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Username Post: 348 performance (lack of!)        (Topic#350622)
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-14-18 01:55 PM - Post#2730969    

I have a 1960 Impala convertible, 348 Tri-power. It runs well, but doesn't put out anything like the power I feel it should. When I got it the (mechanical) throttle linkage was way out of adjustment, so I have fixed that, but from the driver's seat it feels like it is putting out maybe half the power it should. Feels like about 100bhp.
I am getting full throttle on all three carbs.
I haven't done any investigative work yet - I will of course check the timing and dwell, but from the feel of the engine I doubt if it's far out - she runs perfectly, but when you floor the throttle you don't get the surge of power I feel you ought to.
I had an Aston Martin DB6, which weighs only a couple of hundred pounds less than the Impala, and puts out 282 bhp, which I guess is similar to the 348. I'm not expecting Aston Martin performance from my Chevy, but I feel it should pull a lot stronger than it does!
Does anybody have any idea what BHP I should get at the back wheels on a dyno, or what figure a compression test should give?
It did cross my mind that I may have a low compression truck engine, as it's not the original.
As I said, this is just a preliminary post, as I haven't even started investigating it yet.



 
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Tri5man 
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Tri5man
Loc: Possums Crotch, KY
Reg: 06-26-07
04-14-18 03:44 PM - Post#2730980    
    In response to Hugomiller

What is the transmission? If its a Turboglide thats your answer.



 
Gene_Schneider 
Ultra Senior Member
Posts: 12107

Loc: Wisconsin..USA
Reg: 09-27-02
04-14-18 03:50 PM - Post#2730981    
    In response to Hugomiller

What rear axle ratio is in the car?
What transmission and if it is a close ratio 3 speed or 4 speed it will not have a lot starting from a stands still.
When new they went from zero to 60 in about 9 seconds. Today that would be considered a slug.

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


Edited by Gene_Schneider on 04-14-18 03:51 PM. Reason for edit: No reason given.

 
Dave_Neiling 
Senior Member
Posts: 1232

Loc: Custar, Ohio United State...
Reg: 12-10-01
04-14-18 04:51 PM - Post#2730988    
    In response to Hugomiller

My buddy bought a 61 tripower car and same issue no umph when he put the pedal down.On further inspection we found only the primary carb was functional the other 2 were blocked off. Just a thought something to check.



 
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-14-18 08:43 PM - Post#2731003    
    In response to Dave_Neiling

Sorry, I forgot to say it's a three speed manual, column change.
I did check the axle ratio, but I have forgotten what it was - 3.36 maybe? Either that or 3.7. I got underneath and counted the turns on the driveshaft with one of the rear wheels jacked up. I'll have to do it again and maybe write it down this time!
It feels too low in any case - you just get moving in first then have to change up to second - in fact most of the time I pull away in 2nd.
I'm not into racing starts from the lights, but it ought to have a lot more pull than it does, I'm sure.
I will check that the outer two carbs are actually
doing anything!
Thanks for all the tips!



 
Hugomiller 
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Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-14-18 08:45 PM - Post#2731004    
    In response to Hugomiller

0-60 in 9 seconds is pretty respectable. I know today they have 1,000 bhp cars that are much quicker, but that's still pretty good. Maybe I'll try mine & see what she does. Column shift is not ideal for it though!



 
DonSSDD 
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DonSSDD
Loc: Nova Scotia, Canada
Reg: 08-21-01
04-15-18 03:54 AM - Post#2731010    
    In response to Hugomiller

Check the casting number on the back of the block driver side plus the stamped code in front of the passenger side head, those will tell you what it was "built as new". A stock truck block was under 8 in compression.

Check the casting numbers and dates on the heads.

Its fairly unlikely a 348 in a nice convertible was not rebuilt and maybe upgraded a bit. Most car 348's were the 250HP variety with plenty of torque and should pull fairly good with a 3 speed manual, even with a 2 barrel, if tuned properly. If the tripower is not working correctly, your performance will be lousy.

