how much compression

I am starting to peice together a motor for my 67 rs/ss and have a few questions. i am planning on building a 355 with a completely forged bottom end. i am looking at AFR 195 or 210 aluminum heads. my question concerns comp. on pump gas (93 octane, maybe cam 2 before playing at the track). with the aluminum i have read a higher comp. can be used but how much is too much. this will be a wekend toy not a daily driver.

I'd shoot for 9.5.1 give ot take a little if it were mine.

i was actually shootin fo 10-10.5 but wanted to know if11 was too high?

Everything that was mentioned above.

The maximum amount of compression allowed can change depending on a few factors.

Octane rating is the number one factor. Don't expect a 10:1 engine to run without pinging on 87 octane while 91-93 will be a better choice.

Altitude. A 10:1 engine probably won't ping on 89 octane in Denver. 87 it might.

Piston/head design. A dome piston is the best way to increase compression ratio however it also gives the flame front a poor route to travel over the piston. A flat top piston will give the best flame front which will reduce detonation but to get a high compression ratio, a small combustion chamber is needed which will shroud the valves.

Aluminum heads because they can disipate heat quicker, can get away with a higher compression ratio compared to an identical engine with iron heads.

Aluminum heads and 93 octane should survive in the 10-11:1 range without detonation.

im not planning on running 87 octane. i dont run it in my stock 87 iroc, i only run 93. this will only be run on 93/94 octane and occasionally cam 2, (102 octane i think) whats the difference between static and dynamic compression ratio. im new to this field. 1st build up and what not.

I'll take a quick shot.

Static compression is the volume ratio between when the piston is at BDC and TDC. With 10:1 the total volume when the piston is at the bottom is 10 times what it is at the top.

Dynamic compression is the actual pressure inside the cylinder when at TDC. This is affected by things like cam profiles, valve sealing, piston ring sealing etc. Typical good engines should have around 150 psi on a compression test. Leakdown tests are used to determine where a cylinder is leaking by forcing air into the cylinder through the spark plug hole and listening for leaks.

Dynamic and static pressures have nothing in common with each other.

Very well said.

thanks for the replys. i have a better understanding of what the two are now. still a little foggy but much better then when i posted

Originally posted by Stephen 87 IROC and Trey

Static compression is the volume ratio between when the piston is at BDC and TDC. With 10:1 the total volume when the piston is at the bottom is 10 times what it is at the top.

Dynamic Compression is the compression ratio after the intake valve closes.

Typical good engines should have around 150 psi on a compression test. Leakdown tests are used to determine where a cylinder is leaking by forcing air into the cylinder through the spark plug hole and listening for leaks.


DCR calculators can sometimes be off so don't worry about the math cause rod and stroke lenth have some to do with it along with cam profiles and lash, but the DCR is more appropriately stated as the actual amount of volume that is compressed after the intake valve closes. It's as simple as that.

My suggestion is to talk to someone experienced with this and make sure the person you are getting your camshaft from dam well knows what's going on there. The compression ratio choice (both DCR and SCR)of a motor should be discussed with the engine builder or camshaft designed/provider to eak the very last little bit out of the combination and make sure it keeps on living.

Well with high DCR's and good ring seal you will see well over 200psi cranking compression. So it's not just a factor of the cranking compression but that is a good way to explain it to older gear heads.

Obviously the DCR (meaning the camshaft) is going to be a big part of compression vs. the SCR which is just a ratio of volumes, how much depends on a lot of things.

The most important thing is paying attention to details. Just like a high SCR motor could have less detonation than a low SCR motor a high DCR motor could be the same way. The cam does make a big difference in the motor beign detonation prone or not. Fighting off detonation has a lot to do with DCR and Octane, but still things like temps, quench area, chamber shapes, burn rate, plug location, piston dish/dome shape, coatings, A/F Ratio and the TIMING all play as much of a part in it.

I've seen and built motors that run high 8's to low 9's DCR and still run on pump gas, you have to watch things very closely to make sure the motor is not going to eat itself apart.

If the motor is built right you will not need to go out and get CAM2 just to run WOT at the track, the better gas could make more power due to higher oxygen content or chemistry of the fuel but if it's built and tuned right then it can run WOT or at part throttle on 93 octane.

When you get into motors that could have detonation problems it's best to make sure that you pay attention to that in parts selection also, detonation is not good for pistons, pins and rods. So that should be looked after to make sure the motor still wants to live a long time.

With all that being said, you can run SCR into the 13's and not have detonation issues on pump gas.

Bret

With all that being said, you can run SCR into the 13's and not have detonation issues on pump gas.

Bret

Bret,

Old post, but I'm curious about this statement. Do you still agree with this?

Using my own engine specs with a modified 13.0:1 SCR, it would take an IVC event right at 79.5 to get it down to 9.0:1 DCR. As you already know, so far, the cam software gods are offering a ~50º IVC event as the "right" cam for me :no:.

Ryan

I'm not as technically gifted as many on here and other places, so when I built my 388 I tried to use things I've learned.

