Has anyone ever measured the compression ratio on the factory builds? I know the specs say 9:1 and 10.25:1 but has anyone confirmed this?
My '68 "10.25:1" 4v 350 came out to around 9.67, so that sounds about right. It also proves the power ratings were bogus, which we already knew. It also shows that the factory compression height on the pistons were higher than the aftermarket (cast) ones today. That engine would run all day long on 89 leaded regular gasoline of the time. I'd advance the timing and put 93 Aamoco Ultimate in it and it ran like a raped ape (bone stock). Gary
So,. if these figure's are right, that means what I thought was 9.2-1 comp in my sons 70' buick,.. top end build with .030-.040 milled ported heads and steel shim head gasket,.no decking,. stock sbb 350 short block,. So it may only be 8.5-1 or 8.9-1 compression using a t/a 310 cam,.. we should'nt be going as fast as we are then... thats crazy.. 13:0's at 103.+
Actually I think(hope) that these engine wake up tremendously after you get close to actual 9.0 an up. Depending on cam of course. Considering most are closer to 7.7.
Hey Jerrold, how you been? Going to try and get Mart to go to Milan this year and meet you out there with one of your cars. That just goes to show how much time you were wasting on those 455s when if you built a 350 from scratch with 10:1 C/R you could go as fast as a 455. And you want to swap out the 350 and go to a 455 in the '70 post car when all you would have to do is just make the bottom end better. LOL Did you see the wheel stand with the super stock 9:1 C/R sbb 350 car doing the testing on the new S/P intake? Faster than your 455 car, it did 11.17 short shifting to second with the wheels still in the air and still did that time! Just think when the TA aluminum heads come out the 455 will be obsolete because the sbb 350 can handle making more power before the block breaks than the 455 can. Hope to see you at Milan this year, get those cars ready. Derek
Hey Derek,. everything's well,.. sorry just got home,.. yea, let me know when you guys want to go and we'll try to be there... These little motors are a lot fun,. I did see the pics of the car running 11:17, just crazy... I never thought they would see that kind of horsepower... You have to remember,. I don't have that much money tide up in my 455 with cast iron heads... and old speed pro pistons...9 years on that motor, street driving it and still throwing the numbers down at the track... lol My sons car will flat out lay rubber with the sticky hoosier's on at a 25 mph roll and pull hard too 6000 shifts,... that's crazy with a 350 stock short blk... I smile every time.. Have a good one Derek,.
Even though the DCR may be a bit lower than what would be ideal for a mild/stock engine, with the overlap of that cam and the stall/gearing you're using, with freer flowing heads/headers and scavenging etc. creates more cylinder pressure from better VE at higher RPMs which increases your 'effective' compression. It's not ALL about the DCR, though that's more significant than the SCR. What ultimately matters is the 'ECR', or 'effective compression ratio', which is a combination of SCR, DCR, and VE @RPM. Figuring out the octane requirements for a mild/stock engine with 'normal' VE and a particular DCR is less complicated. The dynamics of airflow come more into play when dealing with engines that have better VE, high overlap cams, and scavenging. There's a formula for calculating the effective compression I found online, but I don't think it's accurate because it only fits into a narrow spectrum of VE. Once you venture outside of that, the numbers are way off. There's probably a formula for figuring it out already no doubt. If anyone has such a formula, it would be great if they'd post it here on these forums, unless of course it's some sort of 'trade secret' or some such among competing racers trying to perfect their engines. There's much to be said for tuning skills. You can transform an engine from 'ok' to 'awesome' as you well know. Sometimes you have to toss out the calculators and numbers and just go by feel, then record your results and go back to the calculators and try to figure it out. lol Lots of dynamics involved. Gary
The reason for this is the DCR (and effective compression) which reaches a 'sweet spot' with the particular grade of fuel you're using. You can get a low comp engine to run well too, it just won't be AS powerful. Parts matchup and tuning are crucial. The reason you hear others (quite often) talk about the 'magic 9' where things begin to happen is because it corresponds to the better DCR that any particular cam will have when matched up to this 9+:1 SCR. This is when overlap and scavenging start to become more prevalent and thus the 'switch' is turned on after a certain RPM is reached. You can get a similar effect from lower compression, it just takes more VE and scavenging to achieve it. Most often the reason low comp/big cammed engines are dogs is because the VE isn't much better (if any) than stock and the DCR cannot compensate without the extra airflow, particularly at lower RPMs... Crappy VE=more compression required. Excellent VE=less compression required. If you bump compression with excellent VE, you need higher octane than you can get at the pumps... Gary
