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Anyone that is running a Comp XR276HR-10 have a sound clip they wouldn't mind sharing? I tried looking for one to no avail. I'm starting to put my car back together with this cam and I am anxious to know what to expect.
Here is my Comp 276 hyd roller cam, 0.503/0.510", 224/230 @0.05, 112 LSA, (i didn't like the 110 for EFI). Mine is the XM276HR ("marine") but the specs are exactly the same as the passenger car 276HR cam, but on a 112 LSA.
This is a 327 with retrofit roller setup, Holley Stealth Ram, 24 lb injectors, World Sportsman II heads, long tubes, 2.25" dual exhaust w/ Dynomax Ultraflo's, full length exhaust system
It might sound a little different once I get it back together, I advanced the cam 4 crank degrees and swapped the 1.6 ratio rockers for 1.52. I also changed the ignition timing curve.
Drive away. Just started tuning it in this video. It stalled, that's why the video starts with me starting it, and the puff of black smoke coming out. Haha. http://www.youtube.com/watch?v=QMfJeSDq2qc
It definitely has an aggressive choppy idle. I'm happy with mine. I took it to 6000 RPM, and it pulled strong all the way.
Last edited by IROCThe5.7L; 10-20-2009 at 10:43 PM.
I just put in a XR276HR-10 in this summer. I used 1.6 Rockers, so it's .535/.544.
It definitely has an aggressive choppy idle. I'm happy with mine. I took it to 6000 RPM, and it pulled strong all the way.
Have you had your setup dynoed at all? or ever had it to the track?
Drive away. Just started tuning it in this video. It stalled, that's why the video starts with me starting it, and the puff of black smoke coming out. Haha. http://www.youtube.com/watch?v=QMfJeSDq2qc
It definitely has an aggressive choppy idle. I'm happy with mine. I took it to 6000 RPM, and it pulled strong all the way.
Have you had your setup dynoed at all? or ever had it to the track?
Not with the new setup. I ran this setup all last summer with 2.77's and a stock stall. I can't wait for the 3.73's and 3K stall this summer. It will see the track this summer for sure.
With that xm276hr cam how does your tune look, i have that cam in my 383, not sure if my tune is jacked up or not.... i guess im struggling with it..... it sounds good but hard to tell its cammed... when i have the timing disconnected(little brown wire) i can tell and sounds great, but as i plug it in it than sounds kinda normal... not sure why.... my base timing is set to 6* adv.... also she hunts idle... 10:1 compression flat top pistons/ vortec heads, all we have up here is 91 non eth fuel.
Any ideas or maybe i can Barrow a bin to compare?
Last edited by happyhapka; 12-11-2019 at 11:53 AM.
I have it set at 700 rpm at idle.......... one thing i will mention is when setting the min idle the adj screw is maxed out.....with it maxed out it idles at 600-650 ... it wont stay idling if its any really lower when setting min idle.
...10:1 compression flat top pistons/ vortec heads...
How did you end up with only 10:1 CR?
With a flat top piston (and 5-6 cc of valve relief), putting the piston to deck clearance at .060" (which is a fairly big gap), the CR I arrive at is 10.6:1. With the XR276HR, the dynamic compression ratio (assuming a 5.7 inch rod) is north of 8.5:1. That's going to hard to keep out of detonation. Been there. Tried that.
When i ordered the stroker kit, the guy said it was 10:1 cr, but dont remember if that was with the stock heads, im running vortec. So i guess not 100% sure on my compression ratio.
Well, if it's a "traditional" build, then your CR could be much too high. But that's only speculation. Do you have details of the build? Specifically, piston deck height (how far the piston is down in the cylinder at TDC), head gasket thickness (or part number), piston volume (although you said flat tops so that narrows the relief volume). Was the cam degreed before it was installed (other than the dot to dot method)?
You're chasing a tune and quite likely that's where your issue lies however it'd be a shame to beat your head against the wall in that regard if the engine fundamentals are out of whack.
FWIW, I built a Vortec headed 350 with the XR276HR cam. I DID have a 10:1 CR (measured and verified) and that was possibly the best iteration of that engine that I put together over the years. Idled very well. Good idle vacuum ( by the way, what's your idle vacuum?) Cranking compression (via a tester) was in or around 200 PSI. But, it didn't have that super aggressive idle lope that many people are looking for. You could tell it had a cam but it wasn't crazy by any means. Even my XR288HR has a decent idle although I've worked the tune considerably (carbed and traditional distributor) to make it mellow.
I want to build a similar 383 with small chambered heads (not unlike Vortecs) but a piston dish of 12 -18cc is needed to keep the CR in line and not give up some engine build basics (like quench/piston to head clearance)
If you can gather some assembly data, I'd be happy to crunch some numbers for you.
