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Question: E4ME low rpm throttle response and mileage with mild engine upgrade


Active member
Jan 10, 2011
Cape Girardeau, MO
Bought a decent running 81 several years ago, THEN found out the unique problems that can occur in the rear suspension and hubs/brakes. Yup, I've got em! I mean the problems.

I work away from home most of the time so the car has been sitting since I discovered that. Now, however, I'm doing some research on the engine to do a budget top end and cam upgrade with the goal of as good of gas mileage as I can get with mild and inexpensive mods, so I can take some longer trips after I retire in a few years, but still have some decent throttle response and power up to about low 5k rpm range.

I've got a 670cfm Holley Pro Jection TB efi I used on a small block van a number of years ago to replace a bad 4bbl and was amazed at the increase in gas mileage and lower rpm throttle response. Since then I've educated myself to a fair degree on efi, and have modified the efi in my daily driver with moderate upgrades, altho without getting into the ecu program itself. Sorry, not a Chevy tho.

Anyway, my current plans are a mildly mod'd 200-4R and 2200 lockup from Bowtie Overdrives and 3.31 gears (I have to rebuild the diff anyway) which, with 27" tires, will give me around 1600 rpm at 58 mph and 2200 rpm at 78 mph, so need good crisp low rpm torque and throttle response.

Budget dictates I leave the shortblock alone as it's in decent shape.

Currently looking at EQ Vortec style heads, 64cc chambers, 170cc runners, 1.94/1.5 valves, for about 9.1-1 compr ratio on the stock shortblock. They flow about 148 at 300 lift & 195 at 400 so should maintain good velocity for crisp low end throttle response. That's a little less than i.e. Dart or RHS vortec heads so should be a little crisper at low rpm. I'm concerned more about 3k or 3.5k and down than I am over 4k or 4.5k rpm. EQ's are easy on the budget at just over $300 ea assembled. They have more mat'l around the exhaust valve seat to eliminate the cracking potential and a better exhaust flow to improve on oem Vortec's marginal exhaust to intake flow ratio.
They're allowed and popular in IMCA claimer classes so must be of reasonable good quality and durability.

Thinking about 1 of 2 cams: Started out looking at the XE-262H but kept lowering my sights when I started looking at the rpm of the torque and the hp peaks, plugging in efi compatible. Finally arrived at the Comp XE-249H with 206/212 @ 050, 434/444 lift and with rpm range of 1k to 5k, per the Comp Camquest dyno program roughly 315 hp at 4500 peak, 400 peak tq at 3000, and 325 tq at 1600 which would fall right into my highway rpms. or Comp XE 256H-12 with 6 deg more duration and slightly more lift and with about the same hp and tq figures only the peaks are 500 rpm higher, rpm range 1.2 to 5.2k rpm, and tq at 1600 is about 15 less but still over 300, so either cam should really work quite well with my gearing. I don't want to rev my shortblock over 5200 or so cuz I need it to last, and this is mostly a cruiser vehicle anyway. Neither cam has much overlap which also helps lower rpm.

Now for the carb question. That TB efi setup will support about 325 hp with a 3 lb psi increase in fuel pressure from stock at the built in regulator and there's a closed loop setup available that would keep the mix correct at steady engine speeds like highway, and I've got an Innovate wide band I would use to help tune open loop. Either of those cams would fall within that hp maximum and of course efi is responsive and smooth with sharp throttle response at low rpms almost always more so than any carb.

BUT with this electric carb, properly set up with a little help (I've worked on QJets on stock 350's but they were 70's vehicles where you could adjust things like metering rod height and secondary opening etc) as this one is more complicated than a regular Q Jet
and I wasn't a huge expert on them to start with - any idea on the comparative throttle response and gas mileage from a properly set up carb versus efi? I also will likely go to a Performer type intake as I can't imagine the stocker is much of anything, esp given the heads it's made to match up with, except maybe a couple lbs more torque than a Performer right at the bottom. If I'd go with the efi, I'd go to a regular vac HEI as the ECM wouldn't function anyway.

If the carb would be pretty close to the low rpm throttle response and the smooth power of efi, with good gas mileage, I might keep it. But altho I've had lots of carb experience since I got into hot rodding in the mid 60's, I've never seen a carb that could hold a candle to efi at low rpm, either with response or smoothness or mileage, and I've already got the efi setup.


Appreciate factual info and/or tips from experience which it sounds like a couple of you guys have.

