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Big block compression question

Old 11-11-2018, 04:41 PM
  #21  
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Originally Posted by Vette5311 View Post

781 & 049 are some of the best cast iron heads GM ever made. These are not and should not be confused with "peanut port" heads that came on truck engines. They respond extremely well to mild pocket porting, bigger valves, and gasket matching. A mild angle mill will reduce cc volumn and increase compression. The oval port intake volume make gobs of low rpm torque and are a great street combo with the correct roller cam. I have even run these heads on king street cars running in low 9s. An often overlooked head choice.





. Ronarndt was talking about his peanut engine. I mentioned they run good to about 5k. I have several sets of each. The peanut head has 119cc Chambers. Essentially identical Chambers to the 781 and other open Chambers. Get a piece of plexiglass. About 6 inches square. Drill a hole about 3/8 inch an inch from 1 side. Find something that is accurate to measure liquid. I like Walmart flavor shooters. 4 bucks for a 30cc syringe. Now grease the Plexi and set it on the head, chamber side. With plug and valves installed. Start filling it with the syringe 25cc at a time. Now you know the actual size of your Chambers.

Last edited by derekderek; 11-12-2018 at 01:26 AM.
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Old 11-11-2018, 09:11 PM
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Thanks guys. I can understand how hp and torque increases with compression but I am hung up on the term “sluggish”. This implies to me that two engines with similar hp and torque would respond differently when transitioning from part throttle to full throttle because of compression.
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Old 11-11-2018, 10:30 PM
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Higher compression usually has longer duration cam with more overlap. More overall power, but bigger cam comes on at higher RPMs. Lower compression usually comes with milder cam that makes all it's power at a lower RPM. But you take same 2 454's. Same heads. Same cams. Flat pistons in 1 at 8 to 1 and other with 13.5 domes and 9 to 1, you will get about 4% or 16-20 hp more. Little diff in feel or sound.

Last edited by derekderek; 11-12-2018 at 01:27 AM.
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Old 11-11-2018, 10:32 PM
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At the risk of seriously oversimplifying things; How much you can stuff in = cam+RPM.... How much you squeeze it= how violent the flame front is. My 2 cents
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Old 11-11-2018, 11:46 PM
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Originally Posted by 2mnyvets View Post
Thanks guys. I can understand how hp and torque increases with compression but I am hung up on the term “sluggish”. This implies to me that two engines with similar hp and torque would respond differently when transitioning from part throttle to full throttle because of compression.
I'm not sure how you couldn't see it? That extra power from compression is what creates the response. The piston accelerates quicker due to more cylinder pressure and comes back around for the next firing earlier. You can build cylinder pressure with a seriously advanced cam or a small one. Many mild engines have 175-200 psi cranking compression. But that small and or seriously advanced cam "runs out of breath" early and doesn't allow the engine to continue building TQ at higher RPM..which is where that HP number comes from.

Certainly there is a LOT to throttle response based on intake design and carb/EFI tuning...but given equal setup/tuning...the higher compression engine is going to make more power and accelerate quicker. Go liten to some You Tube videos of some 14.0 compression small blocks to hear what it can do.

Of course you can go further with acceleration improvements with lighter rotating components....crank, pistons, flywheel etc. Won't show much on the dyno..but I can guarantee you that when it's on the street or track it WILL accelerate those lighter parts quicker than heavier ones.

JIM
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Old 11-12-2018, 12:05 PM
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We are also talking apples and oranges here. Jim is used to 9-second ET's and 12 to 1 compression ratios. OP is wondering whether this is an eight or nine to one engine and whether or not it will make all that much difference.
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Old 11-12-2018, 01:26 PM
  #27  
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It's all relative. We're not talking exponential stuff here...but it's all part of the plan. A 10/1 street engine is going to run better overall than an 8/1 version. As I mentioned...if your street use is limited to 3000 RPM then you can use a small cam and build cylinder pressure and not worry about what happens after that. But if you want to make power a little higher and keep response good...the added compression helps. There's a reason every MFG increases compression when they want power, fuel mileage and drivability. When at the track.....you can tell a mega compression engine by sound, throttle response and MPH it turns in. The difference between 13.0 and 11.0 doesn't impact performance as much as 10/1 vs 8/1.

You can certainly have a strong running 8/1 engine. There's some full weight Chevelle guys running 12's with 454's that are well sorted out. If I was buying pistons I'd definitely kick compression up. If I wasn't, I'd do what I could to get it higher...mill heads, run sheet metal gaskets etc. Edelbrock makes some 100cc heads that work well with flat tops.

