Greetings peoples. Back after a very long absence. On the table today is the notion of building a very mild 350 SBC with no more than 8-1 compression for use with very low quality gasoline. What would be the best approach in general and specifically what might be done to minimize the performance hit such an engine would take from a CR that low? The primary targets are a very broad torque band from just off idle, a bulletproof level of reliability, as as stated the ability to tolerate wretched fuel quality. Thanks for any input.
a so called "rv" or towing cam, such as an Isky262 super cam, or even smaller.Small Cfm Carburetor or small Primary Venturis, such as a Q-jet, 390 or 450 Holley, or even an old Economaster Holley, will all keep low end performance somewhat acceptable. Traditional hot rod tricks, such as headers, intake manifold , air cleaner and distributor curve all still apply, and can boost performance. roller rockers, and camshaft components, and a windage tray or crankshaft s****er, and a good double roller timing chain, as well as a stock pressure/volume oil pump can all free up a little extra horsepowe.
I am somewhat worried that the low compression ratio will make torque production fall off a cliff regardless of what other measures are used to lower engine friction. Small carbs, maybe even a 2-barrel or a Q-Jet will doubtless help in keeping mixture speed into the engine as high a possible at low speeds. Few people have much experience with a compression ratio that low these days and if such a combo is built it will be rather on the experimental side. The projected use is vintage rallying which includes a great deal of very low speed off road work in sand and mud as well as higher speeds on roads and dirt roads. Plus the goal for such engines is absolute reliability since international vintage rallys can traverse areas thousands of miles from decent repair facilities. ****py third world gasoline means CRs higher than 8-1 are a big no-no.
I have an o/t street/strip car with a really low compression 355, built specifically to spray a 250 shot plate system on 87 octane pump gas. It has a custom .650" lift 251/261 solid roller with 114 lca. The thing only cranks 90psi, but gets a little over 20mpg with 3.73 gears and a 4spd manual, no overdrive. Part of that is due to using really low tension rings and 5W-20 synthetic oil. It was a real oil burner until i put a vacuum in the crankcase, now i use a sealed pcv system to draw 10"Hg in the pan without a vac pump. I have an electric vac pump on board, but it only comes on when i spray. Timing is locked out with no curve at all, but it does have a retard box that is triggered by the spray. 625 Carter carb, tuned with an onboard wideband logging o2. I wanted a fast car that i could drive anywhere, while getting decent mileage burning cheap gas that i can buy at any gas station. It's enough to get 1.30 60' and run in the 5's in the 1/8mi. I'm more than pleased with the results, and it gets better mileage than my Astrovan.
look at how a 70's pontiac cam is set up , and also use small ports on the heads to keep the velocitys on the a/f mixture up . Pontiacs were known to have7.0- 7.5 compression in the mid to late 70's you want a valve that has a angle to open as much area up quickly at low lifts so the standard 3 way valve job will not work , you have to use a shallow angle to get the surface area to open quicker , and a 4 bbl carb is ok . as once its running the dynamic compression ratio can increase due to flow . its starting the motor that will be a pain in the *** , once the fire is lit it will go .
Flat tappet or Roller cam? If you can run a roller a basic Vortec 350 with low compression pistons and a low speed torque cam would probably be dead nuts reliable. Upgrade the usual things like the timing set for reliability and you have it. You would have to run an electric pump but that isn't a big deal. Some of the local 4x4 guys have V8 Chev powered Jeeps and other Crawlers that can creep along at speeds so low you can barely measure it with tons of torque but I haven't kept up with what they have in the engines. The info's out there and you can either build it spendy or build it on a budget with over the counter parts.
A while back Hot Rod did an artcle " The 350 Chevrolet should have built" It had long rods from a straight 6 Ford ( 6.125" )? Fairly high compression that would run on **** gas and made decent power. I don't remember which issue.
Isky has cams that will work with low compression. I had a Isky 280H Hydraulic in a 350 once, with 8 to 1 compression. Had a good Dairy Queen idle, and ran like crazy.
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