What cam are you running? And what idle vacuum do you get at what RPM?

I'm going to be getting the GM HT383 short block and need to select a cam. I'm looking for a pretty smooth idle, so various cam companies have suggested something like 218° @.050 on a 112 LSA.

I currently have a 212°@.050 on a 110 LSA in my 350 and I get about 13.5 inHg in gear at 650 rpm with 2500 rpm converter.

I know I can go bigger on the cam with a more cubic inches, but the question is how big can I go while still maintaining an equivalent idle.

 

Can I have your old engine

 

Unfortunately It spun a rod bearing... which is the reason I'm doing the engine now.

My dream had always been to one day do a 427 sbc getting into the 550 hp range, but the reality is the car just isn't set up for that kind of power (suspension, exhaust, fuel system, transmission capability, etc) and the amount of money to make it happen on top of the high cost of the engine is just not in the cards, nor could I justify it in terms of family budget.

I figure with the 383, I'll be in a more manageable 430-440 hp range and the cost is at just the hairy edge of affordable right now. I can put it in and not have to really modify anything else on the car.

 

Bummer man, I’m wanting to get together a 383 I have and try comps xe276 and afr heads.
i think vacuum will be 14hg at idle.

 

I figure with the 8 speed trans (TH700 + gear vendors) that kind of power will plenty sufficient for what I do with the car.

And with the little amount of miles i put on jt, it'll probably last me the rest or my life (or at least until the Soviets here in California ban cars).

 

Any pictures or a link to that installation?
​​​​​​

 

I don't have pictures unfortunately. But it's in a 2nd gen Camaro, so the installation relative to the vehicle chassis would be different.

 

Much different. It's the torque arm that gets in the way. The 2nd Gen would be straightforward enough.
Thanks just the same.

 

When I had my 383, and 400, I used a xfi 280.
Don't know what it pulled for vac, but it was good for power with the LT1 intake I had.
Then I sprayed it

 

I am building a mild 383 right now. I am using the Comp Cams XR269HR - 08-502-8. 218/224 @.050" with 1.6 rockers giving .528"/.536" lift.

I too wanted a smooth idle and good drive ability.

I have no idea what vacuum it will make.

With the right tune I would think you can make a wide variety of cams idle well. It is not a chevy but I have a 408" stroker in my 1989 Mustang. The cam has .625" lift and the car idles at 750 rpm dead smooth after tuning on the stock EEC-IV computer.

 

Very cool... when will you have yours up and running? I'm targeting end of the year for mine.

Your cam is on a 112 LSA... so I'm extremely interested to see your results. I was contemplating getting that same duration but on a 114. From what I understand port fuel injected engines seem to like a little wider LSA.

 

I hope to have it going within a month. I have all the parts, I just need to assemble everything. I am using AFR 195cc as cast heads and a Edelbrock Pro-Flo XT efi intake.

 

Awesome... very similar to my setup... short runner manifold like my Miniram.

 

My LT1 intake is like a mini-ram
I had mine with AFR195's, 3.75 gears. Went a best NA of 12.11
With nitrous, went a best of 10.94
Here is the link to the Xfi280
https://www.compcams.com/xfi-230_236...oller-sbc.html
It has a 113 lobe sep, which idles awesome
Intake lift of .576 with a stock rocker.

 

Pretty impressive numbers, but that must have a somewhat rowdy idle...

I found this cam just now... Certainly will make less power, but I'm willing to trade that for a near stock-ish idle,

https://www.summitracing.com/parts/c...make/chevrolet

218/224 on a 113 LSA .570/.565

This one looks extremely tempting...

 

That cam is almost the same as mine. Just a bunch more lift. Practically the same duration @.050" and advertised duration.

 

Yeah, I think it just another deg of LSA, which would probably make the idle a bit smoother.

Those lift numbers are with 1.6 rockers (which is how it's advertised).

I'm also curious to see how noisy/quiet the valve train is when you get it running as well.

 

Cam is very close to the one in my GTA, except I have a 110LSA vs his 113.

Rowdy is definitely a word I could use to describe the idle. However, it will idle around 900rpm fairly well. Just won't make any real usable power until about 2000rpm, just too much overlap.

 

Mine idled fine at 850-900, bc I had more lobe sep.
Also had a higher stall.
It is a must with a bigger cam. One goes with the other

 

218/228@ 0.050, 108 LSA, 104 ICL and 0.578 lift with the 1.7 rockers on my 11:1 383. When it first starts its up around 1,500 rpm on the fast idle cam. Once the choke thermostat drops the fast idle it was about 650 rpm. Idle timing was 31° BTDC. After adjusting the idle mixture screws it held 19 in/hg @ 700 rpm.

Rhoads V-Max lifters were the key to the tame idle quality as well as torque from idle. This 383 pulls smoothly as low as 1,000 rpm.

 

Got the engine out yesterday and the cam out today.

Confirmed it's the Xtreme Marine version of Comp Cams Xtreme energy. 212/218@.050 on a 112 LSA. So I guess that overlap is my baseline in terms of putting numbers to the idle quality I'm after with the upcoming 383 cam selection.

If wider LSA's are good for upper rpm, then I guess that kinda explains why this engine pulled harder at higher rpm (5500-6000 range) than I would have expected out of a cam with only 212 on the intake. The short runner Miniram I'm sure explains some of that higher rpm performance, but still that cam isn't anything to write home about at first glance.

When I went with the wider 112 LSA at the time (vs the 110 LSA on the more typical street cam), I'm sure I was only after the idle quality and wasn't up to speed on all the other performance impacts that LSA has on engines

 

Not exactly.
Now that you've had time think about it, what are your objectives? Idle quality? Outright performance? Fuel economy?

 

Large focus on idle quality. I want as good an idle quality as the 212 cam in my 350. I get 13.5 inHg in gear at 650 rpm and it's very smooth.

Decent power. I figure I should be able to get to 430 even with good idle quality.

Fuel economy will be what it is. Currently I get about 28-30 on the highway with my gear vendors giving the double OD and the highway mode fueling kicked in. On the street I get about 13-ish if driving normally.

 

Do you have all of the specs for that 212 cam? LSA, advertised duration, lift ?
Also the specs for your current 383 cam.

 

For the record, all I'm doing is a little overlap analysis. Since overlap is a major player in your idle quality and vacuum, looking at what worked well for your 350 provides a good jumping off point for the 383. Taking into account the increase in displacement and factoring in the changes in lift (if any), a reasonable assessment of the overlap required can be made.
Then proceed from there.

 

212/218@.050. 112 LSA.

Its the XR264HR-12 extreme marine from Comp Cams.

Appreciate the input.

 

Typically it seems 3.48 to 3.75 stroke you can add 4-6 degs duration to maintain similar properties. Perhaps a bit tighter lsa to get abit more overlap if needed without too much trouble

miniram has its own issues with air fuel distribution and higher overlap may have issues with idle smoothness and tuning etc.

but that said, 268xfi you posted is a great cam. Definitely would be mild enough yet make power. I tuned a blown 383 tpi motor with a custom 224/230 cam and it idled well. You can tell it had a cam but it wasnt overly big. But idle kpa was low so it had to have decent vacuum. Performance was good too, it was well over 500 whp on just 7-8 psi so on a na motor it still be pretty strong. Mid 300’s whp easily my guess.

hotcams have always been good to me. Lt1 cars ive tuned make good power with them and smooth stock like driving experience. Good exhaust covers the cam sound. Did one in a 350 L98 tpi and it was great. Very good idle

 

I made an interesting discovery on my current cam...

I looked up the street version of the Xtreme energy 264 and the street cam is ground on a 106 intake CL.

The Xtreme marine cam doesn't list the intake CL. But evidently you can calculate it by the formula...

(Intake duration / 2) - intake opening angle

So in the case of the Xtreme marine cam, that'd be 264/2 - 22 = 110

Which means the Xtreme marine was timed 4° retarded compared to the street cam? I think that kinda throws a monkey wrench into my expectations on idle quality vs LSA, since now the cam timing is different as well... hmmm....

 

The only real way to determine ICL is to degree the cam. Whether it's "ground in" or not, it's all relative to how it's installed. That said, if you install a cam that's "dot to dot" on the gear set and there's 4 degrees ground in, then that's what you can expect. A 110 LSA with 4 degrees ground in yields 106 ICL, IF it's installed that way. Sounds like you've got that part figured out.
The mystery is the marine cam and the lack of information.
I'd like to dig into this a little more but I haven't had the time to run through the PN's etc.

 

Your math is correct. Based on the cam card I see that installed "straight up" the ICL will be 110. That's two degrees of advance ground in.


You have the option of advancing that cam to bring the ICL to a more advanced position. That of course will also alter the other cam events. But the IVC has arguably the greatest impact . This will in effect provide a boost to the cranking compression (and DCR if you're so inclined).