63 Pontiac Parisienne Sport Coupe(CDN Chev mechanically (409, 4 speed),62 Bel Air SC (sold), 59 El Camino (sold), 62 Bel Air SC(sold), 63 SWC Vette (sold),
Member #2194


 
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-15-18 04:47 AM - Post#2731012    
    In response to DonSSDD

Can't see the casting number - it's hidden right at the back there. The number on the front of the block is T915H (possibly I915H or 1915H). That's all that is there - I have a Corvette with a 283 that has a much longer number stamped there.
Can't see any numbers on the outside of the heads.
I had a peer down the choke tube of the front carburettor; the butterfly opens fully and I can see right down into the intake manifold, and there is a squirt of gasolene when I open the throttle by hand, so on the surface at least it looks like it is doing what it should.
Is there a definitive way to test the outer carburettors?



 
japete92 
"4th Year" Silver Supporting Member
Posts: 938
japete92
Loc: No. Virginia
Reg: 01-18-13
04-15-18 07:51 AM - Post#2731024    
    In response to Hugomiller

  • Hugomiller Said:
I have a 1960 Impala convertible, 348 Tri-power. It runs well, but doesn't put out anything like the power I feel it should. When I got it the (mechanical) throttle linkage was way out of adjustment, so I have fixed that, but from the driver's seat it feels like it is putting out maybe half the power it should. Feels like about 100bhp.
I am getting full throttle on all three carbs.
I haven't done any investigative work yet - I will of course check the timing and dwell, but from the feel of the engine I doubt if it's far out - she runs perfectly, but when you floor the throttle you don't get the surge of power I feel you ought to.
I had an Aston Martin DB6, which weighs only a couple of hundred pounds less than the Impala, and puts out 282 bhp, which I guess is similar to the 348. I'm not expecting Aston Martin performance from my Chevy, but I feel it should pull a lot stronger than it does!
Does anybody have any idea what BHP I should get at the back wheels on a dyno, or what figure a compression test should give?
It did cross my mind that I may have a low compression truck engine, as it's not the original.
As I said, this is just a preliminary post, as I haven't even started investigating it yet.



Here's link that may answer many of your questions and has lots of original engine info:

https://www.gmheritagecenter.com/docs/gm-heritage-...

First page is blank, scroll down to the section you need.

I can't begin to say anything about your 348, but in original form it required 'hi-test' (octane over 100, as I remember) gas which is not available at the pump today which may be a contributing factor.

Also, if you don't have a tach, I'd install one. You may have to rev that engine 'higher' to get the performance out of it. Your 3 speed manual should not be a problem. The shifts are not lightening quick, but performance between shifts should be enjoyable.

Pete




 
Gene_Schneider 
Ultra Senior Member
Posts: 12107

Loc: Wisconsin..USA
Reg: 09-27-02
04-15-18 02:25 PM - Post#2731064    
    In response to Hugomiller

If it peaks in the gears too soon you may have a wide ratio 6 cylinder transmission.

The 6 cyl. trans has a 2.94 ratio first gear and a 1.68. 2nd gear.

Eight cyl. models had a 2.47 first gear and a 1.53 2nd gear. The stock rear end ratio was a 3.36,

I tought your problem was lack of power. What you explained , is an overspeeding of the engine under normal driving starts.
Octane is rated by a different formuls today and if the timing is set as required 92 octane gas would be sufficent.

Zero to 60 in a modern V-6 car is about 6 seconds in a modern car.


ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


Edited by Gene_Schneider on 04-15-18 02:31 PM. Reason for edit: No reason given.

 
Gene_Schneider 
Ultra Senior Member
Posts: 12107

Loc: Wisconsin..USA
Reg: 09-27-02
04-15-18 02:39 PM - Post#2731067    
    In response to Gene_Schneider

In 1960 and engine number ending in "H" was 250 HP 348 with Turboglide. A 280 HP engine would be basically the same except for the tri-carbs.

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


 
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-15-18 04:15 PM - Post#2731083    
    In response to Gene_Schneider

My problem is indeed lack of power. The gear ratios are also in my opinion not ideal. Would work better with a taller axle ratio (it is 3.36 - i checked this morning). First is a bit low & second a bit high to pull away. But that is an issue I understand, and irrespective of that, my problem is that nothing very exciting happens - in any gear - when you floor the throttle.
Not that I drive like that, and the power is perfectly adequate, but I'm sure it is nothing like what it is supposed to be.
My next step is to verify the timing marks on the balancer & then check the ignition timing.
I plan to put it on a dyno, but I'm not sure what figure I should expect at the back wheels. I'll bet it's less than 150 at the moment though.
I run on ethanol-free fuel BTW.



 
japete92 
"4th Year" Silver Supporting Member
Posts: 938
japete92
Loc: No. Virginia
Reg: 01-18-13
04-16-18 07:36 AM - Post#2731144    
    In response to Hugomiller

  • Hugomiller Said:
My problem is indeed lack of power. The gear ratios are also in my opinion not ideal. Would work better with a taller axle ratio (it is 3.36 - i checked this morning). First is a bit low & second a bit high to pull away. But that is an issue I understand, and irrespective of that, my problem is that nothing very exciting happens - in any gear - when you floor the throttle.
Not that I drive like that, and the power is perfectly adequate, but I'm sure it is nothing like what it is supposed to be.
My next step is to verify the timing marks on the balancer & then check the ignition timing.
I plan to put it on a dyno, but I'm not sure what figure I should expect at the back wheels. I'll bet it's less than 150 at the moment though.
I run on ethanol-free fuel BTW.




You have not mentioned the cylinder pressures. Are the cylinder pressures close to the advertised 150 psi?

Ethanol adds octane. I believe the 348 varied from 9.5 to 11.5 compression ratio depending on the engine. I'm not saying the fuel is your problem. It MAY be contributing to it. I do not know the octane you are using (I don't need to know ).

Good luck with your trouble shooting

Pete





 
stanski 
Infrequent Contributer
Posts: 49

Reg: 05-05-16
04-16-18 08:06 AM - Post#2731151    
    In response to Hugomiller

If you have access to a dyno, I would do that right now before messing with all the other stuff. The dyno has no regards for gear ratios, tranny ratios or tire size, just measures the grunt under load. If it turns out the old W is only putting out 150 hp and a bunch of air, then either accept that or start a full evaluation of all the engine components and parameters that produce hp. On the other hand if you're producing 250 hp then you have to evaluate everything behind the flywheel.



 
Shepherd 
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Posts: 1227

Loc: Lake George, NY
Reg: 11-11-15
04-16-18 08:21 AM - Post#2731155    
    In response to Hugomiller

Base timing and working mechanical advance are your first steps.



 
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-16-18 12:28 PM - Post#2731176    
    In response to Shepherd

Thanks for all the tips - I am about to start checking things like timing, advance and cylinder pressures. The timing doesn't feel as though it is far off, but I will report back when I have something to report.
Also need to find a dyno in Central Florida.
Is there a way of testing the contribution (if any) of the outer carbs? It looks like they are working, but it feels as though they might not be.



 
convman1960 
Infrequent Contributer
Posts: 28

Age: 64
Loc: Kansas city Kans
Reg: 07-25-10
04-18-18 03:13 PM - Post#2731458    
    In response to Hugomiller

i put an hei in my 348 and it really helped when the outer carbs kicked in it would run ! plus it started better that was a plus





 
Hugomiller 
Silver Supporting Member
Posts: 108
Hugomiller
Age: 67
Loc: Florida and UK
Reg: 05-02-17
04-19-18 07:22 PM - Post#2731614    
    In response to convman1960

Thanks - but what is an HEI?
I should have checked the obvious things first I guess - it turns out the outer carbs were not opening fully, if at all. When I reached under the hood and worked the linkage, it was all working ok, but there was enough 'give' in the linkage from the pedal, plus heavy return springs on the outer carbs, that they weren't opening properly when you pressed the pedal. I tweaked the linkage and it is now ten times better than it was, but I have a 'flat spot' where the mixture is momentarily weak when the outer carbs kick in. I know that both accelerator pumps are working, as you get a squirt of fuel when you open the throttles, but not enough obviously. I can't see any adjustment for the accelerator pumps - is there one?
Incidentally, I got some surprising figures when I checked the ignition timing; with the vacuum pipe disconnected (as per the book) it was showing 15 degrees advance at a pretty slow idle (the book says 12, but it's not pinging or detonating at all so I'm happy to leave it at that (plus the fact that I don't have a wrench to reach that stupid distributor clamp bolt!). When I re-connect the vacuum pipe, however, I'm getting an astonishing 32 degrees advance at idle, and it's a pretty slow idle at that (about 450 rpm I would guess). Is that normal?
I should add that I recently fitted an after-market harmonic balancer with lots of pretty timing marks on it. I verified that these were in the right place by poking a thin plastic tube down #1 spark plug hole & pushing the car in top gear. From feeling the pipe rise and fall, I verified that the marks were correct, or at least pretty close. Not a very accurate way of measuring it, I know, but I figured that if the marks were that close they had to be in the right place relative to the keyway. It is possible I suppose that the marks could be a few degrees out, but I reckon that is pretty unlikely.



 
Gene_Schneider 
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Loc: Wisconsin..USA
Reg: 09-27-02
04-20-18 07:51 AM - Post#2731646    
    In response to Hugomiller

That engine has full vacuum advance at idle so thr 32 deg. is OK.

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


 
convman1960 
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Posts: 28

Age: 64
Loc: Kansas city Kans
Reg: 07-25-10
04-20-18 01:27 PM - Post#2731665    
    In response to Gene_Schneider

High energy ignition found on 70s small blocks, you neet to buy an adapter for the 348 i got mine from late great chevy.




 
mahlmann 
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Reg: 01-20-11
04-20-18 01:35 PM - Post#2731666    
    In response to convman1960

An easier and invisible way to get HEI is replace the stock ignition points with Pertronix.

Mike Ahlmann
Las Cruces, NM
'50 Tin Woodies (two) Ramon and Willard
'58 Impala Sport Coupe
'69 El Camino SS-396
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'16 Ford Explorer 3.5 EcoBoost - Red Rocket


 
DonSSDD 
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DonSSDD
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04-21-18 07:15 AM - Post#2731737    
    In response to mahlmann

And Pertronics uses the original distributor and cap, so no issues with fitting the HEI. Been using it in a 283 or 409 since 2002.

63 Pontiac Parisienne Sport Coupe(CDN Chev mechanically (409, 4 speed),62 Bel Air SC (sold), 59 El Camino (sold), 62 Bel Air SC(sold), 63 SWC Vette (sold),
Member #2194


 
beagrizzly 
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beagrizzly
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04-21-18 02:03 PM - Post#2731765    
    In response to Hugomiller

Does anybody have any idea what BHP I should get at the back wheels on a dyno, or what figure a compression test should give?
It did cross my mind that I may have a low compression truck engine, as it's not the original.
As I said, this is just a preliminary post, as I haven't even started investigating it yet.

I took a 348/single WCFB carb out of a 58 Impala. I put it into a 57 2 ton truck with steel flat bed on it. That thing was a beast. We started taking it out to the local Saturday night street races, and the only thing that beat us was a 66 Chevelle SS396. You should feel PLENTY of oomph from your 348.

I have also found out that the older we get, the faster we were. Hahahaha.

Griff


if you're gonna be a bear..................

1960 Biscayne (the 6T)
2005 Yukon XL
2007 GMC Sierra Classic 8.1
2009 Silverado
2011 Escalade ESV


 
Gene_Schneider 
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Loc: Wisconsin..USA
Reg: 09-27-02
04-21-18 02:22 PM - Post#2731766    
    In response to beagrizzly

Chevrolet figures are compression 160 pounds at cranking speed with a hot engine.
Net horse power (not gross) 235 HP at 4800 RPM.

A truck engine was designed to give more power at lower engine speeds (like under 3000 RPM)

ChevGene 1934 Master sedan 1939 Master DeLuxe Town Sedan 1950 Styline DeLuxe Power Glide 1957 Nomad 283 PG 1963 Corvair Convertible


 
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