Hmmm..... my last response got lost somehow..... oh well.

In the case of these low revving engines, wouldn't the intake runner length have more effect than the plenum volume? As i understand it, the longer runners help to move peak VE to the lower rpms. Then combine this with a proper cam for these rpms and that's about the best you can do to maximize tq and efficiency on the low end. Correct me if i'm wrong, but wouldn't moving peak VE towards the intended operating range max out your possible gas milege? I know the TPI equipped L98 had a boatload of torque in the lower rpms... more than the LT1 and LS1... but not sure how much different the milege was between them. I heard someone say one time that the TPI was the best truck induction ever offered.... but never put in a truck. Weird

Could swapping to a TPI manifold and better cam be the key to what you're after WS6 TA?

 

If anything I suspect that my cruising rpm (about 1500 at 60mph) is too low.

Uh, Cummins GM???

Either way you missed my point, my truck is a 2 door K1500 Blazer (same as a Tahoe). I need the back seat (it's the only car that I own that can carry more then 2 people) and It has be that short to park it where I have to park it at times.

I agree, but the runner length thing is almost a no brainer, where the volume thing as far as I can tell is almost completely not understood besides top end/high rpm's.

The TPI is one of the few V8 manifolds out there that were designed to tune for the second harmonic (the only other that I can think of is possibly the old Chrysler crossrams). The thing is that the TPI setup doesn't do anything for you below about 3000rpm, and I can't understand why someone would purposely loose any ram tuning, responsiveness and packaging nightmares to hit a roughly 1.5% stronger tuning pulse (as opposed to the 3rd harmonic), especially if you'll have dead areas between the harmonics where you get a negative pulse (BTW, the TPI's 3rd harmonic is around 5200rpm, higher then what the stock heads will flow well to, so you can never actually use it).

FWIW, I currently own an LT1 (what, 2" runners plus whatever is in the head, I seem to remember around 6" total) an L98 (roughly 25" runners), and just got rid of my Crossfire TA (somewhere in the 11-13" range). Sub 4800rpm the LT1 and L98 are very similar, and will run virtually the same times if you shifted below that, the LT1 is crisper and smoother in it's delivery. Above 4800rpm the LT1 will kill it. The crossfire 305 will kill both of them in the "around town stupid fun category" since it's intake is completely optimized for low rpm, but pretty much falls on it's face about where the TPI does.

FWIW, I'm thinking of adapting the crossfire intake to the TBI truck setup, since it seems perfect for the trucks operating range and the intake is cast with a separate top plate which could be easily changed/modified for different TB's (or even a carb) and plenum volumes.

That's my thought also, but I keep having people tell me that you will not get peak gas mileage or even good gas mileage at your peak ve/torque rpm… I don't really think I believe them

 

Hmmm... interesting. I read some of Vizzard's work on ram tuning a while back... but was barely able to follow it cause he got VERY complex with the material. But i was thinking something a little different. My computer crashed a few months ago and i lost a pretty good bit of information. Last year i spent a few months collecting dyno information on the f-body engines as it seemed like it would give some more insight to different setups.... specifically the L98, LT1 and LS1. Now i realize this is a pretty vague comparison since they have different heads.... but since all three were close in displacement and the intakes were of varying length i hoped it could help. I kept the stipulation that the dynos i would use, the car couldn't have anything more that a CAI and catback exhaust (no headers or anything else). I took some averages for each engine and plotted their hp and tq curves on the same graph. The trannys for the L98s were obviously autos so i couldn't get anything below 3000rpm (but at these rpms the convertor shouldn't affect it too much). The LTs, and LSs were mostly M6s so i could get good data down to 2000 or so. One interesting thing i found is that at 3500, all the engines were making the EXACT SAME TORQUE. The L98s were falling hard while the LSs was just starting to pass the LTs. At 3000 the L98's torque curve was already falling by a fair amount so i assumed it peaked around 2500 or so. But even while falling, at 3000 it was still some 15lb ft above the LT, which in turn was about 10 or so above the LS. The LT1's torque curve is flat as a table top from 2300 to about 4500 where it tapers off. The LS1's torque curve looks kinda weird... you can definitely tell where it comes into it's tuning range. The curve had a fairly gradual upward slope up to around 4000 or so where it would "jump" up and hold that torque past 5000rpm before tapering off. I think i used maybe 6 or 7 different LS1 graphs and all resembled this. I printed a copy out of the comparison graph... if i can find it i'll try to scan it.

Anyway, going off this i figured that the intakes were the biggest differences in how they made low end torque rather than the heads. I wish i could find some near stock Vortec graphs so i could compare those to the L98's torque curves. I just figured that the longer runners would be your best best (along with some good heads and a cam).

[edit]- forgot to add... assuming we could figure out the plenum volume for each of the intakes... would it be reasonable to try to make an assumption regarding the plenum volume's effect on each engine? You said the L98's runners were about 25 inches and the LT1s are about 6 inches. I think the LS1's runners are between 10 and 12 inches. But not sure on the difference in plenum volume between each.

Thoughts?

 

What's throwing that comparison off is the fact that you're comparing vastly different head designs. The L98 has what is probably the crappiest head that GM put on an sbc that size on it, the stock LT1 is basically GM's more modern fast burn design which typically flows 40cfm or so more then the L98 and is much less detonation sensitive (even if it wasn't for the reverse cooling and aluminum…), and the LS1 has the benefits of the LT1 plus a killer, completely different ports and geometry changes allowing much better injector positioning…

I don’t think that you could ever get an accurate comparison of the effects of plenum volume and runner length comparing those engines like that. FWIW, if you're L98 is laying down by 3000rpm, then I would suggest that there is something actually wrong with it (besides what GM saddled it with).