The LSA hasn't changed. The amount of overlap, which determines (to a point) the idle quality is still the same.
What happens with the marine cam is the dyno curve is moved slightly up the scale, giving up some low end punch for the sake of higher RPM output.
That said, an earlier IVC, that is the ICL is advanced, will add some pop to the compression and THAT will change the idle quality. You can close the throttle blades a little more for a given RPM. That raises the idle vacuum.

 

There's your later ICL talking to you. A 106 on the street cam vs a 110 for the marine cam.

 

I know I mentioned this to you on another board but I suspected as much!

 

Id venture to say with durations that small, 4 deg icl change wouldnt be that significant in power production but i could be wrong lol. Ive just seen enough cam swapped ls series stuff with all manner of cam specs all make similar power in similar operational rpm bands

 

There's a reason the aftermarket cam manufacturers started grinding in advance into cams. It was a common hot rod trick back in the day to install the cam in an advanced position. Whatever then duration. Now that's done for you.
As for comparing it to experiences with LS engines, all bets are off. I'm seeing that LS specs differ significantly from Gen 1 specs. Largely due to the better cylinder head. Wider LSA is the obvious difference. The overlap needed in a Gen 1 with an average head isn't needed with LS heads. Or so I'm seeing.
But more significantly with the earlier IVC, that will increase torque across the rev range. Up to the point where the duration limits output at RPM.
But I could be wrong lol.

 

FTR: A 4 degree difference in IVC on a 10:1 350 is worth about 10 PSI of cranking pressure and around 2-3 tenths in dynamic compression.
Not a lot but worth power and torque for sure.

 

the point is you can take 10 custom cams with specs from single pattern to wide exhaust splits, some tighter lsa some wider lsa, durations plus or minus 5-7 degs, they all end up being pretty close in power production most of the time. Example
224/224 on a 115
224/232 on a 113
230/236 on a 115
227/240 on a 114

it all seems to be close, 10-15 hp on a 450 hp deal. Most wouldnt really tell the difference while driving

if the cam events are pretty close, minor changes either way dont seem to do alot.

my bbc. 267/272 on a 114 .770” .022 lash. Went to 254/259 109 lsa .730’s” .014 lash. Different lobe family. Makes within 30 hp or so of the bigger cam and more low end average power, which is nothing much difference at the 1600 hp level lol. Granted forced induction is alittle different but sometimes there is no magic in a camshaft

 

Ok, so correct me if I'm wrong, but here are the conclusions I'm drawing from all this regarding the cam selection for my 383...

My 350 idled really nicely with a marine version 212@.050 intake cam and 112 LSA, despite being ground ~6 retarded relative to the street cam on a dot-to-dot installation.

The 383 (by virtue of more cubic inches) should be able to handle a cam size of between 218 and 220 with similar results. And if I at least match the LSA of the current cam, I should be gauranteed at least as smooth an idle quality as my current 350 setup... in fact even more so if it's ground on a more advanced intake CL.

No?

 

Imo yes that would work fine

 

You had mentioned that idle vacuum was your primary concern. Fair enough. Don't add much more overlap as you're doing and you should get what you're after.
But if you want that idle AND increased performance, especially compared to the marine cam, you should be able to refine that spec even further.
Do you have any performance goals? What do you feel is your RPM limit? IIRC correctly the HT 383 is rpm limited. Maybe 5500?

 

my current idle vacuum was about 13.5 inHg at 650 rpm in gear. I have a 2500 rpm converter, so somewhat unloads the engine a little more at idle the a stock converter would.

That vacuum was good for my power brakes (really the only "accessory" being driven off of vacuum (my HVAC system is aftermarket and doesn't use vacuum for the controls like the stock system).

I'm looking to get between 430-450 hp. Like any hot rodder I'd like to get as much as I can, but I'm trying to be realistic about the trade-off between idle quality and performance, otherwise I'd shoot for the moon with a 230-ish duration and try to get up to 500 or something like that. But yeah, I want good stock-ish driving manners.

The heads are 180cc AFR (for cost reasons, I'm reusing them from the 350). Manifold is the Miniram. Exhaust is 1-5/8 primary tube full length headers with a 2.5" dual system.

Again, given this is a 100% street car, and the relatively small heads on a 383 and relatively small exhaust tube primaries, my realistic rpm range is probably up to 5800 rpm.