These are the gaskets i used, i took it to a machine shop here, the guy is very well trusted and installed it to spec, but am not sure what, i wish i really knew more,
Over all the car does run great, the hunting idle isnt that bad but i guess i was just wanting it glass smooth wich i assume is hard to do..... im not sure the piston travel, its a scat crank, i beam rods, the pistons are forged hypertonic flat tops, stock vortec heads, 1.6 ratio rockers.
There are a lot a variables still in question. That head gasket is only .015" thick but without the rest of the details, there's no telling where you're at. A compression test will give some insight in that however.
Has the block been decked? Do you have a piston part number? What I'm seeing is that with a standard deck block and the correct compression height piston (available with the part number) that gasket combined with rest of the specs may have created something unmanageable.
I tried the next step with my Vortec 350 and jacked the compression ratio. The only thing to make it manageable was to kill the timing. That resulted in a bunch of other issues not the least of which was a poor idle. Typically, a 10:1 SBC with a properly spec'd cam will eat up more the 20 degrees of idle timing. That helps smooth the idle. When I went to the 288 cam, idle timing was more than 30 degrees but was very smooth, with cool engine temps and great throttle response.
If you can't sort this out tune-wise you can do a couple of things to help clarify. Do a compression test. And more easily, put a vacuum gauge on it. If you're not seeing nearly 12-14" of steady idle vacuum at your 650-700 RPM, then something is amiss.
Well, if base timing is only 6 degrees, and you're seeing that level of idle vacuum, it does suggest good compression. Really good actually. Which is where I'm going with all of this.
Keep in mind, that without the proper build specs, I'm kinda shooting in the dark here.
I read in another post of yours that you were experience some amount of spark knock. Is there an indication of when and where this happens?
And what would it take for you do a compression test on at least a couple of cylinders?
Other than that, I'm at an impasse with anything really helpful.
Usually dont actually get alot of KR any more , just what i posted earlier, i was alot worse but it really cleaned up, i usually get it at really heavy throttle.
Kr is what the computer pulls out and the regular knock counts i also posted, they were from the other night...
I know I commented on your other thread with some tuning info, but I didn't pay as much attention to your exact combo as skinny z did in regards to your compression ratio. Figuring out what it is will really help you with tuning this thing at higher loads. Is there any paper work from the builder showing what deck height is? For example, on my 383 build, I used flat tops with 5cc reliefs, 65cc combustion chamber heads, deck height was .016", I used an .027" head gasket. Compression came to 10.9:1, with a decent .043" quench. I'm using a 292 cam, which helps lower my dynamic compression. It runs great on pump gas, although I'm a little conservative on the timing through the peak torque range (27 degrees from 3000-5000, and ramping it up to 34 by 6000. If you have a similar deck height, with that .015" head gasket and 64cc chambers, you're in the low 11's or higher on compression, combined with the 276 cam, iron heads, 91 gas, tuning won't be easy. It might be worth the time and little bit of money to pull the heads, measure deck height, and order the appropriate gasket to get your compression down, while still maintaining a decent quench.
Keep in mind too Matt that this is an iron headed engine.
One thing I learned with my experimentation is that timing trumps the compression ratio. Both in power output and fuel economy. The miserable half point in excess that I was dealing with prevented me setting the timing at an optimum level other than WOT. Cruise spark advance was particularly difficult (although take into consideration that I'm all nut and bolts with respect to tuning and no computer).
That said, I agree with your suggestion of going back to the builder and try to get some details. Or pull it apart. Either that or I suppose just tune around it and live with the results. Happy commented that it's not that bad.
OK. I don't see that the block was decked so we can work with that.
64CC head, 5CC piston, .015X 4.100 gasket, 4.030 X 3.75 engine and with no decking , guessing the piston is .025" below deck. Compression ratio works out to 10.9:1!
i believe my block was decked and my heads as well, im not sure how much, ill see if i cant find any of those out.
If the block and the heads were decked, then that will push your CR up even higher. With a stock deck height (arguably .025" down) and the gasket you listed at .015" your piston to head clearance is .040". That's about the limit. If the blocks been decked, that value gets smaller. Dangerously so but seeing as you've already been driving and it hasn't self-destructed, then chances are you're OK there.
But the high CR is what may be giving your issues regarding tuning. Still guessing though.
So based of the knock and the KR pics i sent, should i focus more on taking timing out due to the knock table or should i pay more attention to the KR.
If it were me, I'd pull timing until there is no further evidence of engine knock. Either audibly or through data logging. Then gradually work into what the engine can use.
02-10-2008, 09:42 PM
