Does a non-vac adv dist make a slug at low rpms?
I have a 428FE modified with a fairly mild cam,approx 10:1 CR, and twin Holley's on a Offy 360 deg manifold. Non-vac-advance distributor.
I'm very happy with the engine once I get over 2500 rpm, but it's a dog down low. In fact it's so bad, it's easy to stall when engaging the clutch.
I've been chalking this up to my intake manifold for some time, but recent access to some dyno charts of my manifold vs a highly recommended one, shows little to no difference below 3000 rpm.
I'm using a non-vacuum distributor, and in order to get rid of a bog taking off, I had to bump the timing at idle up to 20 degBTDC. I've kept the max mechanical advance at 38 degBTDC.
I did some experimenting with initial timing, all the while keeping the max adavnce at 38 degBTDC by adjusting the dist internals. Here's what I get:
12 degBTDC; major bog when accelerating down low.
18 degBTDC; bog almost gone
20 degBTDC; bog is gone, but engine is still a dog down low.
24 deg BTDC; seems better, but am close to detonation under load.
28 degBTDC; definite signs of detonation down low
So I picked 20 as I believe if I have it at 24 and I hit the throttle a little harder than normal (in low gears), detonation might occur.
Now it seems to me that a vacuum advance distributor might take care of all these timing issues at the low end.
And I've read some posts here about a vac-advance giving snappier low end response for a street vehicle.
And then there's the old ported vs manifold connection. All I can say on this, is I know my engine idles hot at 12 degBTDC and doesn't at 20 degBTDC, so if manifold vacuum will advance the timing at idle, then that seems to make sense to me.
And of course, I admit that I've stuck with a non-vacuum distributor as I havn't a clue how to determine proper timing for my engine once if I have a vacuum advance canister to deal with. Also, with the exhaust I have, it's difficult ot hear detonation, although if it's bad enough, I can feel the engine fighting the throttle (doesn't want to accelerate).
So.... any comments that might help me determine if a Vac-Advance dist would be a good idea to give snappier low rpm part-throttle performance, and if so, how can I determine the amount of vacuum advance I need?
ps: just to find out where my engine can idle, I found I can set the idle timing up to 38 degrees before the idle suddenly drops off, (mere co-incidence that is the same number as max mech advance). Of course, any load on the engine at that timing would cause a major problem.
At the top end, I have no idea how much past 38 degBTDC I can go at cruise speeds.
I always advise my customers to run a vac advance distrib on the street. full vac. connection. Usually makes for a better low speed response. Streetability..Better fuel milage
- What do you mean by a "fairly mild cam"? I like to see specifications, not qualitative descriptions. The engine's behavior is in the numbers. the bigger the cam the more the engine will want in base advance and the less it will tolerate in the adjustable advance whether that's vacuum or centrifugal or both.
- Is the compression really 10 to 1, that's pretty high for an OEM FE head in iron running unleaded. The FE with rare exception, and that wasn't the 427, aren't very efficient and were very detonation prone back in the days of leaded premium.
- What is the gearing including tire size? This has a huge effect on performance under the torque peak.
- What is the transmission and if an automatic does the converter stall track with the cam timing and lift?
The use of vacuum advance is for cruise at low to moderate RPM with very little throttle opening. Under these conditions the mixture has fairly low density (molecules of air and fuel per some unit of volume) therefore burns
slowly putting the point of maximum pressure rise too late in the crankshaft turning cycle for optimum effort. So the timing is pushed up to start the burn earlier, but this is a unique situation which needs to go away as load on the crankshaft and/or RPMs are increased. If the cam is mild enough under cruise a high vacuum is formed in the intake which can be used to operate a vacuum advance system from which the manifold vacuum provides the control of this unique situation most perfectly. As the cam gets wilder the vacuum falls making a vacuum advance system unusable, this then requires more base and a faster, though amount limited, centrifugal system. Though guys use this with an automatic it really starts to be pretty fussy working much better with a manual gear box and the ability easily to manage engine RPMs independently of road speed and throttle position (crankshaft loading).
- I am of course assuming that the camshaft is timed correctly to the crank as what you're experiencing is also a sign of the cam being retarded among other potential problems.
- What is the carburation dual 4 barrels? If so whose and how big and what is the jetting.
