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Topic Review (Newest First)
02-05-2013 08:10 PM
cobalt327 Prollably not really leaner, but possibly hotter. The plug can look about the same either way. I agree to readjust the idle mixture screws and keeping an eye on the plugs. You might want to go to a one step colder plug if they continue to look "lean". Be sure there are no vacuum leaks, too. Hoses, accessories, you know the drill. If you have a vacuum gauge, you can use it to see if the vacuum picks up while spraying carb cleaner or similar around the intake to head and carb to intake gaskets. If there's a leak and it's bad enough you'll hear the engine idle change.
02-05-2013 07:57 PM
damanx
Quote:
Originally Posted by cobalt327 View Post
Sounds like a plan, as they say.

To clarify, above I might have made it sound like a tight TC will cause the vacuum to drop at the hit of the throttle. The vacuum will drop regardless of the stall speed, but an engine w/a tight converter will will take longer to recover is the main point.

I tend to agree that manifold vacuum is the better choice for the majority of performance engines.

Good luck and keep us posted.
Thanks for the reply.

So, I decided, until I have the time this weekend to dig in a little deeper, was to put the vacuum advance back on the ported supply as this truck is my daily driver, so I have to be a little careful about tweaks as I need to get to work and back.

I did this after pulling a couple of plugs and seeing what appears to be leaner operation with the manifold supplying vacuum in addition to the ported through that can.

Now it has been running decently, but I don't believe optimally. So, this weekend, I will run the vacuum advance to the manifold and tweak the mixture and what not.
02-05-2013 07:22 PM
cobalt327 Sounds like a plan, as they say.

To clarify, above I might have made it sound like a tight TC will cause the vacuum to drop at the hit of the throttle. The vacuum will drop regardless of the stall speed, but an engine w/a tight converter will will take longer to recover is the main point.

I tend to agree that manifold vacuum is the better choice for the majority of performance engines.

Good luck and keep us posted.
02-05-2013 04:29 PM
damanx Well, it seems I've had a brain dump since I wrote the previous post. The reason for that is that I wrote that post when I got home from work and had an idea of what I was going to do before I went to bed, but since I've woken up a little bit ago, I've forgotten......ugh.....

So, what I am going to do, is:

1. Remove the extra vacuum stuff.
2. Verify initial ignition and set to 12 degrees if needed.
3. Verify how much mechanical advance I have.
4. Reconnect vacuum advance to manifold, adjust idle speed and see how it runs.

I've read through both threads plus the "crankshaft info page" and it seems to me that manifold vacuum is the choice on an engine that has no emissions equipment, so that is where I am going to start with it.
02-05-2013 09:23 AM
cobalt327
Quote:
Originally Posted by damanx View Post
So, I left the base timing at 12 degrees and I tubed in one of those distributor vacuum valves that has the manifold port and carb port that feeds vacuum to the distributor. The manifold supply is actually going to a thermal vacuum valve then to the valve.

I noticed that when the engine was warm, the vacuum from the manifold applied the advance at idle bringing the timing to 22 degrees.

Upon acceleration where manifold vacuum dropped, the timing drops suddenly then comes back up until it seems like the mechanical kicks in.

So I think I am on the right track as it seems like the pull is much smoother than on the ported vacuum.

The other thing I'd like to try is placing a delay valve before the advance can to slow down the loss of vacuum a little.

One issue I need to deal with is the secondary boosters leaking also.

Will update once I install the valve.
Using a delay to the vacuum advance can work, but you need to realize that when you floor it and the engine's under a load (pulling up a hill for example), the delay in dropping the vacuum advance could cause it to detonate. There can be a transient ping at the hit of the throttle even w/o a delay.

The rest of the dynamics involved also come into play, so whether this will be a problem or not remains to be seen. But it's something I wanted you to be aware of (and you prollably already were) so you'd be watching for detonation under a load (pinging).

In general, you're seeing one of the downsides to using a lot of vacuum advance. If the amount was 10-12 degrees from the vac. adv, the drop wouldn't be as noticeable, and that would have allowed you to add that timing (all or part) to the initial advance.

If the engine is running on a lot of vacuum advance, it needs to be matched up very close to optimum as far as torque converter stall, gear ratios, cam, etc. goes. Because if there's a mismatch like a too-tight torque converter as an example, that alone can cause the engine to drop vacuum (and it'll take longer to recover) at the hit of the throttle.
02-05-2013 08:23 AM
damanx
Update 2/5

So, I left the base timing at 12 degrees and I tubed in one of those distributor vacuum valves that has the manifold port and carb port that feeds vacuum to the distributor. The manifold supply is actually going to a thermal vacuum valve then to the valve.

I noticed that when the engine was warm, the vacuum from the manifold applied the advance at idle bringing the timing to 22 degrees.

Upon acceleration where manifold vacuum dropped, the timing drops suddenly then comes back up until it seems like the mechanical kicks in.

So I think I am on the right track as it seems like the pull is much smoother than on the ported vacuum.

The other thing I'd like to try is placing a delay valve before the advance can to slow down the loss of vacuum a little.

One issue I need to deal with is the secondary boosters leaking also.

Will update once I install the valve.
02-04-2013 06:05 PM
damanx Thanks for that too!

So, what I decided to do, since I have the parts, was to use a vacuum delay.

This is that part that has a connection for manifold and ported and then to the distributor.

