Hello All.
What's the combination for low RPM torque, high compression and some kind of economy? I've been saddled with a previous rebuild that's around 9:1 with a 7.9 DCR. That's hoping the intake closing is @50 degrees. Stock cam.

Thanks
-andrew

Short answer is that high compression is great for low RPM torque AND mileage. New cars run much higher compression for best MPG and use engine management to make it all work.

The key is getting timing curve and/or fuel type so you don't ping. You can bleed off some compression with a slightly larger cam if you want to run 87 octane, although it might work with what you have if the timing curve is just right.

Personally, I would not build a NA motor with less than 9:1 static comp. My experience with mileage is that you can go pretty wild with cam, head, compression and such and still get similar-to-stock mileage. Again, key is mixture and advance tuning, and having a good car setup.

Lou

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Home of Slant6-powered fun machines since 1988

If I had more time, brains, and money I would develop an ignition box that controls spark much like a modern car's EFI system...but with no fuel control. This box could be used on carb'd cars and allow the spark to be mapped based on RPM, manifold pressure, throttle position, etc.

You could do all of that with an EFI PCM...but by that point you might as well just go to EFI. But it would be nice to have a $400 box that could do it just for ignition.

Hi Greg,

The MSD 6530 does this pretty well. I have been managing spark on my boosted 68 Dart (with MS 1st gen EFI) for about 9 yrs). It's now $530 with the external MAP sensor (I paid $400 back when). The only downside is the range of timing advance/retard is a bit narrower than you might hope for max advance at part-throttle cruise. I run a lean burn (locked) dist.

Lou

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Home of Slant6-powered fun machines since 1988

Back in the '70s one of the cam companies did a mileage kit for sb Chevies that had 12:1 pistons and a cam to work with the high compression. I don't remember them being very successful.

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Official Cookie and Mater Tormentor.

On NA, high compression is good in every combination, low speed torque goals does not make for a exception so there is no "vs" really to be considered. Actually high compression at low speeds can be a problem, because cylinder filling is more efficient, and can lead to knock, etc because of high pressure for the octane in use. Higher speed leads to less time for cylinder filling, and lower effective pre ignition cyl pressure.

I think this should be stated HIGHER compression for low speed torque. As in, increasing compression to the stock specs from the actual as delivered compression would indeed enhance low speed torque. I cut the head on my old stock engine to get to about 8.8:1 CR. Low speed torque was indeed enhanced.

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Thanks Lou, I'll look into that MSD.


High compression is great for power everywhere...if you can support it with good gas.

I remember my dad, back in the 80's, talking about how my uncle had an early 60's Cadillac that had high compression and how smooth it ran...except he had a hard time finding gas for it. I guess someday I might look up just what kind of CR Caddies ran....maybe it was an urban myth lol

Thanks for the thoughts guys. Trying to figure the intake closing point for a stock type cam. I'm seeing 25* ABDC plus 15*, does the valve lash add anymore?

In the early 60s high compression on the big cars was 9.5 to 10.5 to 1. Low compression was in the 7's on a lot of cars.

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What is the Intake valve closing point at lash? (.010 for a factory solid lifter cam)
FYI: I use the actual; Intake Close at lash for solid lifter cams and the.050 lifter rise + 15* method when checking / degreeing hydraulic cams.
DD

and while I cant say how accurate the "advertised" CR was back then but as we got into the 70s and the later 70s it is unusual to find an as built by the factory engine, that came off the line with the CR it was supposed to due to tolerance stack up and the desire to rush em on down the line. so to overhaul an engine and "blueprint" it to even what it "was supposed to be" can often give you a noticeable boost let alone making improvements for "your purposes" from there.... a 1/4 mile screamer or a tow tug or whatever.... IF you know how to match compnents.... which I never claimed to be an expert at.... I've built a few engines that were "lotsa noise" but the same or less than what it originally was..... usually I got into trouble with the cam choice.

Hi GregCon and all,

I had forgotten about the "Microsquirt" controller for ignition control. This is a highly flexible controller with software that is <$400 all in (with cables and onboard MAP). You can control your ignition curves very nicely with this. I believe this is much more versatile than the MSD 6530 I mentioned and can run many types of ignitions.

https://www.diyautotune.com/shop/megasq ... crosquirt/

Hope this is useful,

Lou

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Home of Slant6-powered fun machines since 1988