I used AFR 210 aluminum heads since aluminum disapates heat so well, engine was built with 12.0:1 slugs but after cc'ing the chambers we found them to actually be 69 cc which lowered it a little, block was not decked, I alled called cam company and told them I wanted to run pump gas and was given a cam with actual duration in the 252/262 duration on a 106LSA. actual static compression came in at 11.3:1 and I was shoointg for 11:1 so we got close, I never performed a dynamic compression test to see where cranking psi was at, but when I pulledit apart pistons still looked fresh out of the box after a touch over 1800 passes and 5-6K street miles, same with bearings.

In a nutshell it runs on 92 octane all day long long-literally and the lower octane actually produced better mph than 110, but the 110 smells soo much better in the lanes when you warming everything up:)

Josh,

While I agree with you that real world testing is the final say, I'd like to find something that puts my DCR in the ballpark of 9:1. after calling two "professional" cam companies and giving them my combo I was told a cam with a ~50º intake closing point would be my best bet. That puts the DCR just shy of 11:1. That is not in the ballpark, IMO.

Ryan

Bret,

Old post, but I'm curious about this statement. Do you still agree with this?

Using my own engine specs with a modified 13.0:1 SCR, it would take an IVC event right at 79.5 to get it down to 9.0:1 DCR. As you already know, so far, the cam software gods are offering a ~50º IVC event as the "right" cam for me :no:.

Ryan
Chances are your cam software is looking at what duration would be best and using a icl of 110 to achieve that if you really want to drop it down find a cam profile with a different icl with similar duration or better yet, rather than using software, talk to a cam designer before trying to use the software.

13:1 SCR should have a IVC of at LEAST 80°, maybe even closer to 90°.

Remember this is at .000" (advertised) duration, not .050"

I was just working on a 14:1 motor design (not pump gas) and it will be mid 8's for DCR. Even with that DCR below the magic 9.0:1 it will still be iffy on 116octane.

Part of this is the VE%, if you have a very efficient motor then you can have problems with much much lower DCR than 9.0:1.

IHI that AFR headed motor only had a DCR in the 7.1-7.4 range, which isin't pushing it too much.

Bret

IHI that AFR headed motor only had a DCR in the 7.1-7.4 range, which isin't pushing it too much.Bret

Probably why the stock bottom end lasted soo long:cool: Oh well, next one will be a little "wilder". I gave up on mathmatics in racing (at our measily bracket level anyways) since after having a motor professionally built for the truck and told this wont work and that wont work, slide rule this and slide rule that. We ran it, and I changed a few thinigs over (have one last mod to make which will net the best) with the gearing, best part is everything I suggested to the engine builder and dyno guys that was shot down while it was strapped to the stand, once I got it in the real world and made the changes I wanted it actually made us faster/quicker.

I do respect and enjoy the numbers guys that can plan from paper and execute to reality and make some crazy hp with nothing real exotic-just optimized every last thing. Around here that goes with a minimum $30K price tag so that's outta my budget;)

If you are getting knock at high RPM, you put too much compression in it. I've used aftermarket systems on the dyno to verify knock. Usually it's only at low RPM in well done setups around 100-110% VE max. Once you crank heat into the cooling system and intake air temps the knock will definately increase.

Bret

what is the effect of gapless rings into this equation? it would seem if you are on the ragged edge of detonation, it might not be a good thing to increase the sealing ability of the engine, correct?

Chances are your cam software is looking at what duration would be best and using a icl of 110 to achieve that if you really want to drop it down find a cam profile with a different icl with similar duration or better yet, rather than using software, talk to a cam designer before trying to use the software.

Someone is helping with this as we speak. I was just trying to get a better idea.

Interesting thoughts in this thread.

Ryan

Someone is helping with this as we speak. I was just trying to get a better idea.

Interesting thoughts in this thread.

Ryan

What software did you use to help you pick out the cam specs?

What software did you use to help you pick out the cam specs?

I have no idea. I've been using a DCR calculator I picked up off of this site to play with the DCR and independent variables.

Ryan

Probably why the stock bottom end lasted soo long:cool: Oh well, next one will be a little "wilder". I gave up on mathmatics in racing (at our measily bracket level anyways) since after having a motor professionally built for the truck and told this wont work and that wont work, slide rule this and slide rule that.

Everyone has a tendency to mention that with equations and computer sims there’s a case of garbage in/garbage out, but no one seems to think about that just as important is what is done with/how the output is interpreted.

I know there are truely gifted folks out there that can acutally sim something on paper/computer and have the real product turn out exactly like they predicted, but these types are few and far between and/or really expsensive to obtain this great information from. I dont have a racing budget to afford these types so I just try to make educated guesses myself. Sure I'm leaving alot on the table but it's affordable and dependable which is what bracket racing is supposed to be, if I wanted max output obviously choices would be alot different.

The truck we had built was done using all these equations and slide rules and to put it lightly, the truck is about as mismatched and slow as can be for what it should be IMO. Things they said in the dyno room were debunked on the track. Things I've read about over the years seem to be more ice cream cruiser related and the track has proven these things worng as well.

Totally agree garbage in garbage out, 2+2 does'nt equal 3 or 5 on any calculator I've ever used either LOL;)

Does anyone know the DCR of the stock motors in LT1's? I don't think I've ever seen the complete cam specs anywhere? :confused: