The K&N dropped base open element is a nice unit designed for maximum drop. I have hood clearance problems also. So I took out my 3X14 filter and installed the 2X14 with the lighter weight filtered top to keep the possible air flow high as possible.

As for one of the better dual plane intake manifolds. My choice is the Weiand Stealth with a 1/2 inch wood 4 hole spacer. all of these from summit racing.

 

As noted, you've got more than enough carb. Vacuum secondaries are the right choice unless you're going drag racing. Rule of thumb calcs:

CFM = ((CI x RPM)/3456) x VE

Estimating you'd need to turn 8500 RPM to over-run the carb, and like 4500 in cruise to have the secondaries open in cruise. This is pretty general and always subject for review - but I don't think you need to look at sizing right now. I do think you need to double-check the fuel pressure at the carb if you can plumb a gauge in.

I agree that the rocker change is more of a tune than solving a problem.

I suspect the ignition as well as previously noted. If the distributor is a locked drag distributor you'll want to change it out anyway. And a lousy coil can cause exactly this kind of flat spot as well.

I'd agree that the Performer EPS isn't ideal for this combo, but IMHO it's not the root cause of that kind of power shortfall. There may be 10-15 HP above 4500 or so, but it's not choking off the engine to this extent.

Timing and fuel - I wouldn't make any changes outside of those two until you're making near expected power. Then you can seek perfection

 

Goes a little too rich where it should be making best power. Note the fuel curve after the ~4,500 rpm bump. Also note that the power spikes at the top of the run where it leans out a bit.

 

Just because you have a carb that is rated at 850 cfm, doesn't mean that it is actually doing its job. Borrow another carb and see what happens.

 

i agree with gkull. That old manifold is maxed out on a 350. e-bay the old manifold and this swap is really low cost. Don't forget the 4 hole spacer!
.
the 1.6 RR is good idea too

 

Ok, after some investigating......

The timing was at 34º total. I bumped it to 36º and moved to a lighter spring on one of the weights so total advance is ~2600 rpms now. Unfortunately, this has created a low speed transition ping so I need to remove some vac-advance timing and reset everything again.

Now, the primaries were not opening all the way as suspected. So, I re-worked the throttle cable and I believe I have it corrected now. SO, do the Primaries have to be open fully for the secondaries to open ???


Next....

I checked hood clearence for a better intake and may have discovered another issue. I have about 0.8" worst case clearance from the hood to the top of the air cleaner. So, both the Air-Strike and Air-Gap look like they are only 0.46" taller than the EPS. Which would leave me with ~0.3" of clearence. Is that enough?

I also checked the height of the drop base to the lid on an interior measurement and I only have 0.75" worst case. I suspect this is also causing me a loss in horse power. besides moving to a lid with a built in air filter....any ideas?

Tim

 

Tim

How are you?

Are you considering a different CAM?

-Adam

Corvettes of Dallas

 

Adam - I'm good. Thanks again for having the guys out.

For now, I need to sort out the combo. The cam is not what's holding back the combination. I think I will swap the intake next and possibly the carb. I'm pretty concerned abut the hood clearence to the air cleaner and the air cleaner to the carb clearence. It going to be TIGHT....

Any thoughts?

Tim

 

If you are getting pre-ignition pinging, you have advanced the timing too much. Maybe leave one spring from the original set-up and one from your most recent test in the distributor to optimize the setting. Then set it back to 36 deg. when maxxed out. It may not ping with that setup.

[P.S. I thought you weren't getting a full fuel charge from the carb...]

 

**** Disclaimer, I'm a newbie here so don't take this as advice, just an observation******

Been playing with cam software from Comp cams plugging in your set-up:

Highest #'s with the XE274H : 383.1 HP (5000 rpm) 438.8 ftlb (4000rpm)
Lowest #'s with the 252H : 318.6 HP (4000 rpm) 454.5 ftlb (2000 rpm)

The XE 274H 230/236 @ 0.0500 and 0.490/0.490

So one of the questions is the parasitic loss of the TH400.
15-20hp for the PS/fan/waterpump and other goodies,maybe 50-60hp for the trans would give you app. 300rwhp.
So you'd probably have a little to gain with tuning but with my peanut brain I'd say you're in the ballpark.

 

.300" is plenty of space. If it hits the hood sometimes who cares. It'll just make a little rub mark. My dad has a Performer RPM on his car and it fits ok. It's an LT1 car so it's got the big block hood.

A roller cam would pick you up quite a bit of power but they are expensive. You could keep those same springs I believe. I don't know anyone that has regretted swapping to a roller.

 

Ok, 0.3" of hood clearence is do-able then. Any thoughts on the air-cleaner base to air cleaner lid at 0.75"? Seems like a choking point but I'm not sure how to correct that and get a better intake at the same time.

The current cam is a HR from Lunati - 227/234 @.050

Tim

 

The pre-ignition is "only" at throttle tip in and I think I can correct it by removing 2º from the vac-advance. I'm going to try this first and see. If it doesn't work, it's not that difficult to change it again.

 

Well, I might be in agreement but I am running a hydraulic roller cam and would expect the TQ to be higher than your projection. With Desktop Dyno, the numbers come out to ~ 485 ftlb (3000rpm) and 398 hp (5000 rpm) which is probably very optimistic.

Now, even at 300 rwhp, I'm missing that by ~40 rwhp which is a lot at this point.

But keep the ideas coming.....

 

A roller with less than .500 lift? I didn't know those were available. Definitely get some 1.6 rockers.

Drop-base air cleaners are not the most efficient but it's still better than a crappy intake.

 

When you say "low speed transition ping", are you meaning at full throttle or partial throttle? It would help if you knew what the vacuum was at this point because the vacuum can go to less than ~6 inches at partial throttle. Further, if the vacuum was less than 6 inches, the vacuum advance would be out of the picture anyway so taking out the vacuum advance won't help. Recommendation: Add a vacuum gauge to a source of intake manifold vacuum so that you can read it when you are driving. You'll be able to correlate it with your observations.

The primaries do not need to be open ALL the way for the secondaries to start to open: There is a hole just underneath the primary venturi which carries the venturi vacuum signal to the vacuum canister, and will pull the secondaries open when the venturi vacuum is high enough to overcome the force applied by the spring in the vacuum canister.

Although there is no linkage requiring the primaries to be fully open, they will be nearly full open, and, of course, the air flow going into the engine must be high enough (high rpm) to create enough of a pressure drop in the primary venturis to pull the actuator open.

 

I would do a compression check and leak down and make sure the motor is healthy. That parts combo is matched up pretty well for a mild street motor....I agree the EPS intake is a restriction but...definetly not 40HP worth. You should definetly be putting more then 260rwhp out....

For comparison sakes,

My original 71' 350/270hp motor(8.5-1) with a 327/350hp cam(L79, 222*@050, 440" lift), stock but cleaned port heads and a 140k mile short block made 200rwhp with a Performer intake, q-junk, and 1 5/8" headers. Just your heads and cam alone would get me to your power level(260-270hp) and thats not even talking about more inches, more compression, FRESH rebuild, better tune etc.... Mine was also going through a stock TH400 and 3.08s. It made 194hp through the dual snorkel air cleaner and stock filter, pulled the lid and it gained 6 rwhp...>LOL

I think once you get the bugs worked out, you should be good for 300-325 rwhp... Remember you have around 25% loss due to the TH400 and IRS.

 

Interesting! My suggestion would be to pick Sean's brain...

Tight squeeze = Dremel or sand paper?


-Adam

 

Pull the aircleaner for the Dyno run and see where you are at. The tip in knock, you can try riching up the idle jets and maybe some octane boost may be needed. Stiffer advance springs will let you set the off idle timing higher so you can get more in at the top. You have plenty of low end grunt but you flatten out to quick. I'd try the cheap stuff before throwing money at it IMHO