[Vettezuki]

I'm having a harder time wrapping my head around the relationship of compression to the other parameters. I totally get the relationship of intake to head flow, cam and exhaust and that it doesn't make any sense to have huge intake and restricted exhaust for example. That's all related to air flow. But compression (except for the effects on cylinder filling and scavenging I guess) seems somehow independent of the air flow issues of intake through exhaust. Basically I was under the impression all things equal in an engine that is breathing adequately, more compression equals more cylinder pressure equals more torque equals more power. Where am I going wrong? What physically causes the diminishing return?

[94cobra69ss396]

Originally Posted by Vettezuki

I'm having a harder time wrapping my head around the relationship of compression to the other parameters. I totally get the relationship of intake to head flow, cam and exhaust and that it doesn't make any sense to have huge intake and restricted exhaust for example. That's all related to air flow. But compression (except for the effects on cylinder filling and scavenging I guess) seems somehow independent of the air flow issues of intake through exhaust. Basically I was under the impression all things equal in an engine that is breathing adequately, more compression equals more cylinder pressure equals more torque equals more power. Where am I going wrong? What physically causes the diminishing return?

You are correct. I'm not saying you won't gain hp and tq by adding compression. As you know cam timing plays part in not only airflow but also cranking compression. Because of the low static compression ratios that are used manufactuers use cams with wider lobe seperations (115-120) to build cylinder pressures at lower rpms. Now if you leave that same cam in the engine but add a lot more compression cylinder pressures get too high and you end up with an engine that will detonate at low rpms.

What I'm trying to say is that there gets to be a point where adding more compression just isn't worth it. I think that if you are going to keep everyting the same as you have now and just add more compression I would stick with around 12 - 12.5:1 and no more on E85.

[Vettezuki]

Originally Posted by 94cobra69ss396

You are correct. I'm not saying you won't gain hp and tq by adding compression. As you know cam timing plays part in not only airflow but also cranking compression. Because of the low static compression ratios that are used manufactuers use cams with wider lobe seperations (115-120) to build cylinder pressures at lower rpms. Now if you leave that same cam in the engine but add a lot more compression cylinder pressures get too high and you end up with an engine that will detonate at low rpms.

What I'm trying to say is that there gets to be a point where adding more compression just isn't worth it. I think that if you are going to keep everyting the same as you have now and just add more compression I would stick with around 12 - 12.5:1 and no more on E85.

My cam is 224/224 .566 lift, 114LSA. This is about the limit of cam size and maintaining emissions compliance, but maybe a custom grind just a tiny bit bigger could fly. Of course I don't know what effects E85 (technically an illegal swap) would have on measured emissions. Carbons should be lower but NOX probably higher as I understand it. Anyway, for sake of argument let's say the emissions is not a problem. What about compression for my cam or a tiny bit bigger?

Thanks for pointing out the cam timing cranking pressure issue. I didn't plug that into the equation.

[BADDASSC6]

Originally Posted by Vettezuki

Is there any reason port injection vs. carburation would make a difference?

Port injection is better. The injectors allow for more accurate and even deliver of fuel to each individual cylinder. Carburators will allow you to set your air/fuel ratio, but not for the entire rev band.

I met a guy at the 1/8mile drag at barona that LOVED E85! He stated that he didn't pick that much up top end, but was able to make a bunch of mid range power by advancing the timing.

[Vettezuki]

Originally Posted by BADDASSC6

I met a guy at the 1/8mile drag at barona that LOVED E85! He stated that he didn't pick that much up top end, but was able to make a bunch of mid range power by advancing the timing.

That's what it looks like on the C6R too. The difference in torque and power under the curve for the E85 vs. Gas setup is shocking. But what all they're doing in addition to the fuel and obviously timing, I don't know.

[TimAT]

FWIW: Shafiroff has a SB and a BB that are setup for E-85. Link to the BB:

http://www.shafiroff.com/sportsman/540_825_e85.asp

Looks like he's running that one at 14:1. I just did a quick look maybe his cam specs are there too.

[Vettezuki]

Originally Posted by TimAT

FWIW: Shafiroff has a SB and a BB that are setup for E-85. Link to the BB:

http://www.shafiroff.com/sportsman/540_825_e85.asp

Looks like he's running that one at 14:1. I just did a quick look maybe his cam specs are there too.

Thanks Tim.

[94cobra69ss396]

Nice find. However, it looks like he is using 13:1. Below are the specs for the 540. This also isn't your run of the mill street engine that he just added compression to and is running on E85. It has a custom solid roller cam, Dart Pro 1 heads and is making 825hp at 6800rpms. Take a look at his 540 pump gas motor that makes 695hp at 6300. It uses 10.5 compression and a smaller cam.

Dart Big "M" 9.8" Low Deck 4 Bolt Race Prepped Block w/Splayed Caps
Eagle 4340 4.250" Stroke Steel Crank
Eagle 4340 6.385" H-Beam 3D Steel Rods
Clevite 77 "H" Series Main & Rod Bearings
JE/SRP 13.1 CR Forged Aluminum Pistons
Speed Pro Plasma Moly Ring Set
SSRE Custom E85 Grind Steel Billet Roller Cam
Comp Cams Heavy Duty Solid Roller Lifters
Cloyes Billet True Roller Timing Set w/Torrington Bearing
Dart "New Design" Pro 1 345 Aluminum Heads
Manley Nextek® 1.640" Roller Springs w/Titanium Retainers & 10° Locks
ARP Head Stud Kit w/Parallel Ground Washers
Comp Cams Ultra-Gold 1.7 Aluminum Roller Rockers
FelPro HP Head Gaskets w/Pre-Flattened Steel Wire
Merlin X "New Design" Dominator Intake Manifold
SSRE/Moroso Custom Stroker Oil Pan w/Windage Tray
Moroso Race Prepped Pump w/Tig Welded HD Pickup
GM Performance Chrome Front Cover w/Billet Pointer
SFI Approved Harmonic Balancer
Complete Assembly Internally Balanced


Ben, I personally don't think it is worth the trouble to add compression and run your engine on E85. I also don't think it will pass a sniffer test with 13:1 compression. If you want to experiment with E85 do it on RxSnake and don't ruin your reliable Vette.

[BADDASSC6]

^^^^^Concur!

[Vettezuki]

True nuff.

The truth is that I think I have a fair bit of power sitting on the table as it is in the form of tune and headers. I have a very conservative tune for pump gas (something like 18 degrees total). I'm still running stock manifolds. With this I'm getting 391 WHP.

The same basic engine (exactly the same long block) was putting down 432 in a C5 vs. my C3. The difference is that he had an aggressive tune and headers. But he also had an LS1 intake, not an LS6 like I have. Bottom line, I *should* be able to hit something like 420+ with some decent headers and a better tune with everything I have right now. While this wouldn't be a mountain of extra power, a lot of it would fall under the curve and be noticeable.

I just have a soft spot for kick ass NA motors.