Dumb question re 70 LT-1

[Joe L.]

Re: Dumb question re 70 LT-1

Most Holleys have a small plastic vacuum pot that's connected to the choke linkage. Its purpose is to crack open the choke valve when the engine fires. It sees manifold vacuum full time, which is why it's a good place to tee into when converting from ported to full time vacuum advance. I'm not sure of the 3310 configuration, but since it was a widely used replacement carb, I expect it has a conventional Holley choke system. Your CSM should explain it for the OE Holley and I expect the 3310 is very similar.

Duke

Duke------


I think that some versions of the Holley 3310 have no choke, at all, and none can be installed on such a carburetor. Versions of the 3310 have been manufactured for over 50 years. The only PRODUCTION Chevrolet I know of that originally used the 3310 was the 1965 Z16 Chevelle. I don't know if that carb had a choke, or not.

[Ed H.]

Re: Dumb question re 70 LT-1

OK, heaven help me, LOL... There is an electric choke kit which can be fitted. Allegedly there is a 'stove kit' to work with this choke, but it seems Holley doesn't sell it. So far, it hasn't needed the choke to start.

There is nothing apparent to me in the way of a 'pull off', but this is pretty much uncharted territory to me. I have a Holley book by HP Books, but it's ahhh, a bit light on this stuff. I need to be clear that Holley has been very helpful, it just took a bit.

[Joe L.]

Re: Dumb question re 70 LT-1

OK, heaven help me, LOL... There is an electric choke kit which can be fitted. Allegedly there is a 'stove kit' to work with this choke, but it seems Holley doesn't sell it. So far, it hasn't needed the choke to start.

There is nothing apparent to me in the way of a 'pull off', but this is pretty much uncharted territory to me. I have a Holley book by HP Books, but it's ahhh, a bit light on this stuff. I need to be clear that Holley has been very helpful, it just took a bit.

Ed------


This carb has a hot air type choke, so this carb must be an early version of the 3310, perhaps even one that was used for some PRODUCTION application (other than Chevrolet application). This type of choke does not use a choke pull-off.

[Ed H.]

Re: Dumb question re 70 LT-1

"OMG, the more one thinks one knows, the more stuff one knows one doesn't know... " but I repeat myself... Thank you!

[Ed H.]

Re: Dumb question re 70 LT-1

So a bit of an update:

Primary jets were 72, I've changed to 70. Secondary power valve was 10.5, changed to 8.5. As near as I know, these correspond to the original jet and power valve specs. The primary PV is also 8.5, and the secondary jets are 76. Car running noticeably better. I also adjusted the timing to spec. We did a 100 mile (each way) round trip today, running 75-85 most of the way, and the mileage worked out to 14.75 mpg.

Have a dyno date Wednesday, hoping to get jetting and timing/advance etc spiffed up, but the weather isn't looking promising.

As an aside, and another question I've posed, the LT-1 stripes. I've found some photos which, while probably not admissible in court, pretty much show that the stripes were the later style. Hopefully we'll get them done sometime soon.

Thank you all for your help and comments. Ed

[David R.]

Re: Dumb question re 70 LT-1

My new 1972 LT-1 had 4:11 gears and the Holley 4150. Around town I was frequently down into single digits, on the highway with some conservative driving I could pull 12 or 13. That's about the best she would do . . . but what fun was that?

[Mark E.]

Re: Dumb question re 70 LT-1

Looks like you made progress with fuel efficiency. As mentioned, first check everything works properly, including choke, no vacuum leaks, primary and secondary ignition, tire pressure, brake drag.

Dialing in the ignition timing and primary fuel system makes a big difference, as you've seen.. Don't forget to adjust the idle mixture- it affects A/F in all driving modes.

The secondary fuel system and power valves don't affect fuel economy much since they're only active during high load/ low vacuum conditions.

[Duke W.]

Re: Dumb question re 70 LT-1

So a bit of an update:


Have a dyno date Wednesday, hoping to get jetting and timing/advance etc spiffed up, but the weather isn't looking promising.

I recommend you postpone your dyno test until you've optimized the spark advance map. The 70 LT-1 is an emission controlled engine, so the map was set up for emissions, not performance and fuel economy. Specs are in your CSM and AMA specs, but you need to measure it now because it may have been modified over the years.

Measure head gasket thickness at the corners of the head-block interface. If it's thicker than the OE .018" shim gasket the CR is lower than when it left Flint - up to two points if it was ever rebuilt with "low compression pistons", which many have.

The ideal spark advance map would be the same as the 365/375 HP 327s. You'll need an Airtex 4V1053 (B28) 8" VAC connected to a full time manifold vacuum source. Use the lightest centrifugal springs you can find, and set total WOT advance at 36-40, as high in the range as the engine will tolerate without detonation. This is easy to do with road testing.

Most OE power valves are 6.5", and I recommend you change yours to this value.

Use a shop with a Dynojet inertia type dyno with a wide band O2 sensor. Don't waste your money on a Mustang dyno. Get the digital test files (email, thumb drive... whatever) and download the WinPrep software from the Dynojet Web site. This will allow you to use different correction factors and plot the data in the best format.

Please start the pulls at 1000 RPM in top gear. You can test on the road if the engine will pull smoothly on the dyno by applying WOT from 1000 in third or fourth gear. If the setup is right it will pull without protest.

If you haven't read my tuning seminar, download it off the Web (search duke Williams san diego corvette) which will help you understand what you need to do. Also read my Tale of Two Camshafts article with emphasis on understanding how John McRae's engine was tested and what we learned.

You can simulate cruise conditions by starting a pull at about 1500 using light throttle to keep manifold vacuum above the power valve opening point, and take it up to about 4000. It may take a little practice and you will need to rig up a vacuum gage than can be read in the drivers seat. This can be very educational during road testing, too. It will be all over the place, and you can learn how to accelerate lightly to keep the power valve closed in normal road driving.

Proper preparation and planning your dyno session will go a long way in getting the most for the cost and you'll have a permanent record of the session and what you learned.

Duke

[John D.]

Re: Dumb question re 70 LT-1

The EPA sums it up quite nicely on the window sticker...your mileage will vary.

[Ed H.]

Re: Dumb question re 70 LT-1

When one lives in Massachusetts one must accept some limitations! ;-) But I have downloaded and read your material, and clearly you would be perfect if you lived next door, LOL.

The pistons are .001 oversize, but otherwise as per original 1970 specs, ordered from Chev in 1978. I can get you the Chev part number, but it's not right at my fingertips.
Measuring the gap between head and block, by the serial # pad, .035 is a go, .038 no go. I can check with Aubrey next week to see what he actually used as his paperwork doesn't note that.

Fred, at SMG Motoring uses a Dynojet. New website under construction, but you can google 'SMG motoring' and there's some stuff to look at. Got our signals crossed today because of the weather, so we're on for Friday.

The distributor is as delivered from Dave Fiedler, so I am pretty confident that it is as left GM in 1970. We'll see what Fred finds in that department.

Thank you, as always, for your thoughtful, knowledgeable guidance. Ed

PS: There is a house for sale on this street, at least I think it is still...

[Mark E.]

Re: Dumb question re 70 LT-1

"The distributor is as delivered from Dave Fiedler, so I am pretty confident that it is as left GM in 1970."

The point is the OE timing advance isn't optimal. For economy and performance, map it as suggested above.

[Ed H.]

Re: Dumb question re 70 LT-1

"The distributor is as delivered from Dave Fiedler, so I am pretty confident that it is as left GM in 1970."

The point is the OE timing advance isn't optimal. For economy and performance, map it as suggested above.

Yes, that is what I was saying, but a bit backwards, in other words, it's as it was, which is sub-optimal. "I know you think you Think you understand what you thought I said, but I'm not certain you realize what you heard is not what I meant." (Meant with humor, as was delivered to me when I didn't get someone's meeting.)

I am bringing the details of Duke's suggestions with me to SMG on Friday, and, hopefully, we'll be able to incorporate them as part of the work.

Thank you for ensuring I understood Duke's meaning.

[Ed H.]

Re: Dumb question re 70 LT-1

OK, time for the good, the bad, but no ugly...

My rejetting rates an 'A' (tho it still looks a little bit 'fat' below 2500). Idle mix, accelerator pump, and secondary engagement were all adjusted. I think the idle mix might be a little weak.

We discussed revising distributor advance, but suggestion was to see how the car behaves at this point. Given that the car seems
happy with readily available 93 octane (Note that compression effectively lowered via thicker head gasket.). Max advance is 32 degrees.

Comments? I confess to being surprised at the horsepower curve with a peak at 4500 rpm.

[Duke W.]

Re: Dumb question re 70 LT-1

So you got only one pull from 2500 to 4500?

The redline is 6500. It's not "fat" (rich) below 2500, it's lean, but that's just a transient effect. It's excessively rich above 3800, but this is at WOT. I guess you didn't do the cruise A/F ratio I suggested in addition to starting the pulls at too high revs and not revving it high enough.

32 degrees total WOT advance is well short of the optimum range of 36-40, which is one reason why it peaks so low.

Considering the less than optimum fuel and spark advance maps the numbers aren't bad, but I question whether it has a real LT-1 cam, which, if true, also contributes to the low rev power peak.

But since the chart doesn't list what, if any, correction factor was used, the data lacks full context. It may be there at the top of the chart, but not in the field of view. The defacto standard is SAE. If you obtain the digital data files (which are very small) and download the Winpep software from the Dynojet Web site, you can format the data in various forms including different correction factors, print the graphs and have a permanent digital and paper record.

As a point of reference a "327 LT-1" (started out as a '65 L-76) .030"-over with massaged OE heads, 10.5:1 true CR, and LT-1 cam made peak torque of about 270 @ 4500 (with 80 percent of peak at 2000 and 90 at 2500) and peak power of about 285 @ 6500 with a useable power bandwidth to 7200, which is just below incipient valve float with the common OE valve springs on a Dynojet with SAE correction. A similarly configured engine made about 330 peak torque at 4500 on a lab dyno and about 360 @ 6500 on a lab dyno with STP correction, and both engines easily pull from 1000 revs in top gear. Using my empirically developed conversion factors the data is well correlated.

What did the dyno guys have to say about the performance curves?

Duke

[Ed H.]

Re: Dumb question re 70 LT-1

As always, thank you! Yes, had my glasses on backwards, and being lean at low revs correlates much better to my observation that I think the idle mix is now a bit lean.

There were several runs on the dyno, but this is the only one he printed. Not being experienced in the world of dynos (I think the house in the neighborhood is sold, but I'm sure I can locate one nearby ;-) ), I really didn't ask questions at the time. I have, after looking at the printout, emailed him a bunch, and now I have more.

Not being the original owner, I don't know the car's entire history. I know the engine was apart early on, and I know that owner, so I can ask him re cam. I hope to see the engine builder Monday, and will ask him his opinions as well.