Dynamic compression question
 

How many of you guys are running between 7.5 to 1 and 8 to 1 dynamic compression on pump gas (93 octane) and what is the real limit here. After simulating an engine build on my computer, (bore, stroke, cam specs., it calculates 13.4 static compression ratio and 7.6 dynamic compression. Which means, I think, will run marginally on pump gas. Marginally, defined as, drive easy on the street and splash a little of 110 on race day. My parts consist of:
440 cfm tiger heads
535 IA2
264-270 110 lsa roller cam
2- 800 cfm carbs
4000 stall converter
t-400 trans
9" rear with 3.50 gear
This engine is currently running with 30 cc dish pistons and single 1050 carb, and 6.4 dynamic compression and 130 lbs. of cranking compression. Only 760 HP. Gotta do better than this!

Steve

I've never ran any numbers on my engine for "dynamic" compression, but it easily manages 87 octane fuel at 10.99 to 1 static compression ratio.

The build is a 428 block, 455 crank, std 455 bore size, exactly zero deck height, and SD's Old Faithful 236/245/112 HR camshaft. ICL is 109.5 degrees. The OF cam is around 292/308 duration seat to seat, nearly as I can remember.

Cranking pressure is right at 198-200psi in all cylinders.....Cliff

Cliff, that's good news. Now all I gotta do is get flat top pistons and run the cylinder pressure up like your combo. In the end, I'm looking for 900 hp.

Is that 760hp at the wheels or crank?

It also helps when setting up an engine to run on pump fuel, to move peak torque up in the rpm range, and spread it out as much as possible/practical.

Even with big power engines (N/A), they don't fill as well at high rpm's, as there is less time (more cycles per second). If you can get it to manage pump fuel up to and at peak torque, good to go. Beyond peak torque the cylinders don't fill as well, and dynamic compression will start dropping off.

From what I've seen with this topic, small/tight LSA camshafts that pull power down in the rpm range, narrow up power, and increase peak torque, don't work as well for pump fuel as they do for race fuel.

Of course that comment applies more to street engines than full race, but the laws of physics still apply. I've shown with my own engine that we can easily manage currently available pump fuel at a relativley "high" compression ratio.

I wouldn't be afraid for one second to step up an additional 10-15 degrees @ .050" with the camshaft, and up the compression ratio another half point or so. It wouldn't be any more octane sensitive than it is now, with the ICL at the right point and close control of timing/fuel curves....IMHO......Cliff

With 13.4 to 1 static I wouldn't mess around and run 112, regardless of what you calc for dynamic. You don't want to detonate.

What intake are you running? Is everything matched?

I am not sure that cam is working for you either.

There is most likely a few places to bring the power up just in tuning.

WE built a 535 Tiger head combo at 11.2 Static cr and 8.6 DCR thats about our limit for 93 octane.It made 892HP at 6900rpm and 718 torque at 6100rpms.Bill C.

this is a great combo for E85

Bill C. it made 575 ultimately on a chassis dyno. I ran the A/F ratio from 11:1 - 13:5 and it liked 12:9 the best. I ran the cam from straight up to 8 degree advanced. I liked 8 degree advanced best. That is also where the cranking compression finally went to 160 psi.The current intake manifold is a sheetmetal one that I made with a 1050 dominator carb. I know there is a compromise on the low profile intake but I also know it isn't a 150 hp compromise. In its current state I ran the timing up to 42 before the power started to taper off. I backed it off to 39 to be safe. This is why I feel it could use alot more compression. I have ordered 2 new 800 cfm carbs and will situate them over the port runners and hopefully make that area more efficient. The GOOD news is it lost oil pressure while driving down the street and now has a rod knock, so, now I get to buy new flat top pistons to fix the low compression.
Bill C were you running flat top pistons on that engine?

Ponjon not many E85 stations around here

Dished pistons.You should not need 39 degrees timing,Bill C.

Bill, I know I should not need 39 degrees but that's where it ended up. I think that the compression is too low with the 30 cc dish pistons and 72 cc heads. Maybe I should just put a blower on it.

What RPM was peak power?

Any more specifics on your intake?
runner CSA @ head, CSA @ plenum, runner length, plenum volume?

how much lift at the valve?

Pastry, the intake was designed to fit under the hood. I matched the head port openings and kept the runners the same dimension all the way. I was recommended to do this by another porter. I do not know the plenum volumn but the runners are 2 7/8 x 1 3/8 to match the port opening. I really DO NOT want to cut the super 400 hood that is on it for a taller intake unless it can be proven to vastly outperform this one.The valve lift is .800. Hopefully I can post the dyno sheet and pics of the intake.

Also bare in mind that the above dyno sheet is a chassis dyno.

The increase in CR will definitely help make more power. But you will have to run race gas.

The look from that intake, imo, is small plenum and short runners. You said no taper. That is a higher rpm intake. With the small plenum, it probably needs more carb than you have.

The cam just doesn't seem to match the combo to me. You said you had to advance it 8 degrees for cranking compression. But did you run it on the dyno to see the power differences?

With the intake being limited, etc and the displacement of the engine I think you need a bigger spread and a fair bit more duration to get good cylinder fill. That and a larger carb and the increase in CR should get you smilin ear to ear.

Ron, I did run it on the dyno and I gained 30 hp to the 8 degree advance from the straight up position. I am also going to remove the top of this intake and refabricate to put 2 -800 cfm edelbrock Thunder series carbs situated over the runners to help the air/fuel mixture from trying to negotiate all the tight turns.

Steve

Cliff, does your engine the 10.99 CR have aluminum heads ? It must because you could not get away with it with iron heads. I would ping itself to death.
Its all about cylinder pressure anyway, not necessarily CR.
The cam in the OPs engine is too small for those heads. its puny.
I run a cam almost that big with iron HOs on pump gas and could use more.

Dragncar, so what size cam would you use. I drive this car on the street more than race. It drives good with current cam but never ran a cam larger that this one and am concerned about over camming.

Steve

The cam you are using is very close to the pump gas 535ci street engine I did that made 813 HP.

The cam was 265/272 @.050 on a 110 with 10.75 to 1 static compression.

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Calvin Hill
Hill Performance
708-250-7420