Bee Jay

I've been trying to get my engine and fuel injection sorted out since I installed the Hot Cam. The computer kept telling my that my manifold absolute pressure at idle was hovering around 60kpa. I put the vacuum gauge on the car, and sure nuff, my idle vacuum is only 12". Isn't that kinda low? It was 15" with the more radical Summit 1105. Do I have a vacuum leak somewhere? I've checked everything, it would have to be at the intake manifold gaskets. I thought my irregularities was because I was using a manifold port on the fuel injection for the MAP sensor and for my headlight vacuum. I have hydraboost, no so power brakes. The vacuum hole in the rpm air gap manifold was directly below the fuel injection, so I originally just plugged it. Last night I bought a brass vacuum fitting and filed by hand untill I could get the throttle body bolted on without interference. I started the car this morning, and still 12" vacuum. Here are the cam and engine specs:
GMMP LT4 Hot Cam 218/228 duration at .050 installed straight up with 1.65 ratio roller rockers and Compcam conversion hydraulic roller lifters.
Engine: Stock L82 block and pistions, 9.0 to 1 compression, AFR 195 heads, headers and 2.5" exhaust, RPM Air Gap intake, Holley 4Di 950 CFM Throttle Body injection. 850 rpm idle. Should I pull the plugs and do a compression check or just go ahead and pull the intake manifold. It is not leaking topside, verified by spraying liquid along the runners.
Bee Jay
Here is the Felpro 1206 glued down

Here is the hole in the intake manifold for vacuum, directly under the throttle body.

Here is the brass manifold fitting I had to file to fit under the fi.

Matt Gruber

1st retorque the intake.
where is the base timing?
just turn the dizzy CCW until u get max vac to find best base.

Bee Jay

I did all of that. The ECU controls the timing, so movement of the dizzy is neccessary. The initial right now is 20 degrees. The previous cam liked 16 degrees plus 10 vacuum for a total of 26 at idle. Maybe this cam wants even more than 20.
Bee Jay

TopGunn

As a comparison I had a 223/233 @.050 cam that would give about 12.5" of vac. I think my present 222/230 is about the same, I couldn't locate my tuning notes on it. They're pretty close to yours. I don't think you have a leak but you should see the gage fluctuate all over the place at idle if you have a leak.

chevymans 77

When I talked with Lunati on cam selection they asked if I would be using FI. They recommended using a cam with a wider lobe separation (112*) for higher vacuum and such.

What was the numbers on the old cam, are they the same with the new?

Neal

Matt Gruber

my 224/224/110 355 would like 36 at idle. But, not having the luxury of an ECM, i can only give it ~26 as it will ping driving it. So take advantage of that ECM!
1st find out what it wants, then program it!

Bee Jay

I don't know what the lobe separation was on the Summit 1105. The LT4 Hot Cam is 112. Gordon, or someone who knows, why is a wide lobe separation angle good for fuel injection? That summit 1105 was a great cam, good vacuum, good power, pulled to 6500 rpm. I thought, and still do, that the Hot Cam with slightly less duration and a whole lot more lift would be a better cam, especially since it was a more efficient hydraulic roller. The C4 guys spend big bucks putting this cam into their LT1s and LT4s. Some of those LT4s make a ton of power.

Matt Gruber

1105 is 114
Did u try twisting the dist to see if it like more?

Bee Jay

WTFrug? I took Matt's advice and decided to see just how much advance would make this puppy happy. I added 5 degrees at a time at idle. I figured no driving, no load, eventually it will run badly and die. I kept adding 5 degrees until it had 50 degrees advance at the now 1000 rpm idle. I didn't dare go further. It loved it. Vacuum was up to 15" finally. The first thing I did was pull the #1 spark plug and verified that the harmonic balancer mark was really indicating TDC, it was. Well, I know I can't leave it at 50 degrees, so I dialed it back to 25 degrees, and right back at 12" of vacuum. But really, 50 degrees advance, and smooth idle with 15" of vacuum. WTFrug?
I took it driving with the 25 degrees intitial, 36 degrees total with throttle, 42 degrees at small throttle openings, all in by 3000rpm. It drove fine. The bottom end feels softer. I thought this cam would be stronger down low than the 1105 did. Damn that was a good cam.
Bee Jay

Matt Gruber

(With a carb), as the timing is advanced, the idle speed is lowered to 800 or whatever it was. THEN check vacuum. is it better than 12?
Maybe i'm dreaming, but isn't 1 advantage to efi, is being able to program whatever curve u want? that is what makes it superior to a vacuum advance. In my dream, u set it at say 40 at idle, and when the TPS detects opening of the throttle, it drops back to, say 25, so u can drive off without ping. now time for my nap
.
does the intake have a cut down center divider? BAD! will lose low rpm torque
comp cams are advanced 4*. u could advance the cam, help vacuum & torque. What head gasket? 1094 give 9.5:1? needs more comp.
check with others that have the same cam; what r they doing & getting?