This is a couple years off, but I'm laid off right now so have time to research and plan.
When I read this I think you did your research just right!

The only thing I'm a little worried about is your short block. Mine was also running fine but let it rebuild when I was bussy with a body off. Found out the crankshaft bearings has some oil problem so were to fail anyway... I think, when you're bussy, don't start building on a unknown short block.

Greetings Peter
I tied to think thru that of course, considering not only the work but also the budget I may have to deal with when I start.

After reading this article,

Vortec Small Block 350 Chevy Engine Build - Popular Hot Rodding Magazine

I think it may at least pay to look into a good truck Vortec shortblock and just freshen it up with rings & bearings and timing set, assuming the oil pump would be OK, rather than new pistons and other work on my existing shortblock. I hadn't realized the Vortec shortblocks were available so inexpensively. Then I'd also have better rods and pistons, a 4 bolt block, and the one piece rear main seal for not much different than replacing pistons in my existing block and having rods checked and/or resized, boring, maybe the crank ground, etc.

And could even get all that prepared and on the stand before I even
dig into it.

I'll just have to see how I'm setting when I get closer to actually picking up the wrenches.
With the things you want to do and you're detail to quench I think it's much better to get another short block and start from there :)

Greetings Peter
Oh, there is no doubt at all about that, but with being in a long layoff and a signicant business loss last year, my retirement plans have taken a big hit.

So the budget is the ruling factor, no what I would like to do. I had originally planned for a nice stout 383, so you can see how much I have scaled back.

So unless something unanticipated happens in the next 3 or 4 years, I will be constrained to using that stock shortblock and minimizing my additional investment. That means no roller shortblock, not even any new pistons in this shortblock that I can make quench with.

I have other more important priorities for retirement than doing everything I would like to this car.
The original carb does require some special tools and know how (which is available here) to set up correctly, but once it is set up, it is hard to beat.

As far as your cam choice, you are going too mild! I know the most common mistake in hot rodding is too much cam, but for a Vette, you need more than a 256 (You have a performance vehicle, not a tow truck). The XE262H or XE268H are the best flat tappet choices for a mildly modded Vette. With the oils available today, a roller cam is really the way to go though. You end up spending the difference in cost on additives to keep from wiping a lobe anyway (XR264HR or XR270HR).

God bless, Sensei
In general and in most situations I completely agree with you on cam.

And I do have to consider budget so that ruled out a retro roller, or a later roller block that I'd have to at least do rings, bearings, gaskets, etc in and then still pay over $300 for just the cam.

So when I ended up with flat tappet, I first looked at the 262. A few years ago I had done quite a bit of research and read that the 262 often made nearly as much power and usually as much if not a dab more torque, esp in the under 3500 rpm range, than the 268.

So when I resumed research recently, tht's where I started. But when I started looking at my gearing and calculating road rpms with the OD trans, I found I'd be in the range of from 1600 rpm at 58 to 2200 rpm at 78. That's on purpose for road trip gas mileage, and even a moderate cam can be notchy and soft at those low rpms. And using a stock shortblock without the benefit of new rings, bearings, etc, I don't want to stress it with any higher rpms. Budget again. (A few years ago I had spec'd out a fairly healthy 383 but with the economy & the hit my IRA took, gas prices predicted to go at least $5 this year, and donating a big chunk to help my daughter start a business, I've had to lower my sights significantly.

But thinking back to a stock 300 hp 350 68 Nova I had awhile, 325hp and nearly 330 lbs of torque in a 3500 lb car still will be fun to drive for an old fart like me, esp with the deeper 2.84 1st gear in the 200-4R with a 2200 or so stall. My days of cruising at 85 and then romping on it if I felt like it are over. Can't afford the tickets or insurance anyway. Speeding tickets used to be cheap back in the 60's. I am an expert on that!! :) (3 tickets of 155 in a 55 at age 19 in front of the same judge within 3 months cost me a total of $85 and my insurance never went up from $130 for 6 months full coverage! And that's just a glimpse! ) Not no more!! But, for sure, that was a blast!

Anyway, got on Comp Cams Camquest like I said, and saw the relative torque curves of their cams, and the 262 and esp the 268 are pretty soft torque wise in the lower rpm ranges compared to this one. And considering the efi system I have will support a max of 325 hp, it doesn't seem to make sense to put in a cam tht will make 350+ hp if my efi won't support that, esp if I'm going to lose torque in that important 1600 to ~3000 range doing it. I'd love to have a FAST EZ Efi, but
budget budget budget. That cam has a hp peak of 4000 with an upper rpm effective limit in the low 5's, just what I want to limit my shortblock to, and 300+ torque nearly off idle to the peak of 350 at 3500. At least per CamQuest and I know that's "representative" but it compared very favorably for torque under 3000 to the other larger cams on the same dyno program.

I've had my share of big cam motors in my younger days (600+ hp fully built street hemi, warmed up 375 hp 396 Camaro, mildly upgraded 440 6 pack, etc) so have had plenty experience with driving bigger cams which helps me zero in on what I want with this car. I'll be later 60's (disgusting but there ain't a whole lot I can about it!) by the time I get this on the road, and the main goal is a daily driver I can actually afford to drive and take some longer trips in and that will still have
a little "go" out of the hole - something nobody expects from an old timer :)

Not that I wouldn't fully enjoy a 350+ or 425 hp engine, I've just got to balance what that would cost versus what else I may want to spend that extra money on. And that includes mono-spring for the front, rack & pinion setup, the 200-4R trans, probly a 6 link for the rear, have to re-do the brakes with a stainless brake setup, a little freer exhaust, cam, heads, roller rockers, electric fan, etc. So have plenty of holes to drop my money in! :) Even with me doing all the work.

So - I agree with you as far as it goes, but have to consider all the angles on this one. I ran that one and the 3 bigger ones thru the Comp Cams Camquest dyno thing, and the more I think of the overall picture, it's the one that will fit my needs the best.

If I want to go fast on occasion, I've still got my 1150 bike. :)
It's not a crotch rocket, but it's not a 2 ton Harley or Harley wanna-be either.

I have to agree with Macx,

If you don't want to go into the 5k rpm's no need to put a big cam into the engine. For me the 262 will also be the max for my engine, but for each his own.
Maybe a good idea for Macx to find a dyno2003 program somewhere from the net. Good program to put in all the engine componants and see the "final" outcome. Not that I take the HP and torque numbers to seriously but it's great to comparing two engine setups.

Greetings Peter
I have found sources for Desktop Dyno (haven't downloaded yet) where I intend to do just that to try to get a little closer to probable reality with the numbers.

Sorry to hear about your layoff and financial situation, on a brighter note you have posted a couple really good threads. I've found when you seek advice on a mild build the tendency is for people to give you info that leads to a larger build then you had planned, you get a lot of good info but it really does not fit your needs. I guess it rare for guys to just perform a mild performance rebuild, everyone wants 400+ HP and a ton of expensive parts to get there. My present build on my 81 is much like what you are looking for, a increase in torque and around 300 HP with good gas mileage. I can not give you the specs to compare with but I went with a Melling MTC-1 cam. The Duration at .050 is I 204 E 214, it has a lift of I .422 and E .444 and is a good cam for a lot less money. Do a google on this cam and you will find some good info. Now it is a RV style cam so the upper limits may be less then what you are looking for, I plan on using 1.6 rockers for a little more lift. My engine is not complete yet so I really don't know how this will work out, but I expect it will run well with good gas mileage on 89 oct fuel. The budget for my build was $1000.00 I should come in right around $1300.00. Good Luck Tim
Well, thank you, kind sir!

Yes, most people think in terms of more hp, but most people are younger than I am, too! :-/

Like I said, I've had my fun and now becuz of budget my priority has to be gas mileage with an economical build.

The Comp cam I'm focusing on for now is quite similar to yours, with 206-212 and I think it was 434-444. It's got low overlap and an early intake closing event to trap compression pressure at low rpms.

It will definitely be the smallest cam, by far, that I've ever installed as an upgrade, tho!

Just for some kind of comparison, and it is admittedly apples to oranges, but in my 5,000 lb 4x4 with 31" tires and
3.92's (gives it an equivalent overall ratio to 3.55's and 225 tires) the oem cam in the 5.9 is only 194 intake @ 050.
And the intake and TB are equally geared to low rpm torque.