If E85 was available everywhere...my street toys would all have 13.5 compression!

Is it worth it? I guess only the O/P can decide.

JIM
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Old 11-12-2018, 04:58 PM
  #28  
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Originally Posted by 427Hotrod View Post
You can certainly have a strong running 8/1 engine. There's some full weight Chevelle guys running 12's with 454's that are well sorted out. If I was buying pistons I'd definitely kick compression up. If I wasn't, I'd do what I could to get it higher...mill heads, run sheet metal gaskets etc. Edelbrock makes some 100cc heads that work well with flat tops.
JIM
My flattops and Edelbrock heads work great.



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Old 11-13-2018, 10:12 AM
  #29  
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I don't think the OP wants to go and invest 1500 to $2,000 in heads for an engine that he just bought or is considering buying. If you're going to do that do Pistons instead for significantly less and stay with the 781 heads
that he has looks like they already have some significant work done to them. I believe he is mainly wondering if the engine will be okay and run like a hot rod motor as is. And my opinion is yes. Sure, 10 to 1 would be nicer. But 10 to 1 would be about $2,000 more.

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Old 11-13-2018, 01:17 PM
  #30  
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I built a 454 .030 with those same OC heads. Had to go with a 22cc dome to hit the compression and DCR we needed.

I use the jeepstroker.com compression calculator. Several others are out there. Measuring your actual chamber volume and deck height are a must.

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Old 11-13-2018, 04:18 PM
  #31  
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OP has some decent parts. Sticking a nice HR cam in there will help it work the best. Not having 10/1 won't make it a dog....but having it will help.

Best thing you can do is put in a cam to help the combo.

https://straubtechnologies.com/strau...-6400-camshaft
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Old 11-13-2018, 04:39 PM
  #32  
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To the op...

A basketball with low pressure has a dead bounce (low compression)

A basketball with high bounce has a zing to the bounce...it’s responds better (high compression)

Even within the normal range, 7-9 psi there’s a difference. (8.0-12.0 compression ratio).
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Old 11-15-2018, 08:40 AM
  #33  
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Originally Posted by 427Hotrod View Post
I'm not sure how you couldn't see it? That extra power from compression is what creates the response. The piston accelerates quicker due to more cylinder pressure and comes back around for the next firing earlier. You can build cylinder pressure with a seriously advanced cam or a small one. Many mild engines have 175-200 psi cranking compression. But that small and or seriously advanced cam "runs out of breath" early and doesn't allow the engine to continue building TQ at higher RPM..which is where that HP number comes from.

Certainly there is a LOT to throttle response based on intake design and carb/EFI tuning...but given equal setup/tuning...the higher compression engine is going to make more power and accelerate quicker. Go liten to some You Tube videos of some 14.0 compression small blocks to hear what it can do.

Of course you can go further with acceleration improvements with lighter rotating components....crank, pistons, flywheel etc. Won't show much on the dyno..but I can guarantee you that when it's on the street or track it WILL accelerate those lighter parts quicker than heavier ones.

JIM
I pretty much agree with everything you said, but I still don’t understand “sluggish’ or “lazy”. Unfortunately, I bought this car from a guy who abandoned his project, so I never drove it. It is a 73 rated at 395 ft-lbs torque. With that amount of torque, I can’t see this engine from the factory, being sluggish. I can understand how hp and torque increases with compression and I bumped compression to 9.5 with 22cc domes. Higher compression explains power and efficiency, but not “sluggish”. I am wondering whether sluggishness isn’t caused by the Dynamic Compression Ratio. I raised my compression to 9.5 and dropped the DCR to 7.6 with the cam ( Lunati 60211) I chose. I just went back and calculated the DCR for the factory cam, and it came out to be in the 7.6 range with the 8.25 stock compression ratio. (I guessed at some of the cam card data since I could find actual numbers). Thanks
Greg
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Old 11-15-2018, 10:39 AM
  #34  
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Sluggish/lazy are sometimes subjective terms aren't they? I think you're getting hung up on a term that has no measureable/quantifiable reference point. You can change gear ratios and make a huge difference in "sluggish/lazy" perception. What one person calls sluggish may scare another.