Bee Jay

I had the old Summit 1105 advanced 4 degrees. David Vizard installs all of his cams advanced. The timing chain set I got allows for 2 degrees advance, so maybe that is an easy option. I haven't heard from other LT4 Hot Cam users except 1, Rodknock from the DC Crew, he could not get sufficient vacuum either and yanked it for a milder Lingenfelter cam. The RPM Air Gap has about 1" of the center divider removed.
My Holley 4Di spark advance is rpm and throttle position based. I did reprogram it last night for 25 degrees at idle, but to drop back a little with throttle application. I'll drive it later today to see if it pulls stronger down low. George (G-Kull) says that the AFR 195 heads are fast burn heads and shouldn't need too much timing. I need to go out and do a G-tech 1/4 mile run. Maybe I did give up some low end torque for some monster top end power. Maybe the manifold is partly to blame, but I was running a big LT1/Z28 intake before.
The heads are 65cc AFR and I used the standard Felpro head gasket that AFR recommended. The engine is a stock L-82 with forged 8.9 to 1 flat pistons. I figured 9 to1 compression now, but I've been wrong a few times this year already. The 96 LT4 Corvette had 10 to 1 I think. Maybe I need more compression. How much can I gain with a head gasket swap?
Bee Jay

Gordonm

The wider lobe seperation will produce less overlap. This does not allow the intake and exhaust to be open together for long periods of time. So it does not confuse the O2 sensor teling the computer it is runing rich. The is less exhaust dilution so it runs cleaner and the computer is not telling it to give less fuel. What do some of the C4 guys with LT1 and 4s get for vacuum running that cam. I lile yo uhave heard great things about that cam. Have you posted anything on the C4 forum like this to see what they are getting?

Bee Jay

Yes, I posted over on the C4 board, no immediate response, and then your post sinks to the bottom really quick. Maybe I'll do one of those ttt posts that iritates me so much. Do as I say, not as I do, right.
Bee Jay
Edit: I just realized that I posted on the C4 DC Crew. I'll post over here now.

Matt Gruber

i'm a little confused. Just how many parts did u change at the same time?
was the Z28 manifold cut down? VERY important if it was NOT.
Do a cranking compression test throttle open on at least 2 or3 cyl.. Famous builder Joe Sherman looks for 200 psi for 91 octane. anything 180-220 sounds good to me.
L-82 w/65cc should be ~ 10-1

Bee Jay

Well, while I was in there...... new cam and new Edelbrock RPM Air Gap intake manifold, also new serpentine belt setup. I was ready to do a new hood too, but surprise, the air gap and holley fit under the hood with a 4" air cleaner. I forgot to mention the 1.65 ratio rockers which makes a cam bigger, but Chevy recommends 1.6 with the LT4 Hot Cam. So, I already have 10 to 1. I may do the compression check today or later this week.
Bee Jay

mrvette

BJ, I have a 355 with L98 stock heads, and a modded up LT1 induction...running speed density with a TPIS ZZ9 cam that's been in there some 12 years now....L98 dizzy...850 rpm idle.....

my cam is 212/226 @ 50, and .484/520 at valves, 1.5 rockers...

its a all roller engine... lobe sep is 112.....

I get 17.5 off my suck meter....i'ts cold now so only about 50f in the garage....

The engine been suffering with fuel related and maybe timing chain jitter which should not really be a problem, but the idle is rough, and the suck meter very steady....

Matt Gruber

BJ
i read your old posts and it appears u want a 12sec 79 that can poke along in OD at low rpm.
good plan but it needs some fine tuning.
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1.RPM AIR-GAP- MUST get that center divider welded up flush. Any connection of the high & low plane kills low end that u MUST HAVE for good driving in traffic, so u don't have to constantly downshift for modest accel.
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2. after the comp. test, u may have to advance the cam at least 4*, again to maintain good low speed accel.
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3. may need a 4 hole gasket to properly mate the TBI to the rpm airgap. see #1, same torque problem. do u have a pic of the bottom of the TBI?

Taijutsu

Matt: I have seen other posts about the center divider in a AG. Why is the gap designed in the first place? Someone else made mention of factory advice to get 500rpm extra by modding this area.
Having a 650 Demon, tuning each half as a 2 bbl is needed. Also 4 hole spacers are noted for better low/mid range power. I would like more low end to pull a big OD gear w/o carb problems.

This is the area where bigger gains than the other guy comes in. Few of us can really test & tune to get the most out of our combo. We use a cook book approach to choose our parts and hope for the best! Those that take the time and money to test, usually get the best results. JMHO

R
Anyone care to comment on welding up the divider?

Matt Gruber

It is a racing mod for high rpm power. Lots of guys build drag cars.
But, BJ seems to want a low rpm street cruiser that runs 12's.
One could say he bought the wrong manifold; i'd say it just needs a little welding to work right for BJ.

Bee Jay

What I really want is a C6 Z06. 11 second quarters, 27 mpg cruising, meets smog, and gets all the women. A 12 second quarter is one goal, and I was so close with the old cam and intake. The cam was designed in the 70s and the intake in the 60s. I thought with new technology, and some careful modifications, I could have the streetability and the quick 12 second quarter. I've always been told to eliminate the top 1" of the divider in a manifold. This is a first to be told to weld it back up.
Advancing the cam 2 degrees can be done easy shmeesy this weekend. But I'd like to spend a few hundred miles and gallons getting this combo dialed in as good as possible first.
Bee Jay