- An Offy 360 intake is a very poor selection, these things have more gaps and holes in the RPM band than a 1 acre, 19 hole golf course. A single 4 barrel on an Edlebrock RPM would be a way better choice.
Thanks for the info and thoughts. F-Birdí88; I was writing my response for OldBogie below, when your reply came in, so if you read the dyno chart part, that was written before you infoíd me on the reliability of dyno numbers under 3000 rpm.
O-B: Yes, it is difficult to imagine. Itís been a bit of an annoyance over the years, and I think itís getting close to the time I ought to fix it.
Hereís more accurate and quantitative information;
First of all, although I call it a 428, itís really a 410, over-bored by 0.040Ē. The pistons are TRW forged 390 pistons that were re-machined for the 410 to yield a 9.5 CR. I double checked the head chamber volume way back when I built this engine up, and it worked out with the math.
I have stated 10:1 as I did do a mini-rebuild at one time due to detonation, as I head the block deck and heads faced a bit. Going by what the machinist told me he took off, it looked like an increase to 10:1 based on some math.
I am using a solid lifter Crane cam. It is the old Fireball 294 which has the following specs:
Adv duration 294
Timing @ 0.050ď:
Intake Opens 4 BTDC
Intake closes 43 ABDC
Exhaust Opens 55 BBDC
Exhaust closes 5 BTDC
Intake and Exhaust Duration 227 deg
I have installed the cam at zero degrees. In other words, with a degree wheel, I can get the results listed above working with 0.050Ē
I am using a Richmond Gear 5 speed, and the ratios are as follows:
Differential Ratio: 3.07:1 (donít forget, 1st gear is lower than in the typical 4-speed)
Yes, I am now aware the Offy 360 is a bit of a dog, but at the time I didnít know that. It was given to me along with a pair of used Holleys when I bought some engine parts to sweeten the deal.
I have been under the impression that this manifold is the cause of my trouble, however I recently bought Jay Brownís book on FE intakes, and the well touted Blue Thunder 2x4 Dual Plane dynoís out the same as the Offy 360 once below 3000 rpm, or as best as I can tell as the dyno charts donít go all the way to idle.
I rebuilt the two Holleys which were 650 cfm each with vacuum secondaryís. I also modified them by connecting a line between the two vacuum chambers to ensure all secondaries opened simultaneously. This was important as I have the carbs set up with a progressive linkage.
I always had to have the idle screws almost closed with these carbs for max vacuum, so I eventually replaced them with a pair set up (by Pony Carburetors) to meet the original Ford 2x4 requirements. They seemed to work, and my idle mixture screws are now in a more appropriate range.
I never checked to see what jets are in the carbs. The secondaries use metering plates, and I know Iím using a 6.5 PV in both. Thatís all I know at this point.
PS: This engine is in a 2550 lb kit car, so itís light. If I let the clutch out a little quickly and donít keep the revs up, it can stall. It feels like I have a 4-cylinder in there! On the other hand, if Iím moving in 2nd gear, and am revving close to 2500, I can take off like a rocket. Iím scared to give it full throttle as it feels like I might lose rear end traction at any more than 2/3 throttle.
So, even if this intake manifold is a dog up high, I have more potential Hp than Iíll probably ever use unless I switch to better tires. Therefore, I donít really need to spend $800 on a new manifold for top end power, but I would spend it in a minute if it would fix the bottom end. At this point, I am trying to investigate the problem further (i.e. to determine causes other than the intake manifold) and am looking for advice.
Trying a vac advance distributor is one thought. Advancing the cam 4 degrees might be another. I'm open to suggestions.
This is typical what happen when somebody installed large CFM carbs. You be better off using two VS 465CFM Holley's instead of pair of 650's. It's just two much the motor to handle. The large venturies on the primary will cause poor throttle response at off idle
Use these carbs and just run them 1:1 leakage.
The distributor has nothing to do why you have lousy throttle response.
I'm running non- vacuum advance distributor on my tunnel ram 406 and the throttle response is excellent.
Last edited by lg1969; 04-11-2012 at 05:01 PM.
Well..... I certainly appreciate all the responses. There's been quite a diversity of opinions, so I need to think about all your contributions and try to prioritize my approach based on some of the thoughts presented here.
Thank you for your replies, all of which were very interesting.
ps: Oops... Not that it matters much, but carbs are 600 cfm each, not 650 as I previously posted.
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