Static/Dynamic compression ratio.

[dfarr67]

Moving from premium to regular gas (E10) on a 385 TPI, engine was built around 9.5:1 with AFR alum vortec. Is my spark mapping out to lunch or is this cam an issue for me? Getting lingering ping with iron vortec map but going back to dyno tuned spark map and premium fuel cleans it up. Just wondering if the 'tuned' intake/cam are really my issue here.

[rebelrouser]

Have you done a cranking compression test to see how much you actually have? Not a perfect indicator of dynamic compression, but quick and easy. If you have over about 160lbs cranking compression then you will need a custom timing map.

[F-BIRD'88]

The power price of running low octane gas is you will need to lower net running cylinder pressure thru de tuning and or reduce spark timing....
If thru a combo of mech compression ratio and tuned induction/exhaust, you are getting high cylinder pressure at wot (and high torque output) you will need to change things about that tuneup to get it to run on cheap low octane gas.

[dfarr67]

Tuning for VE now, tried tuning for spark at the same time- didn't work out to well so filled up with premium. I put in different injectors so remaping the fuel areas. If I have to change anything mechanical- it will stay on premium. Haven't done a compression test on this ...yet. I've been told these TPIs like some odd timing.

[Newold1]

Most TPI's are port injected with injectors aimed at the back of the valve so I don't see where the intake will require greatly different timing. The whole system is going to affect some of the timing required but a lot will be determined with head flows, piston speed, combustion chamber shapes and sizes as well as the dynamic compression and of course camshaft design and timing. It 's not difficult with camshaft event changes to move cranking and running compression ratios around enough sometimes to lessen detonation prone builds. Obviously head gasket thicknesses can be increased enough in some cases to also provide similar relief. Don't get short sided just looking at timing changes to create a quick fix.

[dfarr67]

I did the head gaskets a little while ago, stayed with the compressed thickness of .054 which limits everything else but was able to match the bore better as in smaller diameter gasket (GM). Pistons are hyper dished type no idea on cc. TPI is FIRST unit.

[Newold1]

Dave:

I can't get your Excel attachment to open and not sure what it contains?

[F-BIRD'88]

You are going to give up everything you wanted to gain with the new heads.

[My427stang]

I can't imagine that cam is causing a cylinder pressure problem with the compression you stated, but, I think we'd have to see some real numbers to know what is happening.

If the pistons are in the hole a bit you are losing the benefits of quench with that thick gasket, if the compression is higher than you think with those heads, or the cam isn't what you think it is or timed where you expected, or just as likely, it's got a lean condition at that rattle point (or any combo of those), it could easily rattle with the stock curve.

I'd lean toward a lean condition myself, especially if it's a part throttle issue that isn't usually seen when doing a normal peak numbers type of dyno run, but that's a guess with available info.

I would also say that the stock timing curve is likely pretty lazy and wide, although I am not familiar with the stock tuned port curves. Part throttle performance will be much better with something more performance oriented and matching the engine and the use of the car. So sticking with the custom curve and biting the bullet on good fuel may be better in the long run

[pdq67]

Static CR here.

I read where E85 is good up to 17 to 1 in DEDICATED supercharged race engines so I doubt if detonation will be a problem. As for a street engine, 12.5 to 13 to 1 is about right. Some approach 14 to 1!

I would install Felpro's #1094 .015" thick shim head gaskets I(F your engine is a standard SBC to take advantage of any CR increase. This will also tighten up your quench which helps!

Now, the only problem with using E85 is that because to get the most out of it, you need to run these CRs so won't be able to convert back to pump gas unless it is a high CR race fuel.

pdq67

[dfarr67]

Good morning,
- vehice is 1989 chev 4x4, 700r4, 3:73 gearing. This is a 5.7in rod 385.
- E85 isn't available here.
- regular fuel here may contain 5-10% ethanol, so I am favoring 14.3 instead of 14.7 afr, things seem to have improved, I agree I've been running lean for whatever reason- likely the change in injectors.
- the EBL ecu can switch between 10 different tunes, and also has a lean cruise currently set 16.5:1 but not yet enabled.
- With carbon tax 1L regular is now $1.30 premium is around $1.50, tank being 120L.
- as I recall pistons are out of the hole +.015in. Head chambers are 65cc.
- https://www.airflowresearch.com/190cc-s ... nder-head/
- timing tables I have used: 1990 corvette alum head TPI, 1990 iron 350 TPI, 1990 iron 305 TPI, last is iron vortec TPI. So far no issues with dyno tune SA on premium fuel. To be fair I feel I have been running lean up until now.

- Erson cam 4/7 swap
ROCKER RATIO 1.50
ROCKER RATIO 1.50
CAM LIFT INTAKE .340
CAM LIFT EXHAUST .340
LOBE SEPARATION 112
ADV. OR RET. DEG. 4
INTAKE "LASH" DUR. 290
EXHAUST "LASH" DUR. 294
INTAKE DUR @ .050" 222
EXHAUST DUR @ .050" 226
JOURNAL SIZE 1.868
TDC 45
HOT LASH INTAKE 0.000
HOT LASH EXHAUST 0.000
DEFLECTION INTAKE 0.010
DEFLECTION EXHAUST 0.010
GROSS VALVE LIFT INT. 0.510
GROSS VALVE LIFT EXH. 0.510
INTAKE OPEN 37
INTAKE CLOSE 73
EXHAUST OPEN 83
EXHAUST CLOSE 31
OVERLAP@ LASH 68
INTAKE PHASE 108
ANGLE or CENTERLINE
EXHAUST PHASE 116
ANGLE or CENTERLINE
INTAKE OPEN @ .050 3
INTAKE CLOSE @ .050 39
EXHAUST OPEN @ .050 49
EXHAUST CLOSE @ .050 -3
OVERLAP @ .050 0

[David Redszus]

Two factors to consider.

E-85 is not really 85% ethyl alcohol; it can and will vary seasonally and geographically.

Dynamic compression pressure has little effect on combustion.
What matters is the temperature that is produced in the chamber prior to TDC as a result of
dynamic compression pressure. Compression temperature is a function of inlet air temperature.

[slo-svt]

Can you post a picture of some of the timing tables you are using? Also I'm not familiar with the computer your using but have you datalogged delivered timing vs commanded? Many factory computers add or subtract timing based on many variables such as commanded afr/equivalence ratio, intake air temperature, coolant temp, catalyst overtemp, ect. If you computer has those you may find something adding some to you base table.

[pdq67]

Sorry, Thought you were going to use E85 is all, my bad.

At 9.5 to 1, no more than a good old 270/220+ cam like an old isky 270 Mega will do fine. Maybe a 274HO-6 cam from Crane. BUT I am really not up on the modern engines fueling systems and other stuff, I'm still just old-school here..

I figure a 292 cam really needs 11 to 1 to run like it is meant to run is all.

I had a 292/230 RV cam in my Strong Arm 406 that was right at 9.5 to 1 and it was soft until it hit like 2500 rpm, then hold on! M-20 and 3.31 posi- stuff along with Great big old-school L-60 rear tires! I wished I would have went with the old 280/230 cam instead...

pdq67

[dfarr67]

https://www.dynamicefi.com/EBL_Flash.php
Attached is zip file with latest log on premium.
I can get some screen shots of various SA tables later today.
Truck has good bottom end but really pulls 3500 up- ticket territory.