Hot Rod Forum banner
1 - 15 of 15 Posts

·
Registered
Joined
·
74 Posts
Discussion Starter · #1 ·
I've got a little '23 T with a fresh 350 that's beautifully set up, strong, fast, smooth and all, but there's one little thing. As it hits 55-60 mph, the load drops off, the vacuum comes up, and it gets a very light valve rattle until over 60 when the aero load increases and it goes away. I have been from one end to the other with initial advance, cent. adv. and vac adv., lifters from loose to tight, higher octane gas, and nothing makes it go away. It's clearly not hurting the engine or affecting performance, just frustrating. I'm sure the big engine/little car thing has a lot to do with it. Any ideas? Specs follow to keep post short.
 

Attachments

·
Registered
Joined
·
74 Posts
Discussion Starter · #2 · (Edited)
Engine specs

350, +.030, 327 Power Pack heads, Comp 252H kit (cam, lifters, timing set) Delco HEI, Weiand dual plane intake, Holley 600, 3.00 rear end.

18' initial adv, yellow springs (all in by 1600), short vac can with 10' adv.
Total adv is 48' but I have run with much less and more and rattle is always there. I have locked out cent adv to 10' but it runs better with 20'. Even stock vac can runs fine except for brief rattle going thru 55-60 mph.

It's not like the car runs poorly, quite the opposite, great idle, jackrabbit launch, tire-chirping shifts, beautiful shifts and downshifts. But why does it rattle when the load drops off?
 

·
Registered
Joined
·
774 Posts
I would try stronger advance weight springs. If it is a crane kit...try the blue ones or maybe a blue/silver combo.

That's a great little engine! I can see why why it is so snappy...outa be a tire fryer.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #5 · (Edited)
Full vac. Does the same on ported vac too. As for springs, I've been all the way up to 2 blues so the cent comes in at 3500, which this engine rarely reaches. Even locked it down to 10' with a screw through the rotor base.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #7 ·
You may be on to something there. The compression is about 10.25 or so, didn't check pressure but miked the piston at TDC to calculate the CR. I think the cam would prefer lower compression. Oddly, the octane seems to have little to do with it. 93 works fine, octane boost does not help. Accidentally pumped some 87 yesterday (selected wrong grade) and it ran just fine. It never, ever, ever knocks under load. Cam is supposed to be ok up to 11:1 cr, but I suspect it is much happier at the lower end of its range.
 

·
Registered
Joined
·
472 Posts
I believe your compression information is wrong
Increase your street machine's gas mileage and powerband performance!
COMP Cams High Energy cam and lifter kits are designed for street engines with an 8:1 to 9:1 compression ratio. They also work well with all stock components, including intakes, exhaust manifolds, and carburetors. These kits offer good increases in both gas mileage and power throughout the rpm range for most street applications.

Warranty


COMP Cams warrants its products to be free from defects in material and workmanship for one year from the date of purchase. This warranty is valid on camshafts only where new lifters and proper valve springs are used. This warranty is voided on any part used in racing applications or any cam part ending in -5, -6, -7, -8, or -9.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #9 ·
#8 cylinder is fine and no vacuum leaks. PCV to carb base and trans vac to manifold. Some of the rattle may be PCV chattering. Is there a quiet PCV without a moving valve?
 

·
Evil Wicked Mean And Nasty
Joined
·
13,765 Posts
Have you tried backing the timining off alittle maybe? I think you need to tweek the timing. I would go to 14 degrees at 800 at idle and go from there and see what you have then. But those heads are going to restrict the engine IMO.Just courious what rocker arms are you running?............Cole
 

·
Registered
Joined
·
890 Posts
Could be that the vacuum advance is adding too much advance, is it
adjustable? Did you try running it with the canister disconnected?

Another consideration is it might be too lean at that point, may be the
power valve closes leaning t too far.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #12 ·
Good suggestions about the timing, but I thought the same thing and I have already tried those suggestions. Remember this is a VERY light car with a VERY torquey, low-rev engine and it can tolerate enormous amounts of advance without knocking. Retarding the timing and using a short, stiff advance curve gives horrible results, while a very aggressive curve gives excellent results all the way into low 50s. Cam is pro installed and degreed, so it's not cam timing.

Carb has been painstakingly set up too, and runs best on stock jets and power valves with a 25 shooter (31 is stock). Fresh bowl job, float adjust and idle mixture adjust - all good. It did respond very well to the lightest secondary spring.

There may not be an ideal solution to this (admittedly minor) problem. Below 60, the engine is pushing weight and above 60 it's pushing air. It's just the little butterflies at the top of the rollercoaster that makes the valves a little ticklish.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #14 ·
I agree that it's not detonation. I think it comes down to the heads with small chambers and valves and the cam with 110 LSA and very discrete valve events. The car did the same thing with an even milder cam, so it must be the little 1.72/1.50 Power Pack heads creating too much cylinder pressure. On paper, this is a great combination, but I wouldn't be surprised if less extreme heads with bigger valves would work better.
 

·
Registered
Joined
·
74 Posts
Discussion Starter · #15 ·
Still stumped

Well, I got back to the Comp helpline and they couldn't help and handed me off to the expert, Brian, who was stumped as well. I suspected too much compression but he did not think this was the problem since it would show up as spark knock which the car does NOT do under any conditions. Even when I accidentally pumped 87 it did not knock (had some 93 remaining). This is not a car with a knock problem, it has a sweet rumbly idle, peels 1st, chirps 2nd and 3rd, chirps downshifts, and has the exact kind of performance I built it for. It can not be made to knock under any load, but when the load drops off, it gets a little tacks-in-a-jar rattle. Anyone? (crickets) Anyone?
 
1 - 15 of 15 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top