this cam originally. I was just following what hot rod did on a motor. They got good numbers with this cam, so i thought it would be a safe bet since it was my first go at this. I have really struggled getting it to run right with it's ultra low vacuum. I am tired of it bouncing off of the stall.

Do you all think I would be safe on my cam/rod clearance when changing?

techinspector1 10-24-2012 06:32 PM


Originally Posted by hcompton (Post 1602781)
That being said you may need a bit more stall. But dcr should be ok at 9.5 and al heads.

Please be careful when you say these things. At 9.50:1 DCR, this motor would rattle like a can of marbles on pump gas and nothing short of alcohol could make it stop detonating. Of course that's just my opinion.

techinspector1 10-24-2012 06:46 PM


Originally Posted by 1975yellowC3 (Post 1602801)
Do you all think I would be safe on my cam/rod clearance when changing?

If you used Scat Pro Stock rods, they will clear. Any other rods you will have to check the clearance for yourself. Easiest way is to use a long tie wrap. If the tip clears, you're golden.

If you're not concerned with idle quality, then there are better cams than the one I spec'd above. I picked it because of its 106 degree LSA. If you want to give up some lumpy, a cam that wil extend the range out a little further and give a little better idle quality can be chosen, using a 110 LSA.

But before you do anything, you need to take all the options into consideration and decide where to put your money. My opinion right now is that a set of 4.11:1 gears would push you back in the seat hard enough that you would be happy. Even a set of 3.73's would be a day and night improvement. But then, a looser converter would get you up in the power range of the cam sooner. I suggest you get prices on gears and prices on converter and leave the cam alone for now. 8.19:1 DCR ain't all that bad, if you could get up on the cam earlier with either a converter or gears.

1975yellowC3 10-24-2012 07:01 PM

I guess I am just looking for a bigger smile when I hit the go pedal!:thumbup:

I had checked on gears a few weeks ago and they were talking what i thought was crazy money to change them. Around 1200 (2 shops about the same price) dollars if I just dropped the car off. This was figuring on replacing bearings/seals. I was expecting it to cost in the neighborhood of 600! I am thinking about taking the rearend out and just delivering it, which would save me considerable labor money.

techinspector1 10-24-2012 07:25 PM


Originally Posted by 1975yellowC3 (Post 1602817)
I guess I am just looking for a bigger smile when I hit the go pedal!:thumbup:

I had checked on gears a few weeks ago and they were talking what i thought was crazy money to change them. Around 1200 (2 shops about the same price) dollars if I just dropped the car off. This was figuring on replacing bearings/seals. I was expecting it to cost in the neighborhood of 600! I am thinking about taking the rearend out and just delivering it, which would save me considerable labor money.

You're a hot rodder, right? Well, hot rodders figure out how to do stuff. What if you purchased a used pig and changed the gears yourself? You could then change 'em out and sell your stock pig. Here's a reprint of the original factory service manual from Faxon. It's 2 3/4" thick and costs $74....With this manual, a few tools and a little time, you could learn how to do it yourself....
1975 Chevrolet Repair Shop Manual Reprint Set 75 -- All Car Models

1975yellowC3 10-24-2012 07:51 PM

I like your style tech! I have read several posts about changing the gears and it kind of scared me off. But what the heck, I hadn't ever torn down a 350 before I built mine. I put it together and it is still running!

Thanks for the encouragement...I think I might try to give it a whirl!

ssmonty 10-24-2012 07:57 PM

IMO, a thumper cam with its narrow LSA coupled with the 2.73 gears would make for a real dog off the line and give up alot of midrange acceleration even with a slightly higher than stock stall convertor.
A 270HR cam with just a 3:42 gear would make enough of a night and day difference to justify the cost. But then again, I'm not the one paying for it and I don't know if your more concerned with HP numbers or a broad torque range from idle to redline. How often do you turn 6500rpm?
I don't care for the bigger cams on the street as they use alot of gas, don't have the same instant off idle throttle response, and wear out an engine sooner because of the higher optimun rpm range. I much prefer the good driveability of milder cams, but then again it may be because I'm getting too old.
I've attached a link to a site that you can download a Static/Dynamic CR calculator that I think is one of the better ones around. I would try to give you some numbers for the cam I suggested, but I don't know the required parameters of your engine.
I will say that using Comp Cams camquest software and what I'm approximating your engine specs are, that I get about 450 ftlbs of torque as low as 2500rpm with a peak of over 470ftlbs at 4000rpm, and 415HP at 5000rpm useing Comp's pn 12-420-8 270HR. I'm not saying thats the best for your application or driving style, but I know it would put a smile on my face, but thats just me.
Good luck,ssmonty
calculator site location
Dynamic CR

techinspector1 10-24-2012 08:05 PM

Great!! Here are some diff guys if you need parts or help....
Home Page
Moser Engineering - Moser Engineering - Home
Ring & Pinions, Front Axles, Rear Axles, Differential Covers and more from G2 Axle & Gear
PRO GEAR DIFFERENTIALS - San Diego's Original Gear and Axle shop. Differential parts and repair.

vinniekq2 10-24-2012 08:21 PM

I use 3.50 gears in the same car as yours.I went from a 4 speed to a 5 speed.My O/D works out to almost the same as what I had with none o/d and 2.73 gears. The diff is a little bit of a pain to work on. Before you change gear sets,make sure you have the Dana 44 not the Dana 36. These are reasonable weight cars and they hook quite well.If you really need to go 3.73 gears make sure you have the 30% O/D.No reason for you car to be slow off the line.Not a big fan of big hydraulic flat tappet cams but it should work well from 3,000 to 6,000 rpm.
NOTE: when you are under the back of that car,dont be surprised if it needs trailing are work and all the bushings. If you have soft or single fibre glass transverse spring,then that too needs to be upgraded to stop the car from squatting on launch. The car can be very difficult to launch and keep straight. When I did my car I also upgraded the 1/2 shafts and u-joints to big block or better.
Now that you see closer to 2k,you might consider up grading to a true independent rear suspension.The car has 1062 technology in the suspension

vinniekq2 10-24-2012 08:27 PM

couple typos, trailing "arms" and 1962 technology not 1062,sorry. the 1963 corvette has the same suspension we have now.

Also if you have the low profile hood I have to assume your car is down on power as there is not enough room under the hood for a good intake.
You can put 3 inch exhaust through the transmission x member if you do not have side piped

spinn 10-25-2012 08:36 AM

That is a big cam, plus you have an auto with 2.76 gears . Not good for driving fun. If you had a nice restoration take advantage of the long strokes torque boost with a reasonable cam. A cam with duration and lift like that is going to sound like a demented popcorn maker. Weird but that sometimes people like that. They make for poor overall performance, from a drivers perspective, to me at least. If you had .450-.470 lift it would make a sexy bubble. You made what made. Now spend a day swapping the ring and pinion to 4.10 and pay the piper.

If you built it for that cam, you should have gone high stall, big gears. Hot rod did this build?

Nothing to be scared of using a 262. Dynamically it is a better choice.

Finish the combination you made and enjoy it. It will be highwayable with 4.10s , just busy.

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