Having this connected, with the initial set at 12 degrees, put the timing at idle to about 18 degrees at idle. Idle speed was about 200 rpm higher and I did notice a little hard to start unless I pressed the throttle just slightly or closed the choke a little bit.

Will test this out tonight on the drive to work and see how it runs.

Worse case scenario is I pull the vacuum line from the distributor and plug it back into the ported supply on the carburetor.

(In the meantime, I'll be researching what I need for converting to TBI using the model 700 system.)
02-04-2013 05:46 PM
cobalt327 Some more reading here. The page is about GM HEI but the timing info is basically universal.
02-04-2013 05:37 PM
damanx
Quote:
Originally Posted by cobalt327 View Post
Try it and see. It would be better if the vacuum advance wasn't giving so much advance, but even so it may be OK using it on manifold vacuum.

Good luck.
Thanks.

Found this thread when I searched "vacuum advance":

http://www.hotrodders.com/forum/vacu...tml#post330385

Going to read through this thread and see what will work best.

It seems without all the smog/emissions equipment, the better source is the manifold vacuum.

Will read through and report back.
02-04-2013 05:32 PM
cobalt327
Quote:
Originally Posted by damanx View Post
Thanks for the info!

I am thinking that I need to move the vacuum feed to the advance from the ported to manifold to start.

I've been reading a lot and it seems for my application, without the emissions parts that manifold vacuum would be better.
Try it and see. It would be better if the vacuum advance wasn't giving so much advance, but even so it may be OK using it on manifold vacuum.

Good luck.
02-04-2013 05:22 PM
damanx Thanks for the info!

I am thinking that I need to move the vacuum feed to the advance from the ported to manifold to start.

I've been reading a lot and it seems for my application, without the emissions parts that manifold vacuum would be better.
02-04-2013 01:29 PM
cobalt327
Quote:
Originally Posted by damanx View Post
It's just the stock cam in the engine.

I would love to get this truck on a dyno, but I'd love to get the basics in the ball park first.
The mechanical traditionally wants to be all in at or before 3000 rpm. In your case, seeing the 40-plus degrees at light throttle cruise if fine. I do not know what is the best max power timing (initial plus mechanical, w/o vacuum advance added in), but once you find this by track testing or from others who have done this, you can tailor the curve as it needs to be.

The vacuum advance is dropping a lot of timing- looks like 20 degrees. You can use a limiter to limit the amount of vacuum advance to around 12 degrees or so, then add the 8 degrees or so back to either the initial or mechanical timing- or a little of both.

I like to use as much initial as I can. This tends to make the idle better and the off idle response is also usually better. You know you've found the limit when it gets balky to start hot. With a stock cam there's no need to go so far w/initial timing that the starter starts to drag hot, so keep it below that level.

Once you've found the limit to the initial, the rest comes from mechanical. You can start trying lighter springs to bring in the mechanical as early as it'll allow. How early this will be will depend on the fuel, vehicle weight, gearing, etc. and you'll be doing some trial and retrial to get it right.

You prollably don't want the mechanical all in at 2000 rpm, though. I think this would be too low and you'll have problems w/it pinging. When you get on it, the transmission should downshift (passing gear), or you can manually drop it back a gear. This will accelerate the vehicle a lot better than leaving it in a higher gear and pulling from 2000 rpm.

There might be something to be said for an adjustable vacuum advance if one's available for the distributor you're using.
02-04-2013 08:13 AM
damanx
Quote:
Originally Posted by vinniekq2 View Post
what is the cam timing for your application?have you considered buying time on a mustang dyno?
It's just the stock cam in the engine.

I would love to get this truck on a dyno, but I'd love to get the basics in the ball park first.
02-04-2013 06:09 AM
vinniekq2 what is the cam timing for your application?have you considered buying time on a mustang dyno?
02-04-2013 06:05 AM
damanx
Help me fine tune my 3.4l with a Holley 390 cfm.

Hey all, hopefully I can get some help to nail down some tuning issues I am having.

To start, it's a 1985 S-15 with a 1993 3.4l v6 60 degree block. I've put on the Edelbrock 2 piece intake system and topping it off is the Holley 390 cfm carburetor. 700r4 transmission and 3.73 rear.

The ignition system is the original stock system from the 2.8l, which is a distributor and seperate coil.

So far, at 2900 feet, it's running at idle, a very stable 17" vacuum at 900 rpm. In drive with brake applied, it drops to about 15". Initial timing is at 12 degrees.

At cruise speed of 55 mph, 1950 rpm, vacuum is around 12". At this speed, timing is running about 42-45 degrees.

One of my issues is apparently getting the timing all in at 32 degrees, but where? I've read anywhere between 2500 - 3500 for mechanical. If I am cruising 55-65 most of the time, should I set mechanical to be all in at around 2000 rpm?

Fortunately, I have a timing analyzer that I have been using to watch what the timing is doing under different conditions and another issue I need to clarify is what am I wanting the timing to do as soon as I stomp the throttle? For example, at cruise when the timing is full mechanical and vacuum applied, it is at 45 degrees. When I get into it, vacuum goes away and the timing drops to 25 or so. Am I correct in assuming I need to put aftermarket weights and springs for the mechanical to stay at 32 above 2000 rpm?

Also, when I give it gas, with the vacuum advance hooked to the ported outlet on the metering block, it appears that I lose vacuum and maintain the max mechanical advance almost immediately. Should I place a vacuum delay inline to slow the drop?

Thanks for any and all help.

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