But it has "acceptable" torque and throttle response up thru hi 4's or lo 5's and of course great off idle -
esp with my 2400 stall :)

In my 5.8 "hot rod Lincoln" with only 9.0 comp ratio (at best), with Performer heads and GT40 type long tube
torque biased intake, I've got a Trick Flow Stg 1 cam which the mfg described as a mild first step upgrade for stock
5.0 Mustangs (!) even with 221 intake @ 050. In the 5.8 with the other upgrades it idles at 800 and holds a steady
idle speed but does lope (sounds nice, actually :), but the torque is a little soft until nearly 3k rpm when I can really
feel it "come up on the cam". I've got 1.72 roller rockers instead of the stock 1.6 ratio to help make up for the extra cubes a little, so the lift is around .540 (Ford's take more lift for some reason). That torque curve is decidedly higher than what I'm looking for in the Vette. The EQ heads I'm currenty considering flow a dab less than the Performers, and a Performer carb intake isn't as torquey as a long runner efi intake. Surprisingly, with the large dish in the pistons, even tho it's right in the center so the area under the flat part of the comb chamber does have some actual quench, I can run the stock advance curve of the 5.0 Mustang ecm and it does just fine on 87 in cold weather - I run 89 in warm weather just for safety sake but think I could get by with 87 except in really hot weather. I do have a 180 stat and huge electric fan so with the alum heads sucking out more heat than an iron head will is probly why that works. I'm just figuring on 91 for the Vette cuz most of the piston top has a dish (from what I read) so no quench at all and iron heads. That way I can at
least run some decent advance to pick up a little torque and mileage over what I could with even 89. That's why I started looking at the 262 which I had about several years ago as an excellent street cam with good power and toque, then worked my way down.

The 252 is 217 intake with a couple degrees more overlap and an rpm range of 1.3 to 5.3 which wouldn't be bad, either,
and using that Comp Camquest dyno program to compare all their cams within my desired range, it had the same hp and
torque peak rpms and just a tad lower torque at 1500 and at peak in that program. So that would work well, also.
Now both the 256 & the 260 have 218 intake, and the same intake closing event, but both have more overlap and
about the same peak hp and torque but both peaks are 500 rpm higher, and the tq at 1500 is 15 to 20 lower than the
shorter 252 and 249. Stated rpm ranges are quite close.

I was surprised at the calculated hp of both of them at only about 15 less than the 262, but the tq peak on the 249 is
500 rpm lower at 3k with the 252's peak at 3.5k. The 249 has 2 deg less overlap at 050 and a 1 deg narrower lobe angle
which probly accounts for the tq being at a 500 rpm lower peak and about 5 tq more at 1500 than the 252.

Actually, that's really splitting hairs, I know. And, again, only representative as a comparison on that dyno program which
is just in the ballpark as far as nailing down engine specs. But that does illustrate the direction my thinking goes and why.

I've found a much more comprehensive Desktop Dyno program that I'm going to download and run one of these days to
see if it gives different results overall, and how the comparison's match up too.

So in the long run it could be the 249 or the 252. Comp describes both of them as being excellent TBI cams. The 249 is in the XE-H series and the 252 in the XFI series but both are described as computer and efi compatible. I've got plenty of time to play with "what if's" and do the research now, and to me that's a big part of the fun.

I may be dreaming, or just hoping too hard, but with the 3.31 gears and .067 OD in the 200-4R in the 3500 lb (?) car
and that short cam with efi, I'm hoping for mpg in the pretty hi 20's. Heck, I get 23+ in that hot rod Lincoln with quite a bit hotter motor, 500+ lbs heavier, and 3.55 gears, running near 80 in hilly country. And could probly get more if I just bump the static advance about 4 to 6 degrees like the Mustang guys do and run 91.

So as far as the motor goes, my investment will be the ~$650 heads, ~$250 roller rockers, ~$175 to $200 cam & lifters,
and a $50 performance vacuum HEI distrib. Plus gaskets, belts, hoses, T-Stat and maybe a good hi flo water pump.
The trans & converter will suck up around $2000, then there's the diff rebuild, hopefully a mono front spring setup, rack N pinion (I've found a mfg'd bracket that'll work with a Firebird (?) rack) and the stainless brakes upgrade. And a 6 link
rear suspension setup. I want a "complete" package with trans & suspension & steering & brakes, altho without going to the high end parts, so if I can keep my completed motor under maybe $1500 I'll have a chance. :)
When you're really on a budget I'm thinking.. Why do you want to change a perfect front coil springs into a (sorry to say) useless "upgrade" monospring? Second, reading about the rack and pinion aftermarket things for C3's. A rack and pinion should be in front of your front wheel center not behind like a C3 has. I'm happy I just rebuild my steering stuff and not transformed it into a rack and pinoin with the negative side effects. Saves you also alot of money!

Greetings Peter

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