 

Sounds very similar to the 383 I've been talking about building for the last year (don't ask...) or so, so plenty of time to research and ask questions.
What I'm learning is that you CAN have that idle quality and make really good power up to the RPM ceiling you have. The smaller heads tends to limit the high rpm potential. I've about a 180 cc head that flows 255@.550-.600. Cams I'm looking at are tighter on the LSA and lower in duration than you might expect. I'm also targeting the overlap that worked well in my 355 although I'm not at 13.5". Probably closer to 11-12 but that engine is worn out completely. (Cranking compression was 180-190 and now 150-160.)
Anyway, what I'm finding is that there are many methods to selecting the cam you're after. My direction is more outright performance orientated though. Still, predicted idle vacuum with 283/283 on a 107 is still approaching 12". The .050" specs have their place although most of the theory I deal with works with seat to seat. That adv. 283 would fall somewhere around 234 or so depending on the lobe intensity.
You won't go wrong with the thinking posted earlier but there are options.

 

Looking at he HT383, which I have many times, I find the compression is a little wheezy. That said, your cam selection will differ significantly from mine. Despite the RPM range involved and the power output, the best grind will still yield the best results. The margin for error is undoubtedly less though as Orr points out. And the selection method I use is counterintuitive when a CR of 9.1:1 is used as an input.
Are you going to drop this in as is?

 

No I'm only getting the short block.

I also already ran that compression calculation. With my. AFR heads and a .028 gasket from GM, I'm expecting about 9.8:1. I'm running that compression now with a very aggressive timing map and no knocking.

 

I would maybe look at the more aggressive lobe families from comp. can get lift and added .050” duration and .200” duration with similar or even smaller seat to seat timing. Best of both worlds. More area under the curve and since you arent turning alot of rpm, wont need a ton of valvespring to control it

xfi or similar lobe. Maybe even high lift magnum or comp xe high lift. Or some of the newer q stuff. I havent looked at the catalog in a while

 

One thing I'm interested in is minimizing valve train noise too...

I'm hearing that the voodoo cams have lobe designs that gives quiet valve trains.

also been reading that comp cams tend to be noisier

 

What's the chamber volume of your AFR heads?
For the HT383 I'm seeing:
4.00" x 3.80" bore/stroke
64 cc head
Factory typical .025" piston below deck
18 cc piston dish
A .041" x 4.10" head gasket gets you 9.14:1. I guessed at the gasket but it nets the advertised spec CR.
Move to a .028" gasket and the CR moves up to 9.4.
Not to split hairs, but the AFR chambers have to be about 60 cc to get 9.8:1 with the factory deck and pistons. That's with your .028" gasket.

As for the aggressive timing, I'm all for that. I've built to compression before and the timing generally suffers. I'm a firm believer that a full and proper timing curve always trumps excessive compression. And that's running compression which is cam dependent as well.

 

I found the COMP XFI is noisy. Their XR lobes, not so much. That's my experience with a fully rollerized valve train. Which in itself tends to have that "sewing machine" sound.

 

Hmmm.... I must have goofed up on something earlier. I'm getting 9.4 as well now...

The one thing I've been wanting to do though is measure my chambers. I'm going off of AFR's current 180cc heads, but I got my heads almost 20 years ago. I don't seem to recall them being 65... I recall them being more like 62.

https://www.summitracing.com/popup/c...ion-calculator

EDIT: I just noticed 64cc stamped on the deck. I'll still measure just to be sure. But that at least gets me to 9.5:1. Not terrible I guess. A little conservative in terms of guarding against knocking.

The thing is, the HT383 starts out with a .051" gasket (P/N 12557236). So dropping down to .028 is a pretty decent boost.

https://www.gmperformancemotor.com/parts/12557236.html

 

Like I said, from 9.4 to 9.8 is splitting hairs. That said, with one of cam selection programs I use, going from 10:1 to 10.5:1 makes a fairly significant swing in the cam spec. Makes me wonder sometimes.

Here's a link to the calculator I use. It's at the end of the tech paper that unites static compression with running compression. For whatever a person makes of the dynamic compression value (DCR) it has some weight when comparing builds of the same architecture. I.E. Gen 1 SBC with 10.5:1 and decent heads. Go outside of that and although the values are still relevant, they can't be compared. A BBC or LS has different requirements.

http://cochise.uia.net/pkelley2/DynamicCR.html

 

Yeah some of them lobes are gonna be noisey. May wanna talk to comp to what lobes they might have that could give you a bit of everything you want

or hit up custom cam guys like Jones Cams or Straub Tech.