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| How much to cut? https://slantsix.org/forum/viewtopic.php?t=58650 |
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| Author: | vntned [ Tue Nov 03, 2015 10:45 am ] |
| Post subject: | How much to cut? |
Needing to freshen my stock 170 up a bit. Maybe add a little power without sacrificing economy much. Gonna pull the head and have the valves done. How much can I cut safely off of a 170? I know you can cut 0.100" off of a 225, does that apply on the short deck 170 too? Also planning on going to a SuperSix 2bbl setup and maybe a mild cam. Any suggestions on what bump stick to use? The car is a 4spd OD with 3.55s and a 24" tall tire, if that matters.[/list] |
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| Author: | Joshie225 [ Tue Nov 03, 2015 12:07 pm ] |
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If you take .100" off a head on a 170 that's already at 8:1 (it's almost certainly around here or lower) then you get 10.5:1. You are also getting the valves pretty close to the piston. The 170 has different lobe separation requirements than a 225. A 225 with stock size valves needs about 105° LSA for optimum torque across the power band, the 170, 112°. If you go with oversize valves the LSA decreases about 2° in both cases. In my opinion, most performance build 225s have too wide a lobe separation and too much duration, but many are crutched with advanced cam timing. This works okay as the intake valve closing point has a larger effect on how the engine runs than any other valve event such as exhaust opening/closing. Read this: http://www.hotrod.com/how-to/engine/060 ... ft-basics/ If you want a seriously considered cam recommendation you need to tell us what fuel you want to use, the true compression ratio and how much time you're willing to invest in tuning. The condition of the short block is important too as worn cylinders and rings will pass oil and induce ping. |
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| Author: | Joshie225 [ Tue Nov 03, 2015 12:48 pm ] |
| Post subject: | |
After some thinking and figuring I would build the 170 to 9:1 with a cam 248° intake duration, 240° exhaust duration, 112° lobe separation and as much lift as I could get. 1.7:1 ratio rockers on a stock camshaft would be great for a budget build, but they don't exist. A mild roller cam would be ideal, but that's not a budget-friendly choice. Oregon Cam Grinding lobe #819 on the intake and #818 on the exhaust is about the best you can do with a flat solid lifter. |
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| Author: | Charrlie_S [ Wed Nov 04, 2015 3:09 am ] |
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If you are talking dur at .050 being in the 240 range, and a lot of lift, the pistons will need to be notched. If you are talking adv dur in the 240 range, you might be ok, but I would still check. Ballpark with .100 off the head is around .430-.440 lift, at mid 260 adv dur without valve notches on a 170. Of course this varies with the actual cam timing. |
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| Author: | ProCycle [ Wed Nov 04, 2015 6:32 am ] |
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Before you decide how much to take off the head you need to know what head gasket you are going to run. The original steel 'shim' gasket was only .020" thick. Most current aftermarket gaskets are .040" - .050" thick. You'd have to take off the difference just to end up with the CR you started with. |
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| Author: | vntned [ Wed Nov 04, 2015 5:43 pm ] |
| Post subject: | |
First off, I don't mind running premium fuel, (93octane). Second, the car is a DD and is intended to be driven all the time. I don't wanna wreck my economy, as I currently average 25+mpg. However, I'd like a little more power and some hotrod flare. So a bit more compression and a mild cam to go with a little more carb and I should be good. So using a common Felpro head gasket and cutting 0.100" off the head will be equivalent to cutting 0.070"-0.080" off. And that could be border line excessive? So what about cutting 0.080" off, plus the thick head gasket should put me in the 0.050"-0.060" off the head. Would that put me close to the desired 9.0:1scr? And as far as cams go, is there an of the shelf grind that would work for me? |
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| Author: | Joshie225 [ Wed Nov 04, 2015 6:39 pm ] |
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You can't just cut the head. You need to measure and hit the right ratio for the cam you choose. It's unlikely an off the shelf cam will get within 10hp of a custom cam which costs you zero extra dollars. |
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| Author: | vntned [ Mon Nov 09, 2015 4:14 pm ] |
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So I'm not trying to reinvent the wheel here. Simply looking for how much is safe to cut off the head?! Yes I'd be using an off the shelf FelPro head gasket. Yes I'm ok running premium fuel. Yes it has an electronic ignition. No I'm not trying to build a performance motor. Forget the cam upgrade and let's say it's a stock 170ci with a SuperSix 2bbl. Would it be safe to cut .090" off? 0.080"? 0.070"? 0.060"? Etc.. |
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| Author: | ProCycle [ Mon Nov 09, 2015 4:32 pm ] |
| Post subject: | |
In order to answer the question about what will be safe and what would be too much you need to calculate the dynamic compression ratio. There is no reliable 'rule of thumb' but there are plenty of DCR calculators available on the web. If you upgrade to a cam with more performance oriented valve timing you can take off more and reap the benefits. If you leave the stock cam in it you can't go nearly as far without danger of detonation. I know it seems like you are asking a simple question but it is not the thing that anybody would file away as standard knowledge. There are too many variables and most folks would not be trying to boost compression without also upgrading the cam. If I had to guess, I'd say you can't safely take very much off. Certainly .020 or .025 to make up for the difference in gasket thickness then maybe another .020 more? I wouldn't go any further than that without taking 10 minutes to crunch the numbers. |
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| Author: | Charrlie_S [ Tue Nov 10, 2015 4:12 am ] |
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As others have stated, it is best to measure and calculate. BUT as a wild ass guess, you should be fine with .050 off the head, the FelPro gasket, and a stock cam. |
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