Fivethree twinturbo

Hello, i just put together a LS7 short block with callies crank, Eagle rods and 4.130" Wiseco pistons. I have LPE GT21 cam and mast 285 heads. What is highest compression ratio i can run with pump gas or E85? Car is racecar with some street driving.

ChopperDoc

I don't know for E85, but for 93, you want to stay under 8.6:1 dynamic compression ratio. Of course as you probably know, that is set by the intake valve timing. Advance = more compression. Static is largely irrelevant. Only thing that matters is dynamic compression.

I know static for E85 is up around 15 or 16. I have no idea what the dynamic ratio is for E85 though. There are some good articles that talk about E85 being used in applications of 16:1 static compression. Again, don't know what the limit is for dynamic, but that's what actually matters.

You can't compress anything through an open valve.

pantera_efi

iTrader: (1)

Hi TT, What is you area, altitude of engine operation ?

I too would state 16:1 as common for a Compression Ratio. (E Fuel)

THERE MAY BE E-100, E-95 in your area ?

The E-85 fuel, by regulation, can vary from E-70 to E-85 in Ethanol Content.

The Pump gas of 94 Octane (RM) would be a 12:1 C.R.
This would depend on Squish distance of ???
This would depend on Pin Offset of ???

I understand your need as you may have a Flat Top piston ?

My method would be to "blend" Pump/Race fuel using a three to one Ratio.

Lance

Fivethree twinturbo

Hello, i live at sea level. The fuel i'm going to use is E85 or just regular Pump gas. Im not sure about my squish clearance. Im going to use stock head gaskets, maybe mill heads for adding compression. And for the pistons they are high comp. If i dont mill heads CR would be 11.5:1. I think 12:1 is safe for pump gas and 13.5 for E85?

ChopperDoc

I'm at 11.8:1 SCR, with a DCR of 8.2:1. Good to go on pump. The squish distance is set by your valve timing, aka, effective compression ratio. If you know the seat to seat values for your cam, your chamber volume, head gasket thickness, bore and stroke, and static compression, Wallace racing has some useful calculators for figuring that out.

Fivethree twinturbo

Anyone know what is Stock LS7 dynamic CR? It obviously drops when there is cam with a lot of overlap like my GT21?

spanks13

iTrader: (5)

Not only is there a lot of overlap with the GT21 but the IVC is very late. LPE doesn't advance their cams. You'll be able to run your static quite high.

Dynamic compression ratio isn't the end all be all for determining static compression ratio and what octane you can run...just another indicator of what may or may not work. VE and power level needs to be considered too. You can have a low DCR, but a high VE when everything comes into tune and still have issues on 91/93 octane.

ChopperDoc

I agree that there are a ton of other factors. And longer durations will typically decrease the DCR due to later IVC. I understand that it is not the end all be all, as this is actually a very complicated subject. I was just outlining the limit for 93, of 8.6:1 ECR/DCR.

With variations in setup and tune, you could run above a 12:1 static, but as you said, a bigger cam can keep the DCR down. You can also retard the cam to achieve the same thing, at the cost of low end torque. So 12:1 isn't necessarily the limit for 93, whereas 8.6:1 DCR is. If you compress it beyond that it will detonate.

You could also have an engine with 11:1 and a small cam, and with this you could advance the cam beyond 8.6:1 DCR and suffer from detonation too.

Even stock cams close the intake valve well after BDC. In theory, you could advance any cam to the point where the IVC would occur at BDC. That obviously wouldn't be a great idea. Surely stroke and chamber volume plays into that though.

ChopperDoc

Would need all the cam specs, Intake centerline, duration seat to seat, etc... how many cc chamber volume? Head gasket thickness, piston +/- valve if any. The only thing I know on that motor is the bore and stroke off the top of my head. 4.125x4

ChopperDoc

Here is a DCR calculator. Very useful for this subject.

http://www.wallaceracing.com/dynamic-cr.php

Fivethree twinturbo

Thanks!

Fivethree twinturbo

I decided to go with a 12.2:1 CR. I should be ok.

Che70velle

iTrader: (6)

I’m 12.42:1 93 octane. Only been 10 miles or so, but no detonation. Finalizing tuning currently. If we’d ever get a dry day here in N. Ga, I’d be able to complete tuning and get to a dyno. Street car, no strip action.

QwkTrip

iTrader: (23)

Cam card is here,

https://www.lingenfelter.com/PDFdown...L210146006.pdf

You can run some pretty high compression ratio with that cam. Problem is you'll be milling the heads down so much that it might hurt air flow and reach a point of diminishing returns on pump gas. Honestly, you might be better off with a different cam.

About 8.0 DCR by my reckoning. This assumes intake closing of 78 degrees ABDC at 0.006" lift. Your intake closing event is pretty much the same if I did my math right.

Fivethree twinturbo

I put a stock LS7 specs to that calculator and it gave me 10.0:1 DCR. I found that Stock LS7 intake closing is 41 degrees. Then i changed it to 53.5 degrees as GT21 cam card says and 10.0:1 dropped to 9.5:1. Put i noticed i used a .050" lift specs where calculator says "use seat to seat specs". How can i convert those 41 and 53.5 specs to seat to seat specs?

ChopperDoc

Add about 27* to it. There is typically about 55* more duration seat to seat over the .050" value. Varies by cam, but that's a good rule of thumb.

Fivethree twinturbo

Ok i played with that calculator again with right numbers and it looks like that 12-12.2:1 is good. Its 8.3:1 DCR so it will work. If i ever change to E85 i could swap pistons to get 13-14:1 static CR.
Anyways, i ordered set of Pac x1209 springs,
O.D. Outer (in)1.324
I.D. Outer (in) 0.950
Damper No
I.D. Inner (in) 0.680
Install Height (Valve Closed) (in) 160 @ 1.800
Open Load (Valve Open) (LB) 510 @ 1.100
Spring Rate 500
Max Coil Bind (in) 1.000

Any guesses where in RPM range would the top HP be?
Somewhere around 7500rpm? So those springs should be good for that. Im going to run stock LS7 intake and tb for a break in drives but later on im going to use Tunnel ram intake with 102mm tb.

ChopperDoc

I'm not an expert on spring rates or pressures, but I can say that valvetrain weight has more to do with rpm limits than sping pressures. Spring pressure is a cam requirement. More lift = more spring pressure required to close the valve in time to avoid floating valves.

Here is an article on the subject, but basically for every gram of weight loss in the moving parts of a valve train = 35-40 rpm increase.

http://www.enginebuildermag.com/2007...in-technology/

Fivethree twinturbo

Well i have OEM Titanium intake valves, stainless Ex valves, stock rockers with trunnion upgrade. On top of springs obviously titanium retainers. And i am running with hydraulic lifters.

ChopperDoc

Hard to say, but stock is actually pretty light. I have heavier jesel rockers, and ferrea hollow stems, which are also heavier, titanium retainers, and prc double springs. I had a bad spot in my trans tune early on and my car actually missed a shift at WOT, hit 6950 rpm for a second, didn't break anything. Then again I also have valve reliefs, and with checker springs on I get over .100" clearance everywhere with a .634" lift. So even if I float a valve, it can't hit anything.