My car has had 365/377cc ports (different heads), huge cams, giant single plane intakes etc. To many...it would be the recipe for a "sluggish and less responsive" package...but I've let many folks drive it and not once has anyone described it as "lazy". In fact some have mentioned the throttle is "too touchy". Much of throttle response etc is tuning, but of course the whole combination comes into play. I've driven some smaller headed engines with similar cubes...and yes...they seemed to rev quicker...like a chainsaw....at lower RPM right off idle...but they ran out of steam as RPM climbed. If you keep it in that RPM range it will accelerate quicker than a "weaker combo"...but if you want more power and operate in a higher RPM range...you need to be concerned about how "sluggish" it is in that area. My car goes through 4K-5K RPM in about a nano-second...so it really doesn't mean much at WOT...and it drives clean enough at lower RPM that people think it's "touchy"...so it works for me.

DCR is a real thing...it's what all of this is about. That is cylinder pressure before spark......which directly relates to everything we've been talking about concerning fuel mixture burn which creates cylinder pressure AFTER the spark. One thing to remember about all those DCR calculators is they can be accurate for comparing cams/static compression ratio....there can be a HUGE difference in what the cylinder actually experiences. I have to laugh when people lock in on a particular DCR as being pump gas friendly when they have no way of modeling/measuring what the cylinder actually sees under WOT operation. You can drive around with 14.0 compression on pump gas as long as you keep throttle opening minimal...which keeps the cylinders from filling completely. I always point to the old NASCAR Busch series engines that had 9.0 compression and cams in the [email protected] range. If you calculated DCR it would look very weak and definitely pump gas friendly...but once you opened the throttle WOT and those incredible heads/intake tract came into their own...you had better have really good race gas in it because the cylinder pressure was going to be out of sight...even though it had only 9.0 compression. Same thing with the tiny little 390 CFM carbs they used....throw a 750 CFM carb on it and cylinder filling got even better and completely changed things.

Your example is exactly what I've been saying. Yes...your calculated DCR is similar with a larger cam and more compression as it was with a small cam and less compression. If you kicked up compression with the smaller cam you might increase cylinder pressure to the point of having detonation etc. But your new combo should be capable of maintaining similar low speed response yet have the increased potential of better cylinder filling at higher RPM...which increases HP up there.

JIM

Last edited by 427Hotrod; 11-15-2018 at 10:39 AM.
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Old 11-15-2018, 09:21 PM
  #35  
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[QUOTE=427Hotrod;1598339544]Sluggish/lazy are sometimes subjective terms aren't they? I think you're getting hung up on a term that has no measureable/quantifiable reference point. You can change gear ratios and make a huge difference in "sluggish/lazy" perception. What one person calls sluggish may scare another.

Thanks Jim. I think I was coming to the conclusion you suggested. Sluggishness is subjective. I kept hearing that term being thrown around and was looking for a way to calculate or quantify it and there may not be a way. .
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Old 11-15-2018, 11:50 PM
  #36  
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You can measure it...but it's all relative to what you're expecting I suppose. I can take two engines and put them on a dyno and measure acceleration between RPM points and tell you which one accelerates the quickest in that range. That's great info to develop an engine. But again...that's at WOT usually...and that might not be the way people are thinking about it. In fact....I would bet many are discussing part throttle acceleration and how it responds.

One example that stood out to me was when a good friend pulled his tunnel ram 491" engine with EFI and N20 out of his car and installed an LS with a single turbo. I knew it ran high 9's on nitrous with the BBC,,,,and it ran low 9's with the turbo'd LS. He even placed in the top couple of places in Drag Week with it over a few years. I kept asking him how he really liked it and after avoiding the question for a while he finally told me he hated it! I asked how that could be since it was so fast? He said on the street it was basically a low compression small block hauling around a 3800 lb car. Until he stuffed his foot all the way in it and it started making a lot of boost...he felt like it was a slug. He missed the instant TQ of the higher compression BBC. Of course it eventually came out and was replaced with a higher compression 582" BBC with a tunnel ram, EFI and a lot of N20. Now it runs 8.50's and is NOT sluggish on the street!

It's an extreme example..but illustrates that we have to define the parameters and then we can determine what needs to be tweaked to make an engine respond like you want it to. It's all about what YOU want. Some folks will never get over 4000-5000 RPM..so you concentrate the power below that. The OEM's have done incredible work to make engines build ridiculous TQ off idle and below 3000 RPM. They use compression, great flowing heads, relatively small but fairly aggressive cams, very long runner intakes (just like a tunnel ram lying on its side), variable runners, multi ports, cam timing that moves with RPM and load, total EFI fuel management and timing etc etc....and all of that adjustment lets you move it around to make it responsive at any RPM